Overbank Flow Research Articles

Flooding dynamics on the lower Amazon floodplain: 1. Hydraulic controls on water elevation, inundation extent, and river-floodplain discharge

[1] Modeling the routing of flood waters across large floodplains is challenging because flows respond to dynamic hydraulic controls from complex geomorphology, vegetation, and multiple water sources. In this study, we analyzed the topographic and hydrologic controls of inundation dynamics of a large floodplain unit (2440 km2) along the lower Amazon River. We combined land topography derived from the Shuttle Radar Topography Mission (SRTM) with underwater topography derived from an extensive echo-sounding survey to generate a seamless digital elevation model (DEM). Floodplain inundation was simulated using LISFLOOD-FP, which combines one-dimensional river routing with two-dimensional overland flow, and a local hydrological model. For the first time, accurate simulation of filling and drainage of an Amazon floodplain was achieved with quantification of changes in water elevation, flooding extent, and river-floodplain exchange. We examined the role of diffuse overbank versus channelized flows on river-floodplain exchange. Diffuse overbank flows represent 93% of total river to floodplain discharge and 54% of floodplain to river discharge. Floodplain discharge during high-water was four times higher than field observation values when the SRTM v.4 DEM with no correction was used for simulation because of a −4.4 m elevation bias originating from residual motion errors of the SRTM interferometric baseline.

Thinking outside the channel: Timing pulse flows to benefit salmon via indirect pathways

Using models to represent relationships between flow and fishes has important practical applications for managing reservoir releases. Attempts to model such relationships often neglect indirect mechanisms by which flow influences fish. For example, growth of salmon juveniles is measurably faster when flows inundate floodplain and promote higher production of invertebrate prey, but out-of-channel flows have not yet been incorporated into models. The QUANTUS model developed here represents indirect linkages between flow and freshwater survival, mediated by temperature and prey availability, for fall Chinook salmon (Oncorhynchus tshawytscha). Quantiles of spawning time and place were used to define cohorts of salmon in a regulated Central Valley, California river. Survival of these quantile-cohorts was simulated through incubation, juvenile growth, and eventual downstream migration. A genetic algorithm was used to optimize the seasonal timing of pulse flows. Simulated survival was highest for flow regimes that provided a modest, temperature-moderating pulse flow in early summer and, for wetter years, a second, larger pulse of over-bank flow in late winter. For many rivers of the Pacific coast that support fall Chinook salmon, the thermal window of opportunity for spawning and rearing is narrow. Optimized flows made the most of this window by providing access to accelerated juvenile growth and early survival in floodplain habitat, a result that should be verified with field experiments. Timing of optimized pulse flows differed in some respects from the region's natural hydrograph, dominated by spring runoff. This suggests that understanding the mechanisms by which flow influences fishes can be important when shaping flows in the changed context of a regulated river.

Storage-based approaches to build floodplain inundation modelling capability in river system models for water resources planning and accounting

Summary Two conceptual storage-based approaches have been developed for incorporating floodplain inundation modelling capability in two water resources planning models. Approach one is a simple method suitable for data limited environment, in which, flow in a river reach within a floodplain is partitioned into two components, in-stream and overbank flow, based on the in-stream capacity. A flood volume–area relationship derived from the flood inundation time series, which is generated by analysing daily MODIS imagery, is used to estimate flooded area for the overbank flow. The losses due to evaporation and groundwater seepage from floodplain are calculated using the estimated flooded area. This approach was implemented in the floodplain area of the Murray–Darling Basin in Australia. The simulated flow was compared with gauged data in 216 stations. The model has produced daily time series of floodplain stores and fluxes. The mass-balance analysis shows that the long term mass balance error was negligible for all floodplain reaches. Approach two is more comprehensive and suitable for areas with high resolution topography data. LiDAR data is used to divide a floodplain into multiple storages based on pre-defined threshold of floodplain inundation heights. The storage characteristics including disconnected storage volume (or, dead storage) and hydraulic connectivity between floodplain storages and a river reach are derived from the LiDAR data using a spatial data processing technique. This information is used to estimate floodplain inundation area for overbank flow. Approach 2 successfully simulated inundation depths, duration and extents in two reaches of Murrumbidgee floodplain. The performance of Approach 2 is highly satisfactory in terms of flood extent, depth and duration at a very high spatial resolution. The key limitations of Approach one are that this approach is unable to produce the spatial variability of floodplain inundation depth and extent and it does not incorporate dead storage. These limitations can be overcome by integrating it with Approach two. A key advantage of Approach two is the quick run time compared to a two dimensional hydrodynamic model. This makes the approach highly suitable to be incorporated in river system models for various scenario analyses for planning and management, which requires the model to run for over 100 years at a daily time step. The limitations of this approach are the requirement of very high resolution topographic data and the inability to produce flood velocity.

