Stormwater Management Issues: Are We Doing the Right Thing When Dealing with Seemingly Insignificant Increases in Impervious Areas?

Any addition of impervious area within a watershed, big or small, usually results in an increase in peak flow and runoff volume.  This affects both water quality (due to increased pollutant load), and water quantity (issues with existing system downstream conveyance capacity, flooding, siltation and erosion). In meeting the regulatory requirements, identification of stormwater management facilities is critical in addressing both quality and quantity issues.

When dealing with new developments or major re-development projects, whether it is for facilities or linear development projects, complying with regulatory issues of water quality and quantity is easy.  Additional requirements of right-of-way and/or easements, for addressing stormwater management, can be identified early-on in the process.

The compliance issues arise when seemingly insignificant impervious areas are added in a re-development project, such as, addition of a generator pad and a walk way, widening of existing pavement over a small section by 3-4 ft, or addition of a sound barrier wall with a median barrier and gravel dam along the length of the highway.

In most cases, regulatory compliance of water quality can be met by purchase of nutrient credits.

Meeting with regulatory compliance issues of water quantity is more challenging. For each sub-area delineated, the increase in impervious footprint is small.  Allowable capacity of the existing system to absorb the additional increase in peak flow may be limited and/or the older system may not be sized for the current standards.  To satisfy MS-19 requirements, Qpost ≤ Qpre for 2- and 10-yr storm events. Moreover, energy balance for a 1-yr event must to be satisfied.

These regulations do not offer any credit, however, to preserving tree or forest cover.  In designing infiltration practices to the standards specified, existing and well-established trees are to be cleared.  The question arises, are we doing the right thing? The 90th percentile rainfall here in the coastal areas of Virginia is less than 1-inch. For this, tree cover can do wonders for interception and absorption.  The drainage system and facilities are designed for an event rainfall, which is much more than an every-day rain in terms of its intensity and duration.

This paper presents the challenges posed by current regulations and compares the long-term and short-term effects of infiltration practices presently implemented in design to that of tree cover.