Maximum Water Level Research Articles

Exploring the relationship of the winter-spring crop’s rice yield with meteorological drought and water resources in Ben Tre province

Recent environmental changes have significantly impacted rice cultivation in Ben Tre province, the Vietnamese Mekong Delta (VMD) coastal province. This research employed the Mann-Kendall (MK) non-parametric method and Sen’s slope (SS) technique to detect trends in the rice yield of the winter-spring crop (WSC) in Ben Tre province and environmental parameters from 2009 to 2020. These environmental parameters include the meteorological drought index and key water resource metrics, potentially affecting the region’s rice yield. Furthermore, it established the relationship between these environmental parameters and rice yield using multiple linear regression. The results indicated an average decrease of 0.13 ton/ha/year in the WSC’s rice yield, with a severe reduction to 2.86 ton/ha in 2016. This decline in rice yield is likely attributed to environmental shifts, particularly during years with extreme hydrological events. The study found that during the months of dry season, from January to April, there was a decrease of 0.14 per year in the minimum Standardized Precipitation Evapotranspiration Index (SPEI-4min) at the Ben Tre meteorological station; an increase in the minimum daily discharge at the Tan Chau station (QTCmin) by 81 m3/s/year; a decrease in the mean daily water level at the My Thuan station (QMTmean) by 0.22 cm/year; and an increase in the maximum daily water level at the Ben Trai station (HBTmax) by 0.67 cm/year. However, the maximum daily salinity concentration at the Tra Vinh station (STVmax) showed no significant trend, oscillating between 6.5 and 15 g/l, potentially because the three-hourly monitoring intervals might not have fully captured peak salinity levels. The regression model identified SPEI-4min (with a Correlation Index (CI) of 0.450) and STVmax (CI = 0.372) as the primary factors influencing rice yield. Additionally, QTCmin and HBTmax were also significant, though to a lesser extent, with correlation indices of 0.310 and 0.303, respectively. Conversely, QMTmean had a relatively minor influence, with a correlation index of 0.136. These findings are crucial to developing adaptive strategies and policies for improving the resilience of rice production in the study province and throughout the VMD.

Flood modeling in the Ba River basin using a coupled hydrodynamic model—MIKE FLOOD

AbstractThe problem of flooding in Central Vietnam in general and the lower Ba River in particular is one of the natural disasters that frequently threatens people's lives and socioeconomic development in the region. Especially, climate change is becoming ever more prominent and hotter, making extreme natural disasters more unusual and unpredictable. In this research, MIKE‐FLOOD—a model that connects a 1‐dimensional (1‐D) MIKE 11 Hydrodynamics (HD) model with a 2‐dimensional (2‐D) MIKE 21 HD model—was used to set up. The model was calculated for three floods: (1) flood in October 1993, (2) flood in November 2003, and (3) flood in November 2007; these are floods with high frequency and relatively large magnitude. The results show that the 1993 flood rose and receded quickly. The flood peak inundated an area of 22,600 ha, accounting for 52% of the natural area. The flooded areas deeper than 1, 2, 3, 4, and 5 m were 16500, 11,000, 7000, 4200, and 2200 ha, respectively. In the center of Tuy Hoa city, the flooded area at the time of maximum water level was almost 100%.

Experimental study on the dam-break-induced surge impact on an inclined wall

The front faces of coastal structures are not always perpendicular to its foundation. To investigate the surge impact characteristics on an inclined wall, laboratory experiments about the dam-break-induced surge impact were conducted on downstream walls with four different inclination angles, i.e., −30°, −10°, 0°, and 10° (positive for forward inclination). Before the water tongue forms, surge water in front of the wall can be classified into the bottom solid water and the upper aerated water. Although the measured maximum water level is the largest in front of the vertical wall, it contains a large proportion of aerated water. In contrast, the −30° wall exhibits the largest proportion of solid water, which is associated with the force exerted on the wall. As the wall inclines forward, the impact pressure experiences an increase in its peak value and spatial extent, while a decrease in its duration. The maximum impact pressure and the maximum horizontal force induced by the initial impact increase from the backward to forward inclination, while the maximum horizontal force in the transition or quasi-static phase increases as the wall inclines. In addition, it is found that the calculation method for designing breaking wave loads on nonvertical walls underestimates the surge-induced maximum horizontal force. Regarding the potential sliding/overturning failure, the 10°/−30° wall presents the highest risk among the considered cases under the same incoming hydrodynamic conditions. These findings extend our understandings on the surge–structure interactions, being helpful for the safe design of relevant coastal structures.

