Extreme Inflow Research Articles

Increased hydropower but with an elevated risk of reservoir operations in India under the warming climate

Hydropower is a significant contributor to clean global electricity generation; therefore, it plays a crucial role in climate change mitigation. Notwithstanding major hydropower dams in India are in diverse climatic regions and exposed to risks because of the warming climate, potential changes in hydroclimate remain largely unexplored. Using observations and climate projections, we demonstrate the hydroclimatic changes in the upstream catchments and their implications for the hydropower generation of 46 major hydropower dams in India. A warmer (up to 5.0°C) and wetter projected climate with a substantial increase (5.0-33%) in precipitation will lead to an increased (7-70%) inflow to reservoirs of major dams. Increased inflow will enhance (9-36%) the hydropower production for most dams in the future, with a more prominent rise in central India dams. A simultaneous rise in extreme inflow and high reservoir storage conditions is projected under future climate for most dams. However, future climate changes project a favorable hydroclimate for hydropower production, with the associated risks related to extremes.

Experimental Investigation on the Breaching Process of Landslide Dams with Differing Materials under Different Inflow Conditions.

Landslide dams are dangerous because the outburst floods produced by dam failures seriously threaten life and property downstream. In this study, a series of physical flume tests were conducted to investigate the breaching process of landslide dams with fine-grained, well graded, and coarse-grained material under different inflow conditions. The effects of dam material and inflow discharge on the breach development, outflow discharge and erosion characteristics were studied. The erosion resistance of materials and lateral collapses were also discussed. Experimental results reveal that the whole breaching process is determined by the water-sediment interaction. For the fine-grained dams, a general constant downstream slope angle is maintained during the breaching process. For the well-graded dams, a step-pool structure is generated due to the scarp erosion. For the coarse-grained dams, they can remain stable under normal circumstances but fail by overtopping in a short duration under the extreme inflow condition. The final breach of the dam with higher fine content or larger inflow discharge is deeper and narrower. In addition, many fluctuations are observed in the changing curve of the erosion rates along the flow direction for the well-graded and coarse-grained dams. The erosion resistance of materials increases along the flow direction, which needs to be further considered in physically based breach models. Furthermore, the lateral collapse is affected by the dam material instead of inflow discharge. The lower fine content causes more lateral collapses with smaller volumes.

Preparation of Inundation Maps by Developing Unsteady Flow Model of Pamba River- Decision Support System for Disaster Management

Nowadays extreme rainfall events are more frequent across our country. Controlled releases from reservoirs would be necessitated to ensure the safety of dams and people downstream during extreme inflow. Mitigation measures are to be planned and preparedness actions to be taken effectively to reduce the impact of such catastrophe at downstream flood plains of reservoirs. Advance information on the extent of the area likely to be in undated consequent to releases from reservoirs is vital in developing an effective action plan for the timely evacuation of people. In the present study, it has been attempted to develop an unsteady flow model of the Pamba River in Kerala& the flood plain downstream for determining the area likely to be inundated corresponding to a largely controlled release in the tune of 900m3/s. The unsteady flow model is developed in HEC-RAS to study the flow behavior of flood wave caused by the flow hydrograph with peak 900m3/s as it propagates to the downstream plains. From the studies, it is found that the transformation of flow profile of flood wave can be better estimated using the HEC-RAS 2D flow model and the results can be used as important information in the decision-making process for Disaster Management Authorities.

What controls the extreme flow through the Kerama Gap: a global HYbrid Coordinate Ocean Model reanalysis point of view

