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==Major Design Elements==
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#REDIRECT [[Design criteria for Infiltration trench]]
===Physical Feasibility Initial Check===
 
<p>'''Drainage Area:'''' It is ''HIGHLY RECOMMENDED'' that the following [[Glossary#I|infiltration]] practices be designed with the indicated maximum drainage areas:</p>
 
*Dry well – 1 acre.
 
*Infiltration Trench – 5 acres.
 
*Underground Infiltration System – 10 acres.
 
*[[Glossary#I|infiltration]] [[Glossary#B|Basin]] – between 5 and 50 acres.
 
<p>Site Topography and Slopes: Unless slope stability calculations demonstrate otherwise, it is ''HIGHLY RECOMMENDED'' that infiltration practices be located a minimum horizontal distance of 200 feet from down-gradient slopes greater than 20 percent, and that slopes in contributing drainage areas be limited to 15 percent.</p>
 
<p>'''Soils:''' It is ''HIGHLY RECOMMENDED'' that native soils in proposed infiltration areas have a minimum [[Glossary#I|infiltration]]n rate of 0.2 inches per hour (typically Hydrologic Soil Group A, B and C soils). Initially, soil infiltration rates can be estimated from NRCS soil data, and confirmed with an on-site [[Glossary#I|infiltration]] evaluation or geotechnical investigation (see Step 6 of the Design Procedures section for investigation procedures). It is ''HIGHLY RECOMMENDED'' that native soils have silt/clay contents less than 40 percent and clay content less than 20 percent, and that [[Glossary#I|infiltration]] practices not be situated in fill soils.</p>
 
{{alert|It is ''REQUIRED'' that impervious area construction be completed and pervious areas established with dense and healthy vegetation prior to introduction of stormwater into an [[Glossary#I|infiltration]] practice.|alert-caution}}
 
<p>'''Depth to groundwater table and bedrock:''' </p>
 
 
 
 
 
{{alert|It is ''REQUIRED'' that [[Glossary#I|infiltration]] practices be designed with a minimum vertical distance of 3 feet between the bottom of the [[Glossary#I|infiltration]] practice and the seasonally high water table or bedrock layer (see also Step 8 under the Design Procedure section).|alert-danger}}
 
 
 
 
 
<p>Local authorities may require greater separation depths.</p>
 
<p>'''Site Location / Minimum Setbacks:''' It is ''HIGHLY RECOMMENDED'' that [[Glossary#I|infiltration]] practices not be hydraulically connected to structure foundations or pavement, to avoid seepage and frost heave concerns, respectively. If ground water contamination is a concern, it is ''RECOMMENDED'' that [[Glossary#G|groundwater]] mapping be conducted to determine possible connections to adjacent [[Glossary#G|groundwater]] wells. </p>
 
 
 
 
 
{{alert|The minimum setbacks in the table below are ''REQUIRED'' by the Minnesota Department of Health for the design and location of [[Glossary#I|infiltration]] practices. It will be necessary to consult local ordinances for further guidance on siting infiltration practices.|alert-danger}}
 
{{:Minimum setback requirements}}
 
 
 
 
 
<p>[[Glossary#K|'''Karst:''']] It is ''HIGHLY RECOMMENDED'' that [[Glossary#I|infiltration]] practices not be used in active [[Glossary#K|karst]] formations without adequate geotechnical testing.</p>
 
 
 
 
 
===Conveyance===
 
<p>It is ''HIGHLY RECOMMENDED'' that a flow splitter or diversion structure be provided to divert the V<sub>wq</sub> to the [[Glossary#I|infiltration]] practice and allow larger flows to bypass the practice, unless the [[Glossary#I|infiltration]] practice is sized to retain V<sub>cp</sub>, Vp<sub>10</sub> or Vp<sub>100</sub>. Where a flow splitter is not used, it is ''HIGHLY RECOMMENDED'' that contributing drainage areas be limited to the appropriate size given the [[Glossary#B|BMP]] and an overflow be provided within the practice to pass part of the Vwq to a stabilized watercourse or storm drain. It is also ''HIGHLY RECOMMENDED'' that overflow associated with the Vp<sub>10</sub> or Vp<sub>100</sub> storm (depending on local drainage criteria) be controlled such that velocities are non-erosive at the outlet point (to prevent downstream slope erosion), and that when discharge flows exceed 3 cubic feet per second, the designer evaluate the potential for [[Glossary#E|erosion]] to stabilized areas and [[Glossary#I|infiltration]] facilities. </p>
 
