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| + | [[File:Technical information page image.png|100px|right|alt=image]] |
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| + | Pretreatment reduces maintenance and prolongs the lifespan of structural stormwater BMPs by removing trash, debris, organic materials, coarse sediments, and associated pollutants prior to entering structural stormwater BMPs. Implementing pretreatment devices also improves aesthetics by capturing debris in focused or hidden areas. |
| − | <!--[[File:Pretreat sizing general picture.png|200px|left|thumb|alt=schematic pretreatment|<font size=3>A properly sized pretreatment practice, such as this basin, allows for sediment to settle while preventing scouring. Image courtesy University of New Hampshire Stormwater Center; [http://www.unh.edu/unhsc/sites/unh.edu.unhsc/files/pubs_specs_info/2009_unhsc_report.pdf 2009 Biannual Report]</font size>]]-->
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| − | <div style="float:right">
| + | To perform efficiently, pretreatment practices must be properly sized. Given the lack of a consistent approach in stormwater pretreatment sizing in the literature, we developed a sizing approach focused on removing a specific fraction of sediment. This approach is dependent on the settling velocity of different sized particles. |
| − | <table class="infobox" style="border:3px; border-style:solid; border-color:#FF0000; text-align: right; width: 300px; font-size: 100%">
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| − | <tr>
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| − | <th><center><font size=3>'''Quick Guide for pretreatment sizing of basins'''</font size></center></th>
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| − | </tr>
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| − | <tr>
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| − | <td>'''ENSURE YOUR UNITS ARE CONSISTENT AND CORRECT'''
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| − | *Determine, ''v''<sub>''S''</sub>, the settling velocity for the particle size targeted (recommend 80 microns for particle size. Determine v<sub>s</sub> from table on this page)
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| − | *Determine ''FR'', the target percent removal (recommend 50-70%)
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| − | *Determine ''A'', the area of directly connected impervious draining to the pretreatment practice
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| − | *Determine ''I'', the peak rain intensity (0.5 in/hr for a 1-inch event, Type 2 distribution)
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| − | *Calculate the area of the pretreatment basin from A<sub>S</sub> = (FR * I * A * FR) / (v<sub>S</sub> * (1 - FR))
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| − | *Determine the basin depth as the lesser of sqrt(A<sub>S</sub>) or 6 feet
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| − | </td>
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| − | </tr>
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| − | </table>
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| − | </div>
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| − | <div style="float:right">
| + | This page provides an approach to pretreatment sizing. For the technical development of this methodology, [https://stormwater.pca.state.mn.us/index.php?title=Technical_basis_for_pretreatment_sizing_for_basins_and_filter_strips link here]. |
| − | <table class="infobox" style="border:3px; border-style:solid; border-color:#FF0000; text-align: right; width: 300px; font-size: 100%">
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| − | <tr>
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| − | <th><center><font size=3>'''Quick Guide for pretreatment sizing of filter strips'''</font size></center></th>
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| − | </tr>
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| − | <tr>
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| − | <td>'''ENSURE YOUR UNITS ARE CONSISTENT AND CORRECT'''
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| − | *Determine, ''v''<sub>''S''</sub>, the settling velocity for the particle size targeted (recommend 80 microns for particle size. Determine v<sub>s</sub> from table on this page)
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| − | *Determine ''FR'', the target percent removal (recommend 50-70%)
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| − | *Determine ''A'', the area of directly connected impervious draining to the pretreatment practice
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| − | *Determine ''I'', the peak rain intensity (0.5 in/hr for a 1-inch event, Type 2 distribution)
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| − | *Calculate the area of the filter strip from LW = - ((c * I * A) / (v<sub>S</sub>) * ln(1 - FR)) where c = 0.7 for small storms
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| − | *Determine the length (L) and the width (W) from the above computation
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| − | </td>
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| − | </tr>
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| − | </table>
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| − | </div>
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| − | Initial research was conducted to determine whether an approach for {{pop|'''sizing'''|What is sizing?|Sizing typically refers to the volume of stormwater runoff and/or the amount of pollutant a specific structural best management practice (BMP) is designed to retain or treat. This volume or pollutant amount is often called a performance goal. As an example, consider a BMP designed to capture the first inch of runoff from one acre of impervious surface. This BMP must be sized to capture 3630 cubic feet of water. Sizing can be related to physical dimensions, such as the depth and surface area of a BMP, but may also be related to a volume passing through the BMP or the amount of material needed to provide a specific pollutant treatment.}} <span title="Pretreatment reduces maintenance and prolongs the lifespan of structural stormwater BMPs by removing trash, debris, organic materials, coarse sediments, and associated pollutants prior to entering structural stormwater BMPs. Implementing pretreatment devices also improves aesthetics by capturing debris in focused or hidden areas. Pretreatment practices include settling devices, screens, and pretreatment vegetated filter strips."> [https://stormwater.pca.state.mn.us/index.php?title=Pretreatment '''pretreatment''']</span> practices was consistently applied across the country. Stormwater programs for every state were reviewed to determine the approaches being used, as well as a consensus in the approach. The results of this review indicated that a consistent approach to stormwater pretreatment across the country was not used, and some states do not require pretreatment of stormwater runoff at all. Because of the lack of consensus and research-backed sizing criteria for pretreatment, stormwater practitioners in Minnesota determined that guidelines were needed to define specific sizing criteria to provide consistent, effective results.
