Earthworks for Water Conservation and Storage in Permaculture

Permaculture Designers Manual

 

CHAPTER 6 – WATER IN PERMACULTURE

Section 7.3 –

Earthworks for Water Conservation and Storage in Permaculture

 

For the serious small dam and earth tank builder, there is no substitute for such comprehensive texts as that recently compiled by Kenneth D. Nelson (1985).

 

This small classic deals with catchment treatments, run-off calculations, soils, construction, outlets, volume and cost estimates and includes detailed drawings for most adjunct structures.

However, like most engineers, Nelson concentrates mostly on valley dams (barrier or embankment dams) and less on the placement of dams in the total designed landscape.

Few dam builders consider the biological uses of dams and the necessary modifications that create biological productivity in water systems.

A second essential book for water planning in landscape is P. A. Yeoman’s Water for Every Farm – The Keyline Plan (1981).

 

This very important book, written in 1954, is without doubt the pioneering modern text on landscape design for water conservation and gravity-fed flow irrigation. As it also involves patterning, tree planting, soil treatment and fencing alignment, it is the first book on functional landscape design in modem times.

 

There are two basic strategies of water conservation in run-off areas:

The diversion of surface water to impoundments (dams, tanks) for later use and the storage of water in soils.

Both result in a recharge of groundwater. As with all technologies, earthworks have quite specifically appropriate and inappropriate uses.

 

Some of the main productive earthwork features we create are as follows:

Dams and tanks (storages); 

Swales (absorption beds);

Diversion systems or channels; and

Irrigation layouts and in particular those for flood or sheet irrigation.

 

SMALL DAMS AND EARTH TANKS

Small dams and earth tanks have two primary uses.

The minor use is to provide watering points for rangelands, wildlife and domestic stock; such tanks or waterholes can therefore be modest systems, widely dispersed and static.

 

The second and major use is to contain or store surplus runoff water for use over dry periods for domestic use or irrigation.

 

The latter storages, therefore, need to be carefully designed with respect to such factors as safety, water harvesting, total landscape layout, outlet systems, draw-down and placement relative to the usage area (preferably providing gravity flow).

A separate category of water storages, akin to fields for crop or browse production, are those ponds or wet terraces created specifically for water crop (vegetation or mixed polycultural systems of aquatic animal species).

Open-water (free water surface area) storages are most appropriate in humid climates, where the potential (or evaporation is exceeded by average annual rainfall).

There is a very real danger that similar storages created in arid to sub humid areas will have adverse effects, as evaporation from open water storages inevitably concentrates dissolved salts.

Firstly, such salty water can affect animal health. Secondly, the inevitable seepage from earth dams can and does create areas of salted or collapsed soils downhill from such storages.

And in the case of large barrier dams, so little water may be allowed to bypass them in flood time that agricultural soils, productive lakes, and estuaries may lose more productive capacity by deprivation of flush-water and silt deposits than can be made up (at greater cost) by irrigation derived from such lakes. (Figure 7.3)

 

Dry land storage strategies are discussed in Module 10.

What I have to say here is specifically addressed to humid areas and small dams unless otherwise noted.

Earth dams or weirs where retaining walls are 6m (19 feet) high or less, and which have a large or oversized stable spillway are no threat to life or property if well-made.

They need not displace populations, stop flow in streams, create health problems, fill with silt or block fish migrations.

In fact, dams or storages made anywhere but as barriers on streams effectively add to stream flow in the long term.

Low barrier dams of 1-4m (3-13 feet) high can assist stream oxygenation, provide permanent pools be “stepped” to allow fish ladders or bypasses and also provide local sites with modest power generation.

While almost all modern assessments would condemn or ban large-scale dams (and large-scale power schemes) on the record of past and continuing fiascos, a sober assessment of small water storages shows multiple benefits.

Given the range of excellent texts on small dams (often available from local water authorities and by mail order from good bookstores), I have therefore avoided specific and well-published construction details and have here elaborated more on the types, placement, links to and from, and function of small dams in the total landscape.

