Floodplain Slope Research Articles

Healthy Yellow River’s essence and indicators

River’s healthy life is a description of their living conditions, and it is also a comprehensive assessment of river’s functions and relations with the human society. Through analyzing the demands of human being and river ecosystem, the continuous flow, safe river channel for water and sediment transportation, good water quality, sustainable river ecosystem and water supply capacity are regarded as symbols of the healthy Yellow River. Minimum flow, maximum flood discharging capacity, bank-full discharge, transverse slope of floodplain, water quality degree, wetlands area, aquatic ecosystem, and water supply capacity, altogether eight quantitative indicators are set as symbols of healthy Yellow River, and their corresponding standards are determined based on the analysis with historical hydrological data and observed data of 1956–2004.

Causes of River Avulsion: Insights from the Late Holocene Avulsion History of the Mississippi River, U.S.A.

Abstract The emphasis on gradient advantages in studies of avulsion is misleading. While gradient advantages are necessary for an avulsion to occur, the late Holocene avulsion history of the Mississippi River in Louisiana suggests that factors such as substrate composition and floodplain channel distributions are more important. Cross-valley to down-valley slope ratios of the modern floodplain range from 16 to 110 and are typically > 30. The slope ratio is 35 at the location of the Mississippi–Atchafalaya diversion (Old River) yet slope ratios are 83 to 110 immediately upvalley of Old River. All values of Mississippi River floodplain slope ratios are significantly larger than values of avulsion threshold calculated by numerical models. Shallow floodplain cores, 14C dating of organic remains, and geologic mapping show that the Mississippi River has avulsed only four times over the past 5 ky in the southern Lower Mississippi Valley (LMV). Gradient advantages are widespread, yet avulsions are rare. These observations indicate that factors other than gradient advantage control Mississippi River avulsion. Several examples of Mississippi and Red River avulsion by channel reoccupation support the idea that channel distributions and substrate compositions are primary influences on avulsion. Incipient Mississippi River avulsion and development of the Atchafalaya River involved reoccupation of abandoned Mississippi River channels and a Red River crevasse-splay complex. The modern Atchafalaya River also incises buried Mississippi River channel-belt sands. Abandoned channel belts and crevasse-splay complexes consist of sandy substrates that facilitate scour and the development of channels capable of capturing the Mississippi River. Abandoned channels provide ready-made conduits for Mississippi River flow that can efficiently develop into avulsive channels. Multi-storied sheet sandstones in ancient fluvial deposits may provide additional support for the idea that erodible substrates and floodplain channel distributions are critical influences on avulsion. These features record episodic reoccupation of channel belts, which at least in some cases, may simply reflect successive avulsions rather than major changes in aggradation rate or extrabasinal factors such as climate.

Hydro-geomorphic hazards and impact of man-made structures during the catastrophic flood of June 2000 in the Upper Guil catchment (Queyras, Southern French Alps)

The Guil River Valley (Queyras, Southern French Alps) is prone to catastrophic floods, as the long historical archives and Holocene sedimentary records demonstrate. In June 2000, the upper part of this valley was affected by a “30-year” recurrence interval (R.I.) flood. Although of lower magnitude and somewhat different nature from that of 1957 (>100-year R.I. flood), the 2000 event induced serious damage to infrastructure and buildings on the valley floor. Use of methods including high-resolution aerial photography, multi-date mapping, hydraulic calculations and field observations made possible the characterisation of the geomorphic impacts on the Guil River and its tributaries. The total rainfall (260 mm in four days) and maximum hourly intensity (17.3 mm h −1), aggravated by pre-existing saturated soils, explain the immediate response of the fluvial system and the subsequent destabilisation of slopes. Abundant water and sediment supply (landsliding, bank erosion), particularly from small catchment basins cut into slaty, schist bedrock, resulted in destructive pulses of debris flow and hyperconcentrated flows. The specific stream power of the Guil and its tributaries was greater than the critical stream power, thus explaining the abundant sediment transport. The Guil discharge was estimated as 180 m 3 s −1 at Aiguilles, compared to the annual mean discharge of 6 m 3 s −1 and a June mean discharge of 18 m 3 s −1. The impacts on the Guil valley floor (flooding, aggradation, generalised bank erosion and changes in the river pattern) were widespread and locally influenced by variations in the floodplain slope and/or channel geometry. The stream partially reoccupied former channels abandoned or modified in their geometry by various structures built during the last four decades, as exemplified by the Aiguilles case study, where the worst damage took place. A comparative study of the geomorphic consequences of both the 1957 and 2000 floods shows that, despite their poor maintenance, the flood control structures built after the 1957 event were relatively efficient, in contrast to unprotected places. The comparison also demonstrates the role of land-use changes (conversion from traditional agro-pastoral life to a ski/hiking-based economy, construction of various structures) in reducing the Guil channel capacity and, more generally, in increasing the vulnerability of the human installations. The efficiency of the measures taken after the 2000 flood (narrowing and digging out of the channel) is also assessed. Final evaluation suggests that, in such high mountainous environments, there is a need to keep most of the 1957 flooded zone clear of buildings and other structures (aside from the existing villages and structures of particular economic interest), in order to enable the river to migrate freely and to adjust to exceptional hydro-geomorphic conditions without causing major damage.

