HYDROLOGIC ANALYSIS - Wichita, Kansas
CHAPTER4HYDROLOGIC ANALYSISTABLE OF CONTENTS4.1Introduction to Hydrologic Analysis . 14.2Rainfall . 24.3Rainfall Losses . 34.3.1Curve Number Method . 34.3.2Hydrologic Soil Groups . 54.3.3Required Curve Numbers . 64.3.4Composite Curve Numbers . 64.4Time of Concentration . 74.5USGS Regression Methods . 104.5.1Introduction . 104.6Rational Formula Method . 124.6.1Introduction . 124.6.2Equations . 124.6.3Runoff Coefficient (C). 124.6.4Rainfall Intensity (I) . 134.7Hydrograph Methods . 144.7.1General . 144.8NRCS Unit Hydrograph Method . 154.8.1Description . 154.8.2Selection of Time Step . 194.8.3Application . 194.8.4Simplified NRCS Peak Runoff Rate Estimation . 194.9Clark Method . 254.9.1Introduction . 254.9.2Dimensionless Time-Area Curve . 254.9.3Storage Constant and Time of Concentration . 264.9.4Application . 274.10EPA-SWMM “RUNOFF” Method . 274.10.1 Introduction . 274.10.2 Approved EPA-SWMM Hydrologic Procedures . 274.11Routing . 294.11.1 Routing Methods . 29Volume 2, Technical GuidancePage 4 - i
Section 4.1 - Introduction to Hydrologic Analysis4.11.1.14.11.1.2Stream and Floodplain Routing . 30Reservoir Routing . 314.12Downstream Hydrologic Assessment . 364.12.1 Reasons for Downstream Problems . 364.12.2 Methods for Downstream Evaluation . 384.12.2.1 Off-line . 384.12.2.2 On-line Detection Analysis . 394.13Water Quality Protection Volume and Peak Flow . 414.13.1 Water Quality Protection Volume Calculation . 414.13.2 Water Quality Peak Flow Calculation . 45Water Quality Peak Flow Example Problem . 464.13.3 Water Quality Volume Extended Detention . 474.14Calculation of %TSS Removal . 484.14.1 Calculation of % TSS Removal for a site (Controls in Parallel) . 484.14.2 Calculation of % TSS Removal for a site (Controls in Series) . 494.14.3 Calculation of % TSS Removal for Flow-through Situations. 504.14.4 Application of WQv Reductions . 514.15Channel Protection Volume . 514.15.1 Description . 514.15.2 Channel Protection Volume Extended Detention – Centroid Method . 524.15.3 Channel Protection Volume Extended Detention – Simplified Method . 564.16Backwater Conditions . 594.17Water Balance Calculations . 604.17.1 Introduction . 604.17.2 Basic Equations . 604.18Approved Hydrology Models . 65LIST OF TABLESTable 4-1 Point Rainfall Depths (inches) for 24-Hour Design Storms . 3Table 4-2 Pre- and Post-Development Curve Numbers . 6Table 4-3 Calculation of a Composite Curve Number . 6Table 4-4 Average Imperviousness per Land Use (Source NRCS, TR-55) . 7Table 4-5 Velocity Coefficient (K value) for Shallow Concentrated Flow. 8Table 4-6 Calculate Weighted Runoff Coefficients (C) . 13Table 4-7 Dimensionless Unit Hydrograph with Peaking Factor of 484 . 16Table 4-8 Dimensionless Unit Hydrograph with Peaking Factor of 484 . 18Table 4-9 Ia Values for Runoff Curve Numbers . 20Table 4-10 Pond and Swamp Adjustment Factors, Fp . 21Table 4-11 Hydraulic Conductivity by Hydrologic Soils Group . 28Table 4-12 Depression Storage Depths . 29Table 4-13 Volumetric Runoff Coefficients by Land Use and Hydrologic Soil Group . 42Page 4 - iiVolume 2, Technical Guidance
Chapter 4 - Table of ContentsTable 4-14Table 4-15Table 4-16Table 4-17Table 4-18Table 4-19% TSS Removal Values for Structural Stormwater Treatment Facilities . 48Monthly Precipitation Values as % of Average Annual Precipitation . 61Saturated Hydraulic Conductivity. 62Average Monthly Evaporation (inches) . 63Data for Water Balance Example . 64Water Balance Calculation . 64LIST OF FIGURESFigure 4-1 NRCS 24-Hour, Type 2 Rainfall Distribution . 2Figure 4-2 SCS Solution of the Runoff Equation . 5Figure 4-3 Mean Annual Precipitation . 11Figure 4-4 Unit Hydrograph. 15Figure 4-5 Dimensionless Unit Hydrographs for Peaking Factors of 484 and 300. 16Figure 4-6 SCS Type II Unit Peak Discharge Graph . 22Figure 4-7 Clark Concept (US Army Corps of Engineers EM 1110-2-1417) . 25Figure 4-8 Dimensionless Clark Time-Area Curve . 26Figure 4-9 On-Line versus Off-Line Storage . 32Figure 4-10 Example Stage-Storage Curve . 33Figure 4-11 Average-End Area Method . 33Figure 4-12 Example Stage-Discharge Rating Curve . 35Figure 4-13 Illustration of Flat-Pool Reservoir Routing . 36Figure 4-14 Detention Timing Example . 37Figure 4-15 Effect of Increased Post-Development Runoff Volume w/ Detention on a Downstream Hydrograph 38Figure 4-16 Example of the Ten-Percent Rule . 40Figure 4-17 Detention Time vs Discharge Ratios . 57Figure 4-18 Approximate Detention Basin Routing for Rainfall Types I, IA, II and III . 57Volume 2, Technical GuidancePage 4 - iii
