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Introduction Swale

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Introduction SwaleWhat is a Swale?A swale is a relatively broad land drainage depression along which storm-water flows. Its primary purpose is to hold back flows by decreasing the water velocity using grass or other forms of vegetation to slow down the velocity. This in turn, helps to reduce the flood peaks in the receiving waterways. It has a secondary function to improve water quality by reducing contaminant (pollutant) loadings. A decreased flow velocity facilitates the settling of sediments and the vegetative growth traps additional suspended sediment. Swales are most effective as a storm-water treatment device when the water velocity is slow and the swale is densely grassed or vegetated. If a swale requires mowing or cut down, the Council will complete this when the grass gets to 200mm in length as per the Resource Consent guidelines. Guidelines for construction of swales for water quality improvement and flow attenuation of lightly contaminated runoff from poultry farms

What is a swale?Swales are linear depressions formed in the ground to receive runoff and slowly move water to a discharge point. Unlike ditches, they are normally dry out with wet weather and are grassed. Side and longitudinal slopes are gentle. The slow movements of water along the swale, aided by grass and check dams, encourage deposition of solids washed off the hard standing, and help to remove nutrients such as phosphorus. Standard Farming Installation Rule 2.3.3.12 allows lightly contaminated drainage, e.g. from yard areas or roofs to be treated by means of swales or constructed wetlands.Swales dug into even the gentlest of slopes will fill with water after every good rain and allow the moisture to percolate deep into the soil over a day or more. This charges the subsoil over several years allowing the trees planted into the mound to thrive even during the driest of times

What are the benefits of using swales? Improves water quality Cheaper to construct than piped systems Can be incorporated into the landscape Low maintenance Visible operation

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What makes a good swale? The longer the swale the better. A long swale allows plenty of time for settlement of solids contained in the runoff. Swales of more than 70m in length give good, consistent performance. No sharp corners. Swales should have gentle curves. Shallow gradients. A swale which curves too and fro with a shallow gradient is better than a swale which is built directly down slope. Shallow swales with no permanent water have better chance of establishing a grass sward.

Where should the swale be located?This will depend on the layout of the site and the amount of space available, but the swale would usually run either parallel with or at 90 to the houses, with the hard-standing draining towards the swale. To comply with Standard Farming Rule 2.3.3.2 outflow points into the swale should be capable of being stopped with a drain blocker so heavily contaminated runoff, such as may occur when cleaning out, can be diverted to waste effluent tanks. Larger farms may need more than one swale; a second swale could possibly be located between sheds. Roof water can be directed to the swale via surface drains or via smaller swales.

What is a check dam?Check dams are small dams constructed across a swale. The check dam is made from graded broken stone. Runoff will pond behind the dam allowing sediment to settle out. As the check dam is made of stone, it will allow the ponded water to discharge slowly towards the outlet. This improves the efficiency of the swale.

What is the easiest method for designing a swale?To improve water quality, the first flush of dirty water, washed off the hard standing, is trapped behind check dams built across the swale. This first flush is known as the treatment volume. To calculate the number of check dams needed to retain the treatment volume, follow the eight steps listed below.

ActionStep 1. Measure the roof area and area of hard standing, A (m2) Step 2. Establish the length available for the swale based on site layout, L (m) Step 3. Measure the land gradient for the swale (%) Step 4. Calculate the treatment volume Vt (m3) = A 0.012 Step 5. Calculate the number of check dams i) From table A, look up the distance between check dams for the gradient in Step 3. Initially assume the narrowest swale floor width of 5450m2 120m 2%

Examples

5,450 0.012 = 65.4m3 swale floor with = 1m gradient = 2% distance = 25m

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1 m. ii) Divide swale length L in Step 2. by the distance between check dams to calculate the number of check dams. Step 6. Will the number of check dams store the treatment volume? i) From table B. look up the volume held by each check dam ii) Multiply the volume per check dam by the number of check dams. Step 7. If the volume stored by the check dams is less than the treatment volume increas e the swale floor width

120 25 = 5 check dams

Swale floor width = 1m Gradient = 2% Volume per check dam = 10m3 10 5 = 50.0m3 Swale floor width = 1.5m Gradient = 2% Volume per check dam = 13.1m3 Volume of storage = 65.5m3 Treatment volume Vt = 65.4m3 Check dam storage volume = 65.5m3 OK

Step 8. When the volume of storage behind the check dams is greater than or equal to the treatment volume proceed with swale construction.

Note 1: A value of 0.012 - 0.015 represents the quantity of the first flush of runoff that will contain lightly contaminated washings. The values are obtained from Sustainable Urban Drainage Systems: Design Manual for Scotland and Northern Ireland. Table A Distance between check dams Swale Gradient (%) Distance between check dams (m) Table B Volume per check dam Swale Gradient (%) Floor width (m) 1.0 1.5 2.0 1 20.0 26.3 32.5 2 10.0 13.1 16.3 3 6.7 8.8 10.8 4 5.0 6.6 8.1 5 4.0 5.3 6.5 1 50 2 25 3 16.5 4 12.5 5 10

How is the Swale constructed? Excavate the swale to a depth of 750mm. Store the top soil separately. Make the width of the swale floor equal to the width calculated in the 8 steps shown above. Make the side slopes 1 vertical to 3 horizontal or more gentle if there is enough room. Excavate the floor of the swale for a further 150 - 250mm depth and replace the excavated material with the top soil.

