Storm Drainage Design Project Essay Example | Topics and Well Written Essays - 1500 words - 4

Storm Drainage Design Project - Essay Example The hydrograph is shown next page as Figure 2. To facilitate plotting, instead of using the river discharge figures in the vertical axis, the height of the river in meters from the given data was used. For a well-defined hydrograph of the river discharge, river height data were multiplied by 5 during plotting, although actual river height values were used in the computations. In the same vertical axis, the rainfall data in mm. were also plotted. Values were plotted for the hourly data for a four-day period. The time scale is plotted on the horizontal axis. Microsoft Excel (2003) was used to generate a computerised rendition of the hydrograph. The bar graph of the rainfall data showed twin-peaks of 1.2 mm occurring 10 hours apart. The rising limb of the river flood started to accumulate 11:00:00 of October 13 as the first peak of rainfall registered. After 22 hours, the peak of the river flow was reached at 9:00:00 of October 14 when the river height is 0.658 m. It will be observed that the rising limb of the hydrograph is quite steep. Meanwhile, it took 46 hours for the river flood at its peak to return back to base flow. This is defined in the hydrograph by the recession limb. However, the four-day data for the River Cynon did not necessarily return back to base flow. River height before the rising limb was 0.253 m, while the least reading given during the recession limb was 0.306 m. Figure 3 shows the rising limb, peak flow, recession limb and other elements of a typical hydrograph. A significant portion of the Cynon River catchment area is of moderate permeability (65%), which indicates slow infiltration rate and the steep rising limb (British Geological Society, 2005; Gordon, Finlayson, McMahon and Gippel, 2004). The average discharge for the rising limb (Qa) of the river flood is computed as the product of the given river velocity (v), the average river height from the first rainfall peak at 1.2 mm