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). 
The watershed as a whole is home to about 460,000 residents, but population density is much higher in and near the City of Philadelphia. About 230,000 people, or half of the watershed's residents, live in the Cobbs Creek subwatershed (PWD).

There have also been extensive channelizing and relocation of streams by engineering projects. This includes total piping, enclosure, and burial of some streams. These environmentally shortsighted practices have also contributed to increased flooding and decreased water quality. Check out these maps, below, for a before and after snapshot of Philadelphia’s streams!  Adam Levine, a historical consultant, was hired by the PWD to review Philadelphia’s watershed history and his resulting maps are quite shocking.  Mr. Levine also includes a brief history of Cobbs Creek, looking at industry and population trends since the mid-17^th century.

Pre-channelized streams

Red lines are combined sewer and stormwater systems.

PWD and the Darby-Cobbs Partnership’s proposed solutions

What to focus on first?  Over the past 12 years, the watershed-based goal-setting process initiated through IWMP development has taught PWD that the Darby-Cobbs wa­tershed stakeholders generally consider all watershed management goals of almost equal importance; there is no goal of clear “higher rank” than others (PWD’s CSO Long Term Plan). The Green City, Clean Waters program aligns with this equal prioritization by addressing all aspects of watershed management instead of focusing solely on selected in-stream water quality parameters.  These goals outlined in the Green City, Clean Waters plan are the following:

• Stream channel & banks
• Coordination
• Water quality
• Stream habitat and aquatic life
• Pollutant loads
• Stewardship
• Stream corridors
• Flooding
• Quality of life
• Streamflow

The PWD ran a cost-benefit analysis of using “green” infrastructure versus strictly building more capacity in “grey” infrastructure (pipes, treatment facilities) and found that the city can greatly benefit from green infrastructure.  Over a 25-year period, the City of Philadelphia plans to replace impervious surfaces with green infrastructure (that will infiltrate water) for a third of the city’s footprint.  Green infrastructure will reduce stormwater runoff, and consequently combined sewer overflows, and is viewed as a multi-functional tool—addressing most of these goals above.  Some green infrastructure projects have already been implemented in the Darby-Cobbs watershed.  One such project is the porous pavement basketball courts at Second Ward Park Playground in Upper Darby Township.

Second Ward Park Basketball Courts with porous pavement

For more information on Green Infrastructure, view my presentation I created to increase residents’ understanding!

Green Infrastructure and What it Means for City Residents

Some other strategies for improving the Darby-Cobbs watershed are stream restoration, stream daylighting, and monitoring stormwater management plans to evaluate their strictness.