The Choice of Farm Dam Designs
Now that I have described fully the design, construction anduse of one type of farm dam, this chapter will, in less detail, consider the designand construction of other dams, A full appreciation of these dams can be obtained,however, from comparisons with the keyline dam of Chapter XVIII.
In Chapter VII I have classified farm valley dams into threetypes--the high or keyline dam, the reservoir, and the lower valley dam. Both thereservoir and the lower valley dam are larger usually than the high or keyline dam.The larger capacity farm dam water storages are generally not so much larger constructionjobs as their capacities would suggest. The sections of the wall, as described forthe keyline dam, are, with some exceptions, the same as these for the bigger storages.For instance, the same wall section may, in different circumstances of land shape,impound quantities of water varying from 2-1/2 million gallons to 150 million gallons(10 acre feet to 600 acre feet).
A dam with a 20-foot depth of water at the inflow end of thelockpipe could be a keyline dam, or, alternatively, a reservoir, and have a capacityrange of 21 to 100 million gallons. The maximum range of dam capacities varies widely,but there is a closer comparison in the minimum ranges of the various types of dam.The minimum capacity of a dam that is worthwhile for regular and effective irrigationis 2-1/2 million gallons, and this should be a limitation imposed only by land shapeand run-off. The minimum size for a reservoir may be eight million gallons, and fora lower valley dam perhaps twelve million gallons.
A keyline dam on one farm or grazing property may have a greatercapacity than the largest practical lower valley dam on another property. There isa general tendency for the capacity range of dams, all with the same depth of water,to be much wider as the country slope is longer. This length of land was definedin Chapter VI as the distance from the main ridge to the watercourse below.
Reservoir Sites: The reservoir in Keyline is classifiedas a reserve water storage located. at an intermediate elevation between the highor keyline dams and the lower valley dams. A reservoir may be located at the lowend of a large primary valley and just above the point where the primary valley joinsa secondary valley; or where a primary valley flows into a creek, a river, a lake,or flows to a flat plain. A reservoir is also suitably located in the upper areaof a secondary valley and in height somewhere below one or all of the keyline damsin the primary valleys which flow to and together form the secondary valley. Circumstancesof climate, land shape and the association of other storages, or sites for otherstorages, dictate the location of reservoirs.
A reservoir (or several reservoirs), as part of a complete Keylinewater conservation scheme, is designed to remain full during the time the higherand lower dams are used for irrigation. When these storages have been used, the reservoiris still full, despite some evaporation which may have taken place, and so the farmeror grazier in a dry time has at least one or more large reservoirs filled with water.He can plan his irrigation now from the reservoir and regulate stocking and managementaccordingly.
A reservoir is seen then as the largest body of longest lastingwater on the farm or grazing property. It may serve as the basis for the planningfor such lesser but permanent water requirements as stock troughs supplied via pipelinesdirectly from a constant flow take-off on the outlet equipment. It is also the onetype of dam that is most suitable for fish stocking, and one on which other formsof pleasurable water activities may be planned. Dams, such as keyline dams, in whichthe water level is constantly changing, are not so suitable for fish stocking oras pleasure resorts.
The design-for-use suggested for the various types of dams inKeyline (e.g., keyline dams and lower valley dams as general irrigation water andto be put into use at the same time, with reservoirs as reserve water during theuse of these other dams), may be altered to suit individual requirements and convenienceof management. However, the use-pattern for the stored water should be such thatwhen some of the dams have storage capacity available by having been in use for irrigation,and run-off rain occurs, then no water will leave the property until all dams areagain filled.
The design of reservoirs follows very closely the general designfeatures of the keyline dam already discussed. It is assumed that the reservoir ison the same property as our keyline dam. The wall height may be the same or lessaccording to land shape and run-off conditions. Water capacity will be cheaper, thestorage will be larger, and the valley floor slope will generally be flatter. Therelationship between storage capacity and annual run-off can be up to double thatfor the keyline dam.
I have suggested that the keyline dam should be designed tohold 1-1/2 times the annual run-off of its total catchment area, and this capacityis considered more a minimum than a maximum. Where the climate has a very uniformor regular rainfall characteristic the capacity should be somewhat less than thisrecommendation. The relationship of the reservoir size to its catchment area, otherthan in the climatic conditions just mentioned, should be from two to more than threetimes the average annual run-off. The actual ratio of storage to run-off should berelated also to the cost of storage. For instance, if the storage cost in the keylinedam, which may range from under £20 to over £40 per acre foot, is doublethat obtainable in the reservoir site, then it could be good business to build reservoircapacity of double the storage capacity to run-off ratio of the keyline dam. If thestorage capacity cost in the reservoir is more favourably related to the other dams,then it could be made even larger. In this case the reservoir's natural catchmentmay be increased by the use of a special water conservation drain. By keeping suchpoints in mind, the circumstances of climate and land shape applying to each individualreservoir will clearly indicate the answers to these various questions.
For purposes of illustrating design and construction features,one of my own reservoirs may form a background. The design of the reservoir is illustratedin the plan and section immediately following, and titled accordingly. (See Fig.16.)
The site had a general valley floor slope of 1 in 37. The featuresof land shape that determined the reservoir's precise location were, first, a saddleon the ridge on the left bank (the bank on the left of a valley when looking downstream)of the valley was suitable for the spillway for the reservoir and which would alsoallow all overflow water to fall into another dam site; secondly, the site was abovea large area of land convenient for irrigation which would permit the water to beused for either early irrigation or reservoir use; thirdly, the contour shape ofthe valley was good and the reservoir would be in a position to receive the overflowof three other dams.
The depth of the reservoir, because of these valuable features,was fixed at 24 feet; the height of the constructed wall was, therefore, with freeboardand settlement allowance, 29 feet 3 inches, but this was increased to 30 feet asa counter to wave erosion, which, while the dam was new, could be a consideration.The maximum width of the base of the wall was 122 feet, being calculated from battersI in 2; settled height 27 feet, a crest (wider than usual), 14 feet.
The length of lockpipe laid was 130 feet and the baffle plateswere eight feet apart in the forward half of the wall foundation. With these variationsit was constructed as was the keyline dam of Chapter XVIII. (Pictures of the newlycompleted dam and stages of its construction are contained in the Pictorial sectionof this book.)
The lockpipe was placed in position on one side of the valleycentre line one foot below solid ground, and the end of the lockpipe on the insideof the dam was four feet six inches below the solid bottom of the valley. The wholeof the earth for the wall was obtained within the dam site and at a satisfactoryclose distance to the wall, and also without excavating earth below the level ofthe lockpipe inlet.
This reservoir has nine acres within its top water line. Inaddition, there is another acre or more of wall crest, back wall batter, drains andgenerally disturbed ground, which were all covered with the top soil we were ableto strip from the original area. The land within the top water line was keyline cultivatedand together with all areas of disturbed earth was sown with grass seed and superphosphate.The first flow of water into the dam did not move the newly-placed, sown and cultivatedsoil.
The average depth of the reservoir is approximately 10 feet,and storage capacity is 90 acre feet. The cost of water storage was under £17per acre foot, allowing £6 per hour for the 100-horse-power bulldozers whichbuilt it, and 20% of this cost represents the lockpipe equipment and drains for irrigation.The reservoir has a rather special asset value worth many times its cost, yet suchis the variations of these factors in farm dams that the nearest dam to it on thesame property was much less than half this storage price, while another adjacentdam has a storage capacity which cost double that of the reservoir. All three dams,however, are very much more valuable than their costs, particularly so since theyinclude the whole of the equipment and drains for effective irrigation, at the lowestof all operating expenses.
It will be seen that this reservoir has some special features,and this is so with most reservoirs. The recognition and the successful use of theindividual and special features of each site depend largely on an appreciation ofthe relationship of climatic conditions to the shapes of the land. A facility injudging these matters is a great asset to the farmer and grazier. To give anotherinstance of this individual value-of-site feature, we have a reservoir on anotherproperty which gains in usefulness by being precisely located according to the aboveprinciples. The effect of this location is that the spillway of the reservoir wasmade to coincide with the irrigation drain of a keyline dam located in the head orhighest primary valley of the secondary valley formation in which the reservoir islocated. Now we are able to turn water from the keyline dam into the reservoir, orwe can direct the run-off past the keyline dam into the reservoir, or past the reservoirvia its spillway into a lower keyline dam, or we may irrigate from the drain betweenthe reservoir and the second dam. Flood overflow from the first or highest keylinedam can be controlled to flow either into the reservoir or into the second keylinedam, or, still further, if need be, into other lower keyline dams, or even the lowervalley dam near the boundary of the whole region. Again, under drought conditions,after the top keyline dam is emptied by irrigating and its irrigation area. is requiringmore water, this can be supplied by pumping water for the short distance from theoutlet of the reservoir into its own spillway and thence it would flow along theirrigation drain of the first keyline dam.
A reservoir is seen to be a very valuable and versatiletype of dam in Keyline. It is usually a permanent supply, and the best type of damfor a pleasure resort. In a secondary land unit, one or more reservoirs may be employedaccording to climate, land shape and the layout of the rest of the dams. Reservoirs,then, are the best farm insurance for dry times, in that a drought would be welladvanced by the time other water storages were depleted, and with stock held in topcondition, there would still be a large supply of water remaining. At the stage ofthe drought where it would be necessary to commence the use of a reservoir for irrigationthe landholder would be in an excellent position from which to view the prospectsof continuing drought. If a farmer or grazier can sell when most others have nothingfit for sale he can profit considerably. If he is also in a position to buy whenothers must sell, he is even better off.
Lower valley dams, in conjunction with the keyline damsand reservoirs, complete the series of valley dams that are available for constructionon much of the undulating country. On the one property the lower valley dam willusually be the largest of the three types, although on occasions the reservoir mayequal or exceed the storage capacity of the lower valley dam.
In planning for the complete conservation of all run-off water,the lower valley dam is designed to hold the overflow of all dams above it in thesecondary land unit as well as all the run-off from areas below these keyline damsand reservoirs. It represents the last opportunity of conserving all such run-offbefore it leaves the secondary land unit.
