STORMWATER TECHNICAL STANDARDS MANUAL HANCOCK C I
STORMWATER TECHNICALSTANDARDS MANUALHANCOCK COUNTY, INDIANA(Final Draft)July 2018Developed by Christopher B. Burke Engineering, LLC
TABLE OF CONTENTSChapterTitle1INTRODUCTION2METHODOLOGY FOR DETERMINATIONOF RUNOFF RATES3METHODOLOGY FOR DETERMINATIONOF RETENTION/DETENTION STORAGE VOLUMES4STORM SEWER DESIGN STANDARDSAND SPECIFICATIONS5OPEN CHANNEL DESIGN STANDARDSAND SPECIFICATIONS6STORMWATER DETENTION DESIGNSTANDARDS FOR PEAK FLOW CONTROL7CONSTRUCTION SITES STORMWATER POLLUTIONPREVENTION STANDARDS8POST-CONSTRUCTION STORMWATERQUALITY MANAGEMENT STANDARDS9METHODOLOGY FOR DETERMINATIONOF REQUIRED SIZING OF BMPS10MISCELLANEOUS REQUIREMENTSAPPENDIX A : ABBREVIATIONS AND DEFINITIONSAPPENDIX B : STANDARD FORMSAPPENDIX C : CONSTRUCTION BMP FACT SHEETSAPPENDIX D : POST-CONSTRUCTION BMP APPENDICESStormwater Technical Standards - Table of Contents Page i
Chapter OneINTRODUCTIONThis document, the Hancock County Stormwater Technical Standards Manual, preparedby Christopher B. Burke Engineering, LLC, contains the necessary technical standardsfor administering the requirements of 327 IAC 15-13 and the Hancock County StormwaterManagement Ordinance. This document should be considered as a companiondocument to the Ordinance. Whereas the Ordinance contains the majority of theregulatory authority and general requirements of comprehensive stormwatermanagement, this document contains the necessary means and methods for achievingcompliance with the Ordinance. In case there are conflicts between the requirementscontained in this document and the ordinance, the requirements of the Ordinance shouldprevail.This document contains formulas and methodologies for the review and design of bothstormwater quantity and stormwater quality facilities. Chapters 2 through 6 containstormwater conveyance and detention calculations and requirements. Chapter 7 containsinformation on erosion control requirements and other pollution prevention measures foractive construction sites. Chapters 8 through 9 cover calculations required to properlysize and design stormwater quality features that will treat runoff long-term followingconstruction completion. This includes both Conventional and Low Impact Development(LID) approaches. Chapter 10 contains miscellaneous standards regarding grading andbuilding pad elevations, acceptable outlet and adjoining property impact requirements, nonet loss floodplain storage requirements, Floodway and FEH requirements, and therequirements associated with proposed dams or levees. A comprehensive glossary ofterms is provided in Appendix A. Appendix B contains several useful and necessarystandard forms. Best Management Practices (BMPs) for erosion control measures duringthe construction phase are contained in Appendix C. It is the intent of the Hancock CountyDrainage Board and/or Hancock County Surveyor that material presented in AppendicesC will be revised or eliminated once the Indiana Storm Water Quality Manual (ISWQM) isupdated to include details regarding the BMPs currently included in Appendix C.Appendix D contains non-structural and structural post-construction BMP Fact Sheets aswell as Recommended Plant Lists, Recommended Materials, Soil Infiltration TestingProtocol, BMP Maintenance Checklists, and Maintenance Agreement for postconstruction BMPs.The site designer is encouraged to review the LID discussion in Chapter 8 prior to the sitedesign to take advantage of runoff reduction recognitions provided towards water quantitycalculations if LID practices are utilized as part of the site design.TABLE OF CONTENTSStormwater Technical Standards - Chapter 1 – Page 1
Chapter TwoMETHODOLOGY FOR DETERMINATION OFRUNOFF RATESRunoff rates shall be computed for the area of the parcel under development plus thearea of the watershed flowing into the parcel under development. The rate of runoff whichis generated as the result of a given rainfall intensity may be calculated as follows:A.Development Sites Less than or Equal to 5 Acres in Size, With a ContributingDrainage Area Less than or Equal to 25 Acres and No Depressional StorageThe Rational Method may be used. A computer model, such as TR-55 (NRCS), TR20 (NRCS), HEC-HMS (COE), and HEC-1 (COE), that can generate hydrographsbased on the NRCS TR-55 time of concentration and curve number calculationmethodologies may also be used along with a 24-hour duration NRCS Type 2 storm.Note that for the purpose of determining the post-developed conditions curvenumbers, due to significant disturbance to the upper soil layers during theconstruction activities, the initially determined hydrologic soil group for disturbedareas should be