## Preliminary size a ditch

## Preliminary size a ditch

(OP)

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.

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.

## RE: Preliminary size a ditch

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

## RE: Preliminary size a ditch

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.

## RE: Preliminary size a ditch

## RE: Preliminary size a ditch

## RE: Preliminary size a ditch