Aspects of inlet geometry and dynamics

Abstract

This thesis contains new analytical approaches as well as laboratory-and field experiments conducted to understand the hydrodynamic and morphodynamic responses of tidal inlets to forcing from tides, river flow and waves on the time scales of closure, flood or storm events. The thesis outcome is effective new tools for authorities managing coastal zones balancing navigation-, shore protection-and socio-economic development purposes.A new method of coastal inlet classification based on dimensionless parameters is presented. These parameters represent the relative strength of the three main forcing agents: tides, river flow and waves. This new classification is applied to 178 inlets along the NSW coast of Australia, and compared with other widely used classification schemes available in the literature.The inlet hydraulic analysis is presented in Chapter 2 with given inlet geometry and wave climate with overwash discharge (Q over ) added into the usual governing equations. The hydraulic analysis of inlets in terms of the frequency response function for the linearised system is illustrated for cases of monochromatic and mixed diurnal/semi-diurnal tides. This analysis quantifies the influence of the entrance invert level, river flow and bay surface area. A case of inland flooding at Lake Conjola, Australia is used to test different methods resulting in a successful illustration of the importance of wave overwash as a driving force.For each hydrodynamic condition, the inlet system and its elements have a corresponding morphological equilibrium state. New relationships for inlets in equilibrium were constructed based on dimensional analysis and tested on a data set of 36 natural inlets in the USA. These new relations depend not only on the tidal prism but also the tidal period, and mean annual significant wave height s H .During unusual weather, the morphology of tidal inlets runs out of equilibrium.Subsequently, they may return to the previous equilibrium or move towards a new equilibrium or get closed. Inlet morphodynamics analysis is ideally carried out from topographical surveys. These are however costly and usually not available. Process based numerical models are still unreliable. A more economical and reliable new method, a 24.5hour moving window method, is introduced to infer hydraulic-and morpho-dynamic changes from tidal records. The morphological time scales are thus determined from time series of mean water levels, standard deviation, or the gain of the primary tidal components. This analytical method is successfully applied to inlet closure events and flood or storm events.The morphological time scale, T morph has been derived from the 24.5hour moving window analysis for many closure events with bay area (A b <0.7km 2 ) in Australia. The results show a clear trend of T morph decreasing with increasing relative wave strength -i.e., more rapid closure with iii bigger waves. However, for larger inlets or inlets with training works the morphology changes, at the time scale of individual storms, are usua...

Ocean Driven Flooding of a Coastal Lake

“…The first short-coming is that if the wave pump efficiency is used to calibrate the model to match the maximum peak flood level (fig 6), an under-prediction before and over-prediction after the maximum peak flood level occurs. One possible reason for this mismatch could be that the ocean entrance channel increased in size during this storm (Vu, 2014). The second short-coming is timing of model peaks, which are generally matching well except for the largest peak that has a phase shift with that of the model prediction.…”

“…The lake was schematised using two nodes (fig 3) based on previous field measurements indicating that the western region fluctuates approximately uniformly with less range compared to the remainder of Lake Conjola (Vu, 2014). These data were also used to estimate the equivalent sand grain roughness to describe the velocity profile in the entrance channel, see equation (4).…”