Bankfull Depth Research Articles

Variable Shields number model for river bankfull geometry: bankfull shear velocity is viscosity-dependent but grain size-independent

ABSTRACTThe bankfull geometry of alluvial rivers is thought to be controlled by water and sediment supply, and characteristic sediment size. Here we demonstrate a novel finding: when bankfull shear velocity and bankfull depth are correlated against bed material grain size and bed slope, they are to first order independent of grain size and dependent on water viscosity. We demonstrate this using a similarity collapse for bankfull Shields number as a function of slope and grain size, obtained with data for 230 river reaches ranging from silt-bed to cobble-bed. Our analysis shows that bankfull Shields number increases with slope to about the half power. We show that the new relation for bankfull Shields number provides more realistic predictions for the downstream variation of bankfull characteristics of rivers than a previously used assumption of constant bankfull Shields number.

Prediction of Annual Streambank Erosion for Sequoia National Forest, California

AbstractMany bank erosion models have limitations that restrict their use in wildland settings. Scientists and land managers at the Sequoia National Forest would like to understand the mechanisms and rates of streambank erosion to evaluate management issues and post‐wildfire effects. This study uses bank erosion hazard index (BEHI) and near‐bank stress (NBS) methods developed in Rosgen (2006 Watershed Assessment of River Stability and Sediment Supply [WARSSS]) for predicting streambank erosion in a geographic area that is dominated by colluvium and in which streambank erosion modeling has not been previously evaluated. BEHI evaluates bank susceptibility to erosion based on bank angle, bank and bankfull height, rooting depth and density, surface protection, and stratification of material within the banks. NBS assesses energy distribution against the bank measured as a ratio of bankfull near‐bank maximum depth to mean bankfull depth. We compared BEHI classes and NBS to actual bank erosion measured from 2008 to 2012. This index predicted streambank erosion with clear separation among BEHI ratings with R2 values of 0.76 for extreme, 0.37 for high/very high, 0.49 for moderate, and 0.70 for low BEHI. The relationships between measured erosion and BEHI extend the application of BEHI/NBS to a new region where they can inform management priorities, afforestation, stream/riparian restoration projects, and potentially burned area rehabilitation.

Log step and clast interactions in mountain streams in the central Cascade Range of Washington State, USA

Field surveys of log steps in high-gradient streams investigate the interactions between log steps and clasts to assess whether second-growth wood can form log steps capable of diverting flow and acting as significant roughness elements similar to those formed by old-growth wood. We measure the functional log diameter and step height, as well as visually estimate the clast size in the step of 102 log steps in 15 high-gradient streams in the central Cascade Range of Washington, USA. We compare step height data to measurements of maximum bankfull depth to measure the capability of log steps to divert flow relative to their clast size and log diameter. Step height positively correlates with functional log diameter (p<0.001) and clast size. Additionally, a threshold clast size exists above which functional log diameter more strongly correlates with step height, indicating that clast size can limit step height. We find that steps with cobble or larger (>64mm) clasts in the step are 3.7 times more likely to reach or exceed bankfull depth than steps with smaller than cobble clasts in the step. We conclude that it is necessary for steps formed by small logs to have large clasts in the step in order to reach bankfull depth, act as significant roughness elements, and create flow diversions, implying that the potential for second-growth forests to maintain log steps similar to those formed by old-growth logs depends on the size of bed material.

Channel-reach morphology controls of headwater streams based in flysch geologic structures: An example from the Outer Western Carpathians, Czech Republic

