Download page Step 7: Select a Reasonable Scour Depth Based on Calculations, Regional Geology, and Engineering Judgement.

Step 7: Select a Reasonable Scour Depth Based on Calculations, Regional Geology, and Engineering Judgement

The scour equations are simple and empirical, regressions of data from systems that likely diverge from the system you are analyzing in several ways. The scour calculator includes a suite of these equations – in part – to help users and managers to understand the uncertainty in these calculations. The calculator does not automatically combine results into a single scour metric (e.g. average, max, or geometric mean) because these results should inform a broader decision and design process that also includes qualitative information, system observations, expert judgment, and common sense. The ensemble scour calculations provide a range of results that can bound the design scour depth (and communicate uncertainty). But the final scour depth requires the project engineer to take responsibility for the scour depth decision.

These equations are best used in the following decision approach:

1. Exclude equations that are not applicable to the setting or generate unrealistic results (e.g. negative scour). This requires review of the assumptions associated with each equation and deciding if each equation is appropriate for the design setting.

2. Report the median value, and the range. Look for clusters of results (several equations with similar scour depths) but recognize that some equations artificially generate similar results because they have similar forms or structure. Because some equations tend to produce similar results, the scour calculations cannot be treated as "independent observations" for statistical analysis.

3. Use these results with other qualitative assessments, local system expertise, and subject matter experts to agree on a design depth that reflects the likely scour depth, the risk tolerance, and the desired factor of safety.