Design of Culverts - Copy

7/26/2019 Design of Culverts - Copy

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Design of Culverts

Typical cross-sections: Circular, arched, rectangular and oval

Flow Regimes:

1. Submerged-entrance condition

1

h


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Free-entrance conditions

Submerged entrance : DDH 51!1 >

For Type 1 "hhhh fi ++=

Total head loss # $ead loss at entrance% Friction loss % $ead loss at e&it

'eglecting the velocity head at 1 and (

gVh

gVkh

RLVnh eif

!!

!

"

!

()*

!!

===

$ence the velocity can be computed as

1)!

!

()*!++

=

ekRLgn

hgV

andAVQ =

!


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This e+uation or velocity given above is valid or both Type 1 and Type !

lows, which are sometimes called as the lows under outlet control

n Type ( the low is similar to an oriice low and henceghACQ d !=

.!"=dC

or s+uare-edged entrance1dC or well-rounded entrance

n this situation /Type (0 the low is under inlet control

n submerged entrance culverts the low is more as compared to the

unsubmerged entrance ut when it is not advisable to have submergence,

we need to design unsubmerged entrance culvert

For Type * culvert

fi

hhg

V

g

Vh +=+

!!

!!

1

2s may be seen the velocity o headwater is not neglected, becauseDH !1 , Type 1 low, ie eyHh =

! Dye < , assume Type ! low and calculate by the respective

e+uation and DHh =

( Calculate ny in the culvert Dyn > , Type ! low is veriied

* Dyn < , calculate rom the e+uation or Type ( and calculate ny

rom it

5 Dyn < again, Type ( is conirmed

. neither Type ! nor Type ( is veriied, use the minimum o the two

discharges as the capacity o the culvert

Unsubmerged entrance:

1 2ssume Type * and Calculate cn yyQ ,, ecn yyy >> , Type *veriied

! 2ssume Type 5 and Calculate cn yyQ ,, cne yyy > , Type .

veriied

Design Considerations

6eometry o the culvert entrance is important Four standard inlet types are:

1 Flush setting in a vertical wall

! 7ingwall entrance

( 8ro9ecting entrance

* itered entrance set lush with a sloping emban;ment

Selection o the inlet coniguration is based on:

/a0 Structural stability

/b0 2esthetics

/c0


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=SF$72: =S Federal $igh7ay 2dministration

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inimum Culvert si>e is speciied by the local authorities /usually in the

range o (" to ." cm0

inimum 2llowable ?elocity: ". to "@ m)s

a&imum 2llowable ?elocity: ( to * m)s

Autlet protection should be provided or higher e&it velocities

ost common material used is concrete

Example: 7hat is the capacity o a 1!!m by 1!!m concrete bo& culvert

/n#""1(0 with a rounded entrance /;e#""5, Cd#"@50 i the culvert slope is

"5B, the length is (..m, and the headwater level is 1(m above the

culvert invertD Consider the ollowing cases:

/a0 Free-outlet conditions, and

/b0 Tailwater elevation "("*m above the top o the bo& at the

outlet 7hat must the headwater elevation be in case /b0 or the

culvert to pass the low that e&ists in case /a0D

.

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Design of Culverts - Copy