Drainage/Stormwater Runoff
Overview
Stormwater management and its relationship to a proposed new development can be an essential determinant of whether to construct a project. Natural flow, storm sewers, or combined (storm and sanitary) sewers usually remove stormwater from an impermeable surface (e.g., pavement and buildings). The first consideration should be the potential to incorporate design features such as landscaping and the use of pervious rather than impervious surfaces to help limit stormwater runoff. Floodable design features, such as floodable parks, green roofs and rain gardens to retain stormwater, or porous pavers, should be incorporated in communities with increasing flood risk. When stormwater runoff cannot be avoided, the water is usually sent to a surface water body, a permeable recharge area, or temporary storage areas. In assessing impacts to stormwater service facilities, consider the following two factors:
- The proximity of the system to the site
- The capacity of the system to accommodate the project
Important Considerations
- Is there an indication of cross-lot runoff, swales, or drainage flows on the property?
- Are there visual indications of filled ground, active rills, or gullies on-site?
- Will existing or planned stormwater disposal and treatment systems adequately service the proposed development? Will the proposed project be adversely affected by proximity to these facilities?
- Does nearby stormwater infrastructure (e.g., culverts, large drainage pipes) include safety measures like grates or fencing to prevent drownings during floods?
- If the public storm sewer is not available, how will stormwater drainage be handled?
- Is state/regional/local permitting required to control stormwater runoff (e.g., a National Pollutant Discharge Elimination System (NPDES) permit)? If so, what conditions will the permit require?
- Will the project itself cause or substantially contribute to off-site pollution by stormwater runoff, leaching of chemicals, or other pollutants?
- Will drainage and stormwater conditions significantly affect or be affected by the project site? If so, does its design plan include measures to overcome potential runoff problems?
- How will future changes in precipitation patterns affect the above considerations? How can such climate change impacts be mitigated?
Analysis Techniques
To determine drainage and stormwater runoff: First, look at topography—water coming onto the site, pooling on the site, and leaving the site. Second, look at other site conditions such as land use, soil type, slope, and vegetative cover. Third, see where drainage structures could be placed. Fourth, work with the project architect/engineer to understand what is required for the site and to meet city requirements.
Mitigation Measures
If there is excess drainage coming into the site and/or your project has large areas of the site developed with impervious surfaces, consider green infrastructure to lessen the impact. Bioswales, rain gardens, and catchment systems can be used to capture runoff. Pervious paving systems for parking lots can also be utilized to allow large rain events to be handled onsite without the need to overburden the existing stormwater system in the area.
HUD’s Community Resilience Toolkit lists resilience actions that communities can take to limit the impact of flooding.