Physical and economic impacts of sea-level rise and low probability flooding events on coastal communities

Conventionally flood mapping typically includes only a static water level (e.g. peak of a storm tide) in coastal flood inundation events. Additional factors become increasingly important when increased water-level thresholds are met during the combination of a storm tide and increased mean sea level. This research incorporates factors such as wave overtopping and river flow in a range of flood inundation scenarios of future sea-level projections for a UK case study of Fleetwood, northwest England. With increasing mean sea level it is shown that wave overtopping and river forcing have an important bearing on the cost of coastal flood events. The method presented converts inundation maps into monetary cost. This research demonstrates that under scenarios of joint extreme surge-wave-river events the cost of flooding can be increased by up to a factor of 8 compared with an increase in extent of up to a factor of 3 relative to “surge alone” event. This is due to different areas being exposed to different flood hazards and areas with common hazard where flood waters combine non-linearly. This shows that relying simply on flood extent and volume can under-predict the actual economic impact felt by a coastal community. Additionally, the scenario inundation depths have been presented as “brick course” maps, which represent a new way of interpreting flood maps. This is primarily aimed at stakeholders to increase levels of engagement within the coastal community

Flood inundation uncertainty: the case of a 0.5% annual probability flood event

Aging coastal defences around the UK are challenging managers to redesign schemes to be resilient to extreme events and climate change, be cost-effective, and have minimal or beneficial environmental impact. To enable effective design, reduced uncertainty in the assessment of flood risk due to natural variability within the coastal forcing is required to focus on conditions that pose highest threat. The typical UK standard of protection for coastal defences is to withstand a 0.5% annual probability event, historically also known as a 1 in 200 year return period event. However, joint wave-water level probability curves provide a range of conditions that meet this criterion. We examine the Dungeness and Romney Marsh coastal zone, a region of high value in terms of habitat and energy assets, to quantify the uncertainty in flood depth and extent generated by a 0.5% probability event, and to explore which combinations of wave and water levels generate the greatest threat

A temporal waterline approach to mapping intertidal areas using X-band marine radar

Mapping the morphology of intertidal areas is a logistically challenging, time consuming and expensive task due to their large expanse and difficulties associated with access. A technique is presented here that uses standard marine navigational radar operating at X-band frequency. The method uses a series of time-exposure radar images over the course of a two-week tidal cycle to identify the elevation of the wetting and drying transitions at each pixel in the radar images, thereby building up a morphological map of the target intertidal area. This “Temporal Waterline” method is applied to a dataset acquired from Hilbre Island at the mouth of the Dee Estuary, UK, spanning March 2006 to January 2007. The radar gathered data with a radial range of 4 km and the resulting elevation maps describe the intertidal regions of that area. The results are compared with airborne LiDAR data surveyed over the same area and within the radar survey time period. The residual differences show good agreement across large areas of beach and sandbanks, with concentrations of poor estimations around points that are shadowed from the radar or likely to suffer from pooling water. This paper presents the theoretical framework of the method and demonstrates its stability and accuracy. The Temporal Waterline radar method is aimed at providing a useful tool for the monitoring and operational management of coastlines

Modeling impact of intertidal foreshore evolution on gravel barrier erosion and wave runup with XBeach-X

This paper provides a sensitivity analysis around how characterizing sandy, intertidal foreshore evolution in XBeach-X impacts on wave runup and morphological change of a vulnerable, composite gravel beach. The study is motivated by a need for confidence in storm-impact modeling outputs to inform coastal management policy for composite beaches worldwide. First, the model is run with the sandy settings applied to capture changes in the intertidal foreshore, with the gravel barrier assigned as a non-erodible surface. Model runs were then repeated with the gravel settings applied to obtain wave runup and erosion of the barrier crest, updating the intertidal foreshore from the previous model outputs every 5, 10 and 15 min, and comparing this with a temporally static foreshore. Results show that the scenario with no foreshore evolution led to the highest wave runup and barrier erosion. The applied foreshore evolution setting update is shown to be a large control on the distribution of freeboard values indicative of overwash hazard and barrier erosion by causing an increase (with 5 min foreshore updates applied) or a decrease (with no applied foreshore updating) in the Iribarren number. Therefore, the sandy, intertidal component should not be neglected in gravel barrier modeling applications given the risk of over- or under-predicting the wave runup and barrier erosion

Quantification of the uncertainty in coastal storm hazard predictions due to wave‐current interaction and wind forcing

Coastal flood warning and design of coastal protection schemes rely on accurate estimations of water level and waves during hurricanes and violent storms. These estimations frequently use numerical models, which, for computational reasons, neglect the interaction between the hydrodynamic and wave fields. Here, we show that neglecting such interactions, or local effects of atmospheric forcing, causes large uncertainties, which could have financial and operational consequences because flood warnings are potentially missed or protection schemes underdesigned. Using the Severn Estuary, SW England, we show that exclusion of locally generated winds underestimates high water significant wave height by up to 90.1%, high water level by 1.5%, and hazard proxy (water level + 1/2 significant wave height) by 9.1%. The uncertainty in water level and waves is quantified using a system to model tide‐surge‐wave conditions, Delft3D‐FLOW‐WAVE in a series of eight model simulations for four historic storm events

Sensitivity of flood hazard and damage to modelling approaches

Combination of uncertainties in water level and wave height predictions for extreme storms can result in unacceptable levels of error, rendering flood hazard assessment frameworks less useful. A 2D inundation model, LISFLOOD-FP, was used to quantify sensitivity of flooding to uncertainty in coastal hazard conditions and method used to force the coastal boundary of the model. It is shown that flood inundation is more sensitive to small changes in coastal hazard conditions due to the setup of the regional model, than the approach used to apply these conditions as boundary forcing. Once the threshold for flooding is exceeded, a few centimetres increase in combined water level and wave height increases both the inundation and consequent damage costs. Improved quantification of uncertainty in inundation assessments can aid long-term coastal flood hazard mitigation and adaptation strategies, to increase confidence in knowledge of how coastlines will respond to future changes in sea-level

Excavations at Site C North, Kalambo Falls, Zambia:New insights into the mode 2/3 Transition in South-Central Africa

We report on the results of small-scale excavations at the archaeological site of Kalambo Falls, northern Zambia. The site has long been known for its stratified succession of Stone Age horizons, in particular those representing the late Acheulean (Mode 2) and early Middle Stone Age (Mode 3). Previous efforts to date these horizons have provided, at best, minimum radiometric ages. The absence of a firm chronology for the site has limited its potential contribution to our understanding of the process of technological change in the Middle Pleistocene of south-central Africa. The aim of the excavations was to collect samples for luminescence dating that bracketed archaeological horizons, and to establish the sedimentary and palaeoenvironmental contexts of the deposits. Four sedimentary packages were identified with the oldest containing Mode 2 and Mode 3 horizons. In this paper we consider the implications of the luminescence ages for the archaeological record at Kalambo Falls, and place them in a regional context. The reworking and preservation of the archaeological horizons is interpreted as the result of successive phases of meander migration and aggradation. Limited pollen evidence suggests a persistent floodplain palaeoenvironment with intermittent swamp forest and adjacent valley woodland, while mineral magnetic susceptibility data support an interpretation of river flow variability without any significant change in sediment provenance. The dynamics of the fluvial system cannot as yet be linked directly with regional climate change. The age range of ~500–300 ka for the oldest sedimentary package places the Mode 2/3 succession firmly in the Middle Pleistocene, and contributes to an expanding African record of technological innovation before the evolution of Homo sapiens