Annual Water Level Research Articles

Decrease in Fishery Yields in Response to Hydrological Alterations in the Largest Floodplain Lake (Poyang Lake) in China

Habitat degradation has caused reduction in fishery yields in many freshwater ecosystems, particularly recession of water levels in natural lakes. Poyang Lake, the largest freshwater lake and one of the most exploited regions in China, has exhibited a dramatic variation in the water level for decades, especially after the operation of the Three Gorges Dam. We evaluated the long-term dynamics of fishery yields and the relationship to hydrological variability of Poyang Lake from 1990 to 2016. There was a strong positive effect on the annual maximum water level (Hmax), the minimum water level in April (HMmin4), the maximum water level in August (HMmax8), the average water level in October (HMmean10), and the number of days when the water level was above the wet threshold (Wetdays) on fishery yields. The all-subsets regression model identified the best variable combination subset which contains eight hydrological variables (R2 = 0.9493), and the HMmin4, HMmax8, and HMmean10 variables were the most important variable predictor for fishery yields (contributing to 63.03% of the explained variability). The Mann–Kendall test showed that the time series of the fishery yield of Poyang Lake had significant decreasing trends over the past few decades. Moreover, Wetdays, HMmin4, and HMmean10 also showed significantly decreasing abrupt changes, and the abrupt changes’ time of HMmean10 was the same as that of the fishery yield in 2005. The mean fishery yield and HMmean10 dropped from 42,581 tonnes and 14.15 m during 1990–2005 to 27,464 tonnes and 11.78 m during 2006–2016, respectively. This study is critical for implementing effective strategies for the protection of fish resources and lake ecosystems.

The 2014 Zigui Earthquake Sequence near the Three Gorges Dam in China

AbstractSeismicity in the Three Gorges Reservoir (TGR) region increased noticeably as the water level of the reservoir rises, since the Three Gorges Dam (TGD) was built in 2003. Here, we determined moment tensors of six earthquakes in the 2014 Zigui earthquake sequence (ZGS) in the TGR region using local and regional broadband seismic waveform data. We also determined the focal depth of the mainshock using its teleseismic waveform data and then relocated the epicenter using its travel times at local stations. High-precision locations of 64 events in the sequence were obtained by combining the double-difference relative location result and the mainshock’s absolute location. The event locations and moment tensor solutions indicate that the ZGS occurred between depth 5 and 9 km on a previously unknown fault striking southwest and dipping ∼80° to the northwest. The event depth distribution and coulomb stress change estimation suggest that the ZGS were not induced directly by the reservoir water but triggered by the stress change from the annual reservoir water level variation. We estimated that the newly found fault has the potential for a magnitude 5.7 earthquake for which ground motion has a 16% probability to exceed the designed maximum intensity level at the TGD, though it would take more than 100 yr to accumulate the needed amount of slip.

Water level change of Lake Machang in eastern China over 1814–1902 CE

Abstract. Lake Machang, occupying an area of approximately 30 km2 in the city of Jining, eastern China, was a historical reservoir on the Grand Canal from the early 15th century to early 20th century. The premodern monthly water level observation of Lake Machang commenced in 1814 CE and ended in 1902 CE. The available observation data from the monthly records covered 75.6 % of the entire study period of 1814–1902. Although the water level was seemingly artificially influenced by human activities, monthly and annual water level changes still correlated well with precipitation. That is, climate is still the dominant factor in water level changes on seasonal and annual scales. The flooding of the Yellow River in 1871 carried a large amount of silt into Lake Machang, which resulted in the rise of the lake bed and reclamation initiated by local residents. After the reclamation activity was officially approved in 1900, Lake Machang was massively reclaimed and eventually dried up in the early 20th century.