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How is the check dams constructed? Excavate a trench across the width of the swale. Make the trench 200mm deep and 3.3m long. Build up the check dam from 75mm 150mm grade broken stone to a height of 500mm above the floor of the swale. Build the check dam at the down slope end of the trench, leave the upslope end of the trench empty. The side slopes of the check dam should be 1 vertical to 2 horizontal

How is the swale completed? Grass the sides and the floor of the swale with a mixture that requires little maintenance and provides a dense well-knit turf. For example a mixture of 70% creeping red fescue, 20% smooth-stalked meadow grass and 10% creeping bent. Apply the grass mix at about 24 grams per m2. Fertiliser and/or lime should be applied to bring the topsoil nutrient content and

pH to within the recommendations of BS 3992:1994, General purpose topsoil.

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Water Catchment Strategies for Dry lands: SwalesDry lands present a serious problem for agriculture and for human development; and the problem is worsening. Standard agricultural practices have actually created major deserts in the world including the deserts around Mesopotamia. Twenty-five percent of the land on Earth is either desert or threatened with desertification. The problem strikes poor regions (like Africa with 66% of the land affected by desertification), and rich regions alike (as with Texas, where 40% of pastureland is now too arid for use). Almost 70% of the agricultural dry land is degraded by desertification.

Yet, our agricultural practices remain largely unsuitable for most climate conditions exacerbating the problem. Our methods are contributing to salinization and soil erosion. Tragically, many people live in or depend on the agriculture from these areas. Generally, in most arid regions there is enough rainfall to re-green an area, but the rain that does fall quickly washes away taking precious soil with it.

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One of the strategies that have proven affective in combating and even reversing desertification has been reforestation. Trees help the land hold water, are more drought tolerant than most plants, and their detritus helps build up soil. Trees serve as windbreaks which help reduce evaporation and soil erosion, and provide shelter for humans and animals. Trees also make up a significant part of the hydrological cycle, providing virtually 100% of the water for inland continental rainfall.

Consider the situation in the two most populous nations in the world: China and India. China is suffering from encroaching deserts and already has 19% of its land suffering from salinisation. India is chiefly arid or semi arid, though this was not always the case. Eight Thousand years ago, Asia had an estimated total of 15.132 million km2 of forests. Today, that figure is down to only 4.275 million km2, of which just 0.844 million is natural forest (i.e. not tree plantations). To assist in reafforestation, we can make use of the swale. Simply put, a swale is a waterharvesting ditch cut level into the ground on contour. Swales are not diversions ditches. Their job is not to carry water form one place to another. Water will continue to flow over land until it reaches the lowest point it can find. Creating swales is essentially creating low points for water to collect in. The swales catch rainwater running off the land, allowing it to soak into the land. Once in the ground, water is more protected from evaporation a very serious problem in deserts which reduces the problem of salinisation.

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Swales are the most cost effective type of earthworks, catching the most amount of water for the work put into their construction. Once constructed, swales greatly assist the growing of trees by capturing rainwater that would otherwise run off the land, eroding the soil in the process. Additionally, swales also help to recharge groundwater a water source that has become greatly taxed since the proliferation of motorised pumps following WWII.By reducing soil erosion and capturing water that would otherwise have been lost, swales contribute to the creation of health soils. The trees grown in conjunction with swales can further aid in soil stabilsation and can contribute vital nutrients for the soil from this detritus. Trees and associated groundcover plants can provide symbiotic hosts for mycorrhizal fungi, which, in turn, can boost plant growth, protect plants from acid rain, salt and pollutants (see also here), and greatly benefit the fertility of the soil.

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Construction of Swales for Arid Climates Arid climates are characterised by large rainfalls occurring in just a few events each year. As such, swale dimensions in these regions will be a little different than they would be in regions with more rainfall. One easy and inexpensive way to find a level contour is to construct an A-frame as shown in the diagram and hang a string with a weighted end down from the top of the A-frame. Find a level surface and mark on the cross piece where the string points to. This mark will show you where level is as the diagram below shows. You can use this A-frame level to map level areas along the ground. Along the level point, place stakes as markers. At the end, you will have marked out a contour line.

Mark out contour lines about 20 metres apart from one another. (Twenty metres is a rough guide, as the shape of the land will make the contour lines closer in some places and further apart in others). Dig the swales to be 2 to 3 metres across and about 50 cm deep. The excavated earth is used in construction of the swales mound on the downhill side. When digging the trench and when constructing the mound of the swale, do not compact the earth. This will assist in water infiltration. Also take care to make the top of the swale level. In this way, it will be less likely to erode should they overflow with rain. Below is a sample of an actual swale designed by designer Dan Palmer. These measurements can be flexible as long as these swales for drylands are about 2 to 3 metres across.

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