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HomeMy WebLinkAboutVEGETATIVE FILTER STRIP - Henderson Subdivision - Annexation & Rezone to R1 & HDR2• DESCRIPTION Vegetated filter strips are vegetated sloped strips in which flow is distributed broadly along the length of the vegetated area as overland sheet flow. APPLICATION AND LIMITATIONS A vegetative filter strip is designed to provide runoff treatment of conventional pollutants but not nutrients. Also, unlike a biofiltration swale, a vegetative filter strip should not be used for conveyance of larger storms because of the need to maintain sheet flow conditions, plus the filter strip would likely be prohibitively large for this applica- tion. Vegetative filter strips can be effective at pretreating runoff to protect filtration BMPs from siltation. It may also be a viable treatment BMP for small, less intensely developed sites. The maximum recom- mended drainage area for a vegetative filter strip is 5 acres (2 hec- tares). Vegetative filter strips must not receive concentrated flow discharges as their effectiveness will be destroyed plus the potential for erosion could cause filter strips to become sources of pollution. Slope. Vegetative filter strips should not be used on slopes greater than about 10 percent because of the difficulty in maintaining the necessary sheet flow conditions. Note: This does not mean that vegetated buffers are not suitable for slopes greater than 10 percent; I th t ff ti treatment of runoff is unlikel for slo es Targeted Pollutants 80% Sediment 30% Phosphorus Q Trace metals Q Bacteria 0 Petroleum hydrocarbons Fhysical Limits Drainage area 5 ac Max slope 14% Min bedrock depth 5 feet Min water table 3 feet SCS soil type BCD Freezerrhaw fair Drainage/Flood control no it simp y means a e ec ve y p greater than 10 percent. Do not confuse a "buffer zone," which is used to protect streams and other environmental resources, with a "vegetative filter strip," which is a runoff treatment BMP. DESIGN PARAMETERS Criteria has been developed to ensure that a residence time of 20 minutes for the water as it flows across (perpendicular to) the strip. Complete details of the criteria are given below, and Appendix G-1 provides step-by-step procedures for designing both vegetation strips and swales. General Criteria See BMP #38- Vegetated Swale. Specific Criteria for Vegetative Filter Strips 1. Design vegetative filter strips according to the same method detailed for vegetated swales (BMP #38). Calculate the necessary filter strip width (perpendicular to flow) on the basis of the water quality design storm (1/3 of the 2 -year event) and a hydraulic radius (R) approximately equal to the design flow depth (y). Note: The design flow depth (y) will normally be no more than 0.5" (12.77 mm) because of the need to maintain sheet flow over the strip). 2. Calculate the necessary length (parallel to flow) to produce a water residence time of at least • 20 minutes (the length should normally be in the range of 100-200 feet (30 to 60 meters)). Catalog of pormwater Best Management Practices 7:ok2 April 1997 3. Install a shallow stone trench across the top of the strip to serve as a level spreader or make use • of curb cuts in a parking lot. Make provisions to avoid flow bypassing the filter strip. 4. Vegetative filter strips should not be used for slopes in excess of 10 percent, and preferably less, because of the difficulty in maintaining the necessary sheet flow conditions. CONSTRUCTION GUIDELINES See BMP #38 -Vegetated Swale. IdJa11ki Yi:4i!I't1dus) 4 See BMP #38 -Vegetated Swale. II L Is 1997 c:Uod&bmp %mp#3%,. • • Berms placed perpendicular to top of strip prevent concentrated Flows January Residential Street Stone trench acts as level spreader Top elevation of strip is on came contour, and directly abuts trench Vegetated filter strip (Portland, 1995) strip • 0 0 • • • DESCRIPTION Infiltration facilities such as trenches and infiltration basins (BMP #44) are designed to intercept and reduce direct site surface runoff. They hold runoff long enough to allow it to enter the underlying soil. They can include layers of coarse gravel, sand or other filtering media to filter the runoff before it infiltrates the soil. Infiltration trenches are shallow (two to ten feet deep) trenches in relatively permeable soils that are backfilled with a sand filter, coarse stone, and lined with filter fabric. The trench surface can be covered with grating and/or consist of stone, gabion, sand, or a grassed cov- ered area with a surface inlet. Depending on the design, trenches allow for the partial or total infiltration on stormwater runoff into the underlying soil. One alternative design is to install a pipe in the trench and surround it with coarse stone; this will increase the temporary storage capacity of the trench. APPLICATION AND LIMITATIONS An infiltration trench will generally be used in relatively small drainage areas (usually less than 15 acres), such as on residential lots. Trenches are one of the few BMPs that are relatively easy to fit into the margin, perimeter, and other less -utilized areas of developed sites, making them particularly suitable for retrofitting. Unlike infiltra- tion basins (BMP #44) installed at the surface, the land above a sub- surface trench system can be reclaimed and used. A trench may also be installed under a drainage swale to increase the storage of the infiltration system. Targeted Pollutants 75% Sediment 55% Phosphorus Trace metals 0 Bacteria 0 Petroleum hydrocarbons Physical Limits Drainage area 15 ac Max slope 25% Min bedrock depth 44 ft Min water table 3 ft SCS soil type AB FreezelThaw fair Drainage/Flood control