Future hydropower operations in the Zambezi River basin: Climate impacts and adaptation capacity

AbstractSouthern Africa could experience major hydrological changes in coming decades, bringing drastic changes to hydropower. Focusing on four large hydropower dams in the Kafue and Zambezi Rivers (Itezhi‐Tezhi, Kafue Gorge Upper, Kariba, and Cahora Bassa), accounting for over 90% of the installed capacity in the basin, this study analyzed how these dams have altered river flows in the past, and how hydropower generation could be affected by future climate change and water use scenarios based on dynamically downscaled simulations from the Fifth Coupled Model Intercomparison Project for a Representative Concentration Pathway of 8.5 W/m2. Future climate change could affect reservoir levels by decreasing water inflows and by increasing evaporation. Results showed a decrease in median annual water levels for all reservoirs, with largest changes expected at Itezhi‐Tezhi (−1.3 to −3.3 m), followed by Cahora Bassa (−0.8 to −2.8 m), Kariba (−0.2 to −0.9), and Kafue Gorge (0.1–0.2 m). Flows downstream of dams show a consistent negative trend, with mean annual flow changes of −30% to −49% at the furthest downstream gauge. The combined reduction in water levels and flows could result in decreases on average annual hydropower generation and increases in interannual variability; annual generation could decrease by −61% to −28% at Kariba, by −51% to −21% at Itezhi‐Tezhi, by −7% to −39% at Kafue Gorge, and −27% to −9% at Cahora Bassa. Reservoir operation optimization could marginally offset the projected changes in hydropower generation, with largest gains for Kariba (+6%) and Cahora Bassa (+10%). Overall, opportunities to offset losses via operations would probably decrease with the expected drier climate and escalating reduction in river discharge and water availability. Given these negative projections, it is becoming critical to adapt and coordinate reservoir operations to best cope with the expected challenging future in this important river basin.

Remote sensing reconstruction of long-term water level and storage variations of a poorly-gauged river in the Tibetan Plateau

Study regionThe upper Brahmaputra River, China. Study focusThe long-term water level and storage variations of the upper Brahmaputra River (UBR) are not yet clear due to the low accessibility of gauged data in high-altitude and harsh environments. The main objective of this study is to facilitate investigations of long-term river water level and storage variations over the UBR. Here we developed a hypsometric method to reconstruct temporally resolved long-term river water level observations based on remote sensing-derived accumulative water inundation area information and short-term altimetric water level data at 24 virtual stations (VSs) of the UBR. We further analyzed the intra-annual and inter-annual variations of the water level (storage) changes of the studied river reaches. New hydrological insightsThe results reveal that the reconstructed water levels of the UBR are quite consistent with the water levels from the Hydroweb with an average R2 of 0.79 and the reconstructed water levels are generally consistent with the variation of the gauged runoff. The ascending amplitudes of the annual water level cycle of VSs were observed from upstream to downstream of the UBR, varying from 0.58 m to 6.92 m. Additionally, we find that the seasonal change in river water storage approximately accounted for one-tenth of the net mass seasonal amplitude of the UBR basin, as revealed from the Gravity Recovery and Climate Experiment (GRACE) data.

Water shortage and countermeasures of irrigation area around Lake Poyang in dry seasons