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| − | Given the lack of a consistent approach in stormwater pretreatment sizing, analysis was conducted to determine the general guidelines for pretreatment sizing in Minnesota based on the <span title="The rate at which suspended solids subside and are deposited."> '''settling velocity'''</span> of particles in stormwater. Many sizes of suspended solids enter a treatment practice–from clays below 2 μm (2 micrometers or 2000 mm) to silts between 2 μm and 80 μm to sands between 80 μm and 4 mm and beyond. A four order of magnitude difference between the smallest clay particles and largest sand sizes particles entering the practice is noted with a greater order of magnitude difference in settling velocity. A pretreatment practice is designed to settle the largest of these (primarily inorganic) particles in a location where they will be relatively simple to clean out and dispose of. As a result of this pretreatment, required maintenance will be reduced on the primary treatment practice (also called a <span title="a stationary and permanent BMP that is designed, constructed and operated to prevent or reduce the discharge of pollutants in stormwater"> '''structural stormwater BMP'''</span>) because a large portion of the <span title="small solid particles which remain in suspension in water as a colloid or due to the motion of the water,suspended solids can be removed by the sedimentation because of their comparatively large size."> '''suspended solids'''</span> will settle in the pretreatment practice, which will prolong the life span and maintain the design pollutant <span title="Pollutant removal efficiency, usually represented by a percentage, specifically refers to the pollutant reduction from the inflow to the outflow of a system"> '''removal efficiency'''</span> of the primary treatment.
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| − | The rationale for pretreatment is, therefore, to reduce maintenance costs and efforts. A pretreatment practice should serve the following three functions.
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| − | #Settle the larger particles (typically sand, which is greater than 80 μm) into an easily cleaned location to capture roughly one-half of the suspended solids.
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| − | #Keep the sediment from scouring out of the device during high flows, which is typically done by providing sufficient depth of water or using a device to calm high flows that come into the basin.
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| − | #Retain floatable trash and large organic particles (e.g., leaves, grass clippings, and sticks) in the pretreatment practice, which is typically done through a submerged outlet so that floatables are retained and can be cleaned out at a later time.
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| − | [[File:Pretreament sizing PSD.png|300px|left|thumb|alt=image of particle size distribution|<font size=3>Particle Size Distribution of Suspended Solids in Road Stormwater Runoff [Erickson, 2012]. Click on image to enlarge.</font size>]]
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| − | The pretreatment sizing methodology described on this page relies on the settling function of the pretreatment practice (i.e., item No. 1 above) to perform a preliminary sizing of <span title="Basins are stormwater best management practices (BMPs) that are an engineered pool that promotes settling of solids (e.g., pond forebay, sump, hydrodynamic separator, and catch basin)."> '''basins'''</span> and <span title="an area of permanent vegetation or other material used to reduce sediment, organics, nutrients, pesticides, and other contaminants from runoff and to maintain or improve water quality.> '''filter strips'''</span>. Properly designed and sized pretreatment practices will often remove 50 percent of the suspended solids in a typical storm before the water enters the primary treatment practice. However, the amount of sediment removed depends upon the storm intensity. Low-intensity storms will primarily move clay and silt, which do not settle well
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| − | in a pretreatment device, and pretreatment is expected to remove less than 50 percent. High-intensity storms will move most of the particles lodged on impervious surfaces and will result in a higher removal of suspended solids; greater than 50 percent of the suspended solids will settle. Examples of various solids size distributions are illustrated in the adjacent figure. The x-axis covers four orders of magnitude in particle size, and the median particle diameter (at 50 percent finer than) varies from 50 to 700 μm. Therefore, design of a pretreatment facility to remove 50 percent of suspended solids is difficult, unless the average annual particle size distribution is known. Herein, the design sizing goal will be to remove a given percent of a given particle size of sand and silt, which are the primary components of soil.
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| − | ==Basins== | + | ==Pretreatment basins== |
| − | [[File:SS Paul figure 2.png|300px|thumb|left|alt=forebay image|<font size=3>Turfstone Pavers at the Bottom of Seidel’s Lake Park Forebay and Biofiltration Cell Following Construction (Dakota County Soil and Water Conservation District)</font size>]]
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| | [[File: Preserver Photo 1.JPG|thumb|300 px|left|alt=image of Preserver pretreatment device|<font size=3>Heavy Leaf and Debris Loading in Pretreatment Manhole Sump [Ramsey Conservation District, 2017]. </font size>]] | | [[File: Preserver Photo 1.JPG|thumb|300 px|left|alt=image of Preserver pretreatment device|<font size=3>Heavy Leaf and Debris Loading in Pretreatment Manhole Sump [Ramsey Conservation District, 2017]. </font size>]] |
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| | Basins are stormwater best management practices (BMPs) having an engineered pool that promotes settling of solids (e.g., pond <span title="an artificial pool of water in front of a larger body of water. The larger body of water may be natural or man-made. Forebays have a number of functions. They may be used upstream of reservoirs to trap sediment and debris (sometimes called a sediment forebay) in order to keep the reservoir clean."> '''forebay'''</span>, <span title="a pit or hollow in which liquid collects"> '''sump'''</span>, <span title="stormwater management devices that use cyclonic separation to control water pollution. They are designed as flow-through structures with a settling or separation unit to remove sediment and other pollutants."> '''hydrodynamic separator'''</span>, and <span title="A catch basin is an engineered drainage structure with the sole function of collecting rainwater and snowmelt from streets and parking lots and transporting it to local waterways through a system of underground piping, culverts, and / or drainage ditches"> '''catch basin'''</span>). These pretreatment facilities are typically small (relative to the primary treatment practice) but can be sized to settle gravel and sands, and possibly silts, at low flow conditions. For preliminary sizing purposes, the small size results in a basin that is close to well-mixed, where the concentration of suspended solids is constant with depth to the bottom of the basin and the particles are settling out because of zero velocity at the wall. | | Basins are stormwater best management practices (BMPs) having an engineered pool that promotes settling of solids (e.g., pond <span title="an artificial pool of water in front of a larger body of water. The larger body of water may be natural or man-made. Forebays have a number of functions. They may be used upstream of reservoirs to trap sediment and debris (sometimes called a sediment forebay) in order to keep the reservoir clean."> '''forebay'''</span>, <span title="a pit or hollow in which liquid collects"> '''sump'''</span>, <span title="stormwater management devices that use cyclonic separation to control water pollution. They are designed as flow-through structures with a settling or separation unit to remove sediment and other pollutants."> '''hydrodynamic separator'''</span>, and <span title="A catch basin is an engineered drainage structure with the sole function of collecting rainwater and snowmelt from streets and parking lots and transporting it to local waterways through a system of underground piping, culverts, and / or drainage ditches"> '''catch basin'''</span>). These pretreatment facilities are typically small (relative to the primary treatment practice) but can be sized to settle gravel and sands, and possibly silts, at low flow conditions. For preliminary sizing purposes, the small size results in a basin that is close to well-mixed, where the concentration of suspended solids is constant with depth to the bottom of the basin and the particles are settling out because of zero velocity at the wall. |
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| − | ===Development of sizing criteria=== | + | ===Methodology for pretreatment basins=== |
| − | [[File:Rainfall distribution curves.png|300px|thumb|alt=rainfall distribution curves|<font size=3>Rainfall distribution curves (Types 1, 1A, and 2)</font size>]]
| + | Calculate the area of the pretreatment basin from AS = (FR * I * A * FR) / (vS * (1 - FR)) |
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| − | The following assumptions will be made while developing the sizing criteria.
