SPM response to tide and river flow in the hyper-turbid Ems River

In this paper, we analyse the behaviour of fine sediments in the hyper-turbid Lower Ems River, with focus on the river’s upper reaches, a stretch of about 25 km up-estuary of Terborg. Our analysis is based on long records of suspended particulate matter (SPM) from optical backscatter (OBS) measurements close to the bed at seven stations along the river, records of salinity and water level measurements at these stations, acoustic measurements on the vertical mud structure just up-estuary of Terborg and oxygen profiles in the lower 3 m of the water column close to Leerort and Terborg. Further, we use cross-sectionally averaged velocities computed with a calibrated numerical model. Distinction is made between four timescales, i.e. the semi-diurnal tidal timescale, the spring–neap tidal timescale, a timescale around an isolated peak in river flow (i.e. about 3 weeks) and a seasonal timescale. The data suggest that a pool of fluid/soft mud is present in these upper reaches, from up-estuary of Papenburg to a bit down-estuary of Terborg. Between Terborg and Gandersum, SPM values drop rapidly but remain high at a few gram per litre. The pool of fluid/soft mud is entrained/mobilized at the onset of flood, yielding SPM values of many tens gram per litre. This suspension is transported up-estuary with the flood. Around high water slack, part of the suspension settles, being remixed during ebb, while migrating down-estuary, but likely not much further than Terborg. Around low water slack, a large fraction of the sediment settles, reforming the pool of fluid mud. The rapid entrainment from the fluid mud layer after low water slack is only possible when the peak flood velocity exceeds a critical value of around 1 m/s, i.e. when the stratified water column seems to become internally supercritical. If the peak flood velocity does not reach this critical value, f.i. during neap tide, fluid mud is not entrained up to the OBS sensors. Thus, it is not classical tidal asymmetry, but the peak flood velocity itself which governs the hyper-turbid state in the Lower Ems River. The crucial role of river flow and river floods is in reducing these peak flood velocities. During elongated periods of high river flow, in e.g. wintertime, SPM concentrations reduce, and the soft mud deposits consolidate and possibly become locally armoured as well by sand washed in from the river. We have no observations that sediments are washed out of the hyper-turbid zone. Down-estuary of Terborg, where SPM values do not reach hyper-turbid conditions, the SPM dynamics are governed by classical tidal asymmetry and estuarine circulation. Hence, nowhere in the river, sediments are flushed from the upper reaches of the river into the Ems-Dollard estuary during high river flow events. However, exchange of sediment between river and estuary should occur because of tide-induced dispersion.     

Geomorphological effects of catastrophic flooding on a small subtropical Estuary

The Mgeni Estuary is situated on the east coast of South Africa. Tidal salinity changes commonly extend 2.5 km upstream from the mouth. The subtropical climatic regime causes seasonal flooding, the geomorphological effects of which are increased by a steep river gradient. A 120-150 year flood event during September 1987, with an estimated peak discharge of 10800 cumecs, resulted in the erosion of 1.86 × 10⁶ m³ of sediment from the estuary. The normal high tide volume of the estuary (0.35 × 10⁶ m³) increased to 1.85 × 10⁶ m³ after the flood. The post-flood intertidal volume (tidal prism) of the estuary was 0.43 × 10⁶ m³ compared to 0.19 × 10⁶ m³ before the event. The nature and extent of erosion in the estuary was controlled by the composition of the estuary banks and bed material. Gravel and sand substrates were preferentially eroded from the estuary bed before the cohesive bank materials were undercut in sections of the estuary. Mud and fine sand were deposited on overbank areas when the flood waned and a lag of coarse sand and gravel was produced on the estuary bed. Approximately 46 per cent of the bedload sediment supplied to the estuary since 1917 was retained in the estuary until the 1987 flood. The remainder was transported through the system and into the Indian Ocean. Results show that catastrophic floods play an important role in sedimentation in small estuaries.     

