Impact of Management Regime and Regime Change on Gravel Barrier Response to a Major Storm Surge

Abstract

Gravel barriers represent physiographic, hydrographic, sedimentary, and ecological boundaries between inshore and open marine offshore environments, where they provide numerous important functions. The morphosedimentary features of gravel barriers (e.g., steep, energy reflective form) have led to their characterization as effective coastal defense features during extreme hydrodynamic conditions. Consequently, gravel barriers have often been intensively managed to enhance coastal defense functions. The Blakeney Point Barrier System (BPBS), U.K., is one such example, which offers the opportunity to investigate the impact of alternative management regimes under extreme hydrodynamic conditions. The BPBS was actively re-profiled along its eastern section from the 1950s to the winter of 2005, whilst undergoing no active intervention along its western section. Combining an analysis of remotely sensed elevation datasets with numerical storm surge modeling, this paper finds that interventionist management introduces systemic differences in barrier morphological characteristics. Overly steepened barrier sections experience greater wave run-up extents during storm surge conditions, leading to more extreme morphological changes and landward barrier retreat. Furthermore, while high, steep barriers can be highly effective at preventing landward flooding, in cases where overwashing does occur, the resultant landward overtopping volume is typically higher than would be the case for a relatively lower crested barrier with a lower angled seaward slope. There is a growing preference within coastal risk management for less interventionist management regimes, incorporating natural processes. However, restoring natural processes does not immediately or inevitably result in a reduction in coastal risk. This paper contributes practical insights regarding the time taken for a previously managed barrier to relax to a more natural state, intermediary morphological states, and associated landward water flows during extreme events, all of which should be considered if gravel barriers are to be usefully integrated into broader risk management strategies.

Gravel barrier resilience to future sea level rise and storms

“…XBeach-G reproduced observed morphological change on profiles that underwent minimal change, washover, and sluicing washover, but performed less well where profiles underwent barrier crest accretion (here the model underestimated accretion (Pollard et al, 2021)). The model was unable to reproduce barrier breaching leading to the omission of profile 11 which coincided with a breach location.…”

“…Both models demonstrated good agreement with observed water levels (RMSE = 0.23 m), wave height (RMSE = 0.30 m) and wave period (RMSE = 2.5 s). The reader is referred to Pollard et al (2021) for further details of the TELEMAC-2D and SWAN calibration procedure.…”

“…Each calibration run was then ranked on both the BSS and crest height difference. Ranks from each measure were multiplied together (applying an equal weighting to BSS and crest height performance) to obtain an overall rank for each calibration parameter combination (Pollard et al, 2021). Best performance across all the validation cases was achieved using infiltration and bed friction coefficient values of 0.4 and 0.02 respectively.…”

A Study on the Characteristics of Beach Profile Evolution According to the Particle Size Variation of Beach Nourishment

“…However, when an abnormal wave hits the coast, such as a storm surge, the high wave run-up exceeds the morphological threshold and causes large-scale topographic change [1][2][3]. In particular, when high waves along with typhoons hit the coast, the sea level rises, and sand movement is promoted in the backshore [4]. As a countermeasure, many construction methods are applied, and the coastal environment is maintained and changed by the characteristics of each method.…”

Catastrophic overwash and rapid retreat of a gravel barrier spit during storm events (Sillon de Talbert, North Brittany, France)

“…Significant rollover processes occur during extreme events when wave runup overtops, overwashes, or strongly inundates the crest of the barrier over a very short time period (i.e. microscale, <10 0 yr) (Brown et al, 2019; Masselink & van Heteren, 2014; Matias et al, 2012; Phillips et al, 2020; Pollard et al, 2021). The most effective storm‐impact scaling model describing the response of the barriers to storms was proposed by Orford and Carter (1982).…”