Stormwater Library

resources for stormwater professionals

Archive for Design Resources

Compost-based BMPs – EPA Presentation

One of the better presentations that I have come across on the use of compost in stormwater BMPs was done by Chris Newman of the U.S. EPA. Mr. Newman provides some background into the EPA’s initiative to promote compost-based BMPs and points audience to its fact sheets database.

The presentation is a very nice primer on the benefits, effectiveness, and limitations of using compost blankets, compost filter socks, and compost filter berms. He also goes into some detail about the quality standards and certification required for the use of compost products.

Some of the key benefits highlighted in the presentation include:

  • ability to prevent rill erosion
  • runoff volume reduction
  • promoting establishment of vegetation
  • improving water quality through adsorption of nutrients and pollutants

A brief survey of some of the companies out there installing compost-based BMPs illustrated how creative designers can be with the products. Of particular note is the use of compost filter socks, traditionally used in the same manner as straw wattles, in the creation of permanent stream banks, or even as an alternative to rip-rap.

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LID Implementation in Low Infiltration Areas

One of the questions that comes up quite often in the research about Low Impact Development is how to implement the design when you have naturally low infiltrating conditions (i.e. low permeable soils or shallow groundwater/bedrock).  In design scenarios like these you are inevitably going to be faced with major drainage issues, ponding, and vector control problems.  Using the State of California as an example, ponding present after 4 days is going to be the limiting factor on your BMP design.

Daniel Apt of RBF consultants presented a nice discussion on this very subject in 2008.  Mr. Apt summarized the measures to help reduce runoff from these sites as follows:

  • Reduce/Minimize Total Impervious Areas
  • Minimize Directly Connected Impervious Areas
  • Limit use of sidewalks
  • Reduce road/driveway length and width
  • Modify/Increase Drainage Flow Paths
  • Maximize overland sheet flow
  • Conserve natural areas
  • Minimize disturbance
  • Preserve infiltratable soils
  • Preserve natural depression areas
  • Preserve vegetation

The presentation goes into the benefits of using measures such as green roofs, rain barrels, cisterns, bioretention strips, and grassed swales as possible ways to working around infiltration issues on a site, while still drastically reducing offsite runoff.

Review the power point presentation in its entirety here.

Choosing your words: A thoughtful approach to changing how others view stormwater

Understanding who the stakeholders are is quite simply the most important part of the sustainable stormwater discussion. One of the more impactful and concise tools in my search for useful storwater resources is called The Language of Change. Essentially, this brief document helps you frame the green stormwater argument in terms that each of your stakeholders can understand. By choosing your words properly, you improve the chances of connecting with your audience.

Starting with the notion that community stakeholders tend to view stormwater as “dirty”, “a nuisance”, or “threatening”, the document touches on using “rainwater” as a synonym. Stormwater is typically viewed as something to be diverted and hidden away in its pipe until reaching an unseen destination. When the traditional system of handling it is overloaded, then its nuisance or threating side comes out in the form of flooding. Rainwater, however, is viewed as something that can be collected, spread out, harvested, and reused. A simple change in wording, can transform the idea from waste product into valuable and renewable resource.

Getting buy-in from the different stakeholders is critical to the process. Regulators are going to respond more keenly to traditional terms like “TMDLs”, Park & Recreation Districts to “open space”, and engineers like myself will be wooed by “infiltration”. Choosing your words carefully, helps to make the case to each member of your audience.

The document is part of a much larger set of tools organized by the Water Environment Research Foundation at its sustaiable stormwater BMPs website Using Rainwater to Grow Livable Communities. There you can find resouces dedicated to each of the stakeholders, as well as case studies, and a comprehensive toolbox.

Vegetated Swales (Green Street Series)

When designed and constructed properly, vegetated swales can be one of the most effective BMPs available in the stormwater engineer’s toolbox. Due to its linear structure, it is quite effective at treating road runoff in residential areas. However, if it is to be incorporated into an urban street setting with large impervious areas, an important factor to keep in mind is that it should either be constructed in series or be used as a pretreatment feature ahead of other BMPs. Some of the benefits of vegetated swales include:

  • ability to improve water quality by reducing sediments and pollutant loads
  • ability to slow runoff velocities and promote infiltration
  • more cost-effective and easier to maintain than curb and gutter system

The key is proper design. Attention must be given to soil profile – in particular, depth to groundwater, depth to bedrock, and permeability of underlying soils. Following a storm event, a swale should be fully drained within 24 hours, so promoting infiltration is critical.

