Philadelphia’s Green Stormwater Infrastructure Landscape Guidebook

by Caitlin Glagola, Associate ASLA; Tim Linehan, Associate ASLA; and Rachel Streit

PWD’s GSI Landscape Design Guidebook / image: Philadelphia Water Department

The Philadelphia Water Department (PWD) has one of the most progressive stormwater management plans in the country to address the city’s combined sewer infrastructure. PWD’s Green City, Clean Waters program, which begins its 7th year this July, has constructed more than 600 stormwater management practices (SMPs) in the city, including rain gardens, tree trenches, stormwater planters, and stormwater bumpouts. These stormwater landscapes, collectively known as green stormwater infrastructure (GSI), slow, filter, and infiltrate rainfall to help prevent polluted runoff from entering the city’s sewers and waterways. GSI is versatile and fits into the urban fabric of Philadelphia to not only manage stormwater but also to mitigate urban heat, improve air quality, provide habitat, improve human health, increase land value, and improve quality of life for city residents.

Projects installed have been closely monitored to assess system functionality, plant health, resiliency, and overall aesthetic qualities. In the various SMPs, PWD has observed that species vary in their tolerances to moisture levels, concentration of pollutants, as well as sediment and trash. Each design considers these factors by establishing upper zones, lower zones, and entrance zones within a SMP. The boundaries for these hydrologic zones will depend on the designed maximum ponding depth, steepness of the side-slopes, frequency of inundation, and infiltration rate. / image: Philadelphia Water Department

The page above shows some of the approved plants that PWD uses on GSI projects. Each species includes an image of the plant, its botanical name, its sun/shade requirements, and its ability to tolerate different hydrologic zones, as observed by PWD. / image: Philadelphia Water Department

In February 2018, PWD published the GSI Landscape Design Guidebook (version 3.0), which shares important lessons learned and success factors for designing and implementing stormwater landscapes. Informed by PWD’s field-based observations, plant survivorship monitoring studies, and feedback from maintenance crews, the guidebook is a wealth of knowledge for anyone looking to design GSI, especially in the public realm. The guidebook includes criteria for site assessment and plant selection, guidance on addressing different hydrologic zones, consideration of site conditions such as sunlight and available space, as well as design recommendations and guidelines. Also included is PWD’s recommended plant list for GSI, along with important plant characteristics to consider when designing (i.e. seasonal interest and tree canopy form and density). Case studies are presented with their planting plans and before and after photographs to showcase successful PWD projects.

The above image showcases SMPs that PWD frequently implements across the city. Each SMP type has its varying opportunities and challenges that should be considered when designing each system. / image: Philadelphia Water Department

PWD’s Green City, Clean Waters program has plans to build over 9,564 greened acres of GSI throughout the city of Philadelphia by 2036. A greened acre refers to an acre of impervious cover that is retrofitted to utilize GSI to manage stormwater. In October 2017, PWD celebrated the 1,000th greened acre created by Green City, Clean Waters.

This case study features a rain garden in a Northwest Philadelphia park. The community desired colorful plantings in a well-defined and intentional landscape that preserves sight lines through the space. Ribbons of color were incorporated in the mid to upper slopes of the system, while maintaining a simpler basin bottom with mostly grass species. Existing trees in the park were preserved and incorporated into the overall planting scheme for the space. / image: Philadelphia Water Department

To view PWD’s GSI Landscape Design Guidebook, visit: http://documents.philadelphiawater.org/gsi/GSI_Landscape_Guidebook.pdf

by Caitlin Glagola, Associate ASLA, Ground Reconsidered (Consultant to PWD) Caitlin.Glagola@phila.gov;
Tim Linehan, Associate ASLA, Philadelphia Water Department (PWD), Timothy.Linehan@phila.gov;
Rachel Streit, CHPlanning Ltd. (Consultant to PWD), Rachel.Streit@chplanning.com