River Bed Elevation Research Articles

Impacts of the large amount of sand mining on riverbed morphology and tidal dynamics in lower reaches and delta of the Dongjiang River

The sand dredging and its impacts on riverbed evolution and tidal dynamic change in the lower reaches and delta of the Dongjiang River are examined in this paper. The large amount of sand, totally 3.32 billion m3 from 1980 to 2002, was mined from the riverbeds of the lower reaches and delta of the Dongjiang River. Increasing of the channel capacity, lowering of the average riverbed elevation, deepening of the water depth and decreasing of the longitudinal riverbed gradient are the main effects on the riverbed evolution brought by the large amount of sand dredging. Under the strong sand dredging and associated significant riverbed deformation, the notable changes of the tidal dynamic in the lower reaches and delta of the Dongjiang River occurred, including: (1) in the upper reaches of the Dongjiang River delta and lower reaches of the Dongjiang River, tidal level dropped apparently, tidal range widened, flood tidal duration became longer, amplitudes for major tidal components became bigger and tidal dynamics intensified; (2) tidal wave spread faster; and (3) the limits of the tidal level, tidal current and salt water moved upstream.

Do river profiles record along‐stream variations of low uplift rate?

Spatial variations of gradients in landscapes may be used to identify and quantify recent deformation. The problem with doing this is to determine whether tectonic or climatic forcing is responsible for these variations, especially for low uplift rate environments (≪1 mm yr−1) where climate changes may have erased tectonic features. We evaluate the respective contribution of low uplift rate (∼0.1 mm yr−1) and Pleistocene climate oscillations on gradient variations of two comparable river profiles crossing different uplift zones in the southern Upper Rhine Graben. We compare the observed points of discontinuity in river profile (knickpoints) and convex portions (knickzones) with those predicted by a detachment‐limited model that includes stochastic short‐term and cyclic long‐term variations in climate, a bedrock detachment threshold and rock uplift. The detachment‐limited model is chosen as it predicts the development of persistent knickpoints. Differing values of the shear stress exponent, erosion threshold, climate variability and uplift pattern have been checked. Our modeling suggests that climate changes had no significant effects on profiles and that anomalies are more likely due to anticline growth. This surprising result arises from the combination of a very low regional uplift rate and the detachment‐limited assumption. The detachment‐limited model implies an upstream propagation of knickpoints and knickzones generated by uplift at the outlet during dry climate periods of low erosion. The greater the uplift rate, the larger the variations in river bed elevation. Thus, for high uplift rate, knickpoints and knickzones generated by climate oscillations are more likely to hide tectonic features. This result seems counterintuitive because it suggests that tectonic knickzones will be better preserved in low uplift rate environments, provided that the lithology is homogeneous.

COMPUTATIONAL MECHANICS OF A BLOCKING OF GATELESS BOTTOM OUTLET BY DRIFT WOODS

Some flood-control dams have a gateless bottom outlet at a riverbed elevation. A management of this kind of dams is easy, because gate operation is unnecessary and there is almost no environmental problems caused by impounding such as a reservoir sedimentation and water quality changes. The river-bed bottom outlet must be designed to pass a large quantity of driftwood safely during flood events. A three-dimensional free-surface flow is predominant in the neighborhood of a mouth of the outlet. Therefore, the particle method, which can handle a complicated water-surface change explicitly, is effective. In this study, three-dimensional model of floating bodies are proposed to be introduced into the three-dimensional MPS method. A simulation of a blocking phenomena by driftwood at the bottom outlet is carried out.

RIVERBED MANAGEMENT IN MIDDLE REACH OF THE KAMANASHI RIVER

In the middle reach of Kamanashi River, riverbed has a tendency of aggregation. In this study, some development scenarios for maintaining the riverbed elevation in view of the sedimentation trends were examined. The riverbed variations were calculated by using a MIKE21C curvilinear model. First, by comparing calculated results with the actual riverbed variation obtained between 1998 and 2003, the model was calibrated and validated. In addition, plane properties of changes of flow during that period were explained. Next, by using the model, the future riverbed variations starting from 2004 onward under the flow conditions of the 20-year flood return period and under 2 different development scenarios were forecasted. The study result shows that the development scenario 1 where some well planned dredging works are undertaken would be more suitable for riverbed management of this reach.

