DRAINAGE SYSTEM DESIGN
Objective: The drainage system for the project functions to ensure effective transportation of storm water in order to avert the occurrence of flooding of the site. It also helps to safely discharge grey wastewater from buildings to their end points. NATURE OF SITE, LOCATION AND TOPOGRAPHY:
The sight, predominantly sandy, slopes to the south-western part. Due to this topography, water flows to the southern part of the community and since there are insufficient drains to collect the water especially during heavy downpour, flooding occurs. Two U-drains are located along the road that joins the storm drain located at point X on the map. One from the Ayeduase gate with the other at a distance Y, away from the storm drains at X. There are no further drains along the road until a culvert at a point, P on the map. The distance of no drains is where most flooding occurs during heavy downpour of rain. From the culvert is a drain that joins the storm drain with double culvert at a point C. This storm drain connects to the main stream behind Nana Adoma Hostel. From the Ayeduase gate downwards is another drain that joins the main stream represented by a blue line on the map. All drains form a network and later join the storm drain which in turn empty into the stream.
The Road Design Guide by the Ghana Highway Authority was used as the standard for the design of all drains DESIGN RETURN PERIOD
A 20 years return period was used in the design of all drains, both secondary and tertiary
Using rainfall-intensity-duration curve for Kumasi, obtained from the Kumasi Metropolitan assembly, the rainfall intensities for a chosen return periods can be estimated. The total peak flow for each of the catchment areas can also be estimated with the values from the IDF curve. This is used for the sizing of the drains.
CALCULATION OF DISCHARGE
For the discharge of each drain, the rational formula will be used with the assumption that the same rainfall amount will be generated for the storms of equal intensities over the same catchment area. Rationale’s formula
C= Run-off coefficient
I=Rainfall intensity (mm/hr.)
A=catchment area (km2)
SITE RUN-OFF COEFFICIENTS
Due to the fact that the site is commercial, with a high demand of parking facilities, most areas will be paved, hence the run-off coefficient of 0.6 (for pavement) and 0.8(for asphaltic roads). These values are obtained from the KMA for discharge computation.
CALCULATION OF RAINFALL DURATION
Lloyd Davis relation can be used in estimating the Time of Concentration (Tc). This relation is equivalent to the rainfall duration. Lloyd Davis equation;
Tc=58.5L/A0.1 x S0.2
Tc = Time of concentration
L=length of main stream (km)
A= catchment area (m2)
S=slope of mainstream (m/km) A= catchment area (m2)
S=slope of mainstream (m/km)
SITE CATCHMENT AREA
All drains will be sized to accommodate peak flows with the hydraulic discharge been greater than the hydrological discharge. A freeboard of 20% of the drain depth will be used in the drain design. The manning’s formula can then use in computing the discharge capacities of the drains which is the hydraulic discharge. The manning’s formula is given by;
Q = (1/n)*A*R2/3*S1/2
Q=Discharge of drain
A= area of drain
N= manning’s constant (0.015 for in-situ concrete)
S=slope of bed
SLOPE OF BED
A 4% bed slope can be chosen, which is high enough (>1% but <5%) to ensure a self-cleansing velocity in the drains
U-drains of dimensions 600mm*600mm are designed to convey the surface runoffs from the catchments.
Here is a typical pictorial cross-section of the U-drains used in the design.
SAMPLE CALCULATION ON DRAIN SIZING
Considering drain 1
For a given runoff (discharge)
Using a U-drain of dimensions 600 x 600mm to accommodate the...
Please join StudyMode to read the full document