近年来，在以三峡大坝为代表的一系列流域人类活动作用下，鄱阳湖出现了枯水期延长、水位提前消落的现象，导致了一系列生态环境和民生问题.本文从环湖灌溉缺水入手，采用11种概率分布分析1953—2018年星子站水位，进行比较优选后选择Wakeby分布作为理论分布，得到90%保证率下的湖泊年平均水位及枯季9月至翌年2月月平均水位，并据历年平均水位选取代表水文年，进行环湖灌区水资源供需分析，获得缺水量.在此基础上，建立灌区缺水量变化与长江干流流量变化的量化关系，讨论其应对策略.结果表明：自2003年三峡工程运行以来，由于湖泊枯季水位降低，90%保证率下鄱阳湖环湖灌区9月至翌年2月的缺水量已达到2.26亿、2.12亿、0.57亿、0.58亿、0.55亿、0.25亿m³.通过调节长江干流流量满足环湖灌区用水需求的方案受到时段和“五河”来水状况的限制，即该方案仅适用于枯季9—10月“五河”来水减少的情况下.受限于这些条件，调节长江来水并非治本之策，目前仍需采取其他更为有效的手段以恢复鄱阳湖的水位及其过程.;In recent years, due to the influence of a series of human activities around the river basin represented by the Three Gorges Dam, the dry season duration of Lake Poyang was prolonged and early drawdown of water level was observed. This leads to a series of ecological and livelihood issues. To investigate the water shortage of irrigation around the lake, this paper analyzed the water level of Xingzi Station from 1953 to 2018 by applying eleven probability distribution functions. The Wakeby distribution was selected as the theoretical distribution after comparison. Both the mean annual water level and mean monthly water level during the dry season under a 90% guarantee rate were obtained. Based on the mean annual water level over the years, the representative hydrological year was selected to analyze the supply and demand of water resources, thus the water shortage was obtained. Consequently, the quantitative relationship between water shortage change of the irrigation area and water discharge change of the main channel of Changjiang River was established, and the countermeasures were discussed. The results show that the water shortage (under 90% guarantee rate) of irrigation area around Lake Poyang has accordingly reached 226 million m³, 212 million m³, 57 million m³, 58 million m³, 55 million m³ and 25 million m³ from September to February of the next year, since the operation of the Three Gorges Dam in 2003. The solution to meet the water demand of the irrigation area by regulating the water discharge of the main channel of Changjiang River is only applicable to the situation when the water discharge from the ‘Five Rivers’ decreases in September and October. Therefore, to restore the water level of Lake Poyang and its process, other more effective means are needed.

Hydrological System of the Plitvice Lakes—Trends and Changes in Water Levels, Inflows, and Losses

The Plitvice Lakes National Park is inscribed on UNESCO’s World Heritage List. The lake system is composed of 16 cascading lakes of different sizes separated by tufa barriers, which are the park’s key phenomenon. The lakes are characterized by highly diverse trends of the characteristic hydrological indicators—mean annual water levels, discharges, and tufa barrier growth. The analyses carried out in this paper identified that in the period before the early 1990s, Kozjak Lake had a trend of decreasing discharges, together with a trend of increasing water levels and growing tufa barriers. In contrast to this, in the period after 2001, a trend of increasing discharges was recorded, as well as a trend of decreasing water levels and decreasing tufa barriers. A potential cause of the barriers decreasing in size were the extremely high discharges during the last decade, which resulted in increased erosion of the tufa barriers. Losses of water due to the sinking from the lake system as well as the upper Korana course were confirmed, and it was identified that during the analyzed period the losses had not changed significantly. It was determined that the losses of water from Kozjak Lake occurred during low-water periods; however, they depended not only on the quantity of water flowing through the lakes but also on the hydrological conditions underground. The analyses carried out and the methodological procedures used in the analyzed area of the Plitvice Lakes are useful examples for the performance of analyses at similar lakes in karst formed by tufa deposition processes.

Effects of hydrological forcing on short- and long-term water level fluctuations in Lake Huron-Michigan: A continuous wavelet analysis

