ENHANCING COASTAL RESILIENCY OF HYPOTHETICAL LAND RECLAMATION SCENARIOS WITH MANGROVE FOREST AND OYSTER REEF ASSESSED BY ADCIRC AND SWAN STORM SURGE MODEL

This study aims to assess the effectiveness of mangrove forests and oyster reefs on reducing the damages from typhoons in hypothetical land reclamation scenarios in Atimonan, Philippines. Storm surge simulations were ran using ADCIRC and SWAN coupled model on the local government unit’s (LGU) land reclamation plan and the proposed crenulate bay reclamation plan, both with concrete seawall, mangrove forests and oyster reefs. Inputs to the model include modified topography and bathymetry, coastline, land cover, typhoon Durian data and tidal potential constituents. Simulations show that the crenulate bay reclamation plan is better by 39.15% than the LGU’s land reclamation plan on reducing typhoon winds and storm surge inundation extent induced by Typhoon Durian. However, this advantage comes with an additional implementation cost of 11.02%. This study is envisioned to help the land reclamation project of Atimonan LGU to be resilient against typhoon winds and coastal inundation.

Flood forecasting with machine learning models in an operational framework

Google’s operational flood forecasting system was developed to provide accurate real-time flood warnings to agencies and the public, with a focus on riverine floods in large, gauged rivers. It became operational in 2018 and has since expanded geographically. This forecasting system consists of four subsystems: data validation, stage forecasting, inundation modeling, and alert distribution. Machine learning is used for two of the subsystems. Stage forecasting is modeled with the Long Short-Term Memory (LSTM) networks and the Linear models. Flood inundation is computed with the Thresholding and the Manifold models, where the former computes inundation extent and the latter computes both inundation extent and depth. The Manifold model, presented here for the first time, provides a machine-learning alternative to hydraulic modeling of flood inundation. When evaluated on historical data, all models achieve sufficiently high-performance metrics for operational use. The LSTM showed higher skills than the Linear model, while the Thresholding and Manifold models achieved similar performance metrics for modeling inundation extent. During the 2021 monsoon season, the flood warning system was operational in India and Bangladesh, covering flood-prone regions around rivers with a total area of 287,000 km2, home to more than 350M people. More than 100M flood alerts were sent to affected populations, to relevant authorities, and to emergency organizations. Current and future work on the system includes extending coverage to additional flood-prone locations, as well as improving modeling capabilities and accuracy.

An Assessment of Small Wetland Ecohydrological Dynamics Using Sentinel-2 MSI Derived Spectral Indices in Semi-Arid Environments of South Africa

Water presence within small wetlands serve as a determining factor influencing biodiversity productivity and wetland functionality. Small wetlands remain largely unprotected hence, they are more sensitive to frequent exposure to environmental modifications, and are less resilient to changing rainfall patterns, climate change and variability, droughts and changing land use practices. Accurate and up to date spatial and temporal information on changes in surface water and inundation extent becomes imperative for the proper management of these wetlands. Therefore, this study sought to extract and monitor wetland ecohydrological dynamics (surface water and inundation extent) using monthly Sentinel-2 MSI remotely sensed datasets. These dynamics were assessed for the period between July 2020 and June 2021 using the modified normalised difference water index (MNDWI), normalised difference moisture index (NDMI) and normalised difference phenology index (NDPI) derived from Sentinel-2 MSI data. The results showed that the rainy season (Dec 2020 – Feb 2021) had a larger-water coverage extent (10948 m2 (0.05%) to 31594 m2 (0.13%)) when compared to the dry season (July 2020: 19157 m2 (0.04%) and June 2021:14429 m2 (0.03%)). The extent of surface area declined during the dry period due to less rainfall (0.20 mm) and decreased actual evapotranspiration (9.90 mm – 10.43 mm). Further, the NDPI showed a high concentration of wetland vegetation between the months of October 2020 and April 2021. In contrary, higher moisture content was observed between December 2020 to April 2021. The increase in vegetation concentration and moisture content reflects the spatial extent of inundation extent. Wetland water extent, soil moisture, and vegetation condition were assessed with high overall accuracies, ranging between 70.83% and 97.36%. Overall, the results indicate that small wetlands are characterised with significant variations in levels of inundation and productivity throughout the year.

Eco-hydrology as a driver for tidal restoration: Observations from a Ramsar wetland in eastern Australia

Land reclamation projects and the installation of drainage infrastructure has impacted coastal wetlands worldwide. By altering water levels and inundation extent, these activities have changed the viable ecosystems onsite and resulted in the proliferation of freshwater species. As more than 50% of tidal wetlands have been degraded globally over the last 100 years, the importance of this issue is increasingly being recognised and tidal wetland restoration projects are underway worldwide. However, there are currently limited sites where large-scale reintroduction of tidal flushing has been implemented with the explicit aim to foster the growth of a threatened ecosystem. In this study, the tidal restoration of an internationally recognised Ramsar listed wetland in eastern Australia is described to highlight how coastal saltmarsh can be targeted by mimicking inundation depths and hydroperiod across the 410-ha site. Coastal saltmarsh is particularly important to this site as it is part of the east Australasian flyway for migratory birds and the minimum saltmarsh extent, as listed within the Ramsar’s limits of acceptable change, have been breached. To recreate coastal saltmarsh habitat onsite, water level and hydroperiod criteria were established based on similar vegetation patterns within the adjacent estuary. A calibrated 2D hydrodynamic model of the site was then used to test how the preferred inundation criteria could be applied to the largest possible restored wetland area. Once optimised, a synthetic tidal signal was implemented onsite via automated hydraulic controls. The onsite vegetation response over an 8-year period was assessed to highlight the ecosystem response to controlled tidal inundation and denoted substantial saltmarsh expansion during the period. The techniques applied onsite have successfully met the restoration targets and can be applied at similar sites worldwide, offsetting sea level rise impacts to natural inundation hydroperiod.

