Climate benefits of Green Stormwater Infrastructure Revision as of 20:17, 23 May 2018 by Mtrojan (talk | contribs)(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff) Climate benefits of Green Stormwater Infrastructure

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Many of today’s stormwater treatment practices (permeable pavement, tree trenches, bio-retention, swales with check dams, green roofs, sand filters, re-use, constructed ponds) can be designed, constructed and maintained to adapt to climate change. Certain adjustments will need to be made during design and maintenance in order to account for the fluctuation in climate, most notably vegetation selection in vegetated BMPs and materials and design specifications in permeable pavements. Since climate change is largely believed to occur slowly over a long period of time many key adaptation issues related to specific BMPs will depend not on how to deal with potential impacts but instead on when to modify design and maintenance procedures.

Permeable Pavement

Climate change impacts are projected to occur slowly over a long period of time, including changes in precipitation patterns and average temperatures. As a result, there are no immediate changes to the current design, construction, or maintenance of permeable pavement. However, over time as climate patterns change, changes to materials and design specifications may need to be made to adapt to a new climate regime such that pavement does not fail under the expected vehicle/pedestrian load. The Federal Highway Administration (FHWA) recently outlined a study by Washington State that “shows that for a majority of well-constructed, high-volume asphalt pavements, rehabilitation is eventually triggered by rutting distresses, while for most low-volume, pavements, rehabilitation is eventually triggered by cracking distresses⁹. In general, changes in these rends over time may be influenced by climate change and may instigate a strategic change to more cut resistant materials, such as stone matrix asphalt (SMA) and polymer-modified binders in surface courses.” The FHWA has compiled a table outlining climate change adaptation and pavement design-temperature items that can be useful for future design considerations.

Permeable pavement can also be used to reduce stormwater runoff volumes as it is estimated to reduce storm-runoff volume by 70-90%¹⁰. Anticipated climate change conditions project more frequent or intense rains, leading to larger runoff-volumes over a shorter period of time. Permeable pavement can help manage and mitigate the change of rainfall events by slowing runoff leading to flood prevention and reduction in municipal pumping demand and energy costs.

Tree Trench

Trees help intercept and filter stormwater runoff in urban settings and help prevent flooding, provide wind breaks, and help mitigate the heat island effect through shading and evaporation. A typical medium-sized tree can intercept as much as 2,380 gallons of rainfall per year¹⁰. Trees also intercept rainfall during precipitation events that can reduce runoff rates. A study in Sacramento, CA showed that evergreens and conifers intercepted over 35% of rainfall and reduced runoff by 7%. In Oakland, the tree canopy intercepts approximately 4 inches of rain, of 108,000 gallons of water, per acre in a typical year. In certain areas, trees can reduce runoff up to 17% in urban areas.

Trees allow for shaded cover and can reduce surface temperatures considerably. Trees absorb between 70-90% of sunlight in summer and 20-90% in winter. A study showed trees are able to reduce surface temperatures of roods and walls of buildings between 50-70 degrees Fahrenheit. Trees planted around houses can result in an annual cooling energy savings around 1% per tree while annual heating energy use decreased by almost 2% per tree¹⁰.

Bioretention

Studies show that bio-retention cells reduce peak flow by at least 96.5% for small to medium-sized storm events. Denser vegetation in bio-retention cells can increase rainwater retention while greater biomass and plan productivity are associated with greater evapotranspiration losses¹¹.

The University of New Hampshire Stormwater Center 2007 Annual Report documented an 82% and 85% removal of average peak flow in bio-retention cells with a 30 inch and 48” soil base, respectively¹⁰.

Bio-retention basins also reduce the heat island effect and when situated adjacent to buildings can help reduce energy demand for cooling. However, more research must be done to quantify the reduction of energy created by the construction of bio-retention basins.

Swale with Check Dam

Vegetated Swales with check dams help reduce stormwater runoff volumes and aid in heat reduction and energy conservation. The University of New Hampshire Stormwater Center 2007 Annual Report documented a 48% removal of average peak flow in a vegetated swale¹⁰.

In Portland, Oregon, vegetated swales were installed on street project and resulted in a reduced peak flow from a 25-year storm event (2 inches in 6 hours) by 88%, protecting local residents from flooding and a reduction of flow into local sewers by 85%.

Swales also reduce the heat island effect areas and help reduce energy demand for cooling when situated adjacent sidewalks and streets in urban. However, more research must be done to quantify the reduction of energy created by the construction of vegetated swales.

Green Roofs

Green roofs can help reduce energy savings and electricity costs from air conditioning demands and reduce stormwater runoff from buildings. Several studies show that energy savings from green roofs range between 15-45% of annual energy consumptions – mostly from lowering cooling costs¹⁰. It is estimated that green roofs can reduce annual stormwater runoff by 50-60% on average, including peak runoff. Vegetated roofs control between 30-90% of the volume and rate of stormwater runoff, while detaining 90% of volume for storms less than one inch and at least 30% for larger storms.

Constructed Wetlands

Wetlands have been proven valuable as a first line of defense against large storm events and rising sea levels along coastal areas. Following Hurricane Katrina in 2005, the State of Louisiana and the City of New Orleans implemented a wetland restoration project to help protect populated coastal areas from rises in sea levels, hurricanes and river flooding. Wetlands are considered a buffer between the Gulf of Mexico and New Orleans and increase the City’s resiliency to natural disasters¹². Restoration project such as New Orleans’ involves a combination of restoration of natural delta building, marsh creation from use of dredged material, water control structures, and hard structures. Upon completion, the restored wetlands will offer storm surge protection to densely populated areas. Wetlands also are effective in reducing peak flow rates and flooding in inland urban settings.