Determining the effects of a hydropower dam and climate change on salt intrusion in the Gambia estuary up to 2100

Abstract

Due to climate change, temperatures are rising worldwide resulting in sea level rise, higher evaporation and possible changes in precipitation patterns. These effects of climate change influence salt intrusion, particularly by affecting the freshwater supply. In The Gambia also a planned hydropower dam and possibly future irrigation development will affect the freshwater supply. The combined effects of climate change, the hydropower dam and irrigation development on salt intrusion are unknown. A river south of The Gambia, the Casamance, turned strongly hypersaline during the Sahelian droughts, which strongly affected the rivers discharge in the early 80s. The freshwater supply in the Casamance after these droughts was insufficient to flush away the salt and the estuary never restored. This led to the decay of the entire ecosystem that existed in and around the Casamance. The Gambia river is of major importance to the country, the agricultural sector relies on the freshwater it supplies for growing crops, the fishing industry relies on sufficient numbers of fish to catch and the forestry industry makes use of the mangroves growing around the river. All these industries combined account for ±75% of the working force in The Gambia. If the Gambia estuary turns strongly hypersaline such as the Casamance, this would have disastrous results. This thesis aims to clarify the effect that climate change, the hydropower dam and irrigation development have on salt intrusion in the Gambia river. A 1D model, SALNST, is used to model salt concentrations over the river length up to the year 2100. The effects of climate change, the hydropower dam and irrigation development are represented by four model parameters: climate projections, sea level rise, dam operations and irrigation development. Multiple parameter values, ranging from mild to strong conditions or impact are implemented, creating a set of 27 scenarios. These scenarios treat the parameters independently to distinguish the individual parameters effect on salt intrusion. It can be concluded that The Gambia is prone to climate change when considering salt intrusion. Due to the importance of the river, monitoring sea level rise and the development of climate change will be essential for planning and taking counter measures. Understanding the effects that climate change can have regarding salt intrusion can also be used in negotiations to reserve some freedom in the hydropower dam operations. In addition, solutions that limit the increase in evaporative surface for strong sea level rise (e.g., dikes along a part of the Gambia river) prevent extreme salt intrusion lengths and hyper salinity. The hydropower dam causes a reduced range of salt intrusion lengths, this creates easier to predict salt concentrations which is beneficial for aquaculture. Furthermore, the reduced maximum salt intrusion is also beneficial for agriculture that makes use of irrigation.