Paul A. Carling

Sustainable sediment management in reservoirs and regulated rivers: Experiences from five continents

By trapping sediment in reservoirs, dams interrupt the continuity of sediment transport through rivers, resulting in loss of reservoir storage and reduced usable life, and depriving downstream reaches of sediments essential for channel form and aquatic habitats. With the acceleration of new dam construction globally, these impacts are increasingly widespread. There are proven techniques to pass sediment through or around reservoirs, to preserve reservoir capacity and to minimize downstream impacts, but they are not applied in many situations where they would be effective. This paper summarizes collective experience from five continents in managing reservoir sediments and mitigating downstream sediment starvation. Where geometry is favorable it is often possible to bypass sediment around the reservoir, which avoids reservoir sedimentation and supplies sediment to downstream reaches with rates and timing similar to pre-dam conditions. Sluicing (or drawdown routing) permits sediment to be transported through the reservoir rapidly to avoid sedimentation during high flows; it requires relatively large capacity outlets. Drawdown flushing involves scouring and re-suspending sediment deposited in the reservoir and transporting it downstream through low-level gates in the dam; it works best in narrow reservoirs with steep longitudinal gradients and with flow velocities maintained above the threshold to transport sediment. Turbidity currents can often be vented through the dam, with the advantage that the reservoir need not be drawn down to pass sediment. In planning dams, we recommend that these sediment management approaches be utilized where possible to sustain reservoir capacity and minimize environmental impacts of dams.

A physically based model to predict hydraulic erosion of fine‐grained riverbanks: The role of form roughness in limiting erosion

Hydraulic erosion of bank toe materials is the dominant factor controlling the long-term rate of riverbank retreat. In principle, hydraulic bank erosion can be quantified using an excess shear stress model, but difficulties in estimating input parameters seriously inhibit the predictive accuracy of this approach. Herein a combination of analytical modeling and novel field measurement techniques is employed to improve the parameterization of an excess shear stress model as applied to the Lower Mekong River. Boundary shear stress is estimated using a model (Kean and Smith, 2006a, 2006b) for flow over the irregular bank topography that is characteristic of fine-grained riverbanks. Bank erodibility parameters were obtained using a cohesive strength meter (Tolhurst et al., 1999). The new model was used to estimate annual bank erosion rates via integration across the Mekongs annual flow regime. Importantly, the simulations represent the first predictions of hydraulic bank erosion that do not require recourse to calibration, thereby providing a stronger physical basis for the simulation of bank erosion. Model predictions, as evaluated by comparing simulated annual rates of bank toe retreat with estimates of bank retreat derived from analysis of aerial photographs and satellite imagery, indicate a tendency to overpredict erosion (root-mean-square error equals ±0.53 m/yr). Form roughness induced by bank topographic features is shown to be a major component (61%–85%) of the spatially averaged total shear stress, and as such it can be viewed as an important factor that self-limits bank erosion.

A review of open-channel megflood depositional landforms on Earth and Mars

Large freshwater floods on Earth in recent times and in the Quaternary have often been associated with catastrophic out-bursts of water from lakes impounded by glacial-ice or debris (such as moraine). In either case, large-scale depositional sedimentary landforms are found along the courses of the floodwaters. On Mars, similar floods are believed to have resulted from catastrophic efflux of water from within the Martian surface. Within the Martian flood tracts, landforms have been imaged that appear similar to those identified on Earth. These are primarily suites of giant bars – “streamlined forms” – of varying morphology that occur primarily as longitudinal features within the floodways and along the margins as well as in areas of the floodways that were sheltered from the main flow. In addition, flow-transverse bedforms within the floodways have been identified as giant sedimentary dunes or antidunes. Information concerning the flood hydraulics that created these forms may be deduced from their location and plan view morphology. Some other fluvial landforms which have been associated with megafloods on Earth have yet to be identified on Mars. The examples from Earth are described, so as to spur the search for further water-lain landforms on Mars

Role of suspended-sediment particle size in modifying velocity profiles in open channel flows

Previous experimental and analytical studies have revealed that suspended particles can attenuate or enhance turbulence, depending on the particle size in relation to turbulence scales. Incorporating this mechanism, an empirical turbulent eddy viscosity-based closure model is proposed for the mean velocity structure of suspended sediment-laden flow in open channels. The model integrates the sediment particle Stokes number, the ratio of particle-size-to-turbulence microscale, the ratio of particle settling velocity to bed shear velocity, and local sediment concentration. Its good performance is demonstrated in comparison with available laboratory observations. It is characterized that single-phase turbulence closure models can be adapted for sediment-laden flows by implementing sediment particle size effects.

