Alluvial Channels Research Articles

Evaluation of genetic programming-based models for simulating friction factor in alluvial channels

Summary The bed resistance is one of the most complex aspects of water flow studies in natural streams. Most of the existing non-linear formulas for describing alluvial channel flows are based on dimensional analysis and statistical fitting of data to the parameters considered in the functional relationships implicitly, which are partially valid. The present study aims at developing genetic programming (GP) – based formulation of Manning roughness coefficient in alluvial channels. The training and testing data are selected from original experiments, performed in a hydraulic flume using a sand mobile bed. A comparison was also made between GP and traditional nonlinear approaches of resistance modeling. The obtained results revealed the GP capability in modeling resistance coefficient of alluvial channels’ bed.

Shallow sediment transport flow computation using time-varying sediment adaptation length

Based on the common approach, the adaptation length in sediment transport is normally estimated astemporally independent. However, this approach might not be theoretically justified as the process of reaching the sediment transport equilibrium stage is affected by the flow conditions in time, especially for fast moving flows, such as scour-hole developing flows. In this study, the two-dimensional (2D) shallow water formulation together with a sediment continuity-concentration (SCC) model were applied to flow with mobile sediment boundary. A time-varying approach was proposed to determine the sediment transport adaptation length to simulate the sediment erosion-deposition rate. The proposed computational model was based on the Finite Volume (FV) method. The Monotone Upwind Scheme of Conservative Laws (MUSCL)-Hancock scheme was used with the Harten Lax van Leer-contact (HLLC) approximate Riemann solver to discretize the FV model. In the flow applications of this paper, a highly discontinuous dam-break, fast sediment transport flow was used to calibrate the proposed time-varying sediment adaptation length model. Then the calibrated model was further applied to two separate experimental sediment transport flow applications documented in the literature, i.e. a highly concentrated sediment transport flow in a wide alluvial channel and a sediment aggradation flow. Good agreement with the experimental data were obtained with the proposed model simulations. The tests prove that the proposed model, which was calibrated by the discontinuous dam-break bed scouring flow, also performed well to represent rapid bed change and steady sediment mobility conditions.

Mapping past and recent landscape modifications in the Lagoon of Venice through geophysical surveys and historical maps

Abstract The Lagoon of Venice (Italy), the biggest lagoon in the Mediterranean, surrounds the historical city of Venice. Its fate is closely intertwined with that of the city, which is itself endangered today by sea level rise. Major modifications are ongoing to protect the city from flooding with the building of mobile barriers at the lagoon inlets. Other plans exist for future dredging of a large navigation channel close to the city center. In this context, a large geophysical survey explored the lagoon sub-bottom. In this study we reconstructed past and recent landscapes extending from the period before the creation of the lagoon up to present day. Crossing our data with environmental records provided by the city's historical archives, allowed several phases of the lagoon's evolution to be distinguished. Using radiocarbon dating and acoustical sub-bottom reconstruction, we mapped a dense network of palaeochannels corresponding to different hydrologic conditions. One of these palaeochannels related to the alluvial channel that crossed the Venice subsoil. Overall, the results of this study show that the number of channels decreased substantially over the centuries. Explanations for this reduction include natural causes, such as the increase of the mean sea level and natural subsidence, as well as human activities, such as artificial diversion of rivers and modifications of the inlets. Comparison with historical maps showed that this tendency for fewer channels increased dramatically in the last century.

Morphology of small, discontinuous montane meadow streams in the Sierra Nevada

Various fluvial geomorphic models have been developed to characterize the relationships between planform and bedform features of large alluvial channels; however, little information exists for meadow channel morphology. Field investigation of seven narrow, low-energy meadow stream reaches in the northern Sierra Nevada range of California revealed similarities and differences to larger alluvial channels. The average radius of curvature to channel width ratio (5.54) of the meadow streams was almost double that of larger alluvial streams (3.1), with a standard deviation of 4.66. Average meander wavelength to channel width ratio (22.43) was almost triple that of typical alluvial streams (8.5), with a standard deviation of 16.80. Bedform features occurred at an average of 6.72 channel widths, similar to typical pool–riffle spacing of 5-7 channel widths. Grass sod connected a series of scour pools, providing the same energy drop function as riffles or steps. Results suggest that bedform regularity is similar to typical pool–riffle systems, especially as we move to larger watersheds and higher precipitation and runoff, but planform features are less developed and highly influenced by vegetation. Restoration efforts can benefit from considering how planform and bedform channel patterns develop in these meadows.

