Scour Calcs

[MRM]

I'm involved with the design of a new bridge/culvert structure over a small creek. Although the creek is typically small, say flow rate is 15 to 25cfs, the topography and other characteristics of the watershed indicate that during a 50 or 100 year storm event, the flow rate will be on the order of 450cfs.

I'm looking into estimating scour and was wondering if you folks had any words of wisdom for me. I've been using two different methods; 1) USDOT HEC 18, and 2) Army Corp method. Both are giving me similar estimates for equilibrium scour depth during a storm as I described (which is already a miracle), but It's such a complex event, that I wonder what methods you have used. What are your opinions of the methods I've listed?

Thanks for your input.

 

[VAD]

MRM

Are you contemplating using a pier in the creek. From your description you will likely use a single span bridge. For such a structure, and an opening that is narrow, lateral scour would be important as well and could impact your headslopes. Designed heavy riprap would be required as well as posible setback of your abutments depending on prevailing soil conditions etc

If you are using a culvert structure, you will no doubt be designing riprap protection to prevent scour before and at your structure location. Apart from knowing the likely depth of scour the culvert needs to be designed to accommodate the 1 in 100 year or 1 in 50 year flood event in terms of flow capacity.

What you need to do is to obtain some historic records of the site and determine first whether the creek was subjected to such flows previously. Perhaps your scour depth was achieved already. Heve you done any stream bed profiles upstream and downstream of your proposed structure.

Hydrotechnical Engineering (Hydraulics and Hydrology) is somewhat similar to Geotechnical Engineering and certain base line information is required re use of aerial photos etc. Calculations often come last.

With respect to your method of calculation, I would assume that they are satisfactory.

There is a fairly well documented text on scour by some experts. I cannot remember the names but they have reviewed all possible approaches to computations and outline the issues associated with constrictions etc. I think the authors were Aussies or From New Zealand.

Regards

 

[MRM]

VAD,
The structure is indeed a single span bridge with no center pier. Thanks for your helpful comments. As it turns out, the creek in question has been reviewed or modeled in the past in a comprehensive study. So that's one tool we do have. If you recall any of the scour papers/texts, please pass those titles on.

My biggest question at this point is this; to me, "equilibrium scour depth" would be defined as the depth of water required to accomodate a given flow, at a given velocity, with a given size streambed sediment with no further tendency for substantial downstream particle movement, by saltation or otherwise.

If so, that tells me that if the current water level during baseflow conditions is at 595 feet and is 2 feet deep (elev. 593), then during a 100 year storm if the water rises to 599 feet and 6 feet of "scour" or more accurately "equilibrium scour" is anticipated, the actual new elevation of the stream bottom would also be 593 feet. That is, unchanged. Or, would you interpret that 6 feet of scour to be below the current stream bottom at 593-6ft=587feet? The two methods I mentioned in the original post seem to indicate the former. I'd like to get some backup to see if my nomeclature interpretation is correct.

 

[MRM]

I have partially answered my question, I think. Based on some better diagrams I found, the scour depth appears to be measured from the bottom of the existing stream bed using HEC 18. I'm still looking at the scour baseline or datum used in the Army Corp method. It probably has to be the same, or else the methods would be WAY different. Still open for comments.