analysis of condeshire box culvert near rio...
TRANSCRIPT
Analysis & Modeling Report November 24, 2004
Analysis of Condeshire Box Culvert near Rio Bravo
SUMMARY
The Condeshire box culvert, located near Rio Bravo just east of 98th Street, is analyzed for
different developed flow conditions. The double barrel box culvert is steep, with a critical
depth above the culvert for flows greater than 1000 cfs. The HEC-RAS analysis, assuming
supercritical flow is maintained upstream and
downstream of the culvert, indicates the culvert to have
sufficient capacity for developed flow conditions.
However, an inlet control analysis of the culvert
indicates a capacity of 680 cfs without the allowable
headwater depth being exceeded. A 1:24 scale model
was constructed to further analyze this culvert. The
constructed model confirmed the HEC-RAS analysis
showing the CBC to have capacity for 1033 cfs with the
addition of a bullnose splitter wall and ideal c
Furthermore, the model demonstrated that if the inlet
becomes submerged, the capacity of the culvert will
decrease dramatically.
onditions.
Prepared for the Albuquerque Metropolitan Arroyo Flood Control Authority
Julie Coonrod, Ph.D., P.E. Department of Civil Engineering The University of New Mexico
1
INTRODUCTION
A typical trapezoidal concrete channel, with a 10-foot bottom width, runs parallel to
Rio Bravo flowing from west to east. The channel crosses under Condeshire Dr, between
Unser and Coors Boulevards, via a double barrel, 6 foot by 6 foot concrete box culvert
(Figure 1). The anticipated developed flows through the culvert are being altered by upstream
development and other flood control projects. The purpose of this analysis is to determine the
capacity of the culvert.
Figure 1. Vicinity Map and Photo of Condeshire Box Culvert
2
PROVIDED INFORMATION
The following information on dimensions, elevations, and flow rates were compiled from various sources as noted. From construction plans: Double barrel 6 ft x 6ft CBC Plan for road section shows: Upstream invert = 4968.50 HW elev = 4975.34 Profile shows: Upstream invert Sta 268+30.49, elev = 4968.49 -2.00% slope to Sta 269+25, elev = 4966.52 -8.1733% slope to Sta 271+50, elev = 4946.75 Channel approach: 50 ft transition channel 12o wingwalls (measured from plans)
Although plans show a constant channel slope of 2.00%, field inspection reveals several breaks in grade. The channel is a typical trapezoidal channel with BW = 10.0 ft.
From recent survey: Upstream invert = 4968.62 Top of headwall = 4976.86 (8.24 ft above invert) (no invert elevations are shown downstream of the CBC entrance)
50 ft transition channel with 12o wingwalls (measured from plans) Channel invert 50’ upstream of CBC entrance is 4969.59 on north side and 4969.54 on south side, resulting in 1.94% - 1.84% approach slope to CBC
From Doug Hughes letter (11/14/03, Mark Goodwin & Assoc.)
Developed flow = 1151.22 cfs
From revised HEC-RAS plan from Doug Hughes
Developed flow = 992.69 cfs
From Doug Hughes e-mail (8/9/04)
Developed flow = 1032.74 cfs
3
HEC-RAS ANALYSIS
HEC-RAS files were created from the survey. The steady flow analysis includes
water surface profiles for three developed flow rates: 993 cfs, 1033 cfs, and 1151 cfs. The
culvert dimensions were included in the geometric data, and the contraction and expansion
coefficients were increased just upstream of the culvert. The boundary conditions were set by
computing normal depth. Supercritical flow is expected unless something disturbs the flow.
As shown in Figure 2, if supercritical flow is maintained, then the culvert will function.
However, if something happens downstream that causes a hydraulic jump, then the culvert
would be overtopped for the 1151 cfs as shown in the mixed flow regime in Figure 2. The
subcritical regime shows similar results with the culvert overtopping with a flow of 1151 cfs.
The results are somewhat suspect because HEC-RAS may not adequately consider the
headlosses that occur upstream of the culvert. In fact, according to the HEC-RAS Hydraulic
Reference Manual (Figure 3), the culvert should be analyzed using inlet control.
To further analyze the culvert, specific energy diagrams were created, and
CulvertMaster was used.
