Open Collections

The free oscillatory response of fjord-type multi-armed lakes

University of British Columbia

This study examines the structure and frequency of free seiche modes in fjord-type multi-armed lakes in order to generalize features of the response of those lakes. The effect of multiple arms on seiches within a lake is not easy to predict. To do so, this study develops a simplified analytical model (SAM) based on idealized lake geometries. In addition, a characterization of surface (barotropic) modes is compared for two ``Y-shaped'' lakes: Quesnel Lake in Canada, and Lake Como in Italy. Lake Como and Quesnel Lake are studied through a combination of field observations and modelling, both numerically using a Finite Element Method (FEM) scheme and using SAM. SAM demonstrates that multi-armed lakes are subject to two classifications of behaviour: a full-lake response, in which all arms are active; and a decoupled response, in which seiching is constrained to only two arms of the lake. A geometric parameter in each arm, which represents the travel time of a progressive shallow-water wave in that arm, determines the range of behaviours expressed: each lake will either experience only a whole-lake response or it will exhibit alternating whole-lake and decoupled modes. The behaviour predicted by SAM is consistent with modes observed and predicted in both Quesnel Lake and Lake Como. Modal periods are identified from observed water level measurements using spectral analysis. FEM predicted periods agree with observations. SAM correctly reproduces the periods of the lowest frequency modes in both lakes when a constant depth is used for each arm. Mode-shapes predicted by SAM qualitatively match those given by the FEM model. While all modes of Quesnel Lake are whole- lake modes, some of the modes in Lake Como exhibit a decoupled response. The results given here also support generalization of the fundamental mode as being inherently the same structure as Merian-type modes in simple elongated lakes. While the study focusses on barotropic modes, SAM can be similarly applied to internal (baroclinic) modes, and so the general behaviours observed here are appropriate for describing both the barotropic and baroclinic responses of multi-armed lakes.

Text

2017-08-10

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/62577

Civil Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/