FLOOD09884

• County: Crowley
• Community: Ordway
• Stream Name: Arkansas River
• Basin: Arkansas
• Title: Drainage and Street Design Manual for the Town of Ordway
• Date: 1/1/1987
• Prepared For: Ordway
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Project

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Time of Cmcentration

I.ren using the Rati.a1al Method FOl1IIiIa an assunptial is lIIIde t1Bt the DBJrlnun rate of flow
is produced by a coostant rainfall which is oaintained for a tinE equal to the period of
roncentration of no... at the point uroer considerati.cr1. 'Ireoretical1 y, this is the tinE of
car:entratim, which is the tinE required for the surface runoff fran the roost I'elIDte Jllrt of
the drainage basin to reach the point being considered. Ib.ever, in practice the cm:entration
tinE is an empirical value that results in acceptable pa3k no... estiJmtes.

In the app1i.cation of the Rati.a1al Method, the tinE of concentration IlIJSt be estiJmted 00
the average rainfall rate of a CvLtespCllmng duration can be detenn:ired fran the tilIe-intensi.ty-
frequeocy OlIVeS. Far UI'iBn areas, the tinE of c.oncentration consists of an inlet time or
aver land no... tinE (s) plus the travel tinE (~) in the storm sewer, piVed gutter, roodside
draiIlage ditch, or drainage channel. for llCIDJI"iBn areas, the time of concentratim consists of
an overland no... tiJIE (ti) plus the tiJIE of travel in a canbiiled form, su::h as a SJBli swale,
channel, or draiIlage.ay. The latter portiCII (~) of the time of concentration can be est:inated
fran the hydraulic pI VI"'" Lies of the storm sewer, gutter, swa1e, ditch or drainageway. Inlet
tinE, on the other hand, will vary with surface slope, depression storage, surface cover,
antecedent rainfall, and infiltration capacity of the soil, as well as distaoce of surface fID;-.

The time of concentration sba1l be calculated using the following equation for both UI'iBn
and nonuriBn areas:

1:c = lj+ ~

(1.2)

10fere 1:c = tinE of coocentratiCII (minutes)
S = initial, inlet, or overland flow tiJIE (minutes)
~ = travel tinE in the ditch, channel, gutter, storm,
etc. (minutes)

~rrhmli7Prl ~in~

The initial or overland flow tinE (ti) will be calculated using the following equatlCII:
L2

s = 1.8 (1.1 - Cs)

3-r;-

10fere ti = initial or overland flow tiJIE (minutes)
C5 = runoff coefficient for 5-year frequeocy (fran
table 1.2)
L = length of overland flow, (ft., 3Xl-foot nmc:inllm)
S = average 00s:in slope (percent)

(1.3)

The equation shall be used for distances up to JJ) feet. For longer basin lengths, the
runoff will be cmsidered in a canbired fom and the travel tinE (~) shall be cak111ated using
the hydraulic properties of the swale, ditch, or channel, or using Cllart. 1.9. The time of
concentration is then the sum of the initial flow tiJIE (l::i) and the travel tinE (tt) in
occordance with Equation 1.2. The mininun tc sba1l be tal minutes.

lk'00nized &sins

Over land flow in UI'00nized basins can occur fran the back of the lot to the street, in
greenbelt area, or within park areas. The initial or overland flow time (ti) shall be lesser
value calculated using Equations 1.3 and 1.4.

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