The temporal variability of volume transport from the North Pacific Ocean to the East China Sea (ECS) through the Kerama Gap (between Okinawa Island and Miyakojima Island—a part of Ryukyu Island Arc) is investigated using a 20-year global HYbrid Coordinate Ocean Model reanalysis with the Navy Coupled Ocean Data Assimilation from 1993 to 2012. We study the causes of extreme flow events defined as the volume transport through the Kerama Gap exceeding one standard deviation (4.50 Sv) above (inflow, from the North Pacific into the ECS) or below (outflow, from the ECS to the North Pacific) the mean volume transport (1.95 Sv). In our results, the impinging mesoscale eddies from the Pacific Ocean are the most important factor in controlling the extreme flow occurrence, explaining 70% (68%) of the mean extreme inflow (outflow)-mean volume transport anomaly. Extreme inflow (outflow) through the Kerama Gap is associated with the impinging anticyclonic (cyclonic) eddies along the eastern side of Okinawa and/or cyclonic (anticyclonic) eddies to the southwest of the gap. The Kuroshio frontal meanders cause the mean Kuroshio axis to shift away from (toward) the Kerama Gap during the extreme inflow (outflow)-mean events and play a secondary role in determining the extreme flow occurrence, representing 15% (14%) of the mean extreme inflow (outflow)-mean volume transport anomaly. The interannual variability of the flow through the gap also plays a subsidiary role, explaining another 13% (14%) of the mean extreme inflow (outflow)-mean volume transport anomaly.

Improved Forecasting of Extreme Monthly Reservoir Inflow Using an Analogue-Based Forecasting Method: A Case Study of the Sirikit Dam in Thailand

Reservoir inflow forecasting is crucial for appropriate reservoir management, especially in the flood season. Forecasting for this season must be sufficiently accurate and timely to allow dam managers to release water gradually for flood control in downstream areas. Recently, several models and methodologies have been developed and applied for inflow forecasting, with good results. Nevertheless, most were reported to have weaknesses in capturing the peak flow, especially rare extreme flows. In this study, an analogue-based forecasting method, designated the variation analogue method (VAM), was developed to overcome this weakness. This method, the wavelet artificial neural network (WANN) model, and the weighted mean analogue method (WMAM) were used to forecast the monthly reservoir inflow of the Sirikit Dam, located in the Nan River Basin, one of the eight sub-basins of the Chao Phraya River Basin in Thailand. It is one of four major dams in the Chao Phraya Basin, with a maximum storage of 10.64 km3, which supplies water to 22 provinces in this basin, covering an irrigation area of 1,513,465 hectares. Due to the huge extreme monthly inflow in August, with inflow of more than 3 km3 in 1985 and 2011, monthly or longer lead time inflow forecasting is needed for proper water and flood control management of this dam. The results of forecasting indicate that the WANN model provided good forecasting for whole-year forecasting including both low-flow and high-flow patterns, while the WMAM model provided only satisfactory results. The VAM showed the best forecasting performance and captured the extreme inflow of the Sirikit Dam well. For the high-flow period (July–September), the WANN model provided only satisfactory results, while those of the WMAM were markedly poorer than for the whole year. The VAM showed the best capture of flow in this period, especially for extreme flow conditions that the WANN and WMAM models could not capture.

Multipurpose Rule Curves for Multipurpose Reservoir by Conditional Genetic Algorithm

Multipurpose rule curves are suitable guide lines to operate multipurpose reservoirs during a considered time period. This study applied the conditional genetic algorithm and reservoir simulation model by considering multipurpose objective functions. 25 years of historic inflow data (1992-2016) and 50 years of future inflow (2017-2066) under climate change scenario B2 data and land use change data for the Man Oon Reservoir in Thailand have been considered. The extreme inflow, the future inflow and the synthetic inflow of reservoirs were used in evaluating the obtained rule curves. The situations of water shortage water excess as well as stored water at the end of the wet season were shown. The results showed that the multipurpose rule curves with objective function of minimized average excess spill water, can mitigate flood situations more than the other rule curves, furthermore it is possible to have more stored water at the end of wet season than water stored when using the other rule curves.

