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# Difference between revisions of "Swale Design for Watershed Management"

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## Abstract

According to United Nations Environment Programme (UNEP), the volume of freshwater resources is about 2.5 per cent of the total volume of water on earth. Of this freshwater, only 30 per cent is stored in the form of groundwater which accounts for 97 per cent of all freshwater potentially available for human consumption.[1] However, much of the groundwater we rely on is slowly being depleted from underground aquifers through pumping. The overpumping of groundwater is causing water tables to fall across large areas of northern China, India, Pakistan, Iran, the Middle East, Mexico, and the western United States. Scientists have recently estimated in India that groundwater is being depleted across the country's north at a rate of 54 billion cubic meters per year. This is putting the nation's food supply and the region's 114 million people—are increasingly at risk. [2]

As a result of falling water tables, many rural communities around the world are facing problems such as increased pumping costs, drying up of wells, deterioration of water quality from salt water intrusion and reduction of water in streams and lakes. This is a result of a greater amount of water being withdrawn than deposited back into the aquifer. Groundwater recharge is what replenishes underground aquifers and can be increased through infiltration. Infiltration is the process of which of water on the ground surface enters the soil and flows into the unsaturated zone. The following information provides background information and design options for a small-scale infiltration system for the purpose of groundwater recharge. This system consists of constructing swales and berms to trap precipitation run-off and promote infiltration. A step-by-step design is provided as well as external links for additional information.

## Background

Information is provided for general hydrogeology considerations to aid in understanding the purpose of watershed management. The structures involved in swale design for watershed management are then described.

### Hydrology

Drainage Area with Divide

Watershed Delineation
A watershed an area of land where surface water from precipitation drains into a body of water. This area also contains drainage areas in which water from a smaller portion of land drains through a downstream point. A drainage area is bounded by a divide, a line made up of a group of high points around the drainage area. To deliniate a drainage area, start with a contour map and draw a divide. The divide line runs through the highest points and is perpendicular to the contour lines on the map. An example is shown in the image on the right. At a smaller scale, delineation can be used to determine the drainage area for an infiltration trench or swale.

Infiltration
InfiltrationDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. is the process of water seeping through pore spaces in the subsurface soil due to gravity and capillary forces. During a storm, infiltration largely determines how much surface runoff is generated.

Surface Runoff
Surface RunoffDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. is the remaining portion of precipitation that becomes overland flow and accumulates in local streams and rivers. This excess water flow travels in a down-gradient direction and drains to a common point within a watershed. Runoff occurs when the soil had reached its maximum infiltration capacity.

Required Infiltration Area Using Surface Runoff

$R = i - f \,$

R = runoff (m/hr)
i = rainfall intensity (m/hr)
f = infiltration rate of drainage area (m/hr) See Table 1

$D_{runoff} = R \times 24 hours\,$

D = depth of runoff (m)

$V = D_{runoff} \times A\,$

V = volume of runoff (m3)
A = drainage area (m2)

$A_r= V \div (f \times 24 hrs)\,$

Ar = required infiltration area (m2)
f = infiltration rate of swale material (m/hr)

The required infiltration area can be used to determine the dimensions of the swale:

$A_r = W \times L\,$

W = width (m)
L = length of base (m)

The Water Budget
The basic components of the hydrological cycle include precipitation, evaporation, evapotranspiration, infiltration, overland flow, streamflow and groundwater flow. For a large watershed, a conceptual mathematical model of the overall hydrological budget is shown below. Using this model, the factors affecting groundwater recharge are identified.[3]

\mathbf{P}-R-G-E-T=S

P = precipitation
R = surface runoff
G = groundwater flow
E = evaporation
T = transpiration
S = change in storage in a specified period of time

### Hydrogeology

Groundwater Flow

The groundwater flow is represented by Darcy's Law. These equations determine the speed at which the groundwater is flowing in the subsurface. This can help determine the rate at which groundwater recharge is occurring.

$q=\frac{-\kappa}{\mu}\nabla h$

Where $q$ is the filtration velocity or Darcy flux (discharge per unit area, with units of length per time, m/s) and $\nabla h$ is the gradient vector.

$v=\frac{q}{\phi}$

The groundwater velocity ($v$) is related to the Darcy flux ($q$) by the porosity ($\phi$).

Soil Permeability
Permeability is a measure of how easily a fluid, can pass through a porous medium such as soil. It is also governed by soil porosity which is the pore space between soil particles. The pore sizes and their connectivity determines whether a soil has high or low permeability. High porosity soils such as gravel and sand have high permeabilities which allow for greater amounts of infiltration. Low porosity soils such as clay have very low infiltration rates.

The table below shows a range of permeability for various types of soils.

Table 1: Soil Permeability

 Permeability Pervious Semi-Pervious Impervious Unconsolidated Sand & Gravel Well Sorted Gravel Well Sorted Sand or Sand & Gravel Very Fine Sand, Silt, Loess, Loam Unconsolidated Clay & Organic Peat Layered Clay Unweathered Clay Consolidated Rocks Highly Fractured Rocks Oil Reservoir Rocks Fresh Sandstone Fresh Limestone, Dolomite Fresh Granite

### Geotechnical Structures

The combination of following structures can be used to increase infiltration of precipitation for the purpose of groundwater recharge. Further information can be found in Technical Specifications and Installation.

Swales

Swales are a type of structure that promote groundwater recharge. Similar to a trench, swales are dug into the ground in order to retain run-off water. The water then spreads and slows down allowing it to infiltrate into the subsurface. This creates a groundwater plume that replenishes underground aquifers and supports the growth of vegetation in areas down-gradient to the swale.

