Roadside Ditches

[martin888888]

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?

 

[francesca]

Once upon a time all we cared about was maintenance. Now we also care about water quality. When I worked in Tennessee, it was 1% or you concrete lined it and 0.5% was minimum. Standing water was frowned upon. (Those pesky mosquitoes/karst features.) Then we got NPDES Phase 2 and call it a bioswale and all bets are off.

 

[Ryb01]

I don't think that there is a right or wrong answer to your question. A number of things would come into play to determine whether the ditch is graded properly. As francesca, hinted towards, there are a number of municipalities encouraging reduced grading within the ditch to promote infiltration and water quality. LID's are starting to show up more frequently within road right of ways.

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.

 

[TerryScan]

I design in a coastal plain area where things are quite flat. Absolute minimum ditch slopes are 0.3%, but I prefer 0.5%.

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.

 

[gbam]

I see a roadside swale as an onsite conveyance system which is used to capture rural roadway runoff and direct it safely to an outfall. It is no different than a closed conduit system for pavement drainage in urban conditions. Where things get confusing is when they are used to capture offsite flow so lets only address roadway drainage here. If you confirm the primary use of the swale as roadway drainage system then your concern should be with the impact to the roadway and its safety for the traveling public. If your swale is erosive then the roadway prism can be undermined. If it is agrading then sedimentation implies loss of capacity over time which, without maintenance, can cause flooding of the pavement and negatively impact the roadway subbase. A roadside swale is an intregal part of the roadway; the designer needs to confirm that the impact to your roadway is minimal.

 

[beej67]

As mentioned above, it depends entirely on who wrote your design criteria and what they wrote it for.

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.



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[civilman72]

"This is to prevent sedimentation and the growth of unwanted vegatation."

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.

 

[beej67]

I could definitely see how a DOT ditch design standard from the "old school" might want to discourage sedimentation, since sedimentation is an increased maintenance cost. Minimum slopes or minimum velocities for pipe design are usually to ensure "clean out velocity" which is another way of saying "prevent sedimentation."

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...

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http://www.campbellcivil.com