Opening and closure of intermittently open natural inlets

Opening and closure of intermittently open natural inlets

Rijkenberg, L.

Stive, M.J.F. (mentor)
Nielsen, P. (mentor)
Reniers, A.J.H.M. (mentor)
Van Rooijen, A.A. (mentor)
McCall, R.T. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2015-09-18

Tidal inlet systems are found all over the world. These highly dynamic coastal features can be classified into three main types wave-dominated, tide-dominated or river-dominated tidal inlet systems. A tidal inlet is described as an opening in the shoreline that provides a connection between the ocean and a bay and is maintained by tidal currents. Tidal inlet systems are the whole area of the near shore ocean, inlet and bay with river catchment area. Recently, Thuy (2013) developed a dimensionless hydraulic parameter tool to classify tidal inlet systems. The study focusses on wave-dominated tidal inlet systems and specifically the sub-class intermittently open natural inlets. The question arises if the intermittently open natural inlets can be distinct by use of the parameter. This type of sub-class has an alternately open or closed connection to the ocean, which is indicated by the word intermittently. A natural inlet is defined as an inlet with no other human interventions at the inlet entrance other than artificial breaching of the entrance berm when the lagoon water level exceeds a critical value. The distinction for the sub-class can be integrated into the wave-dominated classification parameter, therefore the overall objective is to make a contribution to the wave-dominated classification parameter of Thuy (2013) to make a more accurate classification for the sub-class intermittently open natural inlets. The overall objective is too extensive to be answered completely by only this study, therefore a specific objective is formulated. The specific objective is to predict the lagoon water level of intermittently open natural inlets forced by waves, tide and river discharge. The prediction of the lagoon water level will indicate the important forcing working on the inlet, while the entrance is in an open state. In total nine opening/closure events are found in the data and analysed. Data included, offshore wave climate, tidal water levels, lagoon water level and rainfall. To get a more accurate representation of the wave climate near the research sites, the offshore wave climate is adjusted to get the near shore wave climate. Based on the analysis the following main conclusion is found. The lagoon water level is a crucial parameter for prediction of the entrance state open or closed. Therefore, it is important to understand in which way the high waves will affect the elevation of the lagoon water level. Two analytical models are introduces to understand the forces that work on the open inlet. A basic energy equation, after Nielsen (2009), is the starting point of both models. The basic energy equation describes the river discharge and flood tide as inflow parameters, but the effect of waves is not taken into account. The first introduced model is the extended energy equation. The data analysis indicated that shore-normal waves are dominant during the elevation of the lagoon water level. The shore-normal wave forcing is introduced by means of a wave overwash discharge. The outcome of this model gives an underestimation of the lagoon water level, although the model performs better than the basic energy equation. The second introduced model is an alternative approach of the basic energy equation. The data analysis indicates that the near shore wave heights are very important for the elevation of the lagoon water level. Accordingly the elevation of the lagoon water level is assumed to be a function of the near shore wave height. The alternative model gives a better performance of the lagoon water level than the basic and the extended model. Based on this study it is recommended to further develop the alternative model for further research of different tidal inlet systems. A meaningful addition to the model could be the introduction of an entrance width that varies in time. During closure the inlet entrance gets narrower, which will result in different forcing and a different response of the lagoon water level.

intermittently
tidal inlets
opening
closure
overwash
wave-dominated
New South Wales
Australia
natural inlet behaviour

http://resolver.tudelft.nl/uuid:52e891f7-4588-4c18-bad4-0e4988d92f7c

-33.463999, 151.434996

Student theses

master thesis

(c) 2015 Rijkenberg, L.