A detailed measurement of 93 channel reaches that were classified with an adjusted Montgomery–Buffington (1997) reach-scale system provided comprehensive information of approximately 9 at-a-reach parameters: the channel gradient, the bankfull width, the bankfull depth, d90, the percentage of resistant rocks in the bed sediment, the number of pieces of large woody debris, valley confinement, direct sediment inputs and the presence of fluvial accumulations in the stream channel. In addition, the quantified intensity of sediment transport (i.e. ratio between sediment supply and transport capacity in longitudinal stream profiles) during flood events has been estimated by the one-dimensional bedload transport model (TOMSED), which was validated in two local streams. The principal component analysis of the at-a-reach parameters did not reveal significant groups of channel-reach morphologies; thus, the selected parameters that exclude sediment transport dynamics within stream longitudinal profile cannot reliably distinguish or predict individual channel reach morphologies. Nevertheless, the channel gradient represented the most significant single explanatory variable for stepped-bed morphologies. The addition of bedload transport parameters demonstrated that limited sediment supply streams and streams with limited transport capacities featured different successions of the channel reach morphologies in terms of the channel gradient and, subsequently, the fluvial continuity. The bedrock-cascades and step-pools were significant for the first case, whereas cascade and step-rapid morphology often occurred in higher sediment supply conditions.

Morphological patterns of headwater streams based in flysch bedrock: Examples from the Outer Western Carpathians

A channel-reach morphology of mountain headwater streams results from the balance between fluvial and hillslope processes. A large dataset (n=102) of channel-reaches coming out from flysch mid-mountains was evaluated and compared with streams occurring in other environments. Two new channel-reach morphologies were distinguished with respect to their differences in the ratio of sediment supply to transport capacity: bedrock-cascades and step-rapids. The chaining of channel-reach morphologies by gradient criteria and the parameters of channel gradient and bankfull width related to the basin area show similar trends, when compared to other recent studies. By contrast, bankfull depth indicates its independence on increasing basin area. Significant downstream coarsening of bed material occurs only in alluvial stepped-bed morphologies (cascades, step-pools and step-rapids) mainly due to strong slope–channel coupling processes. Moreover, the specific ratio of bankfull width and d90 predicts the interlocking of boulder steps as well as well-developed step-pool morphology. An amount of large woody debris is closely related to the activity of strict local forest management in evaluated channels, as a significant decrease in large woody debris pieces is observed with increasing basin area.

Response of reach-scale bankfull channel geometry to the altered flow and sediment regime in the lower Yellow River

The channel geometry in an alluvial river usually varies in response to the altered flow and sediment regime from upstream damming. Bankfull channel dimensions have significantly adjusted along the lower Yellow River (LYR) because of recent channel degradation caused by the operation of the Xiaolangdi Reservoir, which has led to longitudinal variability in the cross-sectional geometry. Therefore, describing the bankfull channel geometry of the LYR is more representative using reach-averaged variables. In this study, an integrated approach is proposed for calculating reach-scale bankfull channel dimensions based on the measured section-scale bankfull geometry in the LYR, and the proposed approach integrates a geometric mean based on the log-transformation with a weighted average based on the spacing between two consecutive sections. The post-flood reach-scale bankfull channel dimensions in three different channel-pattern reaches of the LYR were then calculated annually during the period from 1999 to 2012 using the proposed approach and the surveyed post-flood profiles at 91 sedimentation sections. Calculated results indicate that the channel geometry changed in the components of bankfull width and depth in the braided reach, with an increase of about 400m in the reach-scale bankfull width over the past 13years; and the channel evolution occurred mainly in the component of bankfull depth in the transitional and meandering reaches, with the increased reach-scale bankfull depths of 2.4 and 1.8m, respectively. Finally, empirical relationships for the three reaches of the LYR were developed between the reach-scale bankfull channel dimensions and the previous four-year average discharge and incoming sediment coefficient during flood seasons, with higher correlations being obtained when calibrated by the measurements from 1999 to 2010. Furthermore, the developed relations were also verified by the observed data in 2011 and 2012, with encouraging results.

Conceptual Model for Simulating the Adjustments of Bankfull Characteristics in the Lower Yellow River, China

We present a conceptual model for simulating the temporal adjustments in the banks of the Lower Yellow River (LYR). Basic conservation equations for mass, friction, and sediment transport capacity and the Exner equation were adopted to simulate the hydrodynamics underlying fluvial processes. The relationship between changing rates in bankfull width and depth, derived from quasiuniversal hydraulic geometries, was used as a closure for the hydrodynamic equations. On inputting the daily flow discharge and sediment load, the conceptual model successfully simulated the 30-year adjustments in the bankfull geometries of typical reaches of the LYR. The square of the correlating coefficient reached 0.74 for Huayuankou Station in the multiple-thread reach and exceeded 0.90 for Lijin Station in the meandering reach. This proposed model allows multiple dependent variables and the input of daily hydrological data for long-term simulations. This links the hydrodynamic and geomorphic processes in a fluvial river and has potential applicability to fluvial rivers undergoing significant adjustments.