Possibility of GPS precipitable water vapour for reservoir inflow forecasting

AbstractWe investigated the possibility of using GPS precipitable water vapour (GPS-PWV) for forecasting reservoir inflow. The correlations between monthly GPS-PWV and the inflow of two reservoirs were examined and the relationship tested, using a group method of data handling (GMDH) type neural network algorithm. The daily and monthly reservoir inflows were directly proportional to daily and monthly GPS-PWV trends. Peak reservoir inflow, however, shifted from the peak averages for GPS-PWV. A strong relationship between GPS-PWV and inflow was confirmed by high R2 values, high coefficients of correlation, and acceptable mean absolute errors (MAE) of both the daily and monthly models. The Ubon Ratana reservoir model had a monthly MAE of 54.19·106 m3 and a daily MAE of 5.40·106 m3. By comparison, the Lumpow reservoir model had a monthly MAE of 25.65·106 m3 and a daily MAE of 2.62·106 m3. The models using GPS-PWV as input data responded to extreme inflow better than traditional variables such that reservoir inflow could be predicted using GPS-PWV without using actual inflow and rainfall data. GPS-PWV, thus, represents a helpful tool for regional and national water management. Further research including more reservoirs is needed to confirm this preliminary finding.

Water Management in Tarbela Dam By using Bayesian Stochastic Dynamic Programming in Extreme Inflow Season

Existing method of forecasting inflows at Tarbela have some limitations, also system needs an adequate operating policy model to deal with highly volatile inflow of summer months of June, July, August and September. In this paper, historical data of inflows from 1986 to 2014 have been used to forecast upcoming inflows at dam. Bayesian predictive distribution is used to predict future inflows. These forecasted inflows were further incorporated into operating policy model to determine the optimal release during the prescribed months. Weather volatility is a major factor causing unstable inflows. High temperature during summer period cause high inflows at dam. Considering weather volatility, this policy model is proposed for the flood season (15th June to 30th September), in which inflows and outflows are higher than rest of the year. This model maximizes the expected profit from hydro power production, minimizes the expected loss from flood damage and updates the proper estimate of current stage of reservoir storage.

Impacts and effects of a historical high and ENSO linked freshwater inflow in the tropical estuary Nha Phu, southeast Vietnam

The frequency of strong and extreme precipitation events and high river discharges is foreseen to increase in southeast Asia in the future. The aims of the study were to evaluate physical and biological impacts of a historical (one-in-36 years) (1980–2015) extreme freshwater inflow in November 2010 into the small tropical Nha Phu estuary. The extreme event impacts were assessed relative to a dataset of samplings (n=11) through 1.5 year of phytoplankton biomass (chlorophyll a), nutrients, CTD, primary production and suspended particulate matter prior to the extreme event. Results show, that the event significantly decreased salinity, increased chl-a 2–3 times, suspended particulate matter and beam attenuation coefficients by 7 times, and the colored dissolved organic matter absorption coefficient by 3 times. The extreme inflow initiated a phytoplankton bloom fueled by catchment area derived nutrients. The bloom established a considerable carbon production and transport of carbon to the coastal and shelf areas. There was likewise a significant export of suspended particulate matter out of the estuary. Residence time decreased from 4.2 to 3.8 days during the extreme event, and pre-inflow water column conditions were re-established within 4–5 days, shown in a previous study. Annual precipitation in the area was linked to the Pacific ENSO-cycle (El Niño Southern Oscillation) with 2010 as a very strong La Niña year. The entire event accounted for 68% of annual precipitation, 39% of annual discharge, and 53% of annual sediment transport in the Dinh River, which discharges into the Nha Phu estuary.

Optimal scheduling of hydro-thermal power systems considering the flood risk of cascade reservoirs

ABSTRACTThe increased flood risk linked to global warming affects the safety of cascade reservoirs, with direct effects on the stable operation of power systems. In this article, an optimal scheduling framework for hydro-thermal power systems considering the flood risk of cascade reservoirs is presented. First, the extreme value theory-based peak over threshold model is adopted to build a generalized Pareto distribution of extreme flood inflow for a single reservoir. Then, the Copula function is used to build a joint probability distribution function of extreme inflow for cascade reservoirs during a flood period. Based on the superior performance of the conditional value at risk (CVaR) in characterizing the tail risk of the cascade reservoir spillway safety margin, a CVaR constraint for cascade reservoir flood prevention is proposed, and a scheduling model for hydro-thermal power systems considering the flood prevention risk of head-dependent cascade reservoirs is presented. Secondly, Rockafeller and Uryasey reformulation and sample average approximation are employed to transform the proposed model with a CVaR constraint into a convex solvable optimization model. Finally, a modified IEEE 14-node system is used to verify the better performance of the proposed model than that of the models with the chance constraint and with the independent normal distribution of extreme flood inflow. The impacts of flood prevention confidence level and Monte Carlo sample size on the optimal scheduling results are analysed quantitatively.