Yeoman’s (pers. comm., 1978) has stated that he believes that if from 10-15% of a normal, humid, lowland or foothill landscape were fitted with small earth storages, floods and drought or fire threat could be eliminated.

Not all landscapes can cost-effectively store this proportion of free surface water; some because of free-draining soils or deep or coarse sands.

Other areas are too rocky or of fissured limestone and yet others are too steep or unstable. But a great many productive areas of clay-fraction subsoil’s (40% or more clay fraction) will hold water behind earth dams, below grade levels as earth tanks or perched above grade as “turkey’s nest” or ring dams.

 

There are very few landscapes, however, that will not store more soil water if humus, soil treatment or swales are tried; the soil itself is our largest water storage system in landscape if we allow it to absorb.

Almost every type of dam is cost-effective (Figure 7.20) if it is located to pen water in an area of 5% or less slope. However, many essential dams, if well-made and durable, can be built at higher slopes or grades, made of concrete, rock-walled or excavated, if water for a house or small settlement is the limiting factor. Each and every dam needs careful soil and level surveys and planning for local construction methods.

 
 

DAM TYPES AND LOCATIONS

There are at least these common dam sites in every extensive landscape:

SADDLE DAMS are usually the highest available storages, on saddles or hollows in the skyline profile of hills. Saddle dams can be fully excavated below ground (grade) or walled on either side of, or both sides of, the saddle. They can be circular, oblong, or “shark egg” shaped with horns or extensions at either end (Figure 7.4).

Uses: wildlife, stock, high storage.

 
 

RIDGEPOINT DAMS or “horseshoe” dams are built on sub-plateaus of flattened ridges, usually on a descending ridgeline and below saddle dams.

The shape is typically that of a horse’s hoof. It can be made below grade or walled by earth banks (Figure 7.5).

 

Uses: As for saddle dams; only of limited irrigation use but, very useful for runoff and pumped storages.

Note: that both saddle and ridge dams can act as storages for pumped water used for energy generation.

 

KEYPOINT DAMS are located in the valleys or secondary streams, humid landscapes, at the highest practical construction point in the hill profile, usually where the stream profile changes from convex to concave; this place can be judged by eye and a descending contour will then pick up all other key points on the main valley.

Uses: Primarily to store irrigation water. (Figure 7.6)

 

Note: that a second or third series can be run below this primary series of dams, and that the spillway of the last dam in a series can be returned “upstream” to meet the main valley, effectively spilling surplus to streams.

 

CONTOUR DAM walls can be built on contour wherever the slope is 8% or less, or sufficiently flat. Contours (and dam walls) can be concave or convex to the fall line across the slope.

Uses: Irrigation, aquaculture or flood-flow basins in semi-arid areas. (Figure 7.7)

 
 

BARRIER DAMS are always constructed across a flowing or intermittent stream bed. These dams therefore need ample spillways, careful construction, fish ladders on biologically important streams, and are made most frequently used as energy systems, but are also used for irrigation if they are constructed well above the main valley floors where crops are grown (Figure 7.8).

 
 

TURKEY’S NEST DAMS or above-grade tanks; water has to be pumped in to these, often by windmill or solar pump. They are common in flatlands as stock water tanks or for low-head irrigation (Figure 7.9).

 

CHECK DAMS; there are many forms of barrier dams not intended to create water storages, but to regulate or direct stream flow.

Even a 1-3m (3-10 foot) wall across a small stream gives enough head to drive a hydraulic ram, to fit a waterwheel, to divert the stream itself to a contoured canal for irrigation or to buffer sudden floods.

Dams intended to regulate flood crests may have a base pipe or fixed opening in the streambed which allows a manageable flow of water downstream while banking up the flood crest behind the dam itself, so spreading the rush of water over time.