A simple global river bankfull width and depth database

[1] Hydraulic and hydrologic modeling has been moving to larger spatial scales with increased spatial resolution, and such models require a global database of river widths and depths to facilitate accurate river flow routing. Hydraulic geometry relationships have a long history in estimating river channel characteristics as a function of discharge. A simple near-global database of bankfull widths and depths (along with confidence intervals) was developed based on hydraulic geometry equations and the HydroSHEDS hydrography data set. The bankfull width estimates were evaluated with widths derived from Landsat imagery for reaches of nine major rivers, showing errors ranging from 8 to 62% (correlation of 0.88), although it was difficult to verify whether the satellite observations corresponded to bankfull conditions. Bankfull depth estimates were compared with in situ measurements at sites in the Ohio and Willamette rivers, producing a mean error of 24%. The uncertainties in the derivation approach and a number of caveats are identified, and ways to improve the database in the future are discussed. Despite these limitations, this is the first global database that can be used directly in hydraulic models or as a set of constraints in model calibration.

Predictability of In-Stream Physical Habitat for Wisconsin and Northern Michigan Wadeable Streams Using GIS-Derived Landscape Data

AbstractQuantifying spatial patterns of physical and biological features is essential for managing aquatic systems. To meet broad-scale habitat assessment and monitoring needs, we evaluated the feasibility of predicting 25 in-stream physical habitat measures for wadeable stream reaches in Wisconsin and northern Michigan using geographic information system (GIS) derived stream network and landscape data. Using general additive modeling and boosting variable selection, predictions of reasonable accuracy were obtained for 10 widely used in-stream habitat measures, including bankfull depth and width, conductivity, substrate size, sand substrate, thalweg water depth, wetted width, water depth, and width-to-depth ratio. Biased predictions were obtained for habitat measures such as bank erosion, large woody debris, fish cover, canopy shading, and substrate embeddedness. Model predictions for many commonly-used habitat variables were judged acceptable based on several criteria, including correspondence between prediction errors and observed inter-annual and inter-site variability in habitat measures and agreement in correlation analyses of fish assemblage metric data with both predicted and observed values. Prediction of physical habitat variables from widely available GIS datasets represents a potentially powerful and cost-effective approach for broad-scale (e.g., multi-state, national) assessment and monitoring of in-stream conditions, for which direct measurement is largely impractical because of resource limitations. Keywords

Channel morphology and bed-load yield in fluvial, formerly-glaciated headwater streams of the Columbia Mountains, Canada

This study examines channel-reach morphology and bedload yield dynamics in relation to landscape structure and snowmelt hydrology in headwater streams of the Columbia Mountains, Canada. Data collection relies on field surveys and geographic information systems analysis in conjunction with a nested monitoring network of water discharge and bedload transfer. The landscape is characterized by subdued, formerly-glaciated upland topography in which the geomorphic significance of landslides and debris flows is negligible and fluvial processes prevail. While the spatial organization of channel morphology is chiefly controlled by glacially imposed local slope in conjunction with wood abundance and availability of glacigenic deposits, downstream patterns of the coarse grain-size fraction, bankfull width, bankfull depth, and stream power are all insensitive to systematic changes of local slope along the typically stepped long profiles. This is an indication that these alluvial systems have adjusted to the contemporary snowmelt-driven water and sediment transport regimes, and as such are able to compensate for the glacially-imposed boundary conditions. Bedload specific yield increases with drainage area suggesting that fluvial re-mobilization of glacial and paraglacial deposits dominate the sedimentary dynamics of basins as small as 2km2. Stepwise multiple regression analysis shows that annual rates of sediment transfer are mainly controlled by the number of peak events over threshold discharge. During such events, repeated destabilization of channel bed armoring and re-mobilization of sediment temporarily stored behind LWD structures can generate bedload transport across the entire snowmelt season. In particular, channel morphology controls the variability of bedload response to hydrologic forcing. In the present case studies, we show that the observed spatial variability in annual bedload yield appears to be modulated by inter-basin differences in morphometric characteristics, among which slope aspect plays a critical part.

