Scour Calculator: Computing Launchable Volume

Toe scour and undercutting are common bank protection failure modes. Rivers often scour during floods. Scour, local erosion, often concentrated at the outside toe of a bend, can dig beneath placed rock. Riprap can collapse into a scour hole, after which, it no longer protects the bank.Because toe scour is one of the most common bank protection failure modes, toe protection is a critical bank protection design principle. There are several approaches to protecting the toe of a rock protection or bioengineering project. Two primary approaches include:

1. Burying toe protection to the projected scour depth.
2. Placing "launchable" toe protection, sacrificial rock that will dynamically line a scour hole.Both these toe protection designs require an estimate of the scour depth. The project team must place a keyed toe deeper than the projected scour depth or compute the correct volume of launchable rock to line the scour hole. In both cases, estimating the scour depth is critical to the design. However, local scour and bend scour are complex processes that are highly sensitive to the local soil properties and the multi-dimensional local flow field. Physical modeling or careful, 3D/CFD modeling paired with extensive soil data collection are the only way to predict local scour depth with any confidence, and even these approaches can have significant uncertainty if they are not calibrated.

Therefore, the riprap and scour calculator includes a suite of simple scour calculators, that take an ensemble approach to scour calculations. The scour calculator applies the most widely used local and bend scour equations to the riprap design cross section, to provide the range of scour depths expected from these empirical approaches. The design team should filter these results, ignoring equations that are not applicable or results that are unreasonable, and use the suite of results with engineering judgement, field observations, expert elicitation, geologic controls, and other quantitative and qualitative metrics to determine a maximum, likely, scour depth.