Search

Mechanistic models for complex atmospheric and hydrological processes are often used to simulate extreme natural events, usually to quantify the risks that are associated with these events. We use novel extreme value methods to analyse the statistical properties of output from a numerical storm surge model for the North Sea. The 'model data' constitute a reconstruction of the storm surge climate for the period 1955-2000 based on a high quality meteorological data set and constitute the only available source of information on surge elevations at offshore and unmonitored coastal locations over this period. Previous studies have used extreme value methods to analyse storm surge characteristics, but we can extend and improve on these analyses by using a local likelihood approach to provide a non-parametric description of temporal and spatial variations in the magnitude and frequency of storm surge events.

Analysis of sea-level data from the east coast of England identifies local-scale and regional scale factors to explain spatial and temporal variations in the altitude of Holocene sea-level index points. The isostatic effect of the glacial rebound process, including both the ice (glacio-isostatic) and water (hydro-isostatic) load contributions, explains regional-scale differences between eight areas: c. 20 m range at 8 cal. ka BP and by 4 cal. ka BP relative sea-level in Northumberland was above present, whereas in areas to the south relative sea level has been below present throughout the Holocene. Estimates for pre-industrial relative sea-level change range from 1.04 ± 0.12 mm a-1 in the Fenland to -1.30 ± 0.68 mm a-1 (i.e. sea-level fall) in north Northumberland, although this may overestimate the current rate of sea-level fall. Isostatic effects will produce similar relative differences in rates of sea-level change through the twenty-first century. The data agree closely with the patterns predicted by glacio- and hydro-isostatic models, but small systematic differences along the east coast await testing against new ice models. Local scale processes identified include differential isostatic effects within the Humber Estuary and the Fenland, tide range changes during the Holocene, and the effects of sediment consolidation. These processes help explain the variation in altitude between sea-level reconstructions derived from index points taken from basal peats and those from peats intercalated within thick sequences of Holocene sediments.

Analysis of cores collected from Late Devensian (Weichselian) and Holocene sediments on the floor of the North Sea provides evidence of the transgression of freshwater environments during relative sea-level rise. Although many cores show truncated sequences, examples from the Dogger Bank, Well Bank and 5 km offshore of north Norfolk reveal transitional sequences and reliable indicators of past shoreline positions. Together with radiocarbon-dated sea-level index points collected from the Holocene sediments of the estuaries and coastal lowlands of eastern England these data enable the development and testing of models of the palaeogeographies of coastlines in the western North Sea and models of tidal range changes through the Holocene epoch. Geophysical models that incorporate ice-sheet reconstructions, earth rheology, eustasy, and glacio- and hydro-isostasy provide predictions of sea-level relative to the present for the last 10 ka at 1-ka intervals. These predictions, added to a model of present-day bathymetry, produce palaeogeographic reconstructions for each time period. The palaeogeographic maps reveal the transgression of the North Sea continental shelf. Key stages include a western embayment off northeast England as early as 10 ka BP; the evolution of a large tidal embayment between eastern England and the Dogger Bank before 9 ka BP with connection to the English Channel prior to 8 ka BP; and Dogger Bank as an island at high tide by 7.5 ka BP and totally submerged by 6 ka BP. Analysis of core data shows that coastal and saltmarsh environments could adapt to rapid rates of sea-level rise and coastline retreat. After 6 ka BP the major changes in palaeogeography occurred inland of the present coast of eastern England. The palaeogeographic models provide the coastline positions and bathymetries for modelling tidal ranges at each 1-ka interval. A nested hierarchy of models, from the scale of the northeast Atlantic to the east coast of England, uses 26 tidal harmonic

The primary objective of the EC-MAST III project PROMISE (PRe-Operational Modelling In the Seas of Europe) is to develop a framework to optimize the application of existing pre-operational dynamical models of the North Sea towards the focus of quantifying the rates and scales of the exchange of sediments between the coast and the near-shore zone and, thereby, to enable the subsequent application of this framework to other coastal areas and for broader management applications. PROMISE started early 1996 and will end early 1999. The work is structured into 4 components: observations (O), rationalizing existing models (R), wave modelling (W) and turbulence / sediment modelling (S). Here the focus is on wave modelling, with links to the other components and in particular to the data needs for model performance assessment. For the W-component the objective was to develop methodologies for application of existing third generation (3G) models in shallow water, including numerical improvements for high resolution applications and the introduction of interactions associated with tidal and storm surge dynamics, ultimately for use in SPM-modelling. The steps were to establish and disseminate codes to simulate: • Waves in the North Sea – shelf scale • Waves in shallow water – coastal scale • Combined waves and tides in shallow water – coastal scale Waves in the North Sea (shelf scale) have been described in Monbaliu et al. (1997), where an intercomparison exercise between different implementations of the WAM-model on a North-Sea scale was executed. This reports described the code changes which have been brought together to extend the capabilities of the WAM-model in shallow water applications. Combined waves and tides in shallow water will be described in a separate report.