FLOOD STUDIES UPDATE (FSU) PROGRAMME - Hydronet

increases the uncertainty of the peak flow estimated. Hence, the method should be usedas a tool to compare outcomes from other empirical methods.The main advantage of the Rational Method is that there is ample experience in itsapplication over many years of its use. It is also a simple concept and computed without use of computers.2.2Modified Rational MethodThe traditional rational method is limited to considering storms with duration equal tothe time of concentration and provides only a peak flow. It allows calculating peak flowunder the assumption that rainfall intensity is uniformly distributed over the wholestorm event (Hua, Liang & Zhongbo, 2003). The modified rational method can considersingle event storms with changing intensities and longer durations. The modifiedrational method is being developed at different practicing agencies to account for thevariation of rainfall intensity within same storm duration. In some instances runoffcoefficient is modified to account for decrease in soil permeability as rainfall intensityincreases and to adjust for increase in runoff as average slope increases. Three examplesare demonstrated below.Example 1The one developed in California (Co. Alameda, Hydrology Manual, 2003) is of thefollowing form:Q C’IAWhere C’ is runoff coefficient modified by slope and rainfall intensity, and A iscatchment area. Rainfall intensity I is modified as:Ij (0.33 0.091144MAP)(0.249 0.1006Kj)Ti-0.56253Where Ij is rainfall intensity (mm/hr) for return period j, and stormduration I, MAP is mean annual precipitation (mm), Ti is storm duration(hr) (or Tc/60), and Kj is frequency factor to be determined per returnperiod as shown in Table 1.1.Table 2.3: Values of Kj, frequency factor (Source: Alameda Hydrology & Hydraulics Manual)Return period (yrs)Frequency Factor, Kj50.719101.339151.684252.1081003.211The modified runoff coefficient is determined as follows:C’ C Cs CiWhere C is runoff coefficient (as in Rational Method), Cs is slope adjustmentrunoff coefficient (to adjust for increases in runoff as average drainage areaslope increases), and Ci is rainfall intensity adjustment factor (to account fordecrease in soil permeability with an increase in rainfall intensity). Cs and Ci aredetermined by the following equations.Cs [(0.8 – C) (ln(S-1)S0.5)]/56, for C 0.8, Cs 0where S is average (weighted) slope in percent.-5-

1Ci [0.8 (C C s )] 1 , for C Cs 0.8, Ci 01 ln( I 1) e IWhere I is rainfall intensity (mm/hr) equal to Ij above.Example 2The Rational Method generates the peak discharge that occurs when the entirecatchment is contributing to the peak (at a time t tc) and ignores the effects of a stormwhich lasts longer than time t. Another Modified Rational Method developed inVirginia, however, considers storms with a longer duration than the catchment tc , whichmay have a smaller or larger peak rate of discharge, but will produce a greater volumeof runoff (area under the hydrograph) associated with the longer duration of rainfall(Virginia Dept of CR, 1999). Fig.: 2.1 shows a family of hydrographs representingstorms of different durations. The storm duration which generates the greatest volumeof runoff may not necessarily produce the greatest peak rate of discharge.Note that the duration of the receding limb of the hydrograph is set to equal the time ofconcentration, tc, or 1.5 times tc. Using 1.5tc in the direct solution methodology providesfor a more conservative design according to the handbook sited. This is justified since itis more representative of actual storm and runoff dynamics. It is also more similar to theNRCS unit hydrograph where the receding limb extends longer than the rising limb,which will be shown later.The modified rational method allows the designer to analyze several different stormdurations to determine the one that requires the greatest storage volume with respect tothe allowable release rate (which is limited to pre-development peak flow rate). Thisstorm duration is referred to as the critical storm duration and is used as a storage basinsizing tool.Fig. 2.1: Modified Rational Method procedures: Type 1 - Storm duration, d, is equal to the time ofconcentration, tc. Type 2 - Storm duration, d, is greater than the time of concentration, tc. Type 3 - Stormduration, d, is less than the time of concentration, tc. (Source: Virginia Stormwater ManagementHandbook, Vol. 2, First Ed., 1999)-6-