The design of a lower valley dam is similar to that ofthe reservoir already described, but differing in that usually the lower dam is oflarger capacity and has a flatter valley floor slope. Its capacity range on landof widely differing shapes may be from under 12 million to 100 million gallons ormore. To illustrate its design and construction it may be assumed that a lower valleydam site has a valley floor slope of 1 in 55, and that it is to have a depth of waterof 20 feet at the inlet into the lockpipe. Here the flatter valley floor slope necessitatesa different design to that of the reservoir discussed. Such a lower valley dam isillustrated opposite this page, where both a plan and section of the dam are shown.The batters for the wall will be assumed to be 1 in 2, therefore, with a 20 footdepth of water, the final dimensions for the cross section shape of the finishedwall will be the same as that of the Keyline dam construction of Chapter XVIII. Themaximum width of the wall is 104 feet, and, with the flatter valley floor slope of1 in 55, the variation in level of the valley floor inside the dam will be only twofeet above that of the back or downstream toe. Therefore, after allowing for thelockpipe being positioned on one side of the valley and a foot into the solid valleyfloor, its depth below the valley on the inside of the dam will be three feet. Theearth for the construction of the wall cannot be obtained from the valley down tothis level, as was done in the construction of both the keyline dam of Chapter XVIIIand the reservoir recently described. The distance away from the wall where someof the earth required would have to be secured would be too far for efficient bulldozeroperation, so earth will need to be excavated from the inside area of the dam nearthe wall and below the lockpipe level. Other than the changed design of the excavationarea occasioned by the flatter valley floor slope, the design and the constructionare similar to that of the dams earlier described.
The length of the wall of a lower valley dam may be considerablylonger than the wall of a keyline dam having a similar depth of water, and so theconstruction may take a relatively longer time. The excavation of the material forthe wall should be planned in two stages. In the first stage the earth is excavatedonly down to the level of the lockpipe and extending back from the wall toe at thislevel for approximately 100 feet. In this size dam and valley floor slope, earthdown to four feet below the lockpipe level will need to be used, so an excavationis now made to this depth, and, to begin with, in the immediate vicinity of the inletof the lockpipe. If heavy rain falls it would pond near the lockpipe in the mostconvenient position to be pumped out through the lockpipe. Also, if the area excavatedto provide wall material is taken down to the full depth at this point and then extendedoutward from the lockpipe, water ponding in this area may be left there and willnot hamper the completion of the wall.
The spillway of a lower valley dam, because of its generallylarger catchment area, needs to be wider than those of the other two types of dams.Even though the dams above will hold large capacity from heavy run-off, there maybe rare occasions when the lower dams will, when all others in the same land unitare filled, be required to take all the run-off from the whole catchment area. Wherewater conservation drains augment the catchment area these can be blocked and breachedto reduce the excess inflow. There are different means of calculating spillway sizein relation to catchment area, but as these are based on other factors which themselvesare difficult to determine, a more reliable guide is local or farmer informationas to the height that previous floods reached in the valley under consideration.Against this type of information one method often quoted may serve as a check. Thespillway, according to this method, should be a width in feet equal to the figureobtained from the square root of four times the catchment area in acres. Using simplefigures, the spillway of a dam having a catchment area of 36 acres is equal to thesquare root of four times 36, which is 12 feet, and the spillway of a dam havinga catchment area of 400 acres is equal to the square root of four times 400, whichis equal to 40 feet.
On my own properties there has been one occasion when thesewidths of spillway may have been inadequate, and the occasion was not during heavygeneral flood rains. However, subject to local knowledge, and also considering thegreatly improved safety factors provided by an adequate outlet system, it is my opinionthat this formula should serve as a satisfactory guide.
Therefore, if the lower valley dam under discussion has a totalnatural catchment of 400 acres, including the catchments of the dams which wouldoverflow into it, then the spillway will be 40 feet wide on the floor of the spillway.The section of the spillway is 40 feet wide on the level floor, with a slope to thewall of the dam of 1 in 3-1/2 to 1 in 4 and with a similar slope on the side awayfrom the wall of the dam.
Whenever a dam is flowing a large discharge through its spillwaythe lockpipe valve can be opened. "Sour" water, which sometimes lies atthe bottom of a dam, is thus removed, and there is also an additional safeguard fromthis practice.
The use of the lower valley storage is similar to thatof the keyline dam, it being a continuous-working and quick-profit dam. Wheneverit contains water above the level of the lockpipe the dam is used continuously ifirrigation is needed and can be put to advantage. Lower valley dam storage is nota supplementary system but is part of the regular farming and grazing enterprise.
The methods of irrigation from the lower valley dam depend onthe circumstances of land shape and the type of pasture and crop to be grown. Althoughthe dam is low in elevation in its situation in a secondary land unit, there maybe much land below it on some properties, and on others it will be near the farm'slowest boundary. Our own lower dams illustrate these varying conditions. On one property"the Pond", or lower valley dam (16 million gallons) is within a few yardsof our lower boundary fence, so in this case the water of the dam is pumped up toan irrigation drain directly from the lockpipe outlet via a pipeline to a point nearly200 yards away and twenty-five feet vertically above the top water level of the dam.The pump delivers a little over 1,000 gallons a minute (60,000 gallons per hour),and the method of irrigation is again keyline pattern suited to our purpose, whichis generally pasture growing, but on occasions may be the growing of other foddercrops. A lower valley dam on another property is near the lower limit of the landunit and very like the reservoir mentioned in the construction details except thatit is somewhat larger. Whilst it is low in its own land unit, it is high above alarge area of our own property, and so its utilisation follows the lines of thosefor the keyline dam of Chapter XVIII. In this particular dam, when all the waterthat will flow from the outlet for irrigating is used, there will still remain anacre and a half of water six feet deep which lies in the excavation area below thelockpipe, and from which it was necessary to dig earth for the wall construction.Although this water represents only 2% of the capacity of the dam, it also representsa large stock water reserve but one which we will rarely need.
A lower valley dam, as I have said, is a real money-making dam.Apart from keyline pattern irrigation, the dam in various circumstances of climate,land shape and property management can supply water for almost any type of irrigation.In gently to steeply undulating country, the lower dam is usually adjacent to theflattest areas of the farm, and any type of watering now characteristic of the largeirrigation district can be adopted here. These include border checks and bays (smallcrop and pasture), contour furrows (pasture), contour bays (rice, etc.), furrow irrigation(vegetable and orchards), and the basin system of irrigation (crops and pastures).All these systems and methods of irrigation are dealt with satisfactorily in officialpublications, so it is not necessary to include them here. Our lower valley typedam is ideal for any of the methods of spray irrigation (these also are describedin official books), which may be suitable for particular purposes of cropping. Itis suggested that the outlet of the dam is the most suitable point for a permanentor casual pumping set-up. Since the outlet is situated to one side of the low pointof the valley behind the dam and with the water of the dam always above the levelof the pump, pumping would not require a foot valve or any pump priming. The pumpalways works when the engine is started. It is also the best position for a permanentinstallation. Now-a-days, permanent pump and engine irrigation set-ups are placedon the wall of the dam, and after initial troubles have been ironed out, they operatesatisfactorily. Personally, I much prefer to have the wall of a dam kept clear, sothat it can be travelled and its soil and pasture aspects improved at the same timeas adjoining paddocks are being worked. The arrangement whereby a pump and engineare placed somewhere around a dam, and, after pumping for a few hours the outfithas to be reset and often put down in the soft muddy area of the dam, is not a money-makingarrangement, although in drought times the worst layout is better than nothing. Oftenafter a dam has been constructed and has filled with run-off water and irrigatinghas been undertaken, the trouble of chasing the receding water into the mud begins;and it is a frustrating, time-consuming and money-wasting effort. Sometimes, too,a bulldozer is brought in to cut a trench from the deep part of a dam back to a spoton the water line to get at the deep water, so as to avoid changing the pump position.A small drag line excavator may be used for the same purpose. However, these thingsare evidence of not only inexperience, because the inexperienced can make a verygood dam from a good plan, but the lack of proper forethought.
The relatively large volume of water available from most lowervalley dams of this Keyline layout are so valuable that the most economical workingset-up for the use of the water becomes very good business. One method which I haveused, and which in other instances may prove suitable, is the use of a pump-sumpfor the permanent site position of the pump and engine. In this arrangement a sumpor hole two feet six inches square and three feet deep is excavated adjacent to thelockpipe outlet, a few inches lower in level and on one side of the valley. The sumpis lined with wood, brick or concrete and a permanent pipe from the lockpipe entersthe sump and a smaller pipe drains surplus water from the top of the sump. The largerpipe from the lockpipe includes a valve. In operation, the valve on the pipe is opened,the sump filled, and the pump started up. The water inlet control is then adjustedso that there is a continuous small flow of water out through the overflow from thesump.
Some years ago I adopted a system which involved a series oflidded sumps 400 feet apart and located in a drain fed from the outlet valve of ahigh dam. Spray irrigation of pasture was the object and a readily portable pumpand engine pumped the water from the sump adjacent to the land to be irrigated. Associatedwith the set-up was a mechanical means of moving the spray lines, which proved verysuccessful. There were two dams, a larger one high enough for water to flow fromit to a second and smaller dam. To use the system, irrigating commenced by usingany water in the smaller dam first and then irrigating from the last sump (the onenearest the second dam) and moving towards the larger dam, using each sump in turn.For the sump to operate successfully there had to be a small overflow of water, whichflowed via the drain to the smaller dam. Pumping from each sump only involved a liftof two feet and there was little trouble in pump priming. Although I did not continuewith the use of this system, as our keyline flow method for pasture irrigating wasvery much quicker and cost almost nothing, the system may be of some value to others.However, the method applies more to the higher dams, and it could be valuable asa drought method of using water from reservoirs. It is mentioned here by way of illustratingthe effectiveness of sump-pumping, but the obvious and best method of pumping isthe direct one from the outlet of the dam.
A lower valley dam may be constructed with a capacity equalto several years of the average annual run-off and still conserve water at low cost,because of its favourable site characteristics. It may remain empty, or nearly soin some instances, and therefore during this time its land area should not be wasted.On occasions two such dams, the second located in the same valley and immediatelyabove the first, may fill, but then remain empty for a year or two, after using allthe water for irrigation. So the valley shape below such dams should be consideredduring dam construction and a good natural shape made or preserved. If old washoutsor eroded gullies lie below top water line, these should be reshaped to a suitablenatural valley form, so that keyline cultivation (parallel downward from the topwater line) when the dam is empty will be effective to the maximum extent. With agood draining shape so provided, the area of the dam when empty can be sown downto special pasture or other crops. When the area of such a dam is dry enough forcultivation it should be worked up quickly and sown as soon as the soil has sweetened.The empty condition of the dam will usually occur in a dry summer, and so can besown and made to produce its special crop.
