White Volta Water Retention

Abstract

Human interventions in the White Volta River in Ghana increase the flood risk in living spaces of the local communities, causing great challenges in the area. Burkina Faso has built a hydropower dam near the border with Ghana which has a large impact on the discharge extremes of the White Volta River. As a result of poor water management of this dam, water is spilled during the wet season, causing more floods in northern Ghana. Several communities, living near the White Volta River, depend on the river water for their drinking water demand and use the sand in the river bed as construction material. This puts them in a vulnerable position when changes in the natural flow regime of the river occur.

This thesis studies the impact of constructing multiple small dams in the White Volta river in order to decrease the flood risk in villages located close to the river and create an opportunity for controlled sand mining as a result of sedimentation.

Tamale is, with 672.000 inhabitants, the largest city in this area and struggles with the consequences of the floods as well. The drinking water company has trouble meeting the drinking water demand during both the wet and the dry season. When the area is flooded, the water intake point shuts down, and during low water levels, the pumps can get clogged. This clogging occurs even more often as a result of increased illegal sand mining from the river banks by the local communities. This has broadened the river, resulting in lower water levels and a higher turbidity.

A possible solution to ensure the water intake in Tamale and reduce the flood risk in northern Ghana is the construction of multiple small dams in the river bed to flatten the discharge peaks and slow down the water in the White Volta River. At the same time, these dams can create an additional advantage by causing upstream sedimentation that provides a possibility for controlled sand mining in the river bed. To research the effectiveness of this solution, the impact of the dams on flood risk and sedimentation, near seven villages in northern Ghana, was modelled.

A base case of the flood of 2003 was compared to a scenario in which seven dams were implemented at locations close to villages that are often exposed to floods. This was done by building a hydromorphological model of the White Volta River, using the D-HYDRO 1D2D software.
The results of the morphological simulation show that in total, the sedimentation upstream of the dams can fill 81 trucks with sand per day. However, extracting the sediment at the most upstream dams decreases the possibilities for sedimentation near the downstream dams. The results of the 1D2D hydraulic model show that the proposed dams are not able to reduce the flood extent in villages and can even increase inundation depths, resulting in more flood damage.

It is recommended to increase the 2D grid of the hydraulic model in order to receive more realistic backwater curves.