Fig. 2.2: Modified Rational Method procedures continued for several return periods. (Source: VirginiaStormwater Management Handbook, Vol. 2, First Ed., 1999).Fig. 2.3: Modified Rational Method, Trapezoidal Hydrograph Storage Volume Estimate (Source: VirginiaStormwater Management Handbook, Volumes 2, First Edition, 1999).Example 3In the 1980s, the Institute of Hydrology, Meteorological Office and HR-Wallingfordrefined the Rational Method and developed Modified Rational Method which is part ofthe “Wallingford Procedure” to be used in homogenous catchments up to 1.50km2(Shaw, 2004). It has the following form (Chadwick, Morfett & Borthwick, 2009):Qp 2.78 (CvCRiA)Where Cv is the volumetric runoff coefficient, CR is the routing coefficient andthe remaining are same as in the Rational Method.The recommended equation for Cv is:Cv PR/100Where PR is the (urban) percentage runoff which is found from:PR 0.829PIMP 25.0SOIL 0.078UCWI – 20.7Where PIMP is percentage impermeable area to total catchment area,SOIL is a number depending on soil type, and UCWI is the urbancatchment wetness index (mm) related to SAAR.-7-

The recommended value for CR is a fixed value (CR 1.3) for all systems (Chadwick et.al. 2009). The estimation of i requires the knowledge of critical storm duration tc. Theassumption made is that this storm duration is equal to the time of concentration for thecatchment, tc, given by:tc te tfWhere te is the time of entry into the drainage system (between 3 and 8min) andtf is the time of flow through the drainage system.The value of i for given return period and duration may be estimated according to theflowing procedure (Chadwick et. al., 2009). First, values of Jenkinson’s r (M5 60min/M5 - 2day rainfall) and M5 - 60min (rainfall of 5year return period and 60minduration) are read from map. Next, the value of M5 - D/M5 - 60min (where D is therequired duration) is read from plotted data using the value of r to obtain the requiredvalue of M5 - D. The value of MT – D (where T is the required return period) is foundfrom tabulated data relating M5 – D to return period T. this value of MT – D (the pointrainfall of required return period and duration) is next reduced by multiplying it with anareal reduction factor (ARF), which is plotted as a function of duration and area, toobtain the design catchment rainfall depth. Finally the design rainfall intensity (i) isfound from:i (MT – D)/DThe value of tf is computed from L/V (where L is channel length and V is channelvelocity) and the peak flow Qp is calculated from the equation shown above (Chadwicket. al. 2009).The Rational Method is ‘modified’ as it has been shown in the above three practices.The runoff coefficient and rainfall intensity has been modified to account for theirtemporal and spatial variability during a storm event. There is no wide range ofexperience with these methods or the methods are limited to certain regions. It is alsonot clear to what size of catchment area they are applied. However, the WallingfordProcedure was found to be more accurate than the Rational Method when applied in theUK up to a catchment area of 1.50km2 (Mitchell, et. al. n.d.).2.3USGS Regression EquationsRegional regression equations are the most commonly accepted method in the US forestablishing peak flows not only at gauged sites but also at ungauged sites or sites withinsufficient data. Regression equations have been developed to relate peak flow at aspecified return period to the hydrology of a catchment. In the US each state is dividedinto regions of similar hydrologic, meteorologic, and physiographic characteristics asdetermined by various hydrological and statistical measures (McCuen, Johnson &Ragan, 2002).Regional regression equations were developed by USGS as a two-step processinvolving ordinary and generalized least-squares regression techniques (Dillow, 1996).Ordinary least-squares (OLS) regression techniques are used in the first step todetermine the best models relating catchment characteristics listed below to any T-yearreturn period peak discharge estimate. In the second step, the final model identified bymeans of ordinary least-squares regression techniques was used in generalized least--8-

