Water Levels In Estuaries Research Articles

A regional analysis of tide-surge interactions during extreme water levels in complex coastal systems of Aotearoa New Zealand

Tide-surge interaction (TSI) is a critical factor in assessing flooding in shallow coastal systems, particularly in estuaries and harbours. Non-linear interactions between tides and surges can occur due to the water depth and bed friction. Global investigations have been conducted to examine TSI, but its occurrence and impact on water levels in Aotearoa New Zealand (NZ) have not been extensively studied. Water level observations from 36 tide gauges across the diverse coast of NZ were analysed to determine the occurrence and location of TSI. Statistical analysis and numerical modelling were conducted on data from both inside and outside estuaries, focusing on one estuary (Manukau Harbour) to determine the impact of TSI and estuarine morphology on the co-occurrence rate of extreme events. TSI was found to occur at most sites in NZ and primarily affects the timing of the largest surges relative to high tide. There were no regional patterns associated with the tide, non-tidal residual, or skew-surge regimes. The strongest TSI occurred in inner estuarine locations and was correlated with the intertidal area. The magnitude of the TSI varied depending on the method used, ranging from -16 cm to +27 cm. Co-occurrence rates of extreme water levels outside and inside the same estuary varied from 20% to 84%, with TSI modulating the rate by affecting tidal amplification. The results highlight the importance of investing in a more extensive tide gauge network to provide longer observations in highly populated estuarine coastlines. The incorporation of TSI in flooding hazard projections would benefit from more accurate and detailed observations, particularly in estuaries with high morphological complexity. TSI occurs in most sites along the coast of NZ and has a significant impact on water levels in inner estuarine locations. TSI modulates the co-occurrence rate of extreme water levels in estuaries of NZ by affecting tidal amplification. Therefore, further investment in the tide gauge network is needed to provide more accurate observations to incorporate TSI in flooding hazard projections.

The influence of geomorphology and environmental conditions on stratification in Intermittently Open/Closed Estuaries

Intermittently Open/Closed Estuaries (IOCE) have entrances that close during periods of low river flow. A major characteristic of IOCE is stratification of salinity, dissolved oxygen and water temperature. After IOCE open (either naturally or artificially), large changes in stratification occur as water drains from the estuary to the ocean. The rapid change in water level and loss of the top oxygenated layer of the water column during drainage often causes fish kills in IOCE globally and is related to stratification. Despite this, there are a lack of studies that statistically analyse the relationships between environmental variables and stratification and that quantify changes to stratification during the draining period across multiple IOCE. To fill these gaps, we (1) analysed the relationships between environmental variables and stratification using distance-based Linear Models (distLM) and distance-based Redundancy Analysis (dbRDA) in five different IOCE across Victoria, Australia, and (2) measured near-continuous physicochemical depth profiles and changes in entrance morphology, fluvial inflows, and estuary water level following four estuary openings.The distLM results revealed that maximum air temperature, mouth state and fluvial inflows showed statistically significant relationships with stratification in the IOCE studied. The dbRDA suggested that high maximum air temperatures were associated with low values of stratification in small IOCE, more commonly during closed periods. High fluvial inflows were associated with low values of stratification in large IOCE during open periods (except at one site, Curdies River). Field observations of changes in stratification during the draining period revealed two distinct responses. First, a high energy opening with discharge at the mouth between 70 and 182 m3s-1 and fluvial inflows of 0.87–1.85 m3s-1, causing the IOCE water column to mix and become uniform. Second, a low energy opening with discharge from the mouth between 6 and 37 m3s-1 and fluvial inflows of 0.02–0.03 m3s-1, causing the IOCE to remain stratified. These findings were summarised into a conceptual model showing the sequence of changes during openings for different types of IOCE. Over longer timescales (days to years), our results suggest that differences between stratification during open and closed periods are reflected at a shorter time-scale during the draining period (hours to days). These differences further reflect differences in geomorphology and hydrology between IOCE. Our findings will be useful for estuary managers to predict how stratification in different types of IOCE will change during artificial openings and provide a proxy for predicting their response to climate change.

