Formation Of Floodplains Research Articles

River runoff decrease in North-Eurasian plains during the Holocene optimum

Three stages were identified in the development of meandering rivers and the formation of floodplains with natural levees in Northern Eurasia: the development of rivers with size larger than that of the modern ones; the development of rivers smaller than the modern ones; and the development of rivers of the present-day morphodynamic type. Small oxbows of the second stage are widespread in the floodplains of lowland rivers in Northern Eurasia. The largest amount of floodplain segments with such oxbows can be seen in the forest zone, mostly in the coniferous forests of northeastern European Russia. The available radiocarbon datings show that river channel were significantly decreasing in size and the steepness of meanders was increasing during the Atlantic period of the Holocene. Data on changes in the size of river channels were used to evaluate the ratios between paleo- and modern discharges and to construct a map of difference between runoff depths in the Holocene optimum and in the present and assess changes in water runoff volume. The discharges in the basins of the Vyatka and middle Irtysh accounted for as little as 40–50% of their current values. North, east, and west from those basins, the ratio of ancient and present-day discharges increases. During the Holocene optimum, water runoff from the northern megaslope of the East European Plain was ∼180 km3/year, which is 30% less than the present runoff from the same drainage area. The annual runoff in Volga basin was ∼134 km3, which is almost half as large as the present value. The runoff in Don and Dnieper basins during the Holocene optimum was 40% less, and that in the Ob and Irtysh basin was 30% less than the present one. If we accept the hypothesis that the Holocene optimum was a climate analogue of global anthropogenic warming of the mid-XXI century, the obtained estimates of the state of water resources in Northern Eurasia acquire great prognostic importance.

Quantitative technique for assessing the geomorphic thresholds for floodplain instability and braiding in the semi-arid environment

For the semi-arid environment of southern California, a reach-scale classification system and conceptual model were created to synthesize the observed floodplain forms into three basic floodplain continuums: armored, non-armored, and active-regional alluvial fan. These continuums are comprised of three to five alluvial floodplain forms (cascade, step-pool, plane-coarse-bed, plane-mixed-bed, plane-fine-bed, pool-riffle, braided, and dune-ripple). For the non-armored floodplain continuum, a floodplain state diagram has been generated to quantitatively describe the natural downstream progression of floodplain forms and geometry, using specific stream power and the width-to-depth ratio as the state and shape metrics, respectively. Based on field data for 91 cross sections along 14 watercourses, this floodplain state diagram provided the basis to define geomorphic thresholds for floodplain instability and braiding using logistic regression analyses. This method for defining floodplain instability thresholds provides a unique management tool by providing both a systematic and a quantitative means to assess the stability state of watercourses and, thereby, provide a means for assessing the potential for the hazards associated with watercourse instability.

The hydraulic geometry of narrow and deep channels; evidence for flow optimisation and controlled peatland growth

At-a-station and bankfull hydraulic geometry analyses of peatland channels at Barrington Tops, New South Wales, Australia, reveal adjustments in self-forming channels in the absence of sediment load. Using Rhodes ternary diagram, comparisons are made with hydraulic geometry data from self-forming channels carrying bedload in alluvial settings elsewhere. Despite constraints on channel depths caused at some locations by the restricted thickness of peat, most stations have cohesive, near-vertical, well-vegetated banks, and width/depth (w/d) ratios of ∼2 that are optimal for sediment-free flow. Because banks are strong, resist erosion and can stand nearly vertical, and depth is sometimes constrained, adjustments to discharge are accommodated largely by changes in velocity. These findings are consistent with the model of maximum flow efficiency and the overarching least action principle in open channels. The bankfull depth of freely adjusting laterally active channels in clastic alluvium is well known to be related to the thickness of floodplain alluvium and a similar condition appears to apply to these swamps that grow in situ and are formed almost entirely of organic matter. The thickness of peat in these swamps rarely exceeds that required to form a bankfull channel of optimum w/d ratio for the transport of sediment-free water. Swamp vegetation is highly dependent on proximity to the water table. To maintain a swamp-channel and associated floodplain system, the channels must flow with sufficient water much of the time; they not only offer an efficient morphology for flow but do so in a way that enables bankfull conditions to occur many times a year. They also prevent the swamp from growing above a level linked to the depth of the channel. Once the channel attains the most efficient cross section, further growth of the swamp vertically is restricted by enhanced flow velocities and limited flow depths. This means that the volume of peat in such swamps is determined by the hydraulic efficiency of their channels. The development and maintenance of the hydraulic geometry of these swamp channels is biogeomorphic and biohydraulic in nature and yet accords to the same optimising principles that govern the formation of self-adjusting channels and floodplains in clastic alluvium.

