Selecting riverbed section(s) and sand dam location(s)

Based on the riverbed survey, a detailed survey is carried out in the parts of the riverbed which seem most promising. All data is collected at a regular interval, for example 20 metres. The data gathered by the above described survey results in a map of the river section. This map shows information about the river length and width, locations of cross-sectional, longitudinal profiles, water-indicating trees and waterholes.

Example of a map of the river (Nissen-Petersen, 2006)

Also, based on the collected data, profiles (see below) will be made containing the depth of the impermeable layer, width of the riverbed, height of the riverbanks and depth of the water table. The data should be filled in a data sheet as shown below.

Example of a data sheet

Data to be collected and methods to do so are based on Nissen-Petersen (2006) and are described below.

Storage capacity and extraction percentage of sand

Water extraction is more profitable from riverbeds containing coarse sand than from those with fine textured sand. The porosity and extractable capacity of sand can be determined through the following method. A 20 liter container with a plug in the bottom is filled with sand from the riverbed. Then, the sand is slowly saturated with a measured volume of water. Then, the plug is removed from the bottom of the container. The volume of water which has drained out of the sand within one hour is taken as a measure for the extractability. Table 3 gives values of extractability of water in different soils. This shows that the extractability of sand is highest, explaining why coarse sand is preferred as the aquifer material.

Sand fractions, saturation and extraction rates (Nissen-Petersen, 2006)

Height of the riverbanks and width

The height of the riverbanks and the width of the riverbed should be measured using a tape measure. The height of both riverbanks will be drawn in the longitudinal profile (below), whereas the width of the riverbed should be noted in the river map (above).

Gradient of the riverbed

Measuring the gradient of the riverbed can be done by using a circular transparent hose, half-filled with water. One person should stand at the starting point, using the levelling tool. Another person should stand upstream of the person holding the levelling tool with a long pole which is held vertically. The person with the levelling tool has to make sure the water levels in the tube are in one line. He or she should indicate to the person holding the pole where this sight line crosses the pole. The height at which the line of sight crosses the pole should be measured from the surface of the riverbed (parameter y [m]). The distance between point No. 1 and point No. 2 should be measured (parameter x [m]). The height of the eyes of the person holding the levelling tool should be measured (parameter z [m]). Then, the gradient (parameter w [m]) can be calculated using the below formula:
W = ((z – y)/x)*100 = gradient [%]

Usage of levelling tool (after Nissen-Petersen, 2006)

Depth and type of basement and depth of groundwater

The depth of the basement or impermeable layer in the riverbed with respect to the riverbed surface is of importance to decide where the sand storage dam should be build. Also, the size of the aquifer should be as large as possible. The presence of water in the riverbed after a long dry period is an indication that there is an impermeable layer which indeed restrict groundwater flow towards the deeper groundwater.

The depth of the sand in the riverbed can be surveyed by using an iron probing rod. The rod should be hammered into the riverbed until it hits the floor under the sand with a dull sound. Mark the level of the sand on the rod and pull the rod straight up without twisting. If a long probing rod is used, the tripod ladder can be used.

The tools nessesary for this part of the survey:
• Measuring rods made of 16 mm (5/8”) iron rods with notches in it every 25 cm to collect sand samples when the rod is pulled up.
• A tripod ladder
• A mason hammer.

Example longitudinal profile and cross section

The data collected as described above should be put into a profile. The profile will contain the height of the riverbanks, the depth of the basement and the depth of the waterlevel in the riverbed. The profile will be important in deciding where the sand dam will be build.

In choosing the building location, the following key factors should be accounted for.
1. The sand storage dam should be build on a location where the basement is close to the surface (natural dyke). In the below profile, this is the case at probing points 10, 12, 15, 18, 23.
2. The basement upstream of a natural dyke is deep to get a large sand dam aquifer. In the below profile, this is the case at probing points 9, 11, 14, 17, 22.
3. Pay attention to areas where there is still groundwater in the riverbed after the dry period. This is an indication that there is an impermeable layer, and the water is prevented from flowing downstream by a natural dyke. In the below profile, this is the case at probing points 9, 14, 17, 22.
4. The riverbanks on both sides should be sufficiently high.

Example of a longitudinal profile (Nissen-Petersen, 2006)

Based on the above profile and the key factors, several locations are suitable to build the sand storage dam, eg. probing points 10, 18 and 23. At these locations cross profiles should be made, to see whether the natural dyke exists in the whole width of the riverbed. Hereto, probings are done at a 2 metre interval, again note the depth of the impermeable layer, the type of impermeable layer and the depth of the watertable in a data sheet. Also, define the maximum floodlevel based on information from the community. This is of importance because the sand storage dam must be higher to prevent erosion. A cross sectional profile such as the one below is made from all this information.

Example of a cross sectional profile over shallow basement (natural dyke) (Nissen Peterson, 2006)

The profile shows that the basement is continuously shalow (170 centimeter at its deepest point at probing no. 5). This confirms the suitability of the location to build a sand storage dam.

Also, just upstream from the natural dyke, a profile should be made.

Example of a cross sectional profile over deep basement (aquifer) (Nissen-Petersen, 2006)

This profile shows the thickness of the aquifer upstream of the natural dyke, and thus provides another argument to build the sand storage dam at the location of the proposed natural dyke.

The last stage is digging trial pits every 3 meters at the location of the cross sectional profile to verify that the probing information is correct. This also enables a check of the material and quality of the impermeable layer.