Canal fall Notch Type design steps

1. DESIGN OF HYDRAULIC STRUCTURES
CANAL DROP-NOTCH TYPE
DEPT. of CE,VAST
Nirmala Krishnan, AP CE VAST Page 1
CANAL DROP-NOTCH TYPE
Design Steps
I) DESIGN OF TRAPEZOIDAL NOTCH
a. No. Of notches
The top width of the notch should lay between 0.75 H to H-As per this
condition fix the number of notches-
H-Depth of water above the sill of the notch-u/s FSD
As a thumb rule -One notch is provided for each 2m3/s
b. Bottom width and side slope
Depends on the FSL upstream and downstream . .
Find
the type of fall, free or submerged.

2. DESIGN OF HYDRAULIC STRUCTURES
CANAL DROP-NOTCH TYPE
DEPT. of CE,VAST
Nirmala Krishnan, AP CE VAST Page 2
a) Free fall
Take C d=0.962 or 0.7
L-base width of the notch-length of the sill
d- depth of water in meters.
For full supply condition take Q=Q, d = d(FSD)
For half supply condition Q=0.5Q, d=0.75 d (HSD)
If not given, assume a minimum of 1m for calculation
Form two equations and solve for L and n
b) For submerged fall

3. DESIGN OF HYDRAULIC STRUCTURES
CANAL DROP-NOTCH TYPE
DEPT. of CE,VAST
Nirmala Krishnan, AP CE VAST Page 3
fix the dimensions of the notch
2) Design of drop wall
Length of drop wall
i) >7/8 times the u/s canal width
ii) <bed width of the downstream canalTop
width of the notch L’=L + n d
Find the total top width =L’ X no. of notches
Find the remaining length. Distribute the length between the notches
1) Thickness at the top of the drop wall
. b = ((d/2) +0.15) to ((d/2)+0.3 ) meters
.d= depth of flow over notch ie FSD of the U/S canal
2) Height of the drop wall
Depends on the depth of water cushion that is to be provided atthe d/s of
the canal, it can be calculated from

4. DESIGN OF HYDRAULIC STRUCTURES
CANAL DROP-NOTCH TYPE
DEPT. of CE,VAST
Nirmala Krishnan, AP CE VAST Page 4
(d1+x)-effective depth of water cushion
X-depth of water cushion below the bed of channel downstream
. d1 -FSD of the channel D/S
. dc -FSD of the U/S canal
. h- Depth drop-difference in FSD of the U/S and D/S canalFind x
Find (d1+x)-fix the bottom level wrt D/S side-ie T/L of solid apron
Find the height of the drop wall wall=u/s canal B/L-T/L of solid apron
3) Bottom width of drop wall
H-height of drop wall
h- Height of water falling over the notch=U/S FSD
G-sp.gr of masonry. =2.25

5. DESIGN OF HYDRAULIC STRUCTURES
CANAL DROP-NOTCH TYPE
DEPT. of CE,VAST
Nirmala Krishnan, AP CE VAST Page 5
3). Design of water cushion cistern
i) Length of floor from the toe of the drop wall
Subject to a minimum of
ii) Thickness of cushion ie solid apron,
Depends upon the hard soil strata
Assume a thickness; check it with Bligh’s creep theory and t min
Based on Bligh’s creep theory.
 Maximum uplift head =diff. in W/L of U/S and D/S canal
 Assume a thickness say 1m
 Assume a hydraulic gradient of 1 in 4
 Bligh’s creep coeff, C=1/HG
 Total creep length =AB+BC+CD (ref fg)
 Head lost =creep length/C
 Residual head, h =max. uplift head-head lost
Thickness required = h / (ƿ-1), ƿ=2.24

6. DESIGN OF HYDRAULIC STRUCTURES
CANAL DROP-NOTCH TYPE
DEPT. of CE,VAST
Nirmala Krishnan, AP CE VAST Page 6
Minimum thickness required t min =
Whichever is higher provide that thickness, check with site conditioncheck for
residual uplift.
Check for residual head at the end of the solid Apron
The downstream end of the water cushion must be built with
A slope of 1 in 5, so that stone and other debris carried to thecistern must be
washed out
4) Design of notches and notch pier
1. Sill of notches-at B/L of channel U/S
2. Top level of notches-FSL on U/S canal3
.Width of notch pier -FSD/2
3. Top width >FSD/2 of U/S channel
Sketch include
5. Protective Works

7. DESIGN OF HYDRAULIC STRUCTURES
CANAL DROP-NOTCH TYPE
DEPT. of CE,VAST
Nirmala Krishnan, AP CE VAST Page 7
1. Abutment BC
Length=width of drop wall at foundation
Top width of abutment=0.5m
T/L of abutment =TBL of channel D/S.
Provide 1 in 8 slope
Foundation is a continuous structure of drop wall with thickness 1m.
Find the height H
Base width =0.4 H
2 .Upstream wing wall
Top level =TBL of channel U/STop
width =0.5m

8. DESIGN OF HYDRAULIC STRUCTURES
CANAL DROP-NOTCH TYPE
DEPT. of CE,VAST
Nirmala Krishnan, AP CE VAST Page 8
Splay 1 in 1
Foundation not as deep as abutment
Check for hard soil, fix the B/L of foundation slab
thickness=0.5m
4.D/S of wing wall
Top Level-TBL of D/S channel.
Top width =0.5m
Front batter =1 in 8
Foundation depends on channel D/S
Foundation slab provide 60cm thickness
Splay 1 in 1.
5) Pitching
1) Length of revetment and bed pitching
a) U/S-measured from the face of the drop walllength
=3FSD or minimum 3m
c) D/S-measured from D/S edge of solid apron Length
=4(FSD +height of drop) or min 6m
2) Bed pitching
Provide half the length of the corresponding revetments.

9. DESIGN OF HYDRAULIC STRUCTURES
CANAL DROP-NOTCH TYPE
DEPT. of CE,VAST
Nirmala Krishnan, AP CE VAST Page 9