Soulèvement et plissement tectoniques révélés par analyse mathématique empirique de profils longitudinaux de rivières : un cas à Taiwan

Abstract We analyse longitudinal river profiles in southwestern Taiwan. As all necessary data are not available, a physical modelling of river erosion would be subject to large uncertainties. We thus shortcut this modelling and adopt simple empirical exponential equations giving riverbed elevation as a function of downstream distance. We identify a positive altimetric anomaly, which reveals active uplift of an anticline at the front of the fold-and-thrust belt. To cite this article: J. Angelier, R.-F. Chen, C. R. Geoscience 334 (2002) 1103–1111.

OPTIMAL CONTROL OF RESERVOIR RELEASES TO MINIMIZE SEDIMENTATION IN RIVERS AND RESERVOIRS1

ABSTRACT: An optimal control methodology and computational model are developed to evaluate multi‐reservoir release schedules that minimize sediment scour and deposition in rivers and reservoirs. The sedimentation problem is formulated within a discrete‐time optimal control framework in which reservoir releases represent control variables and reservoir bed elevations, storage levels, and river bed elevations represent state variables. Constraints imposed on reservoir storage levels and releases are accommodated using a penalty function method. The optimal control model consists of two interfaced components: a one‐dimensional finite‐difference simulation module used to evaluate flow hydraulics and sediment transport dynamics, and a successive approximation linear quadratic regulator (SALQR) optimization algorithm used to update reservoir release policies and solve the augmented control problem. Hypothetical two‐reservoir and five‐reservoir networks are used to demonstrate the methodology and its capabilities, which is a vital phase towards the development of a more robust optimal control model and application to an existing multiple‐reservoir river network.

Aquifer modelling of the Ganga-Mahawa sub-basin, a part of the Central Ganga Plain, Uttar Pradesh, India

The Ganga–Mahawa sub-basin, which has an area of 1280 km2 forms the western part of the Central Ganga Plain in the Moradabad and Badaun districts of western Uttar Pradesh, India. The Bundelkhand granite forms the basement complex, overlain unconformably by the upper Vindhyan sequence, which is further overlain by the Neogene (Middle and Upper) Siwaliks and finally by Quaternary alluvium. Four geomorphological units, the Varanasi older alluvial plain, Aligarh older alluvial plain, terrace zones and the Ganga recent floodplain, abandoned channels, channel scars and meander scars represent various landforms. The hydrogeological cross-sections indicate the occurrence of a single aquifer down to 120 m. Some influent seepage from the River Ganga could be seen around Gangeswari, but the rest of the River Ganga is effluent. Groundwater-flow modelling was carried out to assess the degree of Ganga river and aquifer interaction. The River Ganga marks the western boundary; boundaries to the northeast and southeast are set as fixed heads to simulate lateral inflow into and outflow from the sub-basin respectively. The eastern boundary is simulated as a no-flow condition. The Mahawa and Badmar rivers are considered to be effluent. The area modelled is covered by a grid of 34 rows×46 columns with three layers, viz., an unconfined aquifer, an aquitard which is underlain by a semi-confined to confined aquifer. The permeability distribution was inferred from morphometric analysis and pumping tests. Natural recharge due to monsoon rainfall forms the main input. The River Ganga stage data at Ahar, Naora and Ramghat has been used for assigning surface water levels and river bed elevations in the model. Abstraction from all existing deep and shallow tube wells has been assigned as output at various cells. A steady state flow simulation was carried out and calibrated against the June 1986 water level; subsequent transient conditions were calibrated up to May 1995. The computed groundwater balance was comparable to that estimated from field investigations. The aquifer modelling study has attempted to integrate all available information and provided a tool that could be used for predictive simulation. Copyright © 2000 John Wiley & Sons, Ltd.

