Weija Reservoir Sediment Studies

Presenter:Frederick Yaw Logah (RS)

CSIR-Water Research Institute, Accra-Ghana15th July, 2014

Funded by

CSIR Water Research Institute

Project Team

Ofori D., F. Y. Logah, K. Kankam-Yeboah and B. A. Amisigo

&

F. Oblim, C. Asante-Sasu & G. Appiah

CSIR-WRIAccra-Ghana 15th July, 2014

Introduction

Objective

Methodology/Activities

Results and Discussion

Conclusion/Recommendation

Outline

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.

Introduction

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 m³Volume at NWL (15 m): 115 M m³Mean 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 months80 %-90 % in the wet seasonMean Inflow into Dam: 54.2 m³/sWater Abstraction 30% of the mean annual inflow to the reservoir (Kumah and Ashley, 2008) .

The Weija Dam

Sedimentation is an important environmental concern, affecting the useful life of a reservoir

Reduction in the carrying capacity of drains and reservoir

Floodingreduction 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

Introduction Con’t

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

Obectives

Material and Method

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 SedimentLocation : AshalajaFrequency: Weekly basisPeriod : Sept 2012 - Aug 2013. Sampling points: 3 points along X-section

Measurement: Suspended SedimentLocation : Weija ReservoirDate: 25th Jan, 2013. 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

Discharge computation

Current meter

Measurement: Flow velocity

Location : Ashalaja

Frequency: Weekly basis

Period : Sept 2012 - Aug 2013.

Discharge computation: Q = VA

On the Weija Dam

The developed suspended sediment rating curve follows the power function.

Developing Rating Curve for Suspended Sediment

where, Qs is suspended sediment discharge in tonne/day, Qw is the river discharge (m3/s), a is an index of erosion severity and b is an exponent equal to the slope of the curve .

Mean monthly inflow of SS into the reservoir at Ashaladja on the Densu River

Suspended Sediment (SS) in the Weija Reservoir Section Range (mg/l) Mean (mg/l)Delta 11.3 - 40.7 24.6Middle 08.0 - 28.1 16.1Dam site 12.6 - 19.8 15.0

Results and Discussions

0

25

50

75

100

Oct/12

Nov/12

Dec/1

2

Jan/13

Feb/1

3

Mar/13

Apr/13

May/1

3

Jun/13Jul/1

3

Aug/1

3

Time

Susp

ende

d Se

dim

ent

(mg/

l)

0

5

10

15

20

Dis

char

ge (m

³/s)

Suspended SedimentDischarge

Results and Discussions

Suspended Sediment Discharge

Suspended Sediment Rating Curve for the Weija Reservoir

y = 2.94x1.06

R2 = 0.84

0

1

10

100

0.01 0.1 1 10 100

Water Discharge (m³/s)

SS D

isch

arge

(t/d

ay)

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).

Mean Specific Suspended Sediment Yield

River Basins Mean Specific Suspended Sediment Yield (t/km2/yr)

Densu * 2.04Pra 8.28Ankobra 4.39Tano 0.19Ochi-Amisa 3.71Ochi-Nakwa 4.25

Estimated of Volume of Sediment in the Weija Reservoir

ASHALAJA ENTIRE BASIN Mean Maximum Mean Maximum SSD (tonnes/yr) 5,300.0 27,184.0 6,890.0 35,339.2 SSD (tonnes/km²/yr) 2.7 13.6 2.7 13.6 Specific density of Sediment (tonne/m³) 1.7 1.7 1.7 1.7 Annual Volume of Sediment Concn in Weija Reservoir (m³/yr) 3,081.4 15,804.7 4,005.8 20,546.0 Volume of sediment during operation period (38 yrs) (m³) 117,093.0 600,576.7 152,220.9 780,749.8 Volume of Reservoir (m³) 115,000,000.0 115,000,000.0% Volume of sediment in the reservoir 0.10% 0.52% 0.13% 0.68%

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/km2/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.

Conclusions

Thank You