Minimum Design Criteria for Implementation of …

1 Minimum Design Criteria forImplementation of Certain BestManagement Practices for StormWater Runoff Treatment Options2001 Edition2001 EditionMinimum Design Criteria - Runoff Treatment BMPPage iTable of ContentsIntroductionBackground1 Purpose1 Organization1 Grass SwalesPurpose and Definitions2 Applications and Limitations2 Design Criteria2 Sizing Procedures3 Maintenance4 Sample Calculation for Sizing Grass Swales8 Vegetated Filter StripsPurpose and Definitions10 Applications and Limitations10 Design Criteria10 Maintenance11 Sample Calculation for Sizing Filter Strips13 Wet PondPurpose and Definitions14 Applications and Limitations14 Design Criteria14 Permanent Pool15 Hydraulic Residence Time 15 Pond Geometry15 Permanent Pool Depth15 Permanent Pool Surface Area16 Sediment Forebay16 Basin Configuration16 Safety Bench16 Vegetative Bench16 Sizing Procedure17 Maintenance18 Sample Calculation for Sizing Wet Ponds24 Dry Detention BasinsPurpose and Definitions26 Applications and Limitations26 Design Criteria26 Sizing Procedure26 Maintenance27 Sample Calculation for Sizing Dry Detention Basins312001 EditionMinimum Design Criteria - Runoff Treatment BMPPage iiConstructed Wetlands for storm Water TreatmentPurpose and Definitions32 Applications and Limitations32 Design Criteria32 Maintenance33 Bibliography36 APPENDIX ( Design Check List)2001 EditionMinimum Design Criteria - Runoff Treatment BMPPage iiiList of TablesTable : Sizing Criteria for Grass Swales7 Table : Recommended Plants for Grass Swales8 Table : Flow Path and Required Filter Strip Width11 Table : Sizing Criteria for Filter Strips11 Table : Sizing Criteria for Wet Ponds19 Table.

2 Sizing Criteria for Dry Detention Basins28 Table : Sizing Criteria for storm Water Treatment Wetlands 352001 EditionMinimum Design Criteria - Runoff Treatment BMPPage ivList of FiguresFigure 1: Grass Swale Sample Layout 5 Figure 1-A: Level Spreader 6 Figure 2: Filter Strip Example Design Layout 12 Figure 3: Wet Pond Example Design Layout20 Figure 3-A: Wet Pond Example Cross-Sections21 Figure 3-B: Wet Pond Safety and Vegetative Bench Cross-Sections22 Figure 3-C: Wet Pond Riser Outlet with Trash Protection23 Figure 4: Dry Detention Basin Example Design Layout29 Figure 4-A: Dry Detention Basin Example Cross-Sections302001 EditionMinimum Design Criteria - Runoff Treatment BMPPage 1 INTRODUCTIONB ackground As a part of the NPDES storm water permit requirements, the JointTask Force (JTF), drafted and adopted the storm Water QualityManagement Guidance manual (October 2000), Guidance manual .

3 The Guidance manual provides planning-level guidance on severalBest Management Practices (BMPs) that may be suitable in Houstonand Harris County, and incorporates additional BMPs by the public review of the Guidance manual , several commentswere received requesting Design -level information for the structuralBMPs included in the Guidance manual . Non-structural BMPs arediscussed in Chapter of the Guidance manual . This document, Minimum Design Criteria for Implementation of Certain BestManagement Practices for storm Water Runoff Treatment Options(2001 Edition), was prepared to address the public comments andprovide Design -level Criteria for certain structural BMPs. It is intendedto supplement the structural BMP information material provided in theGuidance manual . The information contained in this document isadapted from several other manuals prepared in other parts of theUSA.

4 Sources are listed in the reference section of this purpose of this document is to provide Minimum designparameters for certain BMPs for storm water quality features for newdevelopment and significant redevelopment. Use of the BMPspresented here does not guarantee acceptance of a particular StormWater Quality Management Plan or effectiveness of the BMP toreduce pollutants. The BMPs described in the Criteria document areintended to facilitate the plan review process for new development andsignificant redevelopment projects in the City of Houston andunincorporated Harris document contains Design -level Criteria and Design detaildrawings for five structural BMPs, where each BMP has an identifiercode composed of letters and numbers. The document will be revisedand updated as necessary to include additional BMPs. Other BMPsmay be acceptable on a case-by-case basis.

5 In such cases priorconsultation with the reviewing agency is highly recommended. Thisdocument provides Design -level Criteria on the BMPs listed below: Grass Swales Vegetated Filter Strips Wet Pond Dry Detention Basins Constructed Wetlands for storm Water Treatment2001 EditionMinimum Design Criteria - Runoff Treatment BMPPage 2 Grass SwalesBMP SwalesPurpose andDefinition Grass swales are defined as grass-lined, earthen channels, intended toprovide water quality enhancements. Grass swales provide effectivetreatment only when they have low longitudinal slopes, with lowvelocities and shallow depth of flow in the swale. The main pollutantremoval mechanisms in grass swales are filtration as the storm watermoves through the vegetation and bacterial decomposition. Carefulselections of grass species that allow more contact withmicroorganisms living on the vegetation enhance pollutant removal ofgrass andLimitationsGrass swales are applicable best management practices where the peakflow rates and velocities are low.

