Low Impact Development (LID) and Bioretention Techniques

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1 Coastal Training Program North Inlet Winyah Bay National Estuarine Research Reserve P.O. Box 1630 Georgetown, SC Ph. net Low Impact Development (LID) and Bioretention Techniques Highway 707 Murrells Inlet, SC Ph Fax

2 Introduction Low Impact Development Integrated Management Practices Landscaping Tools Bioretention What/Where/Why Pollutant Removal Mechanisms Bioretention Planning & Design Construction Techniques Constructed Wetlands Conclusions Highway 707 Murrells Inlet, SC

3 What is Low Impact Development? Why is it so important to Storm Water Management Definition: Low Impact Development (LID) is an innovative storm water management approach with a basic principle that is modeled after nature: manage rainfall at the source using uniformly distributed decentralized microscale controls. The goal of LID is to mimic a site's predevelopment hydrology by using design techniques that infiltrate, filter, store, evaporate, and detain runoff close to its source. LID addresses storm water through small, cost effective landscape features located at the lot level instead of conveying, managing and treating storm water in large, costly end of pipe facilities located at the bottom of drainage areas. Integrated Management Practices (IMPs) are known as are the building blocks of LID. IMP s can include landscape features such as sidewalks, open space, rooftops, streetscapes, parking lots, and medians. Almost all components of the urban environment have the potential to serve as an IMP.

4 Integrated Management Practices The Building Blocks to Low Impact Development Conservation and Minimization (reduced impervious surfaces) Storage (capture and reuse) Conveyance (alternative discharge strategies) Landscaping (ground cover, bioretention, rain gardens) Infiltration (trenches, exfiltration, biobasins)

5 Integrated Management Practices Landscaping Tools Native Groundcover Landscaping Bioretention Filter Strips Rain Gardens Fish Ponds Green Alleys Image courtesy of

6 What is Bioretention? Definition: An engineered process to manage storm water runoff, using the chemical, biological and physical properties afforded by a natural, terrestrial based community of plants, microbes and soil. Bioretention provides two important functions: (i) water quantity (flood) controls; and (ii) improve water quality through removal of pollutants and nutrients associated with runoff. Image courtesy of Sustainable Stormwater Water Quantity Bioretention facilities act like sponges by retaining rainfall and then slowly releasing excess water during more arid seasons. Their temporary storage capacity helps reduce erosion and limit flooding. Water Quality Bioretention facilities improve water quality by filtering inflow to lakes, rivers, and streams. The vegetation traps sediments and remove nutrients from runoff and surrounding soil. This reduces the growth of invasive species, which deteriorates waterway health by stealing the oxygen that plants and animals need for survival.

7 Water Quality & Bioretention Pollutant Removal Mechanisms Non point source pollutants & their typical sources Sediments (agriculture, erosion, construction, utility installation) Nutrients (fertilizers, manure, organic waste) Oils and Greases (Cars, trucks, equipment) PAH s (Vehicle emissions) Metals (vehicle emissions, manufacturing, fuel burning activities) Pathogens (bacteria/viruses pets, septic systems, wildlife) Pesticides/Toxic Chemicals (specific applications or spills) Physical Capture/Filtering trapping particles in vegetation/detritus, settling out in standing water bodies, adsorption to surfaces Biological Degradation breakdown of pollutants through physical or chemical processes Uptake of Nutrients plant materials utilize nutrients for growth, some plants can absorb metals and toxics

8 Water Quality & Bioretention Physical, Chemical & Biological Mechanisms Courtesy of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Published: May 1999

9 Bioretention Structure and Function Bioretention facilities (rain gardens) may range from simple shallow depressions to more complex designs, but all are structurally engineered to provide the following functions with respect to storm water quantity control: interception, capture, infiltration, filtration, storage, and water uptake by vegetation. The nine major components of the bioretention facility are: Pretreatment Flow Entrance Ponding Area Plant Material Organic Layer or Mulch Planting Soil and Filter Media Pea Gravel Diaphragm Underdrain and Outlet Surface Overflow

10 Pretreatment Pretreatment allows settling and filtering of sediments and suspended solids at the entry point Vegetated Buffer Strips and grass buffer strips provide an extra level of pretreatment Pretreatment methods can include: Forebay Pretreatment swale/channel Mulch layer Sand filter layer Gravel diaphragm Upflow inlets

11 Flow Entrance Flow entrances reduce the water velocity and dissipate erosive energy Sheetflow over grassed area is preferred but not always available due to site constraints Flow entrances can include: Landscape stone Surge stone Riprap aprons Upflow inlets Image courtesy of Bioretention.com