Decadal length changes in the fluvial planform of the River Ganga: bringing a mega-river to life with Landsat archives

The Landsat programme, which was started in 1972, initiated an era of space-based Earth observation relevant to the study of large river systems through the provision of spatially continuous, synoptic and temporally repetitive multispectral data. Free access to the Landsat archive via the Internet from mid-2008 has enabled the scientific community to reconstruct the Earths changing surface and, in particular, to reconstruct the planform dynamics of the worlds largest rivers. The present research reconstructed the planform changes occurring in the lower reaches of one of the Asian mega-rivers, the River Ganga (Ganges), from 1972 to 2010 using the Landsat archive. Sequential river planform maps generated from the time-series revealed the pattern of evolution of the river system over the study period. Specifically, within the observed sequence, the river started as a single-thread channel that then began meandering at four locations. The meander bends increased in sinuosity until chute cut-offs were triggered, returning the river to a state similar to that at the beginning of the sequence. This periodic pattern is constrained by several hard points in the geology, and by the Farakka Barrage, meaning that the observed cyclic pulsing is likely to continue into the future. This constrained pattern has significant implications for local planners who may currently fear that the river is migrating laterally.

Megaflooding on Earth and Mars: Channel-scale erosional bedforms in bedrock and in loose granular material: character, processes and implications

High energy fluid flows such as occur in large water floods can produce large scale erosional landforms on Earth and potentially on Mars. These forms are distinguished from depositional forms in that structural and stratigraphical aspects of the sediments or bedrock may have a significant influence on the morphology of the landforms. Erosional features are remnant, in contrast to the depositional (constructional) landforms that consist of accreted waterborne sediments. A diversity of erosional forms exist in fluvial channels on Earth at a range of scales that includes the mm and the km scales. For comparison with Mars and given the present-day resolution of satellite imagery, erosional landforms at the larger scales can be identified. Some examples include: periodic transverse undulating bedforms, longitudinal scour hollows, horse-shoe scour holes around obstacles, waterfalls, plunge pools, potholes, residual streamlined hills, and complexes of channels. On Earth, many of these landforms are associated with present day or former (Quaternary) proglacial landscapes that were host to jokulhlaups (e.g., Iceland, Washington State Scablands, Altai Mountains of Southern Siberia), while on Mars they are associated with pro-volcanic landscapes that were likely host to mega-floods produced by enormous eruptions of groundwater. The formative conditions of some erosional landforms are not well understood, yet such information is vital to interpreting the genesis and paleohydraulic conditions of past mega-flood landscapes. Thus, examples of erosional bedforms on Earth are presented and reviewed and supposed similar forms on Mars are identified for comparison with the Earth examples. Correct identification of some landforms allow estimation of their genesis, including paleohydraulic conditions

Megaflood sedimentary valley fill: Altai Mounatins, Siberia

During the Quaternary, the Altai Mountains of south-central Siberia sustained ice-caps and valley glaciers. Glaciers or ice lobes emanating from plateaux blocked the outlet of the Chuja-Kuray intermontane basins and impounded meltwater to form large ice-dammed lakes up to 600 km3 capacity. On occasion the ice dams failed and the lakes emptied catastrophically. The megafloods that resulted were deep, fast-flowing and heavily charged with sand and gravel, the sediment being sourced from the lake basins and also entrained along the course of the flood-ways. The floods were confined within mountain valleys of the present-day Rivers Chuja and Katun, but large quantities of sediment were deposited over a distance of more than 70km from the dam site in tributary river-mouths, re-entrants in the confining valley walls (e.g. cirques) and on the inside of major valley bends. The main depositional units that resulted are giant bars which blocked the entrances to tributaries and temporarily impeded normal drainage from the tributaries into the main-stem valley such that minor lakes were impounded within the tributaries behind the bars. Fine sediment from the tributaries accumulated in these lakes as local lacustrine units. Later the bars were breached by the tributary flows and the local lakes were drained. Sections of the giant bar sediments and the local lacustrine units are used to describe the nature of the megaflood valley fill which was deposited primarily in Marine Isotope Stage 2. Although there is evidence of the Chuja-Kuray lake being in existence within Marine Isotope Stage 4 there are no flood sediments unequivocably ascribed to this period. Descriptions of the sedimentology and stratigraphy of the valley-fill are interpreted within a context of proposed flow mechanisms associated with deposition of the various facies and thus provide some indication of the flood dynamics