Forecasting Maximum Water Level Data for Post Sangkuliman using An Artificial Neural Network Backpropagation Algorithm

Neural Network (NN) is an information processing system that has characteristics similar to biological neural networks. One of the algorithms in NN is Backpropagation Neural Network (BPNN). BPNN is an excellent method for dealing with complex pattern recognition problems. In this research, maximum water level forecasting was carried out at Sangkuliman Post using a Backpropagation Neural Network. This research aims to obtain modeling for forecasting maximum water level, as well as forecasting results using the best model. The research results show that the best model is five neurons in hidden layer 1 and 3 neurons in hidden layer 2 with the backpropagation algorithm, the activation function used is binary sigmoid, the learning rate is 0.001, and the maximum iteration is 10,000,000 with the smallest RMSE result being 1.816. The forecast results for the following 12 periods are 1.672, 1.779, 1.523, 1.271, 1.752, 1.692, 1.335, 1.479, 1.750, 1.779, 1.340, 1.269, and 1.754. Forecasting results can be used by various parties in decision-making and planning in multiple fields, as an example to see the patterns of biological and vegetable life around Sangkuliman Post. Based of forecasting results, certain months show an increase in maximum water levels.

The efficiency assessment of short-term maximum flood level forecast methodology in the upper and middle course of the Tsna river

A significant rise in water levels in the rivers during the spring flood and the release of rivers to the floodplain is the main danger in this period for nearby territories and people living there. This phenomenon can lead to flooding of large areas, significant economic losses, environmental damage, and threaten the health and life of local residents. Such negative consequences of high floods are typical for the Tambov region rivers, which indicates the need to develop an effective system for forecasting and preventing maximum flood levels. The climatic changes that have taken place over the past few decades, which are also reflected in the rivers’ water regime, show the need to modernize existing forecasting methodologies. In this paper, the authors have demonstrated the results of the effectiveness assessment of the existing methodology for short-term forecasting of the maximum flood level on the Tsna River at two gauging stations (“Kuzmino-Gat” and “Tambov”). Calculations were made using modern data in accordance with this methodology and a comparative analysis was carried out with the calculations of previous years. Based on this analysis, an assessment of the flood levels forecast methodology accuracy was given. According to the study results, it was found that the existing methodology for short-term forecasting of the maximum flood level on the Tsna River is largely ineffective nowadays with regard to modern conditions of spring flood runoff formation. In the analysis of the Kuzmino-Gat gauging station, the values of the maximum flood levels obtained by the reanalysis method according to the tested methodology using modern data showed significant deviations from the actual observed values. At the same time, for the operational forecast of maximum water levels at the Tambov gauging station, it is possible to use the correlation dependence with the observed water levels at the Kuzmino-Gat gauging station, as before. The correlation coefficient with modern data was 0.96. The authors have highlighted the main drawbacks of the existing methodology and made suggestions for improvement, in particular, what factors need to be analyzed in order to clarify the forecast.

A moderately chocked estuary: Influence of a constriction on the water level variations of the Wouri estuary (Cameroon)

We study the water-level (WL) evolution in the Douala basin (DB), a sub-basin of the Wouri estuary, separated from the ocean by a natural constriction and influenced by tides and river inflow. Our objective is to assess whether floods in Douala city could result from water overflow from the Douala basin due to Wouri river floods. We first evaluate the constriction’s damping effect by analyzing tide amplitude variations from the ocean to the basin. A simplified model for basin WL variation is developed, incorporating a dissipation parameter (Г) that is evaluated using weak tidal amplitude modulation from the ocean to the basin over the dry season of 2023. Results in the DB reveal nondimensional choking-parameter (P) values of 12–19, indicating moderate dissipation. The basin's relatively small area results in a nondimensional river discharge parameter (S) of 0–12, indicating high river discharge regimes. Subsequently, the model is used to evaluate the mean and maximum WL, as well as tide amplitude variations of the DB, as functions of river flow and ocean tides. The mean WL increases, but the maximum tide amplitude decreases as river input increases. The maximum WL in the basin always exceeds the maximum amplitude of ocean tides but is only significantly higher when river input is above 1000 m3/s. In current conditions, where maximum observed river input is around 1800 m3/s, it is unlikely that flooding will extend beyond the basin borders into the city of Douala. For a moderate increase (20%) in future maximum river fluxes, it is also unlikely that the DB will overflow into Douala city. Only a drastic increase in mean ocean level and river fluxes (which are possible scenarios associated with climate change) could potentially lead to a significant rise in basin WL, resulting in severe flooding.