The base opening allows silt scour and so keeps the dam free of siltation (Figure 7.10, Figure 7-11, Figure 7-12 and Figure 7-13).

 

GABION DAMS; In dry lands, permeable barriers of rock-filled mesh “baskets” (gabions) (Figure 7.14) will create silt fields and water spreading across eroding valleys.

The scale of these dams varies, but for farm construction, walls 0.5-2 m {2-6.5feet) high are usual.

As with Figure 7.11, the purpose is not to stop free surface water, but to create a flat area where silt loads can usefully deposit and so form absorption beds in flood conditions.
We can see the landscape (as though sliced into layers through contours) as a set of catchment, storage, usage, and revitalization zones. (Figure 7.15)

 
 

BUILDING DAMS

Although we can build dams or tanks on any site, given enough material resources, commonsense dictates that storage dams are carefully located with respect to:

Earth type (core out a sample pit for assessing clay fractions);

 

Grade behind wall (lower slopes give greatest capacity);

 

Downstream safety of structures and houses (a key factor in large dams);

 

Height above use points (gravity flow is desirable);

 

Available catchment or diversion.

 

Tamped earth with some clay fractions of better than 50% is a waterproof barrier up to heights of 3.6 m (12 feet), not counting the holes behind such walls caused by their excavation.

Therefore we speak of depths of 4.5-6m (15-20 feet) for small earth dams. Few of us will want to build farm dams higher and we must get good advice if we wish to do so.

Slopes to crest should be concave and every 25cm (10 inches) a machine such as a roller or the bulldozer tracks themselves, should ride along and tamp down the earth.

 

This, like the exclusion of boulders and logs, grass clumps and topsoil, is critical to earth stability (shrinkage of well-compacted dams is less than 1%).

Earth so rolled should be neither so dry as to crumble nor so wet as to slump or squash out under the roller.

A key should be cut to prevent shear and cut off any base seepage.

 

This is needed on all walls 1.8 m (6 feet) or higher, otherwise the base should be on a shallow day-filled ditch.

Slopes are safe at a ratio of 3:1 (inner) and 2:1 or 2.5:1 (outer), freeboard at 0.9 m (3 feet), key at 0.6-0.9m (2-3 feet) deep. In suspect soils, the whole core can be of carted clay (Figure 7.17).

 

The wall can curve (out or in), but if carefully made as diagrammed and provided with a broad spillway, should be stable and safe forever, barring explosions or severe earthquakes.

The SPILLWAY base should be carefully surveyed at 1 m below crest and away from the wall or fill itself (don’t try to judge this, measure it) and a SIPHON or BASE OUTLET pipe fitted with baffle plates placed to draw off water (Figure 7.18).

 

 

The efficiency in capacity of dams depends on the flatness of the area behind the wall.

A “V” valley or “U” valley, plateau or field should be as level as can be chosen for greatest efficiency.

The key to efficiency is the length of the dam wall, compared with the “length” of water dammed. If the back-up is greater than wall length, then this is a measure of increasing efficiency of energy used or earth moved for water obtained.

A careful survey of grade plus dam length gives this data before starting the wall.

Some dam sites are very cost-effective, especially those short dams at constricted sites where the valley behind them is flattish.

 

Small dams of this nature are a jewel in the landscape. Fenced and planted to 3m of forest and fruit surround, they will provide biologically clean, if sometimes muddy, water, and if the topsoil is returned, lime used and edges planted, mud will decrease and eventually clear.

For water cleanliness and parasite control, cattle, sheep and other animals should be watered at spigots or troughs, not directly at the dam.

Troughs are easily treated with a few crystals of copper sulfate to kill snails and parasitic hosts; dams stocked with fish will do the same job.

Crests can be graveled and safely used as roads to cross valleys or bogs and special deep areas, islands, peninsulas and shelves or benches made inside the dam for birds, plants and wild-fire-immune houses.

