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Computer Modeling of the Evolution of Linked Fluvial and Coastal Erosion and Deposition

Mark R. Vining
Texas Commission on Environmental Quality Austin, Texas

This work presents a computer model of a river system coupled to the geomorphic and tectonic evolution of its drainage basin, and the coastal environment into which it delivers. Existing quantitative river models, which are limited to short stream segments and the time spans of individual flood events, fail to account for the simultaneous evolution of all parts of the river system. Existing coastal models do not provide process-based and feedback-controlled sediment influx. This work is a forward dynamic simulation of sediment origin, transport, deposition, and erosion, at a time scale involving the composite effects of many seasons of floods, regional tectonic rise or fall, and sea level fluctuation.

The method involves modeling a river system by a series of cells representing stream reaches having relatively uniform flow and sediment characteristics. Water and sediment are supplied from a model of the geomorphic evolution of the drainage area. Sediment properties are represented independently for layers of alluvium, soil profiles, and underlying basement rock (including a moderate degree of stratigraphic complexity). Bedform type, distribution, and dimensions are modeled for representation of bed surface hydraulic roughness. Soil development is modeled from decay of strata underlying the land surface. Vegetation biomass is modeled from temperature, rainfall, and surface conditions, and contributes to surface roughness, stability, and evapotranspiration. Deposition of the fluvial sediment load at the coastline responds to the accommodation space in the coastal system. Sea level and tectonic fluctuations as functions of time must be provided as input.

The method keeps geomorphic causes and effects temporally and spatially correlated, enabling systematic investigation of the pattern of evolution of the whole system. Results yield the history of stratigraphic development of the basin, including distribution and properties of all sediments deriving from the interaction of basin physiography and fluvial processes. Feedback between the two model components occurs as the fluvial model feeds sediment to the coastal model, and the coastal model produces depositional accommodation and sea level rise/fall signals that migrate through the fluvial system over time.