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Hydrodynamics and Sediment Transport in the Duwamish River Estuary.

Record Type:	Electronic resources : Monograph/item
Title/Author:	Hydrodynamics and Sediment Transport in the Duwamish River Estuary./
Author:	McKeon, Margaret A.
Published:	Ann Arbor : ProQuest Dissertations & Theses, : 2020,
Description:	124 p.
Notes:	Source: Dissertations Abstracts International, Volume: 82-05, Section: B.
Contained By:	Dissertations Abstracts International82-05B.
Subject:	Physical oceanography. -
Online resource:	https://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=28094281
ISBN:	9798684670428

Hydrodynamics and Sediment Transport in the Duwamish River Estuary.
McKeon, Margaret A.

Hydrodynamics and Sediment Transport in the Duwamish River Estuary. - Ann Arbor : ProQuest Dissertations & Theses, 2020 - 124 p.

Source: Dissertations Abstracts International, Volume: 82-05, Section: B.

Thesis (Ph.D.)--University of Washington, 2020.

This item must not be sold to any third party vendors.

Nearly two-thirds of the world's major cities have been built around estuaries. As populations have grown, many estuaries have been modified, impacting their hydrodynamic regime. Urbanized estuaries, which are typically among the most highly modified, are also often contaminated due to the proximity of intense industrial activity, motivating the need to better understand their hydrodynamic and transport processes in order to better inform current management and future modifications. This study focuses on hydrodynamics and sediment transport in the Duwamish River estuary, a highly engineered and contaminated tidal salt wedge estuary located next to downtown Seattle, WA. Hydrographic measurements of salt wedge structure and dynamics spanning a 20-fold range of river discharges are used to investigate the seasonal variability in the strongly stratified Duwamish River estuary. The effect of river discharge on salt wedge length, stratification, pycnocline thickness, and intratidal differences in salinity structure are evaluated. The salt wedge decreases in length and becomes more stratified as river discharge increases. The ebb and flood responses to increasing discharge are markedly different; the flood phase structure shows little seasonal dependence, while the ebb structure changes due to an internal hydraulic response that is strongest at two severe lateral constrictions. This response varies with the net barotropic forcing, which is controlled by river discharge. The asymmetry between the flood and ebb structure is explained using a two-layer hydraulic framework, although spatially, tidally, and seasonally variable vertical mixing also modifies the structure. As a result of the persistent stratification and dominance of hydraulic dynamics, channel constrictions influence circulation and transport during ebb tides. These dynamics also generate a flood/ebb asymmetry in salt wedge structure and circulation that is modulated seasonally through discharge. We hypothesize that these dynamics lead to a seasonally modulated residual circulation. In 2001 the lower Duwamish Estuary was added to the national list of Superfund Sites. A Record of Decision was issued in 2014 recommending a $342 million remedial plan for the remaining 412 ac, 57% (235 ac) of which relies on the burial of contaminated sediment by cleaner sediment originating from upstream. We re-examine existing total suspended sediment load data to understand the magnitude and timing of suspended sediment delivery to the estuary. We use three existing datasets, with data ranging from 1966 to 2019, and develop new rating curves using a piecewise approach. The new rating curves have improved R2 values compared with previous attempts. A combined-fit rating curve accurately estimates 1960s and 2010s annual loads, whereas no single rating curve generated here or previously is appropriate for the entire period from 1961 to present. Using the combined-fit, we find that most of the 11.7±4.8x104 mt/year long term mean annual load occurs from November-May during high discharge events. There is evidence that modern annual load estimates may be closer to 9.4±4.4x104 mt/year, down from a 1960s annual load of 16.2±5.5x104 mt/year. Estimates of fine and coarse loads derived from modern data indicate that 70% of the average annual total load is composed of fine sediment.We present new measurements of SSC throughout the estuary from 2011-2013 in the form of transects and bottom-mounted moorings, which we use to investigate sediment sources in the estuary, transport pathways, the role of local resuspension, and relevant timescales. Upstream sediment is delivered to the estuary in only a handful of storm events that last on the order of days. Gravitational settling transfers upstream sediment to the salt wedge during loading events. As the salt wedge retreats during ebb, retained material is resuspended and re-entrained into the upper layer. The upper layer exports some material seaward, but some re-settles into the salt wedge. The flooding salt wedge re-entrains and transports fluffy material landward. Our measurements show that retained material is reworked in the estuary for three to four months after a delivery event until it is depleted through seaward export and consolidation.