 

SEALING LEAKY DAMS

There are several ways to seal leaking dams:

Gley;

Bentonite;

Explosives;

Clay;

Impermeable membranes.

 

GLEY is a layer of mashed, wet, green, sappy plant material sealed off from air.

Although the very green manure of cattle is preferred, shredded, sappy vegetation will also work. It is carefully laid as a continuous 15- 233m (6-9 inch) layer over the base and gently sloping sides (ratio of 1:4) of a pond and is covered completely with earth, cardboard, thick wet paper, plastic sheets or rolled clay and allowed to ferment an aerobically.

 

This produces a bacterial slime which permanently seals soil, sand or small gravels.

Once ferment occurs, the pond is pumped or hosed full of water and the paper or plastic can be later removed.

I have used carpets and odd pieces of plastic sheets overlapped with good results. In cold areas, ferment can take a week or so, in the tropics a day or so.

Lawn or second-cut grasses, papaya and banana leaves, vegetable tops or green manure all serve as the base layer.

I believe that in very good soils, especially in the tropics, it may be possible to grow the gley as a mass of Dolichos bean and just roll it flat before seating it (Figure 7.19).

 

Modifications are:

To pen and feed a herd of cattle in the dry dam until the bottom is a manurial pug; occasional watering assists this process.

 

To strew bales of green hay and manure on ponds that leak slightly, producing algae which seal minor cracks.

To sow down green crop in the dry dam, spray irrigates and feed it off regularly with cattle.

 

BENTONITE is a slippery clay-powder derived from volcanic ash.

It swells when watered and will seal clay-looms if roto-tilled in at 5-7cm (2-3 inches) deep and rolled down.

 

However, it is expensive and doesn’t always work. Cement and tamping plus sprinkling might be preferable or a bituminous spray can be rolled in after tilling.

In clay soils, salt or sodium carbonate can have the same effect.

 

EXPLOSIVES are sometimes used to compact the sides of full dams and consists of throwing in a 3-5 stick charge of dynamite. This works well at times, but is dangerous if you own a retriever or if the dam wall is poorly compacted to start with.

 

CLAY is expensive if it has to be carted in, but it is often used to seal dams near a clay pocket. The clay is spread and rolled 23-30cm (9-12 inches) thick over suspect areas.

 

IMPERMEABLE MEMBRANES can be of welded plastic, neoprene, or even poured concrete. Impermeable membranes are too expensive to use on any but critical dams, which may mean a guaranteed water supply to a house or garden in very porous areas.

 

Using membranes enables banks to be steeper than in any other earth-compaction or gley systems, so that more water can be fitted into a smaller space.

It is not “biological” unless a sand or topsoil floor is also added over the sealing layer, when fish or plants can be added.

 

Earth storage is now the cheapest, easiest and most locally self-reliant method of water conservation.  Unless both cities and farms use such methods, clean water will deservedly become known as the world’s rarest mineral, ill-health will be perpetuated and droughts and floods alike become commonplace. None of these are necessary.”

Costs vary greatly; as a rough guide, water stored in soil and humus is the cheapest and or greatest volume, surface dams are the next cheapest and tanks cost dearly, but still much less expensive than piped water from a main supply.

I can only urge all people of goodwill to promote, fund and investigate water and water storage, water energy and water cleanliness, as the chlorinated, metallic, asbestos-filled, poisonous water of modern centralized systems is producing such epidemic disease and illness as cancer, bone marrow failure and gastrointestinal disorder.

If a 22,500l tank costs 20 units of money, the same units in a sensible earth storage pays for 2,500,000 l, or about 100 times as much water.

Up to 135,000-2,500,000l tanks get cheaper, as less concrete is used for more water.

That is, a large tank is relatively cheaper than a small tank. Above 22,500l, such tanks are usually poured on site; below this, they are carted from a central manufacturing site.

 
 

Dams, in contrast, begin to cost more as the height of the wall rises. About 3m (10 feet) of retaining wall is the limit of cheap dams.