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Section 4.1 - Introduction to Hydrologic Analysis4.1Introduction to Hydrologic AnalysisThis Section addresses surface water hydrology and % TSS Removal Calculation only; it doesnot address groundwater hydrology. Therefore all references in this Manual to “hydrology”means surface water hydrology. Also, in this chapter the terms “subbasin” and “catchment”(used interchangeably) refer to a single drainage area being analyzed by the varioustechniques, independently of any other subbasins or catchments that make up the basin orwatershed being analyzed. For example, the peak flow computed by one of the peak flowonly methods (Regression or Rational), or a unit hydrograph developed for a specific drainagearea (e.g., NRCS), are applied to subbasins or catchments. Subbasins or catchments makeup basins or watersheds, along with channels, reservoirs, and other hydrologic features asapplicable.Hydrology deals with estimating peak flows, runoff volumes, and time distributions of runoff.The analysis of these parameters is fundamental to the design of stormwater managementfacilities, such as storm drainage systems and structural stormwater controls. In thehydrologic analysis of a development/redevelopment site, there are a number of factors thataffect the nature of stormwater runoff from the site. Some of the factors to be considered are: Rainfall amount and distribution; Drainage area size and shape; Ground cover and soil type; Slopes of terrain, streams and channels; Antecedent moisture (or runoff) conditions; Rainfall abstraction or loss rates (initial and continued); Storage potential (floodplains, ponds, wetlands, reservoirs, channels, etc.); Watershed development; and, Characteristics of the local drainage system.There are a number of hydrologic methods available to estimate runoff characteristics for asite or drainage subbasin. Some of those methods provide estimates of peak discharge only.Some provide estimates of runoff volume only. Special methods estimate the runoff volumefor small storms that are not readily analyzed using more conventional methods. Othersprovide estimates of the full hydrograph (discharge versus time, and thus the distribution ofrunoff volume). In addition, there are methods available for estimating long-term runoff for usein water balance applications.Rainfall, of course, is the primary driver for all of the methods mentioned above. The depthand distribution of rainfall during storm events is critical to estimating peak flows as well as thedistribution of discharge over time.Volume 2, Technical GuidancePage 4 - 1
Section 4.2 - Rainfall LossesFor some applications, it is necessary to “route” flow through a control structure such as astormwater pond and outlet. The temporary storage of inflowing water (detention) tends toattenuate the inflow hydrograph, resulting in outflow with a lower peak but increased duration.Thus, routing is fundamental to the design of stormwater storage facilities.This chapter provides descriptions of the hydrologic methods to be used to implement therequirements of this Manual.4.2RainfallA rainfall event-based hydrologic analysis requires an estimate of the amount (depth orintensity) of rainfall that will occur on the site during a specified duration for a specifiedaverage return period. The average return period (sometimes referred to as average returninterval or frequency) is expressed in years. The average annual exceedance probability ofthe event is the reciprocal of the average return period, and is expressed as the probabilitythat the event will be equalled or exceeded in any given year. (The probability lies between 0and 1.) For example, an event with an average return interval of 100 years has an averageannual probability of being equalled or exceeded of 1/100 0.01, or 1%.For methods discussed in this Manual requiring a distributed rainfall event, the NRCS 24-hourType 2 rainfall distribution (Figure 4-1) will be used. Table 4-1 lists the 24-hour point rainfalldepths for various frequency storm events. These values were derived from the NationalOceanic and Atmospheric Administration (NOAA) Atlas 14, Volume 8, Version 2.0 and thePrecipitation Frequency Data Server (PFDS) from the Hydrometeorological Design StudiesCenter. (Note that the -year through 10-year values are partial durations series depths and the25-year through 1000-year values are annual maximum series depths. For the larger storms,there is no significant difference between the partial duration and annual series.)Figure 4-1 NRCS 24-Hour, Type 2 Rainfall DistributionPage 4 - 2Volume 2, Technical Guidance
