Contraction and Expansion Reach Lengths for Bridges

[Kim Kallman]

Is anyone following HEC's method of determing contraction and expansion reach lengths and coefficients for bridges?

If so, I've got several questions for you.

What 'n' values do you use, for your overbank and channel in the expansion reach length equation, if they vary from the downstream face to the expanded area? I am trying to model a bridge right now where the n-values range from .045 - .075 in the overbank over a small region. At first I was using the 'n' value for the downstream face, but I ended up with an expansion reach length of 2000' and at 2000' the 'n' value is .075. Should I just use an average?

Also, for this same bridge, I have another smaller bridge within this 2000' expanded reach. Do I just assume that the larger bridge's expanded reach length takes presendence and ignore the expanded reach length of the smaller bridge that falls within the 2000'?

With this 2000' span I am going to need additional sections between the downstream face and the expanded reach. What is the best way to determine the ineffective flow station and elevation for these sections?

Hopefully someone can help!

Thanks - Kim

[zahi]

Through may reading to the

Application Guide - Example 3 Single culvert (Multiple Identical Barrels)

There is explanation In pg. 3-6 "Estimating of the expansion reach length".

In Fact I tried to understand how the value "B- total floodplain width = 145" and how the Average length of the side obstruction was estimated " (the values 925, 1070) .

Unfortunately i did not under stand where those numbers came from?

[Johnlim]

i can understand part of your problem.

ignoring the second bridge in the expansion reach is not a good options but in fact you may treat it separately by considering dual bridge effect . you can get all those paper in hydraulic bulletin or journals and it is not mentioned in any Hec manual

under the bridge you may define n values as 0.045 and the reach downstream as 0.075. the algoritm of the model take charge of that. there is no necessity to average it out.

Just some thoughts. hope it help

cheers