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Mannings n Calculation for Las Vegas Wash

Mannings n calculation for Las Vegas wash cross sections was carried out based on the

procedure outlined in Open Channel Hydraulics by V.T. Chow (1!". The #alue of n was

co$puted by analy%ing se#eral pri$ary factors affecting the roughness coefficient& the

for$ula as gi#en by&

n ' (n)n1)n*)n+)n," $!

where

n' representati#e Mannings n #alue.

n' basic n #alue for a straight& unifor$& s$ooth channel in the natural $aterials

in#ol#ed.

n1' #alue added to nto correct for the effect of surface irregularities.

n*' #alue added for #ariations in shape and si%e of the channel cross section.

n+' #alue added for obstructions.

n,' #alue added for #egetation and flow conditions.

$!' correction factor for $eandering of channel.

-or the calculation purpose& a basic /section was defined as shown in -igure 1. Then& a

table was prepared to calculate the e0ui#alent Mannings n for each of the Las Vegas

ash cross section as defined in 2O33 4i#erC56. 5 #alue table for all the factors was

obtained fro$ the abo#e boo7 and has been presented as reference. 5ppropriate #alue for

the factors was esti$ated based on the field trip& aerial photography and engineering

8udg$ent. 5 brief description of the criteria to choose appropriate factor #alues is

presented here.

n#alue was chosen based on channel with natural $aterials and for $ost cases a

#alue of .* (earth" was used.

n1#alue was based on surface irregularities and for $ost cases a #alue of .!

($inor irregularity" was used.

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n*#alue was based on #ariation of channel cross sections and for $ost cases a

#alue of .! (alternating occasionally" was used.

n+#alue was based on the relati#e effect of obstruction and this #alue #aried

depending upon the location of the particular cross section. -or o#er ban7 sections

where there were significant a$ount structures to obstruct the flow& a #alue of

.9 (se#ere" was used: while for other cases& $inor to appreciable obstruction

#alues were used. -or $ost of the thalweg sections& a #alue of .1! ($inor" was

used.

n,#alue was based on the effect of #egetation. -or $ost of the thalweg sections& a

#alue of .1! (for $ediu$" was used& while for o#er ban7 sections a #alue of

.! (low" was used.

-or $!& a #alue of 1 ($inor" was used as the distance between successi#e cross

section was * ft and there was not any significant effect of $eandering for this

short reach length.

The roughness calculation for Left O#er 2an7 (LO2"& 4ight O#er 2an7 (4O2" and

Thalweg is presented in Table 1.

The co$posite Manningsn for channel was calculated based on the for$ula gi#en in V.T.

Chows Open Channel Hydraulics. The i$pro#ed Channel cross section was di#ided into

fi#e sections; Thalweg& Left side (L3"& Left 2ase (L2"& 4ight side (43" and 4ight 2ase

(42". The roughness factor for Thalweg was calculated considering different roughness

factors as gi#en in Table 1. The co$posite Mannings n for channel calculation is shown

in Table *.

-or the side slopes and side bases (L3& 43& L2 and 42" following #alues were used.

4iprap ' .+! (9/inch riprap"& ., (1*/inch riprap"& .,+ (1

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Concrete ' .1!

=rass Lined ' .+

Then an e0ui#alent Mannings roughness was calculated based on the gi#en for$ula as

gi#en in the pre#iously $entioned boo7 by V. T. Chow.

The for$ula is based on calculating the $ean of the wetted peri$eters. The for$ula is

gi#en below;

where&

> ' etted >eri$eter for a particular section

n ' Mannings n for the sa$e section

+

+*

+*!..1!..1

++

!..1

**

!..1

11 ".......(

P

nPnPnPnPn

NN++++

=