Analyzing the hydraulic performance of water gates due to the impact of extreme rainfall on tidal lowlands: a case study in Katingan rice fields area, Central Kalimantan

Rice fields in the Katingan tidal lowlands are inundated due to spring tides and extreme rainfall. Excessive inundation in the rice fields causes crop failure. Therefore, the study was conducted to analyze the use of water gates in the secondary canals for controlling the inundation. The unsteady flow simulation boundary conditions are the upstream discharge with a return period of 5 and 25 years and measured tidal. The water flow is controlled by closing the sluice gates at the water level elevation of +3.50 m, and flap gates close during high tide and rainfall. This research shows that controlling the water flow by gates decreases the water level and inundation area. Under rainfall with a return period of 5 years, the inundation area decreased by 79.03 %, and the maximum water level decreased by 0.14 m. In the rainfall with a return period of 25 years, the inundation area decreased by 75.31%, and the maximum water level decreased by 0.11 m. The water gates’ operation incompletely decreases the inundation and water level beneath rice crops’ threshold inundation depth and duration. Therefore, there is a need for additional water control to optimize the decrease of inundation in the rice fields.

Reconstruction of hourly coastal water levels and counterfactuals without sea level rise for impact attribution

Abstract. Rising seas are a threat to human and natural systems along coastlines. The relation between global warming and sea level rise is established, but the quantification of impacts of historical sea level rise on a global scale is largely absent. To foster such quantification, here we present a reconstruction of historical hourly (1979–2015) and monthly (1900–2015) coastal water levels and a corresponding counterfactual without long-term trends in sea level. The dataset pair allows for impact attribution studies that quantify the contribution of sea level rise to observed changes in coastal systems following the definition of the Intergovernmental Panel on Climate Change (IPCC). Impacts are ultimately caused by water levels that are relative to the local land height, which makes the inclusion of vertical land motion a necessary step. Also, many impacts are driven by sub-daily extreme water levels. To capture these aspects, the factual data combine reconstructed geocentric sea level on a monthly timescale since 1900, vertical land motion since 1900 and hourly storm-tide variations since 1979. The inclusion of observation-based vertical land motion brings the trends of the combined dataset closer to tide gauge records in most cases, but outliers remain. Daily maximum water levels get in closer agreement with tide gauges through the inclusion of intra-annual ocean density variations. The counterfactual data are derived from the factual data through subtraction of the quadratic trend. The dataset is made available openly through the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) at https://doi.org/10.48364/ISIMIP.749905 (Treu et al., 2023a).

Reservoir-triggered seismicity: case study of the Dnister Hydro Power Complex (Ukraine)

The work considers the seismic activity around the Dnister Hydro Power Complex in the Dnister Reservoir area from 2012 to 2021. Maximum local magnitude of earthquakes for the studied period is ML=3.4. Hypocenters of earthquakes are at the depth of 1—3 km. They are located nearby to previously established faults and contacts of structures with a different lithological composition. Artificial water level regulation in the reservoir is related to the operation of the Dnister Hydro Power Plant and, according to our results, probably affects the occurrence of seismic events. Although the maximum water level changes only by 8.7 m, the region has increased natural seismicity and is located in the transition zone between 6th and 7th isoseims according to the seismic zoning map of Ukraine. Accumulated natural stresses can be triggered into earthquakes when there is a sudden change in water pressure. The seismicity of the region around the Dnister Reservoir was investigated for 10 years, taking into account the water level data in the reservoir, as well as comparing the location of earthquakes with the State Geological Map of the Crystalline Foundation. The profile of the State Geological Map of the Crystalline Foundation with the projected earthquakes hypocenters allowed to indicate that a significant number of earthquakes are located between different lithological structures. In order to assess the interrelationship between seismicity and water level changes in the reservoir, 111 periods of lowering or filling of the reservoir were selected. Parameters of seismic activity within the periods were calculated and analysed. Correlation between the sum of logarithms of energy with water volume changes and pressure changes in the Dnister Reservoir was established. Considering linear dependence it is possible to predict potential intensity of local seismicity caused by water level changes in the Dnister Reservoir.