Surface water connectivity dynamics of a large scale extreme flood

Summary During flood inundation, river water passes from the main channel into the floodplain through floodplain channels and diffusive overbank flow. This flood water is then distributed within the floodplain depending upon internal connections, barriers and storage, and finally returns back to the river through drainage connections. This surface water connectivity can be complex and is important to many aspects of floodplain functioning, including ecology, sediment movement and flood risk. However, there is currently no accepted way of quantifying this connectivity objectively. We quantify surface water connectivity geostatistically as an objectively measurable characteristic of an observed flood event using a time series of MODIS (Moderate Resolution Imaging Spectroradiometer) surface water product for an extreme large scale flood event (11,000 km 2 flooded area and 6 month duration) during 2011 in Bangkok, Thailand. We develop and apply a new gap filling method that better preserves the dynamic information of the event than simple aggregation methods. Comparison of MODIS results with the higher resolution Earth Observer 1 shows fundamental differences in the resolved connectivity with scale despite similar flooded area. The effect of the passage of the flood wave is directly observable in the river reach, as out-of-bank flooding progresses and increases connectivity along the river during rising water. Around peak flow, there is an increase in connectivity of the floodplain adjacent to the river as low lying areas fill. A step increase in connectivity is correlated with a major levee breach. During recession there is a rapid reduction in along river connectivity in the first week after the peak. This rapid reduction contrasted with a slow decrease in the floodplain connectivity as flooded depressions gradually drained reducing depth, while flood extent remained static for long periods. The connectivity analysis of the threshold in floodplain draining indicates that although spatial flood extent changes are small at this time, there is a reorganisation of the internal surface water connectivity within the flooded area. Thus through this measure of connectivity, we can see a clear structure to the event progression with new insights into flood dynamics that were not anticipated a priori .

Bed stability in unconfined gravel bed mountain streams: With implications for salmon spawning viability in future climates

Incubating eggs of autumn‐spawning Chinook salmon (Oncorhynchus tshawytscha) could be at risk of midwinter high flows and substrate scour in a changing climate. A high‐spatial‐resolution multidimensional hydrodynamics model was used to assess the degree of scour risk in low‐gradient unconfined gravel bed channels that are the favored environment for autumn‐spawning salmon in mountain watersheds such as the Middle Fork Salmon River (MFSR), Idaho. In one of the most important MFSR spawning tributaries, near‐bed shear stresses were relatively low at all discharges from base flows to 300% of bankfull. The highest stresses were found only in small areas of the central flow core and not at spawning sites. Median shear stresses did not increase in overbank flow conditions because poor channel confinement released the excess water into adjacent floodplains. Channel and floodplain topography, rather than discharge, control the maximum near‐bed shear stresses. Over the modeled range of discharges, ~2% of the total surface area of the main stem channel bed was predicted to be mobile. Even in known spawning areas, where shear stresses are higher, ≤20% of the spawning surface area was mobile during overbank flows with a 2 year recurrence interval. Field measurements of little gravel transport during flows that were 93% of bankfull support the numerical model predictions. Regardless of some uncertainty in future climates in these watersheds, there appears to be relatively limited risk of extensive scour at salmon spawning sites in any likely hydrologic regimes.

