Synthetic Unit Hydrograph

7/24/2019 Synthetic Unit Hydrograph

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UH

Synthetic Unit Hydrographs

All the methods of deriving a UH discussed previously requireobservations of rainfall and runoff. However, for some drainage basins

rain gages and/ or streams flow gages are not, therefore, rainfall

runoff data are not available. or those basins, some !ind of

techniques to generate UH "without using rainfall runoff records# is

needed. $he UH so derived is called synthetic UH.

$hree types of synthetic UHs%

" those relate hydrograph characteristics "pea! discharge, time base,

time'to'pea!, etc.# to basin characteristics(

")# those based on dimensionless UH( and those based on models of

watershed storage.


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UH

Snyders Method - 1

*tudy area% U* Appalachian highlands. &+ &+,+++ mi)"-+ -+,+++

!m)#

roperties characteries the response of watershed under various

rainfall inputs%

"a# 0uration of rainfall e1cess("b# 2ag time(

"c# $ime base of UH(

"d# $ime to pea!(

"e# ea! discharge of UH(

"f# *hape of UH.


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UH

Snyders Method - 2

*nyder3s method allows the computations of"a# lag time "t2#(

"b# UH duration "tr#(

"c# UH pea! discharge "qp#(

"d# Hydrograph time width at 4+5 and 645 "74+, 764# of pea! flow


4/12

UH

Snyders Method - 3

&. 2ag time "t2#% time from the center of rainfall e1cess to the UH pea!t28 9&9t"22c#+.-

where t28 $ime :hrs;( 9&8 +.64 for *< unit( &.+ for =nglish unit( 9 t8

9oefficient which is a function of watershed slope and shape, &.>).)

"for steeper slope, 9t is smaller#( 2 8 length of the main channel :mi,

!m;( 2c 8 length along the main channel to the point nearest to the

watershed centroid


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UH

Snyders Method - 4 ). UH 0uration "tr#%

tr8 t2/ 4.4where trand t2are in :hrs;. ( qp8 specific discharge,

:m-/s/!m); or :ft-/s/mi);

$o compute actual discharge, Dp8 Aqp

p

t

p9

)9

pq =

pA

t

p9

)9

pq =


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UH

Snyders Method - 5C. $ime Ease "tb#%

Assuming triangular UH,tb8 9-/ qp

where tb :hrs;( 9-8 4.4B for *< unit, &)F+ for =nglish unit.

4. UH 7idths% or

where

97, 64 8 &.)) for *< unit( CC+ for =nglish unit(

97, 4+ 8 ).&C for *< unit( 66+ for =nglish unit(.

74+, 764 are in hours( Usually, &/- of the width is distributed before UH

pea! and )/- after the pea!

emember to chec! that the volume of UH is close to & cm or & inch

&.+>pq

w,649

64

7 =

&.+>p

q

w,4+9

4+7 =


7/12

UH

Snyders

Method -Example


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UH

SCS

imensionlessUH


9/12

UH

SCS imensionless UH


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UH

!nstantaneo"s Unit Hydrograph #!UH$' 2et the duration of a t'hr UH go to infinitesimally small, that is, t +, the

resulting UH is called the


11/12

UH

H% &S 15-min imensionless UH 0imensionless discharge% D3 8 D"2ag@tr/)#/

0imensionless time% t38 &++ t / "2ag@tr/)#

where 8 surface runoff from &I of rainfall e1cess "in ft-#(

2ag 8 basin lag time "min#

8 time between the centroids of rainfall e1cess

and runoff hydrograph

8&.C6 A+.4C with A 8 basin area "acres#(

tr8 &4min

&4 min

&I

2ag

t

u


12/12

UH

H% &S 15-min imensionless UH

-.4&-J&++;,JC&e1p:B4&.>

J

)+6.C>+F#Jln"J+++C-)B.+J++-).+)C+>.+

&++J6.>&,J+).+B->4.)

6.>&JBC,J

#Jln">C.CC&--6

#Jln"

-).C-6BC>J++B4C6.+J->.B-6B.>+-F-

BCJB.CC,J+-B+>.+-+F-.&

B.CCJ+,J+++&+6>.+J++64>C+F.Ce1p:J

)

4.)

)

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Synthetic Unit Hydrograph