Flooding-erosion interactions: implications for coastal risk management

Pollard, J.A. and Brooks, Susan and Spencer, T. and Christie, E.K. and Moeller, I. (2019) Flooding-erosion interactions: implications for coastal risk management. In: ICE Coastal Management 2019, 24-26 Sep 2019, La Rochelle, France.

Abstract

As routinely experienced coastal hazards, flooding and erosion are key considerations within coastal management plans and policy. Flooding and erosion hazards are often analysed separately, without due attention to their interaction. Low lying barrier islands exemplify coastal environments particularly affected by erosion-flooding interactions. Such environments often support substantial human populations, critical infrastructure, and diverse ecosystems. This study undertakes a high resolution, multidecadal shoreline change analysis at Blakeney Point, a mixed sand-gravel spit on the UK’s North Norfolk coast. The analysis spans two distinct management regimes: the ‘first era’ (1992-2005) where the eastern section of the barrier was periodically artificially reprofiled into a steep-sided trapezoid with a narrow crest; and the ‘second era’ (2006-2016) of no active intervention along the entire barrier. We find that over the past 130 years, Blakeney Point has retreated landward at a mean rate of 0.60 m a-1. Along the eastern section of Blakeney Point, we observe an increase in shoreline retreat rate between the two eras of 3 to 30 times depending on the choice of shoreline proxy (High Water Line, ridge line, or vegetation line). The lower shoreline retreat rates during the first era suggest that reprofiling fixed the shoreline position of the eastern barrier. Termination of the reprofiling regime, in combination with storm surge events in 2007 and 2013, resulted in accelerated retreat along the eastern section towards a more landward position. This has had implications for the western section of the spit which appears to have benefited from alongshore westward transport of sediment during the noninterventionist era, resulting in slowed landward retreat rates during this period. The importance of storm surge events should not be understated. During 2013-2014, overwashing along the eastern section of the barrier resulted in vegetation line retreat of up to 127 m. This retreat behaviour was not matched at lower elevations on the subaerial beach, suggesting lateral spreading of the barrier, a behaviour which has been shown elsewhere to involve increased landward retreat and barrier disintegration. Our findings suggest a change in the morphological character of the barrier under the non-interventionist management regime, with clear implications for erosion, flooding and their interaction

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