squares regression analyses to develop equations that can be used for predictivepurposes.The catchment characteristics taken into account in the regression process are (Koltun,2003): A, drainage area (km2), S, main channel slope (m per km), AOS, average maximum overland slope of the land surface (percent), STRMFRQ, stream frequency or drainage density (1/km), MCE, mean catchment elevation (m), MAPc, mean annual precipitation at the catchment centroid (mm), MAPm, mean annual precipitation averaged over the catchment area (mm), CF2, CF25, and CF100, Climate factors with recurrence intervals of 2, 25, and100 years, respectively (dimensionless), Water, percentage of the catchment classified as water, Wetland, percentage of the catchment classified as wetland (%), Urbanised, percentage of the catchment classified as developed/urbanised (%), Undeveloped (grey area), percentage of the catchment classified as barren (%), Forest, percentage of the catchment classified as forested upland (%), CR, circularity ratio (dimensionless) - a measure of catchment shape (circularversus elongated); determined asCR P/ (4πA)0.5where P is the perimeter of the catchment, in km, and A is the drainagearea in km2.The typical regression models utilized in regional flood studies are of the form:bYT aX 1b1 X 2b2 . X p pWhere: YT is the dependent variable (which is the peak flow for a given return periodT),X1, X2, , Xp are independent variables (which are the catchment characteristics,a is the intercept coefficient (or regression coefficient), and,b1, b2, , bp are regression exponents (determined using a regression analysis).The states in US are divided into regions of similar hydrologic, meteorologic, andphysiographic characteristics as determined by various hydrological and statisticalmeasures (Koltun & Roberts, 1990) which is equivalent to the way Ireland is divided into several hydrometric areas. When the regression analysis is complete, not allcatchment characteristics would be included in the final regression equation. Thevariables are selected based on the influence they incur unto the dependent variable(peak flow). Then each hydrological region would have its own regression equation fora given return period. The peak flow for a 2-year return period for a certainhydrometric area could be, say:Q2 2.52A0.775 (E/1000)3.32 (F 1)-0.504And 5-year return period for the same hydrometric area could be say:Q5 23.00A0.720(E/1000)3.36(F 1)-0.885, and so on.-9-

Where A is catchment area, E is mean catchment elevation, and F is forestedarea.USGS Regression offers several advantages over other methods according to State ofMaine urban and arterial highway design guide (Maine DoT, 2008). It is more accuratethan rainfall-runoff modelling in comparable situations. It is based directly on annualmaximum data, when gauged station is used, and thus does not depend on thequestionable assumption (inherent in rainfall-runoff modelling) that the T-year stormproduces the T-year flood event. However, the regression equations are subject toseveral limitations that it works better at catchment sizes greater than 2.5 km2, and notsteeper than 50m/km slope. It also works well at rural, undeveloped, and unregulated(natural) catchments.2.4The National Resources Conservation Service (NRCS) / TR-55 MethodThe Technical Release 55 (TR-55) or NRCS method formerly known as SCS methodrelates rainfall, retention and effective rainfall or runoff (USDA, NRCS, 1986). Massrainfall is converted to mass runoff by using a runoff curve number (CN). The methodfollows two procedures: graphical discharge method or tabular hydrograph method.When the catchment needs to be divided into sub-catchments because of widelydiffering curve numbers or non homogeneous slope patterns, then the tabularhydrograph approach is used, otherwise the graphical method is used. The graphicalmethod is examined below.The rainfall-runoff relationship in the model separates total rainfall into direct runoff,retention, and initial abstraction to yield the following equation for rainfall runoff:QD (P I a ) 2(P I a ) SWhere QD is depth of direct runoff (mm), P is accumulated rainfall/potentialmaximum runoff (mm), Ia is initial abstraction, and S is retention of rainfall onthe Catchment (mm).Through researches, Ia was found to be approximated by the flowing equation:Ia 0.2SThe value of S is related to soil type and land cover of the catchment through the curvenumber, CN. CN is a function of soils type, vegetation cover, magnitude of imperviousareas, interception, and surface storage. CN has a range of 0 to 100 (USDA-NRCS,1986).1000S α 10CNWhere α is unit conversion constant 25.4 (or 1 for British units)The retention, or potential storage in the soil, is established by selecting a curve number(CN). The curve number is read from tables found in most US hydrologic books, or canbe estimated if rainfall and runoff volume are known.CN 1000/[10 5P 10Qa – 10(Qa2 1.25QaP)0.5]Where P is rainfall (mm) and Qa is rainfall volume (mm).- 10 -

