Effects of extreme water levels on water quality in a large shallow eutrophic lake (Changhu Lake, China)

Changhu Lake, a large shallow eutrophic lake in central China, experienced an extreme low water level event from November 2015 to January 2016 followed by an extreme high water level event in July 2016. In this study, we examined the effects of two extreme water levels on the nutrient status of Changhu Lake over five years. The nutrient concentrations in Changhu Lake showed significant interannual variations and the water quality of sites in the western part of Changhu Lake was better compared to sites at the outlet of the lake. In late 2015, the effect of low water levels led to a significant increase in nutrient concentrations. After July 2016, however, the high water level occurred leading to a marked decrease in nutrient concentrations. These changes in nutrient parameters were strongly related to the water level fluctuations. The dilution effect was the key process that determined the variations of nutrient parameters in Changhu Lake. As extreme water levels are likely to become more frequent during the twenty-first century, this work may provide some insights into the conservation and management of lake ecosystems in the face of climate change and human activity.

Can Managed Realignment Buffer Extreme Surges? The Relationship Between Marsh Width, Vegetation Cover and Surge Attenuation

Managed realignment (MR) involves the landward relocation of sea defences to foster the (re)creation of coastal wetlands and achieve nature-based coastal protection. The wider application of MR is impeded by knowledge gaps related to lacking data on its effectiveness under extreme surges and the role of changes in vegetation cover, for example due to sea-level rise. We employ a calibrated and validated hydrodynamic model to explore relationships between surge attenuation, MR width(/area) and vegetation cover for the MR site of Freiston Shore, UK. We model a range of extreme water levels for four scenarios of variable MR width. We further assess the effects of reduced vegetation cover for the actual MR site and for the scenario of the site with the largest width. We show that surges are amplified for all but the largest two site scenarios, suggesting that increasing MR width results in higher attenuation rates. Substantial surge attenuation (up to 18 cm km−1) is only achieved for the largest site. The greatest contribution to the attenuation in the largest site scenario may come from water being reflected from the breached dike. While vegetation cover has no statistically significant effect on surge attenuations in the original MR site, higher coverage leads to higher attenuation rates in the largest site scenario. We conclude that at the open coast, only large MR sites (> 1148 m width) can attenuate surges with return periods > 10 years, while increased vegetation cover and larger MR widths enable the attenuation of even higher surges.

The Role of Beach Morphology and Mid-Century Climate Change Effects on Wave Runup and Storm Impact on the Northern Yucatan Coast

Wave runup is a relevant parameter to determine the storm impact on barrier islands. Here, the role of the beach morphology on wave runup and storm impact was investigated at four coastal communities located on the northern Yucatan coast. Current wave conditions based on regional wind simulations, topo-bathymetric transects measured at each location, and a nonlinear wave transformation model were employed to reconstruct multi-year runup time series. Dune morphology features and extreme water levels (excluding storm surge contributions) were further employed to determine the storm impact at each site for different return periods. Despite the similar offshore conditions along the coast, extreme water levels (i.e., runup and setup) showed intersite differences that were mainly ascribed to subaerial and submerged morphological features. Numerical results showed that the average surf zone beach slope, sandbars, berm, and dune elevation played an important role in controlling extreme water levels and storm impact at the study sites under the present climate. Moreover, in order to assess the potential effect of climate change on coastal flooding, we analyzed wave runup and storm impact in the best-preserved site by considering wave conditions and sea level rise (SLR) projections under the RCP 8.5 scenario. Modelling results suggest no significant increase in the storm impact regime between the present and future conditions in the study area unless SLR is considered. It was found that to accurately estimate SLR contribution, it should be incorporated into mean sea level prior to performing numerical wave runup simulations, rather than simply adding it to the resulting wave-induced water levels.

PREDICTING EXTREME WATER LEVELS AROUND AUSTRALIA

Throughout history, coastal settlers have had to adapt to periodic coastal flooding. However, as a society we have become increasingly vulnerable to extreme water level events as our cities and our patterns of coastal development become more intricate, populated and interdependent. In addition to this, there is now a real and growing concern about rising sea levels. Accurate estimates of extreme water levels are therefore critical for coastal planning and emergency planning and response. The occurrence of extreme water levels along low-lying, highly populated and/or developed coastlines can lead to considerable loss of life and billions of dollars of damage to coastal infrastructure. Therefore, it is vitally important that the exceedance probabilities of extreme water levels be accurately evaluated to inform risk-based flood management, engineering and future land-use planning. This objectives of this study was to estimate present day extreme sea level exceedance probabilities due to combination of storm surges, tides and mean sea level (including wind-waves) around the coastline of Australia.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/vGaB85VRujs

Effect of Fluvial Discharges and Remote Non-Tidal Residuals on Compound Flood Forecasting in San Francisco Bay

Accurate and timely flood forecasts are critical for making emergency-response decisions regarding public safety, infrastructure operations, and resource allocation. One of the main challenges for coastal flood forecasting systems is a lack of reliable forecast data of large-scale oceanic and watershed processes and the combined effects of multiple hazards, such as compound flooding at river mouths. Offshore water level anomalies, known as remote Non-Tidal Residuals (NTRs), are caused by processes such as downwelling, offshore wind setup, and also driven by ocean-basin salinity and temperature changes, common along the west coast during El Niño events. Similarly, fluvial discharges can contribute to extreme water levels in the coastal area, while they are dominated by large-scale watershed hydraulics. However, with the recent emergence of reliable large-scale forecast systems, coastal models now import the essential input data to forecast extreme water levels in the nearshore. Accordingly, we have developed Hydro-CoSMoS, a new coastal forecast model based on the USGS Coastal Storm Modeling System (CoSMoS) powered by the Delft3D San Francisco Bay and Delta community model. In this work, we studied the role of fluvial discharges and remote NTRs on extreme water levels during a February 2019 storm by using Hydro-CoSMoS in hindcast mode. We simulated the storm with and without real-time fluvial discharge data to study their effect on coastal water levels and flooding extent, and highlight the importance of watershed forecast systems such as NOAA’s National Water Model (NWM). We also studied the effect of remote NTRs on coastal water levels in San Francisco Bay during the 2019 February storm by utilizing the data from a global ocean model (HYCOM). Our results showed that accurate forecasts of remote NTRs and fluvial discharges can play a significant role in predicting extreme water levels in San Francisco Bay. This pilot application in San Francisco Bay can serve as a basis for integrated coastal flood modeling systems in complex coastal settings worldwide.