Above this, costs rise rapidly as greater skills, more expensive and massive materials, more complex controls of levels and much greater environmental risks take their toll.

As noted, “Cheap” water in dams depends on the choice of site, so that very low dams on well-selected sites impound 20-100 times more water than the same earth used on steeper sites, where every unit of earth moved equals a unit of water.

However, even earth tanks excavated below grade are at one-tenth the price of concrete tanks above grade.

Where are tanks, modest dams, and massive dams appropriate?

Tanks are appropriate on isolated dwellings, in flatlands and everywhere in cities and urbanized areas.

Dams of from 22,500 to 4.5 Ml are best built on any good site in country and parkland areas.

 

Massive dams are appropriate hardly anywhere but the rock-bermed or glaciated uplands of solid and forested hills, subject to low earthquake risk and then only for modest domestic (not dirty industrial) power generation.

 

TANKS

Cultural and historical precedent may determine how earth is moved and used or even if it can be moved at all.

Thus, an Australian, accustomed to a great variety of surface storage, is astounded that there are no significant domestic rainwater tanks in Europe, the USA or India (where clean drinking water is rare), that British, American and Brazilian farmers rarely use multiple earth storages of water and that expensive pipelines and bores are the preferred “alternative“, even where local rainfall often exceeds local needs.

The simple forms for making concrete tanks cost a few hundred dollars and may be used hundreds of times.

 

About 22,500l provides a family with all needed water (drinking, showers, cooking, and modest garden water on trickle) for a year; tank water is renewed by rain at any time of the year.

Every roof, whether domestic or industrial, would fill many such tanks, and simple calculations (roof area x average rainfall in millimeters or inches) and conversion to liters or gallons gives the expected yield.

Granted that roof areas themselves can be contaminated by birds, dust, or industry, the first precaution is to reject the first now-off of water and use it on gardens or in swales. Two methods for doing this are shown in Figure 7.21.

 

As for the entry of insects, birds or rodents to tanks (and this includes mosquitoes), a “U” pipe entry and exit, a sealed tank roof and an overflow pipe emptying to a gravel-filled swale all effectively exclude these potential nuisances.

If birds persistently perch on roof ridges, a few very fine wires or thread stretched along the ridge as a 10cm (4 inch) high “fence” will discourage them.

Gutters on roofing can be cleaned out regularly, or “leaf -free” gutters or downpipes fitted (about 3 or 4 types are commercially manufactured; some systems are illustrated in Figure 7.22).

 

Given that most dust and leaves are removed, residual organics are usually harmless.

These “fix” as an active biological velvety film on tank walls and bases. Taps or outlet pipes are normally fitted 15-20cm (6-8 inches) above any tank floor to allow such a film to remain.

Finally, a net or bag of limestone, shell, or marble chips is suspended in the tank.

 

This creates hard (alkaline) water, preventing heavy metal uptake from the water and decreasing the incidence of heart attacks in those using the tank.

Washing and shower water can be soft (acid) but the water we drink is best made alkaline for the sake of health.

It makes far more sense to legislate for such tanks on every roof than to transport exotic water for miles to towns; it will also ensure that clean air regulations are better observed locally, that every house has a strategic water reserve and that householders are conservative in their use of water.

 

SWALES

Swales are long, level excavations, which can vary greatly in width and treatment from small ridges in gardens, rock-piles across slope or deliberately excavated hollows in flatlands and low-slope landscapes (Figure 7.23).

 

Like soil conditioning or soil loosening systems, swales are intended to store water in the underlying soils or sediments.

They are, simply, cross-flow dry channels or basins intended to totally intercept overland flow, to hold it for a few hours or days and to let it infiltrate as GROUNDWATER RECHARGE into soils and tree root systems.

Trees are the essential components of swale planting systems or we risk soil water logging and a subsequent local rainfall deficit caused by lack of evapo-transpiration of the stored water.

Thus, tree planting must accompany swaling in arid areas.

