|Modeling the hydrodynamics in tidal networks|
Alebregtse, N.C. (2015). Modeling the hydrodynamics in tidal networks. PhD Thesis. Utrecht University: Utrecht. ISBN 978-94-6299-259-7. 157 pp.
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This thesis covers tidal propagation through networks of channels. Such networks are widespread and are often subject to discordant human and natural interests. First, the effect of a secondary channel on the tides in a main channel is explained with the use of an idealized model and the responsible physical mechanism is uncovered. The secondary channel can both enhance as well as dampen the tide in the landward part of the main channel, depending on its position. Second, it is investigated what the effects of a secondary channel are on nonlinear tidal characteristics such as tidal range and flood-to-ebb ratio. All tidal constituents are affected through the same physical mechanism, which indicates that the secondary channel locally produces a modification to the tide. This modifying wave subsequently propagates through the main channel. Third, the effect of changes in river discharge and changes to the geometry of a tidal network are investigated. The Yangtze Estuary is used as an example, as this is a characteristic tidal network that experiences strong fluctuations in river discharge and has been subject to a manmade alteration of the geometry. Higher river discharge results in stronger tidal damping of the semi-diurnal tide. Quarter diurnal tides are enhanced due to an enhanced interaction of the semi-diurnal tide and the residual current through quadratic bottom stress. The changes in the geometry of the Yangtze Estuary resulted in a reduction of net water transport through the affected channel of the network, due to a local decrease in river induced transport and stronger landward tidally induced net water transport. Finally, the effect of dissipation of momentum on tidal flats on tidal velocities is investigated, as well as the effect of exchanging water over a tidal flat between different channels on tidal velocities. Tidal flat area, tidal flat cross-sectional profile and the position of the tidal flats along the channel are important parameters that determine how the momentum sink mechanism influences the tidal velocities, however, the overall effect of momentum sink is small. Exchange of water between channels was modeled for various heights of the connecting tidal flat, which allowed for varying durations of exchange between the channels. Lower levels of the tidal flat tend to enhance the flood-to-ebb velocity ratio, meaning flood dominant situations become more flood dominant and ebb dominant situations become more ebb dominant.