Development of a Flow Routing Model Including Inundation Effect for the Extreme Flood in the Chao Phraya River Basin, Thailand 2011

In the second half of 2011, massive flooding in the lower part of the Yom and Nan River Basins and the Chao Phraya River Basin resulted in tremendous loss and adversely impacted on the livelihood, society, and economy of Thailand. The development of reliable and efficient prediction tools is important for prevent similar occurrences in the future. Consequently, this study thus aimed to develop an applicable distributed flow routing model including the inundation effect. A flow routing model using a kinematic wave equation was modified based on the concept of a diffusive tank model, which included the inundation effect. Overbank flow was estimated by using a broad crested weir equation. The inundation model had ten parameters that provided a good fit for observed and simulated discharge in 2011 at the C.2 Station with a Nash-Sutcliffe efficiency of 0.91.

Development of empirical regression-based models for predicting mean velocities in asymmetric compound channels

In this paper, the authors present experimental results of overbank flow in asymmetric rectangular compound channels considering both smooth and rough flumes. The mean velocity distributions in the main channel, floodplain, and the whole cross-section were measured for nine different test models constructed with variable main channel width and step height. The mean velocity measurements are then related to a dimensionless parameter called the relative depth defined as the ratio of the depth above the floodplain bed to the depth above the main channel bed. The variations and interactions of the three outlined mean velocities have been investigated with respect to relative depth. A set of single-variable regression models has been developed for estimating the three mean velocity types using relative depth as the only independent variable. Another set of multiple-variable regressions models has been derived using two additional dimensionless parameters which take into account the width dimensions of the constructed asymmetric compound channel. The application of several key statistics and validation procedures has indicated the high significance and reliability of the developed models in predicting the three mean velocity types. The predicted mean velocities can then be used to estimate the corresponding channel flow rate using the continuity equation.

Beaver dams and channel sediment dynamics on Odell Creek, Centennial Valley, Montana, USA

Beaver dams in streams are generally considered to increase bed elevation through in-channel sediment storage, thus, reintroductions of beaver are increasingly employed as a restoration tool to repair incised stream channels. Here we consider hydrologic and geomorphic characteristics of the study stream in relation to in-channel sediment storage promoted by beaver dams. We also document the persistence of sediment in the channel following breaching of dams. Nine reaches, containing 46 cross-sections, were investigated on Odell Creek at Red Rock Lakes National Wildlife Refuge, Centennial Valley, Montana. Odell Creek has a snowmelt-dominated hydrograph and peak flows between 2 and 10m3s−1. Odell Creek flows down a fluvial fan with a decreasing gradient (0.018–0.004), but is confined between terraces along most of its length, and displays a mostly single-thread, variably sinuous channel. The study reaches represent the overall downstream decrease in gradient and sediment size, and include three stages of beaver damming: (1) active; (2) built and breached in the last decade; and (3) undammed. In-channel sediment characteristics and storage were investigated using pebble counts, fine-sediment depth measurements, sediment mapping and surveys of dam breaches. Upstream of dams, deposition of fine (≤2mm) sediment is promoted by reduced water surface slope, shear stress and velocity, with volumes ranging from 48 to 182m3. High flows, however, can readily transport suspended sediment over active dams. Variations in bed-sediment texture and channel morphology associated with active dams create substantial discontinuities in downstream trends and add to overall channel heterogeneity. Observations of abandoned dam sites and dam breaches revealed that most sediment stored above beaver dams is quickly evacuated following a breach. Nonetheless, dam remnants trap some sediment, promote meandering and facilitate floodplain development. Persistence of beaver dam sediment within the main channel on Odell Creek is limited by frequent breaching (<1–5years), so in-channel sediment storage because of damming has not caused measurable channel aggradation over the study period. Enhanced overbank flow by dams, however, likely increases fine-grained floodplain sedimentation and riparian habitat. Contrasts between beaver-damming impacts on Odell Creek and other stream systems of different scales suggest a high sensitivity to hydrologic, geomorphic, and environmental controls, complicating predictions of the longer-term effects of beaver restoration.

