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

Chapter 4
Special Features and Considerations

4-1. Sediment Control Structures

a. General. Two basic types of cona-ol SlrUCtures
are used:

(1) stabilizers designed to limit channel degradation
and

(2) drop structures desi gned to reduce channel slopes
to effect nonscouring velocities.

These SlruCtures also correct undesirable, low-water.
channel meandering. Gildea (1963) has discussed channel
stabilization practice in USAED. Los Angeles. Debris
basins and check dams are special types of conrrol SlruC-
tures that are used to trap and store bed-load sediments.

b. Stabili:ers.

(I) A stabilizer is generally placed normal to the
channel center line and lraverses the channel invert.
When the stabilizer crest is placed approximately at the
elevation of the existing channel invert. it may consist of
grouted or ungrouted rock. sheet piling. or a concrete sill.
The stabilizer should extend into or up the channel bank
and have adequate upStre:lm and downstre:lm bed and
bank protection. Plate 44 illuslI:lles the grouted sto.ne
type of stabilizer used in USAED, Los Angeles. Stablliz.
ers may result in local flow acceleration accompanied by
the development of scour holes upstre:lm and downstream.
As indicated in Plate 44. dumped SlOne should be placed
to anticipated scour depths. Maximum scour depths
usually occur during peak discharges.

(2) Laboratory tests on sheet piling stabilizers .for t.he
Floyd River ConlrOl Project were made by the Umverslly
of Iowa for USAED, Om:lha (Linder 1963). These stud-
ies involved the development of upSlream and down.
Stre:lm bed and bank riprap protection for sheet piling
stabilizers in a channel subject to average velocities of
14 fps. The fmal design resulting from these tests is
shown in Plate 45. Plate 46 is a general design chart giv-
ing derrick stone size required in critical flow areas as a
function of the degree of submergence of the SlruCture.
Plate 47 presents design discharge coefficients in terms of
the sill submer2ence T and critical depth de for the
channel section: Use of Plates 46 and 47 is predicated on
the condition that the ratio Tide is greater than 0.8. For
smaller values the high-velocity jet plunges beneath the
water surface. resulting in excessive erosion. The top of

EM 1110-2-1601
1 Jul 91

the sheet piling is set at an elevation required by the
above-mentioned criteria. Plate 47 is used with the
known discharge to compute the energy head at 5d,
ups= of the structure. The head H on the SO"ucture
is determined from the energy equation and used with
Plate 46 to estimate the required derrick stone size. The
curves in Plates 29 and 30 should be used as guides in the
selection of riprap sizes for the less critical flow area.

c. Drop structures.

(1) Description and purpose. Drop structures are
designed to check channel erosion by conaolling ~e
effective gradient. and to provide for abrupt changes m
channel gradient by means of a vertical drop. They also
provide a satisiactory means for discharging accumulated
surface runoff over fills with heights not exceeding about
5 ft and over embankments higher than 5 ft provided the
end sill of the drop structure e~tends beyond the toe of
the embankment. The hydraulic design of these structures
may be divided into two general phases, design of the
notch or weir and desi gn of the overpour basin. Drop
SO"uctures must be so placed as to cause the channel to
become stable. The SlruCture must be designed to pre.
c1ude flanking.

(2) Design rules. Pertinent features of a typical drop
structure are shown in Plate 48. Discharge over the weir
should be computed from the equation Q" CLH312 ,
using a C value of 3.0. The length of the weir should
be such as to obtain maximum use of the available chan-
nel cross section upStre:lm from the SlruCture. A
triaI-and-error procedure should be used to balance the
weir height and width with the channel cross section.
Stilling basin length and end sill height should be deter.
mined from the design curves in Plate 48. Riprap
probably will be required on the side slopes and on the
channel bottom immediately downsa-eam from the
structure.

d. Debris basins and check dams.

(1) General. Debris basins and check dams are built
in the headwaters of nood conlrOl channels having severe
upstre:lm ercsion problems in order to trap ~g~ bed-load
debris before it enters main channels. This IS done 10
prevent aggradation of downslream channels and deposi-
tion of large quantities of sediment at Stre:lm ,mouths.
Also. the passage of large debris loads through reinforced
concrete channels can result in costly erosion damage to
the channel. Such damage also incrc:JSes hydraulic
roughness and reduces channel capacity. A general sum.
marv of data on the equilibrium gradient of the deposition
profIle above control structures has been presented by

4-1