Weather-Smithing: Assessing the role of vegetation, soil, and adaptive management in urban green infrastructure performance

HONOR AWARD

Research

Philadelphia, PA, USA | Andropogon Associates | Client: University of Pennsylvania

PROJECT STATEMENT

Landscape architects face increasing pressure to design high-performance landscapes in cities where regulatory requirements and underlying engineering models tend not to reflect the measurable capacity of green infrastructure within different contexts, particularly soil storage and evapotranspiration. Understanding how built green infrastructure performs is critical in informing new engineering models, advocating progressive regulations, and advancing sustainable landscape design. To help address this knowledge gap between science and policy, designers and scientific researchers collaboratively pursued a three-year study at a newly-built, non-infiltrating urban park. The team monitored runoff volume, water quality, soils, and vegetation to better understand the infrastructure’s performance and evaluated the long-term impacts of adaptive management. The designers modified maintenance protocols as needed to increase landscape performance. Findings reveal that the park has the potential to manage more than three times the stormwater that the engineering models predicted.

PROJECT NARRATIVE

Landscape architects experience pressure from municipalities and within the profession to design high-performance landscapes that provide multiple functions, particularly in urban areas where space is scarce. Performance data on individual green stormwater infrastructure (GSI) components has become more readily available, but little performance data exists on: 1) the integration of multiple GSI elements within a site over time; or 2) the impact of adaptive management on landscape performance. Most engineering models and current regulatory requirements do not take into account the capacity of GSI to meet performance requirements within different contexts and typically use a one-size-fits all approach. Landscape architects need field-verified information to understand how GSI projects perform over time and how to best manage the infrastructure to achieve maximum performance. This information can help landscape architects inform new engineering models; advocate for new GSI policy and regulations; and advance sustainable landscape design and management.

To understand the performance of a “treatment train” approach of interconnected GSI elements under an adaptive management program, a joint designer-academic research team deployed a three-year investigation of a 2.75-acre urban park located within an urban university’s campus. The project was designed to offer landscape monitoring curriculum opportunities and contribute field-collected GSI performance data to the greater community of professionals and regulators. By building partnerships with academic, government, and design professionals, this site-level investigation has provided temporal data that helps the research team better understand performance-based landscape design and to establish adaptive management loops to inform landscape management practices that maintain or improve performance of these systems.

WHY THIS SITE?

The non-infiltrating park leant itself to GSI monitoring for the following reasons:

• Academic Setting: The client is a university, dedicated to extending the classroom to the landscape and exhibiting robust sustainable practices on campus.
• Limited Outlet Points: Most of the park required a sub-grade liner, like a green roof, and the runoff overflow was dispensed solely through two combined sewer outlet points.
• Integrated GSI Design: The park design deployed a treatment train approach that linked engineered soils, a carefully selected plant palette, and a stormwater reuse irrigation system. The majority of water that hits the site is routed through GSI systems, including:
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• Sub-lawn sand storage bed
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• 4,700-square-foot rain garden
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• Irrigation system fueled by a 20,000-gallon cistern
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• Native plant assemblage beds
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• Continuous tree trenches
• Adaptive Management Program: The university supported an adaptive management program aimed at improving the site’s stormwater management over time. The program includes compost tea application; irrigation monitoring; use of alternative de-icing salts; soil and plant monitoring; and soil and turf remediation.
• Regional Context: The park lies within a city with an aging combined sewer, thereby underscoring the need for GSI performance research with comparative analysis to traditional engineering models that undervalue the role of high-performance soils and vegetation.

RESEARCH GOALS AND METHODS

The research aimed to evaluate the engineering model assumptions of GSI performance within an urban setting and to provide feedback to the university’s facilities managers to improve campus-wide landscape performance. The key research questions that framed the investigation were:

• How much do soil and vegetation really contribute to stormwater management?
• Can a sand-based soil and organic turf management program promote a healthy landscape, including resisting compaction and maximizing evapotranspiration?
• How can an adaptive management program influence performance?

The research team monitored stormwater runoff volume, water quality, soil, vegetation, and social use to answer these questions and understand how the site functions as an integrated GSI system. Several methods were used to analyze the park before, during, and after construction, to provide insight into changes in landscape performance and inform the management program. Cost-effectiveness and accuracy were balanced when selecting both monitoring methods and instrumentation.

FINDINGS

Key findings from the three-year landscape performance investigation include:

1. This integrated GSI system can manage at least three times more stormwater runoff than the engineering models predicted.

The engineering models required by local stormwater management regulations did not account for the park’s engineered soil (which had a high water storage capacity); high-performance vegetation (which provided significant water uptake via transpiration); or stormwater reuse irrigation system (which recycled cistern water for watering the park’s plantings). The study period’s largest precipitation event occurred on June 7, 2013, when 3.14” of rain fell on the site. No stormwater overflowed to the combined sewer during this event, even though the engineering model’s revealed a maximum of 1” rainwater capture capacity.

Adaptive Management Response:

During the study period the site almost overflowed into the combined storm sewer during seven instances and overflowed once. Six of these overflow close calls occurred during the winter, when the cistern was offline for the season (to protect against de-icing salt intrusion). Had the cistern been online during these months, the park would have been able to handle an overflow, even if slightly more water had fallen on the site. The adaptive management response to this scenario is to install devices that monitor the cistern’s salt content, so the cistern can remain functional during winter.

2. Irrigation system monitoring is critical in preventing cistern overflows to the storm sewer and for sustainable water management.

The other overflow close call and one actual overflow event occurred during dry-weather. The park’s management manual suggests 1-inch of irrigation per week, but during each of these events, the irrigation system was incorrectly programmed to release 4-inches of water.

Adaptive Management Response:

Active monitoring efforts revealed the irrigation problem and consequently provided the landscape managers with proof of overwatering. Unmanaged and mismanaged irrigation systems can lead to significant problems and contribute to unnecessary stormwater overflows to the sewer system.

3. Native floodplain species and un-compacted turf are stormwater workhorses.

Native floodplain species are particularly successful at transpiring water, even within urban settings. Using porometer measurements, the park’s young swamp white oak (Quercus bicolor) was found to transpire up to 35 gallons of water per day per tree during the peak growing season. As the trees mature, their water draw is anticipated to increase over time. Un-compacted turf is also hard at work, although transpiration and compaction are inversely proportional. These data were paired with soil evaporation measurements, taken from the site’s tensiometers, to understand the amount of water leaving the site through evapotranspiration.

Adaptive Management Response:

Preliminary data suggest that when designing GSI systems, water should be held in the rhizosphere as long as possible to let soil and vegetation evaporate and transpire as much water as possible without compromising plant health. Irrigation systems that reuse stormwater and air-conditioning condensate, and continuously cycle water through the vegetated system, can further allow opportunities for water to be transpired and evaporated.

SUMMARY

This study revealed that engineered soils, appropriate plant selection, irrigation reuse, and adaptive management significantly reduced overflows to the combined sewer system by more than three times that projected by the locally mandated engineering models. Effective advocacy for GSI policy and implementation requires more field-tested research to determine which soils and plants have the potential to manage the most stormwater under a wide variety of conditions. This research can not only help advocate for more implementation of GSI systems, but also achieve goals such as net-zero water on a site though programs such as SITES and the Living Building Challenge.

“They built research into the design process. This project is unique in that respect; it’s holistic. The data are shared; the process clearly laid out so it can be replicated and applied by other landscape architects.”

- 2016 Awards Jury

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