Low Impact Development in Northern Nevada: Bioretention

Transcription

1 Nonpoint Education for Municipal Officials Protecting water quality through community planning Low Impact Development in Northern Nevada: Bioretention Fact Sheet FS Benefits of Bioretention IMPROVES water quality REDUCES postdevelopment stormwater volumes and offsite flows INCREASES groundwater recharge COSTS LESS to design, install and maintain than conventional stormwater technologies PROVIDES attractive landscaping ENHANCES wildlife habitat Copyright 2009, University of Nevada Cooperative Extension. All rights reserved. By Susan Donaldson, Water Quality Education Specialist L UNIVERSITY OF NEVADA COOPERATIVE EXTENSION ow Impact Development (LID) is a sustainable strategy for handling stormwater that uses techniques that aim to mimic the way water behaved on the site before development occurred. LID uses a variety of design and pollution-prevention techniques, including bioretention, to compensate for impacts such as increases in runoff flow rate and volume and water-quality impairment. One of the most recognized and widely applied sustainable stormwater management techniques is called bioretention. How do bioretention features work? Bioretention is the practice of routing stormwater to landscaped depressions filled with porous soil and covered with a thin layer of mulch. Unlike typical landscape designs that focus on raised planters or islands, these features store water in gentle depressions. Plants, including grasses, shrubs and small trees, are grown in the bioretention feature. The plants promote evapotranspiration, maintain soil porosity, encourage biological activity and absorb some pollutants. When located over silt- or clay-rich soils with slow infiltration rates, bioretention features typically require perforated drainage pipes called underdrains to facilitate proper drainage. Runoff from an impervious area, such as a parking lot or roof, is directed into a bioretention This residential rain garden captures and filters runoff water. area. The flow can be contained within a planter box located at ground level or be diverted into a larger landscaping feature. The water infiltrates through the plant/mulch/soil environment, providing pollutant treatment. Infiltration rates must be tested prior to installation to ensure water does not remain ponded in the feature for longer Chris Conway than a few days to discourage mosquitoes and other vermin. Supplemental irrigation is needed for most bioretention features in northern Nevada to ensure plant survival during extended dry weather periods. Bioretention features are designed to retain the stormwater runoff volume produced by the most frequently occurring

2 Page 2 Low Impact Development in Northern Nevada: Bioretention LID is used because it is: EFFECTIVE in reducing and treating urban runoff; ECONOMICAL, when less infrastructure is required for LID practices than for traditional stormwater features; SIMPLE! Most LID features are not highly engineered, expensive or elaborate; FLEXIBLE and customized for each site; USEFUL in achieving multiple objectives: stormwater management, landscape beautification, habitat improvement, open space connectivity, etc.; and DESIRABLE, because it adds value when the landscape features used to achieve LID goals are also aesthetically pleasing. This can increase property values. rainfall or runoff events. This is determined from records of storm events in the area. For larger storms, most large bioretention features are designed so that any overflow of stormwater runoff is diverted to a conventional stormwater conveyance system, such as a storm drain. Even though the bioretention feature may not be able to retain all the stormwater from these large events, some settling and filtering of the stormwater occurs before it overflows to the conventional storm drain system. Where can bioretention be used? Bioretention features can be constructed on individual sites, within a new development, or even retrofitted into an existing landscape. This technique can be as simple as directing the downspout of gutters to a landscaped garden bed or rain garden instead of the driveway or street. In a commercial setting, bioretention can be incorporated into parking lot islands and perimeters, street median strips, driveway perimeters, the centers of cul-de-sacs and virtually anywhere landscaping is planned, including planter boxes located next to buildings. While they should be planned in advance, bioretention areas are one of the last features to be installed within a new development. To prevent clogging and provide pretreatment, include grass buffer strips or swales leading into the feature. Consider existing vegetation and drainage patterns and the location of existing or proposed underground utilities, including septic or sewer systems, water lines and storm drain pipes, when designing the system. These systems are most cost-effective when applied to areas with minimal slope, well-draining soils and low sediment loads, which helps to discourage clogging. Mark areas that will incorporate bioretention into native soils before work begins to avoid soil compaction during construction. Vehicular traffic other than what is specifically necessary to construct the facility should not be allowed within 10 feet of planned bioretention areas. If proposed to be located in hotspot drainage areas near gas stations or other businesses that have high potential pollutant loads, or areas with known soil and/or groundwater contamination, bioretention features must be designed as flow-through planters that do not allow infiltration into underlying soils. Flow-through bioretention facilities are designed to temporarily store stormwater within an impermeable container, such as a concrete box or a liner. These facilities collect filtered stormwater in perforated underdrain pipes located at the bottom of the impermeable container and drain to an approved discharge point, such as a ditch or storm drain. Where an approved discharge point is available, flow-through bioretention facilities can also be used in areas that have poorly draining soils, where the water table is within 6 feet of the ground surface, next to Do rain gardens actually work to improve water quality? The pollutant removal effectiveness of bioretention basins has been the focus of several studies. The publication, Design Guidelines for Stormwater Bioretention Facilities, University of Wisconsin-Madison (2006), lists the following typical pollutantremoval rates for these facilities. Pollutant Total phosphorus Metals (copper, zinc, lead) Total Kjeldahl nitrogen Total suspended solids Ammonium Organics Bacteria Removal Rate 80% >90% 65-75% 90% 60-80% 90% 90%