An experimental investigation to calculate flow resistance in a steep river

High gradient streams are characterised by steep slopes, coarse bed material with non-uniform size distribution, flash floods and flow depths of magnitude comparable to the bed material size. Another characteristic of high gradient streams is development of armor layers with cobbles and boulders. In these streams, it is often difficult to measure water discharge or flow velocity directly, particularly at high flows; instead, it is necessary to use the indirect methods in which flow velocity and discharge is calculated using a flow resistance predictor. The problem of predicting flow resistance in high gradient alluvial channels with sufficient accuracy is of great interest to hydraulic engineers and is essential for the effective design of water-resources related projects. In this paper, the flow resistance of a steep river with coarse gravel bed material has been investigated by using 75 accurate measured hydraulic data in a relatively wide flume with different coarse gravel bed materials, flow rates and longitudinal slopes. The main purpose of this article reviews the ability of the resistance empirical equations to predict mean flow velocity in gravel river beds. Also, based on measured data, a new equation to predict the flow resistance in steep river has been proposed. This equation along with the other empirical equations has been used to predict the parameters introducing the flow resistance in steep rivers including Manning roughness coefficient, mean flow velocity and Darcy-Weisbach roughness coefficient. The results were compared with measured and calculated hydraulic data and the best equations to predict the Manning’s roughness coefficient in a steep river with coarse gravel bed material are introduced. Finally in order to show the generality and accuracy of the proposed equation, it has been used for the other field data and the results have been discussed.

Backwater profile during barrage gate operation for variable pond levels

The backwater profile plays an important role in the design and performance of any hydraulic structures. Various methods are already in practice for the estimation of backwater profile. Present study attempts to estimate the backwater profiles due to barrage gate operations considering variable pond levels for alluvial channels using both theoretical analysis and laboratory model tests. The physical model tests were conducted for different pond levels and discharges. Backwater profiles obtained from theoretical and physical model tests data were compared with those obtained from available methods. Finally, a non-dimensional relationship has been established between the pond level and the backwater length.

Magnetic and seismic refraction survey for site investigation of an urban expansion site in Abha District, Southwest Saudi Arabia

Ground magnetic and seismic refraction survey is carried out on an urban extension site in the southwest of Ahud Rufeidah town, southwest Saudi Arabia. The purpose of the survey was to map the surface topography of the bedrock and thickness of the alluvium overburden. The ground magnetic survey based on an assumption that the alluvial sediments is less or non-magnetic relative to the underlying gneiss basement rock. In this context, a total of 3,750 survey stations were measured along 40 east–west survey profiles, striking roughly perpendicular to the extension of the expected structures. In addition, 13 seismic refraction spreads were conducted along four seismic survey profiles across the expected pathways of buried alluvial channels in order to provide additional details about the depth and boundaries of the buried channels. The ground magnetic survey results show the presence of a basin combining two sub-basins filled with alluvium sediments that occupy the middle area of the investigated site. This basin is a closed basin that has no outlet, and there are four small and narrow channels that convey water and sediments from the eastern and southern hills into sub-basins. These channels are represented by narrow and elongated low magnetic anomalies extending towards the basinal area. The thickness of the alluvial sediments is verified using seismic refraction survey that indicates a greater thickness, exceeds 20 m, of low velocity sedimentary overburden inside the interpreted sub-basins and surrounding buried alluvial channels. These soft alluvial sediments can be deceptive and dangerous for urban foundations.