Section 4.3 - Rainfall LossesTable 4-1 Point Rainfall Depths for 24-Hour Design .917.838.7810.1011.10For methods requiring the rainfall intensity of specific durations, a table of point rainfallintensities for Wichita and Sedgwick County has been derived from a regression analysis ofthe data obtained from NOAA Atlas 14, Volume 8, Version 2.0 and the PFDS, presented inAppendix B. The table is for durations of 15 to 120 minutes, and average return periods of 1to 1000 years.The depths and intensities provided herein are point values. Theoretically, these values maybe adjusted for actual basin area. Figure 15 of the National Weather Service TP-40 rainfallatlas may be used for that purpose. However, it is general practice to use point values fordrainage areas of 10 square miles or less. The reduction is minor (approximately 3% or less)for basins up to 20 square miles. Practically, for the majority of studies performed under theprovisions of this Manual, aerial reduction will not be required.4.3Rainfall Losses4.3.1Curve Number MethodExcept for unusual conditions, not all of the rainfall that falls on a subbasin is discharged asdirect runoff. A portion of the rainfall may be retained in depressions, while some mayinfiltrate into the soil. Other portions of the rainfall may be returned to the atmosphere byvegetative interception and ultimately by transpiration and/or evaporation. The portions ofrainfall that do not become direct runoff are referred to collectively as “rainfall losses” or“abstractions” while the rainfall that does become direct runoff is referred to as “excessrainfall” or “direct runoff.”Rainfall loss is a function of soil characteristics, land-use, antecedent moisture (or runoff)conditions, and other factors. There are many methods used for estimating rainfall losses,with varying degrees of complexity from the standpoint of ease of use as well as datarequirements.For the applications of the hydrology methods presented in this Manual, the NRCS (formerlySCS) “Curve Number” (CN) method will be used. The CN indicates the runoff potential. Thegreater the CN value, the higher the runoff potential.An approximate relationship between cumulative rainfall and cumulative runoff was derived byNRCS from experimental plots for numerous soils and cover conditions. The following NRCSrunoff equation (SCS, 1986) is used to estimate direct runoff from storm rainfall:Volume 2, Technical GuidancePage 4 - 3
Section 4.3 - Time of ConcentrationEquation 4-1where:QPIa S P I a 2Q P I a S cumulative direct runoff (in)cumulative rainfall (in)initial abstraction including surface storage, interception, evaporation, andinfiltration prior to any runoff occurring (in)(1000/CN) - 10 where CN NRCS Curve NumberAn empirical relationship used in the NRCS method (SCS, 1986) for estimating Ia is:I a 0.2S(Note that for P Ia, Q 0)This is an average value that could be adjusted if there are calibration data to substantiate theadjustment. Table 4-9 provides values of Ia for a range of CNs.Substituting 0.2S for Ia in Equation 4-1, the equation becomes:Equation 4-22 P 0.2S Q P 0.8S Figure 4-2 shows a graphical solution of this equation. For example, 4.1 inches of direct runoffwould result if 5.8 inches of rainfall occurred on a watershed with a curve number of 85.Equation 4-2 can be rearranged so the CN can be estimated if rainfall and runoff volume areknown (Pitt, 1994). The equation then becomes:Equation 4-3Page 4 - 4CN 1000 10 5P 10Q 10 Q 2 1.25QP 12 Volume 2, Technical Guidance
