No_8_August_1969 — Page 32

Flood balancing reservoirs

Another part of the Plover Cove scheme on which computers were used to advantage, was the design of the Lower Shing Mun reservoir and the various tunnels and intakes.

In the Plover Cove water scheme, storm run-off from the catchments south of Tai Po is taken by tunnel to Lower Shing Mun reservoir near Sha Tin. It is subsequently passed back through the tunnel to Tai Po Tau where it is discharged into the tunnel leading to Plover Cove.

In cases like this, where flood run- off from catchments is to be collected at stream intakes and conveyed through a tunnel to a distant storage reservoir, it can prove economic to provide a balancing reservoir adjacent to one of the intakes. In this way flood water can be temporarily stored locally and passed to the main storage reservoir over an extended period, between storms, with consequent reduction in the size of tunnel needed. To achieve an economical design it is necessary to match the sizes of the intakes and tunnels and the capacity of the balanc- ing reservoir to collect as much water as possible at a cost not exceeding a predetermined figure.

Given data on run-off or rainfall intensities at short intervals, say one hour or less, over a period of several years, it is possible to simulate the system on a computer to test the effect of varying the sizes of any of the com- ponents and to calculate the amount and cost of the water collected.

Yield of water supply schemes

The reliable yield of a reservoir scheme is the greatest rate of draw-off which can be maintained continuously without exceeding a predetermined risk of exhausting the storage in time of drought. By regulating the supply (i.e. applying restrictions) as the storage becomes depleted, a greater rate of draw-off can be allowed in normal times while maintaining a rationed but adequate supply during droughts. This enhanced rate of draw-off is termed the regulated yield.

Yields can be estimated by analysing existing records of rainfall and run-off for the catchments (which are seldom available for more than a short period, generally less than 100 years) and generating a synthetic sequence of monthly run-off figures for a long period (say 500 years) which have the same probability of occurrence as the recorded figures.

This can best be achieved by com-

puter. The generated sequence can then be fed back to the computer as input and the operation of the reservoir scheme simulated in monthly steps. For reliable yield estimation, values of draw-off are tried until the frequency of storage exhaustion corresponds to the risk selected.

For regulated yield estimation, trial values of draw-off and trial rules are used. Many trials are made until a yield is found which meets the pre- determined criteria of frequency and severity of restrictions.

By using this technique on a CDC 3300 computer, the consultants em- ployed on the Water Resources Survey of Hong Kong were able to estimate yields of existing and future water schemes. The program prepared for this purpose was able to take into account seasonal variation in demand and inputs from other sources, and to automatically optimise rules for im- posing restrictions.

Operation of reservoirs

There are 16 main fresh water reser- voirs in Hong Kong which contribute to the water supply of the urban areas.

Where many reservoirs are operated simultaneously to maintain a water supply to an urban area, it is important that before overflow is allowed to occur from any one of the reservoirs the others should be full, or nearly so; otherwise water is wasted which could be stored and which might be very valuable should a drought follow. Rules can be developed for making preferen- tial use of the fuller reservoirs which aim at achieving simultaneous overflow from all reservoirs while allowing the use of the cheapest sources of water where an option exists.

Empirical operating rules can be prepared for each reservoir based on the relation between its catchment area and its storage capacity and on any limitations due to its supply area. Such empirical rules can be tested by simulating the operation of the system over a number of years using historical data of run-off or estimating run-off from rainfall data. A period of ap- proximately 15 years using monthly steps is suitable.

Project appraisal

Where a dozen or more reservoirs are involved, as in the case of Hong Kong, use of a computer for the simu- lation becomes important, and for this reason the CDC 3300 computer in London is being used to develop op- erating rules as part of the Water Resources Survey.

Where there are a number of ways of developing new resources to meet an increasing demand and in particular when the order of development can be varied, the most economical develop- ment sequence can be selected by using the discounted cash flow techni- que. The value and timing of payments for capital and running cost in each selected sequence of development is estimated and they are discounted to obtain their total present value.

The present value is the sum of money which, if invested at the dis- count rate selected, would just enable all payments to be made from interest and capital over the period of develop- ment, leaving no money at the end. The development sequence with the lowest present value is the most eco- nomical.

As part of the Water Resources Survey of the colony several potential developments were considered by the consultants and the present values of various sequences of development were calculated at five discount rates for two possible rates of increase of de- mand, using two possible costing methods.

The calculations were done quickly and cheaply using a PDP 9 computer which was made to print out total annual expenditures and the present value in tabular form, ready for exam- ination without the need for tran- cription.

Transportation studies

With the increasing complexity of traffic estimating procedures for use in urban transportation studies and regional planning studies, the computer has become an essential. Indeed it is due to the computer that the advances in the development of sophisticated techniques have become possible.

Full use was made of computers for the recently completed Hong Kong Long Term Road Study and Hong Kong Mass Transport Study. Similarly, computers were used on the Hong Kong Passenger Transport Survey.

The consultants for the two Hong Kong studies have found the use of computers so important in the whole field of traffic estimation, economic evaluation and operational research that they have installed their own CDC 3300 computer in London, which has been designed to be particularly suitable for work of this nature. The computer has been used on some very large studies, including the London Transportation Study, and found to be invaluable in processing traffic data for the Hong Kong studies.

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Far East BUILDER, August 1969