Storm Drainage Design Project and Flooding Assignment

Storm Drainage Design Project and Flooding - Assignment Example Surface streams and groundwater flows and the wetness of soil before a the storm are the factors to be considered. According to Dr Tim Stott, rainfall prediction is a matter of statistics. No one can predict what the rainfall and catchment conditions will be in the future. With climate change, past records are not reliable for future references. Engineering design will be based on specified probability storm and the runoff generated by catchments In the hydrographs of Cynon River, there was an almost steady flow of water in the river for the first 40 hours. As the height of water start to rise on the 42nd hour, water had a abrupt rise and on the 44th hour the water temporarily had a constant flow and began rising again on the 48th hour. The rise was constantly rising until it reached the peak flow and it was recorded to be 0.65m above the river bed and the rainfall recorded at 1.2mm. In the analyses of the river hydrograph, the rise of water is faster than when it starts to subside. It would take more time to subside, and having a steady downward motion. There are factors that control the shape of hydrographs. The typical shape are shown and the main components are labeled according to Weyman (1975). A hydrograph would show the difference between the peak rainfall from the peak discharge. This is known as the lag time. Then a lag time is greater, there is less chance of flooding whereas a short lag time will show that water had reached the river channel quickly. The rise in discharge or rainfall as shown in the graph is called the rising limb and the decrease in rainfall is the falling limb. There are several factors that affect a flood hydrograph. Areas with large basins receive more precipitation that the small ones and they have a larger run-off(Hoyt, 1936) Larger basins will have a longer lag time as water has a longer distance to travel before it can reach the river trunk. According to Gillesania, 2006, the shape of the basin which is typically elongated, would produce a lower peak flow and longer lag time than a circular one. The effect of the slope is also very important. The flow will be faster down a steep slope, thus producing a steeper rising limb and shorter lag time. Channel design Given Data Note: Given the discharge in the channel, apply the the Manning Q = 1.5 m3/s formula to get a suitable breadth b, of a channel with n = 0.019 depth d S = 1/2500 = 0.0004 d = 0.6 Formula to be used V = where: v = velocity Q = Av R = Hydraulic Radius Q = A S = slope A = bd n = Manning's coefficient R = Q = discharge Computations: A = db = 0.6(b) Q = A R = 1.5 = 0.6b 1.5(0.019) = 0.6b 0.0285 = 0.6b = 0.6 1.425 = 0.6 = 2.375 = (2.375)3 = b3 13.3964 = 13.3964 = 13.3964(1.44 + 2.4b + b2) = 0.36b5 19.2908 + 32.1513b + 13.3964b2 = 0.36b5 19.2908 + 32.1513b + 13.3964b2 - 0.36b5 = 0 b = 3.9798 m. The discharge flow of the river is also to be computed. The computation is necessary because a comparison will have to be made in order to determine