DESIGN OF A GRAVITY DAM A case study of River Nyamamithi in Naivasha constituency F21/2500/2009...
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Transcript of DESIGN OF A GRAVITY DAM A case study of River Nyamamithi in Naivasha constituency F21/2500/2009...
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DESIGN OF A GRAVITY DAM A case study of River Nyamamithi in Naivasha constituency
F21/2500/2009MWAURA WILSON NJENGA
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Overview
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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.
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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.
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PROBLEM STATEMENT
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Contmap of lake naivasha catchment
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Site Analysis
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Objectivesoverall objective Reduce the runoff problem in Naivasha.
Specific objectives Determine the total volume of water from the
catchment Design a gravity dam
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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
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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.
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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
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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
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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
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Cont
- The gravity dam dimensions were determined
- The dam was checked for stability, tension, sliding and compression.
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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
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cont
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cont
1) Hydraulic height (H)
= 2333 – 2330 = 3m
2) Freeboard (FB) = 1.33hw or 5%H
= 1m
m
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cont
3) Structural height (Ht)= H + FB
= 4M
4) Top width (Tw) = 0.14Ht or 0.55H0.5
= 1M
5) Base width (b) = 3m
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cont
initial dam dimensions
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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)
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cont
Tension
e= 0.5m and b/6= 3/6= 0.5m
e=b/6, thus dam is safes safe
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Autocad drawing
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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
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Catchment sketch
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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
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Digital elevation model
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contours
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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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References Auto cad civil 3D 2010, pipelines from alignments, profiles and corridors, Jack Strongitharm,
Autodesk ltd, July 2009 Becht R, Odada EO, Higgins S(2005) lake naivasha: experience and lessons learned brief. Managing
lakes and basins for sustainable use: a report for lake basin managers and stakeholders. Design manual for concrete gravity dams, a water resources technical publication, Denver,
Colarado, 1976. http://en.wikipedia.org/wiki/surface_runoff. King and Brater: Handbook of Hydraulics, Mcgraw Hill Book Company, Inc., New York, Fifth Edition
1963. Merritt: Standard Handbook for Civil Engineers, Mcgraw Hill Book Company, Inc., New York, 1968 Otiang’a owiti GE, oswe IA.(2007) human impact on lake ecosystems: the case of lake Naivasha,
Kenya. African journal of aquatic science 32:79-88 River weirs- good practice guide, Charles Rickard, Rodney Day, Jeremy Purseglove. Streeter: Fluid Mechanics, Mcgraw Hill Book Company, Inc., New York, Fifth Edition. The physical attributes of the Lake Naivasha catchment rivers, Mark Everard, Jacqueline A. Vale,
David .M.Harper and Hakan Tarras-Wahlberg Training on design of hydraulic structures, module 3, design of weirs and pumps, Zemene Tsehay,
September 2009, Bahir Dar.
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