Section B4
Rock Chute Design Guidelines
170
171
CONTENTS
PAGE
B4.1
Introduction
172
B4.2
Design guidelines
195
B4.3
Rock size design charts
200
B4.1 INTRODUCTION
In 1986, McLaughlin Water Engineers Ltd. reported to the Denver, Colorado, Urban Drainage and Flood
Control District on their investigations into the design guidelines for open channel drop structures. The
December 1986 report Evaluation of and Design Recommendations for Drop Structures in the Denver
Metropolitan Area was followed by an Addendum and Errata report dated December 1989.
In the evaluation of sloping rock-lined chutes, the 1986 report states:
“The present Draft Criteria should be revised and upgraded. It would appear reasonable to
consider other drop options entirely [drop options other than sloping loose rock] and dissuade
usage of this type of drop, especially without careful evaluation of all the parameters involved,
and acknowledgement by the owner of the risks and problems involved.
Improvement in the design should include incorporation of a trickle channel through the crest
and the drop. Separate hydraulic analysis should be made for the main drop and the trickle
channel including determination of unit discharge, water surface profile analysis, and jump
analysis.
The determination of appropriate rock size should consider the location in the main drop or the
trickle channel, utilize a Shield’s parameter of 0.091 [Addendum (1989) later recommended 0.07
to 0.09], and a safety factor of 1.5 using equation X.9. A conservative value for angle of repose
should be used. All the following graphs herein [presented in Section B4.3] have used 42o.”
d50 =
J y Se
F (J s - J ) cosD (1/SF - tanD/ tan I )
*
X.9
“Froude number and the relative drop height to critical depth ratio should be checked. The
trickle channel downstream should be utilized to create a depressed stilling basin for the main
jump. Normally, a 1.5 to 2 foot [0.46 to 0.61 m] deep trickle is recommended, which allows
depression of the main basin approximately 1 foot [0.3 m] (with a shallower trickle channel
through the basin).
The trickle channel will create the need for an increase in rock size and the basin length in the
trickle area. As a rule of thumb, we recommend that the width of heavier trickle rock be 3 times
the width of the trickle channel. Also, we would recommend using large boulders which obstruct
the flow of the trickle in the basin, and if necessary, in the main basin to reduce downstream
erosion. Extra care in adjacent rock placement and foundations for the large rock will be
required.”
“The depth of the main stilling basins should be 2 or more the trickle and are generally 1 foot
[0.3 m] for qm > 15 cfs/ft [1.39 m 3/m]. Slopes should be kept flatter than 6:1 and generally flatter
than 8:1. Because of the trigonometric functions involved with increasing slope, the slightest
change makes a significant difference in rock size when using steeper slopes.”
[Drop heights of 1.8 m and 3.65 m] “are presented, largely to allow for guidance in designing
rundowns (channels to carry tributary flows into main channel) for flows less than about 500 cfs
[14.2 m 3 /s]. [The results presented for a drop height of 1.8 m] can be used when drops are
between 4 foot [1.2 m] and 6 foot [1.8 m]. Normally, drops greater than 4 foot [1.2 m] should be
avoided. In the situation where a design channel, say of 4 foot nominal depth, had to fall 6 foot
total, it would be preferable for energy dissipation and reduction of downstream erosion to have
one drop rather than 4 foot and 2 foot drops.”“Much improvement is needed in details,
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specifications, and quality control; including gradation tests, materials and placement. The
concept of dumped riprap is totally misleading. Realizing that all the relationships indicate that
the smaller size fraction will wash away, it is imperative that the d50 and larger material be placed
so that it is exposed and flush with the surface. The interstices should be filled with rock that is
not likely to be displaced and is as large as possible, and/or securely wedged between the larger
pieces in a mixture of smaller material. The remaining riprap should act as a reverse filter and a
levelling course.”
B4.2 DESIGN GUIDELINES
An alternative design procedure and rock size selection procedure is contained in the Standing
Committee on Rivers and Catchments, Victoria (1991) Guidelines for Stabilising Waterways, prepared by
The Working Group on Waterway Management, Armadale, Victoria.
The following design guidelines have been sourced from Evaluation of and Design Recommendations for
Drop Structures in the Denver Metropolitan Area—McLaughlin Water Engineers, Ltd. 1986, pp XII-15 to
19. Design guidelines in Section B2.4 should also be considered when designing a rock chute.
(a) Determine trial layout and rock sizing according to the design charts in Section B4.3.
