Preliminary size a ditch

[slange]

I need to preliminary size a ditch to contain the floodplain and floodway. I used Chezy Mannings to calculate, and it calculated a Q well over what my hydrology calculated. I then placed in the Hec Ras model and it over tops the ditch, roadway, etc. I know that the downstream structure will create a tailwater condition but not sure why the ditch doesn't contain the water. I did a 6' deep trapezoidal channel with 3:1 side slopes and 75' wide. My Hydrology states that there is 1228 cfs being directed to the channel.
Chezy Mannings assumptions
Depth of ditch 5' (want about a foot of freeboard)
ditch bottom width: 75'
sideslopes: 3:1
n: .025 Natural channel
S: .004 Ft/Ft
A: 2(5*15/2)+5*75= 450sqft
R: A/P P=(sqrt(5^2+15^2))2+75=106.62 R=450/106.62=4.22

Q=1.49/.025*(450)*(4.22)^.67*(.004)^.5= 4450.9 cfs
so the ditch should be able to handle 4451 cfs correct or am I doing something wrong. I appreciate any help.

 

[cvg]

you already answered your own question.

I know that the downstream structure will create a tailwater condition

the capacity is limited by the downstream structure which I assume is a too small culvert or bridge. normal depth calculation of the ditch upstream of the bridge is not sufficient to determine the capacity. Your HECRAS model is likely correct

 

[slange]

Thanks,
I was thinking about it this weekend. the condition is a backwater/tailwater condition and there is a small amount of slope on the ditch so the water surface profile will be flat and the velocity slow. So, increasing the size of the ditch area will not have an effect. Basically, the ditch is acting as a pond.

 

[cvg]

your flow rate is controlled by the culvert which is the bottleneck, enlarge the culvert and you could get more capacity, assuming you don't have more small culverts further down...

 

[slange]

agreed that is what is messing up my WSE, unfortunately I can't change the size because it is a DOT culvert and they installed it without doing any analysis or anything.

 

[gbam]

Have you evaluated the pre-project condition? It may show that you do not have an increase resulting form your design. Just a thought.