Stormwater July/August 2012 : Page 42
What is “Pre”? In the example above, the “pre” condition was forested. Thus, the peak fl ow rate reduction derived using the En-ergy Balance method was applied to the peak fl ow rate in a forested condition. However, an argument can be made that regardless of the existing condition, the base from which the fl ow rates should be reduced should be a forested condition. The basis for this argument re-lates to the historical condition of watersheds within the mid-Atlantic region of the US where we primarily work: In most instances, degradation did not begin until land was cleared for agricultural or other purposes. Thus, ac-cording to the argument, the goal should be to return streams to their stable state that existed before colonial settlement. In the mid-Atlantic region, this stable state predominantly comprised a forested condition. However, this goal is not always feasible or desir-able. Therefore, it is necessary to determine the level of Figure 4. Runoff hydrographs for the hypothetical 20-acre site showing the the fl ow rate reductions achieved through the Energy Energy Balance approach Balance approach for any given site. Goals can include not worsening the existing conditions, requiring some level approach allows site development to proceed in a more pre-of improvement over existing conditions, or meeting some dictable manner, while providing real incentive to reducing level of runoff assuming existing conditions are forested (or impacts to downstream receiving waters. The Energy Balance method achieves this goal. Recogniz-a meadow). These different alternatives have been enacted by regulatory agencies, as discussed in the following sections. ing that the pre-and post-construction product of runoff vol-ume times peak fl ow rate must match, the more the runoff volume can be reduced, the less onsite storage volume that Regulatory Solutions With the understanding that accounting for the additional will be required. Because of the expense of storing the in-runoff volume, in addition to the peak fl ow rate, is neces-creased volume, this approach encourages the exploration of sary to protect downstream receiving waters, the question ways to reduce runoff volumes in the site planning process. becomes how to achieve this goal. One option would be to This can include taking advantage of the portions of the site simply mandate that the hydrology in the pre-and post-con-that have soils that can provide infi ltration, if at all possible, struction condition match — that is, no increase in peak fl ow or implementing low-impact-development (LID) techniques, rate or runoff volume would be permitted. However, adoption such as the use of permeable paving materials, rain gardens, of such a regulation would prevent many sites (if not most, in green roofs, or cisterns to capture rainfall to be used for onsite certain areas) from being developed without numerous sub-irrigation. In this way, the Energy Balance method provides jective exceptions granted. This outcome would be particu-incentives for reducing storage requirements for stormwater larly true for intensely developed sites located on soils that do as a way to decrease stormwater management costs. The re-not infi ltrate (hydrologic soil groups C and D). Even for sites sult is a methodology that quantifi es the application of “prac-ticality” instead of leaving its defi nition that do provide some level of infi ltration to the angst of a subjective regulatory (hydrologic soil groups A and B), there process. may not be enough capacity to eliminate The other benefi t of this approach the additional runoff volume that is gen-is that sites that are unable to reduce erated for some land uses—for example, their runoff volumes suffi ciently can still in an urban setting where the buildings www.stormh2o.com/regulatory-issues be developed through the provision of occupy most of the site or where grading suffi cient stormwater storage volume, requirements eliminate use of the perme-but without the politics of an undefi ned able portion of the soil profi le. Because waiver process. Thus the site can be de-preventing site development would not be economically desirable, a system of waivers would have to veloped and the stormwater management program can be be instituted on a case-by-case basis. This approach would add planned early on in the process, as opposed to having to wait signifi cant uncertainty and cost into the development process, for a decision from a regulatory agency. as landowners would not know whether their property could be developed until signifi cant funds had been expended. History of Regulatory Development As an alternative to the blanket mandate discussed above, The regulatory history behind the development of the Energy a performance standard could be developed that encourages Balance method of stormwater management has spanned the goal of matching hydrology in the pre-and post-construc-many years, beginning at the local level and culminating in tion conditions without mandating that it be achieved. This adoption for use throughout Virginia. The process by which For related articles: 42 July/August 2012 www.stormh2o.com
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