Flume Experiments on the Geometry of Local Scour Holes at Boulder-Like
Obstacles during unsteady flow conditions: Part I ¬ Hydrograph Shape and
Intensity
Abstract
Local scouring is a complex phenomenon that is well studied at bridge
piers while less attention is given to the time-dependent evolution of
local scour holes at natural instream obstacles like boulders. This
specifically applies to changing hydraulic boundary conditions at the
obstacle in course of flood hydrographs while physical modelling in
flumes offers the advantage that hydraulic boundary conditions can be
systematically varied. This first companion paper yields novel
experimental data on the impact of hydrograph shape and flow intensity
on local scour hole formation and dynamics at boulder-like obstructions.
Experimental results revealed that (1) intense incision and enlargement
of local scour holes occurs on the rising limb irrespective of
hydrograph shape at shallow flow depth that was comparable to the size
of the obstacle while the impact of peak flow was negligible due to
submergence of the obstacle. (2) Sediment supply due to general
mobilization at high flow intensities partially refilled the emerging
local scour holes and reduced the size of the geometrical length scales.
(3) Two temporal stages of local scouring could be modelled based on
statistical analysis of geometrical relations (depth to length, and
depth to width) in time that were consistent to prior observations on
constant hydraulic boundary conditions. This contribution improves the
understanding of local scouring at boulder-like obstacles exposed to
hydrographs while it is speculated that the identified length and time
scales may be utilized for hydraulic interpretation at field condition.