Watershed Development & Forests: How Much Is Too Much, Too Little?

ChicagoRivershutterstock_146960165If you’ve been depending upon stormwater management, sediment control or other protection measures to preserve your favorite waters from development impacts, then you’re likely in for disappointment.  While these measures definitely provide benefits, they are insufficient to protect sensitive waters from damage.  So far only limits on watershed development intensity and preserving a minimum amount of forest has been successful in maintaining healthy  streams, rivers, lakes and tidal waters.  However, if we watershed advocates can find a way to be far more effective in supporting new technologies like Low-Impact Development and Environmental Site Design, then we may reap the benefits of growth while keeping our waters safe for kids and aquatic ecosystems.

How Much Development Is Too Much

Beginning with a scientific paper published by the author in 1979, a large number of studies have documented the decline in aquatic resource health as a watershed is transformed from rural to suburban and urban.  Most studies use percent impervious area to quantify the intensity of watershed development.

Impervious area includes buildings, streets, parking lots and anything else that prevents rain from soaking into the earth.  By preventing rain from infiltrating the soil, impervious surfaces greatly increase the volume of runoff which makes downstream flooding worse and scours stream channel to a width two- to eight-times greater than predevelopment.  A long list of pollutants settle upon impervious surfaces then wash into nearby waterways come the next storm.  The loss of infiltration reduces the volume of groundwater flowing into wells, streams and wetlands during dry weather.  It is the cumulative effect of each of these impacts which causes aquatic resource health to decline as watershed development increases.

IA GraphA study conducted in the Chesapeake Bay watershed documented the relationship between stream health and the percent of a watershed covered by impervious surfaces.   The aquatic life inhabiting numerous streams was sampled to assess aquatic resource health, which was then rated as excellent, good, fair or poor.

An excellent waterway supports all human uses and highly sensitive fish like brook trout and other salmonids.  A good quality stream is still fit for most uses but may have lost the more sensitive species.  A fair quality stream no longer supports game fish and is unfit for swimming, though wading may still be OK.  A poor quality stream is suited for no uses and has little aquatic life.

Most water quality protection programs seek to preserve good quality conditions.  Excellent quality is set as the goal when uniquely sensitive and important resources are at risk, like trout or endangered species.  The June 27th article on Kid-Safe Waters explains why fair quality should be the minimum acceptable for suburban-urban waters.

IAB&WThis table shows the point at which increasing watershed imperviousness begins to threaten various sensitive resources and human uses.  The threshold is expressed as percent of watershed covered by impervious surfaces.  The equivalent housing density is provided to put the percentages in perspective.

While one might assume that construction phase sediment control and post-construction stormwater management would allow watershed growth to exceed these impervious thresholds, this simply isn’t the case – at least not yet.  One study indicated that these conventional measures might lower impact from that of a watershed 19% impervious to the equivalent of 16%.  In other words, there would be no significant difference in quality.  Following are the reasons why conventional sediment control and stormwater management does not fully protect our waters.

Exposed Soil = Pollution

Far too many construction sites fail to comply with requirements to prevent erosion by blanketing disturbed soil first with straw mulch then grass.  Perimeter controls like black silt fence and ponds simply cannot keep enough soil on site to protect sensitive waters.  Mulch and grass reduce erosion by 90% to 99% whereas perimeter control might trap a third to half the eroded soil.  Fortunately, all Chesapeake Bay watershed states require mulching-seeding within 3- to 30-days of initial disturbance.  Unfortunately, compliance is generally poor.  So, whenever you see exposed soil on a construction site you can assume nearby waters will be polluted come the next rain and that the site is probably in violation.  For further detail see: Exposed Soil = Pollution: How You Can Save 100 Feet of Chesapeake Bay Tributaries in an Hour by Halting Construction Site Mud Pollution

Conventional Stormwater Management Doesn’t Mimic Forest

WP-FullCapacityThere maybe 100,000 stormwater facilities throughout the Chesapeake Bay watershed,  Most of these facilities are ponds, like the one to the left.  A pond removes a third to half of runoff pollutants whereas a forest floor removes 85% to 95%.  A pond allows little water to infiltrate into underlying soils to provide dry-weather groundwater inflow to nearby streams and wetlands whereas about half the rain falling on a forest becomes groundwater recharge.  Replacing a forest with homes can cause flooding to increase a hundred fold.  While ponds can control the height of floodwaters they are poor at protecting channels from the erosion.  It appears that in many localities even these minimal benefits are diminished due to poor maintenance.  For further background see: Auditing Chesapeake Bay Watershed Stormwater Best Management Practices

In summary, conventional stormwater technology cannot provide the same benefits as a forest.  But Low-Impact Development and Environmental Site Design practices are designed to do precisely that.  The problem is that LID and ESD are just being implemented and it may take 20 years to tell how well these approaches work in truly reproducing forest benefits and protecting our waters.  So in the meantime we need to place greater emphasis on limiting impervious area and preserving sufficient forest to safeguard human uses and sensitive ecosystems.

How Much Forest Is Enough?

A Pacific northwest study determined that a watershed could still support a healthy fish (salmonid) population with an imperviousness of up to 10% provided a minimum of 65% remained in forest.