BED VARIATION AND SEDIMENT BUDGET IN THE DOWNSTREAM REACH OF KIZU RIVER

he present study describes field surveys of bed material, findings based on surveyed data and bed variation in a downstream reach, 0km to 37km, of the Kizu River. Field data realize that the river bed elevation changes longitudinally like riffle-pool bed profiles, coarse gravel are dominant in the riffle region, sand particles are deposited deep in the pool region, and sediment particles of pool region are transported from upstream as over-passing load or as selectively entrained sediment on riffle bed. The formula of grain size distribution which is employed in numerical simulation of river bed variation as the upper boundary condition of the study reach is proposed, taking account for the difference between grain sizes in riffle and pool regions as well as for characteristics of bed load transport such as over-passing load.

Stochastic Theory for Irregular Stream Modeling. II: Solute Transport

A stochastic theory is developed for longitudinal dispersion in natural streams. Irregular variations in river width and bed elevation are conveniently represented as one-dimensional random fields. Longitudinal solute migration is described by a one-dimensional stochastic solute transport equation. When boundary variations are small and statistically homogeneous, the stochastic transport equation is solved in closed-form using a stochastic spectral technique. The results show that large scale longitudinal transport can be represented as a gradient dispersion process described by an effective longitudinal dispersion coefficient. The effective coefficient reflects longitudinal mixing due to flow variation both within the river cross section and along the flow and can be considerably greater than that of corresponding uniform channels. The discrepancy between uniform channels and natural rivers increases as the variances of river width and bed elevation increase, especially when the mean flow Froude number is high.

The morphology of graded gravel rivers: a network perspective

Analytical hydraulic geometry equations are combined with a watershed model to quantify downstrem trends in equilibrium fluvial morphology in gravel streams. The hydraulic geometry equations relaet fluvial morphology to discharge, bedload transport rate and grain size; these independent variables are in turn determined by routing gravel supplied at zero-order basins through a channel network. As gravel is routed downstream, it decreases in size according to an empirical power law. The resulting model specifies the discharge, bedload transport rate, grain size, width, depth, slope and river bed elevation at every point in the drainage basin. When the downstream fining law is calibrated using data from central Pennsylvania, dowstream hydraulic geometry exponents for the width and depth are accurately reproduced. Longitudinal profiles calculated from the model fit observed profiles in largerwatersheds, but are inaccurate in smaller watersheds. The model also reproduces Hack's (1957) empirical result relating slope to the 0.6 power of A/ d s, where A is the drainage basin area and d s is the mean grain size. Unfortunately, the model requires an equilibrium time scale which is unreasonably long, suggesting that the channels of the study area are best described as systems with multiple time scales and multiple rates of response. The model probably is successful because the width and depth have relatively short relaxation times, and also because downstream trends in discharge and sediment transport rate are controlled by network topology. Thus, downstream hydraulic geometry equations do not necessarily provide strong evidence for the equilibrium of fluvial landscapes. Rather, they reflect the constancy of network topology and the rapid response times of width and depth.

Stochastic Theory for Irregular Stream Modeling. Part I: Flow Resistance

A stochastic theory is developed for predicting flow resistance in natural rivers. Irregularly varying river width and bed elevation are represented as one‐dimensional spatial random fields. Large‐scale random flow acceleration and deceleration in response to boundary variations are described by the stochastic differential flow equation. Analytical stochastic flow solutions are developed for the case when boundary variations are small and statistically homogeneous. In particular, closed‐form expressions for the effective flow resistance coefficient and flow variance are obtained. The results indicate that flow resistance in natural rivers is strongly influenced by cross‐sectional nonuniformity and mean flow condition, in addition to relative boundary roughness and mean cross‐sectional shape. The results also show that effective resistance is always greater than uniform resistance in a corresponding mean straight channel. This difference increases as the mean Froude number increases for a given mean bed sl...