changed to the next less infiltrating capacity category (i.e., A to B,B to C, and C to D).LID Exception: If Low Impact Development (LID) approach ispursued in satisfying the requirements noted in Chapter 8 (PostConstruction Stormwater Quality Management Standards), thepost-developed CN for the protected undisturbed or restoreddisturbed areas meeting the requirements described in Chapter 8and BMP fact sheets may be determined based on predevelopment underlying soil layer.In the Rational Method, the peak rate of runoff, Q, in cubic feet per second (cfs) iscomputed as:Q CIAWhere:C Runoff coefficient, representing the characteristics of thedrainage area and defined as the ratio of runoff to rainfall.Stormwater Technical Standards - Chapter 2 – Page 1
I Average intensity of rainfall in inches per hour for a durationequal to the time of concentration (tc) for a selected rainfall frequency.A Tributary drainage area in acres.Values for the runoff coefficient "C" are provided in Tables 2-1 and 2-2, which showvalues for different types of surfaces and local soil characteristics. The composite"C" value used for a given drainage area with various surface types shall be theweighted average value for the total area calculated from a breakdown of individualareas having different surface types. Table 2-3 provides runoff coefficients and inlettimes for different land use classifications.Rainfall intensity shall be determined from the rainfall frequency data shown in Table2-4.In general, the time of concentration (tc) methodology to be used for all stormwatermanagement projects within Hancock County shall be as outlined in the U.S.Department of Agriculture (USDA) - NRCS TR-55 Manual. In urban or developedareas, the methodology to be used shall be the sum of the inlet time and flow timein the stormwater facility from the most remote part of the drainage area to the pointunder consideration. The flow time in the storm sewers may be estimated by thedistance in feet divided by velocity of flow in feet per second. The velocity shall bedetermined by the Manning's Equation (see Chapter 4). Inlet time is the combinedtime required for the runoff to reach the inlet of the storm sewer. It includes overlandflow time and flow time through established surface drainage channels such asswales, ditches, and sheet flow across such areas as lawns, fields, and other gradedsurfaces.Stormwater Technical Standards - Chapter 2 – Page 2
TABLE 2-1Urban Runoff CoefficientsRunoff Coefficient “C”(by Storm Recurrence Interval)Type of Surface Hard SurfacesAsphaltGravelConcreteRoof Lawns (Sandy)Flat (0-2% Slope)Rolling (2-7% Slope)Steep (Greater than 7%Slope) Lawns (Clay)Flat (0-2% Slope)Rolling (2-7% Slope)Steep (Greater than 7%Slope) 25 year25 year50 year100 300.330.400.50Source: HERPICC Stormwater Drainage Manual, July 1995, and other sources.Stormwater Technical Standards - Chapter 2 – Page 3
TABLE 2-2Rural Runoff CoefficientsRunoff Coefficient “C”(by Storm Recurrence Interval)Type of Surface Woodland (Sandy)Flat (0-2% Slope)Rolling (2-7% Slope)Steep (Greater than 7%Slope) Woodland (Clay)Flat (0-2% Slope)Rolling (2-7% Slope)Steep (Greater than 7%Slope) Pasture (Sandy)Flat (0-2% Slope)Rolling (2-7% Slope)Steep (Greater than 7%Slope) Pasture (Clay)Flat (0-2% Slope)Rolling (2-7% Slope)Steep (Greater than 7%Slope) Cultivated (Sandy)Flat (0-2% Slope)Rolling (2-7% Slope)Steep (Greater than 7%Slope) Cultivated (Clay)Flat (0-2% Slope)Rolling (2-7% Slope)Steep (Greater than 7%Slope) 25 year25 year50 year100 600.550.660.660.790.830.990.720.790.951.00Source: HERPICC Stormwater Drainage Manual, July 1995, and other sources.Stormwater Technical Standards - Chapter 2 – Page 4
TABLE 2-3Runoff Coefficients by Land Use, Typical Inlet Times, and Storm Recurrence IntervalRunoff Coefficients "C"(by Storm Recurrence Interval)Flat(1)Inlet Time(Minutes)(4) 25year0.7525year0.8350year0.99100year1.00 25year0.83Rolling(2)2550year year0.91 olsSemi DetachedResidentialDetachedResidentialQuarter Acre LotsHalf Acre Land UseCommercial (CBD)Commercial(Neighborhood)IndustrialGarden ApartmentsChurches100year1.00 25year0.91Steep(3)2550year year1.00 1.00100year1.00Source: HERPICC Stormwater Drainage Manual, July 1995, and other sources.(1)(2)(3)(4)Flat terrain involves slopes of 0-2%.Rolling terrain involves slopes of 2-7%.Steep terrain involves slopes greater than 7%.Interpolation, extrapolation and adjustment for local conditions shall be based on engineering experience and judgment.55 - 1010 - 15To beComputed
