'Geomorphic evolution of the Great Yarmouth coastal system: spit sediment dynamics and forcing mechanisms'

The Crown Estate-Caird Fellowship 2009

Hannah M. Evans, British Geological Survey

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Executive summary

The potential for offshore sandbank systems to support resources such as wind farm developments, marine aggregates and sensitive habitats has triggered the need for an understanding of their geomorphic evolution. Such is the case for offshore sandbanks in the region of Great Yarmouth, Norfolk which have experienced a recent spate of investigation. However, despite the likelihood of sediment links, the interaction of these banks with adjacent coastal features has been largely overlooked. An understanding of coastal geomorphology is also central to the management of onshore resources such as buildings, infrastructure, scenic landscapes and sensitive habitats. This is especially pertinent in areas such as the Great Yarmouth region of Norfolk where the coastal geomorphology (the Great Yarmouth spit) protects low-lying land from extensive flooding.

Despite this strategic role in protecting and maintaining offshore and onshore resources, the role of coastal geomorphological features as sediment sinks is often neglected from studies of coastal sediment dynamics. For example, little is known of the volume of sediment held within the Great Yarmouth spit or its short-term fluctuations in sediment storage capacity. Such gaps in coastal sediment budgets mean that the effects of predicted future increases in relative sea level (RSL) and storminess are difficult to forecast. The current study will address this by examining the late-Holocene geomorphic evolution of the Great Yarmouth spit, providing a value for the volume of sediment stored within the feature and placing current morphological changes within an historical context. The specific aims of this research are to investigate: (1) spit volume, (2) spit morphological change (3) potential forcings for this change and (4) likely future morphological trends. Findings disseminated within this report result from the Crown Estate-Caird Research Fellowship, under the joint scheme between The Crown Estate and the National Maritime Museum.

Mathematical modelling of the pre-Holocene topography of the Great Yarmouth area highlights the existence of a wide palaeo-valley cutting the modern coastline between Caister-on-Sea in the North and Gorleston-on-Sea to the South. The Great Yarmouth spit lies across this feature and appears to be grounded against topographic highs contained within the channel. An early form (pre-1613AD) of the Great Yarmouth spit, lying across this estuary and extending southwards to Gunton, contained approximately 213x106m3 of sediment. The current feature holds approximately 190x106m3 and provides a significant sink for sediment within the Great Yarmouth coastal system.

Short-term fluctuations in the Great Yarmouth spit’s sediment storage capacity were identified by investigating morphological changes within the feature’s coastal zone from map and aerial photograph evidence. These fluctuations appear to be site-specific across the Great Yarmouth spit and adjacent areas. Indeed, between 1800 and 2007 Winterton-on-Sea experienced accretion whilst coastline retreat followed by a period of relative stability typified trends at Caister-on-Sea. North Denes shows sea-ward advance of the coastline but this occurs after a phase of erosion prior to 1890.

Great Yarmouth Pleasure Beach and Gorleston-on-Sea have remained relatively stable after a similar phase of pre-1890 erosion. All sites investigated are currently (2008 data) displaying either maintenance of or an increase in sediment storage capacity within the stable spit area (that above normal tidal conditions) with the exception of Caister Point which is undergoing erosion.

RSL change of +1.5mmyr-1 from 1000yrBP to present may account for a general landward migration of the coast planform. However, shorter term perturbations appear better related to individual storm events and coastal engineering works. The most significant event in terms of morphological change was the harbour engineering works of 1613AD which caused a reduction in spit volume of 11% through disruption of littoral drift patterns. The degree of morphological change caused by individual storm events is dependent upon antecedent beach levels, the combination of meteorological and tidal conditions and the state of the coastal defences. Predicted regional changes in sea-level and storminess are likely to cause landward retreat of the coastal planform and reduction in stable spit area. Along defended sections of the Great Yarmouth coast, narrowing of the inter-tidal zone may be expected.