Appendix D - Technical Design Criteria for BMPs City of Wayzata Page 3
On-site infiltration features Definitions and Scope: Infiltration facilities are constructed basins or depressions located in permeable soils that capture, store, and infiltrate a calculated volume of stormwater runoff. On-site infiltration features are considered those which receive drainage from less than 5 acres, and can provide temporary storage for up to one inch of runoff depth from the total contributing impervious area as runoff is infiltrating. Types of on-site infiltration features include dry wells, infiltration trenches, underground infiltration, and rainwater gardens. 1. A percolation test must be conducted on the site to ensure that the soils are suitable for infiltration. General methods include analysis of two pre-soaked holes each 24 inches deep and 6-12 inches wide. The underlying soil type must be suitable for infiltration, with a minimum infiltration rate of 0.2 inches per hour. 2. Soil borings must be done to a depth of at least twice the depth of the designed infiltration feature, or a minimum of 10 feet. 3. Pre-treatment by a sediment basin, filter strip, grass channel, or similar item is required, except for rooftop runoff, which can be infiltrated directly. Sheet flow from impervious surfaces discharged over vegetation or routed through swales for at least 25 feet is preferred. However, use of structural units, such as hydrodynamic separators for sediment reduction upstream of infiltration is acceptable. 4. On-site infiltration systems must be designed off-line and must completely draw down with 48 hours after a rain event to prevent nuisance standing water conditions. 5. Infiltration systems should be designed with a minimum of three feet between the bottom of the system and the seasonally high groundwater or bedrock layer. 6. Setbacks to buildings and property lines must be a minimum of 10 feet. Setbacks to private wells or public wells must be at least 100 feet, while those from septic system drain fields must be at least 50 feet. 7. Underground systems must have an observation well feature to measure water levels in the system. See the most recent edition of the State of Minnesota Stormwater Manual for further guidance (Minnesota Stormwater Manual -- Minnesota Pollution Control Agency) City of Wayzata Page 4
Bio-filtration Definitions and Scope: Bio-filtration facilities are shallow, landscaped depressions that capture stormwater runoff to be filtered through a prepared soil medium. This practice utilizes the chemical, biological, and physical properties of plants, microbes and soil for removing pollutants from runoff. Once the soil pore space of the medium is exceeded, stormwater begins to pool at the surface of the planting soil. These practices differ from infiltration practices in that they have an underdrain system that collects stormwater after it has traveled downward through the prepared soil medium and directs the filtered runoff to the storm sewer system or to a receiving water. The advantage of this technique is that it can be used in areas of soils that are unsuitable for infiltration. The disadvantage is that it is considered to have only a limited impact in reducing stormwater runoff volume or the dissolved form of stormwater pollutants. A biofiltration feature should not receive drainage from more than 2 acres total and more than 1 acre of impervious cover. Common applications of bio-retention practices include as landscaped islands, in cul de sacs, at parking lot margins, within building setbacks, and as street-scape applications between the curb and sidewalk. 1. The recommended depth of prepared soil in the bioretention feature is 30, with 48-52 acceptable if large trees will be incorporated into the design. 2. Extremely careful attention must be paid to the prepared soil media. At this time, the Minnesota Stormwater Manual provides material specifications for a well-blended, homogenous mixture of 50-60% construction sand, 20-30% topsoil with less than 5% clay content, and 20-30% organic leaf compost. Refer to the most recent Manual for the exact specifications. 3. Pre-treatment by a sediment trap, filter strip, grass channel, or similar item is required, except for rooftop runoff, which can be introduced directly to the bio-filtration feature. Sheet flow from impervious surfaces discharged over vegetation or routed through swales for at least 20 feet is preferred. 4. Bio-filtration systems should be designed to drain both the depression and the soil media within 48 hours after a runoff event. 5. Bio-filtration systems should be designed with a minimum of three feet between the bottom of the system and the seasonally high groundwater or bedrock layer to maintain the hydraulic capacity of the practice and provide adequate water quality treatment. See the most recent edition of the State of Minnesota Stormwater Manual for further guidance (Minnesota Stormwater Manual -- Minnesota Pollution Control Agency) City of Wayzata Page 5
