RIVER PATTERNS AND SPATIAL AND

TEMPORAL TRANSFORMATION MODES

NI Jinren1^{, 4}, WANG Suiji ^{2, 4} and WANG Guangqian ^{3, 4}

ABSTRACT

  Five main modes of spatial transformation of the river patterns and various factors influencing the river systems are discussed in the paper. Representative rivers in China and other countries demonstrate the river pattern transformation in response to some controlling factors. Furthermore, the fluvial processes are studied with historic fluvial deposits in ancient rivers. It is found that there are certain relationships between the modern spatial transformation and the ancient temporal evolution in some river systems. The anastomosing river pattern and its deposits are analyzed with the drilled core samples. Various spatial transformation modes of river patterns have been reported and different temporal evolution modes have been recorded in sediments of the ancient rivers. The braided-meandering transformation, either spatial or temporal, is found to occur more often than other modes. The main factors controlling the spatial transformation and temporal evolution of the river patterns are geotectonic characteristics, sea level fluctuation, climate change, vegetation, composition of sediment in banks and floodplains, flow discharge and sediment load. The order of these factors differs for transformation modes under different conditions.

Key Words: Fluvial river pattern, Spatial transformation mode, Evolution, Braided-meandering transformation

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EROSION AND SEDIMENTATION PROBLEMS

IN SOME ASIAN COUNTRIES

Bingnan LIN, Deyi WU and Zhide ZHOU[1]

ABSTRACT

  Some experience in dealing with sediment problems encountered in some Asian countries is presented in this article.  Some of the problems, such as watershed management and collection of river data, are perhaps unique with developing countries.  Others including the recognition of sediment problems, the need of basin-wide planning of water resources development, the maintenance of reservoir life, the effect of sediment on water qualities and the importance of further research should be of general interests.

Key Words: Sedimentation, Erosion, Asia, Watershed management

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A MATHEMATICAL QUASI-2D OVERLAND FLOW MODEL

 W. SUMMER[2]

ABSTRACT

  A physically-based overland flow model focusing on the movement over complex surfaces is presented, e.g. as the basis for relevant sediment transport modelling. The sediment movement over interrill areas is treated as a two-dimensional ''sheet flow'' process. The relevant conservation equations, for slope gradients over 5% reasonably approximated by the kinematic wave, are locally averaged over an interrill section to account for interacting overland flow towards the rills. The transport in rills is understood as one-dimensional ''channel flow'' using again the kinematic wave approximation as plausible as well as economical solving approach. The locally averaged transport equations for rills and interrill areas are then average along the large-scale distance of a hillslope transect or contour line gaining the expected values of the stochastic parameters. The probability of the occurrence of rills in a combined and averaged rill-interrill transport equation finally reduces the quantitative parameter estimation and computation problem occurring in the point-scale methodology. The space-time variation of the spatial rill density, rill geometry and other stochastic parameters during storm events is assumed as constant - these changes are presently not understood at all - but can be incorporated into the model.

Key Words: Overland flow model, Sediment transport, Kinematic wave approximation, Stochastic parameter, Ecosystem, Watershed management

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DEBRIS FLOW CONTROL BY USING SLIT DAMS

Hui-Pang LIEN[3] and Fang-Wu TSAI[4]

ABSTRACT

  Slit dams have been shown to be effective in trapping coarse sediment and retarding the debris flow.  It is necessary to establish the controlling efficiency and the systematic procedures for designing slit dams, as well as reasonable sites for their location.  Using the principle of mass conservation and laboratory experiments, this paper develops the sediment flow-out ratio and sediment concentration ratio to evaluate the effectiveness of the slit dam, and confirms that sediment concentration ratio increases with an increase in relative spacing and slit density.  The sediment flow-out ratio depends on the sediment concentration ratio and sediment concentration before passing through a slit dam.  For sediment concentration before passing through a slit dam, based upon Bagnold’s concept of the dispersive stress generated by grain collisions in flow and bed shear stress, a semi-empirical relationship is obtained.  Compared with other researchers’ studies, this study is found to be in excellent conformity with theirs, when taking 0.1 as the empirical coefficient.  Besides, height, spacing and location factors affecting a slit dam design for debris flow mitigation are established herein.  Furthermore, the design procedures of a slit dam are provided as a useful tool for controlling the debris flow hazards and are illustrated by a numerical example.