Effects of peat on the shapes of alluvial channels: examples from the Cumberland Marshes, Saskatchewan, Canada

An avulsion of the lower Saskatchewan River in the 1870s inundated a large segment of peat-covered floodplain that subsequently has become aggraded with a broad (500 km 2) belt of alluvium deposited by the redirected flow. Routing of water and sediment discharge through the avulsion-affected area has been accomplished mainly by networks of sandy bedded anastomosed channels that have formed, evolved, and abandoned as the alluvial belt prograded down the floodplain slope. These processes continue today, though at a much-reduced rate. New channels, formed by crevassing and basinward extension of distributaries, are initially small and shallow, with bottom elevations situated within the avulsive alluvium but above the pre-avulsion peat (floodplain) surface. Subsequent enlargement and downcutting of many of these channels eventually uncovers the underlying peat layer whose resistance to erosion exerts significant influence on cross-sectional shape and further channel development. Peat-floored channels tend to have rectangular cross-sections, high ratios of average to maximum depth ( D/ D max), and a large range of width-to-depth ratios. If the channel continues to enlarge, the peat layer eventually becomes breached, commonly leading to temporarily irregular cross-sections caused by localized scouring at the breach sites. Eventually, the peat layer is completely eroded from the channel floor by undercutting and slumping, after which channel shape becomes governed mainly by other perimeter characteristics. Channels unaffected by peat, either before the peat layer is encountered during early channel development or after it is entirely removed, tend to have low width/depth ratios and a large range of D/ D max values.

Estimation of discharge capacity in meandering compound channels using artificial neural networks

Flow in compound (or two-stage) channels is very complex and different energy loss mechanisms operate under different geometric and flow conditions. Neither theoretical analyses nor current empirical approaches are sufficiently developed for practical calculation of conveyance for all conditions experienced in practice. An alternative approach, using artificial neural network modelling, has been successfully applied to predict conveyance under a wide range of conditions. The model proposed uses a feed-forward system with one hidden layer and an error back-propagation learning procedure. It predicts a dimensionless discharge using input describing the main channel and floodplain flow depths, vegetation density over the cross section, channel sinuosity, transverse floodplain slope, and floodplain bend tightness. The discharge is dimensionalized by multiplication with the composite discharge calculated assuming frictional resistance only. The model was trained using 45 data sets representing a range of main channel and floodplain characteristics and tested using 15 additional data sets. The discharge prediction error for all the data used in development and testing the model was -0.19% on average and exceeded 15% for one condition only.Key words: compound channels, channel conveyance, flow resistance, neural networks.