| + | For the following calculation, ensure your units are consistent |
| − | #A design storm of 1-inch of precipitation.
| + | *Determine, ''v''<sub>''S''</sub>, the settling velocity for the particle size targeted (recommend 80 microns for particle size. Determine v<sub>s</sub> from table on this page) |
| − | #Permeable surfaces in the catchment {{pop|'''do not contribute runoff'''|Permeable surfaces and runoff|The contribution of permeable surfaces (e.g. grass, forest) to runoff varies with factors such as soil, initial soil moisture content, slope, and rainfall intensity. Hydrologic Soil Group D soils, which have a significant amount of clay, have low infiltration rates and typically produce more runoff than other soils. Runoff from permeable surfaces also increases with slope, higher initial moisture content, and higher rainfall intensity. For a typical 1-inch rain storm, permeable surfaces generally produce little runoff.}} for the 1-inch storm.
| + | *Determine ''FR'', the target fraction removal (recommend 0.50-0.70) |
| − | #The design <span title="Manner in which the depth of rainfall varies in space and time"> '''storm distribution'''</span> over time is [https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1044171.pdf Natural Resources Conservation Service (NRCS) Type II] with an intensity of 0.5 inches per hour (in/hr) during the most intense portion of the storm.
| + | *Determine ''A'', the area of directly connected impervious draining to the pretreatment practice |
| − | #The bottom of the basin does not have sediment scouring.
| + | *Determine ''I'', the peak rain intensity (0.5 in/hr for a 1-inch event, Type 2 distribution) |
| − | #The water column is well-mixed so that the suspended solids concentration is constant over the basin.
| + | *Calculate the area of the pretreatment basin from A<sub>S</sub> = (FR * I * A * FR) / (v<sub>S</sub> * (1 - FR)) |
| − | #The watershed <span title="the time needed for water to flow from the most remote point in a watershed to the watershed outlet. It is a function of the topography, geology, and land use within the watershed."> '''time of concentration'''</span> will be approximated using the simplified 1965 US Federal Aviation Administration (FAA) time of concentration equation, which is meant to be paired with the <span title="The Rational Method is a simple hydrologic calculation of peak flow based on drainage area, rainfall intensity, and a non-dimensional runoff coefficient. The peak flow is calculated as the rainfall intensity in inches per hour multiplied by the runoff coefficient and the drainage area in acres. The peak flow, Q, is calculated in cubic feet per second (cfs) as Q = CiA where C is the runoff coefficient, i is the rainfall intensity, and A is the drainage area. A conversion factor of 1.008 is necessary to convert acre-inches per hour to cfs, but this is typically not used. This method is best used only for simple approximations of peak flow from small watersheds."> '''Rational Method'''</span>.
| + | *Determine the basin depth as the lesser of sqrt(A<sub>S</sub>) or 6 feet |
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| − | Assumption Nos. 1 through 3 are common for rainfall-runoff computations in Minnesota and greatly simplify the design. Assumption No. 4 is correct if the basin is properly cleaned. Assumption No. 5 is applicable with the small size of the pretreatment basin, relative to the discharge and the size of the primary practice and would result in a conservative sizing criteria. This sizing criteria is to be used for preliminary sizing of a pretreatment basin. For a more refined design, a dynamic design criteria similar to [https://stormwater.pca.state.mn.us/index.php?title=Overview_for_pretreatment_settling_devices#Design_assistance_using_SHSAM SHSAM] (Sizing Hydrodynamic Separators And Manholes) is recommended. Assumption No. 6 is a fair assumption with the velocities and depth of the flow over a filter strip and would result in a conservative sizing criteria.
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| − | The NRCS Type II distribution of the 24-hour, 1-inch storm was selected be used in these pretreatment sizing calculations. The most intense hour of this design storm occurs around hour 12, when 50% of the storm total falls in one hour [NRCS 2015]. This design intensity, 0.5 inches per hour for that single hour is used in the following design calculations. The design flow into the basin results from the Rational Method by multiplying the <span title="impervious areas that are hydraulically connected to the conveyance system (e.g streets with curbs, catch basins, storm drains) and to the watershed outlet point without flowing over pervious areas."> '''directly connected impervious'''</span> area of the catchment with this intensity, or
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| − | <u>'''Eq. 1'''</u> <math> Q = cAI </math>
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| − | where ''Q'' is water discharge into the filter strip, ''A'' is the directly connected impervious area of the catchment (square feet), ''I'' is the peak intensity during the design storm (0.5 in/hr or 1.16x10<sup>-5</sup> ft/s), and ''c'' is the runoff coefficient equal to 0.7 for smaller storms on impermeable surfaces.