I have cultivated this type of land and have found that a deepcultivation to gain quick aeration of the soil brings a rapid change and createsadmirably suitable growing conditions; but the soil must not be touched while itis too wet.
The art of land utilisation is largely a matter of making thebest use of water, and these larger lower valley dams offer wonderful opportunitiesin this connection over a very wide range of climates and land types.
Creek dams are classified as farm dams that are constructedin a natural watercourse in which the water flows over raw earth and between confinedand defined banks. The stream may be flowing either permanently or intermittently.(See Fig. 18. )
As the distance down the land from the keypoints of the primaryvalley is increased for the various types of dams, so will the construction problemsthat may be encountered become greater. In the secondary land unit mentioned earlier,of the three types of dams that can be constructed, the highest or keyline dam isusually associated with the best construction conditions. The earth and the foundationsare often better and the stability of the structure can be obtained with the minimumof effort. The reservoir site may have a deeper built-up earth and require a deepercut-off trench into. the more stable materials below. The cut-off trench may haveto be excavated and filled with good earths as an independent job before the lockpipetrench is made. The cut-off trench of a lower valley dam in the same secondary valleymay have further problems. However, the purpose of the cut-off trench and its fillingwith good material is always to prevent the water within a dam from seeping and thenpossibly later flowing under the wall of the dam. If this should happen the wallcould collapse, and in any case the effectiveness of the dam is inevitably reduced.The creek dams, by being sited further down the land, may have these problems accentuated,and so not all creeks are suitable for farm dam construction. While there are waysand means of solving any of these problems, given the necessary finance, this discussionis concerned only with those dams whose construction is within the capacity of farmersand graziers generally, and with dams which will be very profitable to them. In themain, creeks with very large catchment areas are usually not so suitable for farmdams, as are likewise those with deep beds of loose sand below them and also thecreeks of the very flat lands.
If I had continued my classification of valleys in Chapter VIfrom the primary and secondary valley to the next larger system, then this valleywould be the one with the most suitable creeks for the farm creek dams.
The particular circumstances of the land shape and the characteristicof the creek itself should be studied before building any creek dam. For instance,the full benefit of the flow of the creek may often be obtained more economicallyby diverting the flow to an off-the-creek storage, which may be a dam of the typeof the reservoir or lower valley dam, or possibly the contour dam to be later discussed.The kinds of diversion weirs that are suitable for such a purpose vary almost aswidely as the circumstances of their use, but they have one common feature; theyall must be able to take high flood flow of water over the top of the weir.
Good natural sites for creek dams have to be searehed for, andnot only for their largest water-storage capacities, but also for their absence ofconstruction and management problems. On our new property at Orange we have a sitefor a creek dam which has a drainage area of under 600 acres, but if the drainagearea was 4,000 acres the site would still be suitable and there would be no difficultiesin providing cheaply the large spillway capacity that would be required. On the otherhand we have a creek dam on the same property with less than 600 acres of catchmentarea, but the spillway for it had to be excavated, partly with explosives, into rock.A large creek dam may be constructed in some circumstances very cheaply, yet in almostsimilar conditions in the same area it could be an uneconomical task.
The design and construction factors that apply in the constructionof a keyline dam, reservoir and lower valley dam govern theconstruction of the creek dam. If the materials are similar, then wall batters maybe the same as for these other dams. Site preparations are the same as set down forthe keyline dam.
The creek site is often associated with two features not generallyencountered in the other dams; the creek may flow over rock and the earths adjacentmay contain gravel and sands where the creek in earlier times flowed in a differentcourse. It is then the examination of the material available for wan constructionbecomes more important; the cut-off trench in places may need to be deeper to getbelow the loose material, and there is more likelihood that some of the earth forthe wall may need to be obtained outside the area of the dam.
Where water is flowing continuously the whole of the marking-outand site preparation should be completed as far as possible before commencing inthe water. If the bottom of the creek is on solid ground or firm rock the next stagewould be to bulldoze the loose rock and sand straight downstream along the creekbed through the wall site, depositing it a little higher than the creek bottom andon one bank of the creek. Ripping and 'dozing out in the creek bed will form a suitablechannel for laying the lockpipe, which is set with the baffle plates sunk into thecreek bottom. With the lockpipe in position (Chapter XVIII) and completely boltedup, a small earth dam is quickly pushed up with the bulldozer, using the earth nearthe upstream ends of the line. The purpose of this small dam is to hold back theflow of the creek and to force all the flow water through the lockpipe. With thewater under control, the forward half of the lockpipe channel (the creek bed) canbe cleared by hand of rock and excess loose material, which may be deposited in thelower part of the channel towards the back of the wall. In the construction of anyfarm dam it is intended that the seal against the movement of water through the wallis effected in the upstream section of the wall foundation down to the outside (lowerside) of the cut-off trench, and that if water reaches beyond this point it is allowedto get away. If this water were sealed in the wall it could build up enough pressureto blow-out or cause the collapse of a section of the back wall. Therefore, the forwardend of the lockpipe channel is cleaned to back behind the cut-off trench and someloose rock beyond this point is not a disadvantage. The laying of the lockpipe andthe filling of the trench proceeds as in Chapter XVIII.
In a creek where flow is liable to continue for long periodsa dam should have two spillways provided in the design of the dam. There should neverbe any question of the capacity of the main or flood spillway to dispose safely ofthe water. However, it is undesirable that any spillway constructed in earth andcovered with soil and pasture should flow for long periods. The continuous flow eventuallydamages the pasture and soil. If damage is to be avoided where flow is prolonged,then provision should be made for the second spillway, a so-called mechanical spillway.
The mechanical spillway, then, is simply a pipe throughthe wall, and it may be placed some three or four feet below top water level (floodspillway level), with a right-angle bend standing up vertically and having the inletfrom six to twelve inches below the main spillway overflow level. The capacity ofthe mechanical spillway should be from three to six times normal creek flow. Theinlet into it should be approximately three times the diameter of the pipe itselfand be covered with a heavy-gauge one-inch wire mesh. The pipe, after coming throughthe wall, is turned down the wall of the dam and discharges directly into the creekonto a heap of stones. The pipe of the mechanical spillway is set in the wall inthe same manner as a length or two of the heavy pipe sections of lockpipe. The down-pipesection to the creek bed below may be a pipe of lighter gauge.
The main or flood spillway of a creek dam must be completelyadequate for its purpose, that is the disposal of the run-off from the largest floodrains, so that the excess water cannot overtop the earth wall of the dam. The disposalarea where flood run-off re-enters the creek then becomes one of the considerationsof site selection. Very large quantities of water can be returned to the creek viaa pasture ridge developed and pattern cultivated as already described. An adjacentvalley form may be suitable, but whatever means is chosen, the development and preparationof the area should proceed as an essential part of the dam construction.
No land adjacent to a dam should be kept solely for dam purposes,since the larger the flood spillway of a creek dam the more important it is thatit be developed as a good pasture area. Flood overflow does not last long, whilethe mechanical spillway, which has been flowing during the flood, will then divertthe decreasing flow and the main spillway flow will cease. Whenever work is beingdone in the paddock near any dam, improvement of the dam itself can always be givenfirst preference.
So we can see that again with creek dams, as with these otherdams, they offer almost limitless possibilities for widespread improvement of thewhole farming and grazing landscape, and, in addition, they yield those high monetaryreturns which are always a necessary final proof of good farming and grazing practices.
I know of a farm dam so valuable that were its real worth assessed,and allowing for the deduction of its cost, the value of the dam would greatly exceedthe purchase price of the property on which it is situated.
We have a newly constructed creek dam on "Kencarley",at Orange, which serves to illustrate the possibilities associated with planningthat is realistically based on climate, land shape and the farming requirements.The dam has a higher flat catchment, which continues to a lower but steep catchment.Hundreds of feet above this creek dam are other dams with a capacity of 150 milliongallons of water, which will be increased later by another 50 million gallons. Thewater from the dams above may be used on adjacent areas in flow irrigation or turnedinto the creek above the creek dam, which we call "Control" dam. Controlitself is of over 10 million gallons capacity, and full advantage was taken in thedesign and construction of all its natural features. It has a large flood spillway,soil covered and sown to pasture, also an 8-inch mechanical spillway to handle creekflow which is continuous, and with the large dams and irrigation areas above thedam will probably increase in flow within the next two years. For irrigation purposesControl has three lockpipe outlets, one a little to one side from the bottom of thedam, and two 10-inch systems, one on each bank of the valley of the creek and eachlocated at the same level some three feet below the level of the intake into themechanical spillway. The 10-inch lockpipe outlets connect to irrigation drains flowingalong the top of large areas on both sides of the creek. These areas will soon becomevery valuable irrigated land. The steep areas of the dam's watershed are near-verticalslates and schists, which, when their soil fertility and pasture are improved, willabsorb more of the rainfall into the rock below and practically all of which willno doubt add considerably to the high springs which are the present source of thecreek's continuous flow. The developed water capacity of these steep hills, whichwill be consequential to the normal Keyline development, must be the equivalent ofmany millions of gallons of extra storage capacity, and will be used for irrigatingfrom the higher 10-inch outlet systems. Lesser flows and the reserve capacity ofthe dam are available for irrigating from the lower 8-inch system. Any time we wishwe could turn 300,000 gallons an hour extra water into Control dam. Since the entirewater of its catchment is under complete control, we can use the catchment area forirrigation, which, in effect, also will replenish the ground water of the catchmentand maintain and probably soon increase the normal flow of the creek below Control.The two 10-inch systems, apart from the irrigation, could be employed to fill damswhich may later be constructed three-quarters of a mile away from the creek itself.(See Fig. 18. )
In the foregoing discussions it will have been noted that eachtype of dam in turn has come further and lower down the landscape and from the keylinedam at the head of the primary valleys to the generally lesser slope land of thecreek dam. In the flatter country, too, all valley dams can be useful if the valleyspossess sufficient shape. A dam hereabouts will usually follow the design and constructionmethods of the previous ones, but the walls will not be as high and the dams themselveswill have larger surfaces and be shallower in average depth. They will generallybe more affected by evaporation, but this factor is offset by the lower cost storagecapacities. If a keyline dam costing £120 per million gallons of storage capacityis a good business proposition, then a dam in flat country costing £30 per millionand only filling once in two years and losing half its water by evaporation, canstill be an equally good or even better proposition for the farmer and grazier. Apartfrom valley sites for dams, with their very wide suitability over all shapes of land,water can be conserved very economically on gentle or flattish slopes.
