Effects of Beach Nourishment Project on Coastal Geomorphology and Mangrove Dynamics in Southern Louisiana, USA

Relative sea-level (RSL) rise associated with decreased fluvial sediment discharge and increased hurricane activity have contributed to the high rate of shoreline retreat and threatened coastal ecosystems in Port Fourchon, Louisiana, USA. This study, based on QuickBird/drone images (2004–2019) and LIDAR data (1998–2013), analyzed the impacts of shoreline dynamics on mangroves (Avicennia germinans) and marshes before and after the initiation of a beach nourishment project in 2013. The coastal barrier and dune crest migrated landward between 1998 and 2013. Meanwhile, the dune crest height increased between 1998 and 2001, then decreased in 2013, probably due to hurricane impacts. The total sediment volume along this sandy coastal barrier presented an overall trend of decline in the 1998–2013 period, resulting in a wetlands loss of ~15.6 ha along 4 km of coastline. This has led to a landward sand migration onto muddy tidal flats occupied by Avicennia germinans (1.08 ha) and Spartina (14.52 ha). However, the beach nourishment project resulted in the advancement of the beach barrier from Nov/2012 to Jan/2015, followed by a relatively stable period between Jan/2015 and Mar/2019. Additionally, both the dune crest height and sediment volume increased between 2013 and 2019. This set of factors favored the establishment and expansion of mangroves (3.2 ha) and saltmarshes (25.4 ha) along the backbarrier environments after 2013, allowing the tidal flats to keep pace with the RSL rise. However, waves and currents caused shoreline erosion following the beach nourishment project between Oct/2017 and Nov/2019, threatening wetlands by resuming the long-term process of shoreline retreat.

De Zijpe en de Zuiderzee

The Zijpe and the Zuiderzee Dutch geologists and archaeologists disagree about the time the Flevo-lakes in the Wet Heart of the Netherlands were connected with the western part of the Waddenzee. Geologists hold the strong view that this connection originated in the centuries bc. Archaeological research revealed, however, that the pre-urban development of the city of Amsterdam did not start until the last quarter of the 12th century ad. This urban development has been considered as an indication that the shipping route between the mouth of the river Amstel and the Waddenzee had improved shortly before. Therefore, archaeologists are convinced that the improvement of this shipping route was forced by the All Saint’s flood of 1170. To the west of Schagen, the sea is considered to have broken up the coastal barrier which led to the development of the Zuiderzee. Because of the etymological relation between the names Flevo and Vlie, the Flevo-lakes must have been already connected with the Waddenzee in the Roman period. Near Medemblik, however, the water did not become brackish before the 8th century ad and more to the south not earlier than the 9th century. Therefore, drainage must have been the main function of the connection between the Flevo-lakes and the Waddenzee for centuries. From historical documents it is known that somewhere around Stavoren a large amount of land was lost to the sea in 1170. Together with the swift development of Amsterdam it is likely that the Zuiderzee came into being after 1170. A breakthrough of the coastal barrier to the west of Schagen at that time, however, is unlikely. In 1170 there was some damage in the northern part of the province of Holland caused by a storm surge, but the loss of land was limited due to the building of several dikes.

ROLE OF SPATIAL VARIABILITY OF SOIL RESISTANCE IN ALONGSHORE VARIABILITY OF COASTAL BARRIERS RESPONSE TO SUPERSTORM SURGES

Sand dunes and other natural coastal barriers (e.g. barrier islands) represent important components of the defense system against consequences of storm surges. However, in many coastal systems, major storm surges represent important drivers of coastal erosion. Increased extreme events potentially result in accelerated coastal erosion, coastal barrier breaching, and coastal flooding. The response of a barrier to a storm surge is often determined by mutual interaction among the driving hydrodynamics, the subsequent morphodynamics, and the local geology, including spatial variations of subaqueous bathymetry and subaerial topography. However, the effect of alongshore variability of soil properties on the alongshore varying response is not yet considered. Therefore, this study examines soil parameters that may affect coastal erosion during major storm surges. Moreover, it applies a novel extension of the numerical model XBeach that accounts for spatial variation of soil properties to an artificial dune system of spatially varying soil permeability. Results showed that variability of soil permeability alongshore the dune results in alongshore varying resistance to erosion so that breaches may occur at the locations of less resistance that are corresponding to locations of higher soil permeability. Outcomes of the numerical simulations proved also that reduced soil permeability represents a nature-based solution that increases the resilience of natural defense systems during major storm surges by mitigating rates of coastal erosion.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/1ERwbW5BmYA

NATURE-BASED DYNAMIC REVETMENT CONSTRUCTION AT NORTH COVE, WASHINGTON, USA

A dynamic revetment was constructed at North Cove, Washington, USA in December 2018 along a historically eroding 2-km shoreline reach of coastal barrier at the northern entrance to Willapa Bay. The revetment is composed of poorly sorted angular quarry rock ranging in size from pea gravel to small boulders as well as large wood debris and structures, a dune ridge, and native vegetation integrated with the revetment. The design, aim, and maintenance of the dynamic revetment is to simulate the functions of naturally forming cobble berms along composite beaches in the U.S. Pacific Northwest. The dynamic revetment continues to be adaptively constructed over time, enabling the testing of innovative design approaches and concepts that are rarely possible to do at full-scale in the field. The project provides a unique opportunity to explore nature-based engineering principles and design features.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/5w18tEjEePg