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

Plate 39 along with Equation 3-4 using <1>" 40 deg.
Also shown on Plate 39 is the correction for side slope
when D30 is determined from Plate 37. Correction for
the vertical velocity distribution in bends is shown in
Plate 40. Limited testing has been conducted to deter-
mine the effects of blanket thickness greater than
IDUJo(max) on the stability of riprap. Results are shown
in Plate 40.

(2) The basic procedure to determine riprap size
using this method is as follows:

(a) Determine average channel velocity (HEC-2 or
other uniform now computational methods. or measure-
ment)

(b) Find V 55 using Plate 33
(c) Find ~o using Plate 37

(d) Correct for other unit weights, side slopes, verti.
cal velocity distribution. or thicknesses using Plates 38
through 40

(e) Find grnda1ion having ~o(min) ~ computed
D3o-

(3) This procedure can be used in both natural chan-
nels with bank prOlection only and prismatic channels
having riprap on bed and banks. Most bank protection
sections can be designed by direct solution. In these
cases. the extent of the bank compared to the tolal
perimeter of the channel means that the average ch:mnel
velocity is not significantly affected by the riprap. The
fITSt example in Appendix H demonslrates this type of
bank protection.

(4) In some c:lSCS, a large part of the channel
perimeter is covered with riprap; the average channel
velocity, depth. and riprap size are dependent upon one
anOlher; and the solution becomes iterative. A trial riprap
gradation is fITSt assumed atid resistance coefficients are
computed using Equalion 3-2. Then the five steps
described in (2) above are conducted. If the grad:uion
found in paragraph (e) above is equal to the assumed trial
gradation, the solution is complete. If nolo a new trial
gradation is assumed and the procedure is repeated. The
second example in Appendix H demonslrates this type of
channel riprap.

(5) In braided s=s and some meandering
Stre:lms. flow is often directed into the bank line at sharp
angles (angled flow impingement). Guidance is lacking
on determining the imposed force for this condition.

EM 1110-2-1601
1 Jul 91

Until better guidance can be developed, a local velocity of
1.5 times the average velocity in the approach channel is
recommended for use in riprap design.

(6) Transitions in size or shape may also require
riprap protection. The procedures in this paragraph are
applicable to gradual a-ansitions where flow remains
tranquil. In areas where flow changes from a-anquil to
13pid and then back to a-anquil. riprap sizing methods
applicable to hydraulic SlrUCtures (HDC 712-1) should be
used. In converging a-ansitions. the procedures based on
Equation 3-3 can be used unaltered. In expanding aransi-
tions. now can concentrate on one side of the expansion
and design velocities should be increased. For installa.
tions immediately downs=m of concrete channels. a
vertical velocity distribution coefficient of 1.25 should be
used due to the difference in velocity profIle over the two
surfaces.

3-8. Revetment Top and End Protection

Revelrnent top and end protection requirements, as with
all channel prolective measures, are to assure the project
benefits, to perform satisfactorily throughout the project
economic life, and nol to exceed reasonable maintenance
costs. Reference is made to ER 1110-2-1405, with
emphasis on paragraph 6c.

a. Reverm.e1l1 top. WIlen the full height of a levee is
to be proteCted. the revelrnent will cover the freeboard.
I.e., extend to the lap of the levee. This provides protec-
tion against waves. noating debris, and water.surface
irregularities. Similar provisions apply 10 incised channel
banks. A horizontal collar. at the top of the bank. is pro-
vided to protect against escaping and returning nows as
necessary. The end protection methods i1IuslI:lled in
Plate 41 can be adapted for horizonlal collars. Plate 36
provides general guidance for velocity variation over
channel side slopes that can assist in evaluating the
economics of reducing or omitting revetment for upper
bank areas. Revetment size changes should not be made
unless a sufficient quantilY is involved to be cost effec-
tive. Many successful revelrnents have been conslrUcted
where the top of the revelrnent was tenninated below the
design flow line. See USACE (1981) for examples.

b. Revetment end protection. The upstre:lm and
downstre:lm ends of riprap revelrnent should be protected
against erosion by in=ing the revetment lhickness T
or extending the revelrnent to areas of noneroding veloci.
ties and re!:ltively stable banks. The following guid;]p:e
applies to the alternative methods of end protection iIIus-
lI:lled in P!:lte 4 I.

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