Abstract

The joint Ministry of the Environment/Ministry of Natural Resources (MOE/MNR) Interim Stormwater Quality Control Guidelines are intended to promote receiving water quality protection by requiring detention for stormwater discharges of specified volumes for specified detention times. The required detention times (td) are in turn specified by the class of receiving water (e.g. 12-24 hr td for discharge to cold water fishery areas and 72 hr td for discharge to body contact recreation areas). The more recent Stormwater Management Practices Planning and Design Manual (MOEE, 1994) recommends a uniform 24 hour detention time. The usual assumption implicit in specified detention times is that the longer detention time provides a higher level of water quality control. Furthermore, there is usually little quantitative justification for the specified detention times.

This chapter analyses the implications of required detention times of extended detention dry ponds with respect to their actual pollution control performance. Longer detention times are compromised by two factors. First, the longer detention time of any runoff event increases the probability of a larger overflow for the subsequent event. The negative impact of pollution from the Subsequent event overflow can outweigh the positive impact of greater pollution removal provided for the preceding event. Secondly, the marginal gain in removal efficiency of the pond is constantly diminishing with increasing detention times in accordance with sedimentation laws. It is clear by induction that for any given runoff characterization, pond design and settling velocity distribution, an Optimal detention time will exist.

An analysis methodology is presented for determining optimal detention times for extended detention dry ponds based on derived mathematical relationships among the water quality performance of ponds and the runoff, pond and settling velocity characteristics. The relationships are developed analytically derived probability distribution theory. The water quality control performance is based on turbulent settling mechanisms for the removal of particulate pollutants. The numerical results from these models indicate that pollution control performance is maximized with typically short detention times and diminishes monotonically with increased detention time thereafter. Therefore, further increases in detention time can be counter-productive. It is concluded that currently available analysis technology for pond design based on quantitative water quality control measures is preferable to the specification of arbitrary detention times.

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