Assessment of Siltation Impact and Mitigation Strategies for Sustaining Storage Capacity in Lwanyo Dam, Tanzania

Abstract

The ongoing generation, transportation, and deposition of silt in the 
Lwanyo Dam has significantly reduced the storage capacity of the 
Lwanyo Reservoir, originally constructed to support irrigation and the 
surrounding ecosystem. The objective of this paper was to assess the 
extent of siltation in Lwanyo Dam, evaluate its impact on the dam's 
storage capacity, and propose measures to mitigate silt accumulation. 
The upstream catchment area, approximately 39.6 km², includes 
around 128,991 m² allocated for rain-fed crop cultivation and 5.89 
km² for pastoral activities. Frequent overtopping of the reservoir has 
been observed, largely due to siltation reducing its live storage 
capacity. In the reservoir trial pits were excavated and assessed, and 
they indicate that average silt layers range in thickness from 0.54 m 
to 0.98 m per rainy season. The deposited material consists of a silt 
layer from 0 to 540 mm, followed by an intermediate clay layer from 
540 mm to 3100 mm. The impounded silt depth was measured at 
1270 mm, with an estimated siltation volume of 58,349.4644 m³. The 
reservoir's original storage capacity of 210,153 m³ has been reduced 
by 27.765% due to siltation. The reservoir’s structural design 
inadequately addresses silt management, lacking both silt flushing 
tunnels and upstream silt check dams. The analysis indicates that 
storage capacity decreases by 3.085% annually, and if this linear 
trend continues without any intervention measures, the dam will lose 
all storage capacity within 24 years. The study recommends urgent 
measures to mitigate silt accumulation.