Hydrodynamics and sediment transport in the Duwamish River Estuary

Abstract

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±4.8à 104 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±4.4à 104 mt/year, down from a 1960s annual load of 16.2±5.5à 104 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.