Green Infrastructure: Landscape features for stormwater management


Stormwater management in the landscape

In a pre-development landscape one of 3 things happens to rain water that falls on the site.

  1. Evaporation or transpiration can return the water to the atmosphere. Evapotranspiration (ET) in this climate (the Mid- Atlantic) will return about 50-60% of the annual precipitation to the atmosphere.
  2. The precipitation can sink into the ground ultimately resulting in groundwater recharge. This accounts for 30-40% of the annual precipitation in most of Pennsylvania.
  3. Excess water can runoff the site. This generally accounts for only a small portion of the precipitation in an undeveloped site, maybe 10% of the total annual precipitation at most. In many undeveloped sites no runoff occurs.

Once you develop a site there is an increase in runoff to 80-90% for impervious surfaces. Compacted turf and developed landscapes will also have more runoff perhaps as much as 50% of the annual precipitation depending on the site. Increased site runoff not only causes surface runoff and pollution problems but can overwhelm existing stormwater drainage systems causing flooding.

The problem with combined storm and sewer systems.

Fixing the CSO problem

 Pittsburgh Water and Sewer Authority (PWSA) has over 7 million linear feet of sanitary and combined sewer pipes (1/3 of the ALCOSAN system) 

The cost to solve the overflow problem in the total ALCOSAN service has been estimated at somewhere between $10 and $50 billion.

The problems and the costs of the solutions are the same for most cities in North America. Because of the problem, many municipalities and regulatory agencies are looking for natural and distributed green infrastructure solutions to the problem

The Pennsylvania Stormwater Best Management Practices Manual. Publication # 363-0300-002

This manual published by the PaDEP is available on the web.

Click on the link above to follow the link to the manual and skim through the sections. In lab we will be focusing on 3 areas covered in the manual. Read these sections of the manual in preparation for lab.

  1. Non-structural BMPS
  2. Green Roofs
  3. Rain gardens

Green Roofs

The Benefits

Green roof on the Forest Resources Building

A green roof can provide many benefits for a developer, a building owner, the building occupants, and the municipality where it is built.

Aesthetics, habitat and biodiversity: A green roof is an attractive feature that can improve occupant satisfaction. Green views have been credited with improved patient outcomes in hospitals and improving employee moral in commercial buildings. A green roof can often be more attractive than the surrounding landscape and can provide critical urban habitat for pollinators. 

Stormwater BMP:  Research shows that in temperate North America a green roof can restore evapotranspiration to pre-development levels, reducing runoff by 50-60% over the course of a year for a 3.5 to 4” thick roof. A green roof can also delay runoff and in some instances improve runoff quality. For example in parts of North America where acid rain is an issue a green roof will neutralize the acidity in stormwater runoff.

Energy savings and urban heat islands: A green roof functions as an evaporative cooler and can reduce air conditioning demand in a building reducing summer cooling 10-20%. This results in peak energy demand reductions and lower urban summertime temperatures.

Increase roof life 2-3x: A green roof protects the waterproofing underneath from UV and moderates temperature induced expansion and contraction.

Design Considerations 

The building and roof deck must be able to support the added dead load of the green roof. For a simple extensive roof using lightweight media this will be about 7.5 lbs per square foot of roof per inch of media depth. For intensive roofs with heavier media and larger plants this could be much greater. Roofs with a slope greater than 2 in 12 may require supports to stabilize the roof system.

The waterproofing must be chemically stable when wet and should be root resistant. Waterproofing root resistance is tested using a 2 year greenhouse test developed by the  German FLL and adapted into a North American standard by ANSI/SPRI/GRHC (American National Standards Institute). If the waterproofing has not been tested or has failed the root resistance test an additional root barrier is required.

Drainage is needed to allow excess rainwater to flow off the roof. Drainage can be course granular material (i.e. gravel) or a synthetic layer.  Granular drainage may be lightweight aggregate like lava rock or expanded clay, slate, or shale or if the structure can support the weight, gravel or crushed brick or other similar material can be used. Aggregate may provide runoff detention benefits while synthetic egg-crate like or woven drain sheets may be lighter. In side by side comparisons of granular and synthetic drain layers on small research roofs in central PA no statistically significant differences in plant growth or annual stormwater retention were observed. The drainage should be separated from the media by a filter fabric to prevent fines from the media filling the drainage.

Media selection is critical to the success of a green roof. Considerations include weight, stability, water holding capacity, hydraulic conductivity, parent materials etc. Most commercial media combine a lightweight aggregate like expanded clay, slate or shale (70-90% by volume) with organic compost (10-30% by volume or 2-5% by weight). If the building can support greater weight less costly materials like fine gravel and sand can be used. In either case the media should be tested and meet FLL and ASTM  guidelines for the type of roof.

Plant selection depends on the design intent for the roof, the climate and microclimate, irrigation, maintenance constraints and the depth of media. Shallow (less than 5 inch deep) roofs without irrigation will have a relatively limited palate of plants that can be used including sedums and a few other very drought tolerant species. As roof media depth is increased the roof becomes more garden like and can support a broader range of plant species. Intensive roofs with media depth greater than 1-3 ft can support trees and shrubs as well as herbaceous annuals and perennials. More complicated plantings will increase maintenance and management requirements including the use of irrigation.

Examples of green roofs in North America

Follow the link below to see examples of green roofs in North America

© Penn State Horticulture  2021