CONCEPTUAL MODEL FOR ASSESSING THE CHANNEL CHANGES UPSTREAM FROM DAM RESERVOIR

Abstract Most of geomorphological studies related to the impact of dam reservoirs are focused on its effect on the river channels downstream of dams. However, little is known about the evolution of river channels upstream of dams (in backwater areas). In this paper I propose a conceptual model of alluvial channel changes in the backwater. It assumes that three conditions of a channel that may occur in the pre-dam period. These are sediment supply deficit, balance and surplus compared to the river transport capacity. These conditions can be strengthened, stable or weakened. In the post-dam period these conditions is modified by the influence of the dam reservoir, which may affect the trajectory of channel changes, as well as its response and adjustment time in the backwater. The adjustment of channel parameters, in spite of the reservoir operation mode, also depends on the local river, reach characteristics and the magnitude and frequency of floods. Therefore, the use of an appropriate reference reach is inherent in this model. The model suggests that some adverse changes to backwater channels degraded by the regulation can be hampered. The model is based on Lane’s balance concept. It may be useful for explaining and predicting channel changes upstream of reservoirs and forming hypotheses in other studies.

Continental-scale relationship between bankfull width and drainage area for single-thread alluvial channels

We explore the bankfull width (Wbf) versus drainage area (Ada) relationship across a range of climatic and geologic environments and ask (1) is the relationship between ln(Wbf) and ln(Ada) best described by a linear function and (2) can a reliable relationship be developed for predicting Wbf with Ada as the only independent variable. The principal data set for this study was compiled from regional curve studies and other reports that represent 1018 sites (1 m ≤ Wbf ≤ 110 m and 0.50 km2 ≤ Ada ≤ 22,000 km2) in the continental United States. Two additional data sets were used for validation. After dividing the data into small, medium, and large-size basins which, respectfully, correspond to Ada < 4.95 km2, 4.95 km2 ≤ Ada < 337 km2, and Ada ≥ 337 km2, regression lines from each data set were compared using one-way analysis of covariance (ANCOVA). A second ANCOVA was performed to determine if mean annual precipitation (P) is an extraneous factor in the Wbf versus Ada relationship. The ANCOVA results reveal that using Ada alone does not yield a reliable Wbf versus Ada relationship that is applicable across a wide range of environments and that P is a significant extraneous factor in the relationship. Considering data for very small basins (Ada ≤ 0.49 km2) and very large basins (Ada ≥ 1.0 × 105 km2) we conclude that a two-segment linear model is the most probable form of the ln(Wbf) versus ln(Ada) relationship. This study provides useful information for building complex multivariate models for predicting Wbf.

Geologic and physiographic controls on bed-material yield, transport, and channel morphology for alluvial and bedrock rivers, western Oregon

The rivers of western Oregon have diverse forms and characteristics, with channel substrates ranging from continuous alluvial gravel to bare bedrock. Analysis of several measurable morphologic attributes of 24 valley reaches on 17 rivers provides a basis for comparing nonalluvial and alluvial channels. Key differences are that alluvial reaches have greater bar area, greater migration rates, and show systematic correlation among variables relating grain size to bed-material transport capacity. We relate these differences between channel types to bed-material transport rates as derived from a coupled regional analysis of empirical sediment yield measurements and physical experiments of clast attrition during transport. This sediment supply analysis shows that overall bed-material transport rates for western Oregon are chiefly controlled by (1) lithology and basin slope, which are the key factors for bed-material supply into the stream network, and (2) lithologic control of bed-material attrition from in-transport abrasion and disintegration. This bed-material comminution strongly affects bed-material transport in the study area, reducing transport rates by 50%–90% along the length of the larger rivers in the study area. A comparison of the bed-material transport estimates with the morphologic analyses shows that alluvial gravel-bed channels have systematic and bounding relations between bed-material transport rate and attributes such as bar area and local transport capacity. By contrast, few such relations are evident for nonalluvial rivers with bedrock or mixed-bed substrates, which are apparently more influenced by local controls on channel geometry and sediment supply. At the scale of western Oregon, the physiographic and lithologic controls on the balance between bed-material supply and transport capacity exert far-reaching influence on the distribution of alluvial and nonalluvial channels and their consequently distinctive morphologies and behaviors—differences germane for understanding river response to tectonics and environmental perturbations, as well as for implementing effective restoration and monitoring strategies.

