-
-..-
MQceflan~oef a p e ~HL-94-4
S e p t ~ n l e1394
i
US Army Carps
of Englnecrs
Waterways Experiment
Station
Streams Above the Llne: Channel Morphology
and Fioad Control
Proceedings 01 the Corps of Englmrts Workshop on Steep
Streams, 27-29 October 1892, Seattle, Washington
Apprwrk For Public Ralsare; Distributian h U~~)nEmitad
Prepared for Headquarters. U S . Army Cotps of Eh~inaers
where
lCLprrrp d-5for utagrniu b and c -9
M L a a t b n of
mebod P*.emat.d LP EM WO-2-1661Tha abfmatiw af thir
paper *a t o ptrnant zlprrp b r d p 4ar catspry b mb c e t s ~ b m .
for braib.6 s t x e g ~k
'L2ra aajor ahdlmge i n tLpre
tha irposha forea at the i q i a y o c n t point. In the
M 1110-2-1601 method, th. cbracteristie imposed f e t e t for side
acthating
slopr riprap , i r the depth-anrmged ~ l o c i t gar 20 percear of the
nlop. Ieiagdh 4 from rha m e Vm. A t t h q h wrplcwen. tkr met
e m r e baak attack in bratdod ctl:.~yas ir, thought t o occur when tho
vmtrr surface is at ur rlilghcly b e rhs t ~ p ro f the ddchrrrar~
barb. ~t a i r rtase, f i g l o t a confined to ch. W t i p l e chraatls
laboratory channels having 1V:2H side slopes and channel bottoms
with the same riprap size, failure almost always occurred on the
channel bottom i n s t a b i l i t y tests.
In laboratory bendways of the
Riprap Test Facility, US Army Engineer Waterways & p e r b e n t
Station, having 1V:W side slopes, f a i l u r e generally occurred about
P r e l w results from ongoing
halfway up the side slope.
studies of impinged flow having 1V:2H side slopes showed Chat f a i l ures were initiated higher up the side slope than i n the bendway.
This suggests that impinged flow has high velocities well up on the
side slope, and the 1991 field study (Kapnord, in preparation) confirms t h i s observation. The laboratory snrdy of impinged flow is
trying t o determine the appropriate value of C, for intpinged flow.
Until that time a value of C
, of 1.25, which is close t o a bendway
2, is recommended.
having R/U
-
For the Snake River near Jackson, Wyoming, the required riprap size using the procedures presented herein is as follows:
--
-
-
- -
L.6(15,000/2000)
12 Eps, depth at Vm = 10
Input: Vz0
155 pef, C,
1.25, C,
0.30,
f t , specific weight
C
0
S = 1.1 1 V : a side slope, thickness
lDloa, use ETL 1110-2-120 gradations given i n Table
3-1 of M 1110-2-1601,
-
Result:
Required
thickness
-
-
-
= 1.09 ft, ETL r>,,(min)
27 in., Wso(min) 185 lb.
-
1.10 it,
This compares with the exhting rlprap that has an average size of
less than 100 l b according to US Anny Engineer District, Walla
Walla (1987). New riprap placement along the Snake River generally
400 ILb w i t h thickness of 42 Ln. at the toe
uses tiprap having W%
and 24 in. a t the top.
-
for Categorv c Stre-
For single channels or overflow embankments, slopes greater
than 2 percent are outside the range of direct applicability of
EM 1110.0-2-1601because of the importance of the damslope gravity
component and the effect of steep slopes on flow resistance.
Overflow embankment riprap s t a b i l i t y t e s t s have generally been
limited to a maximum slope of 20 percent.
The most recent t e s t s
were conducted by Abt e t al. (1986) and Abt et al. (1988). While a
20 percent slope may seem large f o r loose riprap, highway engineers
have questioned this author about design guidance for riprap placed
on slopes approaching 40 percent. Using Abt's data and dimensional
analysis results i n the following empirical equation
or i n terms of DS0 used i n EM 1110-2-1601
where S is the slope of the bed and q is the unit discharge. Both
equations can be used i n any consistent set of units, both f a l l on
the conservative side of the eta, and both a r e restricted to a
thickness
of.-l.SDzp,,
--- -- - angular rock, specific weight of 167 pcf,
6-in. gravel f i l t e r b e n i a K ?iprapy 'Dss/at5 from 1.7 t o 2.7, slopes
from 2 t o 20 percent, uniform flow--oii2i- downslope iFiXEFno tailwater, and a?l_erage-riprap_.si_ze-----less than 6 in. The comparison of
E q u a t i o s w i t h t;he d a G is shown i n Figure 1. One of the problems
w i t h this approach i s thae different specific rock weight, blanket
thickness, and gradation uuifonaLty 'camrot be used with t h i s
approach.
sL.rw%.w--.-
An alternative approach would be to w e the El4 1110-2-1601
procedure t o address other specific weights, thickness, and gradation but t o include the appropriate factors f o r downslope gravity
effects and a resistance equation for flow on steep slopes. From
Ulrich ( 1 9 8 7 ) , the appropriate 1% factor t o use I n Equation 1 is
Kl
Cos a
7.
