JEawb
Civil/Environmental
- Jul 10, 2009
- 1
I have a project where there is a regional ditch running through the site. We have two road crossings on our plans. On our first submittal, we sized the upstream culvert to convey the 25 year storm via HY-8. We showed it in hydrocad as piece of the TC path. We did model the ditch downstream and between the roads in hydrocad to ensure we accounted for tailwater for our pond discharges. The reviewer asked for us to "add a pond/analysis point at the upstream road crossing to ensure that there are no adverse impacts upstream."
I have having some trouble deciding how to show the pre-development water elevation in the ditch. I cut a section along the property line, and then created a pond with a custom weir that follows that section. Unfortunately, the ditch is not very deep (only 2-3' deep in the defined ditch, and 10-12' wide). So the storage/spillway extends very far beyond the banks of the ditch in the model. So the ditch itself is at elevations 44-46. The storage needs to go out between 47 and 48, and the weir width at the 48 contour is hundreds of feet wide. Meaning that virtually any culvert will cause an increase in upstream ponding water based on the pre-development model just because the weir is so wide as it is currently modeled. I question if a pond is genuinely appropriate here anyway because I think the capacity of the channel itself will constrict the upstream flow and provide additional storage/delays along the way.
To cause a genuine non-impact, I would need to widen the ditch (which is in wetlands) and provide several box culverts, which makes little sense.
I flirted with the idea of showing the upstream ditch as a reach to establish a depth, and then trying to maintain that depth when it is modeled as a pond/with a culvert. I am afraid the reviewer will view that as an apples to oranges comparison. Am I pretty much stuck looking to use HEC-RAS or another software for this analysis? Is there another method to set up the pre-development conditions that I'm overlooking?
I have having some trouble deciding how to show the pre-development water elevation in the ditch. I cut a section along the property line, and then created a pond with a custom weir that follows that section. Unfortunately, the ditch is not very deep (only 2-3' deep in the defined ditch, and 10-12' wide). So the storage/spillway extends very far beyond the banks of the ditch in the model. So the ditch itself is at elevations 44-46. The storage needs to go out between 47 and 48, and the weir width at the 48 contour is hundreds of feet wide. Meaning that virtually any culvert will cause an increase in upstream ponding water based on the pre-development model just because the weir is so wide as it is currently modeled. I question if a pond is genuinely appropriate here anyway because I think the capacity of the channel itself will constrict the upstream flow and provide additional storage/delays along the way.
To cause a genuine non-impact, I would need to widen the ditch (which is in wetlands) and provide several box culverts, which makes little sense.
I flirted with the idea of showing the upstream ditch as a reach to establish a depth, and then trying to maintain that depth when it is modeled as a pond/with a culvert. I am afraid the reviewer will view that as an apples to oranges comparison. Am I pretty much stuck looking to use HEC-RAS or another software for this analysis? Is there another method to set up the pre-development conditions that I'm overlooking?