The Rational Method for Calculation of Peak Storm Water Runoff Rate Article by Harlan Bengtson (9,227 pts ) Edited & published by Lamar tonecypher (2!,"#$ (2!,"#$ pts ) on ep 22, 2!%! Related Guides: 'A he ational *ethod is used to calculate the pea+ surace runo rate or design o storm -ater management structures, li+e storm drains, storm se-ers, and storm -ater detention acilities. /alues /alues or the runo coeicient, drainage area, time o concentration and design return period are needed.
Introduction he ational *ethod is -idely used to estimate the pea+ surace runo rate or design o a 0ariety o drainage structures, such as a length o storm se-er, a storm -ater inlet, or a storm -ater detention pond. he ational *ethod is most suitable or small urban -atersheds that don1t ha0e storage such as ponds or s-amps. 't is best or areas less than %!! acres, but is sometimes used or up to 2 mi 2 areas.
The Rational Method Equation he euation that is the centerpiece o the ational *ethod is3 4 iA, -here is the pea+ surace runo rate in cs, rom a -atershed o area, A acres, acres, and runo coeicient, , due to a storm o intensity, intensity, i in5hr. he units on pea+ runo rate, , are actually acre6 in5hr, but the con0ersion rom acre6in5hr to cs is 0ery nearly one, so the more common unit, cs, is tyically used or . 'n order to calculate a 0alue or pea+ p ea+ runo rate or a gi0en drainage area, 0alues are needed or the three parameters, A, A, , and i. Each o these parameters -ill be discussed separately in the net three sections.
The Drainage rea!
he drainage area, A, is oten determined rom a map -hich includes the drainage area o interest. 't may be necessary to irst determine the boundaries o the drainage area using a contour map. 8nce the boundaries are +no-n, the
area can be determined using the map scale. ince the area must be in acres or use in the ational *ethod euation, a useul con0ersion actor is ",#$! t25acre. A map -ith a drainage area outlined is sho-n at the let.
The Runoff Coefficient! C he runo coeicient is the raction o rainall stri+ing the drainage area that becomes
runo rom that drainage area. 't is an empirically determined constant, dependent on the nature o the drainage area surace. An imper0ious surace li+e a concrete par+ing lot -ill ha0e a runo coeicient o nearly one. A 0ery tight clay soil -ill also ha0e a relati0ely high runo coeicient, -hile a sandy soil -ould h a0e more iniltration and a lo-er runo coeicient. 'n addition to the nature o the surace and the soil, the slope o the drainage area has an eect on the runo coeicient. A greater slope leads to a higher runo coeicient. coeicient. here are many tables o 0alues or runo coeicient or a 0ariety o types o drainage areas in handb oo+s, tetboo+s and on the internet. he table at the right sho-s some typical ranges o 0alues or runo coeicient.
The Design Rainfall Intensit"! Intensit"! i he design rainall intensity is the intensity o a constant intensit" design storm -ith the speciied design return #eriod and duration eual to the time of concentration o the drainage area. 8nce the design return period and duration are determined, the design rainall intensity can be determined rom an ap propriate intensit"$duration$frequenc" graph or euation or the location o the drainage area. As you can see, determining a 0alue or i is the most complicated part o using the ational *ethod. he procedure or doing so is co0ered in a separate article, :alculating :alculating ;esign ainall 'ntensity or
E%am#le Calculation onsider a section o a do-nto-n do-n to-n business area o "#,!! suare eet that drains to a particular storm -ater inlet. he runo runo coeicient or this drainage drainage area has been estimated to be !.=#. Based on a speciied design return period and the time o concentration o the drainage area, the design storm intensity has been determined to be #.% in5hr. >hat is the pea+ runo rate rom this area to be used or design o the storm -ater inlet?
Solution: 4 iA 4 (!.=#)(#.%)("#,!!5",#$!) cs 4 &'() cfs'
he other article in this series gi0e inormation about other aspects o storm se-er design. @or an article -ith a do-nloadable Ecel spreadsheet template or ma+ing storm se-er design calculations, see :torm-ater :torm-ater e-er ;esign using Ecel @ormulas in a preadsheet emplate emplate..:
References and Image Credits eerences or urther inormation3 %. Bengtson, Harlan H., Hydraulic H., Hydraulic Design of Storm Sewers, Including the Use of Excel , an online, continuing education course or course or ;H credit. 2. *cuen, ichard H., Hydrologic H., Hydrologic Analysis Analysis and Design, 2nd Ed ,
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