Answer Happy

5. Allow approximately 32 minutes for this question. Your client intends to develop a 6.2500 Ha site in Long Bay, which is currently being utilised as paddock, into a 110-lot residential subdivision. (a) Using an undeveloped run-off coefficient for the paddock of 0.27 and a developed run-off coefficient of 0.55, calculate both the peak pre-development and post-development volumetric runoff rates for a design storm with an annual exceedance probability of 10 % (i.e. a 1 in 10 year return period) and a peak rainfall intensity of 75 mm/hour. (4 marks) (b) Use a simple triangular hydrograph to calculate the volume of water that must be detained so that the volumetric rate of discharge from the new subdivision will not exceed the peak pre-development discharge rate. Use a post-development time of concentration of 15 minutes. Assume the detention system has a constant flow-rate discharge outlet. (6 marks) (c) A detention pond is proposed to contain the additional run- off volume (due to the development) that was determined in (b). The pond is proposed to have a circular cross-section with a diameter of 9.5 m and have a maximum depth of 2.0 m (excluding freeboard). It is designed such that all of the water empties out of the pond between rain events. Is the proposed pond large enough to store the volume of run- off that needs to be detained? If not, how deep does the pond need to be to provide the necessary storage volume if the diameter of the pond is held constant? (d) The discharge rate from the pond will be controlled by a sharp-edged, rectangular weir located adjacent to the pond. Calculate the minimum required length of the weir so that the peak discharge rate from the pond does not exceed the peak pre-development discharge rate. Take the weir's coefficient of discharge (Ca) to be 0.62 and the depth of flow over the weir to be 0.20 m. (4 marks) (4 marks) (Total 18 marks)