Non-uniform Sediment Transport Estimation in Non-equilibrium Situations: Case Studies

Quantitative estimate of sediment transport in alluvial channels is one of the most important task in river engineering. Even today, numerical models of sediment transport processes are confronted with some difficulties, often of conceptual nature. One of these difficulties is the simulation of non-uniform sediment transport in non-equilibrium situations, which requires the characterization of the ability of the alluvial system to immediately overcome the variations of the sediment boundary conditions. In this work a 1-D numerical model, which includes a new expression of the so-called “adaptation coefficient”, has been applied to test its capability to simulate the transient bed profiles. Specifically, the model has been applied to predict bed- level changes due to sediment overloading and sediment cut-off. The model's application to literature study cases (used by other researchers to assess coupled models) has shown that it gives reasonable results and, thus, it appears suitable for practical applications

Two Dimensional Finite Element Model to Calculate the Influence of Channel Width Variation in Alluvial Channels on Bed Transport Capacity With Constant Value of Manning Coefficient

The effect of change of channel width on bed load and transport load of sedimentation for given discharge are studied. The transport load is a mathematical function of change in channel width . The phenomena of transport load of sedimentation in a meandering alluvial channels are significant problems in river engineering and important factor that effects on the works of river control . The finite element model is developed to determine the effect of change of channel width on bed load capacity to ( 2.5 km ) length of the Euphrates river within hit city used value of manning roughness is ( 0.028 ). Calculated the optimum width actualized maximum transport capacity .

Scale-dependent interactions between wood and channel dynamics: Modeling jam formation and sediment storage in gravel-bed streams

[1] A stochastic model is used to investigate how the geomorphic function of wood changes with watershed scale, assuming wood recruitment occurs due to the mortality of individual trees, not to mass recruitment events such as landslides or episodic bank erosion. The model replicates the downstream decline in total wood load observed in the field, but predicts that the functional wood load peaks in channels having bankfull widths about 33% of the characteristic riparian tree height. The model also predicts that the greatest potential impact of jams on channel pattern—both in terms of sediment stored behind individual jams and the potential for jams to trigger avulsions—will typically be associated with channel widths between 25% and 67% of the riparian tree height. The simulation results are used to refine the categories that describe wood in alluvial channels, and the equivalent terms that describe the size of streams with forested riparian areas: small channels (or channels with large wood) are associated with widths less than 25% of the tree height; large channels (or channels with small wood) are associated with widths greater than 67% of tree height; and medium channels (or channels with intermediate wood) have widths between 25% and 67% of the tree height. We surmise that large wood acts primarily to store bed material (in small channels); intermediate wood tends to form channel-spanning jams, which can induce channel avulsions and create anabranched channel patterns (in medium channels); and small wood may increase the morphologic diversity, but does not store significant quantities of bed material or form channel-spanning jams capable of inducing stream avulsions (in large channels).

Thresholds of stability in incised “Anthropocene” landscapes

Abstract Channel incision occurs in the “Anthropocene,” where natural river processes and climate variation increasingly interact with human activity. Causes of “Anthropocene” incision include landuses that change the ratio of discharge to sediment load, lower baselevels, or human activities that otherwise alter fluvial systems, such as channelization. This paper reports a field study of an alluvial channel incised into valley fill within the northern Coast Ranges of California. At this site, channel slope adjustments associated with incision, indicated by bank heights of ∼5–8 m, increased transport capacity and excess shear stress by over 20%. The incision exposed Holocene valley fill in eroding channel banks. Results of field surveys enabled development of a dimensionless metric “relative incision,” ht/de, that quantifies the ratio of terrace height (ht) relative to effective flow depth (de). The ratio is predicted to be near a threshold value of 1.0 in stable alluvial channels; in incised alluvial channels the ratio is predicted to exceed 1.0. Further application and testing will provide information to aide in managing incised systems where complex feedbacks in coupled human–landscape systems may promote or dampen incision.