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| − | The discharge is also equal to the volume of water in the basin divided by the residence time of the water in the basin, or
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| − | <u>'''Eq. 2'''</u> <math> Q = A_Sh / t_r </math>
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| − | where ''A''<sub>s</sub> is the surface area of the water in the pretreatment basin, ''h'' is the depth of the water in the basin, and ''t''<sub>r</sub> is the residence time of the water in the basin.
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| − | The residence time of water (''t''<sub>''r''</sub>) can be approximated by assuming that settling occurs in a well-mixed basin (Assumption No. 5) from the relation [Gulliver, 2007]
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| − | <u>'''Eq. 3'''</u> <math> 1 - FR = 1 / (1 + v_S t_r / h) </math>
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| − | where ''FR'' is the fraction of a given settling velocity to be removed and ''v''<sub>s</sub> is the settling velocity of a given size particle. Equation 3 can be rearranged to give the residence time
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| − | <u>'''Eq. 4'''</u> <math> t_r = (h / v_S) * (FR / (1 - FR)) </math>
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| − | Substituting to solve for A<sub>S</sub> gives
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| − | <u>'''Eq. 5'''</u> <math> A_S = (cAI / v_s) * (FR / (1 - FR)) </math>
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| − | Note that Equation 5 does not depend upon the basin depth. The basin depth should be sufficient so that scour does not occur. Avoiding scour is not considered in these sizing calculations but should be considered in the sump design. To prevent scour, the depth (''D'') is typically recommended to be equal to the lesser of
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| − | <u>'''Eq. 6'''</u> <math> D ≤ sqrt (A_S) or 6 feet </math>
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| − | For <span title="a bioretention practice having an underdrain. All water entering the practice is filtered through engineered media and filtered water is returned to the storm sewer system."> [https://stormwater.pca.state.mn.us/index.php?title=Bioretention '''biofiltration''']</span>, or <span title="a bioretention practice in which no underdrain is used. All water entering the bioinfiltration practice infiltrates or evapotranspires."> '''bioinfiltration'''</span>, basins that may experience higher flows, a recommended minimum depth to avoid scour is equal to the lesser of
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| − | <u>'''Eq. 7'''</u> <math> D ≤ sqrt (A_S) or 1.5 feet </math>
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| − | The fraction of a given settling velocity to be removed by the pretreatment basin will need to be chosen by the designer. The settling velocity of a given size (typical sand shaped) particle has been developed by Ferguson and Church [2004], as
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| − | <u>'''Eq. 8'''</u> <math> V_S = (gRd^2) / (18 u + (0.75Rd^3)^0.5) </math>
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| | + | ===Example for pretreatment basin=== |
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| − | [[File:Example particle retention curves.png|300px|thumb|left|alt=example particle retention curves|<font size>Example particle retention curves</font size>]]
| + | One of the inlets into a 2-acre pond is draining a directly connected impervious area of 30 acres. The pretreatment basin for the 2-acre pond will be designed to retain 70 percent of the 80 μm fine-sand particles. Using adjacent table, the settling velocity (v<sub>S</sub>) for 80 μm fine-sand particles is 0.017 feet per second (fps). For this case, the values to be input into the calculation are |
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| − | where ''g'' is the acceleration of gravity, ''R'' is the specific gravity of the particle relative to that of water, ''d'' is the equivalent spherical diameter of the particle and ''u'' is the viscosity of water. Equation 8 combines <span title="the force required to move a sphere through a given viscous fluid at a low uniform velocity is directly proportional to the velocity and radius of the sphere."> '''Stoke's Law'''</span> at small particle diameters with the <span title="a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment, such as air or water. It is used in the drag equation in which a lower drag coefficient indicates the object will have less aerodynamic or hydrodynamic drag. The drag coefficient is always associated with a particular surface area."> '''turbulent drag'''</span> coefficient at larger particle diameters and a transition from a dependence upon ''d''<sup>2</sup> to one upon √''d'' as the particle size increases. If we assume that the pretreatment basin is being designed to settle silica-based, inorganic particles (i.e., silt and sand) at 20<sup>o</sup>C, the settling velocities of the particles at various equivalent spherical diameters are given in the adjacent table. Sample results of these computations are given in the adjacent figure.
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| − | ===Basin sizing examples===
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| − | Many types of basins can be used as pretreatment practices for various sizes and circumstances of bioretention/biofiltration facilities, including forebays, sumps, proprietary devices, and basins used as pretreatment for bioretention and biofiltration facilities. The examples used herein will use a forebay, sump, and bioretention pretreatment basin. Various-sized retention criteria can be used, which is the user’s choice. Note that these sizing calculations are preliminary. If justified by the cost, it is recommended that these calculations be used as preliminary sizing methodology, to be followed by a more refined dynamic design criteria similar to SHSAM, which uses watershed routing of storms.
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| − | ====Example A - forebay pretreatment basin====
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| − | One of the inlets into a 2-acre pond is draining a directly connected impervious area of 30 acres in a watershed with a 2-percent average slope and a watershed length to the outlet of 2,000 ft. The pretreatment basin for the 2-acre pond will be designed to retain 70 percent of the 80 μm fine-sand particles. Using settling velocities from the above table, the settling velocity (v<sub>S</sub>) for 80 μm fine-sand particles is 0.017 feet per second (fps). For this case, the values to be input in Equation 5 are | |
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| | *FR = 0.7 | | *FR = 0.7 |
| − | *A = 30 acres | + | *A = 30 acres * 43,560 ft<sup>2</sup>/ac = 1,306,800 ft<sup>2</sup> |
| − | *I = 0.5 in/hr | + | *I = 0.5 in/hr / 12 in/ft / 3600 s/hr = 1.157 X 10<sup>-5</sup> ft/s |
| | *v<sub>s</sub> = 0.017 ft/s | | *v<sub>s</sub> = 0.017 ft/s |
| − | *S = 0.02 ft/ft
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| − | *L = 2,000 ft
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| − | The rainfall intensity is related to the time of concentration and determining the peak intensity for the storm is essential for sizing pretreatment practices. Using the following equation from the FAA [Chow, et al. 1988], we can estimate the time of concentration (t<sub>c</sub> in minutes)
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| − | <u>'''Eq. 9'''</u> <math> t_c = 1.8 (1.1 - c) L^{0.5} / (100S)^{1/3} </math>
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| − | where ''c'' is the Rational Method runoff coefficient of 0.7, ''S'' is the average watershed slope (ft/ft), and ''L'' is the watershed length (ft) to the outlet. The time of concentration is equal to ''t''<sub>c</sub> = 26 minutes. This concentration time is within the most intense hour of a NRCS Type II storm [Merkel and Moody, 2015]. Thus, the intensity of the storm, ''I'', is 0.5 inches per hour.