Tidal straining effect on the suspended sediment transport in the Huanghe (Yellow River) Estuary, China

Tidal straining effect on sediment transport dynamics in the Huanghe (Yellow River) estuary was studied by field observations and numerical simulations. The measurement of salinity, suspended sediment concentration, and current velocity was conducted during a flood season in 1995 at the Huanghe river mouth with six fishing boats moored at six stations for 25-h hourly time series observations. Based on the measurements, the intra-tidal variations of sediment transport in the highly turbid river mouth was observed and the tidal straining effect occurred. Our study showed that tidal straining of longitudinal sediment concentration gradients can contribute to intra-tidal variability in sediment stratification and to asymmetries in sediment distribution within a tidal cycle. In particular, the tidal straining effect in the Huanghe River estuary strengthened the sediment-induced stratification at the flood tide, thus producing a higher bottom sediment concentration than that during the ebb. A sediment transport model that is capable of simulating sediment-induced stratification effect on the hydrodynamics in the bottom boundary layers and associated density currents was applied to an idealized estuary to demonstrate the processes and to discuss the mechanism. The model-predicted sediment processes resembled the observed characteristics in the Huanghe River estuary. We concluded that tidal straining effect is an important but poorly understood mechanism in the transport dynamics of cohesive sediments in turbid estuaries and coastal seas.     

Numerical modelling of morphodynamics—Vilaine Estuary

The main objective of this paper is to develop a method to simulate long-term morphodynamics of estuaries dominated by fine sediments, which are subject to both tidal flow and meteorologically induced variations in freshwater run-off and wave conditions. The method is tested on the Vilaine Estuary located in South Brittany, France. The estuary is subject to a meso–macrotidal regime. The semi-diurnal tidal range varies from around 2.5 to 5 m at neap and spring, respectively. The freshwater input is controlled by a dam located approximately 8 km from the mouth of the estuary. Sediments are characterised as mostly fines, but more sandy areas are also found. The morphology of the estuary is highly influenced by the dam. It is very dynamic and changes in a complicated manner with the run-off from the dam, the tide and the wave forcing at the mouth of the estuary. Extensive hydrodynamic and sediment field data have been collected in the past and provide a solid scientific basis for studying the estuary. Based on a conceptual understanding of the morphodynamics, a numerical morphological model with coupled hydrodynamic, surface wave and sediment transport models is formulated. The numerical models are calibrated to reproduce sediment concentrations, tidal flat altimetry and overall sediment fluxes. Scaling factors are applied to a reference year to form quasi-realistic hydrodynamic forcing and river run-off, which allow for the simulations to be extended to other years. The simulation results are compared with observed bathymetric changes in the estuary during the period 1998–2005. The models and scaling factors are applied to predict the morphological development over a time scale of up to 10 years. The influence of the initial conditions and the sequence of external hydrodynamic forcing, with respect to the morphodynamic response of the estuary, are discussed.     

Morphologic evolution of bifurcated reaches in a macrotidal estuary with mountain streams

A macrotidal estuary with mountain streams(MEMS) is characterized by rapidly rising and falling flood peaks and large tidal ranges and is a typical estuary type with strong flow dynamics that is found worldwide. Understanding the morphodynamic evolution of MEMSs has great significance for river management. The roles of the mountain stream river flood process and macrotides on the morphology evolution still needs further quantitative study. Taking the Oujiang Estuary as an example, the evolution ...     

Morphological controls in sandy estuaries: the influence of tidal flats and bathymetry on sediment transport

The morphodynamics of shallow, vertically well-mixed estuaries, characterised by tidal flats and deeper channels, have been investigated. This paper examines what contributes to flood/ebb-dominant sediment transport in localised regions through a 2D model study (using the TELEMAC modelling system). The Dyfi Estuary in Wales, UK has been used as a case study and, together with idealised estuary shapes, shows that shallow water depths lead to flood dominance in the inner estuary whilst tidal flats and deep channels cause ebb dominance in the outer estuary. For medium sands and with an artificially ‘flattened’ bathymetry (i.e. no tidal flats), the net sediment transport switches from ebb-dominant to flood-dominant where the parameter a/h (local tidal amplitude ÷ local tidally averaged water depth) exceeds 1.2. Sea level rise will reduce this critical value of a/h and also reduce the ebb-directed sediment transport significantly, leading to a flood-dominated estuarine system. A similar pattern, albeit with greater transport, was simulated with tidal flats included and also with a reduced grain size. This suggests that analogous classifications for flood/ebb asymmetry of the tide in estuaries as a whole may not represent the local sediment transport in sufficient detail. Through the Dyfi simulations, the above criterion involving a/h is shown to be complicated further by augmented flow past a spit at the estuary mouth which gives rise to a self-maintaining scour hole. Simulations of one year of bed evolution in an idealised flat-bottomed estuary, including tidal flow past a spit, recreate the flood/ebb dominance on either side of the spit and the formation of a scour hole in between. The erosion rate at the centre of the hole is reduced as the hole deepens, suggesting the establishment of a self-maintaining equilibrium state.     

