Fisheries and Hydropower

Mt Coffee hydropower dam in Liberia is a recently refurbished dam that is the only means of producing electricity in Liberia aside from diesel generation.

Until recently the dam lay in a state of disrepair due to a breach that occurred during the early 90’s. Fishtek consulting were contracted to undertake a fish passage assessment and appraisal for the dam, working with MRAG ltd. who undertook baseline fish population surveys.

It was found that the River Saint Paul supports a very diverse fish population with somewhere in the region of 100 species caught within the vicinity of the site. The options appraisal concluded that a 2 km long naturalised bypass channel with two entrances (one at the spillway and one at the powerhouse) was the preferred upstream fish passage solution at the site to cater for the high species diversity and abundance. Downstream migration solutions included fish bywash and behavioural guidance systems.

An interesting find from the project was the potential impact of the dam on the life cycle of African river prawn Macrobrachium vollenhoveni, which migrate to estuaries to spawn (a process known as catadromy). It has been found at similar sites in the region that where this life cycle has been disrupted, there has been a large decline in the prawn population. This not only impacts on the local economy and protein availability (with the livelihoods of communities surrounding dam sites often dependent on fishing), but also risks human health as the prawns are known to feed on species of freshwater snail that are vectors of the parasitic infection bilharzia. With a reduction in prawn populations snail numbers increase as does the incidence of bilharzia in local communities.

The client was unable to deliver the fish passage mitigation options due to budget constraints but wished to explore options for the delivery of specific prawn pass at the site. Due to a lack of existing data on prawn passage requirements, Fishtek mobilised to site with a bespoke test facility to establish design criteria for a prawn pass. Working with local fishers, prawn was introduced to the test facility as part of a fully factored experiment. The ability for prawn to ascend the facility under different combinations of slope, flow and substrate type was measured. A trials report was produced along with a concept design of a prawn pass based on the flow/slope/substrate combination with the highest passage efficiency. This led to the completion of a detailed design, which the client used to put the construction of the prawn pass out to tender.