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| − | The design discharge is computed from Equation 1
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| − | <math> Q = 0.7 * 0.5 in/hr * 30 ac = 10.5 ft^3/s </math>
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| − | Substituting Equation 1 into Equation 5, the equation for the area of the pretreatment basin becomes
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| − | <u>'''Eq. 10'''</u> <math> A_S = (Q * FR) / (v_S (1 - FR)) = (10.5 ft^3/s * 0.7) / (0.017 ft/s * (1 - 0.7)) = 1442 ft^2 </math> | + | Plugging the these into the equation A<sub>S</sub> = (FR * I * A * FR) / (v<sub>S</sub> * (1 - FR)) yields a value of 1453 ft<sup>2</sup> for the area of the forebay. The depth is the lesser of 6 feet or the square root of the forebay area (square root of 1453 = 38.1). The depth is therefore 6 feet. |
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| − | Finally, in Equation 6, the forebay depth would be roughly the lesser of (√1,441 = 38 ft) or 6 ft, so the preliminary water depth would be roughly 6 ft. With a large impervious area draining into one outlet into a pond, the designer may want to specify a forebay because of the large pretreatment area that is required.
| + | ==Pretreatment filter strips== |
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| − | ====Example B - Sump pretreatment basin==== | |
| − | An inlet to a 3,000 ft<sup>2</sup> pond will be draining a parking lot with a directly connected impervious area of 3 acres in a watershed with a 1 percent average slope and a watershed length to the outlet of 500 ft. The pretreatment basin for the pond will be designed to retain 50 percent of the 150 μm sand particles during the design storm. For this example, the values to put into Equation 6 are
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| − | *FR = 0.5
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| − | *A = 3 acres
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| − | *I = 0.5 in/hr
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| − | *v<sub>S</sub> = 0.049 ft/s
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| − | *S = 0.01 ft/ft
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| − | *L = 500 ft
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| − | As in the previous example, the peak rainfall intensity should be over a time of concentration, ''t''<sub>c</sub>, calculated using Equation 9. The time concentration here is equal to 16 minutes. This time of concentration occurs within the most intense hour of an NRCS Type II storm, which accounts for approximately 50 percent of the storm total. Thus, the intensity of the storm is 0.5 in/hr.
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| − | The design discharge is computed from Equation 1
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| − | <math> Q = 0.7 * 0.5 in/h * 3 ac = 1.1 ft^3/s </math>
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| − | | |
| − | and using an intensity of 0.5 in/hr, ''A''<sub>S</sub> becomes
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| − | | |
| − | <math> A_S = (1.1 ft^3/s * 0.5)/(0.049 ft/s * (1-0.5)) = 22 ft^2 </math>
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| − | | |
| − | The sump would have a diameter of 5.3 ft but scaling up to a standard manhole size of 6 ft is recommended as a good practice. Using Equation 6, the water depth in the sump would be the lesser of (√21) = 5 ft or 6 ft, so the preliminary water depth in the sump would be roughly 5 ft.
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| − | | |
| − | ====Example C - rain garden pretreatment basin====
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| − | A directly connected impervious area of 10,000 ft<sup>2</sup> with a mean slope of 3 percent and a watershed length to the outlet of 300 ft, drains into a bioretention practice. The area is of sufficient size to install a well-sized pretreatment basin, so the designer chose to settle 70 percent of the 80 μm fine-sand particles in the filter strip during the design storm. For this case, the values to be input for Equation 5 are
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| − | | |
| − | *FR = 0.7
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| − | *A = 10,000 ft2
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| − | *I = 0.5 in/hr
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| − | *v_S = 0.017 ft/s
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| − | *S = 0.03 ft/ft
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| − | *L = 300 ft
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| − | | |
| − | As in the previous examples, the peak rainfall intensity should be over a time of concentration, ''t''<sub>c</sub>, calculated using Equation 9. The time concentration here is equal to 9 minutes. This time of concentration occurs within the most intense hour of an NRCS Type II storm, which accounts for approximately 50 percent of the storm total. The design discharge is computed from Equation 1
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| − | | |
| − | <math> Q = 0.7 * 0.5 in/hr * 100,000 ft^2 * 1 ac/43560 ft^2 = 0.08 ft^3/s </math>
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| − | | |
| − | and using Equation 12, ''A''<sub>S</sub> becomes
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| − | | |
| − | <math> A_S = (0.08 ft^3/s * 0.7) / (0.017 fr/s * (1-0.7)) = 11 ft^2 </math>
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| − | | |
| − | Using Equation 7 for bioretention facilities, the pretreatment basin depth would be roughly the lesser of (√11) = 3.3 ft or 1.5 ft, so the preliminary depth would be roughly 1.5 ft.