(b) Determine the unit discharge in the trickle channel and on the main crest. In sizing the trickle
channel, consider the in-stream ecological requirements and the discussions in Section B4.1.
(c) Compute water surface profile analysis on the face of the drop and in basin below. Compute
separately the main drop and the drop through the trickle channel using Manning’s n according to
Equation B4.1. Use the roughness value to compute friction loss in hydraulic analysis. Do not assume
normal depth (numerical model analysis is usually required).
n
where:
x = (R/d90)(d50/d90)
R
B4.1
(d90 )1/6
0.7
26(1 0.3593 (x) )
= Hydraulic radius of flow over rocks [m]
d50 = mean rock size for which 50% of rocks are smaller [m]
d90 = mean rock size for which 90% of rocks are smaller [m]
In a ‘natural’ gravel-based stream, the relationship between d50 and d90 rock size is typically in the region
of:
Natural channel bed rock: 0.2 < d50/d90 < 0.5
In constructed channels where imported graded rock is used, the ratio is likely to be higher, depending
on the grading process.
Graded rock: let d50/d90 = 0.5 to 0.8 unless otherwise known
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(d) Determine the location of the hydraulic jump. If tailwater is greater than the crest elevation, consider
the recommendation presented in the US Corps of Engineers, Hydraulic Design of Flood Control
Channels (1970). Normally, the controlling velocity, depth and energy gradeline slope parameters
reflect conditions at the toe of the drop. The jump is usually located at the toe or on the face of the
main drop. The trickle flow travels downstream in the basin before the jump occurs. From the point
where the jump is initiated, it appears that a length of 5 to 6 times the tailwater depth is appropriate.
This should be reviewed for each case.
(e) Using Equation B4.2, iterate the acceptable solution for d50 based on verification of reasonable
assumptions for trial d50, and the angle of repose of riprap.
d50 =
y Se
*
F (ss - 1) cos D (
Where:
d50
y
Se
F*
ss
D
SF
)
=
=
=
=
=
=
=
=
1 tanD
)
SF tan )
B4.2
mean diameter of rock [m]
deepest flow depth at section [m]
energy gradeline slope [m/m]
Shield’s parameter (0.07 to 0.09 recommended)
relative density of rock (refer to Table B4.1 for a guide)
angle between the bed and horizontal
safety factor (1.5 recommended)
angle of repose of the rock (42o rounded rock and 42o angular rock)
Recommended Shield’s parameter F* = 0.07 for critical drops structures, drop structures with a fall
greater than 1.2 metres, and drop structures that are designed to a flood frequency less than Q100.
Table B4.1 Typical relative densities of rock
Rock type
Relative density (ss)
Sandstone
2.1 to 2.4
Granite
2.5 to 3.1 (most commonly 2.6)
Limestone (crystalline)
2.6
Basalt
2.7 to 3.2
(f) See Section B4.3 for guideline values of rock size. They are based on the angle of repose equal to
42o, other assumptions for trickle depths (0.6 m in the channel, 0.3 m in the basin) and basin
depression (0.3 m). If the required d50's are different from the original assumptions (step a), the entire
process has to be repeated until they equal.
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(g) Use of the guideline values in Section B4.3 should be tempered with the following:
Slopes flatter than 6:1 are inherently much safer. In Equation B4.2, the safety factor is adjusted by
a trigonometric function that rapidly increases the required sizing and shows more sensitivity to
errors in slope.
The maximum graded riprap specified within the Denver, Colorado District is d50 = 600mm.
Basically, use of this riprap will involve flatter slopes. Within Brisbane, loose rock chutes with a
specified rock size greater than 600mm should be avoided, as it is difficult to get such rock.
If steeper slopes or larger d50 are required, the Derrick stone placement approach has merit.
However, this will involve special placement of large boulders to match surfaces and minimise
voids. Place well-graded riprap in the remaining voids so that they are interlocking and cannot
be moved. The stacked boulder approach is similar and viable. The key is to have each boulder
step so that the upstream boulder is [at least] half way below the top of the downstream boulder
and the voids are filled with material that is not easily displaced.
Design charts for drop heights of 1.8 m and 3.65 m are presented largely to allow for guidance in
designing rundowns (channels to carry tributary flows into main channel) for flows less than about
14 m3/s. Rock sizes shown for a drop height of 1.8 m are provided for the case where a 1.2 m to
1.8 m drop is unavoidable.
