Subsurface flow is primarily controlled by the distribution of hydrogeological properties. Spatially correlated random fields, currently the primary means of describing heterogeneity in these properties, underutilizes the existing knowledge of geological systems. A new computer code, the Braided Channel Simulator (BCS-3D), is used to geometrically simulate the spatial heterogeneity of hydrogeological properties using representations of surface topography to estimate the three-dimensional arrangement of subsurface units. Four methods of assigning conductivity were used to generate conductivity fields as a basis for saturated flow simulation. Variograms measured for each of the conductivity fields give no indication of underlying discrete structure. However, particle-tracking simulations showed that flow occurred along paths of preferentially high hydraulic conductivity. Results indicate that in systems where the contrast in conductivity is sufficiently great, the location and magnitude of flow is constrained by discrete internal structure. Additionally, geometrical simulation techniques such as BCS-3D can be used to produce property fields that embody these discrete geological structures. 533 534 WEBB AND ANDERSON: SIMULATION OF PREFERENTIAL FLOW OF PREFERENTIAL FLOW 545

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A computer code, BCS, was developed to simulate the three-dimensional topographical expression of braided channel systems. The model uses a random walk approach to simulate channel braiding and incorporates the hydraulic geometry of stream channels to assign width, depth, and velocity to each channel segment. Input parameters are directly related to observable geometric characteristics of a braided system such as channel system width, depth, and sinuosity. Thirty-one measures of the degree or type of braiding were taken from the literature and used as calibration targets for the model. All targets were matched for a portion of the Ohau River, New Zealand. This simulation technique can be used to assess quantitatively the distribution of channels with their associated hydraulic properties and visualize topographic surfaces associated with braided channel systems. This information provides a foundation for probabilistic (multiple realizations), geometric simulation of the three-dimensional distribution of braided stream sediments. 1.Paper number 95WR01952. 0043-1397/95/95 WR-01952505.00 tems are subject to other sediment transport mechanisms such as debris flows. To reduce the influence of these competing mechanisms and limit the scope, this study concentrates on laterally constrained valley train or glacial outwash systems that are dominated by fluvial transport. In particular, the simulations represent streams that belong to the high-gradient, high-sediment load, high-energy flow regime and that have rapidly avulsing multithread channel systems.The purpose of this paper is to discuss a simulation technique that reproduces both the plan view of braided channel systems (topology) and their associated three-dimensional topographic surfaces. This work builds on previous attempts to simulate braided systems [Howard et al., 1970; Krumbein and Orme, 1972], incorporates system theory [Smart and Moruzzi, 1971], and relies on descriptions of the hydraulic geometry of stream channels [e.g., Fahnestock, 1963; Church, 1972; Park, 1977].The objective is to reproduce known topological and topographical attributes of braided channel systems. These attributes can be divided into two general categories. First, characteristics common to all braided systems can be used to verify that the model produces realistic braided channel systems. Second, attributes that are characteristic of a given system can be used as site-specific calibration targets. The results of this surface morphological simulation technique are used to simulate the three-dimensional subsurface distribution of bounding surfaces and sedimentary unit characteristics in a model known as BCS-3D [Webb, 1994]. This paper describes (1) previous studies on simulating braided channel systems with their accompanying calibration checks, (2) field-based statistical characterization of channel systems; (3) the concepts of hydraulic geometry for stream channels, (4) structure, input, and output for BCS, the braided channel simulation code, (5) calibration targets used to substanti...

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