Channel evolution of Des Moines Lobe till drainage ditches in southern Minnesota (USA)

Some drainage ditches in the intensively managed row-crop agricultural region of southern Minnesota evolved from a trapezoidal form to multi-staged channel forms similar to natural streams. Older ditches constructed in cohesive sediment of the Des Moines Lobe till tend to follow a channel evolution model developed by Simon and Hupp. Site cross sections, longitudinal water and bed profiles and bed material particle size were determined according to Harrelson and others at 24 older ditch reaches, 5 newly constructed ditch reaches and 13 natural stream reaches. Morphological features were hypothesized to change from trapezoidal form to flat bench banks, similar to benches found in natural stream channels. All data were statistically analyzed with respect to drainage area using regression, because channel form is directly related to drainage area for a given climate, geology and land use. Results show similar regression slope and intercept for bankfull channel width and bankfull cross-sectional area (CSA) of older ditches and natural streams compared to typical trapezoidal designed ditches. Evolved ditches developed a small floodplain bench above the ditch bed and adjusted their bankfull widths similar to natural stream channels with respect to drainage area. Old ditches showed a relatively strong R 2 (0.82, 0.68) for channel CSA and width, a weaker R 2 (0.45) for water surface slope, and little to no correlation with bed particle size. Channel form appears to have adjusted more quickly than bed facets and/or bed particle size distribution. However, stepwise regression determined that D84, width/depth ratio and mean bankfull depth explained 83 % of the variability of channel features across varying drainage areas. Findings suggest a possible reduction of long-term maintenance costs if older ditches are allowed to evolve over time. A stable ditch form similar to natural streams is typically self-sustaining, suggesting that prior to a scheduled clean-out, the ditch should be examined for hydraulic capacity, sediment transport and bank stability.

A geostatistical framework for quantifying the reach-scale spatial structure of river morphology: 1. Variogram models, related metrics, and relation to channel form

Fluvial geomorphology is fundamentally concerned with the association between form and process in rivers. Examining these interactions in complex, natural channels requires a means of quantifying the variability and organization of bed topography—this paper introduces a geostatistical framework for characterizing reach-scale spatial structure. Transformation to a channel-centered coordinate system allows topographic variations to be resolved into along- and across-stream components. Dimensionless variables, obtained by scaling distances by the mean channel width and de-trended elevations by the mean bankfull depth, account for channel size and allow spatial patterns to be compared over time or among sites. These patterns are effectively described by the variogram, a spatial statistic that expresses dissimilarity as a function of distance. Fitting a parametric model to the sample variogram provides a rich description of channel form. For example, multiple, nested structures can be combined to account for anisotropy, with varying degrees of spatial variability observed over different length scales along and across the channel. Integral metrics derived from the variogram model yield a more compact summary, and variogram maps a useful visualization. To guide interpretation of these metrics, I used a simple ‘channel builder’ to isolate the effects of specific aspects of river morphology on the variogram. This analysis indicated that geostatistical models were sensitive to changes in the size, shape, and orientation of channel features, but not to a pure translation of the morphology. The results also highlighted the importance of considering streamwise and transverse components jointly rather than in isolation.