Understanding water level fluctuation patterns in the Great Lakes is one of the pillars for designing adaptive management practices that can mitigate the impacts of extreme water levels on shoreline infrastructure and associated economic activities. The present study uses continuous wavelet transformation to conduct a two-dimensional frequency-scale spectral analysis on monthly water levels in Lake Huron-Michigan. Consistent with past work, we detected 1-, 8-, 12-, and 36-year quasi-state periodicities in water level records during the 1860–2015 period. The wavelet analysis revealed diminishing, re-emergent, and/or intermodulation patterns in the long-term periodicities, but the 1-year cycle has persisted throughout the temporal domain studied. While the features of the dominant frequencies are consistent with previous findings, the consideration of wavelet ridges with the continuous wavelet transformation yielded a complete picture of evolution in time–frequency domain that is impossible to achieve with the same accuracy using conventional signal processing methods. Continuous wavelet transformation and analysis of wavelet coherence between hydrological and water level changes signified the role of runoff and evaporation as the primary drivers of water level fluctuations in Lake Huron-Michigan. Evaporation strongly influences the annual water level fluctuations, whereas runoff and precipitation covary with the water levels in both intraannual and interannual cycles. Specifically, analysis of phase differences indicated that runoff leads water level change by π/4 rad from 2-month up to 1.5-year cycles, then runoff starts to become nearly in-phase (with small intermittent lags) up to the 12-year cycle, two main recurring cycles of 0.5- and 1-year. Finally, we conducted a comprehensive assessment of 16,809 regression models, which highlighted the effects of runoff-minus-evaporation in modulating the extreme high or low water levels, which in turn could inundate the areas close to the shoreline or reduce the navigability in shallow parts of Lake Huron-Michigan.

Great lakes urban pocket-beach dynamics: A GIS-based analysis of infrastructure-design influences on geomorphic development

Chicago’s lakefront beaches experienced inundation and coastal erosion when Lake Michigan’s mean annual water level rose by >1.5 m between 2013 and 2019. Understanding beach geomorphic response to this type of lake-level event is important, as future climate predictions call for a continuation of decadal oscillatory patterns. A GIS-based study of beach change was conducted along the city’s urban lakefront, where sand is embayed by groins, jetties, and revetments. Morphologic changes associated with the most recent decadal lake-level rise were evaluated in context of the surrounding infrastructure. Beach morphometrics, derived from historical aerial images and available LiDAR products, were compared against the characteristics of the fixed urban infrastructure. Overwash volumes associated with an ∼1 m-rise in lake level scaled well with beach size (R2 = 0.88), suggesting that the creation of new sediment accommodation and its spatial distribution along the urban lakefront during lake-level rise is an important control on beach morphodynamic behavior. Overwash patterns were influenced by embayment orientation and groin characteristics. Counterclockwise beach rotation of up to 21° occurred in places where shorelines were exposed to the open lake. More tightly enclosed beaches retreated more uniformly along strike due to passive inundation of terrain with lake-level rise. Our insights provide managers with useful information on key beach behavioral patterns and how they are influenced by infrastructure design, allowing for the possibility of mitigation strategies to be emplaced in anticipation of future lake-level oscillations.

Forecasting annual maximum water level for the Negro River at Manaus

AbstractMore frequent and stronger flood hazards in the last two decades have caused considerable environmental and socio‐economic losses in many regions of the Amazon basin. It is therefore critical to advance predictions for flood levels, with adequate lead times, to provide more effective and earlier warnings to safeguard lives and livelihoods. Water‐level variations in large, low‐lying, free‐flowing river systems in the Amazon basin, such as the Negro River, follow large‐scale precipitation anomalies. This offers an opportunity to predict maximum water levels using observed antecedent rainfall. This study aims to investigate possible improvements in the performance and extension of the lead time of existing operational statistical forecasts for annual maximum water level of the Negro River at Manaus, occurring between May and July. We develop forecast models using multiple linear regression methods, to produce forecasts that can be issued in March, February and January. Potential predictors include antecedent catchment rainfall and water levels, large‐scale modes of climate variability and the long‐term linear trend in water levels. Our statistical models gain one month of lead time against existing models for same skill level, but are only moderately better than existing models at similar lead times. All models lose performance at longer lead times, as expected. However, our forecast models can issue skilful operational forecasts in March or earlier. We show the forecasts for the Negro River maximum water level at Manaus for 2020 and 2021.