Flood inundations and risk mapping in a tidal river: a case study for the Kelani River basin, Sri Lanka

The downstream low-lying regions of the Kelani River, including some areas in the Districts of Colombo and Gampaha, Sri Lanka, frequently face severe inundations due to extreme rainfalls in the upper basin. In the present study, 1-D and 2-D hydrodynamic models in HEC-RAS have been used to examine the flood inundations in the tidal influenced Kelani River with ground observations and remote sensing. The HEC-RAS model has been used to produce a flood hazard map for hazard assessment in the lower Kelani River basin under different return periods. Furthermore, expected discharges for different return periods have been estimated using the hydrological model HEC–HMS with the updated intensity depth frequency curves for the Kelani River basin. Sentinel 1 imagery and field survey results are used to validate the simulated flood inundation extent; hydrodynamic model results validated against observed stage measurements; hydrological model validated against discharge measurements. Further, the validated hydrodynamic model showed the high capability to capture flow processes (Nash-Sutcliffe coefficient = 0.90 and Pearson coefficient of correlation = 0.95) along with inundation extent (Success Index = 0.90) of selected historical extreme events. Then the hydrological model is used to predict the flows of the Kelani River basin with a good agreement (Nash-Sutcliffe coefficient = 0.91 and the Pearson coefficient of correlation = 0.93). Finally, flood risk zoning for different return periods are developed based on the present model which would be a useful benchmark to design and implement flood control and mitigation measures for the river basin.

Prediction capabilities of GeoFlood for the delineation of flood-prone areas: the Tiber River case study

<p>Land use and delineation of flood-prone areas require valuable and effective tools, such as flood mapping. Local authorities, in order to prevent and mitigate the effects of flood events, need simplified methodologies for the definition of preliminary flooded areas at a large scale. In this work, we focus on the workflow GeoFlood, which can rapidly convert real-time and forecasted river flow conditions into flooding maps. It is built upon two methodologies, GeoNet and the HAND model, making use only of high-resolution DTMs to define the geomorphological and hydraulic information necessary for flood inundation mapping, thus allowing for large-scale simulations at a reasonable economical and computational cost. GeoFlood potential is tested over the mid-lower portion of the river Tiber (Italy), investigating the conditions under which it is able to reproduce successful inundation extent, considering a 200-year return period scenario. Results are compared to authority maps obtained through standard detailed hydrodynamic approaches. In order to analyze the influence of the main parameters involved, such as DTM resolution, channel segmentation length, and roughness coefficient, a sensitivity analysis is performed. GeoFlood proved to produce efficient and robust results, obtaining a slight over-estimation comparable to that provided by standard costly methods. It is a valid and relatively inexpensive framework for inundation mapping over large scales, considering all the uncertainties involved in any mapping procedure. Also, it can be useful for a preliminary delineation of regions where the investigation based on detailed hydrodynamic models is required.</p>

Connectivity of floodplains in Germany – which floodplain extent is relevant?

<p>Floodplains are transitional ecosystems, rich in biodiversity, endangered and adapted to inundation by floods. Flood magnitude, hydrologic connectivity and elevation define the extent of an active floodplain. In past centuries, active floodplains in Germany were reduced by up to 90% of their original size – in terms of the area that is statistically inundated at least once every 100 years. But, does this area reflect the area relevant for floodplain ecosystems and for evaluating their functioning and the services they provide? Analyzing two scenarios of Flood Hazard Maps (FHM), a German-wide comparison including 78 rivers was carried out to quantify the extent of floods with statistical occurrence intervals of 5 to 25 years, so-called ‘frequent floods’ (T-frequent), and intervals of 100 years, or ‘medium floods’ (T-medium), as well as selected characteristics. The comparison was carried out on the river (basin) level, and based on hydrological catchments. By additionally analyzing measured discharges of relevant gauges from the past 20 years, real inundation was quantified. As a result, even in exceptional wet years these ‘frequent floods’ occur for a few days per year or not at all. The extent of the two FHM scenarios differs for most areas: Only at 13% of gauging units was the T-frequent inundation extent similar to that of T-medium. Furthermore, within T-medium the land use of arable land doubles and that of urban areas more than doubles, showing how disconnected the T-medium floodplain is in many parts. On the other hand, 25% of grasslands are Natura 2000 meadows occurring within the borders of T-frequent but only 6% are Natura 2000 meadows outside these borders, indicating the effect of connectivity and thus inundation, making these habitats valuable in terms of biodiversity mainly in T-frequent. This study provides evidence that, especially for regulatory services like water purification, water retention and climate mitigation, T-frequent might be more suitable for consideration. With more frequent flooding, less intensively used areas are connected more often, holding back water, nutrients and sediments – in addition to comparatively more areas relevant for nature conservation. For flood protection and also as a refuge for flora and fauna and for connection of habitats, of course the extent of disastrous 100-yr floods are important.</p>