Key Words: Slit dam, Debris flow, Sediment concentration, Sediment flow-out ratio

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THREE DIMENSIONAL MATHEMATICAL MODEL

OF SEDIMENT TRANSPORT IN ESTUARINE REGIONS

-A CASE STUDY OF THE HAIHE RIVER MOUTH

BAI Yuchuan[5,6], SHEN Huanting[6] and HU Shixiong[7]

ABSTRACT

  A 3-D numerical model for simulating tidal flow and sediment transport in the estuarine regions is presented in this paper. The model adopts σ-coordinate system and carries out σ-coordinate transformation to the basic equations, which ensure that all horizontal area has equal numbers of calculation points in vertical direction, this method provides higher distinguishable rate to the flow and sediment transport.  Under theσ-coordinate system, the Galerkin finite-element method is applied for horizontal domain by using a new specially interpolating shape functions, while a finite difference approximation is employed over depth. The model can be used even in the situations with a considerable variety in the water depth in the computational domain. The model is verified by a test for which analytical solutions are available and then applied to simulate the tidal current and sediment transport of the Haihe River Mouth. A modeling system of sediment transport is established for applying to muddy estuary of North China. The comparison with the field data shows that the model can well simulate the evolution process in the estuary.

Key Words:  Three dimensional mathematical model, Estuarine region, σ-coordinate system, Sediment transport

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SIZE DISTRIBUTION OF SUSPENDED SEDIMENT IN

MAXIMUM TURBIDITY ZONE AND ITS RESPONSE TO

TIDAL DYNAMICS IN THE JIAOJIANG RIVER ESTUARY*

LI Bogen[8], XIE Qinchun[8], D. Eisma[9], XIA Xiaoming[8]and LI Yan[8]

ABSTRACT

  This paper discusses the change of the size distribution of suspended sediment, both in the region and over the tidal cycle, by means of statistics and calculations of different size fractions and median diameters. It also indicates that the size distribution is mainly influenced by three factors, i.e. sediment sources, re-suspensions of the bottom sediment and flocculated settlings. Of which, the action of the resuspension is more remarkable during the spring tide than during the neap tide, while the action of the flocculation is more pronounced during the neap tide than during the spring tide.

Key Words: Jiaojiang River estuary, Turbidity maximum, Suspended sediment particle size, Response

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STABLE CHANNEL OBJECTIVE FUNCTION

 Prabhata K. SWAMEE[10]

ABSTRACT

  The stable channel geometry has attracted the attention of hydraulic engineers for quite a long time. Various criteria have been proposed to explain the self-formed geometry of a channel. Based on the optimal cost-structure of flow systems, and using geometric programming, the present investigation formulates an objective function to yield Lacey's stable channel dimensions. The function may be useful to reveal many details of the stable channel geometry.

Key Words: Alluvial streams, Canals, Geometric programming, River morphology, Stable channel, Unit power function

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CONTRIBUTIONS OF TUNNEL EROSION TO SEDIMENT YIELD IN

A SEMI-ARID BASIN IN THE LOESS PLATEAU OF CHINA

 T.X. ZHU[11], Q.G. CAI[12] and S.H.LUK[13]

ABSTRACT

  To evaluate the geomorphic significance of piping and tunnel erosion in basin sediment yield in semi-arid areas, we conducted a field monitoring program in Yangdaogou, a subbasin in the hilly loess region in the Loess Plateau of China in 1989 and 1990. Six tunnel systems were monitored through fifteen flow events. Overall, 57 % of total basin sediment was routed through tunnel systems and net tunnel erosion accounted for about 25% of total sediment production of the basin. This study suggests that tunnel erosion can be highly significant in basin sediment delivery and hillslope development in semi-arid areas.

Key Words:  Tunnel erosion, Loess Plateau, Semi-arid areas

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 MORPHOLOGY AND GROWTH PATTERNS

OF THE USRI RIVER POINT BARS, BIHAR, INDIA[14]

Barendra PURKAIT[15]

ABSTRACT

   Morphology and growth patterns of four point bars of the river Usri, Bihar, India were studied by plane table mapping for two to three successive years before and after the monsoon. In plan, all these bars are crescentic in shape with downstream tail. The central parts of the bars are wider than their two ends. The bar represents aggregate of strips of sediments added laterally. The elevation contour lines demarcate the individual strips by abruptly cutting at the junction of two adjacent strips. In transverse profiles, the strips show up as series of steps. General slope of the bar is towards the adjacent channel though this slope is interrupted by these steps. In 3-dimensions, all these bars exhibit avalanche faces near the downstream end. The longitudinal profile of the bar over this avalanche face resembles the profile of an asymmetrical ripple, the lee face being this avalanche face.  Trenches across the avalanche face reveal cross-stratification with gradation of grain sizes within individual cross-strata, similar to the features generated in front of migrating ripples. All these features suggest that the point bar is a mega ripple (macro bedform) which migrates downstream during the high flood when the entire bar is inundated. SEPM bedform classification also records that dunes are bedforms with wave length greater than 100m and height greater than 5m. The equation of relative discharge as worked out by Sundborg (personal communication) from the Chézy and Manning equations, has been applied to establish a relationship between relative discharge and relative volume of sedimentation in point bars.

Key Words: Morphology, Growth pattern, Point bar, Longitudinal profile, Avalanche face, Cross-stratification, Bedform

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