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Impervious Area Information August 2017

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Impervious Area Information August 2017 White Bear Lake Community Development Department White Bear Area Department of Inspections 4701 Highway 61, White Bear Lake, MN 55110 Zoning 651-429-8561/Inspections 651-429-8518/Fax 651-429-8503 www.whitebearlake.org Impervious Area Information August 2017 Within the Shoreland Overlay District, and in the Old White Bear neighborhood, the amount of impervious area per property is limited to no more than 30% of the lot area. This handout is a summary of the requirements and does not contain all of the specific codes for design and construction, and shall only be used as a guide. Plans are subject to review by the City Zoning Administrator and City Engineer. Definition of Impervious Area: An artificial or natural surface through which water, air, or roots cannot penetrate. Examples of impervious surfaces include: All concrete and asphalt, including driveways, sidewalks, and patios Any structure with a roof (including structures on stilts) All permanent swimming pools Stepping stones The following are NOT considered impervious surfaces: Gravel parking lots and driveways Decks, provided there is no concrete, plastic, or other impervious material underneath Landscape rock Porous pavement can be considered 50% pervious, subject to staff approval. Submission Requirements: All projects increasing the amount of impervious area in the Shoreland Overlay District or in the Old White Bear Neighborhood must include a completed impervious surface area worksheet in the permit application. If the impervious area is more than 30%, plans for mitigation of excess impervious area by means of a stormwater infiltration system(s) must be included with the application. See the “Stormwater Infiltration System Submittal Requirements” handout for a list of required exhibits and infiltration design considerations. Impervious Surface Credits: For single-family and two-family uses only, staff may administratively grant an impervious surface “credit” for the use of stormwater infiltration systems such as porous pavement, raingardens, trench drains, and others. The credit granted may vary depending on specifications of the system but may not exceed 50% of the surface area of a porous pavement system. For other types of systems, the system must be designed to infiltrate at least twice as much run-off as the additional hard-surface area generates, based on the 1.1-inch rain event and not accounting for ground absorption if traveling over pervious area to get from the hard-surface to the system. The surface area of a porous pavement system may not exceed 25% of the total lot area, and the overall coverage of the site may not exceed 50%. The final surface and sub-grade design of an engineered infiltration system is subject to approval by the City Engineer. Mitigation Options: To determine the type of impervious surface mitigation best suited to your project, consider the cost, capacity, and ongoing maintenance requirements of the system. Three common infiltration systems and their general maintenance requirements are summarized below. To discuss your options, contact the Environmental Specialist at 651-429-8564. This document is for informational purposes only and is not intended to address every situation for the permitting and plan review process. Porous Pavement: Porous pavement is a hard surface made of asphalt, concrete, or pavers specially designed to allow water to drain through the pavement and subgrade and infiltrate into the soils below. Porous pavement can be used for driveways, sidewalks, and patios, and is best suited in areas with sandy soils. Porous pavement must be regularly maintained to preserve the permeability of the system. Routine maintenance involves vacuuming or sweeping the surface to remove sediment and debris from the void spaces in the pavement. Review the specifications of the particular product you use to ensure proper maintenance. Adjacent planted areas should also be maintained to prevent soil washout onto the pavement. Use tarps to protect the surface if storing soil or mulch on the pavement. Preventing superficial soil and debris from being ground into the surface by frequently sweeping and regularly vacuuming will prevent more costly and time intensive maintenance. Porous pavement reduces the potential for ice buildup in the winter. If ice does occur on the surface, use moderate amounts of salt, not sand, to remove ice. Porous pavement should never be sealcoated. Raingardens: Raingardens are shallow depressions designed to capture and infiltrate stormwater runoff routed to the raingarden from impervious surfaces. Raingardens are planted with deep-rooted plant species that are adapted to intermittent wet and dry conditions. Plants need to be watered the first season until they are established. Well- established plants not only make the garden attractive, but also keep the soil loose, improving infiltration. Plants are highly recommended over seed due to the high initial maintenance required to establish seed. For soils with a high clay content, the raingarden may need to be over excavated and a soil amendment added. Long-term maintenance includes periodic weeding and mulching. Raingardens are most successful when water entering the raingarden is free of sediment and debris such as water runoff from a roof. For curb cut raingardens, a pretreatment device should be installed to keep road sediment from getting into the raingarden. Sediment that collects in the pretreatment device will need to be cleaned out periodically. Trench Drains: Trench drains are shallow trenches lined with filter fabric and filled with washed stone. Stormwater runoff from impervious surfaces is captured in the void spaces between the stone and infiltrated into the soils below. Trench drains are most successful when the water entering the drain is free of soil and debris, which can be accomplished by pretreating the water directed to the drain with a filter system such as a grassed swale, or locating the trench drain directly below the roofline of a building, such as a garage. If soil or debris fills the void spaces, the system will need to be cleaned out to maintain storage and infiltration capacity. In some cases, a more complicated system can be designed that allows grass to be planted over the trench. This document is for informational purposes only and is not intended to address every situation for the permitting and plan review process. .
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