Near the western limit of Boulder, Colorado, stands the Justice Center, a large new building used jointly by city and county. Its base lies embedded in alluvium of the Boulder Creek flood plain. Excavation occurred at two different times: first in the late 1950s for a structure that was never completed, then again in 1975 for the present building. Remaining as a legacy of both digging efforts are boulders that were too large for crushing or removal (Fig. 1). We measured the 89 largest boulders whose three axes were visible (see appendix for axial dimensions and Corey shape factors). The coarsest 25% of this sample had a mean intermediate dimension of 188 cm (6.2 ft), a value we will use for grain size of the coarse alluvial fraction. The largest boulder had an intermediate diameter of 230 cm (7.7 ft).
What kind of flows are needed to move 188-cm (6.2-ft) boulders? This study attempts to answer the question. It is a question of practical importance to communities such as Boulder which are concerned with the effects of exceptional floods.