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Hydraulics Lab: 2004-2005

Condeshire Box Culvert near Rio Bravo


Full modeling report                                                    



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 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.