Normally, drops greater than 1.2 m should be avoided. However, where a design channel, say of
1.2 m nominal depth, had to fall 1.8 m total, it would be better for energy dissipation and
reduction of downstream erosion to have one drop rather than 1.2 m and 0.6 m drops.
(h) Evaluate the stability of the jump below the main drop and along the trickle channel. If further
depression of the basin is not feasible, consider using large boulders that protrude into the flow—
ideally as much as 0.6 to 0.8 times the critical depth (yc)—and creating a couple of bends in the trickle
flow channel to dissipate energy.
(i)
Boulders in horizontal basins can be evaluated based on the velocity just upstream of the hydraulic
jump and by using the Isbash Equation (Equation B4.3). All adjoining rock should be large with care
taken to fill all voids with interlocking riprap and provide a subgrade comprising riprap and bedding.
Heavier rock at the trickle channel should extend laterally so there is a width of heavier rock equal to
3 times the width of the trickle channel. This is an estimate of what might be needed to stabilise the
rock against the diverging/converging flow conditions. Stacked boulders, stepped in a ledge-like
fashion in the trickle and carefully arranged (and placed with adequate riprap to fill voids and provide
subgrade), is a recommended approach.
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d50
Where:
d50
Ub
ss
g
K1
=
=
=
=
=
Ub
2
2 (ss 1) g K
B4.3
2
1
mean diameter of rock [m]
flow velocity over the boulders [m/s]
relative density of rock (refer to Table B4.1)
acceleration due to gravity (9.81 m/s2)
constant = 0.86 for high turbulence, and 1.2 for low turbulence
(unexpected in most design cases)
Note:
The above equation may be reduced to a form similar to that presented in the design
guidelines for rock beaching (Section B2.7), ie. for ss = 2.6 and K1 = 0.86:
d50 = 0.043 (Ub) 2
B4.4
(Also note; the units for d50 are [m], not [mm] as used in Section B2.7 where
d50 = 40 (v) 2 where v Ԙ Ub )
(j)
Absolute minimum thickness of riprap is 1.5d50, with 2.0d50 at the crest (see Figure B4.1).
Generally, the height of the rock on the banks should be critical depth for the main drop plus 0.3
metres.
(k) Very conservative bedding should be implemented. At minimum, use 0.3 metres of material that
is tightly matched to the riprap above it.
(l)
The crest approach should be devised as described in the introductory remarks. There is no
contraction or sill (it only aggrades anyway) and the upstream protection length LA (Figure B4.1)
can be longer (eg. 4.5 to 7.5 metres).
(m) Allow for adjustments and repairs to the rock work, especially where there are potential hydraulic
jump problems.
(n) The ‘owner’ of the structure should be told of potential problems, especially that the forces
involved are random and localised pressure fluctuations can suddenly dislodge rock. Movement
is inevitable, but is dramatically accelerated and magnified by weakness in analysis, design,
material, construction and inspection.
(o) Typically limit dmax Ԥ 1.5 to 1.6 d50. On very steep grades, let dmax Ԥ 1.25 d50.
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Figure B4.1 Sloping loose rock chute (just one possible layout)
[Source: Evaluation of and Design Recommendations for Drop Structures in the Denver Metropolitan
Area—McLaughlin Water Engineers, Ltd. 1986]
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B4.3 ROCK SIZE DESIGN CHARTS
Typi cal D50 rock sizes for various chute drop heights are provided in the following charts. Rock size is
based on:
a rock relative density of 2.6
o
a placed rock angle of repose = 42
trickle depths (0.6 m in the c hannel, 0.3 m in the basin)
basin depression (0.3 m)
chute slope 6:1
a design safety factor of 1.5.
Notes:
1. Slope is the slope of the drop (ie, 5% Slope is 1(V) to 20(H)
2. If D50 is greater than 0.6 m then maximum slope of the drop should be 16.67%
3. D50 values are based on Shields 0.091, Safety Factor of 1.5 and a slope correction
[Source: Evaluation of and Design Recommendations for Drop Structures in the Denver Metropolitan
Area, McLaughlin Water Engineers, Ltd. December 1986]
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Notes:
1. Slope is the slope of the drop (ie, 5% Slope is 1(V) to 20(H)
2. If D50 is greater than 0.6 m then maximum slope of the drop should be 16.67%
3. D50 values are based on Shields 0.091, Safety Factor of 1.5 and a slope correction
[Source: Evaluation of and Design Recommendations for Drop Structures in the Denver Metropolitan
Area, McLaughlin Water Engineers, Ltd. December 1986]
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