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HomeMy WebLinkAboutC2A - CARMEL AND BMP DETAILS - CIVILC2A 1803-021KEELER-WEBB ASSOCIATESCARMEL DETAIL & INFILTRATION TRENCH DETAILS Children's TherAplayPHASE 2REMODEL & ADDITIONSENGINEERING REVISIONS8-23-20192 INFILTRATION TRENCH An infiltration trench is a linear stormwater BMP consisting of a continuously perforated pipe within a sub-surface stone-filled trench wrapped with geotextile. Usually, an infiltration trench is part of a conveyance system and is designed so that large storm events are conveyed through the pipe with some runoff volume reduction. During small storm events, volume reduction may be significant and there may be little or no discharge. All infiltration trenches should be designed with a positive overflow. Sediment pretreatment of runoff from impervious areas should be considered to prevent clogging within the trench, particularly when conveying runoff from roadways and parking areas. Construction Guidelines The following guidelines apply for all infiltration BMPs. •Do not compact soil infiltration beds during construction. Prohibit all heavy equipment from the infiltration area and absolutely minimize all other traffic. Equipment should be limited to vehicles that will cause the least compaction, such as low ground pressure (maximum four pounds per square inch) tracked vehicles. Areas for Infiltration should be clearly marked before any site work begins to avoid soil disturbance and compaction during construction. •Protect the infiltration area from sediment by ensuring erosion and sediment control practices are implemented until the surrounding site is completely stabilized. Methods to prevent sediment from washing into BMPs should be clearly shown on plans. •Runoff from construction areas should never be allowed to drain to infiltration BMPs. This can usually be accomplished by diversion berms and immediate vegetative stabilization. The infiltration area may be used as a temporary sediment trap or basin during earlier stages of construction. However, if an infiltration area is also to be utilized as a temporary sediment basin, excavation should be limited to within one foot of the final bottom invert of the infiltration BMP to prevent clogging and compacting the soil horizon, and final grade removed when the contributing site is fully stabilized. •All infiltration BMPs should be finalized at the end of the construction process, when upstream soil areas have a dense vegetative cover. In addition, do not remove inlet protection or other erosion and sediment control measures until site is fully stabilized. Any sediment which enters inlets during construction is to be removed within 24 hours. •Provide thorough construction oversight. Long-term performance of infiltration BMPs is dependent on the care taken during construction. Generally, plans and specifications must be followed precisely. The designer is encouraged to meet with the contractor to review the plans and construction sequence prior to construction, and to inspect the construction at regular intervals and prior to final acceptance of the BMP. •Provide quality control of materials. As with all BMPs, the final product is only as good as the materials and workmanship that went into it. The designer is encouraged to review and approve materials and workmanship, especially as related to aggregates, geotextiles, soil and topsoil, and vegetative materials. •Excavate infiltration trench bottom to a uniform, level uncompacted subgrade free from rocks and debris. Do NOT compact subgrade. •Place nonwoven geotextile along bottom and sides of trench. Nonwoven geotextile rolls should overlap by a minimum of 16 inches within the trench. Fold back and secure excess geotextile during stone placement. •Install upstream and downstream control structures, cleanouts, etc. •Place uniformly graded, clean-washed aggregate in 8-inch lifts. •Install continuously perforated pipe as indicated on plans. Backfill with uniformly graded, clean-washed aggregate in 8-inch lifts. •Fold and secure nonwoven geotextile over infiltration trench, with minimum overlap of 16 inches. •If vegetated, place a minimum six-inch lift of approved topsoil over infiltration trench, as indicated on plans. •Seed and stabilize topsoil. Maintenance There are a few general maintenance practices that should be followed for all infiltration BMPs. These include: • All catch basins and inlets should be inspected and cleaned at least twice per year. •The overlying vegetation of subsurface infiltration features should be maintained in good condition, and any bare spots revegetated as soon as possible. • Vehicular access on subsurface infiltration areas should be prohibited (unless designed to allow vehicles), and care should be taken to avoid excessive compaction by mowers. •Inspect the basin after major storm events and make sure that runoff drains down within 72 hours. Mosquitoes should not be a problem if the water drains in 72 hours. Mosquitoes require a considerably long breeding period with relatively static water levels. •Inspect for accumulation of sediment, damage to outlet control structures, erosion control measures, signs of water contamination/spills, and slope stability in the berms. •Mow only as appropriate for vegetative cover species. •Remove accumulated sediment from the sediment pretreatment device/forebay as Inspect pretreatment forebay at least one time per year. • If the infiltration basin bottom becomes clogged, scrape bottom and remove sediment and restore original cross section. Properly dispose of sediment. Winter Considerations Most infiltration practices are typically located below the frost line and continue to function effectively throughout the winter. It is imperative to prevent salt, sand, cinder, and any other deicers from clogging the surface area of infiltration practices by avoiding piling snow in these areas. Sand and cinder deicers could clog infiltration devices and soluble deicers, such as salt, can damage the health of vegetation. 2 22 2 2 3 8'ENGINEERING REVISIONS9-20-20193 3 2