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The county will act as a pathfinder for LID in southern California. With the recent adoption of Order R9-2007-001 in January 2007 as issued by Region 9 of the California Regional Water Quality Control Board, the 21 San Diego co-permittees became subject to a new municipal separate storm sewer system (MS4) permit. The County of San Diego has taken the lead permitting role among a group of 18 other incorporated cities, as well as the Unified Port District and Regional Airport Authority. The re-issuance of the San Diego region permit is the first in a series of anticipated MS4 permit changes by the state Water Quality Control Board that affect various jurisdictions within southern California. These jurisdictions include Orange County and Los Angeles County. The newly issued permit represents an extension of an original National Pollutant Discharge Elimination System (NPDES) permit granted in 1990, promulgated by the federal Clean Water Act as well as the California Porter-Cologne Act. Since its original inception, the permit has been reissued only once, in 2001. Four key elements within the revised portion of the permit have captured the attention of the local engineering and building community:
The County of San Diego has initiated compliance with the new permit by forming a technical advisory group to draft a LID Handbook. The handbook, which was recently subject to a 30-day public review period, is intended as a non-regulatory guide to assist local developers in the land planning and design process until a more formal update of the county’s Standard Urban Stormwater Management Plan (SUSMP) can be completed. The LID Handbook incorporates design concepts already successfully used in areas such as Prince George’s County, MD; Santa Clara County, CA; and Australia. LID design principles seek to mimic natural runoff patterns, velocities, volumes, and duration. They are fundamentally based on the concept of disconnecting impervious surface runoff from a direct tie-in to the municipal storm drain system. This can be accomplished by a number of site-design best management practices (BMPs) that promote areas of infiltration within a site, as well as use of pervious materials in roadway and sidewalk areas. Site-design BMPs are by no means new to jurisdictions within the County of San Diego or elsewhere within southern California. What makes LID unique is the integration of multiple site-design BMPs wherever possible throughout a site, as opposed to a single “end-of-pipe” treatment. The implications for site planning and land use are significant, as LID typically requires allocation of additional space to allow runoff from the developed areas to infiltrate pervious areas in much closer proximity to the source. The County of San Diego LID Handbook illustrates numerous BMP design concepts such as clustered residential developments, infiltration and bioretention strategies, and low-impact street and parking areas that feature reduced widths, street trees, curb cuts to vegetated areas, and permeable pavement alternatives. Special consideration has been given to infill and urban redevelopment projects because they are expected to be a substantial portion of the construction effort in the San Diego region over the coming years. To successfully implement LID, the County of San Diego and the 20 other co-permittees must considerably revise many local ordinances and design manuals, most notably each permittee’s local SUSMP. To completely comply with the new MS4 permit, a wide variety of other publications would require revision, such as local hydrology and hydraulic design manuals, stormwater and grading ordinances, regional standard drawings, and, potentially, street design manuals, because LID concepts include reduced street widths and permeable pavement application. In addition, other discretionary-level processing of zoning, land-use entitlement, and environmental review are expected to be impacted among each of the co-permittees. Christine Sloan, watershed and environmental planner for the county, notes, “In the County of San Diego, review and revision of the watershed protection, stormwater management, and discharge control ordinance is just beginning. In addition, county staff anticipates possible modification of other land-use ordinances, including the zoning ordinance, the county parking standards manual, grading ordinance, and landscaping requirements, among others.” In addition to administrative hurdles and an ever-present potential for regulatory conflicts, LID implementation will face a host of engineering and construction challenges. The details related to identifying and solving these challenges have only reached the stages of initial discussion by local industry experts.