N/A Appropriate soil conditions and the protection of groundwater are the most important considerations limiting the use of this BMP. Infiltration rates must be high (0.5 inches/hour or greater). Generally speaking, SCS Type A and B soils will convey water at this rate. Site-specific testing should be done to confirm the infiltration rate. Other soil conditions that will not support the use of infiltration trenches include: • Soils with more than 40% clay content (subject to frost heave) • Fill soils, unless the fill material is specially designed to accommodate the facility • Steep slopes (>25%) which can contribute to slope failures Infiltration facilities are not suitable in many areas where the groundwater table is very shallow. Observe conditions at the site during the winter and early spring, when the water table is at its highest. If the minimum depth to groundwater at these times is 3 feet from the proposed bottom of the infiltration trench bed, and the other noted soil conditions are right, infiltration can be used. If it is difficult to measure the seasonal high water table with any degree of certainty, place the bottom of the bed of the trench 5 feet above the suspected position of the water table. One advantage to trenches is that they have less tendency to become clogged with sediment than do other infiltration BMPs such as basins (BMP #44). However, clogging is still an issue. This BMP should typically be located 'off-line" from the primary conveyance/detention system in order to effectively treat pollutants and protect the infiltration soils from clogging. Infiltration trenches should always be preceded by a pretreatment BMP to remove sediments that could clog the infiltration soils. Catalog of Stormwater Best.Management Practices.' 1 of s April 1997 � �tW&\bmps\bmp#43e BMP #43. INFILTRATION TRENCH Conservatively speaking, the longevity of trenches is expected to be about two years before partial of full • clogging/sealing of the floor. The life span can be significantly increased given good permeable soils and pretreatment to prevent clogging. The relatively short life span of infiltration facilities can be significantly increased through proper design and maintenance. DESIGN PARAMETERS The procedure for sizing infiltration trenches should follow a Darcy's Law approach, as described in BMP #40 (Sand Filters) presented earlier in this Handbook. Additional design parameters specific to infiltra- tion trenches are given below. Soils Investigation. A minimum of one soils log should be collected for every 50 feet of trench length, and in no case less than two soils logs for each proposed trench location. Each soils log should extend to a minimum depth of the high water table below the bottom of the trench, describe the NRCS (SCS) series of the soil, the textural class of the soil horizon(s) through the depth of the log (soil and structures), and note any evidence of high ground water level, such as mottling. In addition, the location of imper- meable soil layers or dissimilar soil layers should be determined. The design infiltration rate, fd, will be equal to one-half the infiltration rate found from the soil textural and structural analysis. Pretreatment. It is recommended that all infiltration trenches be preceded by a pretreatment BMP, such as a presettling basin (BMP #50), a vegetated swale (BMP #38) or a simple sump (see wet/vault tank design, BMP #51). Regular maintenance of the pretreatment device is critical. Drawdown Time. Infiltration trenches should be designed to completely drain stored runoff within one day following the occurrence of the 6 -month, 24-hour design storm. Thus, a maximum allowable draw- down time of 24 hours should be used. This will ensure that the necessary aerobic conditions exist in order to provide effective treatment of pollutants. If a presettling basin (BMP #50) precedes the infiltra- • tion trench, the combined drawdown time for both BMPs should be 24 hours. Backfill Material. The aggregate material for the infiltration trench should consist of a clean aggregate with a maximum diameter of 3 inches and a minimum diameter of 1.5 inches. The aggregate should be graded such that there will be few aggregates smaller than the selected size. Void space for these ag- gregates is assumed to be in the range of 30 percent to 40 percent. Geotextile Fabric. The aggregate fill material should be completely surrounded with an engineering geotextile. In the case of an aggregate surface, should surround all of the aggregate fill material except for the top one foot. Overflow Channel. In general, because of the small drainage areas controlled by an infiltration trench, an emergency spillway is not necessary. In all cases, the overland flow path of surface runoff exceeding the capacity of the trench should be evaluated to preclude the development of uncontrolled, erosive, concentrated flow. A nonerosive overflow channel leading to a stabilized watercourse should be pro- vided. Seepage Analysis and Control. An analysis should be made to determine any possible adverse effects of seepage zones when there are nearby building foundations, basements, roads, parking lots or sloping sites. Developments on sloping sites often require the use of extensive cut and fill operations. The use of infiltration trenches on fill sites is not permitted. Buildings. Trenches should be a minimum of 100 feet upslope and 20 feet downslope from any building. Land Use. Infiltration facilities are not recommended under surfaces that are expected to have traffic loads, such as driveways and parking lots. Soils become too compacted and access is difficult. • Catalog of S[ormiva[er Best Management Practices 3 of 4' April 1997 Oaod&%m�mmpuma.d« FBMP #43.