Barrier island hydro-morphodynamics driven by typhoon events – KUN-SHEN

Taiwan is located in a sub-tropical region exposed to extreme meteorological events and is on the track of numerous major typhoons. More than 38 typhoons crossed Taiwan in less than 10 years. Moreover, the South-Western coast of Taiwan displays well-expressed lagoonal systems that do not exist elsewhere in Taiwan. Sand barriers are well developed, and interesting morphologic features are displayed such as wash-over deposits and storm inlets. In the vicinity of Tainan county, Cigu lagoon and the Wan-tzu-liao sand barrier are one of the most striking examples of such littoral systems around the world. Therefore, KUN--SHEN ANR/NSC project aims at characterizing misunderstood phenomena that occur during typhoon-like events and their consequences on the sand barrier hydro-morphodynamics of Cigu littoral. The challenge of the present ANR project is to join together a field work approach, physical experimentations, numerical modelling and a more general naturalistic point of view with theoretical physics.

In order to identify the role of typhoons on the coastal hydro-morphodynamics of Cigu nearshore and sand barrier, the morphostratigraphy, geomorphological (morphology, altimetry, bathymetry) and environmental constraints of the Cigu littoral will be characterized. Field measurements will be elaborated in order to get hydrodynamic and morphodynamic parameters (e.g. instantaneous water level, velocity profiles, directional wave spectrum, wave characteristics, circulation patterns). Frames and equipments will be adapted to the monitoring of hydro-morphodynamics under extreme conditionss so that they can be securely deployed during a full typhoon season.

Physical experimentation will provide a good frame to validate conceptual and numerical results. THL has a long experience in physical experimentation of coastal and nearshore processes and displays one of the most important set of facilities around the world. The experiments will be performed in a 300 m long flume Super Tank and a 150 m large wave basin forced by fairweather and storm waves, and with a sandy bottom that will mimic a real nearshore area, emerged sand barrier and lagoon. Several sets of extreme forcings (to the scale) will be applied. On that basis, the experimentation aims at providing new informations relative to extreme wave transformation and wave/ wave interaction to the coastline, nearshore morpho-dynamics under extreme events, over-washing and subsequent wash-over deposition processes. Specific attention will be paid to the characterization of the changes of instantaneous water elevation and submersion of the barrier island itself. Several numerical models will be used to characterize the hydro-morphodynamics of Cigu system, from the shoaling zone in the open sea, to the lagoon, including sand barrier domain broken through by inlets created during storm.

The numerical results with realistic forcings will be mainly compared to results from field data campaign for their reciprocal validation. Conversely, other simulations with several sets of typhoon wind/ wave conditions will help to generalize our knowledge of the extreme hydro-morphodynamics of Cigu. On the other side, hydro-morphodynamic models will be used to compute morphodynamic response of sand beach (immersed and emerged) under storm wave conditions, using some recent numerical developments relative to beach modelling.

Last we aim at highlighting the fundamental controls of instantaneous sea level dynamics during extreme events. To tackle this question, explicit expressions of the instantaneous elevation of water will be derived from the shoaling zone to the barrier dune. The same type of conceptual development will be performed for the peculiar questions of extreme wave/ wind interactions, over-washing and subsequent wash-over deposition mechanism. Other fundamental questions such as the existence of threshold phenomena in the beach dynamics driven by extreme events will be discussed.