How do you design a Sand Dam?

UDO has qualified water engineers, called Dam Coordinators, who work with the SHG to site the sand dam, and then draw up the dam design which is submitted with the other paperwork to the government for approval. Here is the basic methodology for the design:

Primary spillway

The primary spillway will center and discharge the normal flow into the normal river channel during the rainy season.

• At the chosen site, find the narrowest place with the best rock bottom. Place a stake here. Now find the center of the river bed at this point.
• Identify the center of the river bed to center the primary spillway directly above this point. Make sure that the dam can be secured to the best rock bed available and the primary spillway will be centered in the centre of the natural river bed.  The river must continue to flow where it flowed before or it will cause erosion. Place a stake here.
• Determine the height of the primary spillway from the historical normal height of the flow of water after rains. Use a line level and mason’s line and identify and mark two points, one on each bank with the same height. This height at the center will be the primary spillway height.
• Determine the length of the primary spillway along this line level and mason’s line. The length of the spillway is the length needed to guide water through this channel and keep it in the normal river channel during the normal course of its running during the rainy season.
• Mark the height and length of the primary spillway. Use a tape measure. Mark its location with pegs, string, and level.

Height:

The primary spillway crest is initially one meter or slightly higher. The chief factor is the height of the normal flow during the rainy season. Other factors are:  the size of the river, the amount of normal flow, the bank heights, and the amount of storage and sand deposition area desired. After the sand has filled the dam to this primary spillway level, the community often decides to raise the level of the dam by another 0.5 meters to accommodate more sand and water. The wings will need to be extended then also.

Length:

The length of the primary spillway depends on the width of the river banks, slope of the river banks, and the amount of water contained in the river. The length of the secondary spillway depends on these factors and the anticipated amount of water that will flow down the river after heavy rainfalls upstream.

Width:

The width of the dam wall at the top is always wide enough for a person to walk on the top, a minimum of one meter. The foundation width at the bottom depends on the river size and flow. The average sand dam is one and a half meters wide at the foundation level and tapers up to the top.  If the river is very big and the dam is high the foundation will be thicker.

Secondary spillway

The secondary spillway is built to keep water guided into the center of the regular river channel when there are heavy rains and heavy stream flow.  The primary spillway keeps the water in the center of the river when there is less rain. The secondary spillway is important to prevent soil erosion as it centers the water in the center of the river bed.

• Determine the height of the secondary spillway. This will be the flood level of the rivers in very heavy rains.
• Use a line level and mason’s line and identify and mark two points, one on each bank, at the flood level of the river in very heavy rains. This height will be the secondary spillway height.
• Determine the length of the secondary spillway. It is the length needed to control the flow of the flood water and keep it within the river channel during a very heavy rain.
• Mark the height and length of the secondary spillway.  Use a tape measure. Mark its location with pegs, string, and level.

Height:

The secondary spillway is usually 1 meter higher than the primary spillway.

Length:

The length extends to the wings, and depends on the width of the flow of the river at flood level.

Width:

The width is usually 1 meter wide.

Tertiary spillway

In the case of a very large and wide sand dam, you may need to lay out a tertiary spillway. The height will be the height of the highest flood in the memory of the members of the SHG and elders in the area. Follow the above method to lay out and level the tertiary spillway.

Wings

Wings are built to keep the flood waters from going around the sand dam and causing erosion and eventual undercutting of the dam walls. They may not be necessary depending on the size of the banks and the volume of flow.

• The wings must be constructed to guide the river water back to its natural course in case of flooding.
• Determine the path of flood waters on both sides of the banks of the river. Mark this path of flood water with stakes.
• Determine the length of the wings, making sure they are long enough and high enough to meet the height of the highest flood in memory of the elders. Use a tape measure and level.  Stake out the wings.

Height:

The wings of the dam go up 1 meter high or more above the secondary spill way to prevent erosion and contain the water in the river bed.  The height of the wings depends on the amount of flow of the river, the curve of the river and the current of the river.

Length:

The wings may extend in length from three meters up to 50 or more meters. If the river banks are very steep the wings may just extend a few meters.  If the banks are very flat, the wings can go many meters. The wings have to be long enough so that the water never diverts around the wings: sometimes we need to go back and extend the wings.

Width:

The base of the wings is thicker than the top of the wings, and tapers off towards the ends of the wings. The wings are made of the same materials as the dam wall.

Make a record of all measurements and keep them.  Check to see that you have all the required measurements for the design.
The Sand Dam Dimensions Design is drawn on site by the Dam Coordinator and used to do the Design Measurements, which is then used on site by the Artisan who constructs the spillways and wings according to these specifications.