MIGRATION, ELECTRICAL RESISTIVITY GROUND IMAGING (ERGI), AND TEXTURAL CHARACTERISTICS OF GRAVEL BARS, MAD RIVER, OHIO, USA

A ~500 m-long study reach of the gravel-bed Mad River developed point bars and a braid bar from an initially straight reach which was channelized prior to 1968. The river has a bankfull width of 50-70 m, a bankfull depth of 3-4 m, and a sinuosity of 1.12. Examination of historical aerial imagery indicates that the channel migrated by expansion from 1968 to 1994, by expansion and translation from 1994 to 2000, and by translation from 2000 to 2014. Downstream translation is ¼ to ½ of the channel wavelength. Short-term migration rates are ~1-10 m/yr, with an average rate of ~8 m/yr from 2005 to 2014.

Along-channel and cross-channel ERGI surveys were conducted on one point bar. Bar deposits are distinguishable from underlying glaciofluvial deposits (resistivities, ρ’s, of ~140-340 Ωm). Well-drained surface gravels (ρ’s of ~160-1900 Ωm) are discernable from gravel-bar deposits below the water table (ρ’s of ~50-150 Ωm). Laterally, resistivities generally vary little, likely reflecting homogeneity of bar deposits due to the preferential preservation of downstream parts of the bar associated with translation. However, some variations are evident, likely reflecting differences in water content possibly associated with variations in grain size, sorting, and/or porosity. Additionally, the finer-grain fill of an inner-bank, bar-tail swale (silty sand, ρ’s of ~17-40 Ωm) associated with accretionary topography is discernible from laterally adjacent and subjacent bar gravels.

Bulk grain size analyses of subsurface bar deposits yield mean grain sizes of 7.3-11.3 mm, and sorting of 1.73 Φ (poor) to 2.20 Φ (very poor). Armor layers are substantially coarser, with mean grain sizes of 16.0-32.0 mm, and better sorted (0.81-1.46 Φ, moderate to poor), consistent with the winnowing of fines. Porosities of subsurface gravels were determined by digital image analysis of successive slices through in situ cores impregnated with phosphorescent epoxy. Porosities range from 23.3-30.3%. Permeabilities of the gravels were calculated using the Kozeny-Carman equation, applying values of porosities and harmonic means of grain sizes (which range from 0.99-1.52 mm). Permeabilities range from 118-560 darcys. Results are important for the effective development and management of aquifers and hydrocarbon reservoirs composed of similar deposits.