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| − | | |
| − | ==Filter strips==
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| | [[File:Burnsville figure 2.png|300px|thumb|left|alt=figure of filter strip|<font size=3>Vegetated Filter Strip at Curb Cutout (Barr Engineering Company, 2006).</font size>]] | | [[File:Burnsville figure 2.png|300px|thumb|left|alt=figure of filter strip|<font size=3>Vegetated Filter Strip at Curb Cutout (Barr Engineering Company, 2006).</font size>]] |
| | | | |
| | Filter strips are primarily designed to enhance settling; physical filtering of particles in a filter strip is minimal. The filter strip is located at the bottom of a shallow, widely spread, open channel flow to allow for particle settling through the media, which is often grass or rocks. The grass or rocks allow space for the settled particles to collect until cleanout is required. | | Filter strips are primarily designed to enhance settling; physical filtering of particles in a filter strip is minimal. The filter strip is located at the bottom of a shallow, widely spread, open channel flow to allow for particle settling through the media, which is often grass or rocks. The grass or rocks allow space for the settled particles to collect until cleanout is required. |
| | | | |
| − | ===Development of sizing criteria=== | + | ===Methodology for pretreatment filter strips=== |
| − | {| class="wikitable" style="float:right; margin-left: 10px; width:500px;"
| + | LW = - ((c * I * A) / (vS) * ln(1 - FR)) where c = 0.7 for small storms |
| − | |-
| |
| − | | colspan="3" style="text-align: center;"| '''Settling velocity according to Equation 8 of various equivalent spherical diameter sand and silt particles at 20°C.'''
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| − | |-
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| − | ! Silt and sand Diameter (um) !! v<sub>s</sub> (m/s) !! Header text (ft/s)
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| − | |-
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| − | | 10 || 0.000089 || 0.00029
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| − | |-
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| − | | 30 || 0.00078 || 0.0026
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| − | |-
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| − | | 80 || 0.0051 || 0.017
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| − | |-
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| − | | 100 || 0.0075 || 0.025
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| − | |-
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| − | | 150 || 0.015 || 0.049
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| − | |-
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| − | | 200 || 0.023 || 0.075
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| − | |-
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| − | | 500 || 0.071 || 0.23
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| − | |-
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| − | | 1000 || 0.13 || 0.43
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| − | |}
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| | | | |
| | + | For the following calculation, ensure your units are consistent |
| | + | *Determine, ''v''<sub>''S''</sub>, the settling velocity for the particle size targeted (recommend 80 microns for particle size. Determine v<sub>s</sub> from table on this page) |
| | + | *Determine ''FR'', the target fraction removal (recommend 0.50-0.70) |
| | + | *Determine ''A'', the area of directly connected impervious draining to the pretreatment practice |
| | + | *Determine ''I'', the peak rain intensity (0.5 in/hr for a 1-inch event, Type 2 distribution) |
| | + | *Calculate the area of the filter strip from LW = - ((c * I * A) / (v<sub>S</sub>) * ln(1 - FR)) where c = 0.7 for small storms |
| | + | *Determine the length (L) and the width (W) from the above computation |
| | | | |
| − | The following assumptions have been made while developing the sizing criteria.
| + | ===Example for pretreatment filter strip=== |
| − | #A design storm of 1-inch precipitation.
| + | A directly connected impervious area of 10,000 ft<sup>2</sup> drains into a bioretention practice. The area is a sufficient size to install a well-sized filter strip, so the designer chose to settle 80 percent of the 80 ''u''m fine-sand particles in the filter strip during the design storm. For this case, the values to be used are |
| − | #The design <span title="Manner in which the depth of rainfall varies in space and time"> '''storm distribution'''</span> over time is [https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1044171.pdf Natural Resources Conservation Service (NRCS) Type II] with an intensity of 0.5 inches per hour (in/hr) during the most intense portion of the storm.
| |
| − | #Permeable surfaces in the catchment {{pop|'''do not contribute runoff'''|Permeable surfaces and runoff|The contribution of permeable surfaces (e.g. grass, forest) to runoff varies with factors such as soil, initial soil moisture content, slope, and rainfall intensity. Hydrologic Soil Group D soils, which have a significant amount of clay, have low infiltration rates and typically produce more runoff than other soils. Runoff from permeable surfaces also increases with slope, higher initial moisture content, and higher rainfall intensity. For a typical 1-inch rain storm, permeable surfaces generally produce little runoff.}} for the 1-inch storm.
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| − | #There is no scour on the filter strip surface.
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| − | #The infiltration rate through the filter strip surface is minimal and relative to the flow over the filter strip.
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| − | #The water column is well-mixed, such that the concentration of suspended solids is constant, with a loss of solids as the water moves down the filter strip in a plug-flow fashion.
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| − | | |
| − | Assumption Nos. 1 through 3 are common for rainfall-runoff computations in Minnesota and greatly simplify the design. Assumption No. 4 is correct if the filter strip does not need maintenance. Assumption No. 5 is made because of the short residence time of the water on the filter strip. Assumption No. 6 is a fair assumption with the velocities and depth of the flow over a filter strip and would result in a conservative sizing criteria.
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| − | | |
| − | The highest discharge for the design storm will be during the peak 0.5 in/hr intensity, based upon Assumption Nos. 1 through 3. The flow into the filter strip results by multiplying the directly connected impervious area of the catchment with this intensity, or
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| − | | |
| − | <u>'''Eq. 11'''</u> <math> Q = cAI </math>
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| − | | |
| − | where ''Q'' is water discharge into the filter strip, ''c'' is a <span title="The Rational Method is a simple hydrologic calculation of peak flow based on drainage area, rainfall intensity, and a non-dimensional runoff coefficient. The peak flow is calculated as the rainfall intensity in inches per hour multiplied by the runoff coefficient and the drainage area in acres. The peak flow, ''Q'', is calculated in cubic feet per second (cfs) as ''Q'' = ''CiA'' where ''C'' is the runoff coefficient, ''i'' is the rainfall intensity, and ''A'' is the drainage area. A conversion factor of 1.008 is necessary to convert acre-inches per hour to cfs, but this is typically not used. This method is best used only for simple approximations of peak flow from small watersheds."> '''Rational Method'''</span> coefficient equal to 0.7 for small storms, ''A'' is the directly connected impervious area of the catchment, and ''c'' is the peak intensity during the design storm (i.e., 0.5 in/hr).