Width adjustment in experimental gravel‐bed channels in response to overbank flows

We conducted a series of flume experiments to investigate the response of self‐formed gravel‐bed channels to floods of varying magnitude and duration. Floods were generated by increasing the discharge into a channel created in sand‐ and gravel‐sized sediment with a median grain size of 2 mm. Flooding increased the Shields stress along the channel perimeter, causing bank erosion and rapid channel widening. The sediment introduced to the channel by bank erosion was not necessarily deposited on the channel bed, but was rather transported downstream, a process likely facilitated by transient fining of the bed surface. At the end of each experiment, bank sediments were no longer in motion, “partial bed load transport” characterized the flat‐bed portion of the channel, and the Shields stress approached a constant value of 0.056, about 1.2 times the critical Shields stress for incipient motion. Furthermore, the discharge was entirely accommodated by flow within the channel: the creation of a stable channel entirely eliminated overbank flows. We speculate that similar processes may occur in nature, but only where bank sediments are non‐cohesive and where channel‐narrowing processes cannot counteract bank erosion during overbank flows. We also demonstrate that a simple model of lateral bed load transport can reproduce observed channel widening rates, suggesting that simple methods may be appropriate for predicting width increases in channels with non‐cohesive, unvegetated banks, even during overbank flows. Last, we present a model for predicting the equilibrium width and depth of a stable gravel‐bed channel with a known channel‐forming Shields stress.

Effects of flooding on recruitment and abundance of Golden Perch (Macquaria ambigua ambigua) in the lower River Murray

SummaryFlooding is often considered a stimulus for production of fish in floodplain rivers. In the southern Murray–Darling Basin (MDB), Australia, however, few native fish species have been shown to use the floodplain for spawning, and recruitment has been positively and negatively associated with flooding. In 2010/11, extensive flooding in the lower River Murray provided an opportunity to investigate the recruitment response of Golden Perch (Macquaria ambigua ambigua) following 10 years of drought and floodplain isolation. Annual variation in Golden Perch abundance and recruitment were investigated in anabranch and main channel habitats at Chowilla in the floodplain geomorphic region of the lower River Murray over a 7‐year period incorporating the flood and 6 years of in‐channel flow. Spatial variation in recruitment in the lower River Murray was also investigated by comparing the age structure of Golden Perch in the swamplands/lakes, gorge and floodplain geomorphic regions. Golden Perch abundance in the Chowilla region increased significantly postflooding compared with drought years. Age structures indicated that increased abundance was due predominantly to fish spawned during the flood (2010/11) and the previous year (2009/10), which was characterised by in‐channel flows. Age structure was similar in the nearby Katarapko Anabranch system indicating a uniform postflood recruitment response in the floodplain geomorphic region. Juvenile Golden Perch from the 2010/11 and 2009/10 cohorts were less apparent in the gorge and swamplands/lakes regions. Golden Perch have flexible life histories and will spawn and recruit in association with in‐channel rises in flow and overbank flows, but significant increases in abundance in the lower River Murray may result from overbank flooding. Contemporary approaches to flow restoration in the MDB emphasise overbank flows and floodplain processes. We suggest, however, that environmental flow management that incorporates floodplain and in‐channel processes, at appropriate spatio‐temporal scales, will result in more robust populations of Golden Perch.

Assessing the impact of climate and land use changes on extreme floods in a large tropical catchment

Summary In the wake of the recent catastrophic floods in Thailand, there is considerable concern about the safety of large dams designed and built some 50 years ago. In this paper a distributed rainfall–runoff model appropriate for extreme flood conditions is used to generate revised estimates of the Probable Maximum Flood (PMF) for the Upper Ping River catchment (area 26,386 km2) in northern Thailand, upstream of location of the large Bhumipol Dam. The model has two components: a continuous water balance model based on a configuration of parameters estimated from climate, soil and vegetation data and a distributed flood routing model based on non-linear storage–discharge relationships of the river network under extreme flood conditions. The model is implemented under several alternative scenarios regarding the Probable Maximum Precipitation (PMP) estimates and is also used to estimate the potential effects of both climate change and land use and land cover changes on the extreme floods. These new estimates are compared against estimates using other hydrological models, including the application of the original prediction methods under current conditions. Model simulations and sensitivity analyses indicate that a reasonable Probable Maximum Flood (PMF) at the dam site is 6311 m3/s, which is only slightly higher than the original design flood of 6000 m3/s. As part of an uncertainty assessment, the estimated PMF is sensitive to the design method, input PMP, land use changes and the floodplain inundation effect. The increase of PMP depth by 5% can cause a 7.5% increase in PMF. Deforestation by 10%, 20%, 30% can result in PMF increases of 3.1%, 6.2%, 9.2%, respectively. The modest increase of the estimated PMF (to just 6311 m3/s) in spite of these changes is due to the factoring of the hydraulic effects of trees and buildings on the floodplain as the flood situation changes from normal floods to extreme floods, when over-bank flows may be the dominant flooding process, leading to a substantial reduction in the PMF estimates.

