Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring

3.5    Vegetated Swales/Filter Strips

3.5.1    Description and Purpose

Grassed swales and filter strips are moderate to low-cost BMPs designed to improve the quality of stormwater runoff by using biological and chemical processes in soils and vegetation to filter out constituents. Both BMPs are well suited to the ultra-urban environment and can be located in medians or along the shoulders of roads.

Grassed swales are carefully engineered grassed channels that not only safely convey stormwater from a roadway but also provide water quality benefits. Grassed swales can also be sized to detain stormwater and address water quantity management needs. The swale designs can be adapted to accommodate in situ soils with differing percolation rates by varying the method of detaining the stormwater within the channel.

Filter strips are evenly sloped vegetated areas that treat stormwater by filtering it through vegetation (grass or wooded growth). Filter strips located on soils with high percolation rates can efficiently address water quality issues over a short horizontal length. This feature makes it feasible to use filter strips as roadway shoulders or safety zones.

3.5.2    Design Alternatives

Grassed swale designs are categorized as either wet or dry designs. Wet grassed swales maintain a volume of water in the bottom of the trench by having the invert located below the groundwater table or by the use of baffles in the trench to detain water. This system is designed to emulate a natural wetland. Water quality improvement is achieved by the settling out of particulates in the water column and by the biological and chemical action of the water. Dry grassed swales are designed so that runoff infiltrates through the bottom of the swale into the ground below. The subsoils must be permeable and possess a high infiltration rate. The treatment efficiency of both grassed swale designs is dependent on the gradient of the swale, the swale size, and the infiltration rate of the subsoils.

As a design option, stormwater quantity management can be achieved in larger swales by the use of check dams to pond the water within the channel. The stormwater is slowly dewatered by a notch in the check dam and released downstream or infiltrated into the subsoils. Pretreatment by a vegetated filter strip or other acceptable method to reduce sediment loads in the system is important so that the capacity of the channel can be maintained and the soil pores are not clogged.

Vegetated filter strips are designed as a water quality measure. As water flows in a sheet across the area, particulates and constituents in the first flush of stormwater are filtered out by the vegetation. They are then infiltrated into the soils or taken up as nutrients by the plants. Removal efficiencies are dependent on the slope length, gradient, and condition of the vegetation. A long slope length and mild gradient provide the most efficient removal rates. A berm is often included at the downstream end to temporarily detain the runoff. If necessary, energy dissipators, such as gravel strips, are used to reduce the velocity of the stormwater from the pavement areas before it enters the filter strip. This helps to spread the water out so that channels and rills, which can cause the runoff to bypass the system, do not develop.