A contour dam may be used to advantage on slopes whichcontain no valley form. This dam is essentially a long earth wall of medium heightconstructed from earth which is excavated from immediately above the dam, and withwing-walls made to taper up land to above the water level.
In the flat lands all design features of the dams are flatter;the dams themselves are shallower; the water conservation and the irrigation drainsare both flatter, but the irrigation drains are built up to flow water slightly abovethe level of the land. The land to be used for irrigation is also flatter and allthe flat land methods of irrigation, already mentioned, can be used from the supplyheld in contour dams.
The critical design feature of this type dam, other than theall-important one of climate and its associated run-off, is always that of slope.Contour dams can be constructed on slopes ranging from 1 in 25 (4% slope) to 1 in100 (1% slope). They may be classed or named "straight", "inside",and "outside", according to their general contour shape. A straight contourdam is one whose wall follows a contour line which is reasonably straight; an insidecontour dam follows a contour curving round a flat ridge shape, the dam being onthe inside of the curved shape; and an outside contour dam is one associated witha flat valley formation where the water lies on the outside of the curve of the wall.
:Features similarly associated with the location of the firstvalley dam are to be looked for in the site selection of a first contour dam fora property. It should be as high on the property as convenient; there must be run-offand sufficient catchment area above the water conservation drain to fill the dam.As to size, it may range from five million gallons (20 acre feet approximately) to23 million gallons or more. To bring the matter to practical consideration, we mayassume a contour dam is to be designed with a capacity of 80 acre feet of water;that the slope of the land is 1 in 50 (2% slope) ; that the depth of water at theinlet to the lockpipe is 12 feet; that the land shape contains large low forms only,and that the contour shape of the dam is "straight". This capacity wouldrequire a wall 900 feet long approximately. Water 12 feet deep on a 1 in 50 slopewould place the water line up this slope 12 x 50 or 600 feet, and the dam would thereforehave an area of a little over 12 acres. The average depth of a contour dam is somewhatover 50% of its full depth, or about seven feet in this case, so that the requiredcapacity is satisfied by this general size.
In the medium-size farm dam, a suitable freeboard height isthree feet, but the circumstances of design in a contour dam suggest that this figurebe reduced to two feet. There is no part of the wall of a contour dam that representsthe main bulk of the earth, as is the case in the valley dam, and a failure of partof the wall is not nearly so serious a matter as in the valley dam; moreover, theinflow of water to the dam is readily controllable. These facts also suggest thatthe minimum or cheapest construction methods may be used in building the wall, andalso with the lower wall height the wall batters may be nearly as steep as the mosteconomical slope that the bulldozer equipment can construct.
Wall height will then be 12 feet depth of water plus two feetfreeboard, and as minimum construction methods are to be employed, an allowance forsettlement and shrinkage will be increased to 10%. The constructed wall height istherefore 15.4 feet. The dimensions of the wall section are as shown on the planand section opposite. The constructed height is 15.4 feet, the settled height 14feet, the width of wall shape at the base is 52 feet, and the crest width is 10 feet.The lockpipe will be placed into solid ground and there will be approximately twofeet of earth above the lockpipe level on the inside of the dam. At distances of80 and 100 feet from the inside toe of the wall there will be four and five feetof earth respectively above this level and more than sufficient for the wall withoutdigging earth below the inlet level of the lockpipe. (See Fig. 19. )
The water conservation drain of a contour dam, like those ofall these dams, does not fall directly into the dam, but is constructed right alongand above the dam, and reaching water level height at the spillway of the dam. Asmentioned, the drain may be flatter than the 0.5 % fall generally employed for theother dams. A flatter drain has less capacity, so that the drain needs to be of largersection. The drain should fall in the down land direction, vide Chapter VI.
The position of the lockpipe may be in any portion of the lengthof the wall according to where the water is to be used. If the area of the gentleslope immediately below the dam is to be irrigated, the lockpipe is placed in themain wall at the end where the conservation drain first reaches the dam; in othercircumstances it will be placed in the opposite end.
The price per yard of earth moved in a contour dam of this wallheight win be considerably less than in the higher wall dams. The average haul willbe less, the push up the batter of the wall is shorter, and more of the operation,which is only shallow digging, can be performed in second gear. A reduction of 40%in earth-moving costs is to be expected.
Marking-out and site preparation should proceed as for a keylinedam, with the clear marking-in of the wall shape on the ground with a furrow line.Top water line for the dam should also be marked. That part of the water conservationdrain which is along the top of the dam could be first constructed to prevent anyrun-off into the area of the dam during construction. This drain may have a slopeof from 0.2% to 0.5%, according to features of the land.
A cut-off trench for the full length of the main wall and thetwo wing walls should be used, but may need to be only a few inches deep. Even whereit may be considered that the cut-off trench is not required, it is still advisableto use a shallow trench, since it helps appreciably in controlling the job and insupervision. The area of the wall site is chiselled along the line of the walls toassist bonding as before.
The cross section of the wall as illustrated (and it is notthe minimum that could be used) represents an area of 48 square yards and (acceptingthe two wing walls as containing the equivalent of 200 yards of full wall section),the yardage in the walls is 24,000 cubic yards. The yardage of water storage capacity,with water depth an average of seven feet, is 135,500, or an earth to water ratioof 5.65. In other words, one cubic yard of very cheaply-moved earth creates aboutsix cubic yards of water-storage capacity. The excavation area should be batteredback into the land slope and covered with soil, cultivated and sown as suggestedfor the other dams. A contour dam of this or a larger size contains an area which,when uncovered, could be very valuable land.
The particular purposes for which the water may be used fromsuch a dam are legion and the value of the dam is considerable. We can consider aparticular case by way of example. The rainfall conditions may have been such thatwith the augmented catchment provided by the water conservation drain, the dam willbe filled each year from winter rains. The dam then could be the basis of the productionof a special spring or summer crop on which an extra 12 inches of water from thecontour dam would ensure a successful crop every year. A crop area of 60 acres maybe used for the purpose or a series of three paddocks each of similar size and irrigatedin rotation. The requirements for the lowest cost irrigation are thus provided andany of several methods already discussed for the other dams may be used, accordingto land form and slopes. If the dam will fill each winter, then the dam water shouldbe used fully each spring or summer.
We may suppose now that the water is all used in producing aspring crop and consider the empty dam. The dam area, which was provided during theconstruction with good draining slopes to the outlet, can then be cultivated, whendry enough, and sown to a summer crop. Because of the deep moisture of the dam area,a good summer crop could be produced in the driest of years. If, when this crop isnearly ready, heavy summer rains occur, the water conservation drain along the higherside of the dam, which filled the dam, can be used effectively to prevent run-offreaching the dam. The water conservation drain is blocked or breached before it reachesthe dam and the section of the drain along the dam is put in order, so that run-offfrom the area immediately above the dam is made to flow out through the spillway.Only rain actually falling in the area of the dam would reach it, and, with the outletopen, even this water would drain away. On the other hand, the landholder may preferto retain the water and so will allow the dam to fill up normally.
Under the same climatic conditions, a grazier may decide touse the conserved water for flood or flow irrigation of a smaller area of pasture,say 20 to 30 acres, so as to enable him to have ample water available throughoutthe season. Likewise, the dam is specially advantageous for spray irrigation of valuablecrops, such as vegetables.
The costs of contour dams will vary considerably, but if I hadsuch a suitable contour dam site on my own property I would expect my constructioncosts to be less than £1,800, including all the irrigation controls and drains.Under the climatic conditions of Richmond or Orange about 60 acres of irrigationcould be provided and the area of the dam when emptied would be so much additionalworth. Contour dams may be much smaller or larger than the one illustrated, but itseems from my own experience that the larger dam will return the higher capital costjust as quickly as the smaller dam returns the lower cost.
We may consider now the storage of water on land that is flatterthan those gentle slopes suitable for the contour dam. As our water conservationstructures are to hold water above the level of some of the immediately adjacentland, then the only way this can be accomplished on land with slopes of only a fewfeet fall in a mile is by completely surrounding the water with a constructed earthwall. In these circumstances, also, the only way to get the water for storage intothe dam is by lifting it into the storage area. This pumped type of storage is alwaysthe last to be considered for storing appreciable volumes of water, so before dealingwith this new type, consideration will be given to slopes intermediate between thosesuitable for the various contour dams and those where water must be pumped for storage.
It has been seen that a contour dam is an efficient water conservationstructure where the land slope is 1 in 50. What changes are necessary where slopesare 1 in 100? A contour wall as described would, if employed on this slope, backwater up to 12 feet (depth of water at outlet) multiplied by a slope of 100, i.e.,1200 feet, so that the main contour wall would be 900 feet long as before and eachof the two wing walls would be 1400 feet long, after allowing for two feet of freeboard.The approximate relationship between the two contour dams, one on 1 in 50 slope andthe other on 1 in 100 slope, is that for the same length of main wall the area andcapacity of the second is double that on the 1 in 50 slope. Though the quantity ofearth of the wing walls has been increased, the extra yardage of earth required hasonly increased by less than 50%. The ratio of earth moved to storage capacity nowapproaches 1 for 8 instead of the other 1 in 50 slope dam, which is 1 for 5.65.
The second contour dam provides most efficient water storagecapacity on this slope of land. The question now is, can any other type of structurebe used satisfactorily for the same slope conditions? Of the various shapes whichmay be used to completely enclose a storage area, the ring shape is the most efficientin yardage of earth moved to capacity enclosed, so a ring dam may be consideredfor the above slope.
