DESIGN OF A GRAVITY DAM A case study of River Nyamamithi in Naivasha constituency

F21/2500/2009MWAURA WILSON NJENGA

Overview

Background information Surface runoff is the water flow that

occurs when the soil is infiltrated to full capacity and excess water from rain, melt water, or other sources flows over the land.

Infiltration excess overland flow. This occurs when the rate of rainfall on a surface exceeds the rate at which water can infiltrate the ground, and any depression storage has already been filled.

cont

Saturation excess overland flow. When the soil is saturated and the depression storage filled, and rain continues to fall, the rainfall will immediately produce surface runoff.

PROBLEM STATEMENT

Contmap of lake naivasha catchment

Site Analysis

Objectivesoverall objective Reduce the runoff problem in Naivasha.

Specific objectives Determine the total volume of water from the

catchment Design a gravity dam

Literature review

Gravity dam is a structure so built that it derives its stability from its own weight to resist external forces

They transfer their weight to the ground by cantilever action and require strong rock foundation

Theoretical framework The rational method

Q= 0.0028CIA

This was used to calculate the total amount of runoff from

the catchment area

Tc= 0.01947L0.77S-0.385

Kirpichs equation was used to calculate the time of intensity.

cont The cone formula was used to calculate the total

volume of the reservoir

Gravity dam equationsHydraulic height (H) =highest contour – lowest

contourFreeboard (FB) = 1.33hw or 5%H Structural height (Ht)=H+ FB Top width (Tw) =0.14Ht or 0.55H0.5

Methodology

Reconaissance survey and topographical surveys were conducted

Led to identification of suitable site for dam construction

River catchment area was estimated using Google earth pro

Runoff coefficient for the catchment were determined

Cont

The peak runoff rate was determined The total volume of water from the

catchment was calculated A contour map of the reservoir area was

prepared Total volume of water the reservoir can

hold was calculated

Cont

- The gravity dam dimensions were determined

- The dam was checked for stability, tension, sliding and compression.

Results and DiscussionElevation (m.a.s.l)

Area H (m) volume Cumulative volume

2330 7926.1 1 9197 9197

2331 10529.3 1 12755.8 21952.8

2332 15120.38 1 17341 39293.8

2333 19660.82 1 18452.3 57746.1

cont

cont

1) Hydraulic height (H)

= 2333 – 2330 = 3m

2) Freeboard (FB) = 1.33hw or 5%H

= 1m

m

cont

3) Structural height (Ht)= H + FB

= 4M

4) Top width (Tw) = 0.14Ht or 0.55H0.5

= 1M

5) Base width (b) = 3m

cont

initial dam dimensions

Dam stability analysis Overturning R.m=36.768t.mo.m=(9+2.71+5.833+4.5) = 22.043 t.m = 36.768/22.043=1.661.66>1.5, thus dam is safe Compression/crushing.

pntoe =9.8t/m2 = 0.98*10^5N/m2

f= 83.333*10^5N/m2

pntoe <f (thus dam is safe)

cont

Tension

e= 0.5m and b/6= 3/6= 0.5m

e=b/6, thus dam is safes safe

Autocad drawing

Catchment characteristicsattribute value source

area 6.74 Google earth pro

Watershed length 10300 Google earth pro

Highest contourLowest contour

2606.42308.6

Google earth pro

Average watershed slope

0.08

Major land cover classesCultivated landGrasslandForest

603010

Soil characteristics Sandy loam

Catchment sketch

Runoff computation Runoff volume (m3 ) = runoff depth

(m) * catchment area (m2)The runoff coefficient method was used to calculate depthR= C.P

C= runoff coefficient p= rainfall (mm) (from climwat)Depth= 9.35mmTotal volume= 63,011.37 m3

Digital elevation model

contours

conclusion The ultimate goal of this project was to

design a gravity dam for flood control on River Nyamamithi. It can be concluded that this projects objectives were achieved as the detailed design of the concrete gravity dam was achieved. The dam has a height of 4m above the foundation and creates a reservoir storage of 57746 m3, this is sufficient to control the downstream flooding.

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