Stormwater Management Tool Evaluation: Porous Asphalt

There are several types of green stormwater infrastructure tools to manage urban stormwater runoff.  Tree trenches, sidewalk bump-outs, planters, pervious pavement, green roofs, rain barrels, and rain gardens are common examples. 

The Darby-Cobbs watershed has implemented some porous basketball courts, as mentioned in my last post, to manage stormwater runoff.  Water can soak into the ground slowly through the porous asphalt rather than rush to a stormwater drain and go through the combined sewer system.  This helps replenish groundwater, prevent flash flooding, and protect rivers and streams within a watershed. 

Porous asphalt, pervious concrete, and interlocking concrete pavers are the three main types of pervious pavement.  The following is a review of porous asphalt as a practice to manage stormwater runoff.  

Porous asphalt (Source: National Asphalt Pavement Association)

Problems troubling the Darby-Cobbs Watershed and Solutions for Cleaner Water

There are many cited problems within the Darby-Cobbs watershed.  Some identified problems are described in the Cobbs Creek Integrated Watershed Management Plan, Darby Creek Watershed Conservation Plan, and the Darby-Cobbs Stormwater Management Plan (as part of PA Act 167):

• Sometimes during dry weather periods, bacteria contamination of the Cobb’s waters prevents achievement of water quality standards that would support swimming or other forms of primary contact recreation in the creek;
• There has been illegal litter and dumping, trash from stormwater discharges, and bank deterioration along the stream corridors;
• Urban runoff deteriorates aquatic and riparian habitat from flash flooding and polluted waters caused by combined sewer overflows after heavy rainfall;
• Impervious areas also reduce the baseflow of streams, which is imperative for aquatic life during drier summer months.

Many of these problems are, in part, from its historic development.  It is an urbanized watershed where development has often occurred at high densities.  Increased impervious (paved) surfaces causes flash flooding, especially in the lower watershed, that erode stream banks, scour away the natural pools and riffles critical to aquatic biota (Darby Creek Watershed Conservation Plan, 2005). 

Assessments, Plans, and Current Data Collection

Plans and assessments for the Darby-Cobbs watershed are a little dated—most are about a decade old.  The key documents that the PWD and its partners use to implement progress towards improving the watershed include the following:

• 2001 Trail Master Plan for Cobbs Creek Park
• 2004 Comprehensive Characterization Report
• 2004 Integrated Watershed Management Plan (IWMP)
• 2004 Stormwater Management Plan, Act 167
• 2005 Darby Creek Watershed Conservation Plan
• 2006 Cobbs Creek Wetlands Assessment

Leadership in the Darby-Cobbs Watershed

The Philadelphia Water Department (PWD) initiated the Darby-Cobbs Watershed Partnership (DCWP) and continues to financially support this important effort to unite watershed stakeholders in a variety of ways.   Philadelphia lies at downstream of their suburban neighbors in Delaware, Chester, Montgomery, and Bucks counties and watershed-wide solutions require coordination these municipalities.  The PWD notes, “without their cooperation and commitment to take similar protection measures, the efficacy of [the PWD’s] efforts would be limited” (PWD 2011).

Defining the Darby-Cobbs Watershed

Darby Creek originates in Easttown Township in Chester County and flows in a south/southeast direction through most of the watershed.  It changes direction in the southern portion of the watershed where it flows west/southwest direction until it discharges into the Delaware River between the Townships of Ridley and Tinicum. 

Darby Creek’s watershed is often referred to as the “Darby-Cobbs watershed” since its largest tributary, Cobbs Creek, is approximately one third of the Darby Creek watershed. The Darby-Cobbs watershed has a total area of 77.2 square miles [Source].

The major tributaries of Darby Creek:

• Cobbs Creek
• Little Darby Creek
• Julip Run
• Ithan Run
• Meadowbrook Run
• Wigwam Run
• Foxes Run 
• Muckinipattis Creek 

Darby-Cobbs Watershed

Although the Schuylkill Watershed is really important, as its size affects many people and wildlife habitats, I've moved my focus just west to study the Darby-Cobbs Watershed.

I have found a surprising wealth of information through the Philadelphia Water Department and Philly H2O, a site displaying the findings of a history consultant's look into the watershed history of the city. The Philadelphia Water Department (PWD)'s website is a great compilation of work that has been done and current status of the watershed.  It has links to its partner organizations and many reports that have been created.

A Redirection

After spending a few weeks exploring the Anacostia watershed, I’ve decided to change my focus to another geographic area.  I found that the AWS had a great program and citizen engagement and DC’s stormwater regulations paired with DC Water’s construction of sewage storage facilities will get the river back on track to better water quality. 

I have an interest in the Philadelphia region and may find myself living and working there in the future, so I’m now looking into any gaps in information, leadership, services, and planning initiatives in the Schuylkill watershed, one of the main rivers that flows through Philadelphia.  Philly also has a combined sewer system (where sewage and wastewater mix and are both treated at wastewater treatment plants), and is focusing on reducing stormwater runoff to prevent contaminated wastewater overflow into waterways.  My analysis will include some spatial data in ArcGIS. 

Image from 2001 Schuylkill Watershed Conservation Plan, http://www.schuylkillplan.org