Published in 2000 by Permaculture Activist (USA)

The cheapest way of storing large volumes of water is in a water storage dam or pond. The potential functionality and aesthetics of a dam is brought through good design, thorough planning and investigation and appropriate construction techniques. This article hopes to address these outcomes by outlining the process that we go through when designing and constructing a dam or series of dams on a client’s property. A civil engineer should be definitely be involved where the dam wall is higher than 5 metres, where dam failure could result in loss of life or serious damage to property, or where the developer’s expertise is limited. As a guide however the following information will act to inform the designer/developer on the issues to be considered in designing and construction an earthen water storage dam.

Fig. 1 METRIC CONVERSIONS (some useful approximate metric/imperial conversions are given below)

Length 25 millimetres (mm) = 1 inch

3 metres (m) = 10 feet

1.6 kilometres = 1 mile

Area 5 square metres (m2) = 6 square yards

4 hectares (ha) = 10 acres

(1 ha = 10 000 m2 or 100m x 100m)

Volume 4.5 litres (l) = 1 gallon

1.25 megalitres (Ml) = 1 acre foot

1000 litres = 1 cubic metre (m3)

1 cubic metres = 0.75 cubic yards

Discharge 1 cubic metre per second (cumec) = 35 cubic feet per second

Nomenclature Dam = Pond

WHY BUILD A DAM?

The sight of water storage dams is a common sight in the Australian rural farming landscapes. Dams construction allows us to store large volumes of water for a multiplicity of integrated uses such. Dams can be used for as broad uses as aquaculture, erosion control, gravity irrigation, stock and domestic water storage, solar passive effects, wildlife habitat, aesthetics and recreation and can be used for these things all at once!

Until relatively recently the low cost, ease of construction and lack of regulation in dam construction has meant that their permanence, effectiveness and aesthetic appeal has often been limited. P.A. Yeomans was the first to capture the true potential of dams on rural landscapes through his integrated Keyline farm design system which he and his son’s developed in the post war years in South Eastern Australia. Keyline has of course been adopted by the Permaculture Designer as the best technique for broader landscape layout design which starts with the use of the Keyline Scale of Permanence (Fig 2) in conjunction with Permaculture Design Ethic and Principles. The key current reference for the Yeomans Keyline System is through the book Water for Every Farm which is available through Keyline Designs Website –www.keyline.com.au.

Fig 2. Keyline Scale of Permanence

Climate Land Shape Water Roads Trees Buildings Subdivision Soil

If you need to store anything less than around 100 000 litres of water or if potable water is needed then a water storage tank is possibly a cheaper and better option. Tanks are construction of various materials and are available and they can also be used around the property as a source of effective gravity storage in conjunction with a lower level dam, stream or ground water source. If the site design process indicates that a high-level tank or hillside dam construction inefficient and therefore uneconomic then a tank is also a better option.

LEGAL PLANNING REQUIREMENTS

The first part of the process in the construction of a dam is to address the appropriate government regulations controlling dam construction and the use of the water stored in the proposed storage (s).

The purpose of any regulations is to protect the community from poorly constructed dams and to ensure that regional water resources are not unfairly distributed. Contact your local state or local government authority to obtain the necessary legal and permit advice and they may also be a source of useful regional and general information. Getting in touch with a local civil engineer experienced with dam construction is very useful and may be a requirement of a planning or building permit being issued.

DAM SITE IDENTIFICATION

Dam site identification should be a result of going through a holistic property planning process using Permaculture Design Principles with an emphasis on the use of the Keyline Scale of Permanence. For the purposes of this article however we will go through some of the perimeters in assessing the potential siting of dams in the landscape.

The first issues to address is what are the water resources available to the property, how they flow, how can they be captured, what is the most cost effective way of storing them and how much is actually needed to be stored?

We always start with a catchment analysis, which identifies just how much water flows through a property. Understanding the land patterns represented by topographical maps is crucial for the effective calculation of catchments. This is achieved by recognizing the contours for their definition of ridges, saddles and valleys/gullies. Define the water divide lines (or centre lines) on the ridges of a particular catchment area as the boundaries of that catchment. Once this is achieved then use a grid paper transparency (grid paper photocopied onto clear transparency) to generate an area statement. If you’re lucky like us then you’ll have a GIS (Geographic Information System e.g. MapInfo, ArcView) software that makes the area statement just a click away. Once you’ve worked out a figure then use the following tables (Tables 1 & 2) to generate the total average run-off figures for a whole or given catchment. An engineer would also ascertain this as part of their investigation.

