Relations for Bankfull Hydraulic Geometry of Sinuous Channels in Submarine and Subaerial Settings
The bankfull hydraulic geometry of river channels has typically been characterized in terms of mean bankfull width Hbf, mean bankfull depth Bbf and mean downchannel bed slope S as functions of bankfull discharge Qbf. In the case of rivers, these parameters, as well as bed grain size, can be directly measured. General relations for rivers characterizing hydraulic geometry have been developed in both dimensioned and dimensionless form. A corresponding analysis is difficult to perform in the submarine case because the parameters that are directly measurable are generally limited to channel width, depth and slope (and bed grain size when cores are available). Neither the characteristic bankfull discharge nor the characteristic volume concentration C of suspended sediment that drives the channel-forming turbidity currents that construct the channels are known in advance. Here we use the following information and tools to reconstruct these parameters: 1) a data set consisting of 250 reaches/cross-sections for (mostly) meandering, sand-bed rivers for which all the relevant parameters are known; 2) a data set for consisting of 180 reaches/cross-sections for meandering submarine channels in which only Hbf, Bbf and S are known; 3) relations for momentum balance, bed shear stress and interfacial shear stress for turbidity currents and rivers. We then back-calculate a single characteristic concentration C necessary for the turbidity currents to follow the same trend in driving force/area versus channel size as observed for rivers. We in turn use this value to calculate the bankfull discharge for each submarine channel. The back-calculated value of C that brings the submarine data into accord with the fluvial data is around 0.0017. The analysis yields a common set of relations for hydraulic geometry for the submarine and subaerial cases. While the submarine channels of our data set tended to be much larger than the subaerial channels in the corresponding data set, the two cases do show a zone of overlap. While it is likely that the channel-forming value of C differs from channel to channel, the analysis a) provides a characteristic estimate of this parameter that has proved otherwise inaccessible until now and b) allows estimation of bankfull discharge for each submarine channel. The relations so derived should provide useful tool in the interpretation of channels in outcrops and seismics.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California