Diversion of Watercourse/culvert by introduction of new culvert

[Graduate engineer]

I am a graduate engineer and I have a task to provide the most sustainable solution that provides both a safe, cost effective and sustainable solution having minimal impact on the existing watercourse. My question is whether I can provide a steep gradient of 1:5











at the inlet of the culvert and then have a linear gradient throughout the rest. The new culvert has a proposed public road over it. How much cover does the culvert require (according to UK regulations it is 1.2m cover but does that include the thickness of the wall? Also is there any other things I have to take into consideration such as Scour protection at the outlet and how to calculate these. What about scour protection at the inlet? The road has also trees planted on an ireland style road where the road narrows? Any answers are much appreciated. Thank you! If you require more detailed information to assist in your answers please do let me know.

 

[saikee119]

I have not designed such a culvert section before so I could only talk about the general points. Before I retired I did have a lot to do with massive culverts systems in power plants.

I believe the 1.2m cover is to the top of the culvert surface if the backfill is soil. This is for the protection of the services from damaging by freezing. You may be able to reduce it within reason if you use structural backfill say lean concrete and beef up the culvert roof for the justification.

From the hydraulic point of view the 1:5 gradient of you culvert base slab is governed by the change of gradient upstream, gradient downstream as well as the cross sectional area.

For example you are mostly likely unable to provide original width of the water course by restricting the flow into a narrower man-made box. You can mitigate the scour effect by having an apron slab before and possibly after the culvert. In any case I would expect the culvert to marry in with the water course with two wing walls so as to fan the flow from a wide stream into the culvert and then widen it out back to the natural water course.

I can't see how anyone, including yourself, can answer your question without carrying a profile calculation. Depending on the downstream slope after the culvert you may have a hydraulic jump then so the apron will have to be designed to withstand it.

You are the designer so it is your god given right to arrange normal, subcritical, critical and supercritical flows in your system. You get this from profile computation which can be done by a spreadsheet. You will then understand how your system behaves and where the risks lie.

Without looking at any code I would say your culvert has to withstand 1 in 50 to 1 in 100 years flood.

 

[GeoEnvGuy]

I don't work in the UK but there should be a design manual for roads and bridges which should outline the design process. Replacement culverts essentially look to see how the last one performed and make adjustments. New culverts require a hydrology assessment and then determine the size required. The construction specifications are all standard road specifications that you need to arrange.

 

[ACtrafficengr]

That steep a gradient would prevent aquatic organism passage, which probably would not satisfy your minimum watercourse impact criterion. That may not be an issue in the UK, but here in NY State, it would only be allowed if no other solution would work.

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/