Project Abstract

Los Angeles is a highly populated and intensely urbanized landscape. Large storm events in impervious urban areas such as Los Angeles can result in stormwater runoff problems including water pollution and beach closures. Green roofs can alleviate some of these problems by intercepting precipitation and thus reducing the volume of runoff. This study addresses the environmental and economic value of green roofs as a stormwater solution in Los Angeles.

Drawing on prior literature and expert opinions, the effect of green roof vegetation types, roof depth, and growing substrate on stormwater runoff from an individual roof was modeled using actual Los Angeles climate data. The model outputs reveal that green roofs could remove between 21 and 64 percent of stormwater runoff, and that they perform best in intermediate size storms. Sensitivity analysis indicates that each individual roof parameter value is highly significant (p≤0.001 or p<0.01) in stormwater attenuation and roof performance. Results were extrapolated to the Los Angeles River watershed using GIS and aerial imagery to determine potential area for green roof application.

A cost-benefit analysis determined that costs currently significantly exceed benefits from stormwater reduction, although recent policy advancements could improve the cost-benefit ratio. Our analysis indicates green roofs provide measurable environmental benefits.

Project Objectives

This project:

• investigated current impacts of stormwater related to non-point source pollution and related public health issues in coastal waters, and
• determined effective vegetation types and growing media for use on Los Angeles area green roofs, concentrating on local sources and suppliers.

The team used a two-pronged approach to address stormwater attenuation:

1.   We applied the existing Earth Pledge Smart Stormwater 2.0 Model, a green roof model, at a single-roof scale for use in the semi-arid Los Angeles River watershed.

·    Incorporated multiple vegetation, growing substrate, and precipitation regimes into sensitivity analysis to generate range of model outcomes with the goal of maximizing stormwater runoff attenuation.

·    Contrasted our results with results from green roof applications in other urbanized settings.

2.      We extrapolated the single-roof model results to the entire Los Angeles River watershed.

·    Used a multiplicative approach to expand from a single roof to green roof application on the flat roofs of commercial and industrial buildings.

We utilized cost-benefit analysis to assess the economic impacts of green roofs and recommend appropriate policies for green roof use in the Los Angeles River basin.

·    Established baseline cost per square foot metric for extensive green roofs incorporating pertinent construction, installation, infrastructure, and maintenance costs.

·    Calculated benefits resulting from green roofs on two scales.

o    Determined public benefits resulting directly from stormwater attenuation by evaluating reductions in costs associated with illnesses and loss of beach use due to coastal water pollution.

·    Based on economic analysis, recommended policy directions for individuals, organizations, and city officials interested in expanding green roof coverage in Los Angeles.