Classification of freshwater ice conditions on the Alaskan Arctic Coastal Plain using ground penetrating radar and TerraSAR-X satellite data

Arctic freshwater ecosystems have responded rapidly to climatic changes over the last half century. Lakes and rivers are experiencing a thinning of the seasonal ice cover, which may increase potential over-wintering freshwater habitat, winter water supply for industrial withdrawal, and permafrost degradation. Here, we combined the use of ground penetrating radar (GPR) and high-resolution (HR) spotlight TerraSAR-X (TSX) satellite data (1.25 m resolution) to identify and characterize floating ice and grounded ice conditions in lakes, ponds, beaded stream pools, and an alluvial river channel. Classified ice conditions from the GPR and the TSX data showed excellent agreement: 90.6% for a predominantly floating ice lake, 99.7% for a grounded ice lake, 79.0% for a beaded stream course, and 92.1% for the alluvial river channel. A GIS-based analysis of 890 surface water features larger than 0.01 ha showed that 42% of the total surface water area potentially provided over-wintering habitat during the 2012/2013 winter. Lakes accounted for 89% of this area, whereas the alluvial river channel accounted for 10% and ponds and beaded stream pools each accounted for <1%. Identification of smaller landscape features such as beaded stream pools may be important because of their distribution and role in connecting other water bodies on the landscape. These findings advance techniques for detecting and knowledge associated with potential winter habitat distribution for fish and invertebrates at the local scale in a region of the Arctic with increasing stressors related to climate and land use change.

Decimeter‐scale in situ mapping of modern cross‐bedded dune deposits using parametric echo sounding: A new method for linking river processes and their deposits

Collecting high‐resolution data to quantify the sedimentary architecture within contemporary alluvial channels remains one of the outstanding challenges in fluvial sedimentology. Here, we present data collected using a new geophysical method, the parametric echo sounder (PES), which can meet this challenge. From surveys over a field of sand dunes in the Río Paraná, Argentina, we demonstrate the unique ability of PES to image the subsurface structure within active channels at a decimetric resolution. These data reveal the bounding surfaces between bars and dunes, as well as the foresets and reactivation surfaces within them. This provides quantitative in situ data for recent work that suggests cross‐strata preserved by dunes may be more related to flow depths less than the commonly assumed bankfull level. These surveys demonstrate that PES can provide hitherto unobtainable data from alluvial channels and presents significant opportunities for more detailed coupled studies of fluvial processes and their deposits.

Experimental study on coarse grain saltation dynamics in bedrock channels

AbstractSaltation of bed load particles on bedrock surfaces is important for landscape evolution and bedrock incision in steep landscapes. However, few studies have investigated saltation in bedrock channels where, unlike alluvial channels, the bed roughness height and the sediment size may be independent. To address this data gap, we measured the saltation hop height, hop length, and velocity of gravel saltating over a planar bed using 80–160 readings from high‐speed photography and direct measurements. Two separate dimensional analyses are used: one leading to a bed shear stress scaling and another leading to a Froude number (Fr) scaling. Our new saltation data coupled with numerous data from previous studies suggest that both shear stress and Fr‐scaling analyses are valid in characterizing bed load saltation dynamics with bed roughness ranging from smooth to alluvial beds. However, the Fr approach has the advantages that (1) there is no need to estimate a critical Shields stress , which alone can vary up to 2 orders of magnitude (e.g., 0.001–0.1) due to changes in relative bed roughness and slope and (2) the Fr‐based scaling fits the saltation data set better in a least squares sense. Results show that the saltation velocity of bed load is independent of grain density and grain size and is linearly proportional to flow velocity. Saltation height has a nonlinear dependence on grain size. Saltation length increases primarily with flow velocity, and it is inversely proportional to submerged specific density. Our results suggest that either or bed roughness coefficient must be properly estimated to yield accurate results in saltation‐abrasion models.

LIVE BED SCOUR AT A SIDE‐WEIR INTERSECTION LOCATED ON AN ALLUVIAL CHANNEL

ABSTRACTThis study aims to obtain both the depth of clear water and live bed scour around side‐weirs located on an alluvial channel for sharp‐crested side‐weirs under subcritical flow conditions. At the downstream end of the side‐weir, a longitudinal bar in the middle of the channel and a scour hole close to the outer bank are formed because of changes in shear stress field and velocities. Dimensionless depth of scour increases with an increase of the dimensionless parameters of flow depth, side‐weir length and sediment size. The dimensionless depth of scour increases almost linearly with the dimensionless parameter of flow velocity for clear water scour, and it decreases for live bed conditions at the straight channel due to the decreasing deviation angle and kinetic energy toward the side‐weirs and bed material coming from upstream of the main channel. Because of the effect of the helicoidal flow created by bends, the dimensionless depth of scour fluctuated with the dimensionless parameter of flow velocity at the maximum value of clear water scour for movable bed conditions at a bend. At the 30° section of a bend, the equilibrium depth of scour is achieved as maximum values due to the deviation on the path of maximum velocity towards the outer bank. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Assessment of waterlogging in agricultural megaprojects in the closed drainage basins of the Western Desert of Egypt