Hydraulic gradient and wave height control the success of estuary artificial entrance openings

Intermittently Open/Closed Estuaries (IOCEs) are dynamic, sensitive estuaries that periodically close as a function of competing marine and fluvial energies. Once closed, IOCEs are often artificially opened to relieve localised flooding. Artificial openings are generally effective as a short-term flood mitigation strategy, however, sometimes IOCEs can close immediately after excavation without draining the lagoon. In this situation, the opening does not achieve its management purpose and reimplementation is required – costing many thousands of dollars at a time and leading to adverse impacts on estuary biota. Using a dataset of 137 openings at 37 IOCEs globally, we show that the hydraulic gradient (or grade) at opening and offshore significant wave height determine whether an estuary will remain open and drain its lagoon. The hydraulic gradient represents the energy slope between the estuary and the ocean, and wave height determines the capacity of waves to infill the channel with sediment to counter offshore erosion. Artificial openings are only successful when IOCEs are opened with a hydraulic gradient steeper than 0.017 m/m (equivalent to a grade of 1:60) and when significant wave height is below 4.30 m. These findings highlight that the estuary water level alone is not a good predictor of opening success, despite it being widely used to decide when to implement artificial openings. Using near-continuous monitoring of geomorphic change at 28 artificial openings, we show that the hydraulic gradient (and grade) at the time of opening controls the rate of channel expansion, outflow velocity at the mouth, lagoon drainage rate, and the time to reach peak channel width and outflow velocities. The tidal stage at opening can be used to either speed up or slow down the rates of change, potentially reducing the occurrence of fish kills associated with rapid drainage. Although the decision to open an estuary is a complex balance between environmental, cultural, socio-economic, and practical factors, we present a simple and cost-effective way to predict if artificial openings will be successful. The thresholds of our study can be used to complement existing decision support tools, but as a standalone method, are best suited for emergency openings where draining the lagoon to reduce flooding is the main management priority.

Coastal windstorms create unsteady, unpredictable storm surges in a fluvial Maine estuary

Storm surges create coastal flooding that can be damaging to life and property. In estuaries with significant river influence (fluvial), it is possible for tides, storm surge, and river discharge to interact and enhance surges relative to the immediate coast. These tide-surge-river interactions were previously identified in a fluvial Maine estuary as higher frequency (>four cycles per day) oscillations to storm surge which were proposed to be incited by enhanced friction and resonance during certain windstorm events (Spicer et al. 2019). The relative contributions to tide-surge-river interaction from atmospheric forcing variables (wind, barometric pressure, and externally generated surge) remains unclear. This work seeks to decompose and analyze a recent windstorm surge event to better isolate the effects of atmospheric forcing on tidesurge- river interaction. Results show total storm surges in the fluvial estuary to be two times larger than at the estuary mouth because of tide-surge-river interaction. Analysis indicated at least 50% of the magnitude of tide-surge-river interactions are created by non-tidal forcing, in the form of wind, enhancing frictional energy in the estuary. The remaining tide-surge-river interaction is likely a result of changes in tidal wave propagation speed due to surge deepening the mean estuary water level.

Methods for predicting berm height at Temporarily Open/Closed Estuaries

This study investigates the berm crest elevation at 20 Temporarily Open/Closed Estuaries (TOCEs) along the wave-dominated, microtidal coastline of South Africa and provides suitable methods of prediction. Several years of berm survey and estuary water level data were analysed for the selected estuaries, resulting in a relatively large record of historical berm crest elevations compared to previous regional datasets. The new dataset was utilised to test and develop multiple methods for the prediction of berm height and consequently flood levels within TOCEs. Long-term wave runup elevations (R2%) were simulated at the estuary berms using existing runup parameterisations to establish the exceedance probability threshold of R2% associated with the maximum berm height (BMAX). It was found that the 5% exceedance probability of runup provided consistent predictions of BMAX at the selected TOCEs with a mean absolute error (MAE) of 0.28 m. A Berm Height Criteria was developed to predict BMAX based on the wave height, beach face slope, sediment grain size and tidal range at the estuary. The criteria-based approach ensures ease of use and is intended as a first estimate of BMAX (with MAE of 0.23 m). Lastly, a stepwise berm growth model was tested according to its potential to track short-term berm growth during inlet closure to breach cycles. The model provides a simplified approach to predict berm accretion based on the overwash potential depth, which is defined as the elevation difference between R2% and the berm height. The use of a site-specific scaling parameter for berm accretion provided predictions of the berm height at breach with MAEs varying between 0.03 m and 0.17 m among estuaries for the short-term scenarios tested. The hydrodynamic and morphological variability displayed by the selected South Africa TOCEs increased the robustness of the results, potentially making the findings relevant to TOCEs in other parts of the world.