Some confusion has arisen as to the names of these dams, sothey are given the general descriptive name of "closed wall" dams, whichindicates that a constructed wall completely surrounds the water storage area. Inthis text they will be given individual names based on the particular shape of theclosed wall. Thus, the closed wall dam of ring shape is the "ring dam".This is sometimes incorrectly referred to as the turkey's nest dam. However, theturkey's nest dam, as is "the overshot", is a dam of long-established usein Queensland's country areas. It is constructed in the same manner as the nest ofthe scrub turkey, from which it takes its name. The turkey's nest dam is built withearth obtained outside the structure, which forms a circular wall, and it is filledfrom flowing or pumping bores. By being above ground level the water of the dam canbe led into stock troughs equipped with automatic float valves, which replenishesthe trough water as it is used. The ring dam, on the other hand, is constructed bydigging earth from the inside to form a wall. The dam then is a larger diameter earth-wall.ring with a channel just inside the wall, the earth excavated from which forms thewall. Inside the ring-shaped channel there is usually a circle of unexcavated materialor natural land surface. According to the depth of the dam, which may be from 11to 16 feet in the channel, there will be from 8 to 12 feet of water over the landsurface in the central area of the ring dam.
A ring dam of the same maximum depth of water above ground onthis slope and having a diameter of 900 feet would have the following characteristics.The wall at its maximum dimension would be identical to the cross section of thecontour dam on the same slope. At the opposite side of the dam, 900 feet up the 1in 100 slope, the ground level would be nine feet higher, so at this point the ringdam would hold three feet of water above ground level. The average depth of the ringdam, and disregarding the excavation, would then be the approximate average betweenits deepest depth of 12 feet and shallowest depth of three feet, or an average ofseven feet six inches deep above natural ground level. The section of the wall atthe shallowest part of the dam would be five feet high (three feet of water plustwo feet of freeboard), 10 feet wide at the crest and 25 feet wide at the base. Calculationsfor both dams show that the ratio of earth moved to storage capacity is somewhatbetter in the contour dam when compared with the ring dam, but the ring shape providesmore water per acre of land under top water level, its average depth is a littlegreater and it has a smaller area of shallow water. Both dams appear almost equallyadvantageous except when it comes to filling the dam with water. If the ring damon this slope had to be pump filled it would not be chosen for the site and a contourdam would then be the only logical choice. However, a ring dam on such a site canbe filled as can the contour dam on the same site, namely, by flow water. For thering dam to fill, the flow water must enter the dam (or a part of the dam or a closedinlet to the dam) at or just above top water level. Top water level would be representedby a position 300 feet further up the slight slope and beyond the dam near its smallestwall section. Water will then need to flow to the dam from this point. There aretwo satisfactory ways of getting this done. One is by extending a pipe of suitablediameter through the shallow wall of the dam and continuing it up the incline tothe height of top water level. A water conservation drain (as for the contour dam)would deliver run-off water to this point, whence it would flow via the large pipe(about 18 inches diameter) through the low wall into the dam. The pipe line, whichwould be a little over 300 feet long, would be underground and a small bank or bayat the height of top water level would need to. be formed around the higher end ofthe open pipe. The alternative method of filling the ring dam from the same waterconservation drain is by extending a pair of parallel walls from the shallow wallsection of the dam up to the water conservation drain. The walls are to be 30 feetapart and join the shallow wall side of the dam where a 30 feet section of the wallwould not have been built to open into the waterway formed by the two parallel walls.The height of these little walls would be on the same level as the wall crest ofthe dam. This is now a "broken ring" dam or "pan-handle" dam,and the pan-handle walls at their water conservation drain end would be so arrangedthat when the dam reached its full planned depth no more water could flow in or overtopthe dam. Spillway area for the disposal of excess water would be required and couldbe arranged on the lines already discussed for the other dams.
Of the two methods of filling the dam, the pan-handle wallsmethod would be the more economical and generally the one to be preferred. This typeof dam, the "broken ring", may be of any suitable shape, because the fullring shape may be influenced by other features, such as a watercourse or a propertyboundary. The ring shape is depicted here, as it is the most efficient of all theclosed wall types of dam. (See Fig. 20. )
The above type of dam, i.e., a broken ring dam, coupled withthe method of filling it, is suitable for slopes slightly steeper than 1 in 100,but is not likely to be preferred to the contour dam. While it may also be used onslightly flatter slopes than 1 in 100, obviously it eventually reaches its flat limitof slope where the pan-handle walls arrangement becomes too long and later completelyimpractical. The specially critical factor of these designs is the slope, and indesigning a dam in these, or, for that matter, in any circumstances, no attempt shouldbe made without first knowing the exact land slopes.
Where the slopes are so flat that the contour dam and the brokenring become unsuitable and not worth consideration there is no way to store waterabove ground level other than with the closed wall dam and pump filling. All otherdams may be filled by natural catchment, by water conservation drains, or by weir-divertedcreek flows, but where a ring or other closed wall dam is the only choice the siteshould be near a watercourse of a particular type. The most usual source of watersupply is an intermittently flowing stream, or, more often, one which flows onlyafter heavy rainfall. The dam may need to be filled during heavy rainfall, thereforethe above facts should be noted when locating the dam and when making the designof the dam and its related filling structures. In some circumstances, though a dammust be filled from a certain watercourse, it may be a disadvantage or even an impossiblehazard to construct the dam close to the watercourse. In other circumstances andfor the sake of efficiency and economy, it may be worthwhile to depart from the ringshape by having part of the wall following a section of the bank of a watercourse.(See Fig. 21. )
However, only after carefully studying the possible locationsfor the closed wall dam and the filling site should the dam site be finally determined.
The most satisfactory filling arrangement is that the dam canbe filled through the lockpipe outlet, and to this end a bay from the watercoursecould be excavated to a suitable common filling and outlet point. A small permanentflood weir in the watercourse may be arranged in such a way that during flow periodsthe weir causes the drain and bay to fill for pumping into the dam, but at the sametime allows flood water to flow over the weir, with no inconvenience to the siteand structure if pumping is not required from certain flood flows.
There is an idea in the minds of some that water has to be pumped"over the top" of the wall of such a dam. On the contrary, this is a disadvantageagainst pumping through a suitable pipe beneath the wall. The higher water has tobe lifted the greater is the power required, or, alternatively, less water will bedelivered for a given power.
The construction of the ring or other closed wall dam followsthe general procedures already given. The size of the lockpipe may be increased accordingto the capacity of the pump which is to fill the dam, but generally a size of 10inches is suitable in nearly all circumstances.
The importance of making proper arrangements for the fillingof ring dams (or any closed wall dam) cannot be overstressed. The full layout shouldbe decided and included in the design of the dam itself; and the creek weir, thedrain and bay, from which the water is to be pumped, should all be constructed andcompleted as part of the dam construction. Again, the elevation to which water hasto be raised in the dam, the pump capacity and power requirement, must be logicallydetermined in relation to the capacity of the dam. Generally, where water has tobe pumped into a dam, time is so limited that large capacity low head pumps are invariablyrequired. The dam for these reasons should be close to the level of creek flow, sothat pumping will take place from only slightly below ground level. The likely lengthof time available after storm rains for pumping should be calculated against thecapacity of the dam, that is to say, when the rate per hour of water delivery requiredhas been estimated, a pump capable of this performance against the height of thetotal lift should be acquired. (See Fig. 21. )
The most suitable dam filling arrangement is a permanent set-upof pump and engine that can be operated under the worst possible weather conditions.A lower initial cost method would be to arrange a permanent pump set-up so that thepower of a farm tractor could be quickly coupled to the pump. With the pump in apermanent position and the suction line in place, the set-up is always ready foroperation, i.e., the delivery pipe from the pump is left coupled permanentlyto the lockpipe at its outlet end. There is also coupled to the larger size lockpipea Y piece which has a control valve on each leg of the Y piece. One leg is the permanentinflow or dam filling side, and the other the irrigation water outlet. Whenever thereis water in the dam at the lockpipe level the pump may be primed very quickly bypartly opening the valve on the pump delivery line, then the engine is started upand the valve opened fully. A large low-head high-capacity centrifugal pump selectedto exactly suit the requirement is inexpensive and will have low running costs.
The water from a closed wall dam may be used for many differentirrigation systems and varying from flood to any type of spray irrigation.
Generally these dams of the flatter lands are uneconomical whenattempts are made to employ them outside their proper land suitability. One practicalexperience of the ring dam was in 1947, when I constructed one for the purpose ofmaintaining a head of water in a long underground irrigation main which was laidalong a high boundary on "Yobarnie". An intended use for the water wasthe spray irrigation of a new orchard. The orchard idea was abandoned and the damthen was used only for pasture irrigation, which included the giant monitor typeof spray irrigation just introduced for the first time into this country. The ringdam was filled by pumping, which involved a considerable lift, via a six-inchunderground pipe line that included a crosspiece and thence under the wall of thedam. The crosspiece was fitted with valves, one of which opened for dam filling,and at other times it was used to flow water from the dam back along the deliveryline, which in turn was equipped with numerous take-offs for irrigating along itslength. Other valves on the crosspiece directed water to underground pipe lines intwo different directions. The mechanics of the whole set-up worked admirably andthe scheme might have been successful in other directions also if the orchard projecthad been continued successfully. However, as a pasture spray irrigation project forbeef cattle production there was little likelihood that it could be economical orever really profitable, and so was abandoned. A ring dam was constructed on a farmin N.S.W. as recently as last year, and in circumstances unsuitable for its use,although adjacent to this particular ring dam (which has a pump lift of about 100feet), there are well-nigh perfect sites for keyline dams. If these were utilisedsimilarly to our dams and irrigation system, they would be highly profitable. Themistakes that have been made, and are continuing to be made, in all aspects of farmwater storage and water use, would be very educational to farmers and graziers ifthey were made known. Many branches of agricultural science have been improved considerablyby utilising knowledge gained from mistakes, and it is hardly an exaggeration tosay that science generally is largely the accumulated knowledge gained from innumerablemistakes.
All dams of whatever type or kind and for any purpose must fitin with and became part of the landscape. They all must have a means of getting waterinto them, but they also need the best and cheapest means of getting water out ofthem and into effective use.
The foregoing discussion will have been most successful if itdraws attention, much-needed attention, to the need and great value of farm damsof these various types and usages in their application to the development of thewhole Australian landscape. It is a development that is not only beneficial to theindividual farm and grazing property but one which adds merit to the work of everyAustralian landman.
The problems of farm dams do not end with their design and construction,so these dams we have been describing need proper care and attention, and this aspect,along with some associated problems, will now be considered.
