EXPERIMENTAL STUDY OF FLUVIAL OPEN-FRAMEWORK GRAVEL

Open-framework gravel (OFG) in river deposits is important because of its exceptionally high permeability, resulting from the lack of sediment in the pore spaces between the gravel grains. Fluvial OFG occurs as planar strata and cross strata of varying scale, and is interbedded with sand and sandy gravel. The origin of OFG has been related to: (1) proportion of sand available relative to gravel; (2) separation of sand from gravel during high flow stage and sediment transport rate; (3) separation of sand from gravel in bed forms superimposed on the backs of larger bed forms; (4) flow separation in the lee of dunes or unit bars. Laboratory flume experiments were undertaken to test and develop these theories for the origin of OFG. Bed sediment (sandy gravel with a mean diameter of 2 mm) and flow depth were kept constant, but flow speed, water discharge, and aggradation rate were varied. Bed forms produced under these flow conditions were bedload sheets, dunes and unit bars.

The fundamental cause of OFG is the sorting of sand from gravel associated with flow separation on the steep leeside of bed forms. Sand in transport near the bed is deposited in the trough of the bed form, whereas bed-load gravel avalanches down the leeside and overruns the sand in the trough. The effectiveness of this sorting mechanism increases as the scour depth and height of bed forms increases. Infiltration of sand into the gravel framework is of minor importance, and occurs mainly in bed form troughs. Temporal changes in flow and sediment transport stage and aggradation rate did not influence whether OFG occurred, but did influence the character of OFG.

The geometry and proportion of OFG in fluvial sandy gravels is influenced by bedform type and size (bedload sheets, dunes, unit bars), variation in height of individual bed forms as they migrate, superposition of small bed forms on the backs of larger bed forms, and aggradation rate. Thick deposits of cross-stratified OFG require large bed forms (dunes, unit bars) and large amounts of aggradation, as occur during high falling stages in the deeper parts of river channels adjacent to compound-bar tails and downstream of confluence scours.