Hydraulic Structures – Baffled Apron Drop
7-Mar-21
Baffled Apron Drops
Reference: United States Dept. of the Interior Bureau of Reclamation, “Design of Small Canal
Structures”, United States Government Printing Office, Denver, 1974. Pages 299-308. Available in
the college library and the course website.
Design Considerations
Capacity
The capacity of the baffled apron drop is a function of the allowable flow rate, q, per foot of width.
See Table 6-1.
Inlet
The inlet should be the same width as baffled apron and should provide a velocity of approach slower
than the critical velocity Vc where splashing must be minimized, the entrance velocity should not
exceed about V c 2 where
V c  3 gq
1. Sill control:
The inlet length should be at least 2d 1 , as shown in Figure 6-2. The required height of the sill
above the inlet floor may be determined from the energy balance between the inlet and
upstream channel. Thus
E s1  E s c  h i  h s
hs  E s 1  E s c  h i
where
hs is the sill height,
E s1  d 1  hv 1 in the upstream channel,
E sc  d c  hv c in the control section at the sill.
hi  0.5hv i

 0.5 hv c  hv 1

V c 2 V 12 
 0.5 


 2g 2g 
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Hydraulic Structures – Baffled Apron Drop
7-Mar-21
Max R=9 ft. A R=6 ft is frequently used.
The sill has a 6” wide slot to provide drainage of the upstream pool.
Baffled Apron Dimensions
1. Set the longitudinal slope of the chute floor and side-walls at 2 to 1 ( tan   0.5 ).
2. Approximate width of structure (B) should be
Q
B 
q
where
B= width,
Q= Maximum total flow rate, and
q= allowable flow rate per foot of width.
3. Baffle block height, hb, should be about 0.9dc to nearest inch.
4. Baffle block widths and spaces should be equal, and not less than hb but not more than
1 12 hb . Partial blocks, having a width not less than 13 hb and not more than 23 hb should be
placed against the sidewalls in rows 1, 3, 5, 7, etc. Alternate rows of baffle blocks
should be staggered so that each block is downstream from a space in the adjacent row.
5. The slope distance, S, between rows of baffle blocks, as shown in Figure 6-5 should be
at least 2hb but not greater than 6 ft.
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Hydraulic Structures – Baffled Apron Drop
7-Mar-21
6. A minimum of four rows of baffle blocks should be used. The baffled apron should be
extended so that the top of at least one row of baffle blocks will be below the bed of the
outlet channel. The apron should be extended beyond the last row of blocks a distance
equal to the clear space between block rows.
7. Upstream faces of baffle blocks are normal to the chute floor. The longitudinal
thickness, T, of the baffle blocks at the top should be at least 8 inches, but not more
than 10 inches.
8. Suggested height of the walls to provide adequate free board is three times the baffle
block height measured normal to the chute floor.
Uplift stability
The net forces causing floatation of the structure should be considered, assuming a sudden
cessation of flow in the channel. The net floatation force is equal to the weight of the
empty structure minus the hydrostatic force remaining in the soil around the structure.
Sliding Stability
Major forces, FS, tending to induce sliding along the horizontal plane, during maximum
flow in the channel are
FS  F1  F2  F3  F4
where
F1= Hydrostatic force on the upstream face of the upstream cutoff walls.
F2= Hydrostatic force on the vertical face of the sill.
F3= Horizontal component of hydrostatic force on the upstream face of the
baffle blocks.
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Hydraulic Structures – Baffled Apron Drop
7-Mar-21
F4= Force of saturated earth load on the upstream face of the downstream cutoff
walls.
Resisting Forces (FR)
Frictional resistance for  L1  L 2  as the sloping portion can move horizontal. Frictional
resistance is a function of structure weight reduced by uplift.
FR   W c W w  U   passive earth force
where,
 is the coefficient of sliding friction, generally assumed to be equal to
0.35,
Wc is the weight of concrete in the inlet portion of the structure, and
Ww is the weight of the water in the inlet portion of the structure.
U is the vertical uplift force as determined by Lane’s weighted creep
method.
Passive earth force is the total force resulting from passive earth
pressure on the d.s. side of cutoff walls.
Factor of safety F .S . 
FR
FS
Miscellaneous Considerations
Gravel or riprap should be provided on each side of structure from the top of the slope to the
d.s. wing wall, extending laterally a distance equal to the wall height to prevent erosion from
splashing.
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Hydraulic Structures – Baffled Apron Drop
7-Mar-21
Design Example
Design a baffled apron drop for the data
Q  120 cfs
R  2.55
b  8 ft
n  0.025
d 1  4.10 ft
A1  58.02 ft
S  0.00035
2
SS  1 12 :1
V 1  2.08 ft / sec
f b  2 ft
hv 1  0.07 ft
hB  6.1 ft
A drop of 6 feet in invert elevation is required.
Solution
1. From Table 6-1 find the recommended flow rate q for 120 cfs.
q  16 cfs per foor
B 
Q 120