Meander hydrodynamics initiated by river restoration deflectors

AbstractRiver restoration projects have installed j‐hook deflectors along the outer bank of meander bends to reduce hydraulic erosion, and in this study we use a computational fluid dynamics (CFD) model to document how these deflectors initiate changes in meander hydrodynamics. We validated the CFD with streamwise and cross‐channel bankfull velocities from a 193° meander bend flume (inlet at 0°) with a fixed point bar and pool equilibrium bed but no j‐hooks, and then used the CFD to simulate changes to flow initiated by bank‐attached boulder j‐hooks (1st attached at 70°, then a 2nd at 160°). At bankfull and half bankfull flow the j‐hooks flattened transverse water surface slopes, formed backwater pools upstream of the boulders, and steepened longitudinal water slopes across the boulders and in the conveyance region off the mid‐channel boulder tip. Streamwise velocity and mass transport jets upstream of the j‐hooks were stilled, mid‐channel jets were initiated in the conveyance region, eddies with a cross‐channel axis formed below boulders, and eddies with a vertical axis were shed into wake zones downstream of the point bar and outer bank boulders. At half bankfull depth conveyance region flow cut toward the outer bank downstream of the j‐hook boulders and the secondary circulation cells were reshaped. At bankfull depth the j‐hook at 160° was needed to redirect bank‐impinging flow sent by the upstream j‐hook. The hooked boulder tip of both j‐hooks funneled surface flow into mid‐channel plunging jets, which reversed the secondary circulation cells and initiated 1 to 3 counter rotating cells through the entire meander. The main outer bank collision zone centered at 50° without the j‐hook was moved by the j‐hook to within and just beyond the 70° j‐hook boulder region, which displaced other mass transport zones downstream. J‐hooks re‐organized water surface slopes, streamwise and cross‐channel velocities, and mass transport patterns, to move shear stress from the outer bank and into the conveyance and mid‐channel zones at bankfull flow. At half bankfull flows a patch of high shear re‐attached to the outer bank below the downstream j‐hook. J‐hook geometry and placement within natural meanders can be analyzed with CFD models to help restoration teams reach design goals and understand hydraulic impacts. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Channel adjustments in response to the operation of large dams: The upper reach of the lower Yellow River

The Yellow River in China carries an extremely large sediment load. River channel-form and lateral shifting in a dynamic, partly meandering and partly braided reach of the lower Yellow River, have been significantly influenced by construction of Sanmenxia Dam in 1960, Liujiaxia Dam in 1968, Longyangxia Dam in 1985 and Xiaolangdi Dam in 1997. Using observations from Huayuankou Station, 128km downstream of Xiaolangdi Dam, this study examines changes in the river before and after construction of the dams. The temporal changes in the mean annual flow discharge and mean annual suspended sediment concentration have been strongly influenced by operation of theses dams. Observations of sediment transport coefficient (ratio of sediment concentration to flow discharge), at-a-station hydraulic geometry and bankfull channel form observed from 1951 to 2006 have shown that, although variations in flow and sediment load correspond to different periods of dam operation, changes in channel form are not entirely synchronous with these. The channel has been subject to substantial deposition due to the flushing of sediment from Sanmenxia Dam, resulting in a marked reduction in bankfull cross-sectional area. Flows below bankfull had a greater impact on channel form than higher flows because of very high sediment load. At-a-station hydraulic geometry shows that the variation of channel cross-sectional area below bankfull in this wide and relatively shallow system largely depends on changes in width. Such at-a-station changes are significantly influenced by (1) events below bankfull and (2) overbank floods. Bankfull depth is the main component of channel adjustment in that depth adjusts synchronously with channel area. The channel adjusts its size by relatively uniform changes in depth and width since 1981. Channel morphology is not the product of single channel-forming flow frequency. It is determined by the combination of relatively low flows that play an important role in fine sediment transport and bed configuration as with relatively high flows that are effective at modifying the channel's morphology. The sediment transport coefficient is a useful index for efficiently guiding the operation of the dams in a way that would minimize channel changes downstream. Sedimentation over the nearly 60years of study period caused the lower Yellow River to aggrade progressively, the only significant exception being the two years following completion of Sanmenxia Dam.