Berms

A berm is an earth barrier separating two areas of land that can be built to serve a variety of purposes. Berms are often used to control erosion and sedimentation by reducing the rate of surface runoff. As a result, berms either reduce the velocity of the water or direct water to areas that are not susceptible to erosion. This helps reduce adverse effects of running water on exposed topsoil. Berms are typically small mounds composed of soil, gravel or rocks and if well maintained can last for many years.

## Regional Considerations

Groundwater recharge can significantly increase water supply levels in underground aquifers, however not all regions in the world use groundwater. Communities that are reliant on groundwater and face yearly decreases in water table levels will experience the most change from groundwater recharge. Benefits communities have seen in the past are listed beneath regions that can benefit.

### Regions/Areas That Can Benefit

• Where geology favors the storing of water in underground aquifers and allows for it to be extracted.
• Riverine systems with high river flows during the monsoon season and experience a dry season.
• Where it is possible to conserve floodwater and flows are not used for crops and run out to sea.
• Locations where it is too expensive to build dams, to transport water over long distances or land cannot be allocated for storage.
• Areas with no soil salinity problems.
• Places where the ground slope varies gradually and where it is not subject to flooding and/or waterlogging.
• Ideal in places where accurate information on hydrogeology and groundwater movement are available.[4]

Most monsoon rains flood fields, filling rivers and streams with water that rushes out—unused—to the sea. This is the very water that could help the farmer year-round.[4]

### Community Benefits of Groundwater Recharge

• Increases in net income per ha for farmers.
• Pumping cost savings from decreases in depth to water table.
• Families can create fertile land from previously dry areas.
• Increases in cropped area and irrigation potential.
• Reduction in energy use.
• Communities gain independence with a reliable water supply.
• Improved water quality from subsurface filtration.

## Design Considerations

### Slope

A slope of about 2.5 cm for every meter in length is enough to move water.[5]

### Soil Type

Three common types of soil include clay, sandy soil and loam. The following are described below.[6]

Clay

• When compacted and dry, clay is hard and forms large chunks. If wet, it is sticky and can be molded into shape.
• Clay and silt hold moisture well, but slow the downward movement of water and can stop water infiltration completely. Clay soils tend to shed water when dry and to provide little ventilation to plant roots when moist. Dry clay soils will make plant root penetration difficult, while wet clay soils can suffocate plants.
• If soil on your property is heavily clay-based, you will probably need to add sand and loam to increase the infiltration rate if you want to install an infiltration system.

Sandy Soils

• Sandy soils contain large particles which are visible, and are usually light in color.
• Sand feels coarse when wet or dry, and will not form a ball when squeezed in your fist.
• Sandy soils allow for rapid water movement because the particles are large and loosely packed. Sandy soils stay loose and allow moisture to penetrate easily, but do not retain it for long term use. The primary concern with sandy soils is their inability to hold water and nutrients, causing them to dry out quickly and for plants to grow weakly.
• If soil on your property is too sandy, you'll have to add clay and loam to slow water flow enough to allow the filtering of pollutants in the water.

Loam

• Loam soil is a mix of sand, silt or clay, and organic matter.
• These soils are loose and look rich. If squeezed, moist loam will form a ball which crumbles when moved.
• Loam is the best soil for healthy vegetation because they absorb water and store moisture well.
• If soil on your property is very loamy, you may have the ideal soil type for rainwater infiltration.

The following table shows percolationDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. rates for different soil types.It is recommended that loam with a percolation rate of 15 mm/h is the minimum used for an infiltration swale. The water will not be able to infiltrate at a fast enough rate to increase groundwater recharge if the percolation rate is too low.

Table 1: Minimum Soil Percolation Rates[7]

Soil Type Percolation Rate
(mm/h)
sand 210
loamy sand 60
sandy loam 25
loam 15

## Required Materials and Tools

Swales and berms are relatively easy to construct and do not require a large amount of materials or tools. To dig the swale, a shovel or any other type of digging tool can be used to remove soil. A measuring tool might also be useful for determining the depth and side slopes of the swale and berm.

For material used to form the berm, low permeability soils such as clay and rocks are recommended. Once dug, the swales can be filled in with bulk materials such as leaves, rotten wood or straw. High permeability material such as gravel can also be used. Usually materials found in the surrounding region are used in the construction of these structures.

## Technical Specifications

The following images are cross-sectional models of the structures in the inflitration system.

## Infiltration on Inclined Areas

This image is a conceptual model for combining a swale and a berm for the creation of an infiltration system. This is a cross section of the swale and it is shown located on a slope, with the run-off flow in the direction of the structure.

Test

## Installation

Once you have done a site evaluation and designed and sized your swale, you are ready to begin digging.

1. With your shovel dig out the shape of the swale to your design. Be careful not to compact the soil while digging, as this reduces the soil’s ability to infiltrate stormwater.

Scarification, or tilling of the soil to a depth of approximately 300 mm, will enhance infiltration; thereby helping to overcome the soil compaction that normally occurs during construction.

1. If sub soil isn’t adequate: Dig an additional depth in the base of 0.5 – 1m for the sub soil bed. Create a loamy sand mix and re-fill the hole.
2. Dig the swale at a slight slope downhill to move the water away from sensitive areas. A slope of about 2.5 cm for every meter in length is enough to move water.
3. Vegetate the soil with seedlings, seeds or sod. Check the Evergreen Native Plant Database for a list of native grasses suitable to your water and light conditions. Choose hardy plants that are drought and flood resistant.

5. At the inflow end of the swale, where water will be entering (e.g. from a downspout), consider using a splash pad and/or piling up stones or gravel to reduce erosion from high speed water flows, especially in the first few years.