Determining ice-jam stage frequency distributions of an ungauged river reach using dendrogeomorphological data

ABSTRACT Ice-jam floods are a common, but dangerous natural phenomena occurring in many northern rivers. These events are even more dangerous when only limited gauged river data is available to be used to predict the severity of events in ice-jam prone areas, especially when these areas are close to a community. This study examines an alternative method for gathering ungauged river data for ice-jam flood hazard assessments, specifically the use of dendrogeomorphological methodologies. The most accurate way to gather this information is through collection of cross-sectional samples from trees that have been scarred by past ice-jam events, rather than the collection of core samples. Therefore, cross-sectional sampling methods were chosen for this study. The years and heights of past ice-jam events were then compared to instantaneous maximum water levels collected from a nearby river gauge to evaluate whether the dendrogeomorphological findings were accurate. Our results show that there is a direct correlation between gauged river data and dendrogeomorphological findings, with signs of ice-jamming events identified in the trees from the five highest years of instantaneous maximum water levels. Additionally, a correlation between the peak events in the two data sources was also revealed.

Reservoir landslide monitoring and mechanism analysis based on UAV photogrammetry and sub-pixel offset tracking: a case study of Wulipo landslide

Introduction: Reservoir landslides undergo large deformations during the early stages of impoundment and maintain long-term persistent deformations during the operational period of the reservoir. The management of reservoir landslides mostly focuses on the early identification, risk assessment during the large deformations, and long-sequence monitoring during long-term persistent deformations, which requires sufficient continuity and integrity of the landslide monitoring data.Methods: Taking the Wulipo (WLP) landslide in Baihetan Reservoir as example, this paper proposes a reservoir landslide monitoring method that integrates field survey, unmanned aerial vehicle (UAV) photogrammetry and global navigation satellite system (GNSS) monitoring, which can effectively eliminate the practical monitoring gaps between multiple monitoring methods and improve the continuity and completeness of monitoring data.Results and discussion: First, this study determined the initiation time of the landslide through the field investigation and collected five period of UAV data to analyze the overall displacement vector of the WLP landslide using sub-pixel offset tracking (SPOT). On the basis of the above data, we compensated for the missing data in GNSS system due to the practical monitoring vacancies by combining the field survey and the landslide-water level relationship. Based on these monitoring data, this paper points out that the WLP landslide is a buoyancy-driven landslide, and whether or not accelerated deformation will occur is related to the maximum reservoir water level. Finally, this study analyzed and discussed the applicability of UAV photogrammetry for reservoir landslide monitoring in the absence of ground control points (GCPs), and concluded that this method can be quickly and flexibly applied to the stage of large deformation of reservoir landslides.

Analysis of the Load‐Bearing Capacity of the Steel‐Shell Concrete Support Structure Formed by the Construction of the Pipe Curtain Freezing Method

The Sanya estuary passage project is the latest and highly technical underground project after the large shield underground construction, which is of great significance to the construction of the Hainan free trade port and underground space construction innovation. In this paper, we use finite element software to analyze the deformation, stress, and surface settlement of the pipe curtain structure during construction under two conditions (original riverbed line and maximum scour riverbed line) in order to provide guidance for the actual project. Numerical simulations show that due to excavation and unloading, the bottom and top of the pipe curtain structure are subject to tensile stress; the bottom rises, the top settles, and converges in the horizontal direction; under the original riverbed line condition, the displacement values of each index are larger; changing the riverbed line does not change the maximum and minimum main stress of the pipe curtain structure significantly, and the stiffness of the pipe curtain structure can meet the maximum water level and the maximum scour in two unfavorable conditions; the pipe curtain structure can meet the structural. The pipe screen structure can meet the structural load‐bearing requirements. Suggestions for the design and construction of the project: strengthen the thickness of the pipe curtain at the side wall position, that is, use larger diameter steel pipes; do a good job of antiseepage and anticracking of the concrete at the corner of the pipe curtain structure and the side wall position during the construction process.