Table 1 – RUNOFF FROM CATCHMENTSi

Runoff as a % of average annual rainfall (Y) Average annual rainfall (R) (mm) Total annual evaporation (mm) Reliability (years out of 10) Shallow sand or loam soils (%) Sandy clays (%) Elastic clays (%) Clay pans, inelastic clays or shales (%) > 1100 8 10 to 15 10 to 15 15 to 20 15 to 25 9 6.5 to 10 6.5 to 10 10 to 13 10 to 16.5 901 to 1100 8 10 to 12.5 10 to 15 12.5 to 20 15 to 20 9 6.5 to 8 6.5 to 10 8 to 13 10 to 13 501 to 900 less than 1300 8 7.5 to 10 7.5 to 15 7.5 to 15 10 to 15 9 5 to 6.5 5 to 10 5 to 10 6.5 to 10 1300 to 1800 8 5 to 7.5 5 to 12.5 5 to 10 10 to 15 9 3 to 5 3 to 8 3 to 6.5 6.5 to 10 401 to 500 1300 to 1800 8 2.5 to 5 5 to 10 2.5 to 5 7 7.5 to 12.5 9 1.5 to 3 3 to 6.5 1.5 to 3 5 to 8 250 to 400 <1800 8 0 to 2.5 0 to 5 0 to 2.5 2.5 to 7.5 9 0 to 1.5 0 to 3 0 to 1.5 1.5 to 5 >1800 8 0 0 to 2.5 0 2.5 to 5 9 0 0 to 1.5 0 1.5 to 3

Elastic clays when dry develop pronounced surface cracking, which reduces runoff.

Inelastic clays are identified, when dry, by a fine dust cover; this dust prevents seepage into the ground and so increases runoff.

For irrigation schemes a reliability of 8 years out of 10 is acceptable, for domestic and stock schemes the aim is 9 years.

Table 2 – ESTIMATED ANNUAL RUNOFF FORMULA

Catchment runoff = 100 x A x R x Y litres

where: A is the catchment area in hectares (ha)

R is the average annual rainfall in millimetres (mm)

Y is the runoff as a percentage of annual rainfall

eg.

A small catchment of 100 hectares is forested and the soil is sandy clay. It receives an average annual rainfall of 750 mm and has an annual evaporation of 1000 mm. What would the estimated yield be for an irrigation scheme?

A = 100 ha

R = 750 mm

Y = 7.5 % (reliability of 8 in Table 2)

Therefore runoff = 100 x 100 x 750 x 7.5

= 56 250 000 litres

= 56.25 megalitres (Ml)

A strong component of the engineering design of a dam is to design the overflow or spillway of a dam so that it can cope with the 1 in 100 chance of the highest possible flood volume passing through the dam site. Consulting with your local water authority or engineer will enable you to calculate the amount of flood flow in cubic metres per second and design a spillway and or trickle pipe set up to cope with these potentially hazardous occurrences. An engineer will calculate the flood flow using a methodology that considers the rainfall intensity, catchment characteristics and size, average slope of the watercourse and the length from the head of the catchment source to the dam site.

Several different methods are available to increase the amount of available catchment to a dam where the catchment of a hillside or offstream storage for example may be too small for the amount of storage you’re after. One way is to design and develop a system of earthen drains that intercept overland runoff and divert water to a water storage – we call these diversion drains. These are cheaply constructed using a grader or even better with a rotary drainer. These drains can also be integrated with road drains as we often use dam wall as an all weather access across wet gullies and drainage depressions. Drains can be placed so that they link two or more of a chain of dams by running overflow water from a higher dam the next dam of a lower elevation and so on. Water reuse drains can also catch excess flood irrigation water and divert it to a storage. Drains need to be designed and constructed considering similar flood flow volumes to those used in designing the dam itself. Again the use of an experienced engineer will assist in this element of the design.

SOIL CLASSIFICATION

An important aspect of the design process is to complete a borehole investigation of the proposed site. We use the services of localised Geotechnical Engineers complete this task. These engineers come out to the site and using a drilling rig take core samples to some depth and then perform several different laboratory tests. These tests determine the suitability of the site material for the construction of a dam and also what the soil profile consists of i.e. Depth of topsoil, clay/sand/silt composition, depth to water table/rock etc. From this the engineer will then make recommendations as to how the dam should be constructed, and whether or not some amending/sealing material such as bentonite or gypsum is required.

The most important tests a geotechnical engineer would undertake would include:

Emerson Test

The Emerson test determines the behavior of clays in contact with water and to what extent they break down in that contact. This test impacts heavily on the suitability of the site material for dam construction.

