Last week’s post compared how the forest ecosystem and Portland’s municipal sewer manage stormwater. The vision offered for closing the gap between these systems is one spin on what has been dubbed low impact development (LID) – a stormwater management approach that emphasizes using natural features and systems to protect water quality. LID carries a promise of improving water quality and lessening our impact on the natural environment while saving municipalities money by reducing the demand on our aging sewers. Because of this potentially big benefit, public agencies have grown increasingly interested in adopting LID methods, especially here in the Portland area.
As a part of the “Stormwater Cinema” series, KPFF has produced an excellent short film that introduces LID and the engineering behind it, the stormwater toolbox.
So, with that, how should we as engineers approach LID design? What are the key design issues that will help create a successful LID project? To oversimplify things a bit, the goal of LID design is to create a man-made environment that mimics the natural one. The LID designer can best address this goal by decentralizing and layering the design and lengthening the time it takes stormwater to pass through the system. The impact of the resulting design can best be analyzed using three unique but closely related dimensions: time of concentration, stormwater quality and stormwater disposal.
Time of Concentration
Time of concentration (TOC) is a site characteristic, much like the amount of shade or amount of vegetation are. While it can change over time, TOC is independent of intensity, duration and other storm characteristics. Measured in time, TOC is defined as the length of time between when it starts raining and when the flow through that site’s storm drain system reaches its peak.
An interesting relationship exists between TOC and the performance of a storm drain system. As TOC increases, the peak flow rate in the system decreases. This is not to say that increasing TOC will reduce the total amount of water leaving the site – though depending on the method it can – but instead that it will result in the same amount of runoff leaving the site over a longer period of time, creating a longer, lower peak. This relationship is illustrated in the Intensity-Duration-Frequency curve at the link below. Such curves are commonly used to calculate peak flow rates.
The quality of stormwater runoff is a measure of how much pollution that water contains. Higher quality stormwater has less pollution while lower quality stormwater has more. Most municipalities require that the pollutant level in stormwater be brought below some defined level before it can be discharged into a public sewer. Many LID designs – including rain gardens, vegetated swales and flow through planters – have become widely accepted methods of addressing this need. Mechanical systems for treating water are also available and are good options when site constraints preclude the use of LID designs.
Less concentrated, slower moving flows typically can help in the quest for higher stormwater quality as they are less likely to result in erosion and more likely to allow suspended sediments to settle out. While pavement and pipes allow for efficient drainage designs, they also increase velocities and concentrate flows, leading to a need for additional downstream water quality treatment. Project designers often have to take conscious measures to prevent water from concentrating in order to keep velocities down.
Every drop of water that falls on a site is ultimately disposed of in one of three ways.
- Offsite disposal – which could include connecting to an offsite sewer system or sending water directly to a water body
- Infiltration – which could include sending water to a rain garden or infiltration gallery
- Onsite use – which could include rainwater harvesting for irrigation and greywater systems as well as evaporation and use by plants
Before large scale human development, much more runoff infiltrated and the current thrust of design is to return to this condition. Of course, this goal can be at odds with the need to have hard, durable walking and driving surfaces, so it is important to carefully consider how these surfaces are designed and what areas can be reserved for stormwater management.
The space available for stormwater management can play a large role in determining which methods of disposal can be considered on a particular project. While connecting to an offsite sewer system is usually the least desirable outcome, it is often the only viable option, especially when stormwater management is not a priority in design decisions. Infiltration is a low cost option for disposing of stormwater, but requires space and works best when facilities can be decentralized and runoff can be directed to them without the use of catch basins and pipes. Onsite use is an attractive disposal option, but the reality for most projects is that the money saved by using onsite water will outweigh the added cost of rainwater harvesting. Further, there is usually much more water available during storms than can be captured and stored for future use, especially considering that there can be little need for irrigation during the wetter times of year.
As stated earlier – and to wind the points above together a bit better – a solid approach to LID design includes:
- Lengthening the amount of time that it takes for runoff to pass through the storm system
- Avoiding concentrating flows and increasing velocities
- Maintaining a decentralized approach to stormwater management
Because individual LID techniques often can not address all of the stormwater needs on a site, many projects can benefit from a decentralized, layered approach to stormwater management. By layering LID elements, designers can address TOC, stormwater quality and stormwater disposal issues in a simpler form, before flows have had the chance to concentrate and accelerate. Many elements – like eco-roofs, trees, pervious pavements or check dams – can add significant value at the beginning or middle of a drainage path, leading to a less complicated design challenge at the end of that path.
Since it can be helpful to approach problems from several different angles, try imagining you are a rain drop falling on your site. What is your first contact with the project? Do you land directly on pavement, where you immediately start picking up speed and pollutants; or is there a chance that your first contact could be with the canopy of a tree? Once on the ground, are you allowed to move in a well spread sheet flow; or are you directed toward a drainage structure where you will become part of a larger, concentrated flow? As you move across the site, do you pass through any areas – either in the air, at grade or underground – that could be considered a missed opportunity for bettering the storm drain system?
Hopefully consideration of these questions and the core design issues raised in this post will help you in developing high quality, innovative designs that address the unique challenges presented by your projects.