Relative Sea Level Rise in the Winyah Bay-Waccamaw River Tidal System Over the Last Thirteen Years

Prediction of sea level rise (SLR) in response to climate change has been the focus of worldwide research, most focusing on the impact by human development. The research has been limited to estuaries and tidal rivers near harbors dealing with the hydrodynamics of reversing tidal flows. This article focuses on the Waccamaw River National Wildlife Refuge in coastal South Carolina where freshwater unidirectional flow is common. We examined the record of water levels in the Waccamaw and Pee Dee Rivers over the period 2007–2019 and the length of record of the United States Geographical Survey (USGS) gauge at Pawleys Island on the Waccamaw River. The Atlantic Ocean, off the southeastern coast of the US, has experienced accelerated SLR since 2000. National Oceanic and Atmosphere Administration (NOAA) tide gauges from Fort Pulaski on Cockspur Island in Georgia to Beaufort, North Carolina, show significant increase in long-term SLR since then with an average since 2007 of approximately 10 mm y-1. Since the study period was less than the 18.6-year cycle of lunar precession, tidal ranges were expanding for much of the study period resulting in the rate of rise of Mean Higher High Water (MHHW; the average of the highest tide levels during each day) being greater than the rate of increase of Mean Lower Low Water (MLLW; the average of the lowest tide levels during each day) in all ocean stations. We examined water levels at NOAA and USGS gauges from Oyster Creek, in North Inlet to Conway on the Waccamaw River and Near Bucksport on the Pee Dee River. We found mean water levels increased more rapidly with distance from the ocean with an apparent SLR > 40 mm y-1 at Conway on the Waccamaw and Bucksport on the Pee Dee. In contrast to the ocean NOAA gauges, the estuary/river gauges showed more rapid increase of daily minimum water level (an approximation of MLLW) than daily maximum water level (an approximation of MHHW) with an extreme of apparent rise of minimum water levels of 58 mm y-1 at Bucksport on the Pee Dee. Nearly 50% of the increase in apparent SLR was due to an increase in the annual average freshwater flow of the Pee Dee and Waccamaw Rivers. Over the past 13 years the Waccamaw National Wildlife Refuge has experienced an apparent SLR that was more than double that observed at the edge of the ocean. The rise has been greater in the height of daily low water than in the height of daily high water. The increase was driven by both tidal hydrodynamics and an increase in the rate of flow in the Pee Dee and Waccamaw Rivers. These findings have important implications for land managers, policymakers, and homeowners in the region as people in the middle to upper estuaries need to plan for rates of relative SLR rise much greater than the frequently discussed rates in the ocean.

Integration of life cycle and habitat conditions in modeling fish biomass in the floodplain of the Lower Mekong Basin

Migratory fishes utilize several distinct habitats during their life cycles, and thus the conservation of such species requires proper habitat management in both spatial and seasonal dimensions. However, their life cycle in the freshwater environment has not been well investigated, in particular in the tropical region. Therefore, our study aimed to elucidate the importance of the integration of life cycle and habitat conditions in modeling the biomass of migratory fish species. Focusing on the hydrological years 2000–2015, we collected the biomass of 74 migratory fish species and their seasonal habitat conditions in the floodplain of the Lower Mekong Basin, including Tonle Sap Lake. Then, generalized linear, generalized additive, and random forest models were applied to the collected data with and without life cycle information. As a result, the models and their performance indicate that the integration of the life cycle and seasonal habitat conditions in modeling the biomass is significantly important for migratory fishes. The results further showed that changing ecohydrological indices in the feeding and refuge seasons negatively affected the fish biomass. Those indices were hydrologic factors (the maximum water levels, the mean water levels, the start date of the flood, and the end date of the flood) and climate factors (water temperatures). Interestingly, the models indicated that the total area of inundated forests negatively influences the biomass of planktivorous fishes while it positively affects the biomass of piscivores. These results clearly showed the importance of a natural flow regime to ensure the fish life cycle and biomass production in the Lower Mekong Basin. The presented approach can be applied to model the biomass of migratory fishes in spatially connected and changing environments.