Soil Dispersivity testing uses the Emerson Soil Dispersivity Method (AS1289, C8.1, 1980) of analysis and classification:

Class 1 exhibits complete slaking in water. Class 2 only some slaking. Class 3 is registered after re-forming the sample, then after immersion and shaking disperses. Class 4 after shaking for ten minutes and left for 24hrs then disperses; Class 5 does not disperse after 24 hrs but does with the addition of Calcium sulphate (gypsum). Class 6 does not disperse after the addition of gypsum but displays some moderate slaking; Class 7 disperses after subsequent shaking. Class 8 completely flocculates after shaking.

Profile Texture

Determines the proportion of Clay, Silt, Sand and Gravel through the soil profile of the test. Once tested you can get an idea of how much suitable/unsuitable material is in the proposed site.

Atterberg Limits

Two tests – The plastic limit is defined as the moisture content at which soil begins to behave as a plastic material. A plastic material can be molded into a shape and the material will retain that shape. If the moisture content is below the plastic limit, it is considered to behave as a solid, or a nonplastic material. As the moisture content increases past the plastic limit, the liquid limit will be approached. The liquid limit is defined as the moisture content at which the soil behaves like a liquid.

Sieve Analysis

Soil is put through a #200 sieve to wash away clays and silts attached to sands and gravels to determine accurately the 15 Group Unified Soil Classification (USC) – which describes the proportions of Gravels G, Sands S, Silts M, Clays C, Organic Soils O and Peats Pt. When formally classified this provides the engineer/designer with the basis for designing the dam wall.

Permeability Test

This tests the moisture holding capacity of the soil. The laboratory test determines the rate of permeability of moisture per centimetre per minute. This is at once the most useful and the most expensive of the geotechnical tests. Another field/home test called the “bottle test” is as follows:

Cut the bottom of a 750ml soft drink bottle Invert the bottle and 1/3 fill with the soil to be tested Fill the bottle with water. If no water seeps through the soil within 24 hours then the soil has good water holding properties.

We use the following soil log for our own field analysis in conjunction with other tests taken by others.

REGRARIANS LTD FIELD SOIL ANALYSIS CHART Name CURRY Date 20/9/97 Conditions SUNNY OVERCAST LIGHT BREEZE Site Ref. BOREHOLE 1 Profile Description A1 A2 AB B1 Horizon Dimensions mm 0-50 50-100 100-400 400-750+ Texture Sand Loam Sand Loam MediumClay Heavy Clay Colour Brown/Black Brown Light Brown Mottled Orange Pedality nil nil med med pH 5.0 5.0 5.0 5.0 Emerson E1 E1 E1 E1 Salinity Nil Depth to Rock Nil Depth to Pan Nil Hole  250mm Hole Depth 750mm

BANK SATURATION

When the dam is full of water, a significant proportion of the dam wall is saturated. It is important to realise that no dam is completely watertight, as some seepage will always occur. To reduce the potential of failure as a result of this phenomenon the dam must have flat slopes (or batters) and by ensuring that the soils are adequately compacted.

SPILLWAYS/OVERFLOWS/FREEBOARD

The first guiding consideration with overflows is that no more than 2.5 cubic metres per second (2500 litres/second) should flow through a well grassed spillway or the risk of erosion is likely. Calculation of the 1:100 flood flow volume is the key to designing a spillway capable of taking overflow water through the spillway with the lowest risk to the embankment. Inlet and outlet widths vary according to the flow volumes available (see Table 5).

As the spillway determines the ultimate water level in a dam, it is important to match its level so that there is adequate freeboard (distance from top of the bank to the water level). Freeboard depth is determined by the amount of fetch (longest exposed water surface on the storage) and should be at least 750mm – 1m for dams where the fetch is under 600m. Otherwise erosive wave action and overtopping (water going over the embankment) may occur causing dam failure and potential damage to life and property. Give consideration to the amount of settlement on the dam wall after its consolidation by increasing the construction height of the wall above the design by around 5%. This is particularly important if the wall is to be used as an access.

A trickle pipe is often used to reduce the likely movement of lower level flood flows through the overflow. This is often a requirement as you should never allow even small flows to go through a spillway beyond several days as this can cause more erosion than short term higher volume flows. We install High Density Polyethylene (HDPE) trickle pipes of between 150mm and 300mm diameter to just below the maximum water level. Inlet and outlets consist of either prefabricated cement collars or endwalls, or using 1m diameter pipe upturned, buried to expose the top with the trickle pipe inserted and sealed through the side. If well sealed the volume of water flowing though the pipe will be quite substantial. A collar or baffle plate will need to be placed around the pipe in the middle of the wall to restrict moisture seepage along the pipe which could lead to tunnel erosion and wall failure. A mesh cover should be placed over the inlet to remove the risk of blockages in the trickle pipe which are sometimes difficult to clear effectively.

