Presenter: Frederick Yaw Logah (RS) CSIR-Water Research Institute, Accra-Ghana 15th July, 2014

Funded by CSIR Water Research Institute Ofori D., F. Y. Logah, K. Kankam-Yeboah and B. A. Amisigo & F. Oblim, C. Asante-Sasu & G. Appiah CSIR-WRI Accra-Ghana 15th July, 2014

Introduction Objective Methodology/Activities Results and Discussion Conclusion/Recommendation

The Weija reservoir supply water to parts:  Accra metropolitan area  Central Region Anthropogenic activities  disposal of waste,  brick/tiles factory,  sand winning,  encroachment,  harvesting of fuel wood,  farming and  fishing  POLLUTION OF WATER  Resulting in high cost of treatment to GWCL.

Location: Lat. 5° 32.5’ N – 5°37’ N & long. 0° 20’ W - 0° 25’ W Surface area of reservoir: 30 km² Volume at MWL (16 m) : 133 M m³ Volume at NWL (15 m): 115 M m³ Mean annual precipitation: 800-1,150 mm Mean monthly Tempt: 23°C in August to 32°C in April (WRC, 2007). RH ranges between 50 % – 60 % in dry months 80 %-90 % in the wet season Mean Inflow into Dam: 54.2 m³/s Water Abstraction 30% of the mean annual inflow to the reservoir (Kumah and Ashley, 2008).

Sedimentation is an important environmental concern, affecting the useful life of a reservoir  Reduction in the carrying capacity of drains and reservoir  Flooding  reduction in water supply  Reduces water quality. This study therefore seeks to assess the quantity of sediments in the reservoir and trends of sediment entering the reservoir via the Densu River at Ashaladja

The specific objectives are to:  Estimate Annual Sediment Entering the Weija Reservoir from the Densu River at Ashalaja  To Develop a Suspended Sediment Rating Curve for the Densu River Basin at Ashalaja  Estimate the Total Volume of Sediment in the Weija Reservoir

Depth Int wading sampler (USDH-48) Sediment analysis using the Oven Dry Method (Tilrem, 1979) at the sediment laboratory of the CSIR-WRI Measurement: Suspended Sediment Location : Ashalaja Frequency: Weekly basis Period : Sept Aug Sampling points: 3 points along X-section Measurement: Suspended Sediment Location : Weija Reservoir Date: 25 th Jan, Sampling points: Dam, middle & delta Weija Reservoir

Flow velocity (V) measurement were taken at 1 m intervals along the cross-section by using either  six-tenths-depth methods or  2-point measurement Current meter Measurement: Flow velocity Location : Ashalaja Frequency: Weekly basis Period : Sept Aug Discharge computation: Q = VA

 The developed suspended sediment rating curve follows the power function. where, Q s is suspended sediment discharge in tonne/day, Q w is the river discharge (m 3 /s), a is an index of erosion severity and b is an exponent equal to the slope of the curve.

Results and Discussions

Suspended Sediment Discharge Suspended Sediment Rating Curve for the Weija Reservoir a = 2.94 and b = 1.06

 Relatively low mean specific suspended sediment yield from this study compared to other basins in Ghana (Akrasi, 2005 & Akrasi, 2008). River Basins Mean Specific Suspended Sediment Yield (t/km 2 /yr) Densu * 2.04 Pra 8.28 Ankobra 4.39 Tano 0.19 Ochi-Amisa 3.71 Ochi-Nakwa4.25

 Assessment of sediment transport into the Weija Reservoir was undertaken to understand the implications for water supply and the multi-purpose uses of water from the reservoir.  Annual sediment discharge into the Weija reservoir is more than 2.6 tonne/km 2 /year and the mean suspended sediment concentration in the water column in the reservoir  Suspended sediment rating curve was developed for the Densu River Basin at Ashalaja. The can be used to predict sediment discharge into the Weija Reservoir  Estimated volume of sediment deposited in the reservoir ranged from 0.1% and 0.7% of the Reservoir Volume of 115 Mm³.

Thank You