How to design a SSD?

After determining the water demand and estimating the water yield at the selected sand dam location, the design can be made. There are different approaches in designing a sand dam, but this manual will focus on the designing approach of SASOL, combined with AFD.

A sand dam can be defined in four main parts:
• the dam;
• the spillway;
• the wing walls;
• and the stilling basin.

Dam height

To determine the dam and spillway height at a specific location, it is very important that the water level and maximum flood level will remain below the riverbanks after construction of the dam. The dam and spill way height are therefore determined by the maximum discharge and maximum flood height.

If the flood level is higher than the riverbanks (Bh) without the sand storage dam, construction of a dam is not advisable in the first place. If this is not the case, it is important that the sand dam height is such that the maximum flood level will still remain lower than the riverbanks after construction of the sand dam (upper figure). If the sand dam is build too high, the maximum flood level will be higher than the height of the riverbanks after construction of the sand dam (lower figure). This will lead to severe erosion of riverbanks, eventually causing dam failure.

Examples of sand dam heights; do and don't

Calculating the maximum discharge can be done using the following equation.
Q = 1/n * A * R^2/3 * S^1/2
Q = maximum discharge in riverbed section (m3/s)
n = Manning roughness of riverbed
A = wetted cross-sectional area (m2) = 1/2*(channel width + riverbed width)* flood height
P = wetted perimetre (m)= B1 + riverbed width + B2
R = hydraulic radius (m)= A/P
S = slope of riverbed (m/m)

Cross section with maximum flood height to determine maximum discharge

Spillway dimensions

The spillway dimensions are dependant on the maximum river discharge as calculated above. The below formula can be used to calculate the spillway dimensions.

Q = c * Ls* H^3/2
Q = maximum discharge in riverbed section (m3/s)
Ls = length of spillway (m)
c = 1,9 (constant depending on spillway shape, here: broad crested weir)
H = height of spillway (m)

Cross section of a sand dam body and its dimensions

Gf = gross freeboard (m)
Lw = length wing wall (m)
Lwe = length wing wall extension (m)
Ls = length spillway (m)
Hf = height freeboard (m)
Hd = total height of dam (m)
Hs = total height of spillway (m)

Wing walls

When determining the distance the wing walls go into the banks, bank characteristics have to be taken into account (Munyao et al, 2004):
• loose riverbanks: approximately 7 metres into the riverbanks;
• hard soils: approximately 5 metres into the riverbanks;
• hard and impermeable soil: approximately 0 – 1 metre into riverbanks;
• rock formation: no need of constructing in riverbanks.

The length of the wing wall (Lw) should be approximately 2 metres into the riverbanks. The length of the wing wall extension (Lwe) should be approximately 5 metres. This is an example of wing wall dimensions in loose riverbanks. Currently a research is carried by SASOL and Acacia on the specific effects of sand dams on groundwater levels within a catchment, which will lead to more specific guidelines on wing wall dimensions.

Stilling basin dimensions

To calculate the dimensions of the stilling basin, use the below equation.
SL = c * L^1/3 * H2^1/2
SL = length of stilling basin (m)
c = 0.96 (constant)
H2 = height of freefall (m) = (height of water level upstream) – (height of water level downstream)

Cross sectional profile of a sand dam body and its dimensions
Hd = height of dam (m)
Fh = height of foundation (m)
Fl = length of foundation (m)
Lt = length of top of dam (m)
Lb = length of base of dam (m) = 1 m
Sl = length of stilling basin (m)
Sh = height of stilling basin (m) (max of 0.3 m)
Ah = height of anchor (m) = 0.3 m
Al = length of anchor (m)