The ground water flow component is the sub-surface runoff component contributing to the river discharge from a particular region after the rainfall. If the major portion of runoff water reaches the river as overland flow , a heavy rainfall would result in quick response in the river and the hydrograph shape would be peak. Thus the risk of floods would be higher in this case. While if the major share is through ground water flow the rise in discharge is slower and the repose of the river is slower. Thus the rate at which the flood waters reaches the river body determines the shape of t he hydrograph. The major factors contributing to these situations are characteristics of drainage basin, type and amount of precipitation, land use pattern in a region, impact caused by human intervention, size and shape of drainage basin and major river management measures adopted (Flood hydrograph, n.d.).
The hydrograph for river Cynon is drawn based on the rainfall and river flow depth data (figure 1). The discharge in the river is plotted along y-axis and the time (in hours) along x axis. Similarly, the rainfall depth is indicated on a secondary y axis in the same graph corresponding to particular instant of time. Based on the flood hydrograph obtained for the river Cynon, it is obvious that the drainage characteristics of the terrain results in the occurrence of peak discharge soon after the rainfall. The drainage basin soil characteristics is said to have relatively low value of permeability and this results in low infiltration rates into the soil. The rising limb of the hydrograph has a very steep rise while the receding limb was not as steep. The receding limb required more time to reach the base flow condition which justified the continued low water flow at very low rates as a result of infiltrated water. It took almost 22 hours to reach the peak flow condition and the receding limb needed 46 hours to attain the base flow state. The discharge rate in the river during rising limb, peak level and receding limb of the hydrograph are shown below.
Discharge computed for the rising limb = Velocity Depth of flow in river (for the rising limb) width of flow = 4 m/s 0.49 m 15 m = 29.4 m3/sec
Peak Discharge computed = 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 the receding limb = Velocity Depth of flow in river (for the receding limb) width of flow = 4 m/s 0.353 m 15 m = 21.18 m3/sec
Thus the potential for flood is very high as there would be a sudden rise the discharge level of the river Cynon. Further, the level of water too recedes fast and hence a small detention facility to accommodate this rise in the river water would help to prevent the damage caused by the flood water. The basic concept adopted for controlling floods is through the methods of retention, detention and sedimentation. The excess runoff is stored in the reservoirs until the basin could accept this volume after the adequate quantities of water have receded.
The total area of catchment for river Cynon is reported as 160 km2 which is