Influence of River Bed Elevation Change to Channel Patterns

Influence of River Bed Elevation Change to Channel Patterns

Hovercraft Utilization for the River Bed Sound Investigation at the Time of Flood

We have stated about the outline of river bed observation on a flood caused recently, and also stated about a study that has been started concerning this year's concave bank's water basin. Another statement that we presented here is observation on river bed elevation for wide area or at deep area along concave bank which becomes possible by using a hovercraft.

Hydrology, hydraulics, and geomorphology of the Upper Mississippi River system

The Upper Mississippi River system has been modified with locks, dams, dikes, bank revetments, channel modifications, and dredging to provide a nine-foot navigation channel. These activities have changed the river's characteristics. The historical changes in the hydrologic, hydraulic, and geomorphic characteristics were assessed and related to navigational development and maintenance activities in the Upper Mississippi River system. The hydrologic, hydraulic, and geomorphic features studied include river discharges, stages, sediment transport, river position, river surface area, island surface area, and river bed elevation. Water and sediment transport effects on dredging were also estimated. It was found that the general position of the Upper Mississippi River system has remained essentially unchanged in the last 150 years except for specific man-made developments in the river basin. The stage, velocity, sediment transport, and river and island areas were altered by development of the 2.75-m navigation system. Dredging requirements are strongly related to mean annual water discharge. Years in which water discharges were great were generally the years during which large volumes of sediment needed to be dredged from the channel. The backwater areas are experiencing some deposition. With implementation of erosion-control measures in major tributaries and upland areas, better confinement of disposed dredged materials, and better maintenance practices, the sedimentation and pertinent problems in the main channel, as well as in the backwater areas, may be reduced with time.

Seepage Control Methods in Dam Foundations and Abutments Consist of Recent Volcanic Rocks in Kyushu

Dams constructed on recent volcanic rocks have many difficult problems on foundations and abutments such as strength, permeability and so on. Authors have been conducting case studies on 25 dams constructed on Neogene and Quaternary volcanic rocks in Kyushu District to find out the most relevant seepage control methods for such dams. Obtained results are as follows.From a seepage control viewpoint, geological features of studied dams are classified into following six large groups: 1) Lava, 2) Alternation of lava and pyroclastic rocks, 3) Tuff breccia, 4) Densely welded tuff, 5) Partially welded tuff-“Shirasu”, 6) Old river deposits etc. On the other hand, permeability of rock mass estimating from ground water level at abutments is classified into following three large groups: A) Ground water level at abutments is relatively high, B) Ground water level at abutments is relatively low but higher than river bed elevation, C) Ground water level at abutments is lower than river bed elevation.Considering these six geological features and three permeability characteristics, the most relevant seepage control methods proposed by authors are shown in Table 7. As shown in Table 7 curtain grouting can be applied for a wide variety of geological conditions. Detailed profile of curtain grouting is shown in Table 8. But both Table 7 and 8 are tentative standards at early planning stage, so practical seepage control methods should be selected taking full account of precise geological conditions of each dams and conducting various in situ tests such as grouting test, Lugeon test, pumping test, etc.As recent volcanic rocks in Kyushu are often distributed laterally, problems on seepage control arise from not only dam sites but also overall storage areas. So it is important to investigate geological conditions of overall storage areas and to point out problems. Then the most relevant seepage control methods should be selected for various geological conditions such as consolidation grouting, limb grouting, branket, concrete cutoff, etc.

Simulation of Developmental Process of Interfluvial Slopes with Reference to Graded Form

A mathematical model of slope development based on the combined effects of the subduing and recessional coefficients is reliable when viewed with regard to the characteristics of the graded form that is derived from it. The coefficients express the net effects of wash, creep, and weathering which play an important role in slope development. A series of developmental processes of interfluvial slopes is simulated, on the basis of the model with the boundary condition corresponding to river-bed elevation. The results show that the slope development is separated into the following three phases: (1) an irreversible process, in which an equilibrium state tends to be established, (2) a steady (equilibrium) state, in which a graded (time-independent) form is maintained; and (3) an irreversible process, in which the equilibrium state and the graded form are destroyed and the slope is worn down to a final peneplanation stage.

