Community LID Workgroup Issue Paper #6 Topic: Hard and Impervious Surface Coverage Limits Objective: Minimize the amount of hard and impervious surfaces in developments to allow more stormwater to infiltrate into the ground on site. Background: Impervious surfaces, by definition, are materials that prevent the infiltration of water into the soil. The most common impervious surfaces in the built environment are roads, rooftops, sidewalks, and patios. While these structures are almost 100% impervious, other features such as gravel roads, compacted soils, and even lawns are impervious to varying degrees, as they allow for less infiltration than forests and other natural ground. As development increases, so does the amount of impervious surface, which leads to changes in the way water is transported and the hydrology of a drainage basin. Stormwater runoff resulting from increased impervious surfaces affects both the quality and quantity of water entering natural water bodies in many ways. Stormwater runoff can lead to severe environmental impacts such as flooding, habitat loss, erosion, channel widening, and streambed alteration. Along with increased runoff comes decreased infiltration, which reduces groundwater supplies and may lead to a lowering of the water table. Ground water provides a consistent water supply to streams, wetlands, and lakes, and decreases in ground water supply may cause a stream or wetland to dry out during months when precipitation is low. As a general rule of thumb, when impervious surfaces exceed 10% of a basin (the entire area that drains into a stream) adverse environmental impacts can be measured in the stream although impacts can occur in rural basins at impervious surface thresholds as low as 2%. Stream basins with above 25% impervious area generally have degraded water quality. Water Cycle Changes Associated with Urbanization Currently, within Thurston County s Zoning Code (Title 20), some zones have impervious surface limits, while others only limit the extent a lot can be covered by buildings (this excludes
from consideration hard surfaces such as parking lots, driveways, or patios). Impervious surfaces also are limited within the Shoreline Master Program for most shoreline lots the limit is 30% lot coverage. In addition, any development in unincorporated Thurston County that fits the criteria below requires a review by Thurston County Water Resources staff to ensure it meets runoff flow-control standards: More than 5,000 square feet new impervious surfaces Converting more than three quarters of an acre from native vegetation to lawn or landscaping Clearing of more than two-and-a-half acres of native vegetation to pasture Depending on soils and property-specific characteristics, flow-control mechanisms may be put into place to manage stormwater. The new stormwater manual requirements from Ecology address the total amount of hard surfaces on a site hard surfaces include impervious, paved areas, as well as semi-pervious areas, including permeable pavements and vegetated roofs. This 2.2 acre property contains a large home, detached garage, and driveway. The impervious area is 10 percent or 9,900 square feet. Analysis: Thurston County s zoning code has an inconsistent approach to addressing impervious surfaces. Where limits have been set they are typically located within sensitive zoning districts, such as the R 1/10, R 1/20, sensitive basins such as McAllister Geologically Sensitive Area, and Green 2
Cove Creek Basin, or on soil types with low infiltration rates. In other rural zoning districts there are sometimes building coverage limits and/or impervious surface limits that range as high as 60% in rural areas, or no mention of limits at all. Sixty percent is much higher than the actual impervious area on an average developed rural residential lot. Indeed, if all new residential development occurred with such a large amount of impervious area, it would likely lead to degradation of stream health and water quality. For a related study, Thurston Regional Planning Council staff digitized the impervious area for 92 properties that contained a single residential dwelling unit built after 1995 and was considered fully developed based on zoning density. Using this sample data set, the following relationship of rural lot sizes to impervious area were developed: Acres Lot Size Square Feet Typical Zoning Density Average Percent Impervious Average Square Feet Impervious Number of Parcels in sample 0.07 to 0.2 3,000-8,700 4-16 units per acre 52% 3,000 9 0.2 to 0.9 8,700-39,200 3-6 units per acre 2 units per acre 27% 3,900 10 0.9 to 1.8 39,200-78,400 One unit per acre 15% 8,600 16 1.8 to 4.6 78,400-200,400 One unit per 2 acres 9% 8,500 11 4.6 to 9.5 200,400-413,820 One unit per 5 acres 5% 13,000 24 9.5 to 19.5 413,820-849,400 One unit per 10 acres 3% 18,300 7 19.5 to 40 849,400-1,742,400 One unit per 20 acres 3% 37,200 6 40+ ac 1,742,400 plus One unit per 40 acres 1% 36,100 9 The table above shows average percent impervious area. The range is quite high depending on how long driveways are and whether the garage is attached or detached. Thurston County staff discussed the following options, related to impervious surface limits: 1. No change. Impervious surface limits would remain as they currently exist in code and are only applicable for some zones. This option would not reduce new impervious surfaces. 3
