H42B-01 10:20h
Effects Of Changing Flow Regime, Sediment