Outslope Road Profile

[PEVT]

I am designing an access road and stormwater system for a small residential development. The access road goes up at an 8% slope and I would like to make an effort to minimize the accumulation of stormwater to the cut side ditch and also encourage the sheet flow to an adjacent gently sloped grass area. I would like to know if any of you have any experience with outsloping of the road cross section. I have found an abundance of literature regarding this type of cross slope grading for rural and forest gravel roads, but not much for very low volume paved roads. Using an outsloping profile would allow me to substantially reduce the ditch runoff, compared to that of a normally crowned road, and thus reduce culvert sizes and spacing. For this particular project the permitting authority is requiring a curb on the sidewalk side, but I can see a real benefit for future projects if a curb is not present. I have attached a basic sketch, just for comments. Thank You.

 

[Debaser]

In terms of the second profile, do you use combined kerb drains such as these?

http://www.aco.co.uk/product_detail.php?id=3

One of the pictures (4th row down 2nd picture from left, also bottom row 3rd from left) shows a detail of a kerb with a pipe from the rear draining to a swale, a detail I have seen modified to take account of a footway between the carriageway and swale.

 

[ACtrafficengr]

We have a two lane road carrying 20,000+ vehicles per day that has a constant slope and no crown. One side gets the runoff from 32' of pavement (2 12 ft lanes + 1 8 ft shoulder). It was built with the intention of eventually building two more lanes on the high side, but that never happened.

We haven't had any safety problems due to wet or icy pavement, even when traffic was still riding on the original 40 year old concrete.

If it works at these volumes, it should work for you.

What soil type do you have? Could you put an infiltration swale between the sidewalk and edge of pavement? We have a few sidewalks about 5 feet from the EOP with no curb, but the soil is well- to excessively-drained sand.

Will that 1:1 slope at the right edge be stable?

Debaser, that's an interesting system, but how do you clean them out? We get lots of leaf debris and deicing grit.

 

[Debaser]

@ ACtrafficengr,

The kerbs come in a number of types, including ones with a metal cover running the length of the kerb that can be removed for easy access for cleaning, to be used at intervals in the kerb run. In addition the holes in the face of the units are fairly large and will generally cope with grit and leaf litter unless left unmaintained for a numbeer of seasons.

(At the top right of that webpage is a link to download the latest brochure, see also the attachment below).

 

[GoldDredger]

I am a bit dubious with either design.

With option one, I see you are trying to get sheet flow from left to right, over the shoulder then down the grassy side. I think what will happen over time is the water won't sheet flow, but eventually channelize somewhere and start to scour the grass down the slope at some point. Also you are forcing your sidewalk to the right side, which you indicated wasn't optimum for whatever reason.

With option 2 you have a curb on the low side, along with the sidewalk. You can install some curb cuts (leave-outs), with a sidewalk under drain to get flows to the hillside. At this point you will have channelized flow though, not sheet flow, and I think you've overcomplicated the design at that point.

I don't think it is a good idea to have flows over a sidewalk in any case.

IMO, I would cross slope your road as in cases 1 & 2 to reduce flow to the upside channel. On the downhill side, I would have curb and sidewalk, however I would add a curb inlet and pipe the collected flows under the sidewalk some ways down the hillside.

 

[PEVT]

Thank you all for your input.

Debaser, thanks for the improved curbing product idea to consider.

ACtrafficengr, thank you for sharing your experience. I would like to include a grass lined swale between pavement and sidewalk, as you suggest. Potentially a nice improvement. Soil is well drained sand and loamy sand, so it would work really well for low to moderate storm events. I have been a little reluctant to include one because the grade of the road is 8% and I worry a bit that it might channelize between the two adjacent hard surfaces. Perhaps I would be best served to use the grass lined swale and install catch basins within it, dropping into the culvert crossing the road, at 100ft or so intervals. This would allow as much infiltration as could occur, but remove surplus stormwater and prevent a channel from forming during larger events.

GoldDredger, thanks for the input. I will take your suggestion to add a collection system, since I think you are correct that it may overcomplicate the design if I were to use interupted curbs and shallow drains beneath the sidewalk.

In all cases I am leaning away from the sidewalk on the ditch, or high, side as an option and it seems you all agree.

 

[MiketheEngineer]

Hope you don't get much snow or ice...

 

[civilman72]

Adding on to Mike's question: are there any sharp horizontal curves?

Combination of constant cross-slope (no crown), 8% longitudinal grade, sharp curve and icy conditions (which will be exacerbated by constant cross-slope) would concern me, particularly if you put the sidewalk on the downhill side.

 

[cnstengineer]

Option #1 looks like the better option to me. If you are looking to elimiate the curb you can possible install the sidewalk with a haunch on the road side and then pave the road. The authority may be okay with this since it is just an access road that will not get much use. If you are looking to move the sidewalk over and put the swale between the road and the sidewalk, keep in mind the constructability of the swale. The swale should be wide enough for machine grading. The difference in cost of grading/topsoiling a swale with a small machine (skid loader, mini exc, etc.) compared to a dozer may be just as much as the cost to install a curb.

 

[gbam]

I'll chime in with my two cents: I like curb in this situation to provide, at least in concept, an implied barrier between the roadway and the sidewalk, unless you plan on setting the sidewalk far enough back from the travel lane (safe Distance). One can use curb cuts or sidewalk scuppers to collect and direct runoff from the street. A concern is the transition from the gentle slope to the 1:1 slope; this may induce head-cutting even in a sheetflow condition (Not knowing the amount of runoff). I think curb with scuppers and spillways down the 1:1 slope would be prudent. You may even be able to flatten that 1:1 slope. Just my two Cents!