FLUVSIM, is a useful tool for multi-disciplinary characterization fluvial aquifers. FLUVSIM simulates sediment accumulation at the channel belt/floodplain scale producing sedimentologic and stratigraphic attributes that it converts to petrophysical properties for ground-water modeling. The three-dimensional distribution of attributes results from self-organized critical behavior of the stratigraphic process-response system given spatial and temporal variation of the accommodation space and sediment supply ratio (A/S), which is analogous to base level. Process parameter functions driving base-level variation include the rate of sea (or lake) level change, regional subsidence rate, and water and sediment flux into the model.

Presentation (ppt) given at ModelCare 2002, Prague, Czech Reoublic, June 2002

FLUVSIM, simulates the stratigraphic process-response system and recognizes that stratigraphic attributes reflect changes in the accommodation to sediment supply (A/S) ratio. Accommodation space refers to the potential for sediment storage and preservation at a particular location and sediment supply refers to processes that produce and transport sediment.. FLUVSIM minimizes the number of input parameters and direct forcing functions. There are just six user-defined input curves: sea level, subsidence, river discharge, sediment flux, and the median grain size and sorting coefficient of that sediment. The first two parameters affect accommodation and the last four parameters affect sediment supply. The user-defined input curves describe fluxes of energy and mass entering the model as a function of time. Given these energy and mass fluxes defined at the model boundary, internal processes redistribute the energy (accommodation) and flux (sediment supply) according to rules specified by the stratigraphic process-response system.

FLUVSIM simulates sediment accumulation three dimensions at the channel belt-floodplain scale, by simulating a variety of processes. When accommodation is high, the typical life cycle of a fluvial system begins with alluvial ridge aggradation. Aggradation on alluvial ridges is a function of the amount of accommodation added to the system through sea level fluctuations and subsidence. As the alluvial ridge grows in height, the lateral gradient may exceed the down-channel gradient. If the discharge in the channel exceeds a threshold during a flood event, a crevasse notch will form in the channel levee. If conditions are right, water and sediment transfer to splay complexes allowing for crevasse splay evolution. Ultimately, the splay complex may capture all of the water and sediment flow from the main channel in a process called avulsion, resulting in abandonment of the main channel. Aggradation of the new main channel commences and the cycle continues. The stratigraphic record in this case is one of isolated main-channel sand bodies surrounded by splay complex deposits.

Under low A/S conditions, aggradation of the main channel does not occur. Splays will not develop without a gradient advantage, so avulsions do not occur. The dominant process in this case is lateral channel migration. As the channel sweeps across the floodplain, it consumes preexisting sediments and replaces them with coarse-grained channel deposits, resulting in laterally extensive sheets of sand.

FLUVSIM does not generate random events. Rather, it uses deterministic, empirically developed geomorphic and stratigraphic process-response relationships. FLUVSIM simulates an avulsion when warranted based on site-specific conditions.

FLUVSIM reproduces the self-organized criticality characteristics observed in natural fluvial systems by incorporating nonlinear dynamics, feedback, buffers, thresholds, memory, and mass conservation.

FLUVSIM uses fuzzy logic to simulate where, how much, and what type of sediment is deposited on the floodplain. Advantages of fuzzy logic include the ability to simulate multiple, nonlinear, interdependent processes and responses; the ability to incorporate qualitative data, empirical generalizations, and knowledge lacking in robust numerical formulations; and the ability to incorporate complex, nonlinear functions without using more precision than necessary to solve the problem.

Output from FLUVSIM is comparable to easily collected sedimentologic (e.g., grain size, environment of deposition) and stratigraphic (e.g., thickness, geometry, facies distributions) observations. FLUVSIM converts sedimentary facies to a 3-D petrophysical model defining horizontal and vertical hydraulic-conductivity, porosity, storage coefficient, and specific yield/effective porosity for simulations of ground-water flow and transport.

Sensitivity analysis of FLUVSIM is underway to evaluate the accuracy, resolution, robustness and applicability of FLUVSIM. Four conditions are evaluated to judge the quality of the model: 1) appropriate sediment volume partitioning while conserving mass; 2) reasonable changes in cycle symmetries through time and space; 3) existence of an inverse relation between facies tract heterogeneity and frequency of stratigraphic discontinuity surfaces; and 4) reasonable geometries of sand bodies. Preliminary results indicate that FLUVSIM is meeting all of these criteria.

FLUVSIM is a tool for use in satisfying one aspect of multi-disciplinary characterization of fluvial aquifers for use with other invertible hydrogeologic, geochemical and geophysical models. Combination of information from many sources reduces uncertainty associated with prediction of ground-water flow and transport.

FLUVSIM is a research code written in C++ designed to execute on a Macintosh G4. We expect to release the code to the geological community in Fall, 2002