The Limitations of Impervious Limits

Andrew Der
March 13, 2012 — 1,052 views  
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The relationship of impervious surfaces to nonpoint source runoff has been in the forefront of water resource regulation and conservation for many years, especially as a planning tool for watershed compliance as well as the basis for TMDL projections and even some NPDES MS4 conditions.  Research indeed shows a relationship between increasing levels of impervious cover and decreasing water quality conditions in receiving streams, and modeling the effects of pre-existing impervious areas with little or no stormwater management can be effective.  But does the presence of newer or projected impervious surface in and of itself degrade a receiving stream to a known and inevitable degree?  The answer is not so simple in today’s rigorously regulated new development, which is subject to newer federal and state criteria of effectively disconnecting and buffering critical flows.  If flow is to be disconnected or, as in Maryland, offset to “woods in good condition”, then requesting EPA to reconsider its inconsistent impervious surface-dependent projections and assumptions regarding TMDL goals is reasonable.

What can be variable is the degree of impairment relative to impervious cover at the site level, which can vary significantly depending on numerous site-specific characteristics such as the extent and nature of best management practices (BMPs), subsurface soils, type of project, location of any impervious cover, the pre-existing quality of the water resource, etc.  More effort and supplemental approaches are needed to appropriately project the effects of impervious cover accurately for an individual project.  While high levels of older impervious cover are historically correlated with impaired stream conditions, low imperviousness is only weakly correlated with unimpaired stream conditions.  In other words, the absence of imperviousness does not guarantee high quality stream conditions and using impervious cover alone to predict compliance goals or stream condition has limitations.

Historic land use is also a key factor in stream quality and, even in areas where riparian buffers and significant forest cover exist; stream quality may be poor due to lack of any previous stormwater management or a predominance of historic agricultural practices.  Single indicators, such as impervious cover do not alone account for the numerous other factors affecting stream quality in some watersheds.  For example, some areas potentially subject to impervious caps are already developed without controls or BMPs and any type of retrofit as part of re-development would be a potential water quality benefit.  Further, the majority of watershed studies cited to justify the use of impervious modeling and caps as a desktop tool were associated with older developed areas with little or no stormwater management.  Projecting effects of impervious surfaces in the present must be done in terms of effective impervious cover – or the runoff that actually will reach a stream after setback buffering and environmental site design (ESD) to the maximum extent practicable (MEP) We simply do not pipe stormwater runoff directly to streams anymore without appropriate management as was done back then.

If impervious surface modeling alone is conclusive enough to establish policy and regulatory decisions based  on the area of existing or projected surfaces, one then could conversely and logically argue if impervious cover is under any established threshold, then no stormwater management of any kind is necessary – but we know better.  Utilizing impervious cover in water quality management and watershed studies can be an effective tool in our toolbox but should be used in conjunction with other technical principles.  For example, in certain situations, managing watersheds via impervious surface criteria may actually be incompatible at the project level if clustering areas might conflict with ESD to the MEP approaches of dividing up drainage within an intermittent and fragmented impervious layout. Another artificiality of a desktop approach is, if a given development plan exceeds an established impervious cover limit determined by percentage of the property area, an additional contiguous property or parcel can be acquired to combine with the property.  This increases the total project limit acreage and raises the corresponding limit so the exact same project imperviousness is now less than the percent threshold.  The impervious area is exactly the same as before but now is considered “safe” – yet the stream does not know the difference.

The effective modeling of impervious surface effects in a watershed cannot be static and must account for numerous other factors such as pre-existing property character, actual applicable watershed boundaries, offsite influences, and master plan goals and zoning. Such factors need to be considered in setting goals and decision making rather than default to the single desktop indicator of existing impervious surface as the primary planning tool.  Such an approach must also include a baseline reference of pre-existing conditions to provide valid relative growth projections while characterizing impervious cover as effective versus non-effective.  Stream impairment relative to impervious cover is an important consideration but using this approach as the primary basis of sweeping regulatory decisions is overly simplistic.

Andrew Der

Whitman Requardt & Associates, LLP

Andrew Der is an Associate and Director of Environmental Services at Whitman Requardt & Associates, LLP head office in Baltimore and practiced in the consulting industry since 2001 previously completing 17 years of service at the Maryland Department