Elliott Richter, RLA
Low Impact Design (LID) refers to managing stormwater runoff at the source (on-site) using green infrastructure techniques. Keeping water on-site reduces the need for much larger and more costly management systems. It also helps prevent flooding by reducing the amount of water that enters surface waterways during flash flood events. Conventional stormwater management uses a pipe-and-pond technique which replaces natural systems with concrete, pipes, ponds, and extensive infrastructure. These systems channelize the water and increase its velocity; thereby greatly increasing the risk of erosion. The goal of LID is to replace these pipes and ponds with soft engineering practices that: filter, infiltrate, store, and evaporate the run-off on-site. Incorporating LID early in the planning and design of a project has proven to be much more cost-effective while also providing a community benefit and a much better-looking project. Each of these major components of LID is discussed below.
Filtering happens early in the management of stormwater runoff within the systems used to convey water to storage and infiltration facilities. In modern engineering, these conveyance systems are typically pipes and concrete channels. However, LID replaces these pipes and concrete with vegetation. Some types of these systems include vegetated filter strips, bioswales, rain gardens, infiltration trenches and basins, and green roofs. Below are examples of how some of these systems work.
- Filter strips are located along large impervious areas such as parking lots. The runoff from the parking lot is directed to the filter strip as sheet flow. The vegetation within the filter strip slows the water down, allowing sediments and other suspended solids to settle and separate from the water. Filter strips should always be located as close as possible to the impervious surface it is treating, as the first flush of stormwater has the highest concentration of contaminants and should be treated quickly. This sheet flow is then typically channelized into a bioswale.
- A bioswale is a vegetated surface channel that treats and conveys stormwater. The grasses and shrubs within the swale act similarly to filter strips by slowing water down and allowing sediments and debris to settle. Check dams are also commonly used in bioswales. Check dams essentially create tiny pools within the swale that dam up water until it eventually overflows into the next pool and continues down the swale. Check dams greatly reduce runoff speed and increase filtration. After flowing through the bioswale the water is deposited into a storage or infiltration facility.
- A rain garden is a low-lying area in which runoff is directed. It is not used for storage but is used both for filtering and infiltrating runoff. A rain garden is filled with native grasses, shrubs, and sometimes small trees which help filter contaminants. To promote infiltration, a combination of organic sandy soils, gravel, and mulch are used. Typically, an underdrain is also installed to handle large rain events that will begin to flood the garden. Rain gardens are good for small drainage areas such as for residential use and small parking lots.
After runoff has been filtered it is then conveyed to an infiltration system. It is these systems that prevent flooding by depositing water underground during a rain event rather than overflowing creeks, streams, and rivers. These systems decrease the impact and cost of developing a site by requiring less stormwater storage infrastructure on-site or downstream. It also helps replenish our groundwater supply rather than let water flow downstream where it will eventually end up in the ocean. Infiltration techniques include pervious surface materials, infiltration basins, rain gardens, bioswales, and infiltration trenches.
- A pervious surface has gaps and spaces that allow water to flow vertically and absorb into the ground rather than horizontally across the surface. Porous asphalt/concrete, interlocking pavers, open-cell pavers, and gravel can replace commonly used impervious materials. These typically have a higher initial cost compared to impervious materials but over time will save money due to reduced construction and maintenance costs.
- Infiltration basins are the LID version of a detention pond. The difference is a detention pond holds water and then releases it once the storm has passed while an infiltration basin holds water until it drains through the soil. Basins must have highly permeable soils for infiltration to occur, and despite having water filtering properties other filtering systems must be used upstream to prevent sediment buildup. A well-designed and properly vegetated infiltration basin can resemble a natural wetland, a much more desirable look than a typical detention pond.
- Infiltration trenches are essentially smaller infiltration basins. They are useful in residential, roadway, or commercial projects with small catchment areas (max. 2 acres). As with basins, trenches must be used downstream of other LID systems so that sediment does not build up and cause the system to fail. Trees can also cause a trench to clog if planted too close.
Each LID component can be used individually to manage stormwater runoff, but the design of these components in a system of infiltration trenches, rain gardens, bioswales, and basins can serve larger-scale developments.
Detention and retention ponds are the most common ways to manage runoff on large development sites. They are effective yet can be repetitive and sometimes boring. A few more unique ways to store stormwater include underground detention, wet vaults, and rainwater harvesting. These systems can be useful for commercial sites where surface area is limited but are not as efficient or cost-effective as detention/retention ponds are on large sites.
- Underground detention can be located beneath large impervious areas such as parking lots. Runoff is received directly from the impervious surface before any conveyance system. It can store water and release it at a much slower rate, reducing peak flows. Water quality is improved through the settling of solids and it can provide infiltration if desired. This system is more costly but is easy to maintain and can be a solution if space is limited.
- Wet vaults act like an underground retention pond. Just like underground detention, they are located directly below the impervious surface in which the runoff is received. However, vaults hold a permanent amount of water at a specific level below full capacity. This permanent retention allows more sediments to be filtered than a detention system which drains completely. Wet vaults provide storage, filtration, peak flow reduction, and rainwater harvesting potential.
Any water that was not able to infiltrate back into the ground due to soil saturation or a severe rain event will hopefully return to the atmosphere by transpiration through vegetation or by direct evaporation. This would put water back into the natural cycle rather than continue downstream, unused, until it reaches the ocean or to increase the risk of flooding.
A FINAL WORD
Some of the techniques described above are better for small scale residential lot use while others are used for entire community stormwater systems. Each has a different purpose, and all can be beneficial. However, the main principles of LID can be applied in any situation: filter, infiltrate, store, evaporate.