Statistical analysis of the evolution of a semialluvial stream channel upstream from an inversion-type reservoir: The case of the Matawin River (Quebec, Canada)

Built in 1930 on the Matawin River (5770 km²) to supply hydroelectric plants located downstream on the Saint-Maurice River, the Taureau Reservoir caused an inversion of the natural hydrological cycle: maximum flows occur in winter and minimum flows occur in spring during snowmelt. This inversion induces a very rapid rise of the reservoir level (base level for the channel upstream from the reservoir) in April–May during the filling-up process. The goal of the study is to analyze the effect of the rapid rise of the base level on the morphological evolution of the 7-km-long sandy section of the Matawin River located immediately upstream from the reservoir and to compare it with Jiongxin's general model. Comparison of aerial photos taken before (1928) and after (1937, 1950, 1965, 1975, 1983, and 1995) reservoir construction reveals no significant change in the bankfull mean width, sinuosity, and bankfull depth of the studied section. It follows that the morphological evolution of the Matawin River channel upstream from the dam is not consistent with the Jiongxin model. The effect of the inverted hydrological regime in fact produces high channel stability. This stability will probably contribute to a slow but progressive reduction of the channel width and an increase of the sinuosity. This morphological evolution seems typical of the semialluvial streams found upstream from the inverted-type reservoir of the Canadian Shield.

Global off-line evaluation of the ISBA-TRIP flood model

This study presents an off-line global evaluation of the ISBA-TRIP hydrological model including a two-way flood scheme. The flood dynamics is indeed described through the daily coupling between the ISBA land surface model and the TRIP river routing model including a prognostic flood reservoir. This reservoir fills when the river height exceeds the critical river bankfull height and vice versa. The flood interacts with the soil hydrology through infiltration and with the overlying atmosphere through precipitation interception and free water surface evaporation. The model is evaluated over a relatively long period (1986-2006) at 1 degrees resolution using the Princeton University 3-hourly atmospheric forcing. Four simulations are performed in order to assess the model sensitivity to the river bankfull height. The evaluation is made against satellite-derived global inundation estimates as well as in situ river discharge observations at 122 gauging stations. First, the results show a reasonable simulation of the global distribution of simulated floodplains when compared to satellite-derived estimates. At basin scale, the comparison reveals some discrepancies, both in terms of climatology and interannual variability, but the results remain acceptable for a simple large-scale model. In addition, the simulated river discharges are improved in term of efficiency scores for more than 50% of the 122 stations and deteriorated for 4% only. Two mechanisms mainly explain this positive impact: an increase in evapotranspiration that limits the annual discharge overestimation found when flooding is not taking into account and a smoothed river peak flow when the floodplain storage is significant. Finally, the sensitivity experiments suggest that the river bankfull depth is potentially tunable according to the river discharge scores to control the accuracy of the simulated flooded areas and its related increase in land surface evaporation. Such a tuning could be relevant at least for climate studies in which the spatio-temporal variations in precipitation are generally poorly represented.

Developing channel and floodplain dimensions with limited data: a case study in the Amazon Basin

ABSTRACTThis research builds on the concept of hydraulic geometry and presents a methodology for estimating bankfull discharge and the hydraulic geometry coefficients and exponents for a station using limited data; only stage‐discharge and Landsat imagery. The approach is implemented using 82 streamflow gauging locations in the Amazon Basin. Using the estimated values for the hydraulic geometry relations, bankfull discharge, discharge data above bankfull and upstream drainage area at each site, relationships for estimating channel and floodplain characteristics as a function of drainage area are developed. Specifically, this research provides relationships for estimating bankfull discharge, bankfull depth, bankfull width, and floodplain width as a function of upstream drainage area in the Amazon Basin intended for providing reasonable cross‐section estimates for large scale hydraulic routing models. The derived relationships are also combined with a high resolution drainage network to develop relationships for estimating cumulative upstream channel lengths and surface areas as a function of the specified minimum channel width ranging from 2 m to 1 km (i.e. threshold drainage areas ranging from 1 to 431,000 km2). At the finest resolution (i.e. all channels greater than 2 m or a threshold area of 1 km2), the Amazon Basin contains approximately 4.4 million kilometers of channels with a combined surface area of 59,700 km2. The intended use of these relationships is for partitioning total floodable area (channels versus lakes and floodplain lakes) obtained from remote sensing for biogeochemical applications (e.g. quantifying CO2 evasion in the Amazon Basin). Copyright © 2011 John Wiley &amp; Sons, Ltd.