Table 5 Spillway Inlet/Outlet Widths

Flood Flow (cumecs) Inlet Width (m) Outlet width @ 24% (m) Outlet width @ 14% (m) Outlet width @ 4% (m) 3 5.5 20 13 6 4 7.5 27 18 8 5 9.0 34 22 10 6 11.0 40 27 12 7 12.5 47 31 14 8 14.5 54 36 16 9 16.5 60 40 17 10 18.5 67 45 19 11 20.0 74 49 22 12 22.0 80 54 24 13 23.5 87 58 26 14 25.5 94 62 28 15 27.5 100 67 30

Nb. Outlet slopes calculated for return slopes of 24%, 14%, and 4% – Seek references or professional advice for further information on different slopes.

Topsoil coverage and seeding of the spillway immediately after construction with grasses such as kikuyu, couch or para grass or others similar will provide a smooth flow of flood waters through the spillway. Mechanical finishing or smoothing of the all excavated surfaces can be completed using a 4WD bike/car, or tractor dragging a section of reinforcing mesh weighed down with old tyres will provide an excellent smooth finish to the dam and assist in the preparation of a seedbed for sowing and regrowth. Further hand finishing of the dam using a shovel and rake is used around the tighter areas – particularly around the overflow. This extra effort is always worthwhile and you will thank yourself for doing it in the years to come such is the effect.

OUTLET PIPES

Outlet pipes are installed for the following purposes:

Gravity supply of water for downstream/downslope uses

Suction pipe water supply for pumping

To empty the dam for repairs, including silt removal and leak location

Allow environmental flows in sensitive catchments or where required by local authorities.

Outlet pipes present some difficulties in the construction phase and are expensive to install. The application however makes them such a useful item that they should be considered where possible.

Collars or baffles are required along the length of the pipe to prevent seepage along the length of the pipe. Generally made of steel plate, each of around 75cm to 1.2m square, at least 3 are needed for pipe lengths of up to 20m. 25m pipes will need 4 baffles, 40m needs 7 and 50m will need 8. The HDPE, rubber jointed concrete or galvanised iron pipe are installed by hand placed into a prepared trench. Soil is compacted around the pipe and then covered carefully with the machinery available (traxcavator, backhoe, or dozer best) and then carefully built up and track rolled. We place star pickets around the inlet and outlets and put an upturned 44 gallon oil drum over them to protect them from damage for the remainder of the construction process. Again a mesh cover is recommended to cover the inlet with both the inlet and outlet secured in an anchor block relative in volume to the diameter of the pipe. The Keyline Designs website and books has some excellent picture of pipe baffles, and inlet mesh guards.

Outlets have gate valves installed to effectively control the amount of flow out of the dam. Valves are place either downstream or upstream of the wall. Downstream valves are more popular although they are likely to leak more due to constant pressure applied to them and as a result are more difficult to repair. Upstream valves are more difficult to access, as they are submerged, and a remote winding spindle will be need to operate it. They however don’t have the same pressures on them and make it easier to repair the pipe where necessary.

Siphons are another way of piping water out of dams are quite common. Prefabrication HDPE siphons are now available and they remove some of the natural difficulties faced by many who have made siphons themselves. Compared to outlet pipes however the volume of water able to discharge is very small using equivalent diameter pipes and this often makes their potential uses limited. Where applicable a small siphon can be a cheap and effective means of discharging small volumes of stored water.

EARTHWORK VOLUMES AND STORAGE CAPACITIES

A significant part of the design process is the calculation of earthworks volumes and storage capacities. We need to calculate these respective volumes to ascertain the cost of earthworks and the efficiency of the storage. The ultimate volume will depend upon the height of the wall, the shape of the gully/slope cross section, and the of the storage reservoir upstream of the embankment.

The most accurate method of estimating earthworks and storage volumes is to get a high quality electronic field survey completed and then have the dam designed using the appropriate civil engineering CAD software. This is what we use with our team and it makes our lives a lot easier as we can build the dam on the screen, see how much it will cost, and see if it is an efficient storage or not. To do that takes about 0.5-2 hours per dam.

