Comparison of tide-gauge data and a saltmarsh-derived reconstruction of mean sea-level for the Mersey Estuary

Using saltmarsh sediment cores, Mills (2011) reconstructed the historic trend of mean sea-level in the Mersey over a period since 1975. The analysis is based on the foraminifera species identified at different levels within the sediment core; each species being associated with a tidal elevation (for example, mean high water neap) identified from present-day vertical distribution of saltmarsh foraminifera at the coring sites. While the reconstruction at Decoy Marsh matched the tide gauge record at Gladstone Dock, the reconstruction at Oglet Bay for the period 1993 and 2003 disagreed. During this period the reconstruction suggested an initial drop in mean tidal level (MTL) of 50 cm followed by a 50 cm rise back to the underlying trend after 2002. Because a local drop in sea-level (SL) is unlikely, and the foraminifera fossils used in the reconstruction are unlikely to have changed their tolerance to inundation, another factor must account for this sea-level anomaly. Here using the 3D hydrodynamic Proudman Oceanographic Laboratory Coastal Ocean modelling System (POLCOMS), the impact of the position of the main estuarine channel and historic sea-level elevations on the tidal dynamics are investigated relative to the conditions in 2008. Changes in the proportion of time that certain elevations at the saltmarsh coring sites are inundated could explain the deviation observed in the reconstruction. Such an effect is hypothesised to occur in response to local changes in the tidal dynamics, i.e. changes in tidal range or asymmetry in tidal elevation. It is found that in response to changes in channel configuration to test the scenario of a northern channel migrating up-estuary through Oglet Bay, a change in inundation characteristics caused by a change in the bank drying phase of the tidal cycle, may well have contributed to the anomalous reconstruction

The impact of tidal lagoons on future flood risk on the North Wirral and Conwy coastline, UK

This report considers the viability of tidal lagoons in the North Wirral and Conwy coastlines, to mitigate future flood risk and reduce the cost of damage in these areas. The report is aims to provide information on the feasibility and benefits of tidal lagoons as mitigation and adaptation strategies to future sea-level rise, as part of the RISES-AM project. Sea-level has been rising since instrumental records began in the 1700s, and has been rising at a rate of 3.0 ± 0.7 mm / yr-1 since 1990 (Hay et al., 2015). Low probability, plausible high-end sea-level rise scenarios, where global average warming exceeds 2oC in respect to the pre-industrial level, estimate up to 0.98 m sea-level rise (SLR) by 2100 (Church et al., 2013). There is a move away from hard defences in favour of strategies which can mitigate flood risk benefit and allow coastal communities to adapt to and benefit from high-end SLR scenarios (Linham and Nicholls, 2010). Tidal lagoons could be one such innovative option. The report aims to assess the impact of the construction of tidal lagoons on flood risk on the North Wirral and Conwy coastline, under future high-end sea-level rise scenarios. Computer simulations of extreme flood events, using a 2D hydrodynamic model called LISFLOOD, will estimate changes in the extent and depth of flooding following the construction of a lagoon under both present day and future extreme climate conditions. The results of LISFLOOD suggest that: • Colwyn Bay and the North Wirral coastline are not areas at increased flood risk under baseline future high-end SLR, due to steep topography and existing defences. • Infrastructure at Stanlow oil refinery and Connah’s Quay in the North Wirral domain and residential areas in the Colwyn Bay domain at Llandudno, Rhyl and Prestatyn experience increased flood risk under RCP 4.5 (0.72 m SLR) and RCP 8.5 (0.98 m SLR) with no tidal lagoon. This is due to low-lying topography. • The presence of a tidal lagoon on the North Wirral provides flood risk benefit to infrastructure at Stanlow and Connah’s Quay as the magnitude of tidal currents is limited through the Dee and Mersey Estuary. However the size of the lagoon and the bathymetry of Liverpool Bay may mean the lagoon in this study may not be financially feasible. • The construction of a tidal lagoon at Colwyn Bay increases extent and depth of inundation at Llandudno, Rhyl and Prestatyn under all sea-level rise scenarios. Increased flood risk in these areas following the construction of a tidal lagoon is reason enough not to build a lagoon in this location. Tidal lagoons have the potential to offer flood risk benefit and become part of integrated strategies to minimise flood risk in coastal areas. The benefits of tidal lagoons are dependent on their shape, size and location, and feasibility studies should consider impacts in the near- and far-field

Application of X-band radars for deriving intertidal bathymetries and characterising coastal behaviours