2. Develop consistent impervious surface limits for all sensitive zones. This approach would develop more restrictive hard and impervious surface limits for the areas identified as most in need of protection, including Long Term Agriculture, Long Term Forestry, RR 1/10, RR 1/20. Impervious surface limits would remain inconsistently applied to other zones, and the limit for RRR 1/5 (which is the largest zone in the county, by area) would remain high at 60%. An alternative that was discussed was to apply limits to all residential and resource zones, but to exclude commercial and industrial zones. 3. Develop consistent impervious surface limits for all zones. This approach would develop hard and impervious surface limits for all zones, including commercial and industrial zones. Limits would be based on typical use within zones. The limits could be partnered with incentives to allow flexibility of design in exchange for certain desirable practices, such as additional tree retention or use of permeable pavements. 4. Develop impervious surface limits based on basins or watersheds, with an associated impervious surface trading program. This approach would rely on basin studies that set an appropriate limit based on environmental conditions in a particular area, similar to how the County has set impervious limits for Green Cove Creek basin and the McAllister Geologically Sensitive Area. Recommendations: Wherever possible, clarify definition of impervious surface and hard surface across different sections of the code to be consistent with that used in the Drainage Design and Erosion Control Manual (DDECM). Use the term hard surface limits rather than impervious surface limits to be consistent with new guidance from Ecology. Option 3 develop consistent hard and impervious surface limits for all zones (see attached table for current and proposed new limits. For residential and resource lots less than 5 acres, limit is 60% or 10,000 square feet, whichever is less. Preexisting hard surfaces or structures can be replaced within the existing footprint For lots greater than 1 acre, an additional 5% may be used for buildings related to agriculture or forestry practices. Use of pervious surfaces can be estimated at 75% of total area for calculating total hard surface area. Hard surface area limit may be increased with a special use permit. Credit Option 1: o Hard surface limits can be increased by 50% under the following conditions: Soils on site allow for full dispersion of stormwater runoff 4
Landscape plan shows retention of existing native trees and vegetation on at least 20% of the site. Up to 20% of these trees can be in a critical area or buffer. The development is a Planned Residential Development (PRD) or Planned Rural Residential Development (PRRD) o Hard surface limits can be increased by 100% in residential and resource zones under the following conditions: Landscape plan shows retention of existing native trees and vegetation on at least 65% of the site. Credit Option 2: limit if landscape plan shows 6,500 square feet retention of native trees and vegetation that are outside of any designated critical areas or buffers. Retained trees would need to be recorded on the final plat and in the abbreviated drainage plan. limit if landscape plan shows 3,250 square feet retention of native trees and vegetation connecting designated critical areas or buffer. Retained trees would need to be recorded on the final plat and in the abbreviated drainage plan. limit if landscape plan shows 3,250 square feet replanting of native trees and vegetation within a riparian or wetland buffer area. Retained trees would need to be recorded on the final plat and in the abbreviated drainage plan. Community LID Workgroup Discussion: What do you think of the recommended changes to impervious surface limits and credit options? Are there additional incentives or credits that would be more effective or desirable? Are limits for commercial and industrial zones too constraining? 5