ISBN: 9798684670428Subjects--Topical Terms:

3168433
Physical oceanography.
Subjects--Index Terms:

Estuary

Hydrodynamics and Sediment Transport in the Duwamish River Estuary.
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245 1 0 $a Hydrodynamics and Sediment Transport in the Duwamish River Estuary.
260 1 $a Ann Arbor : $b ProQuest Dissertations & Theses, $c 2020
300 $a 124 p.
500 $a Source: Dissertations Abstracts International, Volume: 82-05, Section: B.
500 $a Advisor: Horner-Devine, Alexander.
502 $a Thesis (Ph.D.)--University of Washington, 2020.
506 $a This item must not be sold to any third party vendors.
520 $a Nearly two-thirds of the world's major cities have been built around estuaries. As populations have grown, many estuaries have been modified, impacting their hydrodynamic regime. Urbanized estuaries, which are typically among the most highly modified, are also often contaminated due to the proximity of intense industrial activity, motivating the need to better understand their hydrodynamic and transport processes in order to better inform current management and future modifications. This study focuses on hydrodynamics and sediment transport in the Duwamish River estuary, a highly engineered and contaminated tidal salt wedge estuary located next to downtown Seattle, WA. Hydrographic measurements of salt wedge structure and dynamics spanning a 20-fold range of river discharges are used to investigate the seasonal variability in the strongly stratified Duwamish River estuary. The effect of river discharge on salt wedge length, stratification, pycnocline thickness, and intratidal differences in salinity structure are evaluated. The salt wedge decreases in length and becomes more stratified as river discharge increases. The ebb and flood responses to increasing discharge are markedly different; the flood phase structure shows little seasonal dependence, while the ebb structure changes due to an internal hydraulic response that is strongest at two severe lateral constrictions. This response varies with the net barotropic forcing, which is controlled by river discharge. The asymmetry between the flood and ebb structure is explained using a two-layer hydraulic framework, although spatially, tidally, and seasonally variable vertical mixing also modifies the structure. As a result of the persistent stratification and dominance of hydraulic dynamics, channel constrictions influence circulation and transport during ebb tides. These dynamics also generate a flood/ebb asymmetry in salt wedge structure and circulation that is modulated seasonally through discharge. We hypothesize that these dynamics lead to a seasonally modulated residual circulation. In 2001 the lower Duwamish Estuary was added to the national list of Superfund Sites. A Record of Decision was issued in 2014 recommending a $342 million remedial plan for the remaining 412 ac, 57% (235 ac) of which relies on the burial of contaminated sediment by cleaner sediment originating from upstream. We re-examine existing total suspended sediment load data to understand the magnitude and timing of suspended sediment delivery to the estuary. We use three existing datasets, with data ranging from 1966 to 2019, and develop new rating curves using a piecewise approach. The new rating curves have improved R2 values compared with previous attempts. A combined-fit rating curve accurately estimates 1960s and 2010s annual loads, whereas no single rating curve generated here or previously is appropriate for the entire period from 1961 to present. Using the combined-fit, we find that most of the 11.7±4.8x104 mt/year long term mean annual load occurs from November-May during high discharge events. There is evidence that modern annual load estimates may be closer to 9.4±4.4x104 mt/year, down from a 1960s annual load of 16.2±5.5x104 mt/year. Estimates of fine and coarse loads derived from modern data indicate that 70% of the average annual total load is composed of fine sediment.We present new measurements of SSC throughout the estuary from 2011-2013 in the form of transects and bottom-mounted moorings, which we use to investigate sediment sources in the estuary, transport pathways, the role of local resuspension, and relevant timescales. Upstream sediment is delivered to the estuary in only a handful of storm events that last on the order of days. Gravitational settling transfers upstream sediment to the salt wedge during loading events. As the salt wedge retreats during ebb, retained material is resuspended and re-entrained into the upper layer. The upper layer exports some material seaward, but some re-settles into the salt wedge. The flooding salt wedge re-entrains and transports fluffy material landward. Our measurements show that retained material is reworked in the estuary for three to four months after a delivery event until it is depleted through seaward export and consolidation.
590 $a School code: 0250.
650 4 $a Physical oceanography. $3 3168433
650 4 $a Environmental engineering. $3 548583
650 4 $a Fluid mechanics. $3 528155
653 $a Estuary
653 $a Salt wedge
653 $a Sediment transport
690 $a 0415
690 $a 0775
690 $a 0204
710 2 $a University of Washington. $b Civil and Environmental Engineering. $3 2092991
773 0 $t Dissertations Abstracts International $g 82-05B.
790 $a 0250
791 $a Ph.D.
792 $a 2020
793 $a English
856 4 0 $u https://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=28094281