6 They are recommended as a stand-alone best management practices for areas draining 10 acres or lessand where pollutant concentration from the area is low. Indevelopments draining more than 10-acres, grass swales could be usedin combination with other best management CriteriaThe primary Design factors that determine grass swales' effectivenessare peak flow rate, depth of flow, and height of vegetation. The designshould promote pollutant removal rather than transporting flow withgreater possible hydraulic efficiency. Therefore, it is critical thatfiltration, sedimentation, and other pollutant removal mechanisms swales for water quality enhancements should be sized to treatthe flow generated by the Design rainfall intensity from the drainagearea. Table summarizes the Design Criteria for grass swales andFigure 1 depicts a typical layout of a grass swale.

7 Based on theanalysis of the 1-hour precipitation data of rainfall gauges in the HarrisCounty area, an intensity of per hour was computed for 90percent of the storms. Unless amended by additional rainfall dataanalysis the Design flow shall be calculated using an intensity of per hour. The rational formula (Q=CIA) shall be used todetermine the Design flow rate. The flow velocity shall not exceed per second. The longitudinal slope of the swale shall not exceed 2percent and the Minimum slope shall be percent. Because of sitecondition, if the desired slope could not be attained, check dams shallbe used as small drop structures to achieve the desired swale check dams should be reinforced adequately for erosionprotection. The Design of grass swales must encourage sheet flow ofstorm water within the swale. If sheet flow conditions cannot beachieved, a flow spreader (Figure 1-A) must be installed to uniformly2001 EditionMinimum Design Criteria - Runoff Treatment BMPPage 3spread the flow across the width of the swale.

8 The width of the swaleshall be sized to achieve a depth of flow, in the grass swale, of 3inches or half the height of the grass, which ever is less. Table recommended plant list for grass swales. A morecomprehensive plant list is provided in the Guidance ManualsAppendix E. The Design storm shall travel for at least 50 feet on thegrass swale for water quality treatment before its discharge into thereceiving stream or municipal storm sewer system. The length ofswale should be calculated based on a Minimum hydraulic retentiontime of 5 minutes. Discharges from the grass swale to a receivingstream or roadside ditch shall not promote erosion, and erosionprevention devices shall be constructed. The side slope of the swaleshall be 3H:1V or Procedure Determine the drainage area (A) of the site that drains into thegrass swales. Determine the runoff coefficient (C) of the development.

9 Runoffcoefficient values from the City of Houston Design manual ,Chapter 9 - storm Water Design Requirements, shall be used. Based on the Design storm , calculate Design flow rate (wqQ) fromthe drainage area using the rational :Qwq = Water quality Design flow rate (ft3/sec) C = Runoff coefficient (dimensionless) I = Intensity (in/hr) A = Area in (acres) The Manning's roughness coefficient (n) for the swale shall for frequently mowed grass with a height of 6 inches. Determine the longitudinal swale slope (S). Determine the depth of flow (d). Depth of flow shall be 3 inchesor half the grass height, whichever is less. Select a channel shape and a side slope (z). Using Manning s equation, determine the bottom width (b) of theswale by trial and error or use the approximate method given the value of wqQ calculated above for Q in theManning's :Q = Water quality Design flow rate ft3/secn = Manning's roughness coefficient (dimensionless)A = Cross-sectional area (ft2)R = Hydraulic radius (ft)S = Longitudinal slope (dimensionless)2001 EditionMinimum Design Criteria - Runoff Treatment BMPPage 4 Basic hydraulic equations and cross sectional formula for a trapezoidalshape:2zdbdA+=122++=zdbPPAR=z dbT2+=By recognizing that top width (T) is much greater than depth of flow(d) and the square of the side slope (z2) is much greater than 1 andcertain terms are negligible the approximate solution to determine thebottom width of a trapezoidal shape Calculate the cross-sectional flow area.

10 Calculate velocity of flow (V), and if velocity of flow exceeds therecommended maximum, recalculate the bottom width of the swale. Determine the length of the grass swale based on a minimumhydraulic retention time of 5 minutes (300 seconds). The requiredminimum swale length is 50 =Where:V = velocity in feet per secondt = hydraulic retention time 300 secondsMaintenanceMaintenance is the most important factor if water quality grass swalesare to continue to function as originally designed. Until grass is established, grass swales must be inspectedwithin 24 hours after each storm of inch or greater or dailyduring prolonged rainfall events. After grass has been established, the swale should be checkedat least monthly. The grass must be mowed as needed for good growth and tomaintain the desired grass height. A maximum grass height of6 inches shall be maintained.