Section 4.3 - Rainfall LossesFigure 4-2 SCS Solution of the Runoff Equation(Source: SCS, TR-55, Second Edition, June 1986)4.3.2Hydrologic Soil GroupsThe Curve Number method uses the combination of soil conditions and land uses (groundcover and treatment) to assign a CN to an area.Soil properties influence the relationship between runoff and rainfall since soils have differingrates of infiltration. Based on infiltration rates, the NRCS has divided soils into four hydrologicsoil groups (HSGs). HSG A: Soils having a low runoff potential due to high infiltration rates: These soilsconsist primarily of deep, well-drained sands and gravels. HSG B: Soils having a moderately low runoff potential due to moderate infiltration rates:These soils consist primarily of moderately deep to deep, moderately well to well drainedsoils with moderately fine to moderately coarse textures. HSG C: Soils having a moderately high runoff potential due to slow infiltration rates.These soils consist primarily of soils in which a layer exists near the surface that impedesthe downward movement of water or soils with moderately fine to fine texture. HSG D: Soils having a high runoff potential due to very slow infiltration rates. These soilsconsist primarily of clays with high swelling potential, soils with permanently high watertables, soils with a clay pan or clay layer at or near the surface, and shallow soils overnearly impervious parent material.Volume 2, Technical GuidancePage 4 - 5
Section 4.3 - Time of ConcentrationSite plans with subbasin boundaries can be overlain on the hydrologic soil group map to aid inthe determination of Curve Numbers. Please note that the map includes a soil groupidentified as “Urban.” These areas should be considered as HSG D soils unless the reviewingauthority approves an alternate group classification based on a review of field tests providedby the design engineer.4.3.3Required Curve NumbersThe CN values in Table 4-2 shall be used for all pre- and post-development hydrologiccalculations. The pre-developed CN values for pervious areas are an equal blending ofpasture in fair condition and cultivated small grain in good condition. The City of Wichita andSedgwick County elect to use this land use as a realistic basis for the condition of localwatersheds prior to development. The detrimental effect of grading on infiltration rates isacknowledged in the disturbed pervious land use values. This effect can be reduced throughpreferred site design practices that minimize the grading footprint. The impervious CN of 98accounts for the near total runoff of rainfall from impervious surfaces. A CN of 100 is to beused for permanent water surfaces such as lakes and ponds.Table 4-2 Pre- and Post-Development Curve NumbersHydrologic Soil GroupLand UsePre-Developed orUndisturbed PerviousDeveloped orDisturbed 8Composite Curve NumbersFor a subbasin containing subareas of varying CNs, a composite CN is computed. It shouldbe noted that when composite CNs are used, the analysis does not take into account thelocation of the specific land uses within the subbasin, but simplifies the drainage areaconceptually as a uniform land use represented by the composite CN. The composite CN fora subbasin shall be calculated by using an area-weighted averaging procedure as illustratedin the following example:Table 4-3 Calculation of a Composite Curve NumberLand UseFraction of TotalLand Area (1)CN (2)Weighted CN (1 x 2)Disturbed Pervious, “B” Soil0.3080Undisturbed Pervious, “B” Soil0.7071Total Weighted Composite Curve Number 24.0 49.7 73.7Page 4 - 624.049.7Volume 2, Technical Guidance
Section 4.4 - Time of ConcentrationExcept when the simplified method presented in Section 4.8.4 is used, runoff analysis usinghydrograph methods shall be performed on pervious and impervious areas separately, ratherthan using a composite CN that includes the impervious areas for a given subbasin. Theseparation of pervious and impervious areas ensures the most accurate estimate of runoffpeaks and volumes. Free modern software packages like HEC-1, HEC-HMS and EPASWMM allow the entry of a pervious CN and % imperviousness for a subbasin.For large developed watersheds Table 4-4, may be used to estimate the percentage ofsubbasin area that is impervious.Table 4-4 Average Imperviousness per Land Use (Source NRCS, TR-55)Average %ImperviousLand UseUrban Districts:Commercial and business85%Industrial72%Residential districts by average lot size:1/8 acre or less (town house)65%1/4 acre38%1/3 acre30%1/2 acre1 acre2 acres4.425%20%12%Time of ConcentrationThe “time of concentration” is equal to the cumulative time for runoff to travel from thehydraulically most remote point of