Heartmate II Inflow Cannula Position on Chest X-Ray Predicts Inotrope Dependence

<h3>Purpose</h3> We hypothesized that LVAD inflow cannula malposition contributes to numerous negative outcomes after device implantation. We further hypothesized that inflow cannula malposition may be determined by early post-operative CXR. <h3>Methods and Materials</h3> We reviewed CXR from 79 consecutive Heartmate II implantations (bridge to transplant or destination therapy). We measured LVAD inflow cannula position relative to the sternum on 1st upright PA and Lateral chest x-ray. [figure 1] We evaluated the following explanatory variables: age, LV diameter, redo sternotomy, BMI, cannula angle relative to sternum on PA CXR, cannula angle relative to sternum on Lateral CXR. We used multivariate logistic regression to evaluate the following implantation complications: TIA, stroke, post-implantation inotrope dependence, hemolysis, thrombosis, new ventricular tachycardia, and death. <h3>Results</h3> Cannula angle on PA CXR is associated with inotrope dependence (OR 1.12, p=0.001). All patients (n=16) with cannula angles outside of the range of 4° to 54° on PA CXR or -45° to 59° on lateral CXR experienced at least one of the studied complications. Additionally, increased BMI is associated with inotrope dependence (OR 1.14, p=0.045) and death (OR 1.13, p=0.049). Redo sternotomy is associated with new onset ventricular tachycardia (OR 5.97, p=0.03). LV diameter is associated with hemolysis (OR 0.27, p=0.043). <h3>Conclusions</h3> All patients with extreme inflow cannula angles experienced at least one major complication. Across all ranges, inflow cannula angulation on PA CXR predicted inotrope dependence. Secondary variables including BMI, redo sternotomy, and small LV were associated with major complications. Technical aspects of LVAD implantation remain paramount in assuring optimum outcomes.

Extreme inflow events and synoptic forcing in Sydney catchments

The Sydney catchment region encompasses over 16,000km2, supplying water to over 4 million inhabitants. However, few studies have investigated the synoptic and climatic influences on inflow in this region, which are crucial for understanding the vulnerability of water supply in a changing climate. This study identifies extremely high and low inflow events between 1960 and 2008 based on catchment averages. The focus of the study is an analysis of the synoptic cause/s of each extreme inflow event. The events are evaluated to identify any trends and also to determine the concurrent significant climatic influences on rainfall over the catchments. Relationships between catchment inflow, rainfall, tropical SST indices, and other influencing factors such as observed wind and temperatures are investigated. Our results show that East Coast Lows and anomalously easterly flow are the drivers of high inflow events, with low inflow events dominated by westerly wind patterns and the El Niño-Southern Oscillation.

Closure to “Numerical Oscillations in Pipe-Filling Bore Predictions by Shock-Capturing Models” by J. G. Vasconcelos, S. J. Wright, and P. L. Roe

