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Title:Simple numerical models for fluvially-dominant river deltas
Author(s):Czapiga, Matthew J
Director of Research:Parker, Gary
Doctoral Committee Chair(s):Parker, Gary
Doctoral Committee Member(s):Garcia, Marcelo; Best, Jim; Tinoco, Rafael; Viparelli, Enrica; Mohrig, David
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):River, Delta, Morphodynamics
Abstract:Morphodynamic numerical models of 2D fan deltas can predict evolution delta area and extension rates, but these models have struggled to estimate channel features such as width and depth. A re-analysis of recent bankfull hydraulic geometry data provides an empirical closure for the formative, bankfull Shields Number. This closure improves on previous models that assumed a constant value for this term or used a biased regression scheme. By including this extra constraint, 1D models can simultaneously predict channel width and channel elevation changes, where the former is predicted based on physically-based trends of rivers near morphodynamic-equilibrium. New, juvenile channels are formed at the delta periphery and we find that young channels relate to a formative bankfull Shields number that linearly scales to our empirical relation. A new distributed Exner formulation is included that accounts for channel and floodplain elevations along with a geometric mean delta elevation. This addition allows for a basement incision modelling, which is relevant for river deltas growing into shallow, low slope basins, which commonly erode into the pre-delta surface. The incision submodel includes a below-capacity sediment transport framework and a rate-law function for cohesive sediment erosion. Hindcast model runs compared against the evolution of Wax Lake Delta, Louisiana, USA suggest the new features improve the model’s ability to predict width and depth trends over a 35-year period. A second model is introduced that investigates the effect of juvenile channels on the ability to export sediment from the delta topset. This model convolves a spatial bifurcation rate and flow losses related to juvenile channels to induce declining sediment transport down delta; the model is normalized by upstream variables and all variables are defined by only two parameters. Results are compared against volumetric changes of the Wax Lake Delta topset and the model reasonably well predicts channel properties. An exploration of parameter space for these variables indicates the conditions necessary for sediment trapping in any generic fluvially dominant delta.
Issue Date:2018-07-06
Rights Information:Copyright 2018 Matthew J. Czapiga
Date Available in IDEALS:2018-09-27
Date Deposited:2018-08

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