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When rain falls in natural, undeveloped areas, storm water is able to permeate the soil and return to the water table below the surface. Developed areas have a greater concentration of impervious surfaces such as roads, sidewalks, buildings, and parking lots, which slow or prevent water from permeating the soil.

Natural conditions limit runoff
Natural conditions limit runoff
Development increases runoff
Development increases runoff

In developed urban and suburban areas, enclosed drainage systems are often used to collect and transport storm water during and after rain events. When rain falls onto an impervious surface such as a paved road, the water follows the slope of the pavement to a gutter. The gutter directs the water along the curb line to a series of intermittent inlets where it can enter the collection system below the pavement. A system of pipes below the pavement moves the collected stormwater offsite to creeks, streams, or other bodies of water in the surrounding hydrologic environment.

Developed area
How can we make this...
Natural area
function more like this?

Low Impact Development (LID) is an alternative method of stormwater management which is used to decrease runoff in developed areas by increasing surface permeability and seeking opportunities to store or utilize stormwater on site, as shown in the images below (e.g. underground storage, rain gardens, downspouts that empty into landscaping rather than the street).

LID technique
Credit: CH2M HILL
LID technique
Credit: CH2M HILL

Drainage systems for arterial streets will in most cases require a combination of conventional and LID stormwater management methods. Combining these methods can be especially effective in retrofit situations where existing enclosed stormsewer systems are unable to handle the additional runoff created by new development. In this situation, using the LID method can be much more cost-effective than rebuilding the storm sewer system.

LID technique
Credit: CH2M HILL

Effective stormwater management is imperative, regardless of which method or combination of methods is utilized. Flooding of roadways, bike paths, sidewalks, and adjacent properties can create safety and access issues and significant property damage.

Considerations for stormwater management strategy selection:

  • Enclosed drainage systems are typically costly to construct and/or rebuild.
  • Enclosed drainage systems tend to have significant maintenance requirements. Without routine maintenance, debris can build up in inlets, curb and gutter systems, and pipes, limiting the system’s effectiveness.
  • Enclosed drainage pipes increase the velocity at which stormwater travels, which can exacerbate the erosion of stream banks and channels. Installing erosion control devices and/or energy dissipaters creates an additional cost.
  • The design process for an enclosed drainage system is generally more straight-forward than utilizing LID strategies. While there is an established set of guidelines and software to direct the design and sizing of enclosed system pipes and inlets, additional research on LID controls and the development of models to validate proposed solutions is needed.
  • Reviewers and regulators may be hesitant to approve non-standard drainage system approaches due to the lack of existing design standards and procedures. Further development of regional guidance to assist reviewers and regulators and pointing to local models of successful implementation could help reduce uncertainty.
  • LID strategies can result in some puddling which, although innocuous, some may find aesthetically objectionable or bothersome. However, many LID strategies (e.g. landscaping, green roofs, and rain gardens) offer significant aesthetic benefits.
  • Shared use of parking (e.g. daytime use for offices, nighttime use for a movie theater or restaurants) is a strategy which can be used to reduce the amount of space needed for parking, decrease impervious surface area, and presumably reduce stormwater management requirements.
  • Pavers
    Credit: East-West Gateway

    Pervious pavement can also be used in parking lots to further reduce the impervious surface area. Pervious pavement is more permeable than regular pavement and allows stormwater to more easily drain through to the soil. See the EPA Porous Pavement Fact Sheet for a detailed breakdown of the pros and cons of pervious pavement, or visit perviouspavement.com for more information.

  • Detention ponds are most often used to manage stormwater in residential areas and employment districts; however, they can enhance streets in many types of places. Pocket parks, trails, and greenways can be situated adjacent to a detention pond, providing opportunities for recreation and creating a more visually appealing corridor.
  • Landscaped area to increase permeation
    Credit: CH2M HILL

    Opportunities to use LID strategies to supplement enclosed drainage systems are often overlooked or underutilized. For example, landscaping is often placed higher than the surrounding pavement (e.g. a raised median on a sidewalk or in a parking lot), thus limiting its ability to absorb runoff, as in the image at right.

The images at right illustrate two simple LID strategies. In the first image, a downspout from the roof above is channeling rainwater directly into the landscaping instead of directing it through an enclosed stormwater management system.

Parking lot island
Credit: East-West Gateway

This strategy reduces stormwater capacity requirements while helping to sustain the aesthetically and ecologically beneficial landscaping. However, the curb and gutter system used for the landscaping island prohibits stormwater flowing across the surrounding impervious surfaces from being absorbed by the landscaping.

Placing intermittent curb cuts around the island, as shown at right, allows some rainwater to flow from the parking lot into the landscaped area for absorption. Eliminating the curb and setting the landscaped area lower than the adjacent pavement would allow even more stormwater to permeate the natural surface, further minimizing stormwater capacity requirements.

These simple, effective LID strategies can have significant environmental and economical benefits if applied consistently and appropriately in developed areas.

LID State of the Practice Example: 12th Avenue Green Street Project, Portland

The 12th Avenue Green Street Project in Portland, Oregon is a fantastic example of the successful use of LID stormwater management techniques. The American Society of Landscape Architects recently awarded the project a General Design Award of Honor (2006), and stated that the project was “…the best example of this type of work we’ve ever seen.”

Portland 12th Avenue Project
Credit: City of Portland/ASLA

The initiative was part of a street retrofit project started in 2005. The project uses stormwater planters to collect, treat, and distribute stormwater from the street, bypassing the existing enclosed drainage system. The planters aesthetically enhance the streetscape, while providing excellent stormwater management, as shown at right

For more information about the project, visit the 2006 ASLA Professional Awards Page.

Bicycle-Friendly Design Considerations:

Because bicycles are typically expected to use the rightmost edge of the travel lane (a bike lane, paved shoulder, or wide outside lane), they may encounter various obstacles, such as manholes and grates, which are commonly used along the curbside as part of enclosed drainage systems.

It is important to consider the needs and safety of bicyclists when planning the location and design of these items. Bicycle-friendly grates and manhole covers should be used along established bicycle lanes and routes, but should also be considered in areas where the potential for bicycle traffic exists.