A FIELD EXERCISE ON FLOW-REGIME BEDFORMS IN SILICICLASTIC SAND, SCRIPPS COASTAL RESERVE, LA JOLLA, CALIFORNIA

In a sedimentology course for undergraduate and graduate geology students, I emphasized the flow-regime concept; how ripple-scale bedforms grow and transform at the sediment-water interface. Students are exposed to an aspect of physical sedimentology and become engaged in a simple field exercise that enables them to observe the 2-dimensional mechanics and dynamics of bedform development in the flow regime at Scripps Coastal Reserve, La Jolla, California. Here, clear seawater, recycled from research aquaria at Scripps Institution of Oceanography, cascades onto upper foreshore sand and turbulently erodes a pit > 10 cm deep. Downcurrent from the cascade, a gentler current scours a straight to slightly sinuous channel < 10 cm deep and splays seaward into braided rills 1.0-2.0 cm deep in the swash zone. In the process, fine beach sand is shaped into distinct bedforms, from lower flow regime straight-crested ripples to upper flow regime antidunes. Teams of three-to-four students occupy designated intervals along the channel and define a transect with a metric tape that spans the channel width between two garden stakes marked with bright flagging tape. Team members rotate tasks among themselves and at 10-cm intervals along the transect, they: 1) measure channel depth in centimeters with a plastic ruler, 2) identify bedforms based on classroom and field examples, 3) describe sand texture with grain-size cards and hand lenses, and 4) record observations on data sheets provided. They further calculate average flow velocity in the channel from the time taken for a colored cork to travel a given channel distance. Cross-channel profiles, drafted to scale by each team, summarize the sedimentologic and morphologic character of the segment of channel studied. The field exercise showcased herein can be adapted for use with classroom stream tables or for a field study at another geographic location where an accessible alluvial or tidal flow actively scours a shallow channel. Knowing the association between sand size, stream conditions, and flow-regime bedforms is central to basic interpretations of ancient hydraulics and the parent bedforms responsible for stratification preserved in sedimentary rocks.