 

Swales should ideally not exceed in width the total crown spread of the fringing trees planted to use the stored water absorbed into the swale sediments.

Trees over shade and cool the soils of swales, further reducing the risk of evaporation and dissolved salt concentration or water loss.

Although swales can also be grazed, few grasses can effectively remove the absorbed water to re-humidify airstreams.

Swales are therefore widely used in arid to sub­humid, even humid areas, on both fairly steep slopes and flatlands and in both urban and rural areas.

They are appropriate to road and other silty or contaminated run-off harvest (where the dust or tar oils washed off have no adverse effect on tree growth).

 

The essentials of swale construction are simple:

They are all built on contour or dead level survey lines and are neither intended nor permitted for water flow.

Their function is just to hold water.

Unlike dams, swale banks and bases are never compacted or sealed (although small tanks can be sunk in swale bases for watering livestock or trees).

Conversely, the swale soils can be graveled, ripped or loosened to assist water infiltration.

The swale depth and width can be varied to cope with the speed of infiltration locally, so that wider and shallower swales are made in sands, narrower and deeper swales in clay-fraction soils.

After an initial series of rains that soak in a meter or more of water, trees are seeded or planted on either bank or side slopes of the swales.

This can take two wet periods. Thereafter, it takes about 3-5 years for tree belts to over shade the swale base and to start humus accumulation from leaf tissue. (Humus will accumulate, however, by wash down and wind movement from bare or uphill areas.)

Early in the life of an unplanted swale, water absorption can be slow, but the efficiency of absorption increases with age due to root and humus effects.

As this happens, it is possible to admit water to swales from other areas, leading it in via DIVERSION DRAINS (Figure 7.26).

 

This “exotic” water from unused road or rock surfaces or overland flow can enable the planting of high-value trees of higher water demand or a new set of swales to be constructed.

Every sub-humid and arid townscape can, with great energy gain and much reduced cost for roading and water use, fit all roads and paved areas with swales, along which tree lines shade pavement and reduce heat oases while they produce fuel, mulch, and food products.

Every roof tank overflow and some grey water wastes can be led to swals (if boron detergents are not used).

 
 

Swales interpenetrating the suburban development of Village Homes in Davis, California (Michael Corbett, designer) accept all road and excess roof run-off and support hundreds of productive trees in settlement.

(Picture of actual swale in Village Homes on Left)

Water penetrated soils to 6m (19 feet) deep after a few years of operation and swales were self-shaded after 3-4 years of tree growth.

In Hawaii and in central Australia, swales I have designed produced fast growth in trees in volcanic cinder and sandy soils.

Swales in Australian dry lands have consistently grown larger and healthier water run-off fed trees than have open plantings.

In arid areas, it is imperative to plant trees on swales or we risk salt concentration and soil collapse downhill.

All swales are therefore temporary events, as trees supplant their function; they are precursors to rehabilitation of normal forests in their region.

Natural swales in humid forests (Tasmania) not only generate much larger trees and provide level access ways, but support thick humus and specialized plants on the swale floors. Orchids, fungi and ginseng do better in swales.

Most swales should be adjusted (by widening, gravelling or ripping) to absorb or infiltrate all water caught in from 3 hours to 3 days.

Fast absorption will not harm most tree species, although trees such as chestnuts and citrus may need to be planted on nearby spoil banks for adequate root drainage. I believe swales to be a valuable and greatly under-used earth form in most climates, including upland and plains areas of snow drift in winter.

 

In summary, a swale is a large hollow or broad drains intended to first pool, and then absorb all surplus water flow.

Thus, the base is ripped, graveled, sanded, loosened, or dressed with gypsum to allow water INFILTRATION.

Trees ideally over shade the swale. The base can be uneven, vary in width, and treated differently depending on the soil type.

The spoil is normally mounded downhill or (in flat areas) spread. Water enters from roads, roof areas, tank overflows, grey water systems or diversion drains.

