Estimating Volume Requirements for Runoff Storage Facilities: Old Simplicity vs. New Complexity
Stormwater management analysis practices have evolved significantly over the past three decades and, at present, computer simulation is the technique most commonly used by the practising engineer to perform a variety of analyses ranging in scope from preliminary concept plans to detailed design. Typically accompanying the arrival of numerous and sophisticated simulation models into engineering practice is a general loss of knowledge of and appreciation for some relatively simple techniques which were advanced prior to the current era of stormwater management modeling.
An example of such a technique is presented in this chapter which estimates the size of a runoff storage facility such as a detention pond based on only two catchment characteristics (namely, depression storage and the runoff coefficient), allowable release rates and intensity-duration-frequency (IDF) curves. [Assumption is made herein that probability of the storm equals probability of resulting flood - Ed. note] The technology, originally published in the 1940's, converts IDF curves into curves of cumulative rainfall volume as a function of rainfall duration. Graphical or numerical techniques can be used for the solution of critical rainfall durations which yield the storage volume required to avoid spillage for the storm with a return period that corresponds to the IDF curve. The technique is, therefore, based upon event hydrology. It has since been generally recognized that the long term performance of drainage system elements such as storage devices cannot be designed or analyzed adequately using single design storm events based upon IDF relationships. Several analysis technologies have been developed to overcome this limitation including continuous simulation, analytical probabilistic techniques and stochastic simulation techniques, all of which utilize long term rainfall records or meteorological statistics derived therefrom.
This chapter investigates the adequacy of employing an older, simpler analysis method for arriving at preliminary estimates of volume requirements for runoff storage facilities by comparing results obtained using this method with results generated from continuous simulation and analytical probabilistic approaches. This simplified technique is formulated and solved utilizing exponentially decreasing and Chicago-type design storms. The results of this exercise indicate that this simple technique can be used with reasonable confidence for preliminary estimates. This means that less computational effort is required of the engineer producing concept plans for urban land development. The comparison of results also provides additional insight into the performance of the computationally efficient analytical probabilistic models relative to their more sophisticated and complicated counterpart, namely continuous simulation. It is noted, however, that the detailed design of urban drainage systems does deserve utilization of simulation techniques.
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