Design of a water supply system
Information provided is generally adequate; if any assumptions are made, these should be clearly stated giving your reasons.
A small regional town in Queensland is experiencing a severe drought while subjected to an increasing permanent population. Council has enforced water restrictions at 140 litres/person/day but has received complaints that this level is too strict. The water supply system consists of a trunk main from the source to a service reservoir, the service reservoir itself and service mains to the consumption areas. Two consumption areas A and B at different elevations are gravity supplied by the service reservoir C. The reservoir has a capacity of 8 ML and is gravity fed from Source D which is 22 km away. Source D can provide 800ML per annum during the drought period. This supply is supplemented by bore water which is pumped to service reservoir C. The bore is 5km away. Further information is shown in the sketch below.
A. Determine the annual average water requirements needed from the bore to meet the projected demand for the design maximum population assuming the average water consumption of 140 L/p/d. This requirement is to be pumped to the service reservoir. Assume the bore’s safe yield is 1.5 ML/ d, the maximum flow that can be extracted. Sources such as dams and bores need to have enough water supply to meet the annual average demand, in this case, with the strictest applied water restriction. Determine the size of the rising main and required pump size to meet this demand if pumping is carried out for 20 hrs/day. Choose appropriate average and maximum flows to be used in the design of rising mains (Refer Table 2.4); Costing data is provided below; Calculate the net present cost to determine the most cost-effective solution. Assume that the bore can serve the population for 20 years.
Cost of pipeline – $10 per m per 25 mm diameter
Cost of pumps -$2500 per kW installed power
Cost of power – $4 kWh
Annual charge on capital – 8%
Total installed power -150% of required power (assuming 50% standby) at an efficiency of 65% and a power factor of 0.85
Present worth -9.818147407 Million
If the bore’s safe yield is 1.5 ML/day, what level could water restrictions be lifted to and still be sustainable?
B The ground elevations at area A vary between 320 RL and 330 RL, and in area B between 305 RL and 315 RL. The Queensland Guidelines for Planning and Design of Urban Water Supply Schemes require that the minimum allowable pressure in reticulation or service mains should be 22 m head. Other provisions of the Guidelines referred to in the study materials are also to be considered. Minor loss in the service mains is estimated to be 1.5 times the velocity head.
You are required to investigate and report to the Council what populations are adequately serviced for each component of the existing trunks (DC, CA and CB). Calculate the immediate and future shortfall (or excess) of populations served by the current system.
Based on the results of your investigation, suggest one immediate and two long term measures with supporting calculations that may be considered effective to satisfy present demands and at the end of the design period respectively.
C Assume that the service reservoir operating or equalizing storage is 30% of the peak daily demand; the required fire storage is 25 L/s for 4 hours and the breakdown provision is equal to one peak daily demand. Calculate the number of people the service reservoir can serve and comment on its adequacy for current and design population.
This council requires that the total service reservoir capacity (including operating+ fire storage + breakdown provision) must meet the following criteria based on local experience:
2.75 (PD-MDMM) demand
must not empty at 3 days consecutive peak day demand
At average daily demand, the system should have a capacity to refill from empty to full within 10 days of consecutive operation at this demand
Suggest one immediate and long-term measure (if necessary) satisfying ALL of above criteria.
You should summarise the results for each system components in a matrix format supported with detailed calculations in an appendix. Refer to the marking scheme for assignment sheet for the details. Marks will also be apportioned for documentation and clarity. Assume Kinematic viscosity to be 1.139 x 10-6 m2/s.