Flood management along the Lower Mississippi and Rhine Rivers (The Netherlands) and the continuum of geomorphic adjustment

Flood management alters fundamental fluvial processes that have geomorphic consequences for rivers and floodplains. The Lower Mississippi and Rhine Rivers (The Netherlands) are two important examples of intensively regulated large rivers. Understanding the magnitude and direction of change caused by flood management requires a long-term perspective. This is particularly true of large lowland fluvial systems because of substantial lag-times required for adjustment to be manifest in the floodplain geomorphology. This study is a historical analysis and synthesis of the impacts of flood management on the Lower Mississippi and Rhine Rivers (The Netherlands), and investigates the interrelations of flood management with floodplain geomorphology. Although flood management varies between the Rhine and Mississippi on many accounts, the actual techniques of flood (and river) management are somewhat similar, and primarily include dikes, groynes, cutoffs, and bank protection. The implementation and history of these specific types of activities, however, varies considerably. Historical flood management along the Lower Mississippi can be characterized as abrupt, with the major options imposed within about five decades, while historical flood management along the Rhine River in The Netherlands is characterized as incremental and adaptive, with the major options imposed over about eight centuries. Conversely, modern flood management plans are implemented much more promptly along the Dutch Rhine than the Lower Mississippi. Changes to the Lower Mississippi include channel adjustment (width and incision) caused by meander bend cutoffs. The majority of the knickpoint incision in response to cutoffs occurred by 1963. Channel adjustment in some reaches is likely constrained by the presence of resistant alluvium and lithology. Floodplain geomorphic changes include the creation of new oxbow lakes within an embanked floodplain. Embanked floodplain sedimentation of oxbow lakes created from the 1928 Mississippi River & Tributaries Act have rapidly infilled, with 67% of the lake area converted to wetlands. In comparison, older oxbow lakes located outside of the embanked floodplain have undergone much lower amounts of infilling, averaging 37% of oxbow lake area converted to wetlands. The floodplain geomorphology is further modified by numerous large floodplain borrow pits and the selective removal of fine-grained deposits, primarily created for dike (levee) construction and maintenance. The Dutch Rhine has been managed for flooding for over eight centuries and exhibits specific types of humanized embanked floodplain geomorphology that require a greater period of adjustment. Dike breaches create ponds (wielen) and sandy splay-like deposits, which represent distinctive anthro-geomorphic environments along the margins of embanked floodplains. Channel stabilization by groynes and dikes has resulted in the formation of new floodplains along Rhine distributaries. The trapping of flood sediments within the embanked floodplain has resulted in aggradation that has reduced the inundation capacity of the embanked floodplain. This geomorphic alteration reduced the effectiveness of the existing flood management infrastructure and has stimulated a change towards a new flood management approach designed to "work with the river". The major conclusions are placed within a conceptual model, and illustrate that; 1. in many instances specific flood management options were constrained by the type of floodplain deposit; 2. geomorphic adjustment to flood management occurs along a time–space continuum; 3. flood management initiates positive feedbacks with unintended geomorphic consequences that require further management options to minimize flood risk.

Use geopedological methods for the evaluation of sedimentation rates on river floodplains, southern Québec, Canada

Abstract This article pertains to the fluvial dynamics of rivers in southern Quebec, in particular to the aggradation and pedogenetic processes observed in floodplains affected by periodic floods. The frequency of flood events, notably along the Saint-Francois River and its main tributaries, leads to fine materials being frequently deposited on floodplains and affected development of alluvial soils. Particle size and physical and chemical analyses have led to a better understanding of the dynamics involved in the formation of floodplains and the development of soils related to this fluvial environment. Also, sedimentological analyses (layer texture and thickness, microstructure) combined with radiocarbon dating (14C) and isotopic methods (210Pb, 226Rd) enabled the determination of sedimentation rates along the floodplains. The radiocarbon dating results obtained from the organic layers buried in alluvial soils show relatively variable ages, i.e. between 2210 ± 60 and 30 ± 70 years BP. The data gathered reveal an active overbank deposition, which shows evidence of the high flood recurrence in many rivers of the basin. It is estimated that the annual floodplain aggradation ranges from 1.0 to 7.6 mm yr− 1 on average, which causes increases in floodplain elevations (vertical accretion). The sedimentation rates obtained using the constant rate supply dating model (210Pb) show average values that range from 2.1 to 10.7 mm yr− 1. Also, the presence of contaminated layers at the lower level (> 100 cm) of the floodplains suggested an active sedimentation rates along the rivers affected by floods.

Spatial and Temporal Variation in Hydrologic Change Due to Regulation and Tributary Contributions on the Snake River in Grand Teton National Park WY

Channel and floodplain form are primarily determined by the flux of water through a reach and the associated transport of the sediment delivered from the upstream watershed. The dominant paradigm of fluvial geomorphology is that the size of the bankfull channel and the characteristics of the adjacent floodplain are maintained by the current hydrologic and sediment supply regimes (Wolman and Miller, 1960; Andrews, 1980; Leopold, 1994). The linkage among flow regime, sediment supply, and channel and floodplain form is well illustrated on regulated rivers where the flow regime and sediment supply are altered by dams. Bed incision under conditions of sediment deficit has been widely described (Mostafa, 1957; Komura and Simmons, 1967; Galay, 1983; Williams and Wolman, 1984). The longitudinal change from near-dam sediment deficit to sediment surplus further downstream has been described by Andrews (1984) and Grant et al. (2003), although local differences in bed texture, channel organization, and valley confinement affect the magnitude of channel and floodplain change in any specific reach (Lagasse, 1981; Grams and Schmidt, 2002, 2005).