As mentioned, much of San Diego County’s expected construction activity is predicted to be within the realm of redevelopment or infill. Incorporation of LID into redevelopment projects is critical to achieving overall water-quality goals at the regional level. Because most of the region’s stormwater-quality threats come from existing developments, the solution to improving runoff water quality would seem to lie within redevelopment. While accomplishing LID in urban settings is not impossible, it is exceptionally difficult given the minimal opportunity for landscaping, allowable building coverage ratios, and tight setbacks. Scott Molloy with the San Diego Building Industry Association (BIA) cautions, “In order to meet a whole host of other planning objectives—including providing adequate housing for future population growth, utilizing our remaining land more efficiently, and making transit viable, to name a few—redevelopment and infill projects will need to be planned at densities high enough to support multifamily projects. Multifamily projects will not have the ability to preserve significant amounts of land for infiltration and infiltration adjacent to buildings or utilities [since this] will present health and safety hazards that cannot be ignored.” He adds, “The most sensible approach for urban and redevelopment projects might be to identify potential offsite areas within the community that could serve as LID infiltration areas for the community as a whole, as well as [for] future development projects. Community and neighborhood parks, local golf courses, and other large, open landscaped areas are some examples of places where a community-level, as opposed to a site-by-site, approach to LID could be implemented. Health and safety as well as use compatibility issues would need to be addressed with this type of approach. With the growing concern with water quality and the health of our environment, and the increasing need for more energy-efficient, resource-efficient development, it is reasonable to anticipate that all future development will be required to address its stormwater runoff, whether it be through participation in offsite community-level LID strategies or through onsite strategies or a combination of both. Project areas and individual projects will need to be evaluated carefully to determine the most appropriate approach.” As local zoning and development guides are revised to meet the goals of the MS4 permit, the concept of “joint use” may come into significant play, especially as it relates to urban infill and redevelopment. The county LID Handbook acknowledges that in some instances, any given LID concept may not be feasible due to situational constraints, be they administrative or technical. This problem could be overcome with land-planning provisions to incorporate parks and other landscaped areas within urban limits, providing both recreational and water-quality benefits. Joint-use areas could serve multiple projects on a sub-regional level in situations where onsite application is simply not practical. Sloan explains, “Although the County of San Diego’s LID program is still in its infancy, it is looking into the possible use of incentive programs, as well as the possible mitigation of LID requirements through the use of pocket parks using LID techniques or the use of LID-infused landscape designed common areas.” Whether in urban settings, redevelopment situations, or raw undeveloped areas, incorporation of LID will face other engineering challenges related to local soil conditions and other potential conflicts that will require physical improvements. Much of the county consists of soils rated with a hydrologic classification as Type C or Type D (having a slow rate of water transmission, high potential for runoff, and higher water tables). This means that employing any infiltration or bioretention at sites consisting of these soils would involve the importation of select material, which may or may not be economically feasible given the extent of required treatment volume. Concerns have also been raised as to the potential impact from infiltration areas in close proximity to buildings, concrete/pavement, or the underlying groundwater table. These concerns include the potential for differential settlement on building foundations, slope failure, pavement subgrade failure, and possible groundwater contamination from pollutants within the infiltrated surface runoff. The county LID Handbook stresses the importance of accurate geotechnical evaluation for any potential use of infiltration or permeable pavement. Geotechnical considerations are a critical issue that may ultimately restrict the use of LID practices for many types of development. The elimination of excess irrigation is very important when using LID measures, because the volume of runoff from this water source can exceed average annual rainfall. It is also important to note that treatment goals for LID relate to smaller, more frequent design events. However, the county’s new MS4 permit in no way nullifies the present requirements to handle high-volume flood control conditions. A future challenge yet to be tackled is the design of a system of LID low-flow treatment facilities that can work during overflow conditions in concert with traditional storm drain infrastructure as well as other typically anticipated utilities—such as water, sewer, and joint trench. As time has progressed, specialized departmental interest in each of these has led to increasingly greater required setbacks from other facilities. However, a goal within LID would be to minimize road widths. This could require parallel systems of drainage, one for high-volume events and another for low-volume, both of which would have to work without conflict with other water, sewer, and dry utilities in a reduced width section. Additionally, this would all have to be done without compromise to traffic level of service or the ability to maintain emergency vehicle ingress and egress. It is clear that the intended goals of LID are admirable as well as aggressive. So far, there are only a limited number of LID pilot projects in southern California and other arid regions, forcing future ventures to involve a certain amount of trial-and-error effort. Ultimately, only time will tell the effectiveness of LID in accomplishing the intended water-quality goals. Sloan summarizes, “The critical component to successful integration of LID into the southern California development landscape is education. Most people do not understand that LID is a stormwater strategy that has many different components, which involve planning, the use of natural systems, and small engineered solutions. There are a sizable number of folks who get hung up on the infiltration side of LID and miss out on the other LID techniques available that do not involve infiltration. Moreover, confusion exists equating green building, smart growth, and LID. As such, educating the public, the development and engineering communities, and regulators on the usefulness and limitations of LID in southern California’s arid environment is the key to the success for such programs. Further, on-the-ground working examples of LID will help in this education process and highlight this relatively new way of dealing with stormwater.” References County of San Diego, CA. July 20, 2007. Low Impact Development Handbook, Stormwater Management Strategies. Prince George’s County, MD. June 1999. LID Design Strategies, an Integrated Design Approach. Richard Lucera, P.E., CFM, is a project manager and senior associate with RBF Consulting in Carlsbad, CA. He specializes in stormwater management and land development engineering and recently served on the technical advisory committee for the County of San Diego LID Handbook. SW January/February 2008 |
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