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Drainage Design
(HFP Industrial Plot Ibeju lekki)
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General
A hydrological analysis of the area to be drained is done to
obtain information on run-off and
stream flow characteristics required for the hydraulic
design.
The design flow is established by selecting proper combinations
of rainfall and run-off
characteristics that is expected to occur over a stipulated
period. The design criteria is then the
maximum flow in cubic meter per second carried by the drainage
structure with no flooding or
a limited amount of flooding to be exceeded on the average of
once over the design period.
Parameters
Run-off estimates shall be based on the Lloyd-Davies method. In
this method the run-offis related to the intensity of rainfall by
the following formula:
Q = C I A
Where,
Q = quantity of run-off in cubic feet per second (cfs)
C = coefficient of run-off expressed as a percentage of
impervious of the
watershed surface.
I = Intensity of rainfall expressed in inches per hour for the
stipulated period of
Concentration.
A = Area of watershed in acres
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Parameters Estimation
1. Coefficient of run-off
The use of average coefficients for various surface types is
used. However C
is takes to be 0.5 for our analysis.
2. Rainfall Intensity:The value for the intensity of rainfall is
calculated from:
I = Kn
(t + a)b
Where,
I = Rainfall intensity in inches per hour
t = Duration of rainfall in hours
a and b are stationconstants
Kn = A . B Log 10n
;
A and B are station constants
a = 0.3 Ref: Meteorological notes 2 : British West Africab =
0.861 meteorological services Nigeria.
A = 2.18
B = 1.44
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The storm design frequencies for gutters n = 10
Rainfall duration t = 10 hours.
Kn = 2.18 x 1.44 x Log 1010
= 3.14
Therefore,
I = 3.14
(10 + 0.3)0.861
= 0.421
Point 1: A = 2.47 acres
Q = CIA
Q = 0.52 cfs
= 0.147 cubic meter per second; gutter section = 60 x 40;
capacity = 0.24 cms.
Point 2: A = 3.0 acres
Q = 0.631 cfs
= 0.18 cubic meter per second; gutter section = 60 x 40;
capacity = 0.24 cms.
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Point 3: A = 6.79 acres
Q = 1.429 cfs
= 0.404 cubic meter per second; gutter section = 2x 60 x 40;
capacity = 0.48
cms.
Point 4: A = 7.12 acres
Q = 1.5 cfs
= 0.424 cubic meter per second; gutter section = 2 x 60 x 40;
capacity = 0.48
cms.
Point 5: A = 2.41 acres
Q = 0.51 cfs
= 0.143 cubic meter per second; gutter section = 60 x 40;
capacity = 0.24 cms.
Point 6: A = 10.45 acres
Q = 2.16 cfs
= 0.61 cubic meter per second; gutter section = 2 x 60 x 60;
capacity = 0.72 cms.
Point 7: A = 5.93 acres
Q = 1.24 cfs
= 0.35 cubic meter per second; gutter section = 2 x 60 x 40;
capacity = 0.48 cms.
Point 8: A = 15.3 acres
Q = 3.22 cfs
= 0.911 cubic meter per second; gutter section = 120 x 80;
capacity = 0.911 cms.
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