Hydraulic Structures: Fourth Edition

Table 8.1 Types of weirType

Shape

K

n

Remarks

Sharp-crested weir

Rectangular (Fig. 8.6(a))

2/3

Cd

C

(2v

g)

1/2

b

3/2

b

effective width of notch; to measure moderate to

large discharges

C

d



(

h/

P), where



0.602 and





0.075 for

b/

B



1



0.592 and





0.011 for

b/

B



1/2

Triangular (Fig. 8.6(b))

8/15

Cd

C

(2v

g)

1/2

tan

/2

5/2



included angle; to measure small flows

C

d

f(h

/P

,P

/B

,

)

0.58

0.61 (see BSI, 1969a, b, 1986)

Compound weirs (Fig. 8.6(c))

To measure wide range of flows; sensitive toapproach conditions and submergence

Broad-crested weir

Rectangular

0.544

C

Cd

gv

1/2

b

3/2

To measure large flows; less sensitive to approachconditions and submergence

C

d

f(h

, crest length

L

,h

/b

, roughness of the

(Table 9.2 and Worked example 9.1)

crest)



0.85

0.99 (for recent studies, see Ranga Raju (1993))

Spillways:

K

and

n

values are the same as those of sharp-crested rectangular weirs but

C

may vary (Chapter 4)d

Crump weir

Sharp crested

C

Cd

gv

1/2

b

3/2

Fairly constant value of

C

; tod

(Water Resources

with 1:2 upstream

measure moderate flows; less

Board, 1970)

and 1:5 downstream

sensitive to approach

slopes

conditions; good prediction

(Fig. 8.21)

of submerged (non-modular)flows (Worked example 8.3)

Flumes

Venturi

0.544

C

Cd

gv

1/2

b

3/2

b

throat width; to measure wide

C

d

f(L

/b

,h

/L

)

0.95–0.99 (see BSI, 1969a, b, 1986)

range of flows; copes with sediment and

Parshall (Fig. 8.7)

K

and

n

vary with

debris-laden flows; increased non-modular

size of flume; design

flow range with reasonable estimates

tables available

(see Bos, 1976)

(Bos, 1976)

Throatless flume:

K

and

n

values are the

To measure moderate flows (see

raised bed (hump) in

same as those of broad-

Featherstone and Nalluri, 1995)

stream

crested weirs; cheap

Steep slope streamflume: supercriticalapproach flow: specialflume (Harrison andOwen, 1967)

Cv

nh



nH

;H



h

(^2) Va
/2g
;V
a
Q
/B
(h

P), where
H
is the energy head and
Va
is the approach velocity;
Cv
(
1.0) is a function of the discharge coefficient
Cd
,b
/B
and
h/(
h
P),
where
B
is the channel width and
P
is the height of the sill. Solutions for
Cv
(graphical or analytical) are available (e.g. see BSI, 1969a, b, 1986; Ackers
et al.
, 1978)