Models for Water Quality Control by Stormwater Ponds
The design of stormwater management quality control ponds is largely based on empirical practices which have been developed over time by water resources engineers. Of much more value to practitioners, and more importantly the environment they protect, is sound engineering design founded on a scientific basis. At present, the modelling of stormwater management facilities for both quantity as well as quality control is often accomplished using event and/or continuous simulation analysis methods. Analytical probabilistic models, based on long-term rainfall statistics, have been developed as an alternative approach and have proven to be reliable when compared to continuous simulation models for the estimation of stormwater quantity control performance. As more emphasis has been placed on environmental impacts, and how non-point source pollution, namely urban runoff, can be mitigated, these analytical models have been extended for the prediction of water quality control performance of urban drainage systems as well.
In this chapter, these analytical model results are compared to those of continuous simulation models for the quality control performance of stormwater management ponds, namely extended detention dry and wet ponds. The estimates of pollution control performance of these facilities are compared with respect to storage volume for various detention times, catchment areas and pond depths. The objective of this exercise is to illustrate the applicability of the analytical models for use in lieu of, or in conjunction with, continuous simulation models. The results of the analytical models compare favourably with those of continuous simulation models for planning level analyses. For extended detention dry ponds servicing catchment areas of 10 to 100 ha with depths of 1.0 to 1.5 m and detention times of 24 to 48 hr, the difference in predicted suspended solids removal is generally less than 5 to 10% for all practical storage volumes. For wet ponds servicing catchment areas of 10 to 100 ha, with permanent pool depths of 1.0 m and active storage depths of 0.5 m, and active storage detention times of 6 to 48 hr, the difference in predicted suspended solids removal is between 10 and 30% for all practical storage volumes.
The pollution control performance of extended detention wet ponds is estimated using a constant ratio of active storage volume to permanent pool volume. This ratio is demonstrated to substantially impact the long term pollution control performance of the facility since the different storage zones are subjected to different pollutant removal mechanisms. An investigation is undertaken to determine the optimal combination of active storage volume and permanent pool storage volume for a given set of conditions. These results are generated using the analytical models and can be a valuable guide to the engineer in the preliminary design of urban runoff quality control ponds.
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