the subbasin being analyzed, to the design point (outlet ofthe subbasin). In this Manual, the time of concentration is assumed to consist of three typesof flow: Sheet flow; Shallow concentrated flow; Channel flow.Time of concentration (Tc) is computed by summing the travel times for these consecutivecomponents of the drainage conveyance system from the hydraulically most distant point ofthe watershed to the outlet of the subbasin. The following is a discussion of the requiredprocedures and equations (USDA/SCS, 1986 and FHWA, 2001).Travel TimeTravel time is the ratio of flow length to flow velocity, as shown by Equation 4-4 (HEC-22,2001):Volume 2, Technical GuidancePage 4 - 7
Section 4.4 - USGS Regression MethodsEquation 4-4where:TTLV60 TT L60Vtravel time (min)flow length (ft)average velocity (ft/s)conversion factor from seconds to minutesSheet FlowSheet flow shall be calculated using the Equation 4-5, which uses sheet flow roughnesscoefficients found in Appendix A: (SCS, 1986)Equation 4-5where:TT (sheet)nLP2S TT ( sheet) 0.42 nL 0.8 P2 0.5 S 0.4travel time (min)Manning sheet flow roughness coefficient (see Appendix Table A-1)flow length (ft): 100’ maximum2-year, 24-hour rainfall (3.4 inches from Table 4-1)land slope (ft/ft)Shallow Concentrated FlowOverland sheetflow often becomes shallow concentrated flow as it progress down thedrainage area. By definition, sheet flow occurs only over plane surfaces at the head of thedrainage area. Due to surface irregularities, sheet flow will eventually transition to shallowconcentrated flow. The NRCS has determined that sheet flow will never occur for more than300 feet, regardless of the evenness of the surface. When sheet flow conditions exist (orwhen the design exceeds the permitted length) the designer will generally use additionalsegments of shallow flow or channel flow to handle the remainder of the flow path. Theaverage velocity for shallow concentrated flow shall be computed using Equation 4-6 (HEC22, 2001). The equation uses Table 4-5 to determine K-values.Equation 4-6where:VKS V KS 0.5average velocity (ft/s)shallow concentrated flow velocity coefficient (see Table 4-5).slope of hydraulic grade line approximated by the watercourse slope (ft/ft)Table 4-5 Velocity Coefficient (K value) for Shallow Concentrated FlowPage 4 - 8Shallow Concentrated Flow CoverK valueWooded w/heavy litterFallow or no-till cultivationWooded w/light litter2.54.75.0Volume 2, Technical Guidance
Section 4.4 - Time of ConcentrationShallow Concentrated Flow CoverK valueShort grass pastureCultivated straight row cropNearly bare and untilledGrassed waterwayUnpaved (as defined by TR-55)Paved79101516.320.3After determining the average velocity using, use Equation 4-4 to calculate travel time for theshallow concentrated flow segments (TT(shallow)).Open ChannelsVelocity in channels should be calculat
Section 4.1 - Introduction to Hydrologic Analysis Volume 2, Technical Guidance Page 4 - 1 4.1 Introduction to Hydrologic Analysis This Section addresses surface water hydrology and % TSS Removal Calculation only; it does not address groundwater hydrology. Therefore all references in
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BYLAWS OF THE CENTRAL CHRISTIAN CHURCH OF WICHITA, KANSAS ARTICLE I-NAME The name ofthis corporation is The Central Christian Church of Wichita, Kansas (CCC.) It is a nonprofit, religious corporation with its principal office in Wichita, Sedgwick County, Kansas. CCC is not affiliated with any denomination and is under the leadership of the Lord
MARSH MURDOCK AND THE "WILY WOMEN" OF WICHITA 5 5. See H. Craig Miner, Wichita: The Early Years, 1865-1880(Lincoln: University of Nebraska Press, 1982). On "respectability" in towns such as Wichita, see C. Robert Haywood, Victorian West: Class and Culture in Kansas Cattle Towns(Lawrence: University Press of Kansas, 1991). 6. Wichita Eagle, March 18, February 17, 1887.
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Don Strong Peter Stubbs Photos from Banquet BULLETIBULLETINN IN THIS ISSUE . Editor Dr. Sal Mazzullo at salvatore.mazzullo@wichita.edu , whose mailing address is Department of Geology, Wichita State University, Wichita, Kansas 67260. . KS Geological Survey (785) 864-3965 SOCIETY NEWS KGS Library (316) 265-8676 EDITOR EMERITUS .
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