As it was in pre-development conditions, a restored stream network in 4MR should permit the safe conveyance of runoff from the entire contributing watershed. Outside of the park area, the neighborhoods are currently drained by the combined sewers: contributing to combined sewer overflows, basement backups and overland flooding within the Run. A priority goal of this project is to prepare the area with an alternative drainage network so that storm runoff need not become mixed with sanitary sewage, reducing the frequency and volume of combined sewer overflows, and relieving the pressure within the sewer system that causes backups. While the scope of this project does not include any designs or interventions for areas outside of the park, it is crucial to anticipate the expected flows that would result from future stormwater separations upstream.
Our current method of managing stormwater in developed areas is to connect roof downspouts directly to the combined sewers, and to capture street-level runoff in street inlets. Large pipes connect these areas to major trunk lines in the valley. During dry weather, sewage from the trunk lines is discharged to ALCOSAN’s “interceptor system,” which then conveys the waste for treatment at ALCOSAN’s plant on the Ohio River. During wet weather, the interceptor system is overwhelmed and mixed sewage within the trunk line is released to the Monongahela River through the M29 outfall. Furthermore, because the piped connection to the river is also overwhelmed, pressure in the pipes is released in the Run through manholes, street inlets and basement drains. By diverting upstream stormwater contributions away from the piped system for management within a stream network, these unsightly and unhealthy overflows can be reduced or eliminated.
The success of this effort depends highly on the ability to separate rooftop and street runoff from enough of the contributing area, safely convey the runoff to local collection points, and then establish new connections to the park for discharge to the new stream network. Retrofitting along these lines is sometimes tricky, and requires considerable collaboration with property owners and residents. Furthermore, keeping runoff out of pipes generally requires that the runoff be allowed to flow within new surface networks in the public right-of-way. This can be accomplished by a number of techniques, including the use of bioswales (planted channels) along roadsides or median strips, or enhancement of the curb and gutters in a way that allows for water to be conveyed within a narrow channel at the edge of the roadway. If the road slope is sufficiently steep, runoff can be conveyed approximately six to eight blocks before flows exceed the capacity of the surface network.
This implies the possibility of separating sub-catchment areas with a radius of six city blocks around a collection point, and then connecting to points of discharge within the new stream network in the valley. Sometimes making that connection across park or developed areas is in itself a challenge, requiring installation of new dedicated storm sewer pipes or repurposing existing combined sewer pipes that provide redundant services within the existing network.
The team has been assessing the viability of applying these practices within the 4MR watershed. While no specific plans will be developed for upstream areas, we have been identifying developed areas within the watershed where conditions are right for stormwater separation. Criteria include street slope and width, availability of establishing a point of collection with some storage to dissipate the energy of flowing water, and making the final connection to the stream network. Cost plays a large role in this assessment, as well as the willingness of local stakeholders to consider the use of these strategies in their neighborhoods. Up to now we have identified four primary areas within the watershed where separations are technically feasible; two in Squirrel Hill, one in South Oakland, and one in Greenfield. Additional areas could also become separated but at greater expense and effort. Our recommendation is to first address these four areas that are within easy reach, and plan for subsequent expansion in the future. If these efforts are to be successful, public involvement will be crucial, as property owners will be expected to disconnect their downspouts and direct them to the right-of-way channels or bioswales, close area drains, and (where feasible) provide storage or detention of runoff to help attenuate peak flows.
An application of this method is being developed now by PWSA for use at a pilot project at Wightman Park. Runoff from areas uphill of the park will be maintained on the surface and conveyed within the right-of-way for discharge to a detention facility within the park.