Lateral dynamics and associated transport of sediment in the upper reaches of a partially mixed estuary,Chesapeake Bay,USA

Data from time series of transects made over a tidal period across a section of the upper Chesapeake Bay, USA, reveal the influence of lateral dynamics on sediment transport in an area with a deep channel and broad extents of shallower flanks. Contributions to lateral momentum by rotation (Coriolis plus channel curvature), cross channel density gradients and cross channel surface slope were estimated, and the friction and acceleration terms needed to complete the balance were compared to patterns of observed lateral circulation. During ebb, net rotation effects were larger because of river velocity and reinforcement of Coriolis by curvature. During flood, stratification was greater because of landward advection of strong vertical density gradients. Together, the ebb intensified lateral circulation and flood intensified stratification focused sediment and sediment transport along the left side of the estuary (looking seaward). The tendency for greater stratification on flood and net sediment flux toward the left-hand shoal are contrary to more common models which, in the northern hemisphere, predict greater resuspension on flood and move sediment toward the right-hand shoal. These tidal asymmetries interact with the lateral circulation to focus net sediment flux on the left side of the estuary, and to produce net ebb directed sediment transport at the surface of the same order of magnitude as net flood directed sediment transport at the bottom.     

Contemporary hydrodynamics and morphological change of a microtidal estuary: a numerical modelling study

Contemporary hydrodynamics and morphological change are examined in a shallow microtidal estuary, located on a wave-dominated coast (Port Stephens, NSW, Australia). Process-based numerical modelling is undertaken by combining modules for hydrodynamics, waves, sediment transport and bathymetry updates. Model results suggest that the complex estuarine bathymetry and geometry give rise to spatial variations in the tidal currents and a marked asymmetry between ebb and flood flows. Sediment transport paths correspond with tidal asymmetry patterns. The SE storms significantly enhance the quantities of sediment transport, while locally generated waves by the westerly strong winds also are capable of causing sediment entrainment and contribute to the delta morphological change. The wave/wind-induced currents are not uniform with flow over shoals driven in the same direction as waves/winds while a reverse flow occurring in the adjacent channel. The conceptual sediment transport model developed in this study shows flood-directed transport occurs on the flood ramp while ebb-directed net transport occurs in the tidal channels and at the estuary entrance. Accretion of the intertidal sand shoals and deepening of tidal channels, as revealed by the model, suggest that sediment-infilling becomes advanced, which may lead to an ebb-dominated estuary. It is likely that a switch from flood- to ebb-dominance occurs during the estuary evolution, and the present-day estuary acts as a sediment source rather than sediment sink to the coastal system. This is conflictive to the expectation drawn from the estuarine morphology; however, it is consistent with previous research suggesting that, in an infilling estuary, an increase in build-up of intertidal flats/shoals can eventually shift an estuary towards ebb dominance. Thus, field data are needed to validate the result presented here, and further study is required to investigate a variety of estuaries in the Australian area.     

