Stormwater Library

resources for stormwater professionals

Archive for Residential BMPs

Developing a Stormwater Credit System for Using Trees

The following four municipalities have developed a stormwater credit system for developers who incorporate trees into their stormwater plans:

The issue of quantifying reduction rates can be a difficult one due to the different capacities of tree species. For more information on use of trees as a stormwater BMP, please refer to the recent study on Trees and Structural Soils by the Department of Urban Forestry at Virginia Tech University. Developers and design professionals can also refer to the Green Values Calculator, which allows you to create hypothetical designs using trees and other green interventions and quantifies the results in terms of costs savings and reduction in stormwater discharge.

Over the next several weeks, I will be presenting a series on how the municipalities of Pine Lake, Sacramento, Portland, and Indianapolis each went about changing the local building code to include stormwater credits for trees.


Stormwater Retrofits in Urban Settings

The most recent edition of the journal Stormwater included an article on urban retrofitting. The article focused its attention on 3 retrofit projects in Portland, Minnesota, and Seattle. In particular, the article zeroes in on how each project managed the challenge of incorporating the retrofits with limited space.


In the case of the Portland project, a parking garage was redesigned to reroute stormwater to infiltration planters along two sides of the structure. The major challenge was redesigning the plumbing to properly redirect the stormwater to planters. Narrow infiltration planters were constructed with permebale soil mix and native plants to help treat and infiltrate the runoff. The planters are able to infiltrate a minimum of 2 inches of rainwater per hour and can handle practically all of the stormwater from a 2-year storm event.


By taking a street in the town of Burnesville, narrow strips in front of residential properties were carved out, sometimes using retaining walls, to cluster together a series of rain gardens. Getting buy-in from the the residents was the biggest challenge, but an educational outreach program was developed and helped to achieve 80% participation rates. Sidewalk cut-outs were incorporated to help direct street runoff to the raingardens, which were able to accomodate 0.9 inches of runoff from the tributary hardscape. There hasn’t been any concern with vector control issues, as gardens have typically dried in a 3-4 hour period. When compared with a similar nearby street also feeding Crystal Lake, runoff volumes have been reduced by 90 percent.


On a much grander scale, the city of Seattle took on a retrofit project incorporating 32 acres and 15 residential blocks. Working with the existing topography and regrading portions of streets to create a more meandering path through the neighborhoods, the engineers were able to divert stormwater to culverts, catch basins, vegetated swales, raingardens, and cascades. Swales were used in steeper areas to help control runoff velocity and volumes, while raingardens were a common feature in flatter terrain. As with the Minnesota raingarden clutstering, this large project involved networks of stormwater BMP features that worked to slow runoff, treat it, and infiltrate it to help recharge the groundwater tables.


The article touches on several of the challenges to projects like these. Limited space can be one of the largest obstacles and requires a great deal of coordination with project scheduling to prevent disruption to business and residences. It obviously impacts the design and overall effectiveness of the project and requires creative solutions to address proper stormwater management.

One of the other important considerations is educational outreach efforts. Business and homeowners often need to be educated on the need to promote infiltration. Allowing for buy-in keeps the project strong and growing. Homeowners are a critical part of the maintenance to residential BMPs, while business owners can get LEED credits and can promote their reputation as green businesses.

The biggest challenge is funding. The three examples cited in the Stormwater article all had significant funding from city grant programs. These projects require large inflows of funds to carry through all the design objectives. However, more successful projects cropping up like these examples can make it easier in the future for public works agencies to seek the necessary funds to promote urban retrofits on a larger scale.

Onsite Stormwater Management – Homeowner Responsibility in Portland, Oregon

In the city of Portland, homes that were built after 1999 were typically equipped with onsite stormwater systems that help promote infiltration rather than runoff. That is interesting in itself, but what makes the Portland example so unique is that it puts the responsibility of management and mainteneance of the systems on the homeowner. Routine inspections must be done and logs kept recording the types of systems used on the property, condition, and any actions taken.

Portland’s Bureau of Environmental Services puts out a very informative, easy-to-follow manual (including inspection logs) that directs the homeowner on what to look for and steps to take to keep the onsite systems viable. The manual can easily be used as a model for other communities considering such a program or for the invidual homeowner interested in promoting stormwater infiltration.