For calculating regular shapes the prismoidal formula can be used to estimate both storage and embankment volumes and is useful in all earthworks. It is generally written as follows

Another accurate method in calculating gully embankment volumes uses the horizontal slice method. This requires a plot of the dam wall onto a survey plan of the site. For higher accuracy you can then divide the dam and storage into a series of 0.5m slices, then with the aid of a planimeter the volume of each of the slices can then be calculated, with the total volume the sum total of all of the slices. You can also use the grid paper transparency over the survey plan and count the amount of squares for a similar, less accurate result.

The Queensland Water Resources Commission developed the following method which involves the identification of the shape of the gully cross section and then selecting a corresponding shape from a list – each of which has its own coefficient which have values between 0.5 and 1.6.

Table 6 Gully Dam Volume Formulae

Embankment VOLUME FORMULA V = 1.05 x W x H x (H+1) x K Where V = volume of earth (m3) H = height of embankment (m) W = length of the dam wall along crest (m) K= appropriate coefficient for gully shape Water Storage VOLUME FORMULA V = 0.22 x W x D x L x K Where V = volume of water stored (m3) L = longest length of water surface (m) W = width of water across the dam wall (m) D = water depth at the base of the embankment (m) K = appropriate coefficient for gully shape

Figure 7 Gully Cross Section Coefficients

Gully Storage and Embankment Volume Estimation Example

Figure 8 Embankment Dam Dimensions

W=50m

K = 0.5

Embankment Volume (V) = 1.05 x 0.5 (K) x 50 (W) x 4 (H) x 5 (H+1) = 525 m3

Figure 9 Water Storage/Reservoir Dimensions

Water Storage Volume (V) = 0.22 x 0.5 (K) x 30 (W) x 3.5 (D) x 100 (L) = 1155 m3 (1.155Ml)

Table 7 Storage Ratio Calculation & Gully Storage Economy

Total Storage = 525 (Figure 8) + 1155 (Figure 9)

= 1680 m3

Storage Ratio 1680/525 = 3.2

STORAGE RATIO Rating <2 Poor 2 – 4 Moderate 4 – 6 High > 6 Very High

Rectangular Tank Dam Volumes

For excavated tank dams the following formula is can be used for manual calculations of storage volumes:

Tank dam water storage VOLUME FORMULA (refer to Figure 10) V = (A+B+C) x D/6000 Where V = volume of water stored (m3) A = height of embankment (L1 x B1 in) B = length of the dam wall along crest (L1 x B1) C = appropriate coefficient for gully shape (L1 x B1) x (L1 x B1) D = depth of the tank

Ring dam water storage VOLUME FORMULA (refer to Figure 11) V = D (A1 + 4Am +A2) 6 Where V = volume of water stored (m3) D = distance between end faces A1 = area of one end face A2 = area of opposite end face Am = area of cross section running parallel to faces, a mid point

W2

Top Area A1 = π/4 x W1²

Mid Area Am = π/8 x (W1 + W2)²

Bottom Area, A2 = π/4 x W2²

Permaculture Farm Plan – Gather map resources and develop a holistic diagnosis and design planning approach to assess the integrated need for water storage’s per se, the catchments available, where dams fit into the overall landscape. Why do I need a dam? – Is it the most appropriate water storage choice? Legal Planning Requirements – Contact local/state authorities, experienced consulting civil engineers Soil Classification – Take soil samples – best to use Geotechnical Engineers where possible to ensure safest, best outcome Dam Design – Use climate/soil/catchment/site analysis to assess the best options. Again professional consultation wise if the experience or knowledge base is low. May be legally required anyway. Outlet Systems – Do I need them? Does the cost warrant the installation? Look for suppliers etc. Earthwork and Storage Volumes – Calculate them to assess the cost/benefit ratio. Contact local earthmovers – What’s their experience? Check out references and examples of their work. Find out how much they cost per hour or per cubic metre equivalent. How much earth can they move per hour/day. Get set price quotations and quality guarantees. CONSTRUCTION – Finalize construction timetables, Markout, and your personal finances. Read the next issue…..

References:

“Design and Construction of Small Earth Dams”, K.D Nelson, Inkata Press, Melbourne, 1991

“Farm Water Supplies”, Neil Southorn, Inkata Press, Melbourne, 1995

“Surveying for Construction”, William Irvine, McGraw Hill, UK, 1980

“Water for Every Farm – Yeomans Keyline Plan”, P.A. Yeomans, K.B. Yeomans ed., Keyline Designs, Queensland, 1993

www.keyline.com.au – Keyline Designs Website (Ken Yeomans)