Ditch Flooding Impact from additional 1-4 CFS

[Builder604]

We are working on a project that involves a 48" RCP that discharges to an open ditch for a few hundred feet to another 48" RCP under a road. The 48" RCP is at or near capacity for most storms resulting in approximately 111 CFS. There is a problem already with flooding at the 48" RCP under the road. Our project is over designed to detain more than required The 2-yr, 10-yr, 25-yr and 100-yr storm flows are 1.05, 1.22, 1.78 and 4.22 respectively. Assuming that the 48” RCP is at/near capacity and flowing full at each storm the percent increase in CFS in the ditch for the 2-yr, 10-yr, 25-yr and 100-yr storms is only 0.95%, 1.10%, 1.60% and 3.80% respectively.
The contract review engineer for the town made the following comment: Please provide written verification that the impact of the additional stormwater discharging into the existing ditch has been sufficiently analyzed, and that no adverse impacts to the downstream properties are expected by the development.
Our initial response is that the 2-25 year storms would have a negligible impact on the ditch and the 100-yr storm flow could cause a slight increase in flooding potential.
about a 200' section of the ditch is flat and has quite a bit of debris in it which the city is requiring to be cleaned out and regraded. Cleaning out the ditch will cause flooding at the road more quickly which will make it appear that the flow from our site is causing the flooding.
What are your opinions on this?

 

[gbam]

Builder604 - Have you established the exisitng condition (Pre-Project) hydraulics for the ditch? If not do that first and then evaluate Existing Condition with a cleaned/graded ditch. Now that your base is set evaluate the change in water surface(s) in the ditch with the changes from your project. Now provide in your report a comparison of the existing condition & proposed and the exisitng "clean ditch vs proposed to show that there are no significant changes. I have found that reviewers do not like to take "our word" that there are no impacts; they like to see that there are none.