 7.5 ft
q
16
2. V c  3 gq
 3 32.2 16  8.0 ft / sec
dc 
qc 16
  2 ft
Vc
8
Block height hb = 0.9 dc = 0.9×2=1.8 ft, say 1ft 10 inches
Block width and spacing = w
Min. w  hb  1.83 ft
Max. w  1.5hb  1.5 1.83  2.75 ft
3. Determine exact dimensions of baffle blocks and chute width
min .w p  13 hb  13 1.83  0.61 ft
max .w p  23 hb  23 1.83  1.22 ft
For simplicity try w p  12 hb  0.915 ft
B= 2 half blocks + 1 full block + 2 full spaces
Considering width of blocks and spaces equal to the block height = 1.83 ft
B  2  12 w  w  2w  4w
w  hb  1.83 ft
B  4 1.83  7.32 ft
For simplicity use w  2 ft   1.83 ft and  2.75 ft 
And
w p  1 ft   0.61 ft  1.22 ft 
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Hydraulic Structures – Baffled Apron Drop
Then
7-Mar-21
B  4w  4  2  8 ft
q
120
 15 cfs per foot
8
V c  3 32.2 15  7.85 ft / sec
dc 
q
 1.91 ft
Vc
hb  0.9  1.91  1.72 ft
Use hb= 1 ft 9 inches = 1.75 ft
Select T=9 inches > 8 inches < 10 inches
4. Determination of inlet length, L1
L1  2d 1  2  4.1  8.2 ft
Use L1  8 ft 3 inches
5. Determination of inlet sill height, hs
B  8 ft , q  15cfs , d c  1.91ft , hv c  0.96,V c  7.86 ft
hs  E s1  E s 2  hi (where, hi  inlet loss)

 


= d 1  hv 1  d c  hv c  0.5 hv c  hv 1

  4.1  0.07   1.91  0.96   0.5  0.96  0.07 
 0.85 ft use hs  10 inches  0.83 ft
6. Check inlet velocity to minimize splashing
Determine depth, d1, at inlet cutoff
d 1  hs  d c  hv c
 0.83  1.91  0.96
 3.70 ft
The entrance velocity is then
Q
Q
120
V1 


 4 fps
A1 d 1B 3.7  8
Determine critical velocity, Vc, over crest
Q
Q
120
Vc  

 7.85 fps
A d c B 1.91 8
2V 1 O.K.
Inlet velocity is approximately equal to half the
critical velocity, and splashing will be minimized.
7. Determine the sill length, L2
Using R=6 ft and invert slope of 2 to 1
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Hydraulic Structures – Baffled Apron Drop
7-Mar-21
  2634 '
y
 0.4472
z
y
tan    0.5
x
 2  1317 '
sin  
z
R
z  0.2361R  1.42 ft
tan  2   0.2361 
y  0.4472z  0.63 ft
y
 1.26 ft
0.5
L 2  x  z  1.26  1.42  2.68 ft
x 
and
e  hs  y  0.83  0.63  0.2 ft
8. Determine the slope distance, S
S  2hb (min.) but not greater than 6 ft
S  2 1.75   3.5 ft
Use S=6 ft for all blocks equal or less than 3 feet in height.
9. Determine minimum depth of cover, j at outlet to insure that the last row of baffle blocks will
be covered by the backfill, placed in the structure to the evaluation of the downstream grade.
S y  S sin   6  0.4472   2.68 ft
h y  hb cos   1.75  0.8944   1.57 ft
j  S y  h y  4.25 ft
10. Determine apron lengths, L3 and Ls for a drop, F= 6 feet.
Minimum distance,
Ly  e  F  j
 0.2  6  4.25  10.45 ft
Minimum rows of blocks,
L y 10.45
Rows 

 3.9 use 4 (min.)
Sy
2.68
Finally,
Ls  4S
 4  6   24 ft
L y  4S y
 4  2.68   10.72 ft
L3  4S x  4  6 cos    24  0.8944  21.47 ft
11. Determination of overall length of structure, L
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Hydraulic Structures – Baffled Apron Drop
7-Mar-21
L  L1  L 2  L3
 8.25  2.68  21.47
 32.4 ft
12. Determination of wall heights
h1  d 1  1 ft (F .B )  4.10  1  5.1 ft
Use h1  5 ft 2 inches
With a level invert and with the top of the walls level from h1 to h2
h2  h1  hs
 5 ft 2 inches  10 inches
 4 ft 4 inches
Height to chute walls
h3  3hb  3 1.75   5.25 ft
 5 ft 3 inches
13. Determine length, M1, of the upstream wing walls as shown in Figure 6-7. With C1=2.5 ft from
Figure 7-2.
M 1  1.5h1  C 1
 1.5  5.17   2.5  10.25 ft
Use 10 ft 3 inches.
14. Determine length, M3, of downstream wing wall as shown in Figure 6-8.
M 3  1.5h3  C 3
C 3  2.5 ft for channel depth of 4.1 ft from Figure 7-2.
h3 
h3
5.25

 5.87 ft
cos  0.8944
M 3  1.5  5.87   2.5  11.31 ft
Use M 3  11 ft 4 inches
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Hydraulic Structures – Baffled Apron Drop
7-Mar-21
15. Protection
For water depth 4.1 ft,
Inlet protection: Type 1, extending a distance d1 upstream, and up the sloping sides to
an elevation 1 ft above the normal water surface.
Outlet protection: Type 3, extending downstream a distance 4 d1 and up the sloping
sides to an elevation 1 ft above the assumed water table.
Slope protection: Type 1 should be provided on each side of the structure from the top
of the slope to the downstream wing wall extending laterally a distance
equal to wall height, to prevent erosion from splashing.
References
1.
United States Dept. of the Interior Bureau of Reclamation, “Design of Small Canal Structures”, United States
Government Printing Office, Denver, 1974.
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