Coupled 2D Hydrodynamic and Sediment Transport Modeling of Megaflood due to Glacier Dam-break in Altai Mountains,Southern Siberia

One of the largest known megafloods on earth resulted from a glacier dam-break, which occurred during the Late Quaternary in the Altai Mountains in Southern Siberia. Computational modeling is one of the viable approaches to enhancing the understanding of the flood events. The computational domain of this flood is over 9460 km2 and about 3.784 × 106 cells are involved as a 50 m × 50 m mesh is used, which necessitates a computationally efficient model. Here the OpenMP (Open Multiprocessing) technique is adopted to parallelize the code of a coupled 2D hydrodynamic and sediment transport model. It is shown that the computational efficiency is enhanced by over 80% due to the parallelization. The floods over both fixed and mobile beds are well reproduced with specified discharge hydrographs at the dam site. Qualitatively, backwater effects during the flood are resolved at the bifurcation between the Chuja and Katun rivers. Quantitatively, the computed maximum stage and thalweg are physically consistent with the field data of the bars and deposits. The effects of sediment transport and morphological evolution on the flood are considerable. Sensitivity analyses indicate that the impact of the peak discharge is significant, whilst those of the Manning roughness, medium sediment size and shape of the inlet discharge hydrograph are marginal.

Geomorphology and sedimentology of the lower Mekong river

Publisher Summary This chapter provides an introduction to the geomorphology of the Lower Mekong River within a geological and physiographic setting. The Mekong River Basin can be divided into two units. The Upper Mekong Basin (or Lancang Jiang Basin) lies within China and the Lower Mekong Basin (LMB) lies to the south of the international border between China (Yunnan Province) and Laos (Lao PDR). In China, the Mekong River is known as the Lancang Jiang (“the turbulent river”), the headwaters of which drains from an altitude of 4970 m on the Tibetan Plateau and flow for nearly 800 km in Tibet before entering Yunnan province in China, where it flows for a further 1200 km. The mountains and hills of southern Yunnan, eastern Myanmar, northern Thailand, and Laos (Lao PDR) occupy an area between the eastern face of the Shan Plateau and the western part of the Indochinese peninsula. Elevations reach 2800 m and the mountains have a north-south alignment and, in the north, the valleys drain into the Mekong whilst, in the south, four large rivers drain to the Chao Phrya River in Thailand.

Dynamic simulation of catastrophic late Pleistocene glacial-lake drainage, Altai Mountains, central Asia

ABSTRACT Numerical simulations of the catastrophic draining of Pleistocene glacial-lake Kuray–Chuja quantify the discharge history of the draining event in detail. The plan-view basin flows are modelled as water emptied due to the instantaneous failure of the impounding ice-dam when the lake was at maximum capacity. The Chuja Basin water exited as a jet-flow into the Kuray Basin via a narrow conjoining valley. The peak discharge from the Chuja Basin is determined to be 1.20 × 107 m3 s−1, and the peak discharge (3.19 × 107 m3 s−1 > Q ≤ 2.0 × 107 m3 s−1) that flowed from the Kuray Basin at the failed impoundment is also calculated for two limiting conditions. The variations in lake volume and depth indicate complete drainage within 50 h. In both basins, fields of relict gravel bedforms reflect sediment transport due to entrained lake-bed sediments. Thus, in addition to the general overview of drainage, the detailed temporal and spatial evolutions of drainage parameters are reported, including for the locations of the bedform fields. Local flow above the bedforms is considered in relation to thresholds for sediment motion, bedform development, and orientations. Within the simple bathymetry of the Chuja Basin, the flow field was fairly uniform with flow conducive to bedform evolution only occurring close to the exit from the basin, which accords with field evidence. In contrast, within the Kuray Basin, the flow responded sensitively to the complex bathymetry, which included rapid changes in flow direction due to interaction of the Kuray water with the jet-flow from Chuja, and as submerged ridges shoaled. Thus the Kuray flow field was complex but with time-dependent flow conditions in accordance with bedform development. It is concluded that the location of the bedforms can be explained in terms of the flow modelling and suggestions are made as to how future drainage models might be improved.