====Pre-treatment====
 
{{alert|It is ''REQUIRED'' that some form of [[Glossary#P|pre-treatment]], such as a plunge pool, sump pit, filter strip, sedimentation [[Glossary#B|basin]], grass channel, or a combination of these practices be installed upstream of the [[Glossary#I|infiltration]] practice.|alert-danger}}
 
 
 
 
 
<p>It is ''HIGHLY RECOMMENDED'' that the following [[Glossary#P|pre-treatment]] sizing guidelines be followed:</p>
 
*Before entering an [[Glossary#I|infiltration]] practice, stormwater should first enter a [[Glossary#P|pre-treatment]] practice sized to treat a minimum volume of 25 percent of the V<sub>wq</sub>.
 
*If the i[[Glossary#I|infiltration]] rate of the native soils exceeds 2 inches per hour a [[Glossary#P|pre-treatment]] practice capable of treating a minimum volume of 50 percent of the V<sub>wq</sub> should be installed.
 
*If the i[[Glossary#I|infiltration]] rate of the native soils exceeds 5 inches per hour a [[Glossary#P|pre-treatment]] practice capable of treating a minimum volume of 100 percent of the Vwq should be installed.
 
<p>It is ''HIGHLY RECOMMENDED'' that [[Glossary#P|pre-treatment]] practices be designed such that exit velocities from the [[Glossary#P|pre-treatment]] systems are non-erosive (less than 3 feet per second) and flows are evenly distributed across the width of the practice (e.g., by using a level spreader).</p>
 
{{alert|- Use low-impact earth moving equipment - DO NOT Overexcavate|alert-caution}}
 
 
 
 
 
===Treatment===
 
<p>'''Space Occupied:''' Space varies depending on the depth of the practice. Typically, [[Glossary#I|infiltration]] trenches are three to twelve feet deep with a width less than 25 feet. A dry well is essentially a smaller version of an [[Glossary#I|infiltration]] trench, consistent with the fact that the drainage area to an [[Glossary#I|infiltration]] trench is typically five times greater (or larger) than that of a dry well. Underground [[Glossary#I|infiltration]] systems are larger practices that range in depth from approximately 2 to 12 feet. The surface area of all [[Glossary#I|infiltration]] practices is a function of MPCA’s 48-hour drawdown requirement and the [[Glossary#I|infiltration]] capacity of the underlying soils.</p>
 
<p>'''Practice Slope:''' It is ''RECOMMENDED'' that the bottom of all [[Glossary#I|infiltration]] practices be flat, in order to enable even distribution and [[Glossary#I|infiltration]] of stormwater. It is ''RECOMMENDED'' that the longitudinal slope range only from the ideal 0 percent up to 1 percent, and that lateral slopes be held at 0 percent.</p>
 
<p>'''Side Slopes:''' It is ''HIGHLY RECOMMENDED'' that the maximum side slopes for an [[Glossary#I|infiltration]] practice be 1:3 (V:H). </p>
 
 
 
 
 
[[file:Effective infiltration area for side slopes less than 1-3.jpg|thumb|300px|alt=Schematic of Effective infiltration area for side slopes less than 1-3|Schematic of Effective infiltration area for side slopes less than 1-3]]
 
 
 
 
 
<p>'''Depth:''' The depth of an [[Glossary#I|infiltration]] practice is a function of the maximum drawdown time and the design [[Glossary#I|infiltration]] rate. </p>
 
 
 
 
 
