City of Royal Oak Department of Community Development 211 South Williams Street Royal Oak, MI 48067 Permeable Paving Report June 13, 2019 The Honorable Mayor Fournier and Members of the City Commission: Pursuant to the city commission’s request, staff has undertaken an analysis of permeable paving materials and their appropriateness for use in the City of Royal Oak. The report (Attachment 1) presents options for the expansion of acceptable driving surfaces in the city and compares the permeable infiltration system options that could be utilized to meet the requirements of the stormwater ordinance (Ch. 644) and future storm utility ordinance. Respectfully submitted, Timothy E. Thwing Director of Community Development Approved, Donald E. Johnson City Manager 1 Attachment Attachment 1 Pervious Paving Options Background On the instruction of the city commission and city manager’s office, the planning & engineering divisions have undertaken an analysis of pervious or permeable paving options and their stormwater management implications. The objective of this report is two-fold: 1) To present options for the expansion of acceptable parking surfaces in the City of Royal Oak; 2) To compare the permeable infiltration system options that could be utilized to meet the requirements of the stormwater ordinance (Ch. 644) and future storm utility ordinance. Pervious or permeable pavement is a collection of alternatives to traditional pavement that allow for the infiltration of rainwater or snowmelt on-site. These alternative materials include pervious asphalt or concrete, permeable interlocking pavers, plastic grids or reinforced turf.1 When used as part of a larger infiltration system, these materials can be used as an alternative to, or in conjunction with, other stormwater detention systems. Benefits of pervious or permeable pavement can include: Stormwater management, including flood mitigation Erosion control Reduced water pollution by keeping more water on-site Allow urban trees the rooting space needed to grow to full size Current Regulations At present, the City of Royal Oak Zoning Ordinance (Ch. 770) requires all off-street parking and driving areas to be hard surfaced. Development in all zone districts must comply with the requirements of §770-109. Off-street parking design and construction, which states: “All…parking lots, maneuvering aisles, driveways, and loading areas shall be hard surfaced with asphalt or concrete pavement, shall be graded and drained so as to dispose of surface water pursuant to City ordinance and shall be completely constructed prior to a certificate of occupancy being issued.” However, the Planning Division has allowed the use of pervious paving materials on a case by case basis. In terms of use as an acceptable “hard surfaced” driving surface – materials such as pavers or reinforced turf have been permitted. At the Maryland Club development, grass reinforced with a plastic grid was permitted as an alternative to standard concrete for the continuation of a fire lane. Given site constraints, this option was utilized to allow for the passage of emergency vehicles while maintaining more green space and residential density on-site. 1 EPA. (2019). Soak Up the Rain: Permeable Pavement. Retrieved from https://www.epa.gov/soakuptherain/soak-rain-permeable-pavement. 6/13/2019 Attachment 1 Pervious Paving Report Page 2 of 5 Reinforced turf at the Maryland Club development Porous asphalt parking lot at the Detroit Zoo Porous asphalt paving has also been allowed to fulfil the zoning requirement for hard surface. In terms of stormwater detention, if an applicant has demonstrated that the requirements of the stormwater ordinance (Ch. 644 Stormwater Detention) are met, pervious paving infiltration systems have been permitted. A recent example of this is the Detroit Zoo parking lot completed last year (2018). They installed a porous asphalt pavement parking lot with associated outlet structure instead of standard impervious materials. Types of Pervious Paving & Recommendations In recent years, cities across Michigan such as Birmingham, Detroit, Grand Rapids, and Ann Arbor have permitted or incentivized the use of pervious or permeable surfaces as driving or parking surfaces in addition to standard hard-surfaced materials. In general, to achieve intended stormwater management goals, permeable or pervious surfaces are used as part of a larger infiltration system. A permeable hard surface generally sits atop stone aggregate that holds water until it can soak into underlying soils or drain away through some sort of underdrain. These underlying components store water allowing it to infiltrate and structurally support the surface. The components are also separated from adjacent native soils with geotextile fabric