Author(s): Edward Shen

Linked Author(s):

Keywords: Open boundary; Location; Modelling

Abstract: In a hydrodynamic modeling study, the determination of the open boundary location is a fundamental but crucial issue. For years, modelers have been hesitant to determine the location of the open boundary, because it may have impacts on the model performance if the location is too close to the study area, or it may significantly increase the computation effort if the location is placed too far away. The objectives of the present study aim at (1) to examine the effects of varying locations of open boundary on the model quality via the numerical experiment; to answer why and how the boundary location can affect the model results; then (2) to recommend a practice guideline for locating open boundary in a regional model build. In coastal waters, Froude number Fr and seabed shear stress vary depending on water depth and flow velocity. To take account of impacts of Froude number on the uncertainties in boundary dynamic forces and impacts of shear stress on wave reflection at the open boundary, this study introduced a new parameter D50. It is the ratio of the length of open boundary at water depth equaling or deeper than 50m to the total length of the boundary. The D50 represents a water depth condition at the open boundary. Two experiments were carried out for model builds in Brunei Bay and Pearl River Estuary to demonstrate how the effects of varying boundary location on changes of water level, tidal range, current velocity and tidal flux are interrelated to the changing D50. The study shows that the water level and tidal range are better matched with the recorded when D50 is larger than 50% and consistently increases to a certain level. In this range of D50, the tidal range and tidal flux increase with the increasing D50 and the difference in averaged tidal flux can be up to 10% or more. The lower bound of D50 larger than 50% therefore is recommended for the location of open boundary. The upper limit also exists for the location of open boundary due to changes in seabed gradient from the continental shelf to the continental slope. We suggest that the upper limit of open boundary location shall be placed at the approaching end of the continental shelf. Since the width of the continental shelf varies from one country to another, the upper limit for the location of open boundary shall vary as well.

DOI: https://doi.org/10.3850/IAHR-39WC25217119202257

Year: 2022