Roadside Ditches
Roadside Ditches
(OP)
I was reading some literature that the minimum slope of a roadside ditch was normally in the range of .25% to .50%. This is to prevent sedimentation and the growth of unwanted vegatation.
Can someone explain why we would not want sedimentation in the roadside ditch? Isnt a function of the roadside ditch to capture sediment so it is no deposited in waterways?
Can someone explain why we would not want sedimentation in the roadside ditch? Isnt a function of the roadside ditch to capture sediment so it is no deposited in waterways?





RE: Roadside Ditches
RE: Roadside Ditches
Key things to consider:
1) What is the purpose of the ditch? Conveyance, treatment, habitat, etc
2)Soil Conditions
3)Vegetation
These are a few of many things to consider but ultimately it depends your design criteria.
Hope this helps.
RE: Roadside Ditches
My home has a 0.3% roadside swale. The rut from a lawn tractor in damp conditions can cause ponding for 15ft at the next rain event. More significant alterations(driveways, sedimentation etc.) over time cause significant impact to drainage (or the lack therof.)
In my instance, the total swale depth is only 0.5ft. If one has enough depth & capacity, the flatter grade may not be as significant.
RE: Roadside Ditches
RE: Roadside Ditches
For instance..
Project I'm working on right now in North Carolina has no slope requirements, but it has a *maximum* 1 ft/s velocity standard for grassed swales, to ensure water quality treatment out of them, which actually works out to less than 0.3% slope for many Mannings roughness assumptions. Then again, that's an NCDENR standard for water quality BMPs, not an NCDOT standard for water conveyance.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Roadside Ditches
This statement is confusing to me... A lot of engineers/reviewers confuse erosion and sedimentation. Sedimentation is the result of erosion. Grass swale BMPs actually encourage sedimentation within the swale by requiring very shallow slopes (0.2%-0.5%). You would limit "erosion" of the actual swale by limiting the ditch grades, but you would definitly encourage sedimentation.
Assuming this standard is for drainage conveyance, not water quality, and you want the water to flow (to say a downstream culvert), you should design the grass swale at a 2% minimum. 1% will work but will require more water/runoff to push the water through the inevitable low areas within the swale/ditch.
RE: Roadside Ditches
So again, it all depends on who you're designing your ditch for, and what their standard says.
...then you've got other complications, such as old DOT ditches being reclassified as Wetlands or State Waters, which then prevents people from maintaining them...
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com