What is scouring, Bridge Scouring? How to calculate Scour Depth?

Scouring can be defined as a process due to which the particles of the soil or rock around the periphery of the abutment or pier of the highway bridge spanning over a water body, gets eroded and removed over a certain depth called scour depth. Scouring usually occurs when the velocity of the flowing water increases or crosses the limiting value that the soil particles can easily handle.

Scour at bridge Pier and abutment

When Scouring Happens? Causes?

Scouring can be easily observed during the flood being passed through that water body. Scouring is initiated at the nose of the piers or at the sharp bends. Estimation of scour depth is very important for the design of foundation for abutment or pier. Scouring compromises the structural integrity and thus causes failure of the structure. It has been estimated that over 60 % of the highway bridges are being collapsed due to scouring.

When the discharge within the water body passes the allowed discharge; which is usually the case when flood is there; the scour is initiated; the scour depth can easily be obtained after the flood by finding the depth of the scour in reference with the surroundings or existing structures. If this is not possible the mean depth of scour may be easily obtained by a mathematical formula for natural streams in the alluvial depth.

How to Calculate or Estimate Scour Depth?

Hydraulic Engineering Circular (HEC) has published a manual in which the methods are mentioned some of which are empirical equations or graphs to estimate the scour depth based on the discharge, flow velocity or flow depth etc.

Zeller Equation for estimation of scour depth

If the bed is sand bed streams we can use the empirical Zeller Equation for estimation of scour depth;

Ygs = Ymax [(0.0685 Vm^0.8) / (Yh^0.4 Se ^0.3)-1]
Where Ygs = general scour depth (ft), Ymax is maximum depth of flow, Vm is the average velocity of flow(ft/sec), Yh is hydraulic depth of flow (ft) and Se is the energy slope (or bed slope for uniform flow), (ft)
From actual field measuring scour depth charts to estimate scour depth;

Estimate Scour Depth

This method is applicable only for coarser sands where the bed slope ranges from 0.004 ft to 0.008 ft.
Other empirical methods are also there like Neil, Blench and Lacey for estimation of the scour depth.

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