First Advisor

Stefan A. Talke

Date of Publication

Fall 12-16-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.) in Civil & Environmental Engineering

Department

Civil and Environmental Engineering

Subjects

Tides -- Lower Columbia River (Or. and Wash.) -- Mathematical models, Tides -- Lower Columbia River (Or. and Wash.) -- History, Floods -- Lower Columbia River (Or. and Wash.), Floods -- Oregon -- Willamette River -- Mathematical models

DOI

10.15760/etd.7227

Physical Description

1 online resource (xvii, 195 pages)

Abstract

The Portland area has an extensive flood history since it was founded in 1845. In the late 19th century, the Portland area was prone to flooding from snowmelt freshets (3-6 months duration) and brief winter rain or rain-on-snow events. Since that time the magnitude of spring freshets has been curtailed by 45% due to climate change, flow diversions, and reservoir management. Along with changes in hydrology, the bathymetry of the Lower Columbia River has been altered by the dredging of the navigation channel, diking, and land reclamation. To understand how these changes in hydrology and bathymetry have affected tidal and flood wave propagation, I developed two hydrodynamic models, a modern model, and a model with bathymetry characteristic of the late 19th century. I then simulated a Columbia River spring freshet similar in duration to that of 1880. The results show that increased depth has increased the tidal range for low discharge conditions, and reduced the river slope for low and moderate river discharge. In a major spring freshet of 25x103 m3 s-1 magnitude, reduced floodplain access and confinement from higher modern levees results in similar peak water levels for the historical and modern bathymetry. The confinement in the modern system would result in a 30x103 m3s-1 magnitude spring flood (similar to the 1948) having 0.5m higher peak water levels than its historical counterpart. At 35 x103 m3s-1 discharge (similar to the 1894), modern levees would be likely be overtopped and the increased floodplain inundation would cause similar peak water levels in the modern and historical system.

Most large floods in Portland since 1948 have, however, been brief winter floods, primarily in the Willamette River. Therefore, using the modern model, I then simulated a recent rain-on-snow event, the February 1996 Willamette River flood. I then estimated future flood magnitude by incorporating sea level rise and increases in discharge due to climate change. The results show that 0.6m of sea level rise increases peak water levels in Portland by 0.12m, and 1.5m of sea level rise increases peak water levels by 0.39m. These increases in peak water level represent just 20-26% of the increased sea level at the coast. The mechanism limiting the increase in peak water levels in a sea level rise scenario is a reduction in frictional damping due to an increased depth. The reduction in damping results in a drop in the river slope in the Lower Columbia River. Scenarios incorporating a 10% increase in runoff due to climate change produced a 0.78m increase in peak water levels. Thus, model results suggest that projected changes in runoff due to climate change are likely to cause larger increases in peak water levels in Portland than the projected increases in sea level rise. In scenarios with both increased discharge and sea level rise, there are increases in peak water levels of 0.87m and 1.08m for 0.6m, and 1.5m of sea level rise respectively. Coastal processes such as storm surge and the tidal phase are significant factors affecting flood magnitude, particularly in the estuary and the middle tidal river, but not in Portland. Finally, I found that some locations in the middle tidal river (Longview, Beaver) may be affected by both increases in flood magnitude, and coastal perturbations (tidal phase, storm surge magnitude).

In the last chapter, I analyzed how interactions at the three river junctions around Sauvie Island affect bed stress and water levels in the February 1996 flood. The Willamette River branches into a distributary, the Multnomah Channel (Junction A). The Willamette River flows into the Columbia River to form a confluence (Junction B), and the Multnomah Channel flows into the Columbia River at Junction C, downstream of the other two junctions. The results show that the Multnomah Channel plays a role in reducing flood risk by conveying Willamette River discharge downstream to the Columbia River at Junction C. The degree to which the Multnomah Channel can convey flow is limited by the inundation of the floodplains on the northern segment of Sauvie Island and overbank flow dispersing the flood wave. Backwater effects are also seen in Junction B due to Columbia River discharge and in the Columbia River downstream of Junction B due to inundation and overbank discharge. Constriction of the Columbia River channel downstream of Junction C raises the upstream water level gradient. The rating curve of water level versus discharge upstream of Junction C is characterized by hysteresis, water levels are dependent on the discharge history, i.e., for a given discharge water levels on the rising limb are different from water levels on the falling limb. In locations upstream of the confluence of the Multnomah Channel and the Columbia River (Junction C), water levels are 0.8-1.3m higher on the falling limb than the rising limb. In St Helens, at Junction C, the water levels are ~0.1m higher on the rising limb than the falling limb. In a sea level rise scenario, hysteresis is reduced by 16-25%, due to increased baseline water levels reducing the bed stress.

Persistent Identifier

https://archives.pdx.edu/ds/psu/30801

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