Porous pavers/porous pavement systems Definitions and Scope: Permeable pavements provide alternatives to standard asphalt and concrete, which are completely impervious surfaces. Permeable pavements allow water to infiltrate or pass through them. They often contain a gravel storage layer underneath the pavement surface, which often doubles for structural support. Permeable pavement systems are typically best suited for paved areas with low traffic volumes, including patios, residential parking pads, driveways, fire lanes, overflow parking areas, and some daily parking areas. 1. The pavers and/or pavement system must be installed according to the manufacturer s specifications. 2. A percolation test must be conducted on the site to ensure that the soils are suitable for infiltration of runoff that pass through the permeable pavement system. General methods include analysis of two pre-soaked holes each 24 inches deep and 6-12 inches wide. The underlying soil type must be suitable for infiltration, with a minimum infiltration rate of 0.2 inches per hour. 3. The bottom of storage layer beneath the permeable pavers should be designed with a minimum of three feet between the bottom of the system and the seasonally high groundwater or bedrock layer. 4. The area must have low traffic volumes (< 1,000 trips per day for parking lots). 5. The slope of the permeable paver system must be less than 2%. 6. The minimum base layer void space must be 40%. 7. The minimum base layer depth is 9 inches. 8. For all applications for commercial, industrial, multi-family, and institutional sites, a long-term maintenance agreement clearly specifying parties responsible for maintenance is required. City of Wayzata Page 6
Dis-connection of impervious surface Definitions and Scope: Dis-connection of impervious cover spreads runoff from small parking lots, courtyards, driveways, and rooftops onto adjacent pervious areas where it is filtered or infiltrated into the soil or can transpire. This involves looking for areas of the site where the flow from these surfaces can be diverted onto turf, lawns, or unmaintained vegetative areas that can act as filter strips. If done properly, the volume and rate of stormwater runoff can be reduced significantly. Dis-connections should be restricted based on the length, slope, and soil infiltration rate of the pervious area in order to prevent the dis-connected runoff from reaching the storm drain system in another direction. In some cases, minor re-grading of the site may be needed to promote overland flow and vegetative filtering. For dis-connection of rooftop runoff: 1. Projects involving the subdivision of land must include the maintenance and preservation of the dis-connection as part of a recorded deed. 2. The dis-connection must be designed to adequately address the issue of basement seepage. 3. The contributing length of rooftop to a discharge location shall be 75 feet or less. 4. The roof-top contributing area to any one discharge location cannot exceed 1,000 square feet. 5. Dis-connections will only be credited for residential lots greater than 6,000 square feet. 6. The entire vegetative dis-connection shall be on a slope less than or equal to 5% and must not channelize flow. 7. Where provided, downspouts must be at least 10 feet away from the nearest impervious surface to discourage re-connection. 8. Where a sub-surface drain is used, the drain cannot be directly connected to the storm drainage system. 9. No soil evaluation is required for dis-connections that drain to hydrologic soil groups A or B. Dis-connections draining to hydrologic soil groups C and D shall have a soil evaluation to demonstrate that additional flows will not create nuisance conditions. For dis-connection of non-rooftop runoff: 1. Projects involving the subdivision of land must include the maintenance and preservation of the dis-connection as part of a recorded deed. 2. The site must be graded to promote the flow of runoff to pervious areas. 3. Maximum impervious flow path length shall be 75 feet. 4. The length of the pervious area to which the dis-connected impervious flow is discharged must be at least as long as the contributing impervious length. 5. The vegetated pervious area must be on a slope less than or equal to 5% and not channelize flows. 6. No soil evaluation is required for dis-connections that drain to hydrologic soil groups A and B. Dis-connections to hydrologic soil groups C and D shall have a soil evaluation to demonstrate that directing additional flows will not create nuisance conditions. City of Wayzata Page 7