The discussers would like to congratulate the authors for a very interesting paper, in which the gradual flow regime transition i.e., in the absence of a pipe-filling bore front —an outstanding issue in interface tracking models for flow regime transition—was addressed. The proposed model is in this regard an improvement on the family of models presented by Song et al. 1983 and Cardle and Song 1988 , among others. Based on the discussers’ experience in modeling tunnel systems, gradual-flow regime transitions should probably be more common than pipe-filling bores, even though some extreme inflow conditions, which could be very important for design purposes, may require pipe-filling bore modeling. The discussers would like to make a few comments and raise some issues: 1. Geysering events are not necessarily characterized solely by water jets, as investigated by Vasconcelos 2005 . Experiments and field evidence indicate that such phenomena could also be triggered by air pocket expulsion through water-filled vertical shafts in stormwater systems. Careful consideration of the air pocket entrapment is required when dealing with flow-regime transition events. 2. There is already available an alternative to model flowregime transition other than Preissmann slot or interface tracking-based models, namely the TPA model of Vasconcelos et al. 2006 . The TPA model overcomes the limitations of Preissmann-slot models of modeling subatmospheric flows, while retaining a single set of equations to simulate both free-surface and pressurized flows. Indeed as the authors mentioned, flow-regime transition models constructed with shock-capturing approaches may develop numerical oscillations in the vicinity of pipe-filling bores. However, Vasconcelos et al. 2006 propose an approach to attenuate such oscillations. More recently, Vasconcelos et al. 2009 address this issue, and other techniques to attenuate these numerical oscillations are proposed. 3. The discussers doubt that the proposed model would be able to handle a depressurization scenario in which pressurized flows are succeeded by free-surface flows. In such cases, the flow regime interface would be characterized by air intrusion

Discussion of “Case Study: Numerical Evaluation of Hydraulic Transients in a Combined Sewer Overflow Tunnel System” by M. Politano, A. J. Odgaard, and W. Klecan

The discussers would like to congratulate the authors for a very interesting paper, in which the gradual flow regime transition i.e., in the absence of a pipe-filling bore front —an outstanding issue in interface tracking models for flow regime transition—was addressed. The proposed model is in this regard an improvement on the family of models presented by Song et al. 1983 and Cardle and Song 1988 , among others. Based on the discussers’ experience in modeling tunnel systems, gradual-flow regime transitions should probably be more common than pipe-filling bores, even though some extreme inflow conditions, which could be very important for design purposes, may require pipe-filling bore modeling. The discussers would like to make a few comments and raise some issues: 1. Geysering events are not necessarily characterized solely by water jets, as investigated by Vasconcelos 2005 . Experiments and field evidence indicate that such phenomena could also be triggered by air pocket expulsion through water-filled vertical shafts in stormwater systems. Careful consideration of the air pocket entrapment is required when dealing with flow-regime transition events. 2. There is already available an alternative to model flowregime transition other than Preissmann slot or interface tracking-based models, namely the TPA model of Vasconcelos et al. 2006 . The TPA model overcomes the limitations of Preissmann-slot models of modeling subatmospheric flows, while retaining a single set of equations to simulate both free-surface and pressurized flows. Indeed as the authors mentioned, flow-regime transition models constructed with shock-capturing approaches may develop numerical oscillations in the vicinity of pipe-filling bores. However, Vasconcelos et al. 2006 propose an approach to attenuate such oscillations. More recently, Vasconcelos et al. 2009 address this issue, and other techniques to attenuate these numerical oscillations are proposed. 3. The discussers doubt that the proposed model would be able to handle a depressurization scenario in which pressurized flows are succeeded by free-surface flows. In such cases, the flow regime interface would be characterized by air intrusion

Ocena zawartości azotanów w wodach rzeki Dunajec

Human activities transform the natural environment and cause an extreme biogen inflow to surface waters. The current investigation was aimed to estimate the effect of agricultural and domestic human activities at lowland area of the Dunajec watershed on water quality, both from the spatial and time perspective, which was assessed considering their nitrates content. Water was sampled during the vegetation periods 2003 and 2004 in one of the Dunajec River section flows through the rural areas between Zgłobice, above the industrial center in Tarnów and below Czchów dam reservoirs. Nitrates contents in the Dunajec waters depended both on the date and point of sampling and ranged between 0.38 and 4.84 mg NO3-N dm-3. In 2004 nitrates content on average was higher than in 2003 by 27%. The highest nitrates concentration was stated below Brzozowianka draining rural areas with poor sewage system. High nitrates concentrations were found at Zgłobice, a point closing typical of the agricultural section. The least nitrates content was found at Janowice below Pale nianka draining extensive farming areas. Markedly larger variations of nitrates content were observed in 2003 than in 2004. The highest nitrate concentrations were registered after intensive rainfall and top-dressing with nitrogen, wheras their lowest levels were found in the summer because of intensive development of vegetation. Water enrichment with nitrates in the agricultural area was observed at spring dates, but the reverse relation was noticed at other dates. Nitrates content fulfilled recommended values for A1 category of consumption water but most samples may be considered as sensitive to eutrophication because of nitrates concentration.

