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 (Qover) 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 Hs .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, Tmorph has been derived from the 24.5hour moving window analysis for many closure events with bay area (Ab<0.7km2) in Australia. The results show a clear trend of Tmorph decreasing with increasing relative wave strength – i.e., more rapid closure with bigger waves. However, for larger inlets or inlets with training works the morphology changes, at the time scale of individual storms, are usually not significant enough to be measurable via the tidal records. The moving window analysis is an effective way to analyse surge- or flood events to clarify if the system gets higher hydraulic efficiency due to inlet scour or reduced gain due to nonlinear friction effects and/or increased bay area due to elevated estuary water levels.Regarding the non-seasonal opening/closing of inlets in NSW, the fraction of time the inlet is open and the average time it stays open, T open are quantified in terms of the dimensionless relative tidal strength ˆQpotentialgHs5. The new relationships for inlet in equilibrium are applied to illustrate the use of a new, analytical inlet evolution equation based on the impulse response function for an inlet under the effects of variable waves and spring/neap tide variation. An assessment of the state of the art of numerical, morphological modelling was made by applying the US Army Corps’ CMS model to Pensacola Pass during and after Hurricane Katrina. The model underestimated the morphology changes observed and results did not reach an asymptote under steady, normal forcing after the event. Insignificant erosion of the ebb tidal delta in the model output, compared to observations, is attributed to (i) improper model assumptions on direction of sediment transport (ii) underestimation of role of waves in sediment transport and (iii) difficulties with numerical bed updating balancing stability versus accuracy.Laboratory experiments on barrier development under waves and currents were carried out. Analysis of combinations of two wave cases with shorter period shows that the sediment transport direction is opposite to the net flow direction. The direction of sediment transport (qs) is not consistent in the combinations including other two wave cases with the longer period. These cases show clearly that qs cannot be generally assumed to be in the direction of the net flow as in the CMS model.Based on the results of the laboratory experiments, the applicability of five existing state-of- the-art sediment transport (qs) formulae to inlet morphodynamics has been assessed.