Storm Drainage Design Project, Flood Hydrographs Essay

Storm Drainage Design Project, Flood Hydrographs - Essay Example In addition hydrograph also carries information of base flow, which is the initial flow condition of the river existing in river prior to any storm. The base lag refers to the time repose of the river with respect to a rainfall which signifies the drainage potential of the catchments. The time interval between the peak rainfall the time of occurrence of peak discharge is reported as the basin lag. The rivers with short basin lag are more prone to floods as the respond very quickly to rainfall and hence the discharge in the river increases tremendously sooner the catchment receives a rainfall. The two major component of the runoff from the catchment that contributes to the volume of water in the river are the surface flow, which includes all the flows through the surface of the river and sub surface flow component which incorporates all the ground water flow discharges into the river after the onset of a storm. The slow rise is the level of river flow depth signified the majority of v olume reaching the river as ground water flow. The properly constructed hydrograph would be able to considerable amount of information of the behaviour of drainage basin with respect to a rainfall. ... The flood hydrograph for river Cynon is prepared by plotting he discharge through the river along the Y-axis and the time along x-axis. In order to plot the rainfall distribution, the depth of rainfall is plotted along a secondary y axis with time along the x - axis. The hydrograph hence plotted is shown in figure 1 and provides the detailed information on the drainage characteristics of the basin. The reports available have said that the subsurface conditions of the river Cynon basin is low permeability soil that could result in low infiltration rates. Thus these conditions would result in high surface runoff rates to the river and as a result the depth of flow rapidly rises. Thus the hydrograph obtained have steep rising limb followed by early peak discharge levels. Further, the receding limb is less steep than the rising limb justifies the discussions presented earlier. Also, the lag time is estimated from the hydrological observations as 20 hours. The flood volume of the river is computed from the total volume of the water in river during the period of runoff, by multiplying the depth of flow in the river with the mean river width given as 15 m and the average river velocity given as 4 m/s. The discharge computation from the hydrograph is as given below Particulars of computation Equation Calculation Discharge computed for the rising limb Velocity Depth of flow in river (for the rising limb) width of flow 4 m/s 0.48 m 15 m = 29.1 m3/sec Peak discharge Velocity Depth of flow in river (for peak discharge) width of flow 4 m/s 0.658 m 15 m = 39.48 m3/sec Discharge computed for receding limb Velocity Depth of flow in river (for the rising limb) width of flow 4 m/s 0.34 m 15 m = 20.4 m3/sec Total area of the catchment for the river Cynon From records