ForestThe Chesapeake Bay watershed study referenced earlier documented the relationships depicted in the two graphs to the left.  The graphs show the amount of watershed forest and the extent of streamside forest buffer associated with each stream health rating.  To have an Buffersexcellent quality stream, about 51% of the watershed should be covered with forest and a 100-foot thick buffer of trees should extend from both banks along 77% of the streams draining the watershed.

From the table above, we know that if the goal is to achieve the excellent conditions needed for a healthy trout population then no more than 4% of the watershed can be covered with impervious surfaces.  We also need to ensure that a 100-foot forest buffer extends along at least 77% of the streams draining the watershed.  To preserve healthy fisheries in tidal waters impervious area should be 5% or less.  It is also likely the fishery would do best with a minimum of 51% forest cover.

Low-Impact Development & Environmental Site Design As A Panacea?

While LID, ESD and other innovative approaches offer great promise, all are in the early stages of implementation.  The long term effectiveness of these approaches remains to be seen.

It takes about twenty years to develop enough of a watershed with innovative measures to assess effectiveness.  But even in the early years of ESD and LID we see a wide range in the implementation of both.  In some areas local officials require that development proposals make full use of the highly-effective LID/ESD practices.  In other jurisdictions projects appear to get approved with essentially no LID or ESD practices, despite laws to the contrary.

West County BioretentionThere’s another major issue with LID and ESD which remains to be addressed.  LID/ESD practices, like the bioretention facility to the left, tend to drain a much smaller area than the ponds which dominated stormwater management pre-LID/ESD.  Previously a typical 20-acre project might have two or three ponds or other practices, but with LID/ESD the same site might need 20 or 30 practices.  We were having difficulty maintaining an inspection staff large enough to keep the old practices working properly.  No one has a good answer yet for how we’re going to inspect and maintain the explosion in practices we’ll soon see as more and more LID/ESD sites come online.

Another limitation with regard to the Maryland ESD approach is that while the state law and regulations clearly require an effort to preserve sufficient forest to protect sensitive waters, on most sites only 15% to 20% remains in forest.  As shown in the graph above, a minimum of 30% of a watershed must be in forest just to maintain poor stream quality.  Maryland officials need to consider innovative ways of encouraging applicants to explore options for preserving more forest onsite.

Until LID/ESD Are Proven, Think Land Use Limits To Preserve Sensitive Waters

The photo at the beginning of this article is of the Chicago River.  While no one would suggest an impervious area limit for this lower portion of the river, it would make sense for rural tributaries which remain healthy and provide the high quality inflow critical to making the Chicago River an asset to all that high-priced downtown real estate.

Montgomery County, MD has been using such an approach since 1994.  Keep in mind that Montgomery County adjoins Washington, D.C. and is the 41st most populous county in the nation.

Since 1994, Montgomery County has established four Special Protection Areas which occupy about 10% of the county.  Three of the SPAs have impervious area limits of 8%, 10% and 15%, though there is a movement to reduce the 15% limit to 6%.  The areas with 8% and 10% limits were established to protect self-sustaining trout populations.  Today, nearly two decades after the SPAs were established, both areas still support trout despite a considerable amount of development.  The County also requires the use of highly-effective sediment and stormwater measures backed by very effective enforcement and maintenance.

The Montgomery County SPA approach would be a good model to consider for sensitive waters of concern to you.  Hopefully we watershed advocates will begin providing government with the public support key to making LID and ESD work, which would reduce the need for land use limits.  In the meantime, the only reliable approach for preserving human uses and sensitive aquatic ecosystems is to limit impervious area while retaining minimum amounts of watershed forest and streamside buffers.

Master Plans, Zoning & Impervious Area

Preserving our remaining high quality waters will be far easier if master plans and then zoning is adjusted to ensure impervious area limits are not exceeded in the watershed of our most sensitive waters.  But this would also be a valuable tool in suburban areas where there’s some evidence indicating streams remain safe for human contact (childhood play) up to about 13% impervious area or an average of one house per acre.  Maryland has a planning process which seems ideally suited for this purpose – the Water Resources Element.  But most plans examined impervious area at the level of very large watersheds.  Seldom does impervious area rise to critical thresholds when dealing with such large areas.  Instead the analysis should have focused on the headwater (1st order) stream level.  Fortunately, most WREs will be due for a six-year update in two years (2015) so watershed advocates will have another excellent opportunity to employ one of THE most effective mechanisms for safeguarding aquatic resources.

How You Can Help Hasten the Day When LID/ESD Becomes Reliable

We all know that watershed groups have a tremendous amount of work to do with severely limited volunteers, staff and funds.  While most groups are active in policy areas – advocating for new laws and programs – few review recently approved development plans to verify that the most effective aquatic resource protection measures are being required.  And even fewer seem to get out in their watershed to verify that these measures are properly installed and maintained.  Both of these tasks can be carried out without creating enemies or trespassing on private property.  Until more watershed advocates become active in monitoring ESD plans and maintenance, it is unlikely we’ll see significant improvements.  Detailed advice on both tasks is provided in the following CEDS publications:

For further background on Environmental Site Design visit: ceds.org/esd

Also, consider a Watershed Audit to quickly reduce pollution inputs to your favorite waterway: ceds.org/audit

If you have any questions, contact CEDS at 1-800-773-4571 or Help@ceds.org.


About Richard Klein

President of Community & Environmental Defense Services (ceds.org)
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