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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Original Message:

Sent: 12-05-2019 14:34

From: Richard Evans

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

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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