Downstream effects of diversion dams on sediment and hydraulic conditions of Rocky Mountain streams

AbstractReduced streamflow via flow diversion has the potential to limit the sediment‐transport capacity of downstream channels and lead to accumulation of fine sediments and habitat degradation. To investigate, we examined the effects of variable levels of flow diversion on fine‐sediment deposition, hydraulic conditions and geomorphic alteration. Our study consisted of a detailed field analysis pairing reaches above and below diversion dams on 13 mountain streams in north‐central Colorado and southern Wyoming USA. Diversions are ubiquitous across the American West, yet previous comparative studies on the effects of flow diversion have yielded mixed results. Through application of strict site‐selection criteria, multiple fine‐sediment measures, and an intensive sampling scheme, this study found that channels downstream of diversions contained significantly more fine sediment and slow‐flowing habitat as compared to upstream control reaches. Susceptibility to fine‐sediment accumulation was associated with decreasing basin size, decreasing bankfull depth and smaller d84, and it appears to be magnified in streams of less than 3% slope. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Physical Basis for Quasi-Universal Relationships Describing Bankfull Hydraulic Geometry of Sand-Bed Rivers

Empirical data indicate that hydraulic geometry relationships for single-thread sand-bed rivers (i.e., rivers with median bed-material size between 0.062 and 0.50 mm) can be delineated such that bankfull width, bankfull depth, and channel slope are related in consistent ways to bankfull discharge. Such relationships ought to be the external expression of physical relationships intrinsic to sand-bed river dynamics. In this study, a back-calculation is performed to identify parameters (exponents and coefficients) for three relationships taken to be intrinsic to sand-bed rivers: (1) a generalized Manning-Strickler resistance relationship; (2) a relationship for channel-forming Shields number; and (3) a relationship for sand yield at bankfull flow. To back-calculate parameters for the physical relationships, first the hydraulic geometry relationships are expressed in suitable dimensionless form. Second, the physical relationships are expressed with coefficients and exponents that are analytically related to p...

Towards a Quantitative Method for Estimating Paleohydrology from Clast Size and Comparison with Modern Rivers

Abstract An outcrop-based methodology is presented to reconstruct bankfull depth of fluvial channels from clast size based on well established geomorphological procedures. Results obtained from a Lower Old Red Sandstone (Ludlovian to Emsian) example from the Midland Valley Basin, Scotland, are calibrated against existing methodologies for reconstruction of bankfull channel depth in fluvial sandstones which utilize preserved channel thickness and preserved dune foreset height. The proposed methodology can be applied to pebbly sandstones and conglomerates of fluvial origin. It does include significant uncertainties, but these are of a range similar to the existing approaches for reconstructing bankfull depth in sandy fluvial systems. Estimates of bankfull channel depth can be used to reconstruct paleo-drainage area using paleo-discharge data, derived from geomorphological “regional curves.” These regional hydraulic-geometry curves are constructed for modern rivers with specific hydro-physiographic catchment properties and constrain the relationship between bankfull depth and discharge, and bankfull depth and drainage area. By comparing the results of outcrop datasets with the regional curves from similar inferred climate regimes, an estimate of paleo-discharge and paleo-drainage area can be derived. Utilizing these paleohydrological approaches from outcrops of fluvial strata allow quantitative constraints to be placed on paleogeographic reconstructions, give a better understanding of paleoclimate and help to constrain provenance studies. In addition, comparison of the drainage areas and bankfull discharges obtained from this analysis for Lower Old Red Sandstone fluvial systems with global data for modern rivers reveals good agreement. This indicates that using region-specific hydraulic-geometry curves to reconstruct ancient fluvial systems can be applied to large rivers interpreted to have a dryland setting possibly sourced from another climatic region, as is commonly the case in modern dryland river systems. It is recommended that by integrating several techniques more accurate estimates of paleohydrology can be obtained to better reconstruct ancient fluvial environments.