DESIGN OF A TRUNK URBAN STORM SEWER HARVESTING SYSTEM A CASE STUDY OF GILGIL TOWN
Documentation : 

Gilgil town is a water scarce town despite water being treated at Turasha area in Gilgil constituency. With rationing taking place thrice a week in the town especially during the dry months, most of the activities that require water either stall or the owners have to buy water from water vendors. When the rains fall, storm sewer washes away all the pollutants in its path. The storm sewer finds its way into business premises since there lacks a storm sewer harvesting system.
The main objective of this project was to design an effective storm water drainage system that shall help in controlling and harvesting floods in Gilgil town.
To achieve this, the design phase incorporated an estimation of the frequency and magnitude of flooding in Gilgil town. Pertinent parameters that included the utilizable runoff of our catchment area in Gilgil town, the weighted coefficients, the rainfall intensity, and the time of concentration were calculated and applied in the rational method formulas. The catchment area was digitized using QGIS to give the exact area of our catchment. Information obtained from the calculations helped in the design of the conveyance channels and the storm sewer storage system.
After all the pertinent parameters were considered, the catchment area considered was 25721.14998 m². A weighted coefficient was computed and found to be 0.629495. The rainfall intensity adopted was 36mm/hr. The time of concentration was computed and found to be 32.26 minutes. The above parameters were used to calculate the utilizable runoff that was found to be 1377.53m³. This utilizable runoff was used to compute for the depth of the storm storage structure that was found to be 4.502 metres deep. The results obtained were used to plot engineering drawings as shown in the report.
The sewer system if implemented would help in harvesting the storm sewer for non-potable uses and minimizing pollution in Gilgil town.

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