Episodic sediment accumulation on Amazonian flood plains influenced by El Niño/Southern Oscillation.

Continental-scale rivers with a sandy bed sequester a significant proportion of their sediment load in flood plains. The spatial extent and depths of such deposits have been described, and flood-plain accumulation has been determined at decadal timescales, but it has not been possible to identify discrete events or to resolve deposition on near-annual timescales. Here we analyse (210)Pb activity profiles from sediment cores taken in the pristine Beni and Mamore river basins, which together comprise 720,000 km2 of the Amazon basin, to investigate sediment accumulation patterns in the Andean-Amazonian foreland. We find that in most locations, sediment stratigraphy is dominated by discrete packages of sediments of uniform age, which are typically 20-80 cm thick, with system-wide recurrence intervals of about 8 yr, indicating relatively rare episodic deposition events. Ocean temperature and stream flow records link these episodic events to rapidly rising floods associated with La Niña events, which debouch extraordinary volumes of sediments from the Andes. We conclude that transient processes driven by the El Niño/Southern Oscillation cycle control the formation of the Bolivian flood plains and modulate downstream delivery of sediments as well as associated carbon, nutrients and pollutants to the Amazon main stem.

Formation of braided river floodplains, Waimakariri River, New Zealand

ABSTRACTFloodplains along the braided gravel‐bed Waimakariri River are discontinuous although generally extensive landforms composed predominantly of gravel bars capped with vertically accreted fines. In order of importance, three mechanisms lead to floodplain formation. River bed abandonment by lateral migration of the braid‐train initiates the formation of the largest floodpiains, which usually occur downstream of tributary fans and valley bedrock spurs. In the headwater reaches, localized river bed aggradation during high magnitude events leads to floodplain formation by producing a surface resistant to erosion by lesser events due to either the coarse nature of the deposit, or by its elevation over the river bed, or both. The least important mechanism initiating floodplain development is localized river or channel incision. A six stage model is proposed for the sequential development of floodplains on the Waimakariri River from: (i) active river bed; (ii) stabilizing bar; (iii) incipient floodplain; (iv) established floodplain; (v) mature floodplain; and to (vi) terrace. Two mechanisms, lateral migration by the braid‐train and reactivation of abandoned channels within floodplains, operating separately or in combination, are responsible for floodplain reworking and their relatively young age (<250 years). Clearly, braided rivers can construct substantial areas of well developed floodplain.

Contemporary River Landforms and Sediments in an Area of Equatorial Rain Forest, Gulong Mulu National Park, Sarawak

The river Melinau drains a part of Borneo which has a complete cover of equatorial rain forest. The contemporary river landforms and sediments comprise an integrated assemblage of channel and floodplain forms and patterns, extending across a region of mountainous headwaters and extensive alluvial plain. The long profile is adjusted throughout to peak discharge and bedload transport. Bedforms and floodplain forms range from boulder bars in a rock gorge through braided channels with a braided floodplain, braided channels with a fine-grained, predominantly overbank floodplain, to a meandering channel with a fine-grained overbank floodplain. Similar associations exist in the main tributary valleys, but the smaller tributaries which drain the lowland alluvium are different with patterns and sediment characteristics determined by erosion of alluvium deposited initially by the main stream. In all areas of river confluence the minor streams show landform and sediment associations that are determined by the behaviour of the trunk stream. Currently the alluvial section is undergoing aggradation and the channel position is highly unstable in the braided section with the result that there is considerable likelihood of the river shifting catchments to reach the South China Sea some 125 km east of the present river mouth, at the east rather than the west side of Brunei. The Melinau drainage system is considered typical of equatorial rivers in regions where bedrock outcrops and high precipitation events induce extensive mass movement of materials from hillside slopes to the adjacent river channel.

Welsh floodplain studies: II. Application of a qualitative inundation model

The relationship of inundating waters to patterns of floodplain relief is assessed in terms of an inundation model involving types of input, output and transfer processes. This model is applied to floods observed in the rivers Dyfi and Teifi in mid-Wales from 1973–1977. These show complex patterns of actual inundation sequences, involving hysteretic effects in the relationship between river stage and flood extent, and varying effects arising from human floodplain modifications, different rates of stage fluctuation, and the local incidence of floodplain forms. Further sequential studies of inundation for contrasted floodplain types should allow better understanding of floodplain conductivity as an aid to flood-routing procedures and as a help in understanding discharge-inundation relationships.