Three-dimensional model of alluvial stratigraphy; theory and applications

ABSTRACT A three-dimensional model of alluvial stratigraphy has been developed to simulate the spatial distribution, proportion, and connectedness of coarse-grained channel-belt deposits in alluvial strata as a function of channel-belt width, floodplain width, bankfull channel depth, channel-belt and overbank sedimentation rates, avulsion location and period, compaction, and tectonism (tilting and faulting). In this model, a floodplain surface of variable width and length is occupied by a single channel belt. Changes in floodplain topography are produced by spatial and temporal variation of channel-belt and floodplain deposition rates and by compaction and local or regional tectonism. The location and timing of avulsions are determined by local changes in floodplain slope relative to channel-b lt slope and by flood magnitude and frequency. The diverted channel belt follows the locus of maximum floodplain slope. At the end of each simulation, architectural parameters are calculated, including channel-deposit proportion and connectedness and the dimensions of channel-belt sandstone bodies. Three-dimensional perspective diagrams, mesh surfaces, and two-dimensional stratigraphic sections can be plotted to illustrate depositional surfaces (time planes) and the location and geometry of coarse-grained channel-belt deposits within finer-grained overbank deposits. The model predicts that channel-belt proportion and connectedness and dimensions of sandstone bodies vary as a function of distance from avulsion points and cross-section orientation. Upstream from avulsion points, sandstone bodies have low width/thickness ratios because of aggradation in a fixed channel belt. Immediately downstream from avulsion points, channel belts tend to be connected, resulting in sandstone bodies with high width/thickness ratios. Avulsion sequences develop where points of avulsion shift up valley with a progressive decrease in avulsion period. Such sequences may produce successions in which channel-belt proportion and connectedness vary vertically with a cyclic period of 103 to 105 years. Down-valley increases in aggradation rate or down-valley decreases in floodplain slope (for example, associated with a rise in base level) may result in an increase in channel-belt proportion and connectedness because of high avulsion frequencies in down-valley regions of the floodplain. Down-valley decreases in aggradation rate (as in alluvial fans, foreland basins, and during base-level fall) may result in high avulsion frequencies in up-valley parts of the floodplain. Tectonic tilting and faulting locally increase avulsion probabilities, and channel belts generally shift toward areas of maximum subsidence. Under certain conditions, however, depositional topography may cause channels to shift away from areas of maximum subsidence. Channel-deposit proportion and connectedness are gen rally high near downthrown areas of the floodplain, but distribution (clustering) of channel belts may not be a reliable indicator of fault geometry or displacement. Models of alluvial architecture that consider only sediment accumulation rate as the main controlling factor are oversimplified. The three-dimensional model presented here predicts many of the features of channel behavior observed in modern rivers, but there is a pressing need for better models and adequate natural data to test them.

Indigenous Farmer-Managed Irrigation in Sonjo, Tanzania

Despite wide interest expressed in indigenous farmer-managed small-scale irrigation in Sub-Saharan Africa, there are relatively few studies of such systems. This paper offers a preliminary description of that practised by the Sonjo in northern Tanzania. Sonjo irrigation involves simple canals carrying water diverted by stone and brushwood dams from spring-fed streams and rivers. Land is irrigated on floodplain and piedmont slopes, and a wide variety of crops is now grown. There is an established system of water allocation. Sonjo irrigation is an example of hill-furrow irrigation of a kind occurring in several places in East Africa. Fuller information of the extent, organization, management and agronomy of these systems is vital as development initiatives are established which attempt to 'improve' or replicate 'indigenous knowledge' about irrigation.

Evolution of miocene fluvial systems in the himalayan foredeep through a two kilometer-thick succession in northern Pakistan

Sedimentological variations through a kilometer-thick coarsening-upward succession in the Miocene Himalayan foredeep basin (fluvial Chinji and Nagri Formations of the Siwalik Group) are documented in the Chinji Village area on the Potwar Plateau of northern Pakistan. From the Chinji Formation upwards into the Nagri Formation: (1) the proportion of major sandstone bodies increase relative to mudstone-dominated beds; (2) average thickness and mean grainsize of channel deposits increase; (3) sediment aggradation rates increase; (4) there is a reported change in sediment providence; (5) paleocurrents are generally to the southeast and vary little upsection; and (6) paleosols are more distinct lower in the section where there is also more evidence of ponding of water on the floodplain. Smaller-scale (100 m thick) cyclic variations in the proportion of major channel sandstone bodies also occur in both formations. In the modern Himalayan Basin, sediment dispersal away from the mountain belt is associated with: (1) major rivers that flow along the basin axis; (2) large rivers that drain substantial areas of the mountain belt and deposit low relief sediment fans extending hundreds of kilometers into the basin; and (3) smaller rivers in fan and interfan areas that drain local areas and carry finer sediment loads. Sediment variations through the Chinji-Nagri Formation boundary can be related to a shifting of depositional environments within such fan and interfan areas. Major basin-axial rivers are not exposed in this area. It appears that kilometer-thick upsection variations in these deposits record the progradation of the sediment fan formed by a large river system (river discharge on the order of 10 4 m 3/s) over the floodplain of a smaller river system (river discharge on the order of 10 3 m 3/s). Large-scale vertical sediment variations over hundreds of meters to kilometers appear to reflect changes in the basin subsidence and the rate of sediment input to the basin over time scales on the order of 10 5–10 6 years. Upsection variations can not easily be related to climatic change or eustatic variations in sea level. The most likely explanation for upsection changes is tectonism. Deposition rates and shifting positions of river systems within the basin responded more quickly to changes in basin subsidence rate, whereas increases in the proportion of sandstone bodies and increases in floodplain slopes lagged behind. Deposit variations in the Siwalik Group clearly record variations between different-scale river systems delivering sediment to the basin as well as the overall basin evolution.