Characteristics of the Annual Maximum and Minimum Water Temperatures in Tonle Sap Lake, Cambodia from 2000 to 2019

The Tonle Sap Lake contains unique hydrological environments and ecosystems. Although water temperature is an important consideration in lake management, information on the water temperature of the lake is limited. Thus, we investigated the characteristics of the daytime water temperatures of the Tonle Sap Lake from 2000 to 2019 using MOD11A1, a Moderate Resolution Imaging Spectroradiometers (MODIS) product. Moreover, the relationship between water temperature fluctuations and hydrological–meteorological conditions was analyzed. The maximum and minimum daytime water temperatures were recorded in May (30.7 °C) and January (24.6 °C), respectively, each a month after the maximum and minimum air temperatures were recorded. The annual maximum, average, and minimum water levels showed a downward trend (−0.14, −0.08, and −0.01 m/y, respectively). The annual maximum water temperature increased at a rate of 0.17 °C/decade, whereas the annual minimum water temperature decreased at a rate of 0.91 °C/decade. The annual maximum daytime water temperature had a strong negative correlation with water level change (flood pulse) and a weaker correlation with air temperature. The annual minimum daytime water temperature presented the strongest positive correlation with water level change (flood pulse) in the previous year. However, there was no correlation between the annual minimum daytime water temperature and air temperature. These results indicate that water temperature in the Tonle Sap Lake is mainly affected by water level fluctuations, which are mainly driven by flood pulses.

Uncovering the mechanisms of seasonal river–groundwater circulation using isotopes and water chemistry in the middle reaches of the Yarlungzangbo River, Tibet

Alpine catchments are severely impacted by climate change and have been extensively studied from glaciological and hydrological perspectives. However, groundwater in such catchments is rarely mentioned, and the role of the groundwater system in sustaining streamflow in alpine catchments remains unknown. This study used an integrated framework with multi-tracer data (major ions and isotopes (2H, 3H, 18O, and Sr)) to identify seasonal water sources and to uncover the recharge–discharge mechanism of the river–groundwater system in the middle reaches of the Yarlungzangbo River (YR). The results show that surface waters collected in the dry season major ions exhibit higher solute concentrations and stable isotopes (H and O) are more enriched in river water than in the wet season. In addition, river δ18O values exhibit a distinct spatial variation that decreases and then increases along the flow direction in the dry season, but this is greatly attenuated during the wet season. Estimation results show that groundwater maintains the winter–spring baseflow (contributing 45.1%), including the discharge of deep circulating groundwater through macro structures, such as south–north oriented active tensile faults (contributing 19.6 ± 9.9%). During the wet season, precipitation (contributing 31.0 ± 3.6%) and tributaries (contributing 30.0 ± 8.2%) are the principal recharge end members of the river water. The rapid recharge of phreatic groundwater occurs in alpine regions through faults and fissures. Like the YR basin, the phreatic groundwater system is primarily maintained by modern meltwater and precipitation originating from oceanic moisture. The groundwater is younger than 20 years and recharges at a seasonal scale. Precipitation is likely the principal mechanism governing annual river flow and water level variations. Therefore, groundwater storage and total runoff in the YR Basin will increase greatly with climate warming in Tibet. Simultaneously, hydrological accidents are also likely to occur, requiring advanced monitoring and prevention measures.