B.Development Sites Greater Than 5 Acres in Size or Contributing Drainage AreaGreater than 25 Acres or With Significant Depressional StorageThe runoff rate for these development sites and contributing drainage areas shall bedetermined by a computer model that can generate hydrographs based on theNRCS TR-55 time of concentration and curve number calculation methodologies.Note that for the purpose of determining the post-developed conditions curvenumbers, due to significant disturbance to the upper soil layers during theconstruction activities, the initially determined hydrologic soil group for disturbedareas should be changed to the next less infiltrating capacity category (i.e., A to B,B to C, and C to D).LID Exception: If Low Impact Development (LID) approach ispursued in satisfying the requirements noted in Chapter 8 (PostConstruction Stormwater Quality Management Standards), thepost-developed CN for the protected undisturbed or restoreddisturbed areas meeting the requirements described in Chapter 8and BMP fact sheets may be determined based on predevelopment underlying soil layer.The 24-hour NRCS Type 2 Rainfall Distribution shall be utilized for runoffcalculations. 24-hour Rainfall depth for various frequencies shall be taken fromTable 2-5. The NRCS Type 2 distribution ordinates are found in Table 2-6.Examples of computer models that can generate such hydrographs include TR-55(NRCS), TR-20 (NRCS), and HEC-HMS (COE). These programs may bedownloaded free of charge from the associated agencies’ web sites. Other modelsmay be acceptable on a case by case basis.Stormwater Technical Standards - Chapter 2 – Page 1
TABLE 2-4Rainfall Intensities for Various Return Periods and StormDurationsIntensity (Inches/Hour)DurationReturn Period (Years)251025501005 min10 min15 min30 min1 hr2 hr3 hr6 hr12 hr24 .220.242Source: NOAA, National Weather Service, "Precipitation-Frequency Atlas of the United States",NOAA Atlas 14, Volume 2, Version 3, rev 2006, for Greenfield, Indiana. (values for intermediatedurations can be logarithmically interpolated.) (All rainfall intensities are based on Annual Maximumseries)TABLE 2-5Rainfall Depths for Various Return PeriodsDepth (Inches)Duration24 Hrs.Return Period rce: NOAA, National Weather Service, "Precipitation-Frequency Atlas of the United States",NOAA Atlas 14, Volume 2, Version 2, rev 2006, for Greenfield, Indiana. (Rainfall depths for 2through 100-year storm are based on annual maximum series. 1-year rainfall depth is based onpartial duration series)Stormwater Technical Standards - Chapter 2 – Page 2
TABLE 2-6NRCS Type II Rainfall Distribution OrdinatesCumulativeStormTime (hr)CumulativePercent ofStorm DepthCumulativeStormTime (hr)CumulativePercent ofStorm DepthCumulativeStormTime (hr)CumulativePercent ofStorm .497.79898.398.698.999.299.599.8100Source: National Resources Conservation Service (NRCS), "TR-20 Computer Program for ProjectFormulation Hydrology", page F9, May 1982.NOTE: For use only when SCS Type II rainfall distribution is not a default option in the computer program.C.Development Sites with Drainage Areas Greater than or Equal to One squareMileFor the design of any major drainage system, as defined in Appendix A, thedischarge must be obtained from, or be accepted by, the IDNR. Other portions ofthe site must use the discharge methodology in the applicable section of thisChapter.Stormwater Technical Standards - Chapter 2 – Page 3
Chapter ThreeMETHODOLOGY FOR DETERMINATION OFRETENTION/DETENTION STORAGE VOLUMESThe required volume of stormwater storage for all development sites shall be computedusing a computer model that can generate hydrographs based on the NRCS TR-55 timeof concentration and curve number calculation methodologies. Examples of computermodels that can generate such hydrographs include TR-55 (NRCS), TR-20 (NRCS), andHEC-HMS (COE). Other models may be acceptable on a case by case basis.A. Post-Development Hydrologic ParametersNote that for the purpose of determining the post-developed conditions curvenumbers, due to significant disturbance to the upper soil layers during theconstruction activities, the initially determined hydrologic soil group fordisturbed areas should be changed to the next less infiltrating capacity category(i.e., A to B, B to C, and C to D).LID Exception: If Low Impact Development (LID) approach ispursued in satisfying the requirements noted in Chapter 8 (PostConstruction Stormwater Quality Management Standards), thepost-developed CN for the protected undisturbed or restoreddisturbed areas meeting the requirements described in Chapter 8and BMP fact sheets may be determined based on predevelopment underlying soil layer.B. Design Storm & Allowable Release Rates for Development Sites with Onsite Detention FacilitiesThe 24-hour NRCS Type 2 Rainfall Distribution shall be utilized to determinethe required storage volume. The allowable release rates shall be determinedbased on methodologies provided in Chapter 6 of these Technical Standardsdocument.C. Design Storm & Allowable Release Rates for Development Sites within aDesignated Drainage Area Serviced by a Regional Detention facilityDevelopment sites within Designated Drainage Areas making use of regionaldetention ponds are typically not required to provide on-site detention; as aresult, the requirements of Chapter 6 of these Stormwater Technical Standardsdo not apply to development sites where no on-site detention is planned.Stormwater Technical Standards - Chapter 3 – Page 1