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| − | | |
| − | The discharge is also equal to the volume of water on the filter strip divided by the residence time of the water on the filter strip, or
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| − | | |
| − | <u> '''Eq. 12'''</u> <math> Q = (LWh) / t_r </math>
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| − | | |
| − | where ''L'' is the length of the filter strip, ''W ''is the width of the filter strip, ''h'' is the depth of water on the filter strip, and ''t<sub>r</sub>'' is the residence time of the water on the filter strip. Combining Equations 11 and 12 results in a relation for filter strip surface area (''LW'')
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| − | | |
| − | <u> '''Eq 13''' </u> <math> LW = (cAI * t_r) / h </math>
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| − | | |
| − | Rearranging for ''t<sub>r</sub>'' gives the resident time of water on the filter strip
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| − | | |
| − | <u> '''Eq 14''' </u> <math> t_r = (hLW) / (cAI) </math>
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| − | | |
| − | The residence time of water on the filter strip (''t<sub>r</sub>'') can also be approximated by assuming a well-mixed water column (Assumption No. 6) with settling and <span title="In plug flow, the velocity of the fluid is assumed to be constant across any cross-sectional area perpendicular to the direction of flow."> '''plug flow'''.</span> down the filter strip from the relation [Gulliver, 2007]
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| − | | |
| − | <u> '''Eq 15''' </u> <math> FR = 1 - exp(-(v_St_r) / h) </math>
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| − | | |
| − | where ''FR'' is the fraction of a given settling velocity to be removed and ''v<sub>S</sub>'' is the settling velocity of a given size particle. Equation 15 can be rearranged to give the residence time
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| − | | |
| − | <u> '''Eq 16''' </u> <math> t_r = - (h/v_S)ln(1 - FR) </math>
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| − | | |
| − | Substituting Equation 16 into Equation 13 results in
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| − | | |
| − | <u> '''Eq 17''' </u> <math> LW = - ((cAI)/v_S) Ln(1 - FR) </math>
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| − | | |
| − | The natural log of a fraction will result in a negative number, so the area will be a positive value. Note that Equation 17 does not depend upon the flow depth. If the depth is large, the velocity is correspondingly low so the particles are allowed to settle. It is also noteworthy that the surface area of the filter strip (''LW'') is important, and not just the length of the filter strip. This provides flexibility to designers, as a short filter strip, within reason, can provide adequate pretreatment with a sufficient width to meet the overall surface area requirement. The roughness of the filter strip is unimportant for settling. However, the roughness should have sufficient porosity to hold and avoid sediment scour.
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| − | | |
| − | The fraction of a given settling velocity to be removed by the filter strip will need to be chosen by the designer. However, for a limited area of filter strip, Equation 17 can be rearranged to result in the fraction retained for a given filter strip area
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| − | | |
| − | <u> '''Eq 18''' </u> <math> FR = 1 - exp(-(v_SLW)/(cAI)) = 1 - exp(-(v_SLW)/Q) </math>
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| − | | |
| − | The <span title="The rate at which suspended solids subside and are deposited."> '''settling velocity'''</span> of a given size (typically sand-shaped) particle was developed by Ferguson and Church [2004], as
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| − | | |
| − | <u> '''Eq 19''' </u> <math> v_S = (gRd^2) / (18''u'' + (0.75Rd^3)^{0.5}) </math>
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| − | | |
| − | [[File:Filter strip sizing.png|300px|thumb|alt=filter strip retention images|<font size=3>Examples of particle retention for filter strips.</font size>]]
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| − | | |
| − | where: ''g'' is the acceleration of gravity, ''R'' is the <span title="the ratio of the density of a substance to the density of a standard, usually water for a liquid or solid, and air for a gas"> '''specific gravity'''</span> of the particle relative to that of water, ''d'' is the equivalent spherical diameter of the particle, and ''u'' is the <span title="The viscosity of a fluid is a measure of its resistance to deformation at a given rate. "> '''viscosity'''</span> of water. Equation 19 combines <span title="the force required to move a sphere through a given viscous fluid at a low uniform velocity is directly proportional to the velocity and radius of the sphere."> '''Stoke's Law'''</span> at small particle diameters with the turbulent drag coefficient at larger particle diameters and a transition in between. The transition is between a dependence upon ''d''<sup>2</sup> to one upon √''d'' as the particle size increases.
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| − | | |
| − | Assuming the filter strip is being designed to settle silica-based, inorganic particles (i.e., silt and sand) at 20<sup>o</sup>C, the settling velocity of the particles at various equivalent spherical diameters are presented in the above table specifying settling velocities.
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| − | | |
| − | The designer must first choose the fraction of a given silt or sand size that they want to remove through settling into the filter strip. The settling velocity is then chosen from the table or using Equation 19. The design storm intensity has been chosen herein at 0.5 in/hr, and the directly connected impervious area is given by site characteristics. The surface area of the filter strip (''LW''), can then be computed. If the space is limited for a filter strip, the fraction of the targeted particle size removed can be computed. Sample results of these computations are given in the adjacent figures.
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| − | | |
| − | ===Filter strip sizing examples===
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| − | The following section demonstrates the impact that available space has on the sizing of the pretreatment device. The length of the filter strip greatly determines the amount of pretreatment it can provide downstream BMPs. In Example D, an aggressive filter strip is highlighted, which settles 80 percent of the fine particulates that enter the BMP. Example E will demonstrate a filter strip which is fit into a limited space.