EXPLORATION OF HELOPHYTE GROWTH DYNAMICS IN THE PSKOVSKOE LAKE LITTORAL ZONE USING REMOTE SENSING DATA

The article presents the results of an assessment of the spatio-temporal dynamics of the Pskovskoe Lake littoral zone overgrowing with helophytes in the period from 1989 to 2019. Lake Pskovskoe is a shallow body of water with a high trophic status, annual water level fluctuations, and an extensive littoral zone (about 15 % of the water surface area). The said factors, coupled with global warming, create favorable conditions for the expansion of higher aquatic vegetation. A detailed analysis of the history of research of the lake water area is provided. The types of the lake overgrowing in different years with indication of the prevalent ecological groups and dominant species are suggested. Lake Pskovskoe is characterized by the helophytic – “fragmentary-belt” type of overgrowing. Data from the literature and own field data were analyzed; multispectral satellite images taken in different years were thematically processed. The process of littoral overgrowing in this period has been fluctuating, with a positive trend component. In low-water years, the occurrence of helophytic plants increased. The expansion of helophytes in relation to the averaged shoreline configuration ranged from 26.56 to 641.9 ha, i. e. 0.04–0.9 % of the lake total area. The largest helophyte expansion was noted at Talabsk station, where the overgrown area gained 7.5 hectares in the period from 2007 to 2017. Amongyear changes of the main structural parameters of the southern reed (Phragmites australis (Cav.) Trin. Et Steud., 1840), the species playing the key role in the overgrowing of Lake Pskovskoe, are analyzed. The density and biomass of the reed stands have increased over the period of observations.

Regulation of Vegetation and Evapotranspiration by Water Level Fluctuation in Shallow Lakes

Water level fluctuations play a critical role in regulating vegetation distribution, composition, cover and richness, which ultimately affect evapotranspiration. In this study, we first explore water level fluctuations and associated impacts on vegetation, after which we assess evapotranspiration (ET) under different water levels. The normalized difference vegetation index (NDVI) was used to estimate the fractional vegetation cover (Fv), while topography- and vegetation-based surface-energy partitioning algorithms (TVET model) and potential evaporation (Ev) were used to calculate ET and water evaporation (Ep). Results show that: (1) water levels were dramatically affected by the combined effect of ecological water transfer and climate change and exhibited significant decreasing trends with a slope of −0.011 m a−2; and (2) as predicted, there was a correlation between water level fluctuation at an annual scale with Phragmites australis (P. australis) cover and open-water area. Water levels also had a controlling effect on Fv values, an increase in annual water levels first increasing and then decreasing Fv. However, a negative correlation was found between Fv values and water levels during initial plant growth stages. (iii) ET, which varied under different water levels at an annual scale, showed different partition into transpiration from P. australis and evaporation from open-water area and soil with alterations between vegetation and open water. All findings indicated that water level fluctuations controlled biological and ecological processes, and their structural and functional characteristics. This study consequently recommends that specifically-focused ecological water regulations (e.g., duration, timing, frequency) should be enacted to maintain the integrity of wetland ecosystems for wetland restoration.

The Influence of Lake Level Fluctuations on Fisheries in Lake Van

The impact of hydrological regimes on the productivity in lake ecosystems has received considerable attention in recent decades. This study tested the hypothesis that water level fluctuations influence the fisheries productivity of Lake Van. Monthly water level data from 2000 to 2014 were obtained from Hydroweb and fisheries data corresponding to the same time period were acquired from the Turkish Statistical Institute. In order to test whether water level fluctuations demonstrated significant concordance with fish landings, landings data as a dependent variable, and seasonal water level amplitude and mean annual water level as independent variables were used in a linear regression analysis. The regression analysis proved insignificant results. The general trend of a linear decline, with a rate of -575 tonnes per year, observed in the landings did not match the seasonal and inter-annual water level variations which occurred in the lake during the same period. The consistently declining yields might result from a prolonged overexploitation and/or a constant recruitment failure. The estimated values of the seasonal and inter-annual relative fluctuation index (0.402 and 0.097, respectively) were rather low indicating that Lake Van is hydrologically stable with limited aquatic/terrestrial transition zone interactions and a relatively low nutrient load.

Distribution, sources and risk assessment of PAHs in soil from the water level fluctuation zone of Xiangxi Bay, Three Gorges Reservoir.