Where a regional detention facility is servicing or is planned to service a subwatershed area, and that sub-watershed has been declared a DesignatedDrainage Area to raise the necessary Infrastructure Development Fees in lieuof providing on-site detention, the following method shall be used to determinethe size of storage space that would have been required on-site.The 24-hour NRCS Type 2 Rainfall Distribution shall be utilized to determinethe required storage volume. The stormwater model should not include an onsite detention facility. The outflow hydrograph at the downstream-most point inthe site’s stormwater system shall be compared to the allowable release ratefor the Designated Drainage Area to determine the required storage volume.The allowable release rate for a Designated Drainage Area shall be as definedby the resolution forming the Designated Drainage Area.The storage volume shall be determined by calculating the volume of outflowfrom the site that exceeds the given allowable release rate. For example, if a50-acre site is located in a Designated Drainage Area that has an allowablepost-development 100-year release rate of 0.2 cfs/acre, the required storagevolume for the site would be equal to the volume of water represented by thecross-hatched area in Figure 3-1.FIGURE 3-1Required Storage Volume Determination ExampleStormwater Technical Standards - Chapter 3 – Page 2
Chapter FourSTORM SEWER DESIGN STANDARDSAND SPECIFICATIONSAll storm sewers, whether private or public, and whether constructed on private or publicproperty shall conform to the design standards and other requirements contained herein.Proposed storm sewer systems must be sized and designed to convey at least the 10year frequency on-site stormwater runoff, as well as the anticipated 10-year frequencytributary off-site stormwater runoff based on the future developed condition (using Table6-1 of the Technical Standards). An analysis of the emergency routing of stormwaterrunoff through the subject development must be provided to confirm that the developmentwill not obstruct the free flow of floodwaters from the tributary off-site property in its currentcondition and after development. In addition, the Drainage System Overflow Design mustbe completed in accordance with Section M of this chapter to ensure the safe routing offlood waters through the subject development with the tributary off-site property in itscurrent condition and after development.A.Design Storm Frequencies1.All storm sewers, inlets, catch basins, and street gutters shall accommodate(subject to the “allowable spread” provisions discussed later in this Section), asa minimum, peak runoff from a 10-year return frequency storm calculatedbased on methodology described in Chapter 2. Additional discharges to stormsewer systems allowed in Section L below of this Section must be consideredin all design calculations. For Rational Method analysis, the duration shall beequal to the time of concentration for the drainage area. In computer basedanalysis, the duration is as noted in the applicable methodology associated withthe computer program.2.Culvert capacities for conveyance under interior local, collector, or arterialstreets without roadway overtopping shall be the runoff resulting from the 25year, 50-year, and 100-year frequency storms, respectivel
Stormwater Technical Standards - Chapter 1 – Page 1 . INTRODUCTION . This document, the Hancock County Stormwater Technical Standards Manual, prepared by Christopher B. Burke Engineering, L, contains the necessary technical standards LC for administering the requirements of 327 IAC 15- 13 and the Hancock County Stormwater Management Ordinance.
NPDES: Stormwater Best Management Practice— — Stormwater Wetland Stormwater Retrofit A stormwater retrofit is a stormwater control (usually structural) that a community puts into place after development to improve water quality, protect downstream channels, reduce flooding or meet other specific objectives.
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