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| − | | |
| − | ====EXAMPLE D. AGGRESSIVE FILTER STRIP SETTLING====
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| − | A directly connected impervious area of 10,000 ft2 with a mean slope of 3 percent, and a watershed length to the outlet of 300 ft drains into a bioretention practice. The area is a sufficient size to install a well-sized filter strip, so the designer chose to settle 80 percent of the 80 m fine-sand particles in the filter strip during the design storm. For this case, the values for Equation 25 are
| |
| | *FR = 0.8 | | *FR = 0.8 |
| − | *A = 10,000 ft2 | + | *A = 10,000 ft<sup>2</sup> |
| | *I = 0.5 in/hr | | *I = 0.5 in/hr |
| | *v<sub>S</sub> = 0.017 ft/s | | *v<sub>S</sub> = 0.017 ft/s |
| | *c = 0.7 | | *c = 0.7 |
| − | *S = 0.03 ft/ft
| |
| − | *L = 300 ft
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| − |
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| − | As in the previous examples, the peak rainfall intensity should be over a time of concentration, ''t''<sub>''c''</sub>, calculated using Equation 9. The time concentration here is equal to 9 minutes and occurs within the most intense hour of an NRCS Type II storm, which accounts for approximately 50 percent of the storm total. Equation 17 then provides the values of LW
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| − |
| |
| − | <math> LW = - ((0.7 * 1000 ft^2 * (1 ac / 43560 ft^2) * 0.5 in/hr) / 0.017 ft/s) * ln(1-0.8) = 7.6 ft^2 </math>
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| − |
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| − | Assuming a 3-ft wide inlet, the filter strip would need to be a minimum of 2.5 ft (L = LW/W = 7.6 ft<sup>2</sup>/3 ft = 2.5 ft) in length. by 2.5-ft. To settle 80 percent of the fine sand particles entering this bioretention practice, the filter strip is recommended to be 3 ft wide by 2.5 ft long.
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| − |
| |
| − | ====Limited Space Filter Strip Settling====
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| − | A directly-connected impervious area of 10,000 ft<sup>2 </sup> with a mean slope of 3 percent and a watershed length to the outlet of 300 ft drains into a bioretention practice. The space for a filter strip is limited to a width of 2 ft and length of 2 ft. The designer decided to compute the percent of the 100 m fine-sand particles that will settle into the filter strip during the design storm. For this case,
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| − | *A=10,000 ft<sup>2
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| − | *I = 0.5 in/hr
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| − | *v<sub>s</sub> = 0.025 ft/s
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| − | *LW = = 2 ft * 2 ft = 4 ft<sup>2</sup>
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| − | *c=0.7
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| − | *S=0.03 ft/ft
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| − | *L=300 ft
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| − |
| |
| − | As in the previous examples, the peak rainfall intensity should be over a time of concentration, ''t<sub>c</sub>'', calculated using Equation 9. The time concentration here is equal to 9 minutes and occurs within the most intense hour of an NRCS Type II storm, which accounts for approximately 50 percent of the storm total.
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| − |
| |
| − | The value of FR for a 100 ''u''m fine-sand particle is
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| − |
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| − | <math> FR = 1 - exp( -(0.025ft/s * 4 ft^2)/(0.7 * 10,000 ft^2 * (1ac/43560ft^2/ac) * 0.5 in/hr) = 0.71 </math>
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| − |
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| − | The filter strip would retain 71 percent of the 100''u''m sand particles during the design storm. This retention for the design storm will still be worth considering because for smaller storms, the retention will be greater. For example, at a 0.25 inch/hour intensity and the other conditions given above, the retention would be 93 percent of the 100 ''u''m fine-sand particles. The disadvantage of this limited space filter strip is that more silt and sand will settling downstream in
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| − | the bioretention practice, requiring more frequent maintenance to maintain the water quality benefits of the BMP.
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| | | | |
| − | ===Filter strip recommendations=== | + | Plugging these values into the equation LW = - ((0.7 * 10000 ft<sup>2</sup>) * 0.5 in/hr) / 0.017 ft/s) * ln(1-0.8) = 7.6 ft<sup>2</sup> |
| − | If the site has adequate space, installing a filter strip that would retain 80 percent of the 80 ''u''m fine-sand particles during the design storm is recommended. This method will keep the bioretention or bioinfiltration practice from filling up with fine and coarse sand particles too quickly. Both the aggressive treatment and limited space methods are acceptable if the designer is aware of the limitations of the limited space method (i.e., increased frequency of maintenance on the downstream
| |
| − | bioretention/bioinfiltration practice). It should be noted that, even though a filter strip will retain more particles per surface area than a basin, filter strips typically require a greater level of effort to clean the pretreatment practice. The cost of pulling and washing rocks versus lowering the level of a grass filter strip to clean out a basin should be compared before a decision is made.
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| | | | |
| − | ==References==
| + | Assuming a 3-ft wide inlet, the filter strip would need to be a minimum of 2.5 ft (L = LW/W = 7.6 ft<sup>2</sup>/3 ft = 2.5 ft) in length by 2.5-ft. To settle 80 percent of the fine sand particles entering this bioretention practice, the filter strip is recommended to be 3 ft wide by 2.5 ft long. |
| − | *Chow, V.T., D.R. Maidment, and L.W. Mays, 1988. Applied Hydrology, McGraw-Hill, New York, NY.
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| − | *Erickson, A.J. 2012. Advanced Stormwater Treatment: Dissolved Pollutants.
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| − | *Ferguson, R.I. and Church, M., 2004. “A Simple Universal Equation For Grain Settling Velocity,” Journal Of Sedimentary Research. Vol 74, No. 6. pg 933-937.
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| − | *Gulliver, J.S. 2007. Introduction to Chemical Transport in the Environment. Cambridge University Press, Cambridge, England.
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| − | *Merkel, W., and H.F. Moody. 2015. NOAA Atlas 14 Rainfall for Midwest and Southeast States, prepared by the Natural Resources Conservation Service, Washington, D.C.
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| | | | |
| | <noinclude> | | <noinclude> |
| Line 383: |
Line 123: |
| | *[[Photo gallery for pretreatment]] | | *[[Photo gallery for pretreatment]] |
| | | | |
| − | [[Category:Pretreatment]] | + | [[Category:Level 2 - Best management practices/Pretreatment practices]] |
| | </noinclude> | | </noinclude> |
