FLOOD03351

• County: Statewide
• Basin: Statewide
• Title: Engineering and Design Hydraulic Design of Flood Control Channels
• Date: 7/1/1991
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

(9) Determine the length of IranSloon and invert
slope required to reduce the channel width from be to
~ without exceeding the criterion y{Ye ,,; 0.90 in the
lransition. Convergence rates should be in agreement
with those recommended in paragraph 2-4.

f. Side drainage inlets. Flow disturbances occur
where stonn drains or indusaial waste lines discharge into
nood conlrOl channels, commonly referred to as "inlets."
Small side-drainage flows are commonly conveyed in a
pipe storm drain system. Criteria for box and pipe culvert
inlet design are given in h below. Economical design for
intermediate tributary nows normally requires free surface
SlrUClUreS. A side":hannel spillway type of inlet for this
range of discharge has been developed by USAED, Los
Angeles, which reduces disturbances to a minimum in the
main channel. This type of junction is described in g
below. The conventional confiuence structure described
in d above should be used for large tributary discharges.

g. Side-channel drainage inlet.

(I) The side..:hannel spillway type of drainage inlet
was developed and model tested by USAED. Los Angeles
(196Ob). The recommended SlrUCture consists of a com-
mon wall between the side channel and the main channel.
A weir notched in this wall allows the tributary flow to
enter the main channel with minimum disturbance. A
typical desi gn of this type of SlrUcture is i1luslrated in
Plate 58. A small drain should be placed at the lowest
point of the side channel. The objective of this design is
to discharge the side flow with reduced velocity into the
main channel gradually over a relatively long spillway
inlet Model tests (USAED. Los Angeles. 1960b) indicate
that this effectively reduces wave action and disturbances
in the main channel for all flow combinations. Satisfac-
tory operation may require periodic sediment removal
from behind the weir.

(2) The procedure for designing the side.channel
spillway inlet SlJ'Ucwre follows:

(a) Set the spillway crest 0.5 ft above the parallel to
the design watersurface level in the main channel.

(b) Determine the ~uired length L of the crest by
the equation. L = Q{(CH /2) , so that the maximum H
is not greater than 1.5 ft with critical depth over the crest
C equal to 3.097.

(c) Determine the side..:hannel flow depth d at the
UpStre:lm end of the spillway.

EM 1'10-2-1601
1 Jul 91

(d) Set the side-channel invert so that the spillway
apprcach depth is equal to d. H .

,
(e) Determine the side-channel convergence required
to maintain a constant flow depth in the side channel
behind the spillway. nus should result in a fe:lSCnably
constant unit discharge over the spillway equal to that
computed by the equation in (b) above.

(I) Plot the computed side..:hannel alignment points
obtained from step (e) on the channel plan and connect
them by a smooth curve or slraight line to intersect the
main channel wall so that the side channel has a mini.
mum width of 2 ft behind the spillway.

(g) Adjust the side-channel convergence and repe:lt
step (e) if the spillway length in step (f) does not approxi-
mate that determined in step (b).

h. Box and pipe culvert inlets. Gildea and Wong
(1967) have determined design criteria for pipe inlets.
The variables to be considered in the design are width of
the main channel, angle of enlrance of the storm drain,
size of the storm drain, volume and velocity of flow. and
elevation of the storm drain with respect to the channel
bottom. Model tests (USAED, Los Angeles, 1960b,
1964) have shown that flow disturbances in the main
channel are minimized when side-drain openings are small
and side- drainage flows are inlrOduced reasonably par-
allel to the main flow. The following criteria should be
used for design:

(1) The maximum angle of enarance for side culverts
should be:

(a) 90 deg for diameters of 24 in. or less,

(b) 45 deg for diameters from 24 to 60 in.

(c) 30 deg for diameter 60 in. or greater.

(2) The culven invert should be pl:1ced no more than
18 in, above the main channel invert to give the maxi-
mum submergence prncticable.

(3) Automatic floodgates or flap gates should be
inslalled when damage from backflooding from the main
channel would exceed that resulting from local pondage
caused by g:lte operation. These gates should be recessed
to prevent projecting into the main channel flow when in
a full-open position. Head loss coefficients for fup gates
are given in HDC 340-1.

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