 

The distance between swales (the run-off or mulch planted surface) can be from three to twenty times the average swale width (depending on rainfall).

Given a useful swale base of 1-2m (4-6 feet), the inter-swale space should be 3-18m (12-60 feet).

In the former case, rainfall would exceed 127cm (50 inches), and in the latter it would be 25cm (10 inches) or less.

In humid areas, the inter-swale is fully planted with hardy or mulch producing species. In very dry areas, it may be fairly bare and exist mainly to run water into swales.

Mulch blows into, can be carried to, or is grown and mown in swales. Fine dust and silts build up in swale bases and domestic wastes can be buried here as a mulch-pit for hungry plants.

 

The swale and its spoil-bank make a very sheltered starting place for plants on windy sites and the lower slope swales can be planted mainly to Casuarina or leguminous trees to prevent upslope winds.

Ridges should always have windbreak and condensation plants of hardy and useful species (Casuarina, Acadia, Leucaena, silky oak, pine, cypress).

Windbreaks can occupy every sixth to tenth swale on sites where wind is a limiting factor.

It is better to plant on the down slope side to allow mulch collection in the swale base for use elsewhere.

 

Swale sections can be over-deepened, so that although the swale lip is surveyed level, its floor may rise and fall.

Deepening is most effective in clay fraction soils and may result in shallow ponds for water-needy crop. Widening is most effective in sand or volcanic-fraction soils and readily admits water to the ground table.

Two other pit systems are useful in swales:

One-mulch and manure-filled for heavy nutrient feeders (yams, bananas, etc.) and

 

the other to hold oil drums, plastic liners, or tire ponds as a sealed water reservoir watering young plants. These can be planted with lotus, kangkong, watercress, Chinese water chestnut, or like crop.

 

Keeping the swale width to the tractor, donkey cart, foot track or wheelbarrow access width that one has planned, sections can be widened at regular intervals to take assemblies of plants, to dig ponds or mulch pits and to plant trees of higher water need. This leaves access open and enables many assemblies, species and constructs to be built along the swale as need dictates.

DIVERSON BANKS AND DRAINS

Diversion drains are gently sloping drains used to lead water away from valleys and streams and into storages and irrigation systems or into sand beds or swales for absorption.

If low earth-walls are raised across the flow channels of larger diversion drains, these then act as a series of mini-swales for specific tree sites, while surplus water flows on to storages.

Diversion drains differ from swales in that they are built to flow after rain (from overland flow or from feeder streams).

They are the normal and essential connectors of dam series built on the Keyline systems, so that the overflow of one dam enters the feeder channel of the next. (Figures 7.16 A and B)

Such diversion systems need careful planning and survey, with drain bases sloping as little as 1:6000 in fine sands.

Dam crests and irrigation drains need equally careful placement. Even without a stream intake, diversion drains will gather water from overland flow in as little as 1-1.5cm (1/2 inch) of rain over 24 hours, so that isolated dams are normally fitted with diversion drains even in quite dry country.

Diversion drains can be led to broad level swales in dry lands, or made of simple concrete or stone walls across solid rock faces.

In the Canary Islands, these gather rock run-off which is led to underground cave storages or large open tanks.

 

 

Simple sliding gates across or in the downhill banks of such drains allow controlled flow, controlled irrigation, and (in floods) by-passing of dams.

Spill gates can be FIXED (concrete slide holders and aprons) or MOVEABLE (plastic sheets weighted with a chain sewn into the foot, and supported across the drain by a light pipe sleeved into the top).

These latter are called “Flags“(Figure 7.27).

 

Slide gates can open on to ridge lines and water then spreads downhill, while plastic flags can be placed and taken up at any point along a drain.

This enables one or two people to water 200-240ha (400-600 acres) in a morning. It is also an effective wildfire control system in forested areas.

For sophisticated wildfire or irrigation control, both slide gates and dam-base gate valves can be remotely operated, by radio signal and storage batteries or buried electrical conduits, to power small motors or hydraulic slides.