The effect of asymmetric dune roughness on tidal asymmetry in the Weser estuary

The bed of estuaries is often characterized by ripples and dunes of varying size. Whereas smaller bedforms adapt their morphological shape to the oscillating tidal currents, large compound dunes (here: asymmetric tidal dunes) remain stable for periods longer than a tidal cycle. Bedforms constitute a form roughness, that is, hydraulic flow resistance, which has a large-scale effect on tidal asymmetry and, hence, on hydrodynamics, sediment transport, and morphodynamics of estuaries and coastal seas. Flow separation behind the dune crest and recirculation on the steep downstream side result in turbulence and energy loss. Since the energy dissipation can be related to the dune lee slope angle, asymmetric dune shapes induce variable flow resistance during ebb and flood phases. Here, a noncalibrated numerical model has been applied to analyze the large-scale effect of symmetric and asymmetric dune shapes on estuarine tidal asymmetry evaluated by residual bed load sediment transport at the Weser estuary, Germany. Scenario simulations were performed with parameterized bed roughness of symmetric and asymmetric dune shapes and without dune roughness. The spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics, that is, river discharge and tidal energy, is shown to be complex but substantial. The contrasting effects of flood- and ebb-oriented asymmetric dunes on residual bed load transport rates and directions are estimated to be of a similar importance as the controls of seasonal changes of discharge on these net sediment fluxes at the Lower Weser estuary. This corroborates the need to consider dune-induced directional bed roughness in numerical models of estuarine and tidal environments.     

Suspended sediment fluxes at an intertidal flat: The shifting influence of wave,wind, tidal,and freshwater forcing

Using in situ, continuous, high frequency (8–16 Hz) measurements of velocity, suspended sediment concentration (SSC), and salinity, we investigate the factors affecting near-bed sediment flux during and after a meteorological event (cold front) on an intertidal flat in central San Francisco Bay. Hydrodynamic forcing occurs over many frequency bands including wind wave, ocean swell, seiching (500–1000 s), tidal, and infra-tidal frequencies, and varies greatly over the time scale of hours and days. Sediment fluxes occur primarily due to variations in flow and SSC at three different scales: residual (tidally averaged), tidal, and seiching. During the meteorological event, sediment fluxes are dominated by increases in tidally averaged SSC and flow. Runoff and wind-induced circulation contribute to an order of magnitude increase in tidally averaged offshore flow, while waves and seiching motions from wind forcing cause an order of magnitude increase in tidally averaged SSC. Sediment fluxes during calm periods are dominated by asymmetries in SSC over a tidal cycle. Freshwater forcing produces sharp salinity fronts which trap sediment and sweep by the sensors over short (∼30 min) time scales, and occur primarily during the flood. The resulting flood dominance in SSC is magnified or reversed by variations in wind forcing between the flood and ebb. Long-term records show that more than half of wind events (sustained speeds of greater than 5 m/s) occur for 3 h or less, suggesting that asymmetric wind forcing over a tidal cycle commonly occurs. Seiching associated with wind and its variation produces onshore sediment transport. Overall, the changing hydrodynamic and meteorological forcing influence sediment flux at both short (minutes) and long (days) time scales.     

Small river basin and estuarine sediment fluxes: The magnitude necessary for coastal dispersion and siltation effects on a coral reef

Increasing continental suspended sediment influx to coral reefs is an example of land-sea coupling that requires the identification of sources, magnitude of transport, and controlling processes. In Brazil, a small coastal basin (Macaé River) was identified as a source of suspended sediment to a coral reef on the coast of Cape Armação dos Búzios. Biannual suspended sediment loads were measured at the basin as were fluxes within the estuary and towards the coast during eight tidal cycles. Particle load and yield from this basin were typical of small coastal basins, showing high to moderate slopes and transitional land management. However, the magnitude of the river loads was lower than the sediment transport within the estuary, indicating that the estuary amplifies river fluxes and sustains the transference of suspended sediment alongshore to the coral reef. Nonetheless, the estuary displays both suspended particle retention and export capacity and, therefore, fluxes to the coast and the coral reef occur as episodic events.     

Longitudinal variation in lateral trapping of fine sediment in tidal estuaries: observations and a 3D exploratory model