A neuro-fuzzy model for inflow forecasting of the Nile river at Aswan high dam

River flow forecasting is an essential procedure that is necessary for proper reservoir operation. Accurate forecasting results in good control of water availability, refined operation of reservoirs and improved hydropower generation. Therefore, it becomes crucial to develop forecasting models for river inflow. Several approaches have been proposed over the past few years based on stochastic modeling or artificial intelligence (AI) techniques. In this article, an adaptive neuro-fuzzy inference system (ANFIS) model is proposed to forecast the inflow for the Nile River at Aswan High Dam (AHD) on monthly basis. A major advantage of the fuzzy system is its ability to deal with imprecision and vagueness in inflow database. The ANFIS model divides the input space into fuzzy sub-spaces and maps the output using a set of linear functions. A historical database of monthly inflows at AHD recorded over the past 130 years is used to train the ANFIS model and test its performance. The performance of the ANFIS model is compared to a recently developed artificial neural networks (ANN) model. The results show that the ANFIS model was capable of providing higher inflow forecasting accuracy specially at extreme inflow events compared with that of the ANN model. It is concluded that the ANFIS model can be quite beneficial in water management of Lake Nasser reservoir at AHD.

Congenital pulmonary sling, aorto–pulmonary window and pulmonary vein obstruction as a diagnostic and therapeutic challenge in an infant with VACTERL association

In a newborn with anal atresia and urethral valves an incomplete VACTERL association, was diagnosed and a colon anus praeter was placed. Sweating and heart murmur led to cardiac diagnostics. By 2D colour Doppler echocardiography a huge atrial septal defect and pulmonary venous stenoses were diagnosed. Additionally, a pulmonary sling combined with an aortopulmonary window (AoPAw) was suspected and later confirmed by angiography. Heart failure developed and closure of the AoPAw, transplantation of the left pulmonary artery and closure of the atrial septal defect was performed at the age of 4 weeks. But the patient did not improve. Pulmonary hypertension with suprasystemic pressure in the right ventricle originating from a stenosis of the new orifice of the left pulmonary artery and obstruction of the right pulmonary veins was diagnosed by echocardiography and confirmed by angiography. At the age of 8 months the orifice of the left pulmonary artery was enlarged by a patch, the obstruction of left sided pulmonary veins was opened, and the rightsided pulmonary veins were newly implanted into the left atrium. Finally the inter-atrial communication was closed by a patch plastic. Again, postoperatively the patient improved only slightly. At the age of 10 months trans-septal catheterisation was performed. Angiography revealed a successful balloon dilatation of the long obstruction of the right pulmonary veins. Later on, an absorbable magnesium stent was implanted into the right upper pulmonary vein and medication with an endothelin antagonist was started. Temporarily the patient improved significantly. Within 6 weeks, right ventricular systolic pressure was again suprasystemic due to extreme inflow obstruction of the right pulmonary vein diagnosed by echocardiography. Severe heart failure developed and at the age of 1 year the patient died.

General Characteristics of Solutions to the Open-Channel Flow, Feedforward Control Problem

A dimensionless formulation of the open-channel flow equations was used to study the feedforward control problem for single-pool canals. Feedforward inflow schedules were computed for specified downstream demands using a gate-stroking model. The analysis was conducted for various design and operational conditions. Differences in the shape of the computed inflow hydrographs are largely related to the volume change resulting from the transient, the time needed to supply this volume, and the time needed by the inflow perturbation to travel down the canal. The gate-stroking method will fail to produce a solution or the solution will demand extreme and unrealistic inflow variations if the time needed to supply the canal volume change is much greater than the travel time of the upstream flow change. As an alternative, a simple feedforward-control flow schedule can be developed based on this volume change and a reasonable delay estimate. This volume compensating schedule can deliver the requested flow change and keep water levels reasonably close to the target under the range of conditions tested.

