(a) The maximum
velocity in feet per second during the design storm for water flowing in a
vegetated earthen auxiliary spillway shall be determined from the following
table:
Slope of the exit channel
Vegetation Erosion-resistant soils Easily erodible soils
0% to 5% 5% to 10% 0% to 5% 5% to 10%
Bermuda grass
and Bahia grass 8 7 6 5
buffalo grass,
Kentucky blue
grass, smooth
brome grass, tall
fescue, and reed
canary grass 7 6 5 4
sod-forming
grass-legume
mixtures 5 4 4 3
weeping love
grass, yellow
bluestem, and
native grass
mixtures 3.5 3.5 2.5 2.5
(b) The maximum design velocities specified
in subsection (a) may be increased by not more than 10 percent if the design
frequency of use of the auxiliary spillway is not more than two percent. The
maximum design velocities may be increased by not more than 25 percent if the
design frequency of use of the auxiliary spillway is not more than one percent.
(c) For exit channel slopes
greater than 10 percent, the applicant shall provide analyses showing both of
the following:
(1) There is no more than 0.5
foot of erosion depth within 20 feet of the control section for the one-percent
chance storm.
(2) The auxiliary
spillway does not fail by breaching during the spillway stability design event
indicated in the following table:
Hazard class Size class Spillway stability design event
A 1, 2, or 3 0.3 PMP
A 4 0.4 PMP
B 1, 2, 3, or 4 0.5 PMP
C 1, 2, 3, or 4 PMP
(d) The provisions of paragraphs (c)(1) and
(2) may be used for slopes of 10 percent or less in lieu of the maximum values
specified in the table in subsection (a).
(e) The maximum allowable design velocity for
water flowing over the following types of materials shall be determined from
the following table:
Material Maximum velocity allowed in feet per second
stratified rock 8.0
sound rock 13.0
(f) Channel lining materials not reliant on
vegetation, including concrete, riprap, and grouted riprap, may be used if the
applicant demonstrates that the lining will not fail during the spillway
stability design event specified in paragraph (c)(2).
