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.