4
4968
4970
4972
4974
4976
4978
4980
4982
CondeshireCBC Plan: Plan 01 8/10/2004 El
evat
ion
(ft)
Legend
EG PF 3
EG PF 2
EG PF 1
Crit PF 3
Crit PF 2
Crit PF 1
channel ll to Ri upstreamConde
Top of road
Ele
vatio
n (ft
)
ion(f
Ele
4968
4970
4972
4974
4976
4978
4980
4982
4968
4970
4972
4974
4976
4978
4980
4982
vat
t)
F
Figure 2. HEC-
Supercritical Flow Regime
500 550 600 650 700 750
WS PF 3
WS PF 2
WS PF 1
Ground
5
Legend
EG PF 3
EG PF 2
EG PF 1
Crit PF 3
Crit PF 2
Crit PF 1
WS PF 3
Top of road
5
Fl
RA
Mixed ow Regime
00 550 600 650 700 750
WS PF 1
WS PF 2
Ground
EG PF 3
EG PF 2
EG PF 1
WS PF 3
Crit PF 3
Crit PF 2
Top of road
Subcritical low Regime00 550 600 650 700 750
M ain Channe l Distance (ft)
WS PF 2
WS PF 1
Crit PF 1
Ground
S profiles for Q=993, 1033, and 1151 cfs
5
Figure 3. Flow Chart for Outlet Control Computations (HEC-RAS Hydraulic Reference)
6
UNIFORM FLOW CALCULATIONS
Table 1. Uniform flow calculations for approach channel and box culvert
BW (ft) Side slopes (H:V)
Bottom slope (ft/ft)
Normal depth (ft)
Critical depth (ft)
Normal velocity (fps)
Approach channel and transition
Q = 1151 cfs, n = 0.013
10 2:1 0.0189 2.91 5.28 24.96
11 2:1 0.0189 2.80 5.13 24.75
12 0:1 0.0189 3.57 6.59 26.88
Q = 1033 cfs, n = 0.013
10 2:1 0.0189 2.75 4.99 24.21
11 2:1 0.0189 2.64 4.85 23.99
12 0:1 0.0189 3.31 6.13 26.03
Q = 993 cfs, n = 0.013
10 2:1 0.0189 2.69 4.89 23.94
11 2:1 0.0189 2.59 4.74 23.72
12 0:1 0.0189 3.22 5.97 25.72
Concrete Box Culvert
Q = 575.5, n = 0.013 (half of 1151 cfs in each barrel)
6 0:1 0.02 4.11 6.59 23.33
6 0:1 0.081733 2.42 6.59 39.69
Q = 516.5, n = 0.013 (half of 1033 cfs in each barrel)
6 0:1 0.02 3.78 6.13 22.78
6 0:1 0.081733 2.23 6.13 38.53
Q = 496.5, n = 0.013 (half of 993 cfs in each barrel)
6 0:1 0.02 3.67 5.97 22.57
6 0:1 0.081733 2.17 5.97 38.11
7
SPECIFIC ENERGY DIAGRAMS
Specific energy diagrams are shown for the upstream trapezoidal channel and for a 12-
foot rectangular channel (the channel just upstream of the box culvert opening). The energy
diagrams for the 12-foot channel are shown at two flow rates to emphasize how the diagram
changes with increasing flow. The normal depth is plotted on each curve to illustrate the
amount of energy that can be lost without choking the flow. The flow upstream of the culvert
is rapidly varying and cannot follow these diagrams exactly but the diagrams are useful in
explaining what happens to the flow. For example, the difference between the energy at the
normal depth and the critical depth is approximately 4.5 ft for the 12 foot rectangular channel.
Normal velocity is 25.72 fps, resulting in a velocity head (V2/2g) of 10.27 ft. If the loss
coefficient is greater than 4.5/10.27 = 0.44 then the flow will choke.
0
1
2
3
4
5
6
7
8
9
10
11
12
5 6 7 8 9 10 11 12 13 14 15 16
Specific Energy (ft)
Dep
th (f
t)
12 ft rectangular channel
10 ft trapezoidal channel
Q=1151 cfs Q=993 cfs
normal depth
Figure 4. Specific Energy Diagrams for Simplified Channel Sections
8
RESULTS FROM CULVERTMASTER BY HAESTED METHODS
Inlet control
Outlet control
9
Using CulvertMaster, the capacity of the culvert is approximately 700 cfs depending on the
entrance loss coefficient that is used. The value for the allowable headwater was computed
by adding 8.24 ft (the differences
in height shown on the survey) to
the invert. However, after
reviewing photos taken during a
site visit, it is apparent that the
headwall is quite a bit taller than
the side wall.