After Care of Farm Dams: All newly-constructed farm dams,including those described in this book, are subject to change, The covering of allraw earth with soil and the sowing of grasses on every part of the dam and its immediatesurroundings must be considered a part of the construction of the dam itself. Grassmay grow and quickly cover the wall and the surroundings, but, even so, the wallwill shrink and crack, and so needs inspection, and especially so during the firstyear of its useful life.
The earliest and best check on the general performance and accuracyof the newly completed dam takes place with the first occurrence of heavy rainfall.If it is heavy enough to promote considerable run-off, so much the better. To learnall that rain can teach, the farmer should get out in the rain with a longhandledshovel. He should walk the wall of the dam and look for little ponding areas on thewall crest, ponds which will later break out in one particular spot and flow waterin a small but concentrated stream down one or other batter of the wall, and cuttinglittle gutters. These first little gutters, if they are left, will form real flowpaths for the continuing rain and so increase quite rapidly in size. It is thereforenecessary to fill up the little ponds on the crest of the wall with earth from higherspots. The shape of the wall preserved at this very early stage ensures an even andharmless flow of water in the heaviest of rainfalls. Next, the four areas shouldbe inspected where the constructed wall joins the banks or sides of the valley. Oftenwater may flow along these junctions in small concentrated streams, and so shouldbe diverted and spread away from the wall. The water flowing into the dam will notcause even a slight movement if the cultivation pattern below the waterline is properlydone. A small soil movement is not of much consequence, but an inspection is a goodcheck on the work, since it will illustrate, as nothing else can, the effectivenessof keyline cultivation. Now the water conservation drain should be inspected. Ifthere are low spots where water is threatening to overflow or is likely to overflowwith heavier rain, the low places should be repaired. A low section in a drain indicatesthat the drain at that place is slightly downhill and off its proper line. The lowplace is therefore repaired in such a way that the drain position is moved slightlyuphill to its correct position. This is done very simply by shovelling earth fromthe uphill batter of the drain and placing it, not on the very top line of the bankof the drain, but just inside this line, so that the bank line is moved uphill slightly.If the low place is merely raised by the placing of new earth on the top of the drainbank, the slightly incorrect position of the drain is preserved and more earth willbe needed to raise it to the appropriate height. Since the water conservation drainis to be one of the really permanent man-made structures on the property, earth shouldnot be shovelled indiscriminately in adjusting the section of the new drain; thecross section shape of the drain should be preserved in the shovelling and repairwork.
When water is beginning to break out of a water conservationdrain, first, a high spot is looked for on the dam side of the break, which may bedirectly causing water to pond back and overflow. The high place should be fixedbefore the low spot. A high place in a drain indicates that the whole drain positionat that point is slightly uphill and off its true position. Repairing the high placeis done by digging earth at the downhill edge of the stream of water and spreadingthe earth either well uphill or down behind the drain bank, whichever is indicatedby the circumstances. With the high spot adjusted, the overflow can be treated. Sometimesa bad break in a water conservation drain cannot be controlled directly. A suitablespot should then be selected upstream where the drain is in strong section. Herethe drain is blocked with earth, and so, by causing water to overflow, relieves thebad break, which may then be repaired to its full section. The block is then removedand the drain. adjusted.
When heavy dam-filling rain occurs a new dam is worth watchingfrom many points of view. I have watched a man who, seeing a new and larger dam thanhe had experienced before filling rapidly, rush to break the water conservation drainto stop the flow into the dam and to open the outlet valve to let water out of thedam. But the larger dam is built only to hold more water and so should be allowedto fill as quickly as it can. If the dam was built reasonably well, and wall weightand shrinkage properly allowed for and with a suitable spillway, there is littlecause for apprehension. If anything should go wrong, then the measures suggestedin this book can be taken to correct matters. However, the new wall of a dam constructedof earth which was too dry should be controlled to fill more slowly, since the materialof such a wall often lacks cohesion until it becomes slightly moist right through.If the first cracks on the water side of the wall of a new dam discloses dry, powderyearth in the wall, the water level may need to be lowered immediately. The very wetearth can slip off the dry, deeper material into the dam, and in doing so fracturethe full section of the wall and result in the loss of a large part of it.
After or during the first heavy rain on a new dam, the firstnotable shrinking and cracking of the wall may take place. These movements are normalin a farm dam. They are a part of the design and construction of the dam, since thewall costs have been reduced by about half, because, instead of going to the expenseof using sheepfoot rollers or pneumatic-tyred rollers to get complete compactionof the wall material, the natural compacting forces of settlement and shrinkage areallowed to operate. There are two types of wall cracking, and they are named accordingto their mode of occurrence--longitudinal cracking and cross cracking. The earlylongitudinal cracks usually occur near the outer edge of the crest of the wall andare often associated with the paths the tractor made along the wall. The looser earthon the outside of the path will pull away or shrink away from the more settled earthwhere the weight of the tractor compressed it. Such cracks are rarely a hazard, butthey should be treated by raking earth to fill them a day or two after rain, whenthe wall dries out a little. just sufficient earth is raked to fill the crack. Neglectedlongitudinal cracks become larger and could, after further heavy rain, hold waterin such quantities, which, in finding its way out through the wall, could cause aslip in the wall.
Cross cracking or cross-the-wall cracking is not usually associatedwith early settlement and shrinkage of a new wall. It may occur only after a damhas been first filled, and then all the water used and the wall has started to dryout. Though rarer, it is a more serious form of cracking if neglected or overlooked.These cracks may form a continuous split across the wall or be in the form of shortcracks from the front and back of the wall to a longitudinal crack, and so form acrooked path through the wall. The cross cracks never or very rarely reach down thewall to the water level. Danger lies in cross cracks forming when the water levelis reduced and the crack reaching down near to water level. Should flow water causethe dam to rise above the cracks, water will flow through the wall. If this flowoccurs below spillway level, and is undetected, quite considerable damage to thewall can occur. Prevention of damage lies in filling in the cracks as with longitudinalcracks, but paying particular attention to the crack on the inside of the wall whereit appears above the present water line of the dam. Here the crack should be rammedafter filling, then filled again after ramming. Dry, more so than moist earth, isalways to be used for filling cracks. Fine dry earth is probably the best.
I have experienced occasions where the cracks, by first flowingas a very small stream, have become closed on the surface by stock walking the wallin wet conditions and the leak later developing into a pipe-type of flow. To repairsuch a flow after it has occurred, adjacent earth is shovelled into and around theopening under water, and no attempt is made to fill the full break across the walluntil water flow has been stopped. Later the crack is filled with dry earth.
The ordinary settlement and shrinkage cracking of a new wallmay be effectively filled by a 2-inch to 3-inch cultivation with a chisel plow. Thecultivation may be a part of the soil and pasture development of the area. However,occasional inspection of the wall will ensure that all is well.
The often-recommended procedure of planting special wall-bindinggrasses can act against the safety of a wall. The generally coarse nature of suchvegetation may hide dangerous cracks. Personally, I favour planting only the usualpasture mixture on the wall and fencing the dam off, or, alternatively, fencing thedam into a smaller paddock. Stock can be put on to graze the wall and the surroundingsof the dam as part of the improvement programme, and the grazing should be controlledproperly to these ends.
A well-designed and constructed dam such as any of those discussedis a very safe and permanent asset. The methods of construction are low cost, andthese after-care considerations are for the purpose of seeing that natural forcesin compacting and consolidating the dam do their work without creating damage.
Although things do not go wrong when all the above methods andprocedures are adopted with good supervision, we may, however, consider some of theproblems that are the result of less effective design and construction methods orthat arise out of the use of the less suitable earths.
Failures in farm dams are generally presumed to arisefrom three main causes. The first cause is inadequate spillway size, which failsto convey the overflow water and forces the water over the wall of the dam. Soona channel will be cut in the wall by the water, which, once it has got down belowthe spillway height, causes all the water entering the dam to flow through the breakin the wall. The second presumed cause is from a low spot near the central area ofthe wall crest caused by inadequate or no allowance for shrinkage in the constructionof the wall. The effect is the same as before; water flows over the low place inthe wall, cutting a channel and destroying the wall. The third cause is presumedto be inadequate compaction of the wall, material, which allows heavy seepage tobuild up into a strong flow through the wall. All the water may be lost and the wallremain in position, or the flow through the wall may cause the wall above the holeto collapse into the flow and leave a break in the wall from the bottom to the top.Overtopping may destroy a new wall in 20 minutes and an older wall in two hours ormore.
Many wall breaks resulting from the first two causes, and inspectedafter the failure, are attributed to the latter cause, because the material insidethe now broken wall does not appear to be well compacted. The breaking of the wallitself often makes this aspect appear bad, but in my opinion the failures resultingfrom this cause are very few when compared to the other two causes of failure. Ihave inspected broken walls of dams that were said to have failed because of poorcompaction, but in each case the breached wall remaining and the obviously inadequatespillway provisions clearly showed that overtopping of the wall had occurred. Manyof these failures were in dams of some age which were holding water previously andhad been washed out in later flood rains.
Dams may fail as mentioned earlier by cross cracking in thewall. Faulty dam construction or a batter design that may be too steep in relationto the stability of the wall material, may cause slumping or slipping of the walland reduce the wall height and so overtopping may then breach the wall. Of all thefailures in farm dams, the large majority are caused by faulty design. The earthof the shale area of the County of Cumberland--Sydney and hinterland area of N.S.W.--holdswater well even with the most unsound construction methods, yet in one day duringthe heavy rains of 1956 between 40 and 50 dams were stated to have breached. Althoughthe rain was heavy, the fault in all or at least the large majority of cases musthave been poor design, since none of the new dams were constructed in drought conditionswhen earth walls could be too dry to stand sudden filling. No doubt many of thesefailures were due to the type of advice given to farmers on farm dams, which invariablyemphasises good construction as "most important", when, quite obviously,good construction can only follow better design. All aspects must be given theirrightful attention, and then good construction becomes a counterpart of good design.
We had one leaky dam on "Yobarnie" and another recentlyreconstructed dam that still could leak. As we are discussing problems in dams, thestory of these two dams may serve as practical illustrations.