that prevents the stone voids from being filled with silt and sand. Diagram from Natural Resources Conservation Service Iowa Attachment 1 Pervious Paving Report Page 3 of 5 The type of permeable driving surface or porous pavement infiltration system to use for a location is dependent upon several factors. Considerations include: Proposed use – is this a residential driveway or a well-trafficked commercial parking lot? Soil conditions – how well can underlying soils hold water? Is there known contamination to consider? Location of water table Topography – how much grading would be required to achieve stormwater management goals? Maintenance – vacuuming and flushing is required for porous concrete or asphalt; vegetation in-between pavers must be edged and weeded Climate – road salt migration, infiltration runoff freezing and causing frost heave Cost – pervious or permeable systems can be more costly upfront, but may present stormwater management savings down the road The table below gives an overview of types of pervious paving and presents recommendations for their adoption in the City of Royal Oak. Type Details Assessment Porous asphalt or pervious concrete Porous asphalt or -Acceptable for one- or concrete is similar in two-family residential appearance to standard driveways. asphalt or concrete but -Acceptable for uses with is produced without higher vehicular traffic retaining fine when part of larger aggregates, allowing for infiltration system and increased permeability. designed for those higher volumes and weights. Permeable pavers Permeable pavers are -Acceptable for one- or an interlocking grid of two-family residential units (often concrete) driveway. that provide surface -Acceptable for uses with area to be filled with higher vehicular traffic some sort of pervious when part of larger material such as gravel infiltration system and or vegetation.2 designed for those higher volumes and weights. The image to the left is from the 11 Mile Parking Structure egress lane. 2 SEMCOG. (2008). Low Impact Development Manual for Michigan. Retrieved from https://semcog.org/Reports/LID/index.html. Attachment 1 Pervious Paving Report Page 4 of 5 Reinforced turf Grass or turf that is -Acceptable for low- reinforced with plastic trafficked areas such as or concrete geocells. fire access lanes. Loose Aggregate - Gravel Loose aggregate or -Not recommended. rock fragments, ranging Requires proper in particulate size. maintenance, gravel can generate dust and debris, and requires more care during the winter months. Loose aggregate can easily be spread onto sidewalks and roadways causing a hazard to pedestrians, bicyclists and vehicles. It can also end up in city sewers causing blockages. Zoning Ordinance Amendments Per the information in this report, the planning division has allowed permeable or pervious materials to be used for parking, drive aisles, and driveways, in some cases. If the commission desires clarification to be added to the zoning ordinance the following could be referred to the planning commission for its consideration and processing. §770-109 (A) Off-street parking lot design and construction. Current Language All such parking lots, maneuvering aisles, driveways, and loading areas shall be hard surfaced with asphalt or concrete pavement, shall be graded and drained so as to dispose of surface water pursuant to City ordinance and shall be completely constructed prior to a certificate of occupancy being issued. Revised Language All such parking lots, maneuvering aisles, driveways, and loading areas shall be surfaced with asphalt, concrete, or an acceptable pervious pavement, as determined by the Zoning Administrator. Acceptable pervious pavements include porous asphalt or concrete and Attachment 1 Pervious Paving Report Page 5 of 5 permeable pavers. Gravel and other loose aggregates are strictly prohibited. All such parking lots, maneuvering aisles, driveways, and loading areas shall be graded and drained so as to dispose of surface water pursuant to City ordinance and shall be completely constructed prior to a certificate of occupancy being issued. Implications for Stormwater Ordinance Currently, the objective is to expand the allowable driving surfaces in the City of Royal Oak for lower-traffic locations, such as residential driveways. To consider a non-engineered system as permeable, the proposed treatment should be tested for infiltration using a recognized method (USGS or other). There are standard methods for measuring permeability that could be a requirement for any proposed alternate surface to be considered a permeable. For projects where stormwater detention is triggered, utilization of a pervious pavement infiltration system will be permitted, if the requirements of the stormwater ordinance are met. .