BRIDGE SCOUR FIELD PROCEDURES

Channel cross-sections are typically obtained by wading.  In cases where the channel depth is too great, a simple kayak has been appropriately equipped with a fathometer and prism rod to acquire channel bathymetry.  A simple conversion based upon individual load and displacement is subsequently applied to the height of the prism rod which extends vertically from the bow of the boat.  The boat operator simply adds the converted rod height to the displayed depth.

Sediment samples are obtained from the channel overbank and the channel bed.  The median grain size diameter (D50) is used in the scour analysis to quantify channel bed and overbank contraction scour and local pier scour.  Grain size diameters are typically determined from a settling tube (VA) analysis.  The finer sediment (typically silts) often requires a hydrometer test to determine the median diameter.   In specific cases where footing exposure is evident, it is necessary to consider a D84 grain size diameter which acknowledges the possible armoring effect of channel beds.

Surveying is accomplished by use of a Total Station instrument.  Sections including exit, full valley, bridge, road, and approach are required surveyed sections used for contracted opening computations by the one-dimensional surface-water model WSPRO.