University of Twente Student Theses


Modeling deltaic response to changes in fluvial sediment delivery

Nienhuis, Jaap (2011) Modeling deltaic response to changes in fluvial sediment delivery.

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Abstract:River deltas are extremely dynamic and complex depositional features, shaped by marine and fluvial processes. Due to growing social, environmental and economic pressures, such as population growth and sea level rise, understanding of these systems becomes increasingly relevant. This study aims at identifying and characterizing long-term (centennial) deltaic response to changes in fluvial sediment load. Two types of changes are distinguished: (i) permanent elimination of the sediment supply and (ii) time periodic sediment supply. Thus: how does the shoreline of a wave-dominated delta develop having time-periodic or elimination of fluvial sediment delivery? A numerical model utilizes the relation between waves and littoral transport to calculate shoreline displacement. Aimed at studying wave-dominated deltas, the addition of fluvial sediment somewhere along the shore builds a delta. (i) The directionality and energy of waves determines to a large extent how a delta develops after the elimination of fluvial sediment supply. There are four distinct modes in which a wave-dominated delta can be abandoned. A diffusive mode erodes deposits near the river mouth. Higher wave direction asymmetry creates a discontinuity that propagates downdrift after elimination of riverine sediment input. One of these modes creates a spit that erodes large portions of the shoreline. The Ebro delta, Spain, is an example of that mode. The current shape of these spits can reveal the abandonment conditions of historical lobes. Large trains of downdrift sandwaves before and after lobe abandonment characterize a sandwave-mode. (ii) More regular variation in fluvial input influences downdrift migrating sandwaves. The frequency and magnitude of the riverine "forcing" can initiate an equal pattern that migrates away from the river mouth. There exists a select range of climate forcing frequencies and magnitudes in which that translation is one-to-one. Longer period signals are shredded by autogenic-formed sand waves. Input variation also affects the depositional structure of the delta. Unstable downdrift shorelines, such as the time-periodic megadroughts that influence the Godavari delta, have a highly non-linear response on input signals. The coupling of input signals with updrift deposition is much simpler. Regular updrift erosion creates convex beach ridges up to a distance that is determined by the riverine variation. Understanding and recognizing these conditions helps determine the style and results of historical, current and future delta evolution.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:43 environmental science
Programme:Civil Engineering and Management MSc (60026)
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