5.2C – Managing Hydromodification Impacts – Low Impact Development

Management Measure

1. Conserve and protect natural resources.
2. Maintain or restore the pre-development hydrograph of the site by utilizing distributed, small-scale structural and non-structural management practices. Integrate the non-structural management practices during the site planning phase, before construction starts.

Low Impact Development (LID) is essentially a paradigm shift in the way storm water is managed. LID uses a watershed-scale as the planning unit and aims to protect, conserve, and enhance natural resources, especially those that enhance water quality and aquatic resources. Small-scale LID is designed to achieve the pre-development hydrograph of a site using a variety of distributed green infrastructure technologies that capture and treat storm water on-site, thus preventing polluted runoff from discharging into local waterways. Conventional storm water is managed using a centralized approach; storm water is funneled through various catchments (i.e. storm drains) and conveyance systems. In older cities, combined sewer and storm systems may be routed to a waste water treatment plant for treatment, however in most California cities and towns storm water is discharged directly into streams, bays, estuaries, and other local bodies of water though storm water outlets. Combined sewer and storm systems were very common prior to the early 1900s, e.g. older parts of Sacramento and San Francisco have combined sewer and storm systems. These combined sewer and storm water systems can overflow during storm events causing untreated sewage and storm water to flow directly into nearby waterbodies, thus resulting in violations of municipal National Pollutant Discharge Elimination System (NPDES) permits and causing harm to aquatic life and humans. Separate sanitary sewer systems also have the potential to overflow during high storm events due to system leaks and groundwater infiltration. Both combined storm and sewer, and separate sewer overflows are regulated under Municipal NPDES permits.

Storm water contains various pollutants which are discharged into water bodies without any treatment. Refer to the urban land-use category (MM 3) for more information on urban storm water.

Management Practices

Use watershed-based continuous rainfall simulation models such as EPA’s Storm Water Management Model (SWMMM) or Hydrologic Simulation Program Fortran (HSPF) to help with site design.

LID is a component of green infrastructure which includes regional or large-scale and distributed or small-scale management measures. More information on relevant management measures and practices can be found in various land-use categories within this Encyclopedia.

The following management measures are considered regional or large-scale green infrastructure components of LID and are included in the following sections of this Encyclopedia:

• 2 - Forestry:
  • Plant trees to absorb carbon and nutrients, and to retain sediment from eroding into nearby waterbodies, see MM 2H, Re-vegetation of Disturbed Areas, and
  • Maintain forested buffers surrounding streams to help with pollutant attenuation, flood control, and shading, see MM 2B, Streamside Management Areas.
• 6 - Wetlands, Riparian Areas and Vegetated Treatment Systems:
  • Protect open space, especially riparian habitat and wetlands by developing conservation easements, parks, or conjunctive-use management plans, see MM 6A, Protection and Conservation of Wetland and Riparian Areas;
  • Enhance or restore riparian habitat and wetland functions, see MM 6B, Restoration and Mitigation of Wetland and Riparian Areas;
  • Construct and maintain: vegetated filter strips, grassed swales, and vegetated buffers or storm water wetlands, see MM 6C, Vegetated Treatment Systems; and
  • Protect and restore natural drainage features, see MM 3.1A, Watershed and Groundwater Protection and MM 3.2D, Grading and Excavation.

The following management measures are considered small-scale green infrastructure components of LID and are included in the following sections of this Encyclopedia:

• Minimize (limit, reduce, or mitigate) and disconnect impervious surfaces. Smart growth and urban infill development includes limiting, mitigating, and/or reducing impervious surfaces. Management practices are discussed in MM 3.1C, Impervious Surfaces and MM 3.1E Land Development.
• Use bioretention, i.e. phytoremediation and natural biogeochemical cycling, to transform and attenuate common pollutants from storm water runoff at its source or as close to the source as practical. Green infrastructure components such as urban forestry, bioswales, rain gardens, and general landscaping are discussed in MM 3.1D, Landscaping.