This study investigates the longitudinal variation of lateral entrapment of suspended sediment, as is observed in some tidal estuaries. In particular, field data from the Yangtze Estuary are analysed, which reveal that in one cross-section, two maxima of suspended sediment concentration (SSC) occur close to the south and north sides, while in a cross-section 2 km down-estuary, only one SSC maximum on the south side is present. This pattern is found during both spring tide and neap tide, which are characterised by different intensities of turbulence. To understand longitudinal variation in lateral trapping of sediment, results of a new three-dimensional exploratory model are analysed. The hydrodynamic part contains residual flow due to fresh water input, density gradients and Coriolis force and due to channel curvature-induced leakage. Moreover, the model includes a spatially varying eddy viscosity that accounts for variation of intensity of turbulence over the spring-neap cycle. By imposing morphodynamic equilibrium, the two-dimensional distribution of sediment in the domain is obtained analytically by a novel procedure. Results reveal that the occurrence of the SSC maxima near the south side of both cross-sections is due to sediment entrapment by lateral density gradients, while the second SSC maximum near the north side of the first cross-section is by sediment transport due to curvature-induced leakage. Coriolis deflection of longitudinal flow also contributes the trapping of sediment near the north side. This mechanism is important in the upper estuary, where the flow due to lateral density gradients is weak.     

Analysis of dry/wet conditions using the standardized precipitation index and its potential usefulness for drought/flood monitoring in Hunan Province, China

Local dry/wet conditions and extreme rainfall events are of great concern in regional water resource and disaster risk management. Extensive studies have been carried out to investigate the change of dry/wet conditions and the adaptive responses to extreme rainfall events within the context of climate change. However, applicable tools and their usefulness are still not sufficiently studied, and in Hunan Province, a major grain-producing area in China that has been frequently hit by flood and drought, relevant research is even more limited. This paper investigates the spatiotemporal variation of dry/wet conditions and their annual/seasonal trends in Hunan with the standardized precipitation index (SPI) at various time scales. Furthermore, to verify the potential usefulness of SPI for drought/flood monitoring, the correlation between river discharge and SPI at multiple time scales was examined, and the relation between extreme SPI and the occurrence of historical drought/flood events is explored. The results indicate that the upper reaches of the major rivers in Hunan Province have experienced more dry years than the middle and lower reaches over the past 57 years, and the region shows a trend of becoming drier in the spring and autumn seasons and wetter in the summer and winter seasons. We also found a strong correlation between river discharge and SPI series, with the maximum correlation coefficient occurred at the time scale of 2 months. SPI at different time scales may vary in its usefulness in drought/flood monitoring, and this highlights the need for a comprehensive consideration of various time scales when SPI is employed to monitor droughts and floods.     

Channel morphology and sediment movement in a tidal river,pitt river,British Columbia

The Pitt River is a meandering river channel linking the Fraser River estuary and Pitt Lake. The lake acts as a temporary reservoir for tidally diverted Fraser River flow. Stage level can fluctuate 2 m in Pitt River and as much as 1.2 m in Pitt Lake on a tidal cycle. Stage data from three locations in the system, used in conjunction with velocity measurements (profiles and tethered meter), revealed large tidal and seasonal variations in discharge. Calculations indicate that during the flood, basal shear stress peaks earlier in the cycle and reaches higher values than during the ebb. Thus, sediment moves farther forward on a flood flow than it moves back on the succeeding ebb. An upstream movement of sediment in Pitt River from the Fraser River is indicated by: (1) the identical mineralogy of the two rivers, (2) a decrease in median grain size from the Fraser to Pitt Lake, and (3) a predominance of flood-oriented bedforms in the river channel. A delta, 12 km² area, has accumulated at the lower (draining) end of the lake. Studies of the river channel using hydrographic charts revealed regular meanders (λM = 6100 m) and evenly spaced riffles and pools which appear to be scaled to the strongest flow, winter flood current (2400 m³/s). The winter flood is thus considered to be the effective discharge. Meander point bars are accreting on the ‘upstream’ side indicating deposition by the flood-oriented flow. The three dimensional geometry of the large-scale bedforms which cover the sandy thalweg of both river and delta channel was determined by echo sounding and side-scan sonar. Three distinct sizes (height/spacing = 0.8 m/10–15 m; 1.5 m/25–30m; 3m/50–60 m) of large-scale bedforms (sand waves) were found; their linear relationship of height vs. spacing on a log-log plot suggests a common genesis. Their occurrence by size does not appear to be related to depth of flow but rather to their position in the channel with respect to large scale features which alter flow.     

