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Condeshire Box Culvert near Rio Bravo**

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SUMMARY

The Condeshire
box culvert, located near Rio Bravo just east of 98^{th} 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 conditions. Furthermore, the model demonstrated that if the inlet
becomes submerged, the capacity of the culvert will decrease dramatically.

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.