Reservoir Sedimentation. II: Reservoir Desiltation and Long‐Term Storage Capacity

During the past 30 years, China has implemented structural and operational procedures to maximize sediment discharge from the impounded reaches of rivers, thereby reducing, arresting, or reversing reservoir sedimentation at many sites. Hydraulic methods used in China to preserve reservoir capacity are outlined in this paper, including sediment routing during floods, sediment flushing during floods, emptying and flushing, and density current venting. A method to compute retrogressive erosion during the drawdown of a reservoir is developed, and an approximate method for estimating the amount of sediment that can be vented from a reservoir as a density current is proposed. The computed values are compared with field data. Field data on the floodplain slope formed during flood season and the bottom slope of the main channel formed during nonflood season are analyzed. Empirical formulas for both slopes are presented for estimating long-term storage capacity.

Neotectonic effects on sinuosity and channel migration, Belle Fourche River, Western South Dakota

AbstractShort‐term instability in the behaviour of a small, meandering alluvial channel is identified from the relation between sinuosity and either floodplain slope or channel slope within 17 reaches along an 81‐kilometre section of the Belle Fourche River in western South Dakota. In reaches 1 to 4 and 11 to 17 the channel is relatively stable and sinuosity varies inversely with channel slope. In reaches 5 to 10, sinuosity is positively related to floodplain slope. Sinuosity increases markedly in reaches 5, 6, and 7 (which are immediately downstream from a discontinuity in the long profile of the floodplain) in association with an increase in floodplain slope. Immediately upstream from the discontinuity, bankfull channel depth and sinuosity decrease and the area of the floodplain reworked by meander migration between 1939 and 1981 increases, in association with a decrease in floodplain slope. Channel behaviour in reaches 5 to 10 is best explained as a consequence of neotectonic activity, as indicated by changes in elevation recorded along geodetic survey lines that cross lineaments that may delimit the eastern boundary of the Black Hills uplift. Sinuosity acts as a barometer of the effects of neotectonic activity on alluvial channels. Initial indications of channel and floodplain instability due to neotectonic activity may be derived from evidence of anomalously active channel migration, as documented from photographic or topographic sources.

Forest Succession and Sedimentation on a Meandering-River Floodplain, Northeast British Columbia, Canada

An examination of primary forest succession within two migrating bends along the Beatton River in northeast British Columbia provides information on the interrelationships between the geomorphic and biotic components of a meandering-river floodplain. Overbank sedimentation rates are estimated from changes in the wood structure of partially buried balsam poplars, and from distinct changes in floodplain slope. Particular emphasis is placed upon the changing age-structure of dominant tree species to describe successional changes within the floodplain forest. Rapid sedimentation on the youngest ridges favours the establishment of a dense, non-reproducing balsam poplar stand of uniform age, and prevents the establishment of white spruce seedlings. Following an abrupt decline in overbank sedimentation on surfaces approximately 50 years old, white spruce rapidly colonize the bare mineral soil beneath the poplar canopy and form a dense, relatively even-aged stand. With the death of mature poplars on surfaces 100-150 years in age, the spruce seedlings are released and develop into a mature, non-reproducing stand. This stand persists until its senescence on ridges 350-400 years old. Here, a second, similar, although less even-aged spruce stand establishes, maturing in a like manner to the flrst, to eventually decline on ridges 500-550 years in age. On this older surface there is the suggestion of yet a third generation of spruce seedlings. These oscillations are reflected in changes of the vascular understory-species during succession, and compare to the cyclic regeneration and climax instability described by Watt (1947) and Whittaker (1953). Tree densities and overbank sedimentation achieve minimum values on floodplain surfaces greater than approximately 200 years in age. A reasonable explanation to account for negligible sediment deposition on these older surfaces is that the more open trunk density there permits relatively rapid overland flow, preventing the deposition of suspended sediment load. Sedimentation and vegetation appear to be strongly interrelated components within the total floodplain system.