Abstract. This paper investigates the development of waterlogging in the cultivated and arable areas within typical dryland closed drainage basins (e.g. the Farafra and Baharia Oases), which are located in the Western Desert of Egypt. Multi-temporal remote sensing data of the Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper (ETM+) were collected and processed to detect the land cover changes; cultivations, and the extent of water ponds and seepage channels. The Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) has been processed to delineate the catchment morphometrical parameters (i.e. drainage networks, catchment divides and surface areas of different basins) and to examine the spatial distribution of cultivated fields and their relation to the extracted drainage networks. The soil of these closed drainage basins is mainly shallow and lithic with high calcium carbonate content; therefore, the downward percolation of excess irrigation water is limited by the development of subsurface hardpan, which also saturates the upper layer of soil with water. The subsurface seepage from the newly cultivated areas in the Farafra Oasis has revealed the pattern of buried alluvial channels, which are waterlogged and outlined by the growth of diagnostic saline shrubs. Furthermore, the courses of these waterlogged channels are coinciding with their counterparts of the SRTM DEM, and the recent satellite images show that the surface playas in the downstream of these channels are partially occupied by water ponds. On the other hand, a large water pond has occupied the main playa and submerged the surrounding fields, as a large area has been cultivated within a relatively small closed drainage basin in the Baharia Oasis. The geomorphology of closed drainage basins has to be considered when planning for a new cultivation in dryland catchments to better control waterlogging hazards. The "dry-drainage" concept can be implemented as the drainage and seepage water can be conveyed through the inactive alluvial channels into certain abandoned playas for evaporation.

Regime theories in gravel‐bed rivers: models, controlling variables, and applications in disturbed Italian rivers

AbstractDownstream hydraulic geometry relationships describe the shape of alluvial channels in terms of bankfull width, flow depth, flow velocity, and channel slope. Recent investigations have stressed the difference in spatial scales associated with these variables and thus the time span required for their adjustment after a disturbance. The aim of this study is to explore the consequences in regime models considering the hypothesis that while channel width and depth adjust quickly to changes in water and sediment supply, reach slope requires a longer time span. Three theoretical models were applied. One model incorporates an extremal hypothesis (Millar RG. 2005. Theoretical regime equations for mobile gravel‐bed rivers with stable banks. Geomorphology 64: 207–220), and the other two are fully physically based (Ikeda S, Parker G, Kimura Y. 1988. Stable width and depth of straight gravel rivers with heterogeneous bed materials. Water Resources Research 24: 713–722; Parker G, Wilcock PR, Paola C, Dietrich W, Pitlick J. 2007. Physical basis for quasi universal relations describing bankfull hydraulic geometry of single‐thread gravel‐bed rivers. Journal of Geophysical Research 112, DOI: 10.1029/2006JF000549). In order to evaluate the performance of models introducing the slope as an independent variable, we propose two modifications to previous models.The performance of regime models was tested against published data from 142 river reaches and new hydraulic geometry data from gravel‐bed rivers in Patagonia (Argentina) and north‐eastern Italy.Models that assume slope as a control (Ikeda et al., 1988; or Millar, 2005) predict channel depth and width reasonably well. Parker et al.'s (2007) model improved predictions because it filters the scatter in slope data with a relation slope–discharge. The extremal hypothesis model of Millar (2005) predicts comparably to the other physically based models.Millar's model was chosen to describe the recent changes in the Piave and Brenta rivers due to human intervention – mainly in‐channel gravel mining. The change in sediment supply and recovery was estimated for these rivers. This study supports the interpretation that sediment supply is the key factor guiding morphological changes in these rivers. Copyright © 2013 John Wiley &amp; Sons, Ltd.