Table 2.4: Sample Curve Numbers (CN) according to land cover used in the US (Source: USDA,1986, Urban Hydrology for Small Watersheds, TR-55)CN for hydrologic soilgroupLand cover descriptionCover type and hydrologic conditionAverage %impervious areaFully developed urban areas (vegetation established)Open space (lawns, parks, golf courses, cemeteries, etc):Poor condition (grass cover 50%)Fair condition (grass cover 50% to 75%)Good condition (grass cover 75%)Urban districts:Commercial and businessIndustrialResidential: average lot sizeRow houses, town houses, and residentialwith lot sizes 0.05 ha (1/8 ac) or less0.10 ha0.14 ha0.20 ha0.40 ha0.81 ha and so on Where A, B, C and D respectively are: 7785909238302520126157545146 7572706865 8381807977 8786858482 Group A soils, which have a low runoff potential due to high infiltration rates.Group B soils, which have a moderately low runoff potential due to moderateinfiltration rates.Group C soils, which have a moderately high runoff potential due to slow infiltrationrates.Group D soils, which have a high runoff potential due to very slow infiltration rates.For multiple land use/soil type combinations within a catchment, aerial weighing is usedto compute composite CN.The peak flow estimation equation is:Qp QuAQDFpWhere Qp is peak flow (m3/s), Qu is unit peak flow (m3/s), A is catchment area, QD isrunoff depth (mm) and Fp is adjustment factor given in table to reflect the storage inlakes or swamps that are not along the tc flow path.The unit peak flow is computed from:Qu α10 C0 C1 log( tc ) C2 [log( tc )]2Where C0, C1 and C2 are regression coefficients which are a function of the 24 hourrainfall distribution type and various Ia/P ratios given in table, tc is time ofconcentration, and α is conversion constant 0.000431 (or 1 in British units).- 11 -

Table 2.5: Storage adjustment factor used in the Peak flow estimation formula (Source: USDA,1986, Urban Hydrology for Small Watersheds, TR-55)Area of lake or swamp (%)0.00.21.03.05.0Fp1.000.970.870.750.72The TR-55 method has tabulations only for the US rainfall distribution maps. Therefore,non-US users need to determine whether a typical 24-hr rainfall that resembles a Type I,IA, II, III or which rainfall distribution type best matches the user’s region.While TR-55 gives special emphasis to urban and urbanizing catchments, theprocedures apply to any small catchments in which certain limitations are met (USDA,1986). The TR-55 method has a number of limitations where these conditions are notmet, the accuracy of estimated peak discharges decreases. The method should be usedon catchments that are homogeneous in CN; where parts of the watershed have CNs thatdiffer by 5, the watershed should be subdivided and analyzed using a hydrographmethod, such as TR-20. The TR-55 method should only be used when the CN is 50, orgreater and the tc, or is greater than 0.1 hour and less than 10 hours. Also, the computedvalue of Ia/P should be between 0.1 and 0.5. The method should be used only when thecatchment has one main channel or when there are two main channels that have nearlyequal times of concentration; otherwise, a hydrograph method should be used. Neitherchannel nor reservoir routing can be incorporated.The NRCS has also released the WinTR-55 computer software package, whichcalculates peak flows for catchments with areas smaller than 65km2 (US DoT, 2008).2.5The NRCS Dimensionless Unit Hydrograph MethodUnit Hydrograph Methods may be used to compute storm water discharges for all sizesof catchments, where storm water discharge is produced by catchments, where storm

- 3 -(Watts & Hawke, 2003). It relates peak flow (m 3/s) to catchment area (km2), rainfall intensity (mm/hr) and runoff coefficient. It has the form of: Q CiA Where Q is the peak flow rate, i is the rainfall intensity, A is catchment, area and C is the runoff coefficient. The method is based on the assumptions that rainfall intensity and storm duration is