Information on PAH distribution in the water level fluctuation zone (WLFZ) of Three Gorges Reservoir is limited. In this study, we investigated PAH distribution and sources and assessed PAH risks, over one annual water level fluctuation cycle (June 2017-June 2018) at four elevations spanning the WLFZ (145m, 155m, 165m and 175m) at seven locations in the water level fluctuation zone along Xiangxi River. The mean total PAH concentration in June 2018 (953ngg-1) was significantly higher than in June 2017 (494ngg-1), and the horizontal and vertical distributions of PAHs changed significantly. The changes in distribution patterns provided evidence for the cause of increased PAH levels, which were attributed to construction of the Xiangxi River Bridge. Thus, this study of PAH dynamics in the WLFZ soils of Xiangxi Bay also provided valuable information on the impact of bridge construction on WLFZ soils. The change in PAH levels among stations implicated sediment disturbance resulting from bridge construction as the major contributor to the increased PAH levels. Source characterization, based on the ratios of certain PAHs, indicates that PAHs are mainly from the combustion of petroleum fuels, biomass and coal. These ratios indicated that the proportion of PAHs from fuel combustion increased from 2017 to 2018, implicating the heavy equipment used during bridge construction as another source of the increased PAH levels. The incremental lifetime cancer risk (ILCR) model was used to assess the health risk of the PAHs and the range among all age groups (10-5-10-4) indicates a potential health risk. The mean effects range-median quotient (M-ERM-Q) was used to assess the ecological risk of PAHs and the range (0.1-0.5) indicates low to medium risk. The increase in PAH levels from 2017 to 2018 increased the risk to public health and the environment. The results of this investigation provide a reference for ecological restoration of the WLFZ and support development of effective policies for environmental and public health. Further, the results provide information on the impact of bridge construction on WLFZ soils and identify research needed to more fully understand PAH dynamics in WLFZ soils.

A framework for determining lowest navigable water levels with nonstationary characteristics

The Lowest navigable water level (LNWL) is an important indicator used for navigation design to balance the relationship between navigation safety and economic benefits of a waterway. However, it is a challenge of accurately estimating LNWLs due to the nonstationary characteristics of observed water level data series. In this study, a comprehensive framework was developed for handling this issue. In this framework, inter-annual variabilities in both the mean and variance of water level series were described by decomposing original series and were eliminated by composing new series. Intra-annual variability was determined by detecting indicators describing intra-annual water level distributions. Considerations of inter- and intra-annual variabilities were combined by designing annual water level processes for the past and current environments. Shipping risks during both annual and multi-annual periods were considered in the framework as well. The framework was demonstrated in estimating LNWLs at the Gaodao and Shijiao stations in the North River basin, southern China. The recommended LNWLs at the Gaodao station were 22.32 m for 95% guaranteed rate and 21.84 m for 98% guaranteed rate; LNWLs at the Shijiao station were 0.27 m for 95% guaranteed rate and 0.15 m for 98% guaranteed rate. The impact of variance variability on estimations of LNWLs was also evaluated. Results indicated that the recommended LNWLs would have errors of 0.11 ~ 0.48 m at the Gaodao station and 0.03 ~ 0.04 m at the Shijiao station if the variance variability was not considered. The proposed framework was then compared with Nonstationary synthetic duration curve (NSDC) method, and results illustrated that the duration curves plotted by NSDC method were unreasonable, leading to inaccurate design values. Overall, the developed framework is more reasonable and suitable for designing LNWLs of waterways where the variabilities of the water levels at different time scales are different or where the historical water level data contain various variations .

Barrier beaches and breaches: Historical changes on the Point Pelee foreland

The Great Lakes are non-tidal, but experience significant water level fluctuations at hourly, seasonal, and decadal scales. A rise of 0.83 m was observed between 2013 and 2020 on Lake Erie with the annual water level near the previous record high of 174.89 m set in 1986. In response to elevated water levels, barrier beach-ridges located on the eastern shore of the Point Pelee foreland, in the Lake Erie’s central basin, experienced accelerated erosion including overwash and breaching, removal or burial of vegetation, and damage to infrastructure and sensitive habitat. Historical aerial imagery between 1931 and 2020 and annual average lake levels between 1920 and 2020 were examined to characterize barrier retreat and breach initiation, expansion, and closure. Results indicate that the barriers have transgressed at a rate of >1 m y−1, but the rate of transgression and progradation, overwash, and breaching are dependent on decadal-scale variations in water level. Ephemeral breaches and delta deposits were found to be a significant source of landward sediment transport if breaches remained open for multiple years. This may represent a self-reinforcing cycle that promotes breach fill-in processes, even during high lake levels, and increases the barrier width and resiliency in response to further lake level fluctuations. While common patterns of barrier dynamics were found throughout the time series, human impacts may modify historic barrier transgression rates and breach and recovery processes.

