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Stormwater Structures

Stormwater runoff is a major cause of water pollution in urban areas. When rain falls on our roofs, streets, and parking lots in cities, the water cannot soak into the ground as it should. Conventionally, stormwater drains through gutters, storm sewers, and other engineered collection systems and is discharged into nearby water bodies.  An alternative, green stormwater infrastructure, is a nature-based approach to improve water quality that urban stormwater runoff causes and provides greater benefits than conventional stormwater solutions. Green stormwater infrastructure are soil-water-plant systems that intercept stormwater, infiltrate a portion of it into the ground, evaporate a portion of it into the air, and in some cases release a portion of it slowly back into the sewer system or nearby water body. In addition to improved water quality, green stormwater infrastructure provides benefits such as beautified communities, improved public health, and creation of wildlife habitat.

There are 34 structural and non-structural green stormwater infrastructure practices located within Roger Williams Park.  There are 13 infiltration basins, 2 sand filters,  5 bio-retention rain gardens, 3 bioswales, 4 pavement removal projects,  and 7 buffer plantings.  The stormwater structure MAP provides details of the structures' locations.  This SPREADSHEET provides technical details on the stormwater structures. For an interactive introduction to some of the stormwater structures located near Picture Posts, check out our walking/biking tour.

Infiltration basin

An infiltration basin is a shallow impoundment that is designed to receive and infiltrate storm water.  Infiltration basins use the natural filtering ability of the soil to remove pollutants in storm water runoff.  Infiltration practices store runoff until it gradually infiltrates into the soil and eventually into the groundwater. 

Sand filter

A sand filter is a stormwater management system designed to maximize the removal of pollutants from stormwater. It consists of a pre-treatment zone and a treatment zone, which includes the sand bed, and in underdrained systems, and the underlying components. Pollutants are treated through settling, filtration, and adsorption by the sand bed.

BMP 19C (sandfilter) in Roger Williams Park, February 2020
Sand Filte Diagram

bioretention - rain garden

Bioretention – Rain Gardens are a method of treating storm water by pooling water on the surface and allowing filtering and settling of suspended solids and sediment at the mulch layer, prior to entering the plant/soil/microbe complex media for infiltration and pollutant removal. Bioretention techniques are used to accomplish water quality improvement and water quantity reduction. Rain Gardens are planted with native species which enhances habitat that is beneficial for insects and wildlife

Buffer Plantings

Buffer plantings play a significant role in decreasing pollutants (nitrogen, phosphorous, sediment) and improving overall water quality.  Shoreline plantings in Roger Williams park also create a deterrent for geese in the park.  Geese climbing in and out of the water can cause shorelines to erode and increased sedimentation in the ponds. Goose feces also contributes to an increase in nitrogen, phosphorous, and bacteria concentrations in water bodies with large populations of geese.  


Bioswales are green storm water runoff systems that slow velocities, decrease sedimentation and pollutant concentrations before entering a natural waterbody or sewer system. Bioswales improve water quality by enhancing infiltration of the first flush of storm water runoff and filtering the large storm flows they convey.

Impervious area management

Increased stormwater runoff volume, velocities, erosion, and flooding can be directly correlated to an increase in impervious area in a watershed.  Degradation of water quality is also linked to an increase in impervious area in a watershed. Removing impervious area allows for greater infiltration and pollutant removal of stormwater runoff.