UNIVERSITY OF TWENTE Modelling the impact of storm-induced barrier island breaching on the morphodynamic evolution and stability of multiple tidal inlet systems Author: Tessa Andringa 24th November 2017 UNIVERSITY OF TWENTE MASTER’S THESIS in Water Engineering and Management Faculty of Engineering Technology Modelling the impact of storm-induced barrier island breaching on the morphodynamic evolution and stability of multiple tidal inlet systems Author T.E. Andringa BSc [email protected] Location and date Enschede, 24th November 2017 Graduation committee Graduation supervisor Prof. dr. S.J.M.H. Hulscher Daily supervisors Dr. ir. P.C. Roos Ir. K.R.G. Reef External supervisor Dr. ir. A. Dastgheib iii Abstract Barrier island systems occur all over the world and are important in defending the mainland. Next, they are connected to human activities such as navigation, industry and recreation. This makes it important to gain knowledge of the processes and the morphodynamic behaviour of these systems. The tidal inlets separate the barrier islands from each other. Water flows to and from the basin through these inlets because of the tides. As a result, sediment is transported out of the inlets. Waves, on the other hand, cause a longshore sediment transport that results in a sediment import to the inlets. The interplay between waves and tides thus determines the change in inlet morphology. There is still little knowledge on the long term morphodynamics of tidal inlet systems and their response to external changes. With an idealized model, Roos et al. (2013) demonstrated the existence of multiple stable inlets by accounting for spa- tially varying water levels. Here, we extend this by including storm-induced inlet formation to identify the evolution of new inlets and their effect on the equilibria of the system. The first part of the study contains a review of storm-induced barrier island breaching and a historical overview of breach events. The impact of a storm on barrier islands and the occurrence of breaches depend on both hydrodynamical and morphological properties of the environment. Overtopping water can carve chan- nels on the island which can cause a breach. Many of these breach events have been documented in the past, but little data of the breaches and associated storms are available. Analysis of observations show that breaches occur during storms with at least Beaufort Force 10 and that the breach area ranges between roughly 50 and 510 m2. The second part of the study contains the long-term modelling of tidal inlet sys- tems and storm-induced breaches. Three probability distributions are implemented in the model to describe breach occurrence and the location and size of the breach. Because of the stochastic forcing, a Monte Carlo analysis is used which produces statistic details of the system’s response to the storm-induced breaches. We start the simulation with one inlet at a random location in the system. Every time step there is the possibility that a breach occurs and a new inlet is created. The cross-sectional area of all inlets change in time until the system reaches its equilib- rium state with equilibrium value of total inlet area. Breaches can either close within several years or grow in size which causes existing inlets to decrease in size to re- store the equilibrium state. The initial evolution of individual new inlets depends on three factors: proximity to equilibrium, distance to neighbouring inlets and breach size. The lifetime of inlets is higher for inlets further away from existing inlets and with higher initial widths. This generic model study gives insight in the qualitative evolution of breaches and the interaction among inlets. It helps policy makers showing the possible con- sequences of a breach. Further research is recommended to reduce the uncertainties and improve (quantitative) precision of the simulation. v Preface This document is the report of my MSc thesis which is the last part of my study of Water Engineering and Management at the University of Twente. I enjoyed my study in Enschede and learned a lot the last few years. I think I have combined all I learned in this research and I wish you much pleasure reading this report. Though it is an important topic, especially nowadays with challenges as sea level rise, only few is known about the long-term behaviour of tidal inlet systems. I liked to dive into this broad and interesting subject and I hope my results contribute to the knowledge of or interest in tidal inlet systems. I would like to thank the people who helped me performing this research and guided me trough the graduation process. First of all thanks to my supervisors Su- zanne Hulscher, Pieter Roos, Koen Reef and Ali Dastgheib. I received critical but helpful feedback from all of you. From each person from a different point of view what really improved my thesis. Sometimes the number of comments were a lot, but the smileys that were drawn next to them by Pieter always encouraged me to work further. Besides, I would like to thank my friends for the nice walks and talks when I needed distraction, my parents for