ReFresh

1. Motivation

Recent extreme storm surges induced for instance by hurricanes Irma and Katrina that respectively hit Florida (FL) on 2017 and Louisiana (LA) on 2005 have proven that coastal areas are highly vulnerable environments to coastal floods as they may experience severe impacts from coastal storms. During extreme storm surges, coastal defences (Fig 1) are directly attacked by shortwaves riding on the surge, possibly resulting in coastal barrier breach(es) (Elsayed et al., 2018; Elsayed and Oumeraci, 2016). As a result, a wide extent of the hinterland is inundated leading to vertical saltwater intrusion (VSWI) as the propagating saltwater infiltrates vertically into the freshwater aquifers (Elsayed, 2017; Elsayed and Oumeraci, 2018). For instance, approximately 80% of New Orleans (LA) were flooded for weeks due to the flooding induced by levee failure under Hurricane Katrina. The toll a year later was listed as 1,118 dead and 135 missings, more than 400,000 citizens fled the city, many never to return and property damage reached tens of billions of dollars (Wu et al., 2011). Less known, however, is that the same flood event resulted in the increase of the aquifers salinity in western and eastern coastal Louisiana, causing significant damages to the agriculture sector. Williams (2010) reported that the preliminary estimates of the economic impact from Hurricane Katrina to Louisiana agriculture due to salinity increase may exceed $1 billion dollars.

Coastal aquifers as indicated in Fig 1 represent an important source of fresh water in coastal areas. On many atoll islands, such aquifers are the only source of freshwater. However, they are extremely sensitive to saltwater intrusion, particularly the vertical intrusion induced by coastal floods. Therefore, groundwater resources threatened by coastal floods require increasing research efforts to address VSWI processes. Considering that Europe is often surrounded by coastal defence systems (e.g. dunes and dykes), addressing the VSWI processes and their modelling requires further research to improve the knowledge and modelling in two interrelated directions: (i) Processes underlying the erosion/breaching of coastal barriers under extreme storm surge conditions and (ii) Vertical saltwater intrusion (VSWI) into freshwater aquifers resulting from barrier breaching and subsequent flooding.

2. Objectives

The primary objective of ReFresh is to create a knowledge base for an improved understanding of the physical processes associated with the breaching of coastal barriers such as dunes under extreme storm surges as well as with those associated with the response to the subsequent inundation (e.g. VSWI and degree of contamination) and recovery (e.g. up to potable water) of freshwater aquifers. The new knowledge and the prospective predictive formulae will be implemented in state-of-the-art surface and subsurface numerical codes (e.g. XBeach and OpenGeoSys respectively), and particularly for properly coupling these two codes. In summary, the following specific objectives of ReFresh might be specified:

• OBJ1: to perform a comprehensive analysis and summary of the current knowledge specific to erosion and breaching of highly grain-stabilized or consolidated barriers (Elsayed and Oumeraci, 2017), spatial variability of soil resistance and response of coastal aquifers to coastal floods and their post-flood recovery.
• OBJ2: to develop new mathematical formulae describing and accounting for the physics of the processes mentioned in OBJ1.
• OBJ3: to provide, in the light of new quantitative experiments for VSWI, an improved understanding on how the remediation of contaminated freshwater aquifers works.
• OBJ4: to implement a novel full coupling between existing surface and subsurface opensource models (e.g. XBeach and OpenGeoSys, respectively).
• OBJ5: to validate the model obtained from OBJ4 for one pilot site in Germany.

3. Methodology

To achieve these objectives, the work programme is divided into five distinct work packages (WP1 - WP5). As outlined in Fig. 2, each WP address one of the five aforementioned objectives.

The financial support of the German Research Foundation (Deutsche Forschungsgemeinschaft; DFG) for the ReFresh project (EL 1017-1/1) is gratefully acknowledged.

Elsayed, S.M. (2017): Breaching of Coastal Barriers under Extreme Storm Surges and Implications for Groundwater Contamination. PhD dissertation, Leichtweiß Institute for Hydraulic Engineering and Water Resources, TU Braunschweig. Available at: dx.doi.org/10.24355/dbbs.084-201710161043, Braunschweig, Germany. https://doi.org/10.24355/dbbs.084-201710161043

Elsayed, S.M., Oumeraci, H. (2018): Modelling and Mitigation of Storm-Induced Saltwater Intrusion: Improvement of the Resilience of Coastal Aquifers Against Marine Floods by Subsurface Drainage. Environ. Model. Softw. 100, 252-277. doi.org/10.1016/j.envsoft.2017.11.030

Elsayed, S.M., Oumeraci, H. (2017): Effect of beach slope and grain-stabilization on coastal sediment transport: An attempt to overcome the erosion overestimation by XBeach. Coast. Eng. 121, 179-196. doi.org/10.1016/j.coastaleng.2016.12.009

Elsayed, S.M., Oumeraci, H. (2016): Combined Modelling of Coastal Barrier Breaching and Induced Flood Propagation Using XBeach. Hydrology 3, 34. doi.org/10.3390/hydrology3040032

Elsayed, S.M., Oumeraci, H., Goseberg, N. (2018): EROSION AND BREACHING OF COASTAL BARRIERS IN A CHANGING CLIMATE: ASSOCIATED PROCESSES AND IMPLICATION FOR CONTAMINATION OF COASTAL AQUIFERS. Coast. Eng. Proc. 1, 107. doi.org/10.9753/icce.v36.papers.107

Williams, V.J.V. (2010): Identifying the Economic Effects of Salt Water Intrusion after Hurricane Katrina. J. Sustain. Dev. 3, 29-37.

Wu, W., Mustafa, S., Altinakar, S.F., Bradford, Q.J., Chen, S., Jennifer, G., Duan, D., Michael, G. (2011): Earthen Embankment Breaching. J. Hydraul. Eng. 137, 1549-1564. doi.org/10.1061/(ASCE)HY.1943-7900.0000498