The size and shape of converted borrow pits are important. In the case of new borrow pits, the size and shape are largely determined by the availability of material for excavation and by the way excavators and dump trucks operate. Borrow pits often have irregular shapes. It is important that the original pits be modified and properly landscaped as part of the transition to their new function in water storage. This is done preferably by deploying the earthmoving equipment of the operator/contractor that formerly operated the borrow pits.
The following conditions apply with respect to the shape of converted borrow pit:
- Modify or remove potentially dangerous heaps and sides.
- Ensure stable slopes (see Box 7.1).
- Consider the overall shape of the converted borrow pit to optimize storage. In the case of unlined borrow pits turned into storage ponds, a convex shape is preferable. Convex pits are ideal for water storage, because they allow for maximum storage commensurate to the efforts of excavation. They are also inherently more stable than ponds with odd and square angles. Lined ponds are preferably trapezoidal in shape. For borrow pits lined with geotextile, a trapezoidal shape is preferred; the geotextile can be more easily installed. If a large number of borrow pits are to be lined with geotextile in a certain area, it is preferable (when possible) to use standard dimensions for the converted borrow pits: this will allow the lining to be stitched together centrally, generating cost savings.
- Depth: Where borrow pits are not constantly recharged from shallow groundwater seepage but depend on surface runoff, depth is an important consideration. This is particularly relevant during hot, dry periods when evaporation from borrow pits is high. The deeper the borrow pit, the less water lost to evaporation. A depth of 7 m or more is preferred. However, deep borrow pits are also likely to cross through several layers of rock/soil with different hydraulic properties. If water retention is the primary goal, the borrow pit should not extend below the impermeable layer.
- Access: Different water users will use the borrow pit, so the shape should allow access through an access ramp. Access may need to be provided to people or trucks. According to current guidelines in Mozambique, borrow pits are to be provided with single-access roads to allow for better control. Truck ramps can be easily modified to provide safe access for livestock and people. If possible, it is advisable to stone pitch the ramp to enhance its stability against cattle trampling.
- Contamination reduction: Special levees can be installed with small pumps to lift water out of the borrow pit. Although not ideal, special water troughs for livestock or sub-ponds, fences, or trenches can be installed to prevent livestock from coming into direct contact with the water, reducing the risk of contamination.
- Spillway facilities: Room for a spillway, especially in sloping terrain, is recommended. As borrow pits collect water, they may also spill over once they are filled, such as after a heavy rainstorm. It is important to consider a spillway that can release excess water to a natural drain in the layout of the pit. In some cases, the borrow pit is large enough in comparison to its catchment area and no spillway is required as it is unlikely to fill.
- Inlet reinforcement: Inflowing water often causes erosion and structural damage at the inlet–reservoir interface. Water dropping from the inlet into the storage structure can easily carve out borrow-pit walls and quickly trigger gully formation upstream of the water drop. Stone pitching, steps and masonry can be used to reinforce the inlet.
Box 7.1. Slope of the converted borrow pit
The preferred slope of the converted borrow pit in combination with a pond depends, among other things, on the type of soil. In the ANE Environmental Guidelines, a minimum slope of 1:4 is prescribed. The material of the borrow pits may necessitate much gentler slopes. As a rule of thumb, the following values can be used:
- Clay loam 1:5 – 2:1
- Sandy loam 2:1 – 2.5:1
- Sand 3:1
The average slope of the pond can be calculated as follows:
Y = The average slope %
C = The total contour length cm
I = The contour interval cm
A = Drainage area cm2
Preferably, the slope is gentle and regular.
The following considerations should be followed with respect to the size of the borrow pit:
- The size of the borrow pit and its use for domestic purposes, irrigation, or livestock are closely related. Borrow pits often serve as a source of water where there is no alternative. Therefore, they should have sufficient capacity to provide water over a large part of the dry period. In some areas the pit may fill several times per year, depending on the rainfall pattern; in other areas the borrow pits are only filled once or twice per year. In the latter case, the borrow pits will be large.
- While converting abandoned borrow pits into water-storage ponds, it is important to make a good assessment of the runoff in the area and the required usage. Expected runoff can be calculated with the simplified rational method (see Box 7.2).
- Where the borrow pit primarily serves to recharge groundwater, it should be large enough to accommodate a large part of the runoff (with scope for some water to escape through spillways or other overflow structures into the feeder canal).
Box 7.2. Calculating the size of the converted borrow pits: supply and demand
To calculate the preferred capacity of a converted borrow pit, two factors should be kept in mind: the water supply and the water demand.
To determine the supply, it is necessary to:
- Calculate the average rainfall over at least the last 20 years. Rank the cumulative seasonal rainfall (rainy season) in descending order.
- Calculate the probability of each event using the equation:
with P being the probability of occurrence, m the rank, and N the number of observations. Plot the probability against the amount of rainfall for each season.
- From the obtained curve, determine the rainfall with a 67 percent probability of occurrence (probability of occurring twice every three years).
- Multiply the catchment area’s obtained rainfall by the runoff rate (0.10 for permeable soils, 0.9 for paved roads). In this way it is possible to roughly estimate the amount of runoff reaching the borrow pit.
This is a rough estimation of expected water inflow. The borrow-pit design must also take into consideration the water demand in the area, expected losses (seepage and evaporation), and the need to dispose of excess water through a properly designed spillway.
V = I + E + S + H + L
V = Volume of water to meet local needs m3
I = Water volume to meet irrigation needs m3
E = Water losses due to evaporation m3
S = Water losses due to seepage m3
H = Domestic water demand m3
L = Livestock demand m3
Ideally, the volume of inflowing water must exceed V in order to provide a year-round water supply.
When diverting water into a decommissioned borrow pit, the volume of inflowing water may be larger than the volume of the borrow pit. In this case, the spillway is necessary and should be planned ahead. The borrow pit can also be enlarged to store more water.