Formation of a cohesive floodplain in a dynamic experimental meandering river

ABSTRACTField studies suggest that a cohesive floodplain is a necessary condition for meandering in contrast to braided rivers. However, it is only partly understood how the balance between floodplain construction by overbank deposition and removal by bank erosion and chutes leads to meandering. This is needed because only then does a dynamic equilibrium exist and channels maintain meandering with low width–depth ratios. Our objective is to understand how different styles of floodplain formation such as overbank deposition and lateral accretion cause narrower channels and prevent chute cutoffs that lead to meandering. In this study we present two experiments with a self‐forming channel in identical conditions, but to one we added cohesive silt at the upstream boundary. The effect of cohesive silt on bank stability was tested in auxiliary bank erosion experiments and showed that an increase in silt reduced erosion rates by a factor of 2. The experiment without silt developed to a braided river by continuous and extensive shifting of multiple channels. In contrast, in the meandering river silt deposits increased bank stability of the cohesive floodplain and resulted in a reduction of chute cutoffs and increased sinuosity by continuous lateral migration of a single channel. Overbank flow led to deposition of the silt and two styles of cohesive floodplain were observed: first, overbank vertical‐accretion of silt, e.g. levee, overbank sedimentation or splays; and second, lateral point bar accretion with silt on the scrolls and in the swales. The first style led to a reduction in bank erosion, while the second style reduced excavation of chutes. We conclude that sedimentation of fine cohesive material on the floodplain by discharge exceeding bankfull is a necessary condition for meandering. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Experimental modeling of depositional turbidity currents in a sinuous submarine channel

Submarine channels with intricate meandering patterns and extensive levees are recognized as products of density-driven flows known as turbidity currents. Compared to the fluvial meandering channels, understanding of the flow and morphodynamics of submarine channels is limited. In this paper, we present experimental results on the morphodynamic and stratigraphic evolution of a submarine channel from sedimentation due to the passage of successive flow events. A pre-formed sinuous channel with multiple bends, a trapezoidal cross section, and an initial thalweg slope of 0.43° was emplaced in a large tank. A total of 29 runs, each lasting an hour, were made by releasing heavier fluids containing salt and silica powder at a constant rate in the tank filled with fresh water. The overbank flow was restricted to curvature-induced flow stripping.The following observations were made from the experimental measurements; (i) asymmetric channel cross sections developed due to higher deposition rates on the outer bank of the channel bends, (ii) an apron or sediment wedge with overlain bedforms appeared near the channel entrance that prograded and aggraded with successive runs, (iii) overbank flow due to flow stripping at bend apices resulted in lobe-shaped deposits, (iv) a higher concentration of solute resulted in flow confinement, and (v) the flow velocity during later runs increased due to channel narrowing and steepening of the gradient.

River–floodplain hydrology of an embanked lowland Chalk river and initial response to embankment removal

Rivers have been channelized, deepened and constrained by embankments for centuries to increase agricultural productivity and improve flood defences. This has decreased the hydrological connectivity between rivers and their floodplains. We quantified the hydrological regime of a wet grassland meadow prior to and after the removal of river embankments. River and groundwater chemistry were also monitored to examine hydrological controls on floodplain nutrient status. Prior to restoration, the highest river flows (∼2 m3 s−1) were retained by the embankments. Under these flow conditions the usual hydraulic gradient from the floodplain to the river was reversed so that subsurface flows were directed towards the floodplain. Groundwater was depleted in dissolved oxygen (mean: 0.6 mg O2 L−1) and nitrate (mean: 0.5 mg NO3 −-N L−1) relative to river water (mean: 10.8 mg O2 L−1 and 6.2 mg NO3 −-N L−1, respectively). Removal of the embankments has reduced the channel capacity by an average of 60%. This has facilitated over-bank flow which is likely to favour conditions for improved flood storage and removal of river nutrients by floodplain sediments. Editor Z.W. Kundzewicz; Associate editor K. Heal Citation Clilverd, H.M., Thompson, J.R., Heppell, C.M., Sayer, C.D., and Axmacher, J.C., 2013. River–floodplain hydrology of an embanked lowland Chalk river and initial response to embankment removal. Hydrological Sciences Journal, 58 (3), 627–650.