Coastal monitoring techniques aim to capture the relationship between physical forcing factors and morphological change, at a range of timescales to understand ongoing coastal processes and identify areas prone to erosion and flooding hazards posed by storms. Standard marine radar provides temporally and spatially continuous monitoring data over a wide area in all conditions, and images can be processed to generate intertidal bathymetries to assess morphological change across event (days-years) timescales. This research applies a series of intertidal bathymetries derived from a standard marine radar deployed at Camber Sands, southeast England in XBeach, a process-based, storm response model, to assess wave runup hazard at the coast during a high energy storm event from the deployment period. Wave runup is dependent on offshore wave climate and beach slope and used here as a proxy to explore the influence of nearshore morphological variability, represented by different processing techniques to derive intertidal bathymetries from the marine radar images, on a coastal hazard. XBeach is used in combination with beach survey data from the site to first demonstrate reasonable skill in reproducing wave runup observations. Intertidal bathymetries are derived from the marine radar images using either a local or regional water level signal, and an average of 1, 5, or 10 days of images preceding the storm event. Modelled wave runup shows up to 0.32 m sensitivity to input intertidal bathymetries, which could be important for overwash predictions. The slope and resolution of the radar-derived intertidal bathymetries is sensitive to the water level time series used. This research is the first time that radar-derived intertidal bathymetries have been used to assess a coastal hazard in a process-based model, and results show that ideally users would have a locally measured water level to accurately generate intertidal bathymetries, and extended beach surveys for ground truthing

Modelling coastal erosion and sediment transport on the Dungeness Foreland, UK

Ageing coastal defence across the UK is challenging managers to redesign schemes to be resilient, cost-effective and have minimal or beneficial environmental impact. We take Dungeness and Romney Marsh, a region of high value in terms of habitat and energy infrastructure, as a case study that could potentially be a site for a ‘sandscaping’ project, i.e. an innovative, large-scale beach recharge scheme. At present, this location has both modified gravel barrier defences and engineered structures. We present results for a feasibility study to improve understanding of how ‘working with natural processes’ to manage coastal flood and erosion risk could provide and support defences protecting this site, ensuring an energy supply that is resilient to climate change. This modelling study investigates the impact of re-engineering the coastline with a series of sandscaping options that mimic the natural shape and former evolution of the Dungeness coastline. Particle tracking is used to show the potential pathways of recharged sediment (fine and medium sand) movement along the coastline in both calm and stormy conditions. A coastal evolution model is also applied to assess the alongshore impact of different intervention designs. It is found that the main sediment drift is likely to be towards the north along the coast and that considering larger interventions could possibly provide increased protection for up to 100 years. Further, a series of three smaller sandscaping interventions offers the greatest immediate reduction in erosion rate. The natural drift within the system causes the initial peninsula-shaped intervention to form a recurve that could potentially create additional areas supporting essential natural habitat within the area

The provenance of sediments in the Wash using uranium-series disequilibrium

Available from British Library Document Supply Centre- DSC:D86007 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Height pairings on elliptic curves

SIGLEAvailable from British Library Document Supply Centre- DSC:D062071 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Coastal resilience and late Holocene tidal inlet history: The evolution of Dungeness Foreland and the Romney Marsh depositional complex (U.K.)

Dungeness Foreland is a large sand and gravel barrier located in the eastern English Channel that during the last 5000 years has demonstrated remarkable geomorphological resilience in accommodating changes in relative sea-level, storm magnitude and frequency, variations in sediment supply as well as significant changes in back-barrier sedimentation. In this paper we develop a new palaeogeographic model for this depositional complex using a large dataset of recently acquired litho-, bio- and chrono-stratigraphic data. Our analysis shows how, over the last 2000 years, three large tidal inlets have influenced the pattern of back-barrier inundation and sedimentation, and controlled the stability and evolution of the barrier by determining the location of cross-shore sediment and water exchange, thereby moderating sediment supply and its distribution. The sheer size of the foreland has contributed in part to its resilience, with an abundant supply of sediment always available for ready redistribution. A second reason for the landform's resilience is the repeated ability of the tidal inlets to narrow and then close, effectively healing successive breaches by back-barrier sedimentation and ebb- and/or flood-tidal delta development. Humans emerge as key agents of change, especially through the process of reclamation which from the Saxon period onwards has modified the back-barrier tidal prism and promoted repeated episodes of fine-grained sedimentation and channel/inlet infill and closure. Our palaeogeographic reconstructions show that large barriers such as Dungeness Foreland can survive repeated “catastrophic” breaches, especially where tidal inlets are able to assist the recovery process by raising the elevation of the back-barrier area by intertidal sedimentation. This research leads us to reflect on the concept of “coastal resilience” which, we conclude, means little without a clearly defined spatial and temporal framework. At a macro-scale, the structure as a whole entered a phase of recycling and rapid progradation in response to changing sediment budget and coastal dynamics about 2000 years ago. However, at smaller spatial and temporal scales, barrier inlet dynamics have been associated with the initiation, stabilisation and breakdown of individual beaches and complexes of beaches. We therefore envisage multiple scales of “resilience” operating simultaneously across the complex, responding to different forcing agents with particular magnitudes and frequencies