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Scour- Thoughts on Lacey, Blench, and Neill equations?

  • 1.  Scour- Thoughts on Lacey, Blench, and Neill equations?

    Posted 12-05-2019 03:35 PM
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    A recent paper (see reference below and attachment) assessed the differences between the Neill, Blench, and Lacey regime equations for predicting scour. The paper looked at the differences between the equations as reported in USBR and NRCS design manuals and the equations as originally documented.

    Following this paper, have any of you modified your approaches to using these equations?

    Reference:
    Richards, D.L. (2018). "Assessment of regime equations for predicting general scour", Proc., World Environmental and Water Resources Congress, Minneapolis, June 3-7.

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    Richard Evans P.E., M.ASCE
    Project Engineer
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  • 2.  RE: Scour- Thoughts on Lacey, Blench, and Neill equations?

    Posted 12-10-2019 11:16 PM
    Hi Rick,  we have not modified our approach.  That being said, all of the equations give approximate answers and we have never relied upon a single equation for our estimates. We usually incorporate results from different equations (including those in Richards' article) as well as experience on similar projects in the past in our recommendations....

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    Martin Teal HG, P.E., D.WRE, F.ASCE
    Vice President
    WEST Consultants, Inc.
    San Diego CA
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  • 3.  RE: Scour- Thoughts on Lacey, Blench, and Neill equations?

    Posted 12-11-2019 04:24 PM
    This topic of stream channel design (have not looked into it for long time) is very interesting. Thanks to both Richard and Martin for bringing in the topic and discussing them. Design processes of channels in regime (where things are in balance over longterm), have yielded many useful and simple behavioral models (more on the USBR website). These relations do not come with a erosion/scour thresholds. But I always had disagreements on the definitions of, and distinctions between erosion and scour.

    In coastal works, similar confusion persists. Scour results from nearfield motions of water (mostly obstruction induced), or vortices that scoop out sediments from bed or bank. Erosion, on the other hand is a farfield phenomenon and is due to straight shearing action. The closest analogies of the two processes are like this: the scouring is like the action of a tornado - while erosion is like the ground-parallel wind picking-up sand.

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    Dr. Dilip Barua, Ph.D, P.Eng, M. ASCE
    Vancouver, BC, Canada
    Website: https://widecanvas.weebly.com
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  • 4.  RE: Scour- Thoughts on Lacey, Blench, and Neill equations?

    Posted 12-14-2019 10:36 PM
    Hello,
    I have used the empirical regime equations to predict regime channel flood flow width and depth since the 1970's with good results, and less often to predict scour depths during flood emergency operations and prior to computers.The  challenge is that we often do not know what the actual scour depths are, so one can seldom confirm if scour predictions are reasonable and which method is best for a given site condition. However, during field response efforts during Hurricane Irene in 2012, Lacey scour predictions were similar to observations of failures and non-failures.

    The regime scour equations typically first predict regime mean flood depths (sometimes the maximum flood depths), then multiply the values by a local coefficient that  ranges from 0 to 2.5 to predict maximum total scour depths. The published coefficient values  (Richards 2017) for the Lacey equation appear to agree with Lacey's 1930 paper which I have used, but I was unable to confirm the coefficients listed for Blench (from Pemberton, table 7), the latter appear to be lower than in the Blench papers.

    In either case, we like to confirm scour predictions with quantitative analytical models such as SAM and RAS, but here the limitation is that we seldom have confidence in sediment loads and concentrations, so although computer models are great, we are limited by input data and can still make preliminary field analysis with a slide rule.

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    James Mac Broom P.E., M.ASCE
    V P
    Milone & Macbroom Eng Inc
    Wolcott CT
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  • 5.  RE: Scour- Thoughts on Lacey, Blench, and Neill equations?

    Posted 12-16-2019 11:36 AM
    James-

    It's interesting that the Lacey scour predictions came out similar to observations after Hurricane Irene. Was this based on your internal work or was there a study done?

    As you probably know, the post-flood scour that you see is usually less than the peak scour that occurs during the peak of the storm. So again, it does seem very hard to substantiate the maximum scour.


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    Richard Evans P.E., M.ASCE
    Project Engineer
    Bradford VT
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