Potential effects of climate change on streambed scour and risks to salmonid survival in snow‐dominated mountain basins

AbstractSnowmelt‐dominated basins in northern latitudes provide critical habitat for salmonids. As such, these systems may be especially vulnerable to climate change because of potential shifts in the frequency, magnitude, and timing of flows that can scour incubating embryos. A general framework is presented to examine this issue, using a series of physical models that link climate change, streamflow, and channel morphology to predict the magnitude and spatial distribution of streambed scour and consequent risk to salmonid embryos at basin scales. The approach is demonstrated for a mountain catchment in the Northern Rocky Mountains, USA. Results show that risk of critical scour varies as a function of species and life history and is modulated by local variations in lithology and channel confinement. Embryos of smaller‐bodied fall spawners may be at greater risk because of shallow egg burial depths and increased rain‐on‐snow events during their incubation period. Scour risk for all species is reduced when changes in channel morphology (width, depth, and grain size) keep pace with climate‐driven changes in streamflow. Although climate change is predicted to increase scour magnitude, the frequency of scouring events relative to typical salmonid life cycles is relatively low, indicating that individual year classes may be impacted by critical scour, but extirpation of entire populations is not expected. Furthermore, refugia are predicted to occur in unconfined portions of the stream network, where scouring shear stresses are limited to bankfull stage because overbank flows spread across alluvial floodplains; conversely, confined valleys will likely exacerbate climate‐driven changes in flow and scour. Our approach can be used to prioritize management strategies according to relative risk to different species or spatial distributions of risk and can be used to predict temporal shifts in the spatial distribution of suitable spawning habitats. A critical unknown issue is whether biological adaptation can keep pace with rates of climate change and channel response. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Coupling of a one-dimensional river routing model and a three-dimensional ocean model to predict overbank flows in a complex river–ocean system

River–ocean coupled models were developed to calculate overbank flows in a complex river–coastal ocean system comprising the Wu River and the adjacent coastal ocean of central Taiwan. The hydrodynamic quantities were exchanged between one-dimensional and three-dimensional domains during runtime to resolve the real-time exchange and interaction between the coastal waters and the river system. The coupled model was calibrated and verified with the observed water levels using data from three typhoon events. The results indicate a reasonable agreement between the model simulations and the observed data. The model was then used to calculate hydrographs of the overbank flow caused by Typhoon Kalmaegi, which occurred in 2008. The simulated overbank flows quantitatively agreed with the reported information. We also demonstrated that most of the overbank flows occurred at the upper reaches of the tributaries of the Wu River system, except for the tributary in the Fazi River.

Application and Validation of Regression Analysis in the Prediction of Discharge in Asymmetric Compound Channels

A series of laboratory experiments was performed to present the overbank flow in asymmetric rectangular compound channels. For this purpose, two different sets of asymmetric models with rectangular compound cross sections were tested for a wide range of discharges. The first set consisted of nine compound cross-section models formed by a combination of three step heights and three main channel widths. The second set consisted of six compound cross section-models formed using a combination of two step heights and three main channel widths. The mean flow measurements were then related to a dimensionless parameter called the relative depth defined as the ratio of the depth above the floodplain bed to the depth above the main channel bed. The variations and interactions of the three outlined mean flows were investigated with respect to relative depth. A set of single-variable regression models has been developed for estimating the three mean flow types using relative depth as the only independent variable. Another set of multiple-variable regression models was derived using two additional dimensionless parameters, which take into account the width dimensions of the constructed asymmetric compound channel. The application of several key statistics and validation procedures indicated the high significance and reliability of the developed models in predicting the three mean flow types.