Effect of Hydrogel on Physico- Chemical and Microbiota Properties of Potting Media for Roof Top Cultivation of Ridge Gourd

The present study was conducted to evaluate the physico-chemical and microbiota properties of growing media containing hydrogel, sand, cocopeat, vermiculite, perlite and vemicompost for cultivation of ridge gourd on rooftop. The result of study revealed that rowing media GM5 (Sand + vermicompost + hydrogel + cocopeat + vermiculite and perlite with the ratio of 3: 2.5: 2: 2: 0.5 showed significant effect on Physical properties of the growing media EC (1.85 dS m-1), pH (5.03), total organic carbon (0.843%), physical properties of the growing media bulk density (0.425g cm-3), total porosity (51%) and maximum level of water holding capacity (66.76 mm). Similarly, bacteria population (72.76 (x 106cfu g-1), 77.44(x 106cfu g-1), fungus population (40.56cfu, 44.56cfu) and actinomycetes population (77.44(x 104cfu g-1), 77.44(x 104cfu g-1) at 75th day and 150th day was significantly highest in GM5 sand + vermicompost + hydrogel + cocopeat + vermiculite and perlite (3: 2.5: 2: 2: 0.5) respectively as compared to control. The obtained results claim that sand + vermicompost + hydrogel + cocopeat + vermiculite and perlite act as bio stimulant to nutrient.

The hydraulic investigation of harmonic plan weirs

Existing dams and reservoirs need to be revised to meet safety standards. For the existing reservoirs where flood control is important and the maximum water level is limited, the best method to increase reservoir capacity is to build a labyrinth weir on the dam. In labyrinth weirs, the collision of two nappe flows produces nappe interference, and this reduces the weir performance. In this study, the semi-circle weirs are placed in an open channel harmonically to increase the efficiency of the labyrinth weirs to make the areas more efficient and increase the flow capacity because of the nappe interference and to ensure the flow moving perpendicular to the weir as possible. In the experimental study of the asymmetrically placed circular weirs, 3 different weir heights (P = 20 cm, 30 cm and 40 cm) and 3 different labyrinth cycle numbers (N = 2, N = 3 and N = 4) were used. As a result of the experiments, although there was a 13 % decrease in the discharge coefficients (Cd) compared to the linear weirs, it provided a 57 % increase in the net crest length (Lnet). Discharge coefficient (Cd) is decreased 3.4 %, as the number of cycles (N) increases. As the weir height (P) decreases, Cd increases 7.5 % due to the vacuum effect.

UAV AERIAL SURVEY IN MONITORING OF COASTAL GEOSYSTEMS IN THE SOUTHERN IRKUTSK AMPHITHEATRE

Estimating the dynamics of exogenous geological processes and understanding their controlling factors is an urgent task faced by many regions of the world. One of the methods of its solution is monitoring. The paper presents the UAV monitoring results for short-term dynamics of complex exogenous processes at five key sites in the coastal zone south of the Bratsk reservoir. The study area is a part of the platform structure in the south of the Irkutsk amphitheater, which is considered to be relatively stable in terms of geodynamics. There has been developed a methodical scheme for obtaining data series on the dynamics of complex exogenous geological processes in the coastal geosystem. A flight altitude of 50–60 m is optimal to obtain orthophotos with a resolution of 1.1–2.2 cm/px, sufficient to estimate the areal dynamics of the processes, and a DEM with a resolution of 2.6–5.4 cm/px to estimate the volumetric dynamics.The eroded coastal area at the Rassvet site was measured to be 6900 m2, which corresponds to an average erosion width of 3.45 m per linear meter of coastal length with maximum values of up to 6.51 m. A high rate of coastal erosion in the period 2021–2022 is related to the maximum water level in the Bratsk reservoir, close to the normal headwater level. The assessment of the volumetric dynamics of the coastal gullies yielded negative values, as most of the gully mouths were eroded. The secondary valley-bottom gullies (Mamontov and Barany sites) and coastal gullies (Khadakhan site) are characterized by positive dynamics both in area and volume growth (12–20 m2 and 1.3–35.0 m3, respectively). At the Khadakhan site, the volumetric growth of new and previously observed suffusion sinkholes was 0.45 m3.The obtained areal and volumetric rates of the exogenous processes can serve as a basis for their modern assessment and prediction of their development in order to prevent and reduce socio-economic risks.