>`INFILTRATION `TRENCH I • Observation Well. An observation well should be installed for every 50 feet of infiltration trench length. The observation well will serve two primary functions: it will indicate how quickly the trench dewaters following a storm and it will provide a method of observing how quickly the trench fills up with sediments. The observation well should consist of perforated PVC pipe, 2 to 4 inches in diameter. It should be located in the center of the structure and be constructed flush with the ground elevation of the trench. The top of the well should be capped to discourage vandalism and tampering. More specific construction information can be obtained by contacting Idaho Department of Water Resources (IDWR) or DEQ. CONSTRUCTION GUIDELINES Construction Timing . An infiltration trench should not be constructed or placed into service until all of the contributing drainage area has been stabilized and approved by the appropriate agency. Trench Preparation. Excavate the trench to the design dimensions. Excavated materials should be placed away from the trench sides to enhance wall stability. Care should also be taken to keep this material away from slopes, neighboring property, sidewalks and streets. It is recommended that this material be covered with plastic if it is to be left in place for more than 30 days. Fabric Lavdown. The geotextile fabric (a fabric that is defined as "non -woven, spunbonded and nee- dlepunched") must be cut to the proper width prior to installation. The cut width must include sufficient material to conform to the trench perimeter irregularities and for a 12 inch minimum top overlap. Place the geotextile over the trench and unroll a sufficient length to allow placement of the fabric down into the trench. Stones or other anchoring objects should be placed on the geotextile at the edge of the trench to keep the lined trench open during windy periods. When overlaps are required between rolls, the upstream roll should overlap a minimum of 2 feet over the downstream roll in order to provide a • shingled effect. The overlap insures geotextile continuity and allows the geotextile to conform to the excavated surface during aggregate placement and compaction. U Stone Aggregate Placement and Compaction. The stone aggregate should be placed in lifts and com- pacted using plate compactors. As a rule of thumb, a maximum loose lift thickness of 12 inches is rec- ommended. The compaction process ensures geotextile conformity to the excavation sides, thereby reducing potential soil piping, geotextile clogging, and settlement problems. Overlapping and Covering. Following the stone aggregate placement, the geotextile fabric should be folded over the stone aggregate to form a 12 inch minimum longitudinal overlap. The desired fill soil or stone aggregate should be placed over the lap at sufficient intervals to maintain the lap during subse- quent backfilling. Care should be exercised to prevent natural or fill soils from intermixing with the stone aggregate. All contaminated stone aggregate should be removed and replaced with uncontaminated stone aggregate. Voids Behind Geotextile. Voids that may be created between the geotextile and excavation sides should be avoided. Native soils should be placed in these voids at the most convenient time during construction to ensure fabric conformity to the excavation sides. Soil piping, fabric clogging, and possible surface subsidence will be minimized by utilizing this remedial process. Unstable Excavation Sites. Vertically excavated walls may be difficult to maintain in areas where the soil moisture is high or where soft or cohesionless soils predominate. These conditions require laying back of the side slopes to maintain stability; trapezoidal rather than rectangular cross-sections may re- sult. This is acceptable, but any change in the shape of the stone reservoir needs to be taken into con- sideration in size calculations. catalog of slormwater Best Management Practices 3 of 4' April 1997 c:XW&\b.,Nbm,#4Ud BMP #43. INFILTRATION TRENCH Traffic Control. Heavy equipment and traffic should be restricted from traveling over the infiltration • areas to minimize compaction of the soil. The trench should be flagged or marked to keep equipment away from the area. MAINTENANCE Inspection Schedule. The observation well should be monitored for water quality periodically. For the first year after completion of construction, the well should be monitored after every large storm (greater than one inch in 24 hours), and during the period from October 15 to April 15, inspections should be conducted monthly. From April 16 through October 14, the facility should be monitored on a quarterly basis. A log book should be maintained by the responsible person designated by the local government indicating the rate at which the facility dewaters after large storms and the depth of the well for each observation. Once the performance characteristics of the structure have been verified, the monitoring schedule can be reduced to an annual basis unless the performance data indicate that a more frequent schedule is required. Sediment Removal. Sediment buildup in the top foot of stone aggregate or the surface inlet should be monitored on the same schedule as the observation well. A monitoring well in the top foot of stone ag- gregate should be required when the trench has a stone surface. Sediment deposits should not be al- lowed to build up to the point where it will reduce the rate of infiltration into the trench. 1J 0 Catalog of Stormwater Best Management Practices 4 of 41 c Vadds%.,p m,#4J dx