By Steve Birkinshaw
We are using the Shetran physically-based distributed modelling system to simulate river flow, groundwater levels and soil moisture in the arid community of Folovhodwe, South Africa for current baseline and future scenarios. However a key aspect of the hydrological water balance for this catchment is two reservoirs upstream of Folovhodwe. These are the Luphephe (capacity 14,800,000 m3) and Nwanedi reservoir (capacity 5,310,000 m3). Water is released from these reservoirs but this is also occasional overtopping of the reservoirs. Just downstream of the two reservoirs is the Nwanedi and Luphephe rivers join together to join the Nwanedi river which flows past the community of Folovhodwe.
As part of this work the ability to model reservoirs has been added to the Shetran. Previously lakes could be added by blocking a river channel and allowing the water to gradually fill in a lake, which would eventually overtop.
The Shetran reservoir model can be seen below. The river channels at both dams have been blocked and the ground surface elevations increased to the dam height, which allows for overtopping when the dam exceeds capacity.
In this version water can now also be released from both reservoirs to the corresponding downstream river. There is a constant release if the water is above a specified depth but a reduced release as the water depth decreases with the release proportional to the water depth. Thus for the Luphephe dam the releases (which are abstracted at the deepest section of the reservoir) are shown below. Similar releases are prescribed for the Nwanedi reservoir except the maximum release is 0.2 m3/s (as opposed to 0.4 m3/s for the Luphephe reservoir).
This gives the follows water depths when the dams are full (with a maximum depth of about 22m):
The time series of water depth for the deepest bit of the Luphephe reservoir can be seen below:
This gives the following releases from the two dams:
In eight winter seasons there is overtopping of the reservoir with flows of up to 100 m3/s. Together with the normal reservoir releases this gives the total discharge in the Nwanedi river downstream of the reservoir:
Don’t hesitate to get in touch if you have any questions or to tell us about how you are using this model.