Assessment of the impact of the rise in Lake Victoria water levels on urban flooding using a GIS-based spatial flood modelling approach

ABSTRACT During the period 2018–2021, Lake Victoria experienced a rapid rise in water levels that caused significant urban flooding consequences. However, there are no studies that have extensively investigated the effect of the rising lake water levels on urban flooding. This research responds to this by developing and applying a GIS-based spatial flood modelling approach to simulate the resulting flooding impacts in Entebbe City, Uganda, corresponding to the May 2021 observed maximum lake water level of 13.49 m. Using the validated GIS-based flood model, the simulated flooded area and average flood depth were determined as 9.1 km2 and 1.52 m, respectively. Furthermore, through application of GIS overlay analysis, the total number of affected buildings and length of urban roads were quantified. The proposed methodology provides a robust and computationally efficient approach for identification of at-risk buildings along the lake shoreline and for testing the effectiveness of potential adaptation strategies.

Uncertainties and discrepancies in the representation of recent storm surges in a non-tidal semi-enclosed basin: a hindcast ensemble for the Baltic Sea

Abstract. Extreme sea level events, such as storm surges, pose a threat to coastlines around the globe. Many tide gauges have been measuring the sea level and recording these extreme events for decades, some for over a century. The data from these gauges often serve as the basis for evaluating the extreme sea level statistics, which are used to extrapolate sea levels that serve as design values for coastal protection. Hydrodynamic models often have difficulty in correctly reproducing extreme sea levels and, consequently, extreme sea level statistics and trends. In this study, we generate a 13-member hindcast ensemble for the non-tidal Baltic Sea from 1979 to 2018 using the coastal ocean model GETM (General Estuarine Transport Model). In order to cope with mean biases in maximum water levels in the simulations, we include both simulations with and those without wind-speed adjustments in the ensemble. We evaluate the uncertainties in the extreme value statistics and recent trends of annual maximum sea levels. Although the ensemble mean shows good agreement with observations regarding return levels and trends, we still find large variability and uncertainty within the ensemble (95 % confidence levels up to 60 cm for the 30-year return level). We argue that biases and uncertainties in the atmospheric reanalyses, e.g. variability in the representation of storms, translate directly into uncertainty within the ensemble. The translation of the variability of the 99th percentile wind speeds into the sea level elevation is in the order of the variability of the ensemble spread of the modelled maximum sea levels. Our results emphasise that 13 members are insufficient and that regionally large ensembles should be created to minimise uncertainties. This should improve the ability of the models to correctly reproduce the underlying extreme value statistics and thus provide robust estimates of climate change-induced changes in the future.

Chronic flooding events due to sea-level rise in French Guiana

As sea levels are rising, the number of chronic flooding events at high tide is increasing across the world coastlines. Yet, many events reported so far either lack observational evidence of flooding, or relate to coastal areas where ground subsidence or oceanic processes often enhance climate change-induced sea-level rise (SLR). Here we present observational and modelling evidence of high-tide flooding events that are unlikely to occur without SLR in French Guiana, where sea-level rise rates are close to the global average and where there is no significant ground subsidence. In particular, on 16 October 2020, a well-documented flooding event happened in Cayenne under calm weather conditions. Our probabilistic assessment of daily maximum water levels superimposed on SLR shows that this event can be modelled and is a consequence of SLR. As sea levels will continue to rise, we show that the number, severity and extent of such high-tide flooding events will increase across several urban areas of French Guiana, with an evolution depending on the topography. As concerns are growing regarding the economic impacts and adaptation challenges of high-tide chronic events across the world, our study provides new evidence that this early impact of SLR is emerging now.