Research on the influence of diversion flow ratio on the water environment in Nanjing

AbstractIn this paper, we take the Qinhuai River Basin in Nanjing as an example to study the influence of diversion flow ratio on the water environment. The daily precipitation of 20 rainfall stations in the basin was collected from 1962 to 2006, and the rainfall under different assurance rates was calculated by P-III frequency curve to calculate the surface runoff in different typical years. In the meantime, according to the downstream furcated estuary water level in different typical years, the diversion flow ratio under different frequencies was calculated by using a mathematical model of the water environment. The results show that the diversion flow ratio of a branch channel increases with the increase of water level difference, and the growth rate decreases gradually. The other branch channel diversion flow ratio decreases with the increase of water level difference, and the decreasing rate decreases gradually. The pollutant concentrations in the bifurcated rivers are equal, but the concentration decreases exponentially with the increase of the upstream flow. Under different rainfall frequencies, the diversion pollutant ratio and diversion flow ratio are equal in different months of different years.

Physical-chemical quality analysis of Serayu River water, Banjarnegara, Indonesia in different seasons

This article discusses the analysis of the physico-chemical parameters of water in the Serayu River, Banjarnegara, Indonesia during the dry and rainy seasons. The parameters to be measured are heavy metals, alkalinity, pH, temperature, TDS, TSS, and corrosivity. The sampling location is at the mouth of the Serayu River, where Serayu River water is one of the Mrica Reservoir water sources used for hydroelectric power plants. (PLTA). When the samples were taken in the rainy and dry season in 2018. The results showed that in the rainy season, the surface water pH of the Serayu River estuary was 6.61; the TDS was 178 mg/l; the TSS was 62 mg/l; the BOD was 6.66 mg/l; the COD was 33.31 mg/l; and the nitrate was 4.03 mg/l. Meanwhile, in the dry season, the pH was 8.15; the TDS wa 121 mg/l; the TSS was 55 mg/l; the BOD was 6.35 mg/l; the COD was 31.77 mg/l, and the nitrate was 3.08 mg/l. All physical and chemical parameters meet the quality standards required in Indonesian Government Regulation No. 82 of 2001 concerning Management of Water Quality and Class III Water Pollution Control. The surface water of the Serayu River estuary is weak corrosive which means it is safe for hydropower activities. In general, in the rainy season the chemical physics parameters of the Serayu River estuary water level are higher than the rainy season.

Analytical approach for determining the mean water level profile in an estuary with substantial fresh water discharge

Abstract. The mean water level in estuaries rises in the landward direction due to a combination of the density gradient, the tidal asymmetry, and the backwater effect. This phenomenon is more prominent under an increase of the fresh water discharge, which strongly intensifies both the tidal asymmetry and the backwater effect. However, the interactions between tide and river flow and their individual contributions to the rise of the mean water level along the estuary are not yet completely understood. In this study, we adopt an analytical approach to describe the tidal wave propagation under the influence of substantial fresh water discharge, where the analytical solutions are obtained by solving a set of four implicit equations for the tidal damping, the velocity amplitude, the wave celerity, and the phase lag. The analytical model is used to quantify the contributions made by tide, river, and tide–river interaction to the water level slope along the estuary, which sheds new light on the generation of backwater due to tide–river interaction. Subsequently, the method is applied to the Yangtze estuary under a wide range of river discharge conditions where the influence of both tidal amplitude and fresh water discharge on the longitudinal variation of the mean tidal water level is explored. Analytical model results show that in the tide-dominated region the mean water level is mainly controlled by the tide–river interaction, while it is primarily determined by the river flow in the river-dominated region, which is in agreement with previous studies. Interestingly, we demonstrate that the effect of the tide alone is most important in the transitional zone, where the ratio of velocity amplitude to river flow velocity approaches unity. This has to do with the fact that the contribution of tidal flow, river flow, and tide–river interaction to the residual water level slope are all proportional to the square of the velocity scale. Finally, we show that, in combination with extreme-value theory (e.g. generalized extreme-value theory), the method may be used to obtain a first-order estimation of the frequency of extreme water levels relevant for water management and flood control. By presenting these analytical relations, we provide direct insight into the interaction between tide and river flow, which will be useful for the study of other estuaries that experience substantial river discharge in a tidal region.

Sea level variability in South African estuaries

Sea level variability in the coastal ocean affects water levels in adjacent estuaries over a wide range of periods and to varying distances landward. An assessment is given here of the responses of a number of South African estuaries, focusing on tidal and subtidal periods where the dominant effects are found and the necessary data are available. Shorter period inputs tend to be limited to mouth regions and require more detailed, higher frequency data. Subtidal fluctuations, in particular those caused by coastal trapped waves, play an important part in the dynamics of estuaries in the Western Cape and Eastern Cape. In many cases their influence dominates tidal oscillations farther upstream, and it is apparent that such fluctuations must be considered in any assessment of estuarine characteristics, and indeed their upstream extent. Nonetheless, it should be emphasised that each estuary has unique properties, and must be assessed individually. Sea level variability changes dramatically north of the Kei Estuary, and particularly off the coast of KwaZulu-Natal. Unfortunately, few measurements were available from these estuaries, but the mouth conditions of the estuaries and the build-up of river flow appear to be important factors in resulting estuary water levels. These results show that using only tidal variations in the sea can result in very misleading conclusions about concurrent water levels in estuaries.