The anomalous tides near Broad Sound

Observations of tidal current and height, in conjunction with theoretical mathematical models are used to investigate the propagation of the tide near Broad Sound, a narrowing estuary situated on a wide section of continental shelf toward the southern end of the Great Barrier Reef. The observations indicate that the dense offshore reefs severely inhibit tidal flow, with the result that tides flood toward Broad Sound from the north and from the south, along the main lagoon. There is a local magnification of the semi-diurnal tides within Broad Sound itself.Models of flow across reefs confirm the effectiveness of dense, shallow, and broad reefs in acting as a barrier to the tide. The diffraction of tides through large gaps in the reef is modelled using conformal mapping techniques and with the inclusion of energy leakage, the diffraction model predicts magnification of the semi-diurnal tidal heights by a factor of about 4 and a phase lag of 3 h on the shelf near Broad Sound, these values being consistent with observation. The observed convergence of the tide close to, and within Broad Sound itself is consistent with the proximity of the semi-diurnal tidal period to the natural period for flow in Broad Sound, considered as a narrowing estuary. This results in further amplification, by an additional factor of about 1.5, so that the tides in Broad Sound are increased by a factor of between 5 and 6, altogether, compared with those elsewhere on the east Australian coast.     

Insights into bedload transport processes of a large regulated gravel‐bed river

A comprehensive monitoring programme focusing on bedload transport behaviour was conducted at a large gravel‐bed river. Innovative monitoring strategies were developed during five years of preconstruction observations accompanying a restoration project. A bedload basket sampler was used to perform 55 cross‐sectional measurements, which cover the entire water discharge spectrum from a 200‐year flood event in 2013 to a rare low flow event. The monitoring activities provide essential knowledge regarding bedload transport processes in large rivers. We have identified the initiation of motion under low flow conditions and a decrease in the rate of bedload discharge with increasing water discharge around bankfull conditions. Bedload flux strongly increases again during high flood events when the entire inundation area is flooded. No bedload hysteresis was observed. The effective discharge for bedload transport was determined to be near mean flow conditions, which is therefore at a lower flow discharge than expected. A numerical sediment transport model was able to reproduce the measured sediment transport patterns. The unique dataset enables the characterisation of bedload transport patterns in a large and regulated gravel‐bed river, evaluation of modern river engineering measures on the Danube, and, as a pilot project has recently been under construction, is able to address ongoing river bed incision, unsatisfactory ecological conditions for the adjacent national park and insufficient water depths for inland navigation. Copyright © 2017 John Wiley & Sons, Ltd.     

Effect of ENSO events on sediment production in a large coastal basin in northern Peru

Although the importance of ENSO on hydrological anomalies has been recognized, variations in sediment fluxes caused by these extreme events are poorly documented. The effect of ENSO is not limited to changes in sediment mobilization. Since ENSO events can affect terrestrial ecosystems, they may have important effects on sediment production and transport in river basins over time spans that are longer than the duration of the event itself. The Catamayo‐Chira basin is an interesting casestudy for investigating these geomorphic implications. The objectives were: (i) to study the effect of ENSO on stream flow and sediment yields in the basin, (ii) to investigate if ENSO events affect sediment yields in the post‐ENSO period and (iii) to understand which factors control the ENSO and post‐ENSO basin response. During strong negative ENSO periods, mean annual stream flow discharge at the inlet of the Poechos reservoir in the lower basin was 5.4 times higher than normal annual discharges, while average sediment fluxes exceeded those of normal years by a factor of about 11. In two heavily affected periods, 45.9% of the total sediment yield in the 29 years observation period was generated. Sediment fluxes in the post‐ENSO period are lower than expected, which proves post‐ENSO event dynamics are significantly different from pre‐event dynamics. Our analysis indicates the increase of vegetation growth in the lower basin is not the main reason explaining considerable sediment flux decrease in post‐ENSO periods. During strong ENSO events, sediment in alluvial stores in the lower part of the basin is removed due to enlarging and deepening of channels. In post‐ENSO periods, normal discharges and persisting sediment supplies from the middle/upper basin lead to river aggradation and storage of large amounts of sediment in alluvial plains. The decrease in sediment export will last for several years until the equilibrium is re‐established. Copyright © 2011 John Wiley & Sons, Ltd.     

