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

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Presentation on theme: "Presenter: Frederick Yaw Logah (RS) CSIR-Water Research Institute, Accra-Ghana 15th July, 2014."— Presentation transcript:

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

2 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

3 Introduction Objective Methodology/Activities Results and Discussion Conclusion/Recommendation

4 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.

5 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).

6 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

7 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

8 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

9 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

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11  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.

12 Results and Discussions

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

14  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

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16  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.

17 Thank You


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