Estuarine margins vulnerability to floods for different sea level rise and human occupation scenarios

Rilo, A., Freire, P., Guerreiro, M., Fortunato, A.B., Taborda, R. 2013. Estuarine margins vulnerability to floods for different sea level rise and human occupation scenarios. In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 820-825, ISSN 0749-0208. Sea level rise (SLR) is increasing the risk of flooding in marginal low-lying estuarine areas. High water levels in estuaries are associated with the simultaneous occurrence of various phenomena ranging from high tidal levels to large fresh-water discharges. The intense and diverse human occupation of some of these marginal areas increases the flood vulnerability, endangering human lives and causing heavy economic and environmental losses. This issue becomes even more relevant since the European Floods Directive establishes mandatory guidelines to all member states regarding flood risk assessment, integrating flood maps with territorial characteristics. The present study aims to assess the vulnerability to floods of a specific area in Tagus estuary, by integrating hydrodynamic modelling results with land use cartography. Results suggest that about 16.1% of the estuarine marginal fringe could be flooded for the 2050 scenario, rising up to 23.7% for the 2100 scenario. Moreover, industrial and urban areas will be the most affected sites. Considering future development scenarios planned for that site, the results point to an increasing vulnerability of urban areas and therefore to the relevance of this knowledge in future planning and management guidelines as well as measures to adapt to SLR. The uncertainties and implications of the study are also discussed, and future research topics are indicated.

The 2004 Sumatra tsunami effect on the Itajaí-Açu estuary water level, Santa Catarina, Brazil

December, 2004 off the coast of Sumatra, in the Indian Ocean, was generated by a megathrust earthquake of magnitude 9.3 on the Richter scale, and felt worldwide (Fig. 1A; TITOV et al., 2005). It was the longest-lasting ever recorded in history, of around 600 seconds, giving rise to a 1,500 km long failure in the seabed (NIO, 2011). The earthquake produced the largest trans-oceanic tsunami in over 40 years, and killed more people than any other tsunami in recorded history (NOAA, 2010), in Indonesia, Sri Lanka, India, Thailand, Myanmar and Somalia. According to Titov et al. (2005), the recorded worldwide effects revealed the unprecedented, truly global reach of the waves generated on 26

Surface evolution and carbon sequestration in disturbed and undisturbed wetland soils of the Hunter estuary, southeast Australia

The aim of this work was to quantify the soil carbon storage and sequestration rates of undisturbed natural wetlands and disturbed wetlands subject to restriction of tidal flow and subsequent rehabilitation in an Australian estuary. Disturbed and undisturbed estuarine wetlands of the Hunter estuary, New South Wales, Australia were selected as the study sites for this research. Vertical accretion rates of estuarine substrates were combined with soil carbon concentrations and bulk densities to determine the carbon store and carbon sequestration rates of the substrates tested. Relationships between estuary water level, soil evolution and vertical accretion were also examined. The carbon sequestration rate of undisturbed wetlands was lower (15% for mangrove and 55% for saltmarsh) than disturbed wetlands, but the carbon store was higher (65% for mangrove and 60% for saltmarsh). The increased carbon sequestration rate of the disturbed wetlands was driven by substantially higher rates of vertical accretion (95% for mangrove and 345% for saltmarsh). Estuarine wetland carbon stores were estimated at 700–1000 Gg C for the Hunter estuary and 3900–5600 Gg C for New South Wales. Vertical accretion and carbon sequestration rates of estuarine wetlands in the Hunter are at the lower end of the range reported in the literature. The comparatively high carbon sequestration rates reported for the disturbed wetlands in this study indicate that wetland rehabilitation has positive benefits for regulation of atmospheric carbon concentrations, in addition to more broadly accepted ecosystem services.

Stochastic Dynamic Approach to Predict Water Levels in Estuaries

A method is presented to predict water levels in tidal estuaries using Kalman filtering techniques. The method is illustrated by predicting the water levels in the Eastern Scheldt. This estuary appears to contain many of the features inherent in modeling flows in estuaries. It is seen that, although the geometry is very complicated due to numerous gullies and banks, a very simple schematization is sufficient. Furthermore, it is shown that nonlinear effects in the mouth due to the complicated geometry and the interaction between the tidal wave along the coast and the water movements in the estuary can be compensated by using stochastic dynamic methods applied to internal boundary conditions.