having faith in me and encouraging me to go study, Esther for always being there and Sander for all his help and cheering up the past half year. Tessa Andringa, Enschede, November 2017 vii Contents Abstract iii Preface v 1 Introduction1 1.1 Background..................................1 1.1.1 Barrier coast systems.........................1 1.1.2 Storm-induced formation of tidal inlets..............2 1.1.3 Modelling...............................3 1.2 Research set up................................4 1.2.1 Knowledge gap and objective....................4 1.2.2 Research questions..........................5 1.2.3 Methodology.............................5 1.3 Reading guide.................................6 2 Storm-induced inlet formation: theory and historical data7 2.1 Review of island breaching caused by storms...............7 2.2 Historical overview of breaches.......................9 2.2.1 Example: Long Island........................9 2.2.2 Summary and findings of historical breach events........ 11 2.3 Subconclusion................................. 13 3 Model set-up 15 3.1 General model set up............................. 15 3.1.1 Model formulation.......................... 15 3.1.2 Parameter settings.......................... 17 3.2 Storm-induced breaches - Stochastic modelling.............. 17 3.3 Subconclusion................................. 18 4 Results 21 4.1 Model response - example run........................ 21 4.1.1 Evolution of entire system...................... 21 4.1.2 Inlet area per inlet.......................... 21 4.1.3 Equilibria............................... 23 4.2 Monte Carlo Analysis............................. 23 4.2.1 Equilibria............................... 23 4.2.2 Evolution in time........................... 24 4.2.3 Analysis of closing inlets...................... 25 4.3 Comparison to model without storm-induced breaches (Roos et al., 2013)...................................... 26 4.3.1 Equilibrium values.......................... 26 4.3.2 Sensitivity to basin width and tidal amplitude.......... 28 4.4 Predicting evolution of new inlet...................... 28 viii Contents 4.4.1 Three stages in time......................... 29 4.4.2 Distance to closest neighbouring inlet............... 29 4.4.3 Breach width............................. 30 4.5 Sensitivity................................... 31 4.5.1 Sensitivity of breach width and breach frequency........ 31 4.5.2 Widening of inlets during storms.................. 33 4.6 Subconclusion................................. 34 5 Discussion 35 5.1 Link between storm and breach....................... 35 5.2 Model assumptions and simplifications.................. 35 5.3 Results..................................... 36 6 Conclusions and recommendations 39 6.1 Conclusions.................................. 39 6.1.1 Under what conditions do barrier islands breach, what are the spatial characteristics of the breaches and how do they develop? 39 6.1.2 How can storm-induced breaches be described in a stochastic way and implemented in the idealized model of Roos et al. (2013)?................................. 39 6.1.3 How does the model (as set up in research question2) respond to the stochastically created breaches and can the inlet stability concept be expressed in a dynamical way?............ 40 6.2 Recommendations.............................. 40 Bibliography 43 A Complementary figures 47 1 Chapter 1 Introduction 1.1 Background 1.1.1 Barrier coast systems Around 10 percent of the worlds open coasts are barrier coasts, formed a long time ago as a result of sea level rise (Stutz and Pilkey, 2011). Islands occur all over the world but mostly in areas with a low gradient in the slope of the shore and tecton- ically inactive zones. A barrier coast consists of six elements: mainland (or coast), backbarrier lagoon (or basin), inlet and inlet deltas, barrier islands, barrier platform and shoreface (shore zone of the sea floor) (Oertel, 1985). The inlets separate barrier islands and connect the basin to the sea, so water and sediment can be transported from and to the basin. The geometry and processes in and near these inlets con- stitute the tidal inlet system. The geomorphology and processes of the tidal inlet system are shown in Figure 1.1. FIGURE 1.1: Processes and geomorphology of tidal inlet system - (De Swart and Zimmerman, 2009). Showing coast, basin, inlet and inlet deltas, barrier islands and sea 2 Chapter 1. Introduction Barrier coasts differ in geometry and can be classified based on geology, climate and the wave tide regime of the area (Stutz and Pilkey, 2011). Barrier islands vary in length between 1 and 50 kilometres whereas the cross sectional area of inlets varies between 103 and 105 m2 (De Swart and Zimmerman, 2009). The cross sectional area of inlets is related to the tidal prism: the volume of water flowing through the inlet between high and low tide (Tran et al., 2012). Larger basins attract more water and thus let the inlets deal with a greater tidal prism (Oertel, 1985). The evolution of the tidal inlet system can be de- scribed by the morphological loop as shown in Fig- ure 1.2.