Another important factor in design is the selection of proper vegetation. Native plants and grasses should be selected to ensure long-term success of the swale. The vegetation acts as an impediment to flow in the swale. By slowing down the runoff, it allows for treatment and infiltration within the swale. It is vital that the vegetation take hold in the soil to ensure that the sloped sides will not erode.

Maintenance of a properly-designed vegetated swale is relatively simple. Once vegetation has matured and taken hold, grasses should be kept at 3 to 4 inches in height. Following storm events, swale bottoms should be inspected and cleaned of debris and sediment buildup to ensure best performance. Special attention should be given to any areas within the slope that may be underperforming and leading to erosion.

A good resource to review before considering incorporating vegetated swales in your green street design can be found through the EPA stormwater BMP fact sheets.

Green Infrastructure Scorecard

The Center for Neighborhood Technology summarized BMP performance data for green infrastructure. Data provided by the Portland Bureau of Environmental Services, Milwaukee Metropolitan Sewarage District, and the University of New Hampshire Stormwater Center indicate the BMPs significantly reduced stormwater volume, flow, and pollutant loading.

Additional data are provided to show the impact that BMPs have on the watershed. Projects conducted in Minnesota and Seattle showed 88 to 98 percent reduction to runoff volumes by using rain gardens and vegetated stormwater detention systems in place of traditional curbs and gutters. Futhermore, the Seattle project concluded that savings of 20% or more were realized versus installation of traditional systems.

Developing Narrower Streets (Green Street Series)

The first approach in green street design comes before consideration of any BMPs. In the planning stages, a look at how to reduce the overall impervious area is going to make the biggest impact on effective stormwater management. According to a 1995 Center for Watershed Protection study, nearly half of all impervious cover in residential developments comes from streets. Narrowing of streets can offer as much as a 20% reduction of this impervious cover.

Addressing the misconceptions

Considerations such as emergency and large vehicle access, parking, and safety are often assumed to be at loggerheads with narrow street design. When designed properly, fire and emergency services vehicles have the ability to manuever successfully. This has also been the case with larger service vehicles like garbage and delivery trucks. Parking demands in most residential areas typically can be satisfied through use of one parking lane (rather than two) coupled with driveways. Safety studies have indicated that narrower streets have lower auto and pedestrian accident rates primarily due to speed reduction.

Obvious benefits

The significant reduction in impervious cover is going to help control maintenance costs. An even bigger savings is going to be realized when installing narrower streets. By reducing the overall width of a street by even one foot, $10,000 is saved per residential mile. Stormwater management savings associated with this street reduction should be significant, as well.

Zoning roadblocks

The challenge to implementing narrow street design comes when facing zoning laws which force the hand of many designers to follow street width minimums. While some municipalities have adopted narrower street width requirements, greater steps will need to be taken to involve more decision-makers in the process. Going forward, studies of successful narrow street programs in cities such as Portland, Oregon, need to be published to help build the case for these decision-makers.

Green Streets

A strong resource to include in your library is the Green Streets website created and maintained by the Low Impact Development Center. With respect to green infrastructure, the LID center has been leading the way in research, design, guidance documents, training, and sustainable stormwater planning. The focus of the Green Streets website is to outline the five major approaches in design. Over the next several weeks, I would like to touch on each of these approaches in more detail.

Examples and photos of the five green street design approaches are found at the website and highlighted below:

  • Alternative street design – One of the best ways to address the stormwater problems begins before the process of considering BMPs. Through careful planning and consideration of existing hydrologic features, overall impervious area can be reduced.
  • Vegetated swales – These are successful at reducing flow through overall channel roughness, reducing volume through infiltration, and improving water quality through vegetative and soil filtration.
  • Bioretention features – A good feature to include in street design is a tree box, which forms a bioretention cell that reduces volume, attenuates flow, and treats the stormwater through biological/chemical reactions in the mulch, soil, and root zone.
  • Permeable pavement – Success of permeable pavement systems is well-documented. While providing structural support, these systems allow runoff storage, as well as pollutant removal through filtration.
  • Sidewalk trees – Considering their ability to reduce stormwater runoff, attention needs to be paid to the overall soil volume provided to trees. Inadequate tree box size and overcompaction of the soil during pavement installation are cited as two examples threatening the ability of trees to properly assist in reducing runoff.