- 7,-
When a is the angle of the channel bottom from horizontal and 4
is the angle of repose of the riprap revetment.
From Maynord
(1988) the appropriate 4 for riprap revetmentx is about 53 degrees.
Using Abt e t al. (l986), flow resistance data and dimensional
analysis r e s u l t i n the following modification of the Strickler
equatfon
-
-
which is applicable t o slopes between 2 and 20 percent. Combining
Vd, and Ds0
1.ZDS0 results i n
Equations 1, 4, and 5 , using q
where
Maynord
hL'
k i ~A,<
r> L
Note the similarity of Equation 6 and Equations 2 and 3 and that
Equation 6 was derived without using Abt's et al. stabiliq data.
Also note that the slope effect in &pation 6 is also part of the
Kl factor in the denominator. Equation 6 is limited Co slopes from
2 to 20 percent, angular rock, 6-in. gravel filter beneath riprap,
uniform flow on downslope with no tailwater, and average riprap
size less thaa or equal to 6 in. The comparison of Equation 6
using Sf
1.1 (minimum safety factor), C
,
1, Cr
0.84 (for
0.30 (fox angular rock) , specific stone
thickness
1. 5DX0,), C,
weight- 167 pcf, and 4
53 degrees w i t h Abt's et al. data is shown
in Figure 1 as the modified EM 1110-2-1601 curve. Equation 6 fits
the observed data as well as the empirical approach given by Equation 2 or 3 and allows variation of stone size with unit weight,
blanket thickness, etc,
L
--
-
--
-
Abt et al. (1988) presents a flow concentration factor that
varies from 1 to 3 that is* multiplied by the unit discharge when
the inflow is not uniform across the approach channel. Although
guidance is lacking on the amount of flow concentration for a given
geometry, some degree of flow concentration should be expected.
Riprap on steep slopes should be relatively uniform with Da5/DL5
2.5.
Additional studies are needed to extend Equations 2, 3, or 6
to larger riprap sizes.
Consider a 10-ft-wide downslope hawing a 10 percent slope and
a total discharge of 25 cfs. Rock protection will be placed to a
thickness of 1.5Dxao, and have a rmft weight of 165 pcf. Using a
flow concentration factor of 1.25 results in a unit discharge of
1.25(25/10)
3.13 cfs/ft.
Using Equation 3, the required DS0
0.37 ft,
Using the modified M 1110-2-1601 procedure given by .
Equstion 6, the required Dgg
0.34 ft. In either case, a typical.
gradation having &o(min) 2 0.34 ft wauld have Dloo(max) of about
9 in. and a blanker thickness of 1 . 5 ( 9 )
13-14 in.
-
Summaw
-
-
and Conclusions
. Riprap design for single channels, nonixnpinging flow, and
sl$&is
less than 2 percent should use guidance presented in W
1110-2-1601.
Riprap design for braided channels, impinged flow, and slopes
less than 2 percent should use the velocity estimation method presented herein
and C
,
1.25 in the EM 1110-2-1601 procedure. -
-
-
Riprap design for single channels or overflow embankments,
nonimpinged flow, slopes between 2 and 20 percent, uniform flow on
a downslope w i t h no tailwater, and average rock size less than or
equal to 6 in. should use either the empirical method in Equation 2
or 3 or the modification of the EM 1110-2-1601 method given in
Equation 6.
meter per second
-?7
at,S. R., Buff, J. F., Khattak, M. S . , Wittler, R. J., Shaikh,
&., and Nelson, J. D. 1986. DEnvirolunental assessment of uranium
recovery activities-riprap testing, Phase I," Report No. CER85-
86SRA-JFR-MSK-RJW-AS-JDN24,
Department of
Colorado State University, Fort Collins, CO.
-3
-
Civil
Engineering,
Abt, S. R., Wittler, R. 3 . . Ruff, J. F., LaGrone, D. L., Khattak,
M. S., Nelson, J. D., Hinkle, N. E., and Lee, D. W. 1988. "Development of riprap design criteria by riprap tasting in flumes:
Phase IT,* NUREG/CR-4651, O~/TM-l0100/V2, Vol 2, Prepared for
US Nuclear Regulatory Commission by Colorado State University, Fort
Collins, CO, and Oak Ridge National Laboratory, Oak Ridge, TI?.
Headquarters, US Army Corps of Engineers. 1991. "Hydraulic Design
of Flood Control Channels,a EM 1110-2-1601, US Government Printing
office, Washington, DC .
Maynord, S. T. 1988. "Stable riprap size for open channel flows,"
Technical Report HL-88-4, US Amy Engineer Waterways Experiment
Staeion, Vicksburg, MS.
MBynord, S. T. "Flow impingement, Snake River, Woming" (in preparation), US Army Engineer Waterways Experiment Station, Vicksburg,
MS.
Ulrich, T. 1987. "Stability of rock protection on slopes," Jour.
nal of Hydraulic Engineering, ASCE, 113(HY7), 8794391.
8-7
Maynord