Deposition of Sediment in Transient Flow

An analytical method of predicting the change in riverbed elevation at transient phases is developed. A computer solution is presented along with the method in order that a sample computation can be made to check the stability and convergency of the method. For channels in which the sediment-laden water can be considered completely homogeneous and in which the velocity can be considered uniform over the cross-section, equations of continuity and the equation of motion of sediment-laden water are presented. Introducing the assumption that the difference between the original bed slope and the new bed slope within a short period is very small, these three basic partial differential equations are successfully reduced to two partial differential equations. The method of characteristics is then employed to solve for the mean velocity, v , and the mean depth of flow, h . In order to estimate the sediment deposit thickness, z , the equation of continuity for sediment is transformed directly into finite-difference form.

Navigation Model Studies of New Ohio River Locks

Utilization of hydraulic models to investigate navigation conditions, arrangements of locks and dams and appurtenant structures, and operation of dam gates; as indicated by model studies old movable-wicket dams, with sills at river bed elevation are being replaced by smaller number of high gated dams which will create deeper and longer pools; each pool will have main lock 110×1200 ft and 110×600 ft auxiliary lock.

Navigation Model Studies of New Ohio River Locks

Utilization of hydraulic models to investigate navigation conditions, arrangements of locks and dams and appurtenant structures, and operation of dam gates; as indicated by model studies old movable-wicket dams, with sills at river bed elevation are being replaced by smaller number of high gated dams which will create deeper and longer pools; each pool will have main lock 110×1200 ft and 110×600 ft auxiliary lock.

SEDIMENTATION IN THE MIDDLE RIO GRANDE VALLEY, NEW MEXICO

Surveys have shown sediment accumulation at a rate of 12,000 acre-feet per year, or 22.5 million tons, from 1936 to 1941 in the Middle Rio Grande Valley of central New Mexico. Throughout most of the upper 133 miles of the Middle Valley, the Rio Grande is confined within a levee-bordered floodway, which was aggraded at a rate equivalent to 0.08 foot per year, or about 1 foot in 12 years. This was due to deposition on the berms between the levees and river, chiefly during major floods in 1937 and 1941, for there was practically no net change in average river-bed elevation through the floodway. The relation between berm and river-bed deposition is believed to be temporary, resulting from the scouring effect of a major flood shortly before the 1941 surveys, for comparisons of water-surface elevations indicate river aggradation from 1917 to 1936 at about the same rate as the 1936–1941 floodway aggradation. About 55 per cent of the 1936–1941 sedimentation was in the lower 14 miles of the valley, between the lower end of the floodway and the head of Elephant Butte Reservoir. This sediment concentration was due chiefly to headward growth of the reservoir delta, several channel avulsions that happened to occur during the short period of record, and perhaps some influence of the floodway levees in confining flood waters and forcing transportation of sediment into the lower part of the valley. The sedimentation and river aggradation are serious problems affecting the future of irrigation, drainage, and flood-protection works essential for the productivity of more than 100,000 acres of agricultural land, and increasing flood hazards for about half the city of Albuquerque, many smaller communities, parts of a transcontinental railway and one of its major branches, and several major highways. The Middle Rio Grande may have been aggrading for centuries, but the rate of sedimentation is believed to have increased greatly since about 1870 as a result of arroyo erosion in the alluvial valleys of many intermittent tributaries. The arroyo erosion apparently was caused by depletion of the natural vegetation and trampling of trails by cattle and sheep. Some combination of detention reservoirs and channel improvements may provide temporary relief, perhaps for many years, but a successful long-term remedial program evidently will require stabilization of the eroding arroyos. Unfortunately it has not yet been demonstrated that effective measures for arroyo stabilization are practicable and economically feasible.