AQUIFER HETEROGENEITY IN CHANNEL-BELT DEPOSITS, PART 1. RATIONALE FOR GEOMETRIC SIMULATION

Many important aquifers are composed of gravelly sediments deposited in the channel belts of braided rivers. Recent work on modern rivers and ancient sediments has led to a conceptual model of these types of deposits, a model that can be applied to the deposits from river systems that vary greatly in size. Importantly, these deposits can be organized in a hierarchy and the dimensions of units at all hierarchical levels scale with the size of the river system. In a variety of previous studies, the three-dimensional structure in active channel belts has been quantified by (1) ground-penetrating radar combined with cores and trenches; (2) study of channel deposits in frozen rivers; and (3) time series of aerial photos and satellite images. The quantitative, three-dimensional model that has emerged (summarized in Bridge, 2006) would not be known from any one of these data types alone, and was possible only through their synthesis. The geometric simulation approach described here depends on this depositional model.

At the largest scale, channel belts are one component of floodplains; our focus is on channel-belt deposits. Channel belts are composed of compound bars and channels. Contrary to most representations, the volumetric proportion of compound-bar deposits greatly exceeds that of channel fills and, in many river systems, the channel fills will be lower in permeability. Compound bars are composed of unit bars and cross-bar channel fills. Unit bars have distinct forms built from the deposits of various bedforms, with those of dunes representing the greatest volumetric proportion. At the smallest scale, these deposits are composed of sand, sandy gravel, and open-framework gravel.

To represent important aspects of heterogeneity in gravelly channel-belt deposits, we utilized a geometric simulation approach to create a synthetic deposit representing a region ~2.5 km in length and width and ~10 m in thickness. Archetypal shapes of the depositional units at each level of this hierarchy were created based on published observations and measurements. They are combined in ways that fill space and conform to rules that reflect whether their boundaries in nature are erosional or depositional.