Flume Experiments on the Geometry of Local Scour Holes at Boulder-Like
Obstructions during Unsteady Flow Conditions: Part II - Enlargement
Processes and the Impact of Hydrograph Chronology
Abstract
Local scouring is typically a bed load transport phenomenon that is well
studied at bridge piers while less attention is given to the enlargement
processes of local scour holes at natural instream obstacles like
boulders. This specifically applies to changing hydraulic boundary
conditions at the obstacle in the course of flood hydrographs while
physical modelling in flumes offers the advantage that hydraulic
boundary conditions can be systematically varied. This second companion
paper yields novel experimental data on the role of hydrographs of
different shape and flow intensity onto enlargement processes of local
scour holes at boulder-like obstructions and evaluates the impact of
hydrographs chronology on the local scour hole geometry. In total 48
unscaled process-focussed flume experiments at a range of flows
(subcritical, clear-water and live-bed conditions) were performed.
Experimental results revealed that (1) the enlargement of local scour
hole length and width depends on scour hole depth rather than on actual
flow conditions while an empirical model is presented that
mechanistically predicts enlargement in local scour length and width
based on (i) scour depth, (ii) the inclination of scour slopes and (iii)
the planform area of the scour hole bottom. (2) In the course of
subsequent hydrographs memory effects (i.e. the dependence on antecedent
sizes of local scour depth, length, and width) have to be considered.
This contribution improves the understanding of local scouring at
boulder-like obstacles exposed to hydrographs while it is speculated
that the identified memory effect may be utilized for hydraulic
interpretation at field condition.