Programs

• Stormwater Quality Design Manual for the Sacramento and South Placer Regions. May 2007. LID is discussed in Chapter 2 (and Step 3 in Chapter 3) promotes the conservation/use of natural site features, an important LID principle. Chapter 4 describes how site specific, lot-scale source control measures can keep pollutants from contacting runoff and leaving the site. Chapter 5 focuses on runoff reduction control measures integrated into site design; many of these techniques are intended to reduce site imperviousness, another important LID principle. And finally, Chapter 6 includes a variety of treatment control measures, some of which are intended to be applied at the lot level for pollution and runoff control.
• Contra Costa County, New Development/Provision C.3. requirements include LID BMPs such as "Project site designs must minimize the area of new roofs and paving. Where feasible, pervious surfaces should be used instead of paving so that runoff can percolate to the underlying soil. Runoff from impervious areas must be captured and treated. The permit specifies ways to calculate the required size of treatment devices."

Information Resources

• Bay Area Hydrology Model (BAHM) is a tool for analyzing the potential hydrograph modification effects of land development projects and sizing structural solutions to mitigate the increased stormwater runoff from these projects. This software was developed for use in three counties in the San Francisco Bay Area: Alameda, San Mateo and Santa Clara.
• Building Green has information on LEED accredited buildings and the green infrastructure and LID technologies that can help achieve LEED certification.
• Low Impact Development Center contains technical references for implementing low impact development techniques and has case studies of sites where these practices have been successfully implemented.
• Urban Design Tools, provides watershed managers with a new set of tools and techniques that can be used to meet regulatory and receiving water protection program goals for urban retrofits, re-development projects, and new development sites.
• Alameda County. 2005. Hydrograph Modification Plan, Part A: General Provisions For Hydromodification Management.
• BASMAA. May 2003. Using Site Design Techniques to Meet Development Standards for Stormwater Quality. This guidance document is for planning and design for new development.
• Bay Area Stormwater Management Agencies Association (BASMAA). 1999. Using Site Design Techniques to Meet Development Standards for Stormwater Quality,. Residential site planning and design guidance manual for stormwater quality.
• Bicknell, Beyerlein, and Feng. The Bay Area Hydrology Model - A Tool for Analyzing Hydromodification Effects of Development Projects and Sizing Solutions, presented at the California Stormwater Quality Association Annual Conference, September 26, 2006.
• California Landscape Contractors Association is a non-profit trade organization of licensed landscape and landscape-related contractors. Also included among its approximately 3,200 members are landscape suppliers, landscape architects, public officials, educators, and students.
• City of Emeryville. December 2005. Stormwater Guidelines for Green, Dense Redevelopment - Stormwater Quality Solutions for the City of Emeryville.
• Department of Water Resources A guide to Irrigation Water Needs of Landscape Plantings in California: The Landscape Coefficient Method and WUCOLS III provides information on estimating the irrigation water needs of landscaping plantings in California using the landscape coefficient method and provides data on the specific water use data on landscaping plants using WUCOLS III (Water Use Classification of Landscape Species).
• Friends of the San Francisco Estuary. August 2005. Protecting Water Quality in Development Projects - A Guidebook of Stormwater Best Management Practices in Action.
• Santa Clara Valley Urban Runoff Pollution Prevention Program, Hydromodification Management Plan (HMP) was prepared in response to the NPDES Permit C-3 requirements to address the impacts from new and redevelopment projects on stream morphology, habitat, and erosion potential.
• Stormwater, Funkhouser L. July/Aug 2007. Stormwater Management Adaptation to Climate Change. Discusses the role of LID in sustainable development.
• UC Davis Extension Center for Water and Land Use, Low Impact Design provides tools, resources, courses, and workshops on low impact design techniques.
• US Green Building Council has some information relevant to smart growth on their research and publications page.
• USEPA Green Infrastructure web pages has information and web URL links to basic information including a glossary of terms; partnership and funding opportunities; tools such as models and calculators; case studies; and other technical information.

References

USEPA. 2007. National Management Measures Guidance to Control Nonpoint Source Pollution from Hydromodification. EPA 841-B-07-002. U.S. Environmental Protection Agency, Washington, DC. (http://www.epa.gov/owow/nps/hydromod/index.htm)

(Updated 10/21/09)