Sediment transport in Swiss torrents

Sediment loads have been measured in six Swiss mountain torrents over several decades. Most of these torrent catchments are situated in the prealpine belt. They have catchment areas of between 0·5 and 1·7 km². Bedslopes at the measuring sites vary between 5 and 17 per cent, and peak discharges up to 12 m³ s⁻¹ have been recorded. Geophone sensors installed in the Erlenbach stream allow bedload transport activity to be monitored and sediment volumes associated with each flood event to be determined. A detailed analysis of the measurements in this stream results in an empirical equation in which the sediment load per flood event is expressed as a function of the effective runoff volume (discharges above the threshold for bedload motion) and of the normalized peak discharge. For the total of 143 investigated flood events in the Erlenbach stream, the deviation of the predicted from the measured value is within a factor of two for more than two-thirds of all events. A distinction can be made between summer and winter events in analysing the bedload transport events. The summer events, mainly caused by thunderstorms, transport comparatively larger sediment loads than the winter events. For the other investigated streams, the periods of the deposited sediment volume surveys cover in general several flood events. An analysis is performed analogous to that for the Erlenbach stream. The sediment loads show a similar dependency on the two factors effective runoff volume and normalized peak discharge. However, the exponents of these factors in the power law expressions differ from stream to stream. A comparison of the investigated stream shows that some of the variation can be explained by considering the bedslope above the measuring site. The inclusion of a bedslope factor is in agreement with laboratory investigations on bedload transport. © 1997 John Wiley & Sons, Ltd.     

An integrated approach of flash flood analysis in ungauged Mediterranean watersheds using post-flood surveys and unmanned aerial vehicles

This study analyzes the flash flood event of two ungauged ephemeral streams in Olympiada region (Chalkidiki, North Greece), which occurred at the 21–22 of November 2019. Aim of the study is to reconstruct the specific flash flood event, investigate the causes of flood generation mechanisms, evaluate the performance of SCS-CN hydrological and HEC-RAS hydraulic models, investigate the relation between extreme flash floods and human intervention, using the combination of ground and aerial observations obtained from the field survey and unmanned aerial vehicles (UAVs), respectively. The results of the specific discharge ranged between 9 and 11 m³ s⁻¹ km², values that are typical for flash flood events in Mediterranean region. The comparison between the observed and simulated values of flood extent showed sufficiently good performance of the hydraulic model (CSI = 82%). However, the statistical analysis of the observed and simulated flood depths displayed a flood depth overestimation by the applied model, despite that the values of the used statistic indexes are acceptable (RMSE = 0.35 m, SD = 0.53, NSE = 0.56, PBIAS = 11.26%). The model overestimation of flood depth was attributed to the DEM low resolution and quality. Ground and aerial observations depicted the alluvial fan activation, the alternation of flow paths and the huge sediment transport. Human intervention in main streams, urban sprawl, wet AMC and sediment transport were among the main factors that contributed to the flash flood generation. This integrated approach revealed the necessity of the constant evaluation and validation of hydrological and hydraulic models in small ungauged Mediterranean watersheds and ephemeral streams. The use of UAVs in combination with ground observations and hydraulic simulation could significantly contribute to the enhanced understanding of flash flood mechanisms, in the direction of flood risk mitigation, improvement of the planning efficiency of flood prevent measures, flood hazard estimation, evolution of flood warning systems and floodplain geomorphology analysis.     

SHRINKAGE OF THE ESTUARINE CHANNELS OF THE HAIHE DRAINAGE SYSTEM AND ITS INFLUENCES ON FLOOD HAZARD

1INTRODUCTIONTheHaiheRiverBasinislocatedinNorthChinawithareaof262.6km2.Itisaquicklydevelopedareawithmanyimportantcitiesandindustrialhubs,includingBeding,Tianjin,Tangshan,Cangzhou,DezhouandHuanghua.Theareawatchedfastprogressesinurbanizationinthepastdecades,andhumanactivitieshaveresultedingreatinfluencesontheenvironment,riverhydrologyandsedimentbudget.Theareaisprojectedtobemoreprosperouswithmoreoilandgasfields,chemicalindustrybases,anddenserrailwaysandexpresshighwaysinthenextcent'Ury.T…