Evaluation of the impact of the 2010 pyroclastic density currents at Merapi volcano from high-resolution satellite imagery, field investigations and numerical simulations

The 2010 pyroclastic density currents (PDC) at Merapi have presented a rare opportunity to collect a uniquely detailed dataset of the source, extent, lateral variations and impact of various PDC deposits on a densely populated area. Using traditional volcanological field-based methods and a multi-temporal dataset of high-resolution satellite imagery, a total of 23 PDC events have been recognized, including 5 main channeled flows, 15 overbank flows derived from overspill and re-channelization of the main PDCs into adjacent tributaries and two main surge events. The 2010 PDC deposits covered an area of ~22.3km2, unequally distributed between valley-filling (6.9%), overbank (22.4%) and surge and associated fallout deposits (71.7%). Their total estimated non-DRE volume is ~36.3×106m3, with 50.2% of this volume accounting for valley-filling deposits, 39.3% for overbank deposits and 10.5% for surge and associated fallout deposits. More than 70% of the total volume was deposited during the third eruptive phase (4–5 November), and only 16.6%, 11.5% and 0.9% during the first (26–29 October), second (30 October – 3 November) and fourth phase (6–23 November), respectively. The internal architecture and lithofacies variations of the 2010 PDC deposits were investigated using data collected from 30 stratigraphic sections measured after one rainy season of erosion. The results show that complex, local-scale variations in flow dynamics and deposit architectures are apparent and that the major factors controlling the propagation of the main flows and their potential hazards for overbanking were driven by: (1) the rapid emplacement of several voluminous PDCs, associated with the steady infilling of the receiving landscape after the two first phases of the eruption; (2) longitudinal changes in channel capacity following increased sinuosity in the valley and decreased containment space; and (3) the effects of varying source mechanisms (gravitational dome collapse, vertical or lateral dome explosions and column-collapse) and source materials involved during individual PDC-forming events. Integration of these data into numerical simulations of the channeled and overbank PDCs of the third eruptive phase (4–5 November) using two well-established geophysical mass flow models, Titan2D and VolcFlow, allow us to evaluate the ability of these models to reproduce the main features of the natural deposits and some of the flow overbanking processes observed in the field. Using such a multi-technique approach, the dataset obtained in this study not only characterizes the PDCs and related hazards at Merapi, but will allow comparisons with similar events at other volcanoes around the globe.

Movement and mortality of Murray cod, Maccullochella peelii, during overbank flows in the lower River Murray, Australia

Conservation of Murray cod (Maccullochella peelii), a large endangered fish species of Australia’s Murray–Darling Basin, relies on a detailed understanding of life history, including movement patterns and habitat use. We used radio-tracking to investigate the movement of 36 Murray cod in main channel and anabranch habitats of the lower River Murray during a flood and associated hypoxic blackwater event. During a flood peak of ~93 000 ML day–1, dissolved oxygen decreased to 1.2 mg L–1. Four movement types were observed: (1) localised small-scale movement, (2) broad-scale movement within anabranch habitats, (3) movement between anabranch and main channel habitats, and (4) large-scale riverine movement. Murray cod exhibited high fidelity to anabranch habitats but also moved extensively between anabranches and the main channel. Fish were consistently located in the main channel or permanent anabranches, suggesting that use of ephemeral floodplain habitats is limited, and highlighting the importance of connectivity between off-channel and main channel habitats. Mortality of radio-tagged fish was considerable (25%) in association with low dissolved oxygen concentrations, indicating that hypoxic blackwater may have had a substantial impact on Murray cod populations in the lower River Murray.

Turbulence statistics in compound channels with deep and shallow overbank flows

The results of large eddy simulations (LESs) of turbulent flow in a compound open channel with deep and shallow flood plain depths are presented. These LESs are validated with experimental data, resulting in a good agreement between measured and calculated data. The floodplain-depth-to-main-channel-depth ratio is an important parameter affecting first- and second-order turbulence statistics. The streamwise momentum balance is analysed with respect to the momentum transfer at the interface between the main channel and the floodplain. Depth-averaging of the streamwise momentum equation reveals the dominance of turbulent shear stress terms in the main channel and of secondary currents in the floodplain. Comparisons of the LES results with an analytical solution of the depth-averaged streamwise momentum equation that demonstrate the importance of case-specific calibration of the parameters of the analytical solution. The turbulence anisotropy is quantified and its role in generating secondary currents is illustrated.