The first of these two dams was constructed in an unfavourablesite in that the valley was small and steep and the available material for the wallwas a medium hard blue shale. The site was chosen for a variety of reasons; it fittedin well with the other dams, and there was a very good-shaped piece of land adjacentat the right height and suitable for pattern flow irrigation. Also, I wanted, tobuild a dam wall with the hard blue shale as a test of this material under the worstpossible conditions, namely, to construct a high wall as steeply as the bulldozerscould operate. The planned depth of water at the lockpipe was 34 feet, freeboardwas three feet and shrinkage allowance three feet, giving a total height of 40 feetto the new wall at its largest section. The wall was built, but before the feederdrain could be made very heavy rain filled the dam and a large flow of water wasrunning out through the spillway. There was also a considerable flow from five differentplaces through the back of the wall. We raked up and down the inside of the wallwith a rake attached to a pole and found one leak by the simple process of walkingalong the wall batter in the water in gum boots and finding oneself suddenly sinkinginto a semi-fluid hole. Earth shovelled from the crest of the wall was tramped intothe hole with the boots and closed this high leak. However, the raking had no apparenteffect on the rest of the considerable flow, although this method of stopping seepageis often successful. The leaking areas were evidently too deep below water. So Iprepared explosives. Six plugs of 60% gelignite were cut in half and each fused.I decided to explode the charges in the water about 20 feet from the wall water lineand two feet above the bottom, which was a part of the underwater batter of the wall.A piece of board was tied as a floater to each half plug with a length of stringabout 12 feet long, and then, holding the plug, string and board together, each fusewas lit in turn, and together with the charges and floater, thrown out into the waterabout 10 feet apart. There was a series of dull thumps and rather convincing vibrationsthrough the wall on which I was standing, as the charges exploded about 10 secondsapart. The result was a considerable reduction in the flow of all but the deepestleak. Two more shots were used, each a full plug, and thrown further out and attachedwith a longer piece of string to a floater. (The length of string and floater checksthe depth to which the charge sinks.) The dam was still leaking a little but heldthe water, which was used in the summer for irrigating. There had been also someslumping of the back batter of the wall, and as the leaks were not completely stopped,a bulldozer was put into the dam when it was empty to work on the leaking area andto trim the job up. By this time there had been a considerable breakdown of the shalematerial with now sufficient clay to make the wall impervious.
Explosives are often valuable for farm purposes and are simpleto use, but no one lacking experience should touch them without first studying theinstruction book issued by the suppliers of explosives.
The recently reconstructed dam mentioned was built in 1945 andhad a four-inch outlet under the wall. The dam held about seven million gallons (28acre feet or 42,000 cubic yards) and had been used for various types of flow, floodand spray irrigation for many years.
After the first three years of the development of Keyline asa planning guide for, among other things, the siting and location of farm irrigationdams, it was decided that a keyline dam would be built above it. This old one wouldbe reconstructed and enlarged with an eight-inch lockpipe placed under the wall andthe enlarged dam would then become a reservoir and be kept filled as a reserve whilethe water of the three keyline dams and the large lower valley dam in this one catchmentarea was used first for irrigation.
The wall of the dam was cut with a big V-shaped excavation afterall the water had been used and the dam stood empty. The keyline dam was then constructedhigher up the valley. The eight-inch lockpipe, 120 feet long, was placed in deadlevel in the V cut of the old wall and the closing of the V and the enlargement ofthe wall started. A height of nine feet was reached above the lockpipe when continuousand heavy flood rains commenced.
I saw the work a week later and realised that a lot of troubleand waste of time confronted us. The bulldozer operator had left the job with wavesof loose earth everywhere when he finished for the day. Rain was not expected, butnow ponds of water were in the waves of loose earth and the work became super saturated.Fortunately, the lockpipe was left open, and this prevented the flood water fromfilling the dam to the point where it could overflow the unfinished V cut fill. Somemillions of gallons flowed out the lockpipe in the flood rains which continued, yetno earth was washed away and lost.
Months later, anxious to finish the wall and seeing the surfaceof the earth dry and cracking, the bulldozer operator started work again. The wholewall was like jelly underneath and all he succeeded in doing was pushing a few yardsof earth on to the wall at one part, which promptly settled back to its originalheight, and blocking the lockpipe with a fluid mud. Lengths of two-inch pipe werejoined together and pushed through the lockpipe from the back of the wall to clearit. Again it rained, and once more the lockpipe carried more millions of gallonsof water away, saving the earth and preventing a bigger mess. Later again, with thewall dried out somewhat, although still too wet, the bulldozer operators with a lotof perseverance succeeded in raising the wall to its planned height. Sticking tothe bulldozer blade, the clay material would not spread evenly and went into thewall in lumps. Everyone by now was tired of the dam; however, it was completed. Theoperator relaxed for lunch before moving off, then when he looked at the wall againthere was a great bulge on the inside batter; the whole wall section had slumpedabout five feet. Work was delayed while it dried out further, and eventually thewall was raised again to the finished level. The lumpy clay material which was placedin this wall looked bad enough for us to prophesy that the wall was likely to leak.
The sad story of this dam's troubles arose from the fact thatthe construction was not supervised. By neglecting to trim the wall, water got intoand jellied the clay. The story adds point to my suggestions on supervision and thetrimming of work before finishing each day. Yet it could have been even worse. Ifthe lockpipe had not been used, the partly constructed dam would have been lost andthe whole valley floor for 800 yards below would have been covered with the washed-outmud and earth which would have resulted from the heavy and continuous overflow ofthe loose bank. Instead, we now had a good dam and during the dry period which followedthe final work the wall had shrunk, settled and cracked. The wall crest was cultivatedon three occasions to fill the cracks, and no doubt the long dry spell helped thewall a great deal by allowing settlement to take place before much water enteredthe dam.
With regard to heavy rainfall on a new dam, it is to be rememberedthat a new dam, when filled rapidly in continuous heavy rain, imposes the worst conditionson the back (downstream) batter of the wall. The front wall batter is safe, beingassisted by the water in the dam, but with the dam filled and the back saturated,the critical condition for the back batter of the wall occurs. Slips may follow.Small slips that start on the crest, well away from the water line, usually reacha stage where they stabilise or balance themselves. They present no particularlyurgent problem, for they can be repaired later by filling the slump-hollow with earth.The bulge formed by the slip near its lower end down the batter of the wall can beleft, as it acts as a wedge against further movement. Such smaller slips are repairedfrom above and not by pushing up from below with a 'dozer. The opposite extreme ofa back-of-the-wall slip is the major one and the worst possible. Here the slips startfrom under water inside the dam, moving a section of earth right across the crestof the wall and bulging out lower down the back of the wall. Pushing earth onto thewall crest and down into the water is dangerous. The earth falling in the water standsup very straight, and when the water level drops, or even before, the concentratedweight of the new earth on the saturated wall below water will invariably start aslip of the material inside the dam. Earth that is pushed into the water must bespread out under water. In the large back-of-the-wall slip or slump water can immediatelyflow around one or both edges of the break, and, because the material in the pathof the water is loose and fractured from the movement of the slip, the earth. movesrapidly down the wall. This most serious of all slips is treated by immediately openingthe lockpipe and then breaching the feeder drain. The lockpipe flow is continueduntil the water level drops below the break. When the wall and slip area dries out,the slip may stabilise itself and not move any further. If the slip is never as softagain as it was when the slip occurred, it will usually remain stable. Repairs aredone in dry weather by filling the cavity of the slip area back to the original wallprofile. The bulge area of the slip, if it is not too unsightly, could be left undisturbed.
While the critical stage in the back batter occurs in heavyrain with the dam filled, the same stage for the front batter occurs when the damhas been filled and is later emptied. The inside batter of the wall can slump orslip even before the dam is quite empty. Usually, inside slumping is a slower process.The first signs may show up as a series of little slumps and cracks over a sizeablearea about as much as one-third of the whole inside batter of the wall above water.The little cliffs formed by the movement may be only an inch or two high, but inthe following week or two can become cliffs three or more feet high.
The prospect of serious inside slumping is greatly increasedif, during the construction of the dam, material has been excavated from under theinside toe of the wall, as mentioned in the discussion on dam design in Chapter XVIII.
The complete repair of a serious inside slump can only be madesatisfactorily when the dam is empty and the slump area has dried out. The earthunder the slump is usually very wet, even jellied and unstable, though it may appeardry on the surface. A pole should be pushed into the deeper earth through the cracksto test the material below. If it is wet and sloppy it will need further time todry out properly. All the water of the dam should be released as irrigation water,and, if there is a pond below lockpipe level, this will need to be pumped out. Abulldozer will be required if the slump is large and serious. The wet sloppy materialon the bottom of the dam should be pushed back and away from the inside toe of thewall and an area of more stable material selected in the bottom of the dam that issuitable for pushing up into the slump area of the wall. The bulldozer operator shouldtest the slump area by backing the tractor from the solid bottom of the dam straightup the wall and moving slowly and carefully while watching for excess sinking ofthe tracks. It is very easy to bog a bulldozer in the slump, so the effect of thetracks should be inspected to see if the slump is stable enough for safety. If thereis any doubt as to the stability of the wall supporting the weight of the bulldozer,a load of drier earth should be pushed forward up the wall and the blade raised graduallyto spill earth under the tracks as the machine travels. If the slump area is at allboggy, constant care is necessary to see that there is always new earth being droppedahead of the tracks of the bulldozer. The path of travel up the wall is constantlychanged slightly, so that a uniform compaction of all the unstable material of theslump is produced.
The repair of an inside slump should produce the planned profileof the wall with a finished height of the crest about 5% higher to allow for settlement.
An inside slump may be repaired by smaller equipment or by handwork and working from the crest of the wall. This type of repair aims at using thebulge of the slump as a stabiliser. The slump is allowed to dry out as much as possibleand the cracks and cliffs of the slump filled with new earth. The bulge is not cutdown, but is left as it is, only being covered with earth where it is cracked. Thebulge at the bottom of the slump acts against further movement. Such a repair mayslump a little further after the dam has been filled and then emptied again. However,the slump will be much smaller and may soon become stable. It is important in handrepair work to allow the bulge to act against further slumping, and not to placeany more earth than is necessary to reform the wall profile by filling the hollowof the slump.
