Site Suitability Considerations
Site conditions and constraints may significantly influence the selection and design of GI practices. Common conditions and constraints affecting GI selection and design are space availability, existing infrastructure, existing soils, shallow groundwater drainage area and pollutants of concern. The site suitability considerations addressed for each GI practice are defined below.
Tributary Area
Tributary area is considered the amount of land directly draining stormwater runoff to the GI facility. Confirming the area upstream of the GI practice will ensure proper selection, design, sizing, and functionality. Increasing the tributary area will require larger facility footprints and void space and correct planning for bypass of runoff. Available space will be a limiting factor and installing multiple facilities might be necessary. Facilities can handle varying tributary area loading ratios, however, that decreases the facility's ability to detain or treat the runoff appropriately.
Site Slope
The grading and slope of a site must be considered when selecting and designing a GI facility. A slope that is too flat may cause ponding and backwater effects, which may cause premature sedimentation and clogging of inlets. A slope that is too steep may cause scour at the inlets and outlets of a facility or could be a hazard when detaining water near the top of the slope. Some designs may be modified to accommodate steeper slopes, adding practices such as check dams and energy dissipaters to minimize erosion or liners to limit infiltration.
Hydrologic Soil Group and Soil Properties
Soil is an integral part of the hydrologic cycle, as it regulates the processes of surface runoff, infiltration, storage and percolation. The type of soils at a site may influence the type and design of GI technologies. The recommended Hydrologic Soil Group per Natural Resource Conservation Service (NRCS) is provided for each GI practice. GI that relies on infiltration must have well-drained underlying soils, and sufficient depth to bedrock to prevent excessive ponding or wetting of roadway. Site-specific measurements of infiltration should be made whenever infiltration-based measures are being considered. Promoting infiltration over contaminated soil should be avoided. Additionally, a high water table can constrain infiltration practices.
Maintenance
Maintenance is required to preserve the intended water quality benefits and stormwater conveyance capacities for implemented GI. Typical maintenance includes conducting recurrent inspections and providing routine and major maintenance activities as necessary. Each GI fact sheet identifies the recommended inspection frequency and anticipated routine and major maintenance activities. Maintenance provides both functional and aesthetic purposes. Functional maintenance is important for performance and safety reasons, while aesthetic maintenance is important for public acceptance of GI but not necessary for performance.
Functional maintenance can be divided into two components: preventive maintenance and corrective maintenance. Preventive maintenance is performed on a regular basis and can include actions such as maintaining vegetative cover to prevent erosion or accumulated sediment removal and disposal. Preventative maintenance is generally characterized as ‘minor’ since it is routine and vegetation management constitutes the majority of maintenance activities. Corrective maintenance is required on an as-needed basis because of unforeseen operational problems that may be encountered, such as structural repairs or erosion repair; it is generally characterized as ‘major’ since its magnitude and frequency are not easily predicted.
Aesthetic maintenance enhances the visual appearance of a GI facility and can include the removal of trash and debris or management of vegetation. Some trash and debris removal may also be functional maintenance if it could contribute to clogging or other function disruption.