10
Allowable HW Ke Q (cfs)
74.99 0.20 557
74.99 0.50 487
76.73 0.20 747
76.73 0.50 680
Although the HEC-RAS analysis shows the culvert to have capacity for a developed
flow of 1033 cfs, the analysis is suspect. Traditional culvert analysis does not account for an
upstream supercritical channel. Typically, a designer checks for inlet control and outlet
control. The analysis giving the highest headwater for a design discharge (or the lowest
discharge for the allowable headwater) controls the design. Following that methodology, an
inlet control design should be chosen. However, the typical inlet control design with inlet and
outlet unsubmerged assumes the flow to go through critical depth at the entrance to the
culvert, i.e. the channel upstream is subcritical. As Ven te Chow says in his Open Channel
Hydraulics text (1959, p. 493), “The characteristics of the flow [in a culvert] are very
complicated because the flow is controlled by many variables, including the inlet geometry,
slope, size, roughness, approach and tailwater conditions, etc. Hence, an adequate
determination of the flow through a culvert should be made by laboratory or field
investigations.” Following Chow’s advice, a 1:24 scale model, based on Froude number
similitude, was constructed of plexiglass and wood in a one foot wide by 10 foot long flume
located in the Fluid Mechanics Laboratory in the Department of Civil Engineering at the
University of New Mexico. Following are photos that summarize the experiments.
11
1:24 scale model constructed in flume
Plan view of model with splitter wall in place
12
Oblique view of model with splitter wall in place
No splitter wall, flow height exceeds headwall in vicinity of the wall dividing culvert barrels
(Q=1033 cfs)
13
Splitter wall with square edges, flow height exceeds headwall in vicinity of the splitter wall
(Q= 1033 cfs)
Splitter wall with bullnose, flow does not exceed headwall (Q=1033 cfs)
14
PHYSICAL MODELING SUMMARY
The flume has two built-in pumps that re-circulate the water. To estimate the flow
through the culvert, the normal depth in the culvert was measured. The pumps were adjusted
until the 1033 cfs design flow was achieved. The first experiment included the culvert as
currently constructed with no splitter wall. The flow exceeded the headwall elevation just in
the vicinity of the dividing wall between barrels. To avoid this situation, a parabolic shaped
splitter wall was added to the model. The wall had square edges and actually worsened the
situation of flow exceeding the headwall.
M: 3.5” P: 7.0’
As a rem
further i
rounded
shaped w
wall ext
M: 3.5”P: 7.0’
Parabolic Shaped Splitter Wall X = 0 ~ 14’, Y= 0 ~ 7’ Y = (-0.25 X2+49)0.5
Model: 7.0” / Prototype: 14.0’
edy, the edges were rounded (full bull nose) and a triangular splitter wall extending
nto the flow was created. Both splitter walls worked well. Both splitter walls were
such that the water flow would not come in contact with a sharp edge. The parabolic
all extended the equivalent of 14 feet into the flow; whereas the triangular shaped
ended the equivalent of 21 feet into the flow. The shape and length of the splitter wall
were less important than
making sure that the wall
was rounded.
Model: 10.5” / Prototype: 21.0’
Triangular Shaped Splitter Wall
15
The splitter walls were constructed such that they could be easily inserted and
removed from the model. In doing so, we had several runs where we had not placed the
splitter wall directly in the center, perpendicular to the headwall. When this occurred, one
barrel would fill quickly. The culvert would no longer function as an open channel, the entire
inlet would become submerged, and the flow through the culvert quickly decreased.
RECOMMENDATIONS
The culvert can operate as an open channel if there are ideal conditions in the field.
Less-than-ideal conditions can create large headlosses with the corresponding high velocity
head caused by supercritical flow. The critical depth for the design flow of 1033 cfs is above
the culvert. Thus as headlosses increase, the depth in the channel will approach critical depth
causing the entrance to the box to submerge. Once the entrance is submerged, the capacity of
the culvert (as currently constructed) is reduced to about 550 cfs.
If ideal conditions can be assumed, then a splitter wall (either parabolic or triangular
shaped) should be built, with a bull nose, extending 12 feet into the flow. If ideal conditions
are not expected (as shown below), then there are several options that can be considered to
increase the culvert capacity:
1) Increase the height of the headwall and daylight the elevation back to the channel.
2) Increase the slope on the culvert at the upstream end of the culvert such that the headwater on the throat (rather than the face) will control the flow capacity. This is typically referred to as a slope tapered culvert.
3) Reconstruct the opening to provide a larger area to be used in the orifice equation. This is typically referred to as a side tapered culvert.
4) Some combination of (2) and (3) such that the culvert becomes a slope tapered, side tapered culvert.
16
Photo: Loren Meinz
17