{{alert|The REQUIRED drawdown time for [[Glossary#I|infiltration]] practices is 48 hours or less, and so the depth of the practice should be determined accordingly.|alert-danger}}
 
 
 
 
 
 
 
{{alert|'''[[Glossary#G|Groundwater]] Protection:''' It is ''REQUIRED'' that runoff from potential stormwater [[Glossary#H|hotspots]] (PSHs) not be infiltrated unless adequate [[Glossary#P|pre-treatment]] has been provided. Infiltration of [[Glossary#R|runoff]] from confirmed [[Glossary#H|hotspot]] areas, industrial areas with exposed significant materials, or vehicle fueling and maintenance areas is ''PROHIBITED''.|alert-danger}}
 
 
 
 
 
 
 
<p>'''Aesthetics:''' [[Glossary#I|infiltration]] [[Glossary#B|basins]] can be effectively integrated into the site planning process, and aesthetically designed as attractive green spaces planted with native vegetation. Infiltration trenches are less conducive to site aesthetics, but the surface of trenches can be designed with turf cover crops if desired.</p>
 
 
 
 
 
===Landscaping===
 
{{alert|It is ''REQUIRED'' that impervious area construction be completed and pervious areas established with dense and healthy vegetation prior to introduction of stormwater into an [[Glossary#I|infiltration]] practice.|alert-danger}}
 
<p>It is ''RECOMMENDED'' that vegetation associated with [[Glossary#I|infiltration]] practices be established to blend into the surrounding area, that native species be used wherever possible. It is ''HIGHLY RECOMMENDED'' that deep rooted plants such as prairie grass be used, because they increase the [[Glossary#I|infiltration]] capacity of the underlying soils. Dry wells and [[Glossary#I|infiltration]] trenches can be covered with permeable topsoil and planted with grass to match the surrounding landscape.</p>
 
<p>Due to soil compaction concerns, it is ''HIGHLY RECOMMENDED'' that [[Glossary#I|infiltration]] areas not be used for recreational purposes unless a soil amendment is used to off-set compaction.</p>
 
<p>It is ''HIGHLY RECOMMENDED'' that vegetation associated with [[Glossary#I|infiltration]] practices be regularly maintained and bare areas seeded. Mowing practices can be used to maintain native vegetation.</p>
 
<p>It is ''RECOMMENDED'' that soil testing be conducted in [[Glossary#I|infiltration]]practices, to determine if fertilizer application is warranted. Incorporating mulch or compost into the soil or planting with salt tolerant grasses can counter soil fertility problems caused by high chloride concentrations</p>
 
{{alert|It is ''HIGHLY RECOMMENDED'' that designs include an observation well consisting of an anchored six-inch diameter perforated PVC pipe fitted with a cap to facilitate periodic inspection and maintenance.|alert-caution}}
 
 
 
 
 
===Safety===
 
<p>Dry wells, [[Glossary#I|infiltration]] trenches and subsurface [[Glossary#I|infiltration]] systems do not pose any major safety hazards. [[Glossary#I|Infiltration]] [[Glossary#B|basins]] should have similar side slope considerations as ponds and [[Glossary#W|wetlands]].</p>
 
{{alert|If a dry well or [[Glossary#I|infiltration]] trench is greater than five feet deep, it is ''REQUIRED'' that OSHA health and safety guidelines be followed for safe construction practices.|alert-danger}}
 
 
 
 
 
<p>Additional information on safety for construction sites is available from [http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10930 OSHA]. </p>
 
 
 
 
 
<p>When riser pipe outlets are used in i[[Glossary#I|infiltration]] basins, it is ''HIGHLY RECOMMENDED'' that they be constructed with manholes that either have locks or are sufficiently heavy to prevent easy removal.</p>
 
<p>Fencing of dry wells and i[[Glossary#I|infiltration]] trenches is neither necessary nor desirable. [[Glossary#I|infiltration]] [[Glossary#B|basins]] may warrant fencing in some situations.</p>
 

Latest revision as of 16:22, 10 April 2013

This page was last edited on 10 April 2013, at 16:22.