3 Low Impact Development in Northern Nevada: Bioretention Page 3 buildings, on steep slopes, or plants will be best adapted to has failed, it can be excavated at sites with other constraints the local climate, better able and rebuilt at the same site if that prevent infiltration of to withstand pests, require less infiltration rates are sufficient or stormwater into underlying soils. maintenance and provide habitat underdrains are constructed. How are bioretention features maintained? for wildlife. Select plants that are able to withstand submergence for a few days during extended Rain gardens One of the most common Rain gardens are more than just depressions in the ground containing plants. They Maintenance needs include storm periods. See FS-09-28, bioretention designs for capturing must be carefully designed typical landscape maintenance LID in Northern Nevada: Plant runoff in a landscaped area is to capture the appropriate tasks, such as pruning, mowing, Materials, for suggestions. called a rain garden. The rain volume of stormwater runoff, adjusting and monitoring In addition to routine garden is depressed below drain quickly, avoid erosion irrigation systems, hand weeding, landscape maintenance, be adjacent impervious surfaces, and blend with the landscape. trash collection and replacement sure to inspect bioretention such as concrete driveways The design should include of dead plants. The mulch layer areas regularly for accumulated or paved roadways. The a safe overflow area, such should be removed if it is reducing sediment, damage and standing water drains into the garden, as a storm drain, ditch or the water storage volume, and the water. Check cells 48 hours where it gradually infiltrates open space area, in case surface soils should be excavated after a rainstorm to ensure they into the soil bed. Pollutants stormwater exceeds the if clogging has occurred. Minimize have completely drained. Each are removed by a variety of capacity of the rain garden. the need for fertilization by month, inspect and repair eroded processes, including adsorption, selecting well-adapted or native areas. Remulch bare areas as filtration, volatilization, ion plants. In most cases, native needed. If a bioretention area exchange and decomposition. USDA NRCS, 2007

4 Page 4 Low Impact Development in Northern Nevada: Bioretention For a description of various Low Impact Development practices, consult the following Fact Sheets in the Low Impact Development in Northern Nevada series: LID: An Introduction, FS Soil Considerations, FS Rainwater Harvesting, FS Bioretention, FS Vegetated Swales and Buffers, FS Green Roofs, FS Plant Materials, FS Porous Pavement, FS Roadway and Parking Lot Design, FS Maintenance, FS Rain gardens are used nationwide as a technique for cleaning up stormwater. In northern Nevada, they are also useful in conserving water for landscape use, which may become even more important as water costs rise. In addition to filtering runoff pollution and conserving water, rain gardens have many benefits. They create habitat for birds and butterflies, enhance sidewalk appeal, add interest to the landscape and reduce highwater-use turf areas. In addition, where native soils drain well, they recharge groundwater. Rain gardens are more than just depressions in the ground containing plants. They must be carefully designed to capture the appropriate volume of stormwater runoff, drain quickly, avoid erosion and blend with the landscape. The design should include a safe overflow area, such as a storm drain, ditch or open space area, in case stormwater exceeds the capacity of the rain garden. After identifying an appropriate location to which runoff from roofs or pavement can be diverted, design the rain garden to capture and contain the water-quantity volume from the site - generally the first half-inch of runoff from the contributing drainage area. Typically, the soil surface should be depressed 6 to 12 inches to create space for water to temporarily pond. Excavate soil from the basin to make space for the soil mix, fill the hole with water, and observe how fast the water soaks in. Refer to Rain Garden Design Tips Don t build a rain garden within 10 feet of building foundations. Keep rain gardens at least 100 feet away from septic system leachfields and water wells. Don t make the depressed area in the rain garden so deep that a tripping hazard might occur. Most are 6 to 8 inches deep. Redirect rooftop runoff using underground piping, a vegetated swale or channel, or a gutter extender. Don t let the runoff flow across bare soil, as erosion and clogging of the rain garden may occur due to excessive sediments in the runoff water. To avoid damaging tree roots, don t excavate an extensive rain garden under large trees. Avoid rain gardens in areas where the seasonally high groundwater table is within 6 feet of the ground surface, to guard against potential groundwater contamination. Don t build rain gardens in areas with slopes greater than 20 percent. FS-09-23, LID in Northern Nevada: using the soil that was excavated Soil Considerations, for smallscale testing methods, or consult site slopes, the berm should be to make the depression. If the the Truckee Meadows Structural highest at the down-slope side. Controls Design Manual for Do not attempt to construct approved methods. Rates should a rain garden on steep slopes be greater than one-half inch per of 20 percent or greater. hour. If native soil infiltration rates The recommended soil are slower, place an underdrain replacement mix is 50 to 60 surrounded by clean drain rock at percent clean sand, 20 to the bottom of the basin to drain 30 percent topsoil and 10 to the system and prevent extended 20 percent certified organic ponding. Connect the underdrain or pesticide-residue-free to a ditch or part of the storm compost, thoroughly mixed and drain system. Place a 4-inch layer approximately 30 inches deep. of pea gravel in the bottom of If the existing soils are welldraining sands, loamy sands, the excavated basin to maintain drainage into the underlying sandy loams or loams, the native soils. Avoid using filter percentage of sand and topsoil fabric in the bottom of the basin in the soil replacement mix can and around the underdrain, be reduced. Complete removal as it tends to clog with time, and replacement is often needed requiring complete excavation and when existing soils are silts or replacement of the rain garden. clays. Leaf compost is a preferred Next, construct a berm around type of amendment because the the edge of the rain garden, nutrient content in the leaves is