The long-term salinity field in San Francisco Bay

Data are presented on long-term salinity behaviour in San Francisco Bay, California. A two-level, width averaged model of the tidally averaged salinity and circulation has been written in order to interpret the long-term (days to decades) salinity variability. The model has been used to simulate daily averaged salinity in the upper and lower levels of a 51 segment discretization of the Bay over the 22-yr period 1967–1988. Monthly averaged surface salinity from observations and monthly-averaged simulated salinity are in reasonable agreement. Good agreement is obtained from comparison with daily averaged salinity measured in the upper reaches of North Bay. The salinity variability is driven primarily by freshwater inflow with relatively minor oceanic influence. All stations exhibit a marked seasonal cycle in accordance with the Mediterranean climate, as well as a rich spectrum of variability due to extreme inflow events and extended periods of drought. Monthly averaged salinity intrusion positions have a pronounced seasonal variability and show an approximately linear response to the logarithm of monthly averaged Delta inflow. Although few observed data are available for studies of long-term salinity stratification, modelled stratification is found to be strongly dependent on freshwater inflow; the nature of that dependence varies throughout the Bay. Near the Golden Gate, stratification tends to increase up to very high inflows. In the central reaches of North Bay, modelled stratification maximizes as a function of inflow and further inflow reduces stratification. Near the head of North Bay, lowest summer inflows are associated with the greatest modelled stratification. Observations from the central reaches of North Bay show marked spring-neap variations in stratification and gravitational circulation, both being stronger at neap tides. This spring-neap variation is simulated by the model. A feature of the modelled stratification is a hysteresis in which, for a given spring-neap tidal range and fairly steady inflows, the stratification is higher progressing from neaps to springs than from springs to neaps. The simulated responses of the Bay to perturbations in coastal sea salinity and Delta inflow have been used to further delineate the time-scales of salinity variability. Simulations have been performed about low inflow, steady-state conditions for both salinity and Delta inflow perturbations. For salinity perturbations a small, sinusoidal salinity signal with a period of 1 yr has been applied at the coastal boundary as well as a pulse of salinity with a duration of one day. For Delta inflow perturbations a small, sinusoidally varying inflow signal with a period of 1 yr has been superimposed on an otherwise constant Delta inflow, as well as a pulse of inflow with a duration of one day. Perturbations in coastal salinity dissipate as they move through the Bay. Seasonal perturbations require about 40–45 days to propagate from the coastal ocean to the Delta and to the head of South Bay. The response times of the model to perturbations in freshwater inflow are faster than this in North Bay and comparable in South Bay. In North Bay, time-scales are consistent with advection due to lower level, up-estuary transport of coastal salinity perturbations; for inflow perturbations, faster response times arise from both upper level, down-estuary advection and much faster, down-estuary migration of isohalines in response to inflow volume continuity. In South Bay, the dominant time-scales are governed by tidal dispersion.

Seston transport and deposition in Pelorus sound, south Island, New Zealand

Abstract Transport of seston (suspended sediment) in Pelorus Sound is controlled by tides and freshwater inflow. During high freshwater inflow, a moderately stratified estuarine circulation may be superimposed on the tidal circulation, but the latter dominates and transports seston seawards and landwards with the ebb and flood phases respectively. With extreme freshwater inflow, the estuarine circulation gains impetus and most seston is rapidly transported seaward in the low saline surface layer. Irrespective of circulation there is a persistent trend in seston concentrations. Highest values occur at the sound's head because of the influence of nearby Pelorus and Kaituna Rivers and because of resuspension of bottom sediment by strong tidal currents. Seston concentrations wane along the sound until near the entrance, where values increase as a result of greater production of biogenic seston and because additional seston is brought in from Cook Strait with the flood tide. This trend parallels variability in...