International Journal of Engineering Research (ISSN : 2319-6890) Volume No.2, Issue No.2, pp : 38-43 01 April 2013 IJER@2013 Page 38 Inlet Morpho-Dynamics During a Storm Event Inferred from Tidal Records: A Case Study Of The Brunswick River, NSW, Australia Thuy T. T. Vu 1,2 , Peter Nielsen 1 , David P. Callaghan 1 1 School of Civil Engineering, University of Queensland, Brisbane, QLD 4072, Australia 2 Hydraulic Engineering faculty, Water Resources University, Hanoi, Vietnam. vuthuyau@gmail.com t.vu4@uq.edu.au; p.nielsen@uq.edu.au; david.callaghan@uq.edu.au Abstract: A 24.5h moving window is used to analyse storm effects on the tidal dynamics. Compared to earlier studies using window lengths varying between 2h (for tsunami analysis) to 14 days for tidal inlet analysis, the present method provides good resolution of variations of hydraulic efficiency during storms or floods. The importance of de-trending before carrying out the harmonics analysis has been studied and it was found to be highly important. De-trending involved removing the 24.5h mean before harmonic analysis. The main results are the morphodynamic timescale T morph and the varying response functions F 1 and F 2 of the diurnal and semi-diurnal tidal constituents through a storm event at the Brunswick River, Australia. T morph is inferred from time series of primary semi-diurnal gain G 2 (t)= |F 2 | with an exp(-t/T morph ) curve fit. The results show only insignificant morphological change. Thus, the changes in tidal response for this medium sized catchment are mainly due to hydraulic effects. These include the influence of river flow Q f via the non-linear friction term and increased estuary surface area during the flood. The traces of both F 1 (t) and F 2 (t) in the complex plane show an equilibrium before the storm, dynamic change during the storm and relaxation mimicked by exp(-t/T morph ) after the storm. Keywords: Brunswick Heads, morphodynamic timescale, 24.5 hour moving window, morphodynamics, tidal response. 1. Introduction Tides, waves, freshwater inflow and sediment supply determine the morphology of tidal inlets. Significant changes to one or more of these causes the inlet to move towards a new equilibrium. The time scale at which an inlet responds to such changes is called the morpo-dynamic timescale T morph . The difference between the actual state and the equilibrium state is an exponentially decaying function e -t/Tmorph if the rate of change is proportional to the distance from equilibrium. The time constant T morph in the exponential function varies from days to weeks in small systems like Avoca Lake, NSW, AU cf. Thuy et al. (2012) to months for seasonally closing inlets like Thuan An and Tu Hien lagoons on the Central coast of Vietnam, Lam (2009). The recovery time for the coast and river mouths along Miyagi prefecture, Japan after the March 2011 Tsunami ranges from 25 days for the Arahama Coast to 75days for the Akaiko Coast, or 180 days for the Nanakita River, depending on sediment supply (Hitoshi, 2012). T morph can also be as long as 350years for Dollard’s tidal flat in The Netherlands, as it responds to slow process of sea level rise (Eysink, 1990). Morpho-dynamic time scales are usually derived from inlet throat areas and the volumes of flood/ebb tidal deltas, which are costly to measure. The alternative of using process- based morphology models suffers from the lack of real predictive skills of such models. This paper infers hydraulic- and morpho-dynamic changes from tidal records using a 24.5h moving window on the May 2009 storm event Brunswick Heads, Australia. Figure 1: The Brunswick Heads River entrance with the tide gauge about 630m from the ends of the breakwaters. While the entrance banks (rock walls) are steep, much of the estuary has very flat slopes near MSL. 2. Description of study region and storm event Brunswick Heads is located at 28 o 32’17.22”S and 153 o 33’29.65”E, in northern New South Wales (NSW), Australia (Figure 1). The Brunswick River is a medium sized system with catchment area ca 200km 2 , surface area of 3.3km 2 and spring tidal prism 1.94x10 6 m 3 (Roper et al., 2011). The physical features of the catchment vary from steep, heavily vegetated slopes to open grass flood plain and flat swamp land behind the coastal dunes Webb (1986). The entrance is trained by break waters with the length about 300m. It has a shallow bar fronting the breakwaters which is expected to wash out during major floods. (Hanslow et al, 1996) The storm considered here occurred between 20/5/2009 and 24/5/2009, however the data analysis period was extended further before and after the event from 15/5/2009 to 5/6/2009 to gain a general view of the hydraulic and morpho-