Guidance on Setup, Calibration, and Validation of Hydrodynamic, Wave, and Sediment Models for Shelf Seas and Estuaries

© 2017 Jon J. Williams and Luciana S. Esteves. The paper is motivated by a present lack of clear model performance guidelines for shelf sea and estuarine modellers seeking to demonstrate to clients and end users that a model is fit for purpose. It addresses the common problems associated with data availability, errors, and uncertainty and examines the model build process, including calibration and validation. It also looks at common assumptions, data input requirements, and statistical analyses that can be applied to assess the performance of models of estuaries and shelf seas. Specifically, it takes account of inherent modelling uncertainties and defines metrics of performance based on practical experience. It is intended as a reference point both for numerical modellers and for specialists tasked with interpreting the accuracy and validity of results from hydrodynamic, wave, and sediment models

Changes in coastal sediment dynamics due to managed realignment

Consolidation of the ecosystem services approach and concerns about climate change impacts are leading to a paradigm shift in the management of coastal erosion and flooding risk. Working with nature approaches aiming to restore the adaptive capacity of environments to respond to dynamic conditions are now promoted in a growing number of local and national strategies. In England, for example, Shoreline Management Plans foresee 10% of the coastline to be realigned by 2030 and 15% by 2060. Despite over 100 projects implemented in Europe, only few studies discuss the effects of managed realignment on coastal and estuarine sediment dynamics. This paper presents a conceptual model for the longer term evolution within and adjacent to management realignment sites and contrasts with evidence from published field data

X-band radar system to support coastal management decisions

The difficulties and costs associated with the acquisition of hydrodynamic and bathymetry data in the nearshore is widely recognised. While technological advances have enabled in situ measurements at increased precision, the limited spatial and temporal resolution of data continues to hinder evidence-based coastal management. This paper presents selected results from the ‘X-Band radar as a coastal monitoring tool’ (X-Com) project which tests the suitability of a land-based X-Band radar system to provide data required for practical coastal management applications. Results are shown from a radar installation at Thorpeness (Suffolk, eastern England) from August 2015 to October 2016. At this location, coastal erosion threatens clifftop and beach front properties, and the lack of understanding about the local nearshore-shore-cliff interactions has been identified as a key factor limiting the development of a sustainable coastal strategy. A model is used in a preliminary examination of surface current data from the radar. X-Com results indicate that X-Band radar systems have an unrealised potential and could become a cost-effective tool for coastal management applications pending improvements in the automation of data processing, assessment of data accuracy and end user-friendly output formats

Modelling long-term morphodynamics in practice: uncertainties and compromises

Two problems limit the ability of morphological models to reproduce the real-world behaviour of geomorphological systems: (a) the dominance of short-term observations which fail to capture the full character of morphological evolution and cannot quantify fully the primary phenomena and mechanisms of change; and (b) incomplete understanding of processes at all relevant scales. Present efforts to reduce uncertainty in morphological models assume that: (a) observations are the key to locate, quantify and reduce uncertainty; and (b) means used to quantify, minimise and control uncertainty will improve model performance. This paper first outlines the principles underpinning morphological model development concerning coastal sandy environments. Then it discusses the use of morphological models and considers approaches that improve predictions through reduced uncertainty. The discussion is supported by two examples that illustrate the compromises that must be reached between what a morphological model is required to predict and what the model can deliver in practice

Formation of Shore-Normal Grooves (Gutters) in Sandstone by Wave Action.

Regularly spaced incised shore-normal grooves (gutters) on plane consolidated surfaces in littoral and sublittoral zones are widely observed in the marine geological record. Despite their common occurrence there are few investigations into their origins in contemporary marine environments. While their formation is often attributed to wave-induced currents related to wave swash and backwash on the beach-face, no conceptual model has been advanced to explain the presence of gutters, their morphology or their quasi-regular alongshore spacing. The paper examines gutters cut in soft sandstone at Medmerry near Selsey, UK and argues that their formation is related to wave breaking and swash zone processes during an unprecedented sequence of storms in the winter of 2013/14. During this period exceptionally high near-shore waves (Hs around 6m) were recorded for the south coast beaches and these storm conditions persisted periodically through to mid-February 2014. The consequence was extensive beach erosion and the exposure of underlying substrates. In this study gutter morphology was quantified using terrestrial lidar and a wave-resolving numerical model was used to defined the nearshore wave conditions and swash characteristics. Three of the largest storm events during the winter of 2013/14 were modelled: (a) moderate waves coincident with an exceptionally high tide; (b) exceptionally high waves occurring during neap tides; and (c) high waves occurring during spring tides. The model results showed swash zone shear stress is a dome-shaped function of distance across the beach-face thereby controlling gutter depth. Further, high-speed sheet flows characterised by periodic, shore-normal, high and low speed streaks alongshore are thought to be implicated in the mechanism controlling gutter spacing. However, in any situation, the specific spacing of gutters is moderated by both the local sheet flow characteristics and the larger-scale morphological forcing. Together these factors indicate that gutter spacing is an emergent property which makes spacing unpredictable

The Role of Coral Reefs in Coastal Protection: Analysis of Beach Morphology

This paper evaluates the effect of a fringing reef on the morphodynamic behaviour of adjacent beaches in terms of profile stability and cross-shore sediment exchange. Variations in subaerial beach morphology along 39 cross-shore profiles at Pontal do Cupe beach (Northeastern Brazil) were analysed, using modelled wave data and monthly beach topography acquired from November 2014 to September 2016. Pontal do Cupe has a reef to the south but is exposed to waves in the north, making this an ideal location to assess the sheltering effect of the reef. Beach volume and beach width data were used to compare the reef-fronted profiles with those of the exposed adjacent beach. Seven groups of profiles were identified by applying Principal Component Analys is to the topography dataset. A simple numerical model was used to quantify the role of the reef in dissipating wave energy, showing a reduction of approximately 50% in incoming wave energy to the shore. The reef-fronted beach is significantly more stable than the exposed beach. Total beach volume is similar for both the exposed and the reef-fronted beach. The results of this survey can be used as a proxy for the ecosystem service of coastal protection provided by reefs

Modelling storm responses on a high-energy coastline with XBeach

The XBeach model has been used to simulate the morphological impacts of storms on sandy and gravel beaches. Taking as a case study Rossbeigh Spit located on the high-energy coast of western Ireland, the study reported here tests the capacity of XBeach to reproduce barrier breaching during a storm in December 2008. It demonstrates that predictions of the breaching event agree reasonably well with observations. However, the main focus of the paper is to establish using the model results, site-specific critical wave and water level conditions giving rise to dune erosion, overwashing and breaching. By deriving simple-to-use expressions to define hydrodynamic thresholds the study advances the ability to predict the impacts of infrequent and rarely observed storm events and is considered to provide useful coastal management tool for assessing the vulnerability of sandy barriers to breaching high-energy during storms

Monitoring nearshore processes and understanding significant coastal change using x-band radar

Remote sensing through X-band radar can provide wave and current parameters and bathymetric maps in a 4-km radius from a land-based deployment. This paper explores the use of radar to monitor changes in nearshore bathymetry at Thorpeness, Suffolk, UK. The method presented enables significant nearshore changes to be identified based on the analysis of standard deviation of sediment volume. Seasonal changes in bathymetry can reach 4 m but depths tend to be consistent in each season. A storm power index was calculated for periods of time preceding the significant changes in bathymetry. Results indicate that impact on the nearshore is not directly linked to storm power. Storm clusters and antecedent nearshore conditions seem to be important factors, as larger volume changes were measured as a result of the first and smallest storm of a cluster