12 Ponding Area Ponding area provides surface storage of the runoff and allows evaporation to occur Settling of particulates occurs in the ponding area for additional treatment of runoff Ponding design depths are kept to a minimum to reduce hydraulic overload of soils and mulches and maximize surface area/facility depth ratio Maximum desired ponding depth 6 12 Pooling times of 4 6 hours are met by using sandy soil mediums for bed

13 Plant Material Plant species bind nutrients and other pollutants through uptake Plants remove water through evapotranspiration Plant roots create pathways for infiltration Roots provide a media for bacteriologic growth A variety of plants increases biologic diversity and reduces insect and disease infestation Planted islands provide wind breaks, reduce heat island effects, and improve aesthetics for parking areas Plant materials support wildlife habitat and absorb noise

14 Organic/Mulch Layer Organic/mulch layer provides a medium for biological growth microorganisms feed on organic pollutants Mulch is sustained by decomposition of organic materials added to site Organic/mulch layer provides a site for adsorption and bonding for heavy metals and other pollutants Surface layer of mulch protects soils from drying and erosion; and filters particles/pollutants in runoff Organic/mulch layer should not exceed 3 in depth (too much mulch can affect oxygen transfer to roots)

15 Planting Soil and Filter Media Planting soil provides a location for water and nutrients to transfer to plant roots Upper soils zones are designed to increase biological activity and stimulate root growth Macrofauna (bugs and worms) reduce organic matter into smaller parts as part of decomposition process and provide soil aeration and redistribution of particles Bacteria and fungi break down complex organic compounds into simpler forms and deliver nutrients in a form usable by plants Plant roots contain symbiotic microbes that enhance water retention and nutrient uptake Soil fines provide physical bonding sites for heavy metals, hydrocarbons, nutrients and other pollutants

16 Pea Gravel Diaphragm Pea gravel diaphragms separate the soil medium from the gravel blanket around the underdrain Pea gravel diaphragms create a filtering mechanism that is less likely to clog compared to geotextiles Pea gravel diaphragms are typically between 2 to 9 thick depending on gradation Pea gravel can also be used in place of the underdrain gravel if pipe perforations are less than ¼

17 Underdrain/Outlet Underdrains ensure proper drainage for the plant materials and allow for proper infiltration Underdrains provide a discharge point to ensure that the hydroperiod does not stress the plants Required for sites with tight clay soils or highly visible locations Proper drainage enhances biological activity by providing an aerobic condition in the root zone and reduces the potential for mosquito problems Vertical location of underdrains is key to creating anaerobic zones that process nitrates in runoff

18 Surface Overflow A surface overflow should be included in all designs to allow excess runoff to safely discharge downstream Overflow design should consider storm event, connected impervious surfaces, and erosion risks Image courtesy of Bioretention.com Overflows typically include a stormdrain inlet or catchbasin to allow safe discharge of runoff

19 Putting It Together Bioretention is one of several Integrated Management Practices that can be utilized on new projects to reduce the peak runoff rates Retrofit projects can implement multiple Integrated Management Practices at one site based on need and fit Goal is to mimic natural hydrology by retaining on site, infiltration of stormwater and treatment of runoff Techniques used will depend on factors such as size of site, soils, budget, and client Image courtesy of lowimpactdevelopment.org

20 Moss Park Constructed Stormwater Wetlands Native species are selected to ensure success Imported hydric soils add seedstock and rootstock to the plantings to improve success Planting helps to reduce invasive species

21 P.O. Box 1630 Georgetown, SC Ph. net Moss Park Constructed Stormwater Wetlands

22 Ricefields Created Wetlands EARTHWORKS designed a created wetland to replace the form and function of the previous finger in a location near the original site to maintain lots Proper design and construction provided project success in 2 years and additional monitoring was not required Success was achieved by providing proper soils and hydrology to allow vegetation to thrive. Import of hydric soil provided seedstock and rootstock that eventually outgrew the planted materials

23 Conclusions LID is simple and effective Integrated treatment and management measures Distributed across the site at the source Mimics existing hydrology and water quality regime LID is economical Costs less than conventional stormwater management systems Lower maintenance costs Increased lot yield or open space Enhanced property values LID is flexible Offers a wide variety of structural and nonstructural techniques Provides both runoff quality and quantity benefits Works in highly urbanized constrained areas Any feature of the urban landscape can be modified to control runoff LID is a balanced approach Advanced, ecologically-based land development technology Integrates the built environment with the natural environment Maintains predevelopment watershed and ecological functions