Downstream Effects of Stormwater Management Basins

Article Review

McCuen, R.H. (1979) “Downstream effects of Stormwater Management Basins” Journal of the Hydraulics Division, Vol. 105, No. 11.

Prof. McCuen addresses the problem of hydrological alteration due urbanization. The use of impervious materials decreases natural storage, increasing peak rates, decreasing time-to-peak, and increasing the volumes of runoff and sediments. To mitigate these detrimental effects storm water management (SWM) is required. However, according to the paper, many SWM strategies fail in mitigate the urbanization effects because “they only attempt to limit peak flow rates and sediments volumes” to predevelopment conditions. This is in general attempted by the use of detention basins. This strategy, however, is not sufficient. This problem lead to main question of the article: “Are SWM basins effectively meeting the intent of stormwater management?”

“Urban development causes a decrease in natural storage, including infiltration, interception, and depression storage”. SWM basin is an attempt to restore the loss of natural storage in the watershed. This volume can be artificially restored, but the timing characteristics introduced by detention basins are significantly different from the predevelopment conditions. In natural conditions, the major part of the storage volume is filled during the storm event, while in a detention facility this storage is rapidly filled during the risen portion of the hydrograph.

A second effect introduced by detention basin is that the increased volume of direct runoff is released at a higher rate when compared to natural storage. This cause increase in the duration of bankfull flows.

Due the reasons explained so far, the work examines the hypothesis “that, when changes in both timing characteristics and flow volumes are not corrected by SWM, they may have an adverse effect on flow conditions downstream from the SWM basin.”

A 2.12 square mile watershed was simulated using the Soil Conservation Service TR-20 hydrological model. The predominant soils are silt loams, characterized as deep, well-drained, and moderately erodible. Simulations indicate that peak flow increases from 11.7 to 59.3 cfs (factor of five!!!) from pre-development to urbanized condition of one catchment for the 10 yr-storm. In terms of volume, there is an increase of 100%. The time-to-peak decrease 7 minutes. Introducing a detention basin, the peak flow is reduced to 13.08 cfs, but the time-to-peak is 36 minutes greater than that which existed prior to development. A compared analysis is done for the 100 yr-storm. The effects are the same, but the differences are much lower.

Looking further downstream in the watershed, the computation indicates that for the 100 yr-storm the SWM basin increases the peak flow compared to the post development with no control. This is true for approximately 5 miles downstream the storage facility. The reason for that is changes in timing characteristics affects the runoff conditions downstream.

After the hydrological analysis, the author has approached the bedload transport. Although its is recognized the difficulty in estimating bedload discharges, a theoretical model can be used to assess different scenarios and trace comparisons between them. A model developed by Goncharov was used to predict the sediment discharge as a function of mean flow velocity, mean sediment diameter, the critical velocity of sediment, and the mean depth. The previous discussed scenarios were evaluated to generate estimative of sediment yield. The results indicate that even with the SWM basin there is a significant increase in total volume of sediment. This is due the sustain flow introduced by the basin, which induce higher velocities than the critical velocity over a longer period.

The author finalizes the article with some considerations related with SWM policies and designs. He considers three aspects important:

(1) “The recognition of a comprehensive interpretation to the definition of the intent of SWM;

(2) A policy statement that reflects this interpretation and provides the guidelines that are required to institute proper design methods;

(3) Design methods and construction practices that are representative of the state-of-art and the fully consider hydrologic changes that go along with the urbanization process.”

He suggests that alternatives like grass-lined swales, rooftops detention, and porous pavements are SWM measures that better represents the natural storage found in predevelopment conditions, both in volume and temporal characteristics. A second alternative is to make change in the basin designs, although that may cause conflicts between flood control and pollution/sediment control, for example.