Complete wildfire control can be achieved by dams and sheet irrigation and using infrared sensors and automatic spill-gates.

Such systems remove the risk for fire-fighters and allow forests to regenerate in semi-arid areas.

 

Interceptor drains

These drains-or rather, sealed interceptors- act in the opposite sense to diversion drains. These earthworks are specifically designed (by Harry Whittington of West Australia) to prevent overland water flow and water logging, which has the effect of collapsing the dry land valley or down slope soils of a desert soil catena.

 

Thus, they ideally totally intercept overland flow and direct it to streams or valley run-offs. They can be cross-slope as are swales, but they differ from diversion drains and swales in that their construction always involves the ramming (by bulldozer blade) of subsoil layers hard against the downhill bank.

This effectively prevents or impedes water seepage through the downhill wall of the interceptor bank.

Moreover, they are always 1.5-2.5m (5-8′) deep, carefully spaced and effectively stop not only overland flow but also salt water seepages in shallow sand seams.

In effect, they isolate large blocks of soil from water logging and salt seepage.

After this preparation, trees can be planted in previously desertified soils. Where deeper sand seams carrying salty water are located, these can be trenched out and stopped with a vertical plastic barrier, backed by compacted clay on the uphill side.

Made to flow at 1:600 to 1:1500 cross-slope interceptors effectively cut up incipient or degraded croplands subject to desertification into blocks of 100m (330 feet) to a maximum of 300m (985 feet) wide, isolating each block from flooding and the salty “cascade flow” from uphill surplus water is carried off in streams.

Interceptor banks also cut off seepage from salt lakes and can divert early (salted) overland flow around saltpans, letting later fresh floodwater fill the pans or shallow lakes.

Spreader drains or banks are intended to spill a thin sheet of surplus (overflow) water down a broad grassy slope, either for irrigation or (in deserts) to prevent channel scour and gullying.

 

They are normally made to take any overflow from swales and dams and may be tens or hundreds of meters long. Spreader banks have the lower side dead level and compacted or even concreted.

Water enters from a dam, swale or minor stream and leaves as a thin sheet flow down slope. Spoil is piled uphill, preferably in mounds or removed to allow downhill sheet flow to enter the spreader ditch (Figure 7.28).

 

For irrigation areas of flatlands, the bank is often pierced by a series of dead-level pipe outlets, each feeding an irrigation bay (itself planed flat), to which water is confined by low side walls (STEERING BANKS).

 

At the lower end of that bay, a TAIL DRAIN- a surplus water drain-leads off excess water to a stream or secondary storage dam.

 

In such cases, the uphill or primary feeder drain is called the HEAD RACE and has cross-slides that block flow; this causes the leveled pipes to flood out into the bays.

Sometimes the pipes are replaced with level concrete sills and in old-established spreader banks, the whole of the lower lip may be concreted to give a permanent level spill immune to breakage by cattle or vehicles.

 

Spoil lines uphill from spreader banks are best piled or at least broken by frequent openings to allow downhill overland flow to enter the drain without carrying silt loads.

In any landscape, a subtle and well-planned combination of dams, drains, spreader banks, swales, and appropriate pipes, gate-valves, spill gates, flags or culverts will harvest, store and use surplus flood or overland flow waters.

These are used to flush out salts from soils, spread water evenly over crop, put out wildfires and modestly irrigate land.

It is a matter of first deciding on, then surveying any or all these systems, and above all of considering the long-term effects on the immediate landscape and the soils.

All earthworks can be regarded as REHABILITIVE and remedial if they replace salted and eroded lands with perennial browse or forests.

They can be seen as DAMAGING and exploitative if used to irrigate high watered demand crop (like lucerne/alfalfa) in dry lands, to cut off flow to dry areas or to run water for unessential uses to large urban centers devoted to lawns and car washes.

 
 
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