Throughout these discussions the bulldozer has been consideredas the type of equipment most suitable for farm dam construction. Although I believethis to be so, contrary opinions are often expressed which favour the use of largescoops or scrapers. The reasons given in preference for the latter machines are thatthis equipment imparts better compaction to the wall material. In my own experienceof building dams of the type and size of the useful farm dam with both scoops andbulldozers, I believe the only worthwhile advantage of one over the other dependson the length of haul. Where the earth has to be transported distances beyond 200feet the advantage lies with the scoop and continues so as this distance is increased.As for the scoop providing the best compaction, there is little significance to thisview. What generally happens is that the use of the scoop allows the earth to bespread in thin layers (also a feature of good bulldozer operation), which aids uniformityof texture in the material; but the path of travel, being longitudinal along thewall, provides compaction from the tracks and scoop wheels only in this direction.In the early stages of building a dam wall the scoop and tractor equipment is workingon a wide and rising wall crest and compaction can be made uniform by regulatingthe various paths of travel of the equipment. As the wall approaches its full heightit becomes impossible to travel the material uniformly, and in the final stages,with the wall crest 10 to 14 feet wide, only narrow bands of earth across the longsection of the wall are affected by the wheels. Hard compacted earth then is adjacentto loose fill and the contact between the different textured material forms largerlongitudinal cracks. Later, the wall will still be satisfactory, but not becauseof the type of equipment or for the reasons given in its favour.
I have seen the loosest walls settle into good impervious wallsand I believe that in the relatively small earth wall what is called proper compactionis necessary only on comparatively rare occasions. However, this aspect of farm dambuilding will be further tested over a range of different materials, as we proposeshortly to build experimental farm dam walls with loose earth. "thrown"into the site. In these experiments no equipment will travel over the earth wallexcept possibly to trim the wall to shape after all the earth has been placed. Wehave now a special machine for the tests. Another aim of this new series of experimentsis to devise ways of building large farm dams efficiently with smaller equipment.Even now smaller equipment can build sizeable farm dams by using the designs of thisbook and the lockpipe system to prevent constant flooding by providing good drainagefor the much longer period of construction. Our experiments with the new piece ofsmall equipment which will operate in conjunction with on-the-farm machines, should,if successful, extend the scope and expedite the work of building farm dams.
With the coming of the large bulldozer many men are unawareor have. forgotten what they can do without the bulldozer. While the bulldozer isa wonderful tool, farmers, by becoming too bulldozer conscious, tend to let a jobwait, when the smaller equipment already available on the farm or even their ownhand work can be used cheaper and more profitably. There is too great a tendencyfor farm workers to let a job or an earth works repair await the arrival of a bulldozer,a job which, considering the high hourly cost of a £10,000 to £15,000 bulldozer,they can do cheaper without it.
Wave erosion in the larger farm dam is yet another problemof maintenance; that is, if it has not been allowed for as a factor of design. Waveerosion does not operate evenly along the length of a wall of a dam filled with water,but tends to concentrate its larger waves on smaller sections of the wall. Wave actioncan destroy a wall on occasions by this larger wave concentration cutting right throughthe crest, or, with the waves breaking across the wall, causing a flow of water overit. Wind and heavy rainfall together would accentuate the danger. Then the continuousflow of water across the wall would have the same effects as any other destructiveovertopping. The dam could now be lost unless effective action is taken. However,preventative action is simple and requires only the opening of the outlet valve fullyand the blocking or breaking of the water conservation drain to prevent further inflowinto the dam. With farm dams of the designs of this book this action would quicklyreduce the water level below the breach, when it can be repaired later. A temporaryrepair in such a breach, once the earth is brought to the full height of the wall,will hold back all the water of the dam even if the repaired section of the wallis only one or two feet wide at the top.
However, the best cure for destruction from wave erosion liesin preventing it or providing effective counter measures in the original design andconstruction of the dam. Wave erosion as a serious and destructive force in the largerfarm storages is reasonably predictable and can be duly considered and planned forin the design of the dam.
The accepted engineering counter to wave erosion is the provisionof a riprap (heavy loose stone or rock) covering over the inside batter of the wall.However, the provision of riprap for a farm dam is usually an expensive item andmore often than not it would cost more than the earth wall itself. This cost wouldno doubt be fully justifiable if no less expensive counter to wave erosion couldbe obtained.
In our own case, in designing the larger farm irrigation dams,we counteract wave erosion by quite simple and effective measures. There are threemain considerations. One, the prevailing winds in relation to the shape and lie ofthe dam will indicate the portion of the wall where the highest waves will strike.The wave crests do not usually advance as a straight line or as a wave line withall parts at the same height, but rather as a curved line, curved according to thedrag on the ends of the wave caused by the shallow bottom near the shores of thedam in relation to the wind direction. Winds from various points over a 90-degreeare of the compass, will drive the high crest of the waves over a relatively shortlength of the wall and rarely more than one-third of its total length. It is possibleto estimate approximately the length of wall that will be affected by wave erosionand to provide for it in design. In the earth wall farm dam the material that ismost readily available and cheapest to handle is earth, not rock; therefore, earthin suitable quantities at the right place becomes the obvious counter to wave erosion.If a large farm dam built without considering wave action in the design, is beingdamaged by wave erosion, no doubt the farmer would repair the damage, which we mayassume is reducing the effective width of the wall, by placing further earth at thereduced section. The cheapest means of providing extra earth is surely placing itthere during the building of the wall. The design should therefore include an allowanceof extra earth in the wall as an increased wall section to counter wave erosion.While there is a great deal known about the action of waves, there does not seemto have been any experiments conducted to determine effective counter measures towave erosion of farm dams by the use of earth.
We built a farm dam which had a water surface of 350 yards extendingout in front of the wall, and we made an arbitrary adjustment to our design to counterwave erosion. The width and height of the wall crest was increased by one foot alongthat section of the wall where, from the shape and lie of the dam in relation towinds, we considered wave erosion would be most damaging. The wall of the dam wastreated by covering the crest and rear batter, but not the waterside batter, withsoil and planting the ordinary pasture mixture. Wind shortly after the dam filledcaused waves, which soon cut into the wall and formed perpendicular cliffs of earthwith a long, almost flat, terrace on and just below the water line. Meanwhile, thegrass was growing on the crest and back batter of the wall. Later, the level of thedam was reduced by about three feet in irrigating, and waves again formed the perpendicularcliff and the nearly flat terrace below on the new water line. This time the waveerosion was moving primarily some of the earth of the first terrace formed by theearth eroded in the formation of the highest cliff. Again the water level was reducedby irrigating and a new line of cliff with its associated line of terrace was formed.By this time grass had started to cover the highest terrace and soon appeared onthe next lower terrace.
Many people had seen the experiments, and although its purposehad been explained, some sent special grasses to us to help stabilise the wall. However,we made no attempt to influence the course of the development of the cliffs and terraces,and the use of the water and the consequent lowering of the water level was dictatedonly by our irrigation requirements.
It was quite obvious to us from this experiment that we could,by releasing water via the lockpipe, determine the course of the developmentof the cliffs and terraces. However, a definite stability formed naturally againstthe damaging action of the waves, and as seen in the grassing of the terraces. Itis now apparent that, even without attention, wave erosion would have to act fora very long time before it could affect the efficiency of the wall. There would bea gradual movement of earth from the crest of the wall down towards the bottom ofthe dam, which would occur each time the water of the dam was reduced from the filledto the nearly empty stage.
A recommendation in the design of dams that are of a size wherewind erosion is likely to be a factor is to design the wall with an extra foot ofheight and an extra foot of width for the one-third distance of the wall where thewaves are most likely to affect the wall. Thereafter, cover over, as, with all dams,the crest and both the front and rear batters with a couple of inches of soil andsow down with pasture grass seeds with fertilisers. The first cliff should be allowedto form, but when the level of the water is next reduced, the cliff is smoothed offby hand work, allowing the earth to fall on the terrace below, where more fertiliserand grass seed are then scattered. When further earth is required to maintain a goodtop section of wall, earth would be taken from the extra foot of wall height providedin the design for this purpose. Only the top cliff and terrace will need to be sotreated.
These suggestions should be effective in dams with a water surface400 yards in front of the wall; and the allowance of extra width and height couldbe doubled for those very rare occasions of a farm dam having half a mile of watersurface extending out in front of the wall.
In bringing these discussions to a close, it is not to be assumedthat the subject of suitable structures for the farm conservation of water has byany means been exhausted. Indeed, there are so many opportunities in farm dam constructionit is a marvel that we have not taken more advantage of them. There are other interestingtypes and kinds of structures, for instance the log dams mentioned earlier, whichwill be suitable for particular occasions. There will no doubt be other problemsof materials and sites and circumstances which are not touched on here. Yet it ishoped and expected that a study of the shapes of land in relation to agriculturalpursuits generally and to the efficient conservation of water on rural propertiesin particular, together with the detailed design and construction methods of ChapterXVIII and this present Chapter, will assist the farmers and graziers in doing thiswork for themselves. I hope my experiences will lead to many farmers attacking theseproblems more confidently and that my mistakes and experiments will obviate mistakesin their own work. I hope also I have said enough to justify my earlier contentionthat the farm dam is a job worthy of as specialised attention for its own sake asthat of the "big" dam.
In agricultural areas where there are no existing farm dams,or farm dams have a reputation for failing to hold water, then I would suggest thatthe best procedure for the landman would be to construct his first dam high up onthe property or at the keypoint of a valley where, I have found, the best dambuildingmaterials are likely to be discovered. When it becomes necessary to employ specialanti-seepage techniques the method known as mechanical stabilisation could be investigatedfirst. It is generally desirable that the design of a dam in which these methodsare to be used should provide for flatter batters, so that all the area of the insideof the dam can be worked with suitable equipment. Mechanical stabilisation involvesseveral cultivations of the inside of the dam with chisel implements to fine-up thematerial. Several such workings should be followed by harrowing and rolling. Thefine materials tend to move with seepage water to seal the leaking areas. The solutionof the problem of the first dam may quickly lead to a wide use of other sites anda wonderful development of valuable land. But before all this farm development cantake place on a wide and national scale there is this further aspect to be considered.It is quite certain that if the work of providing all the farm dams which the countryneeds is not to be done largely by our landman himself as the chief supervisor andexpert, then the job will not be done soon enough. When one realises that the farmersand graziers are the largest force of executives in the nation, employing a hugeand capable army of helpers who work always closely with them, and that the landof these directors of our rural industries is the nation's greatest capital, thenone may be confident that the great challenge of the Australian landscape--the needfor more water--will have many acceptors; and, moreover, in accepting this individualchallenge, no farmer or grazier needing help, whether from governmental or privatesources, should ever have it denied him.