Hydrology of annual winter water level drawdown regimes in recreational lakes of Massachusetts, United States

Carmignani JR, Roy AH, Stolarski JT, Richards T. 2021. Hydrology of annual winter water level drawdown regimes in recreational lakes of Massachusetts, United States. Lake Reserv Manage. 37:339–359. Annual winter water level drawdown (WD) is a common lake management strategy to maintain recreational value by controlling nuisance macrophytes and preventing ice damage to shoreline infrastructure in lakes of the northeastern United States. The state of Massachusetts provides general guidelines for lake managers to implement and practice WDs. However, WD management reporting is not required and as such empirical water level records are scarce, making it difficult to assess guideline adherence and link these management actions to littoral habitat conditions. We monitored water levels bihourly in 18 lakes with ongoing WD regimes and 3 non-drawdown lakes over 3–4 yr. Our results show an interlake drawdown magnitude gradient of 0.07–2.66 m with intralake consistency across years. Corresponding WD magnitudes generated exposure of 1.3–37.6% for entire lakebeds and 9.2–71.1% for littoral zones. WD durations averaged 171 d and ranged widely from 5 to 246 d. Longer recession and refill phase durations and faster recession rates were moderately to strongly correlated with drawdown magnitudes. WDs were predominantly initiated prior to the state of Massachusetts 1 November starting guideline (83.1%) and refilled to summer reference levels after the recommended date of 1 April (70.6%). To minimize ecological impacts while still meeting recreational goals, WD performance guidelines may require a more fine-scale approach that integrates local hydrogeomorphic features and the presence of WD-sensitive littoral biotic assemblages. However, climate change model projections of warmer and wetter winters in the Northeast indicate increasing uncertainty for WD as an effective and worthwhile macrophyte control tool.

Comparison between the skew surge and residual water level along the coastline of China

A better understanding of the storm surge phenomenon is essential for coastal protection, future planning, and coastal ecosystem conservation. Using hourly sea level data from 15 tide gauges along the coastline of China the metrics, skew surge and residual water level, which are used to represent storm surges, are compared in this study. The relationship between residual water level, skew surge and predicted high tide were analyzed using two statistical measurements. The results showed that unlike the annual maximum residual water level, the annual maximum skew surge and the astronomical high tide tended to coincide more frequently (more than 50% of the annual maximum skew surges occurred at high tide). Thus, the skew surge was more suitable than the residual water level for use in risk-based coastal planning frameworks. Using the monthly maximum value and indices of the storm surge (storm surge number, duration and intensity), we compared the performances of skew surge and residual water level in expressing changes of storm surges. These two metrics can both capture changes in storm surge activities on seasonal, decadal and long-term scales, and correlation coefficients of storm surge intensity between residual water level and skew surge were greater than 0.80 at 11 tide gauges. At most tide gauges, the skew surge was significantly correlated with 4–5 nearby tide gauges while the residual water level was correlated with only 1–2 tide gauges, the skew surge may be a better metric for risk-based coastal planning frameworks if the interpolation method is used. The El Niño-Southern Oscillation (ENSO) can affect the storm surge frequency along the coastline of China. The effects of the ENSO were more obvious when the residual water level was selected. In conclusion, the skew surge can be selected for use in risk-based coastal planning frameworks, while the residual water level may be a better metric for capturing the relationship between storm surges and climate changes.