5 Low Impact Development in Northern Nevada: Bioretention Page 5 typically minimal during the fall when leaves drop. This reduces leaching of nutrients out of the system. Do not use compost that includes animal or food wastes. consult LID in Northern Nevada: Plant Materials, FS Mulch the soil surface after installing plants. Mulching helps conserve water and trees and shrubs for pests or diseases, and treat as needed or replace. Prune and weed when appropriate. Remove litter and debris on a regular basis. For additional detailed planning and design information, refer to the latest versions of the Truckee Plant the berm or use landscape fabric and mulch to decreases the potential for soil erosion from the garden. It Tree box filters Meadows Low Impact Development Handbook stabilize the soil and reduce also helps suppress weeds. In more urban or developed and the Truckee Meadows weed problems. Design an Maintenance is similar to that settings where space is limited, Structural Controls Design irrigation system for regular for any landscape bed. Monitor tree box filters can be used to Manual available at watering from May through irrigation to make sure plants are add bioretention while providing October and between rainfall events. Drip systems are ideal for rain gardens. Select plants that will survive stresses such not stressed. From May through October, monitor to ensure standing water does not persist for seven or more consecutive landscape aesthetics. Filter boxes are mini-bioretention systems that provide landscaping benefits without requiring a high degree Additional information about bioretention can be found at: as periodic inundation or very days to avoid mosquito breeding. of maintenance. Tree box filters dry periods, and avoid species Complete drainage within 48 can be designed as flow-through Bioretention.com, that are invasive. Consider a mix hours is preferable. Replace planters with underdrains that An Online Reference of perennial flowering plants, mulch as needed to keep the do not allow infiltration into for Designers, shrubs and ornamental grasses, or soil surface covered. Inspect underlying soils, or they can be Bioretention at North Carolina State University, topic/bioretention Low Impact Development (LID) Urban Design Tools, Prince George s County, Maryland, Bioretention Manual, Government/AgencyIndex/ DER/ESG/Bioretention/ bioretention.asp Stormwater Manager s Resource Center, www. stormwatercenter.net Susan Donaldson Tree filter box under construction on Virginia Street in Reno, Nevada., near the Truckee River. Runoff from the street enters the box through the curb cut.

6 Page 6 Low Impact Development in Northern Nevada: Bioretention References: Atchison, D., K. Potter and L. Severson, 2006, Design Guidelines for Stormwater Bioretention Facilities, University of Wisconsin-Madison Water Resources Institute, Madison, WI. USDA-NRCS, 2007, Rain Gardens, Montana NRCS, 06/07/MT. designed with drainage holes that allow infiltration. As with all landscape features, tree box filters require routine maintenance. Check the curb cut inlet areas and the surface of the soil inside the box several times a year to make sure they are free of trash and debris, there is no standing water, and the filter box continues to function as designed. Susan Donaldson A completed tree filter box in downtown Reno (above). Rain garden designs for parking lots (at left and below). National NEMO Network National NEMO Network Technical review provided by Chris Conway, Certified Professional in Sediment and Erosion Control. Copyright 2009, University of Nevada Cooperative Extension. All rights reserved. No part of this publication may be reproduced, modified, published, transmitted, used, displayed, stored in a retrieval system, or transmitted in any form or by any means electronic, mechanical, photocopy, recording or otherwise without the prior written permission of the publisher and authoring agency. The University of Nevada, Reno is an Equal Employment Opportunity/Affirmative Action employer and does not discriminate on the basis of race, color, religion, sex, age, creed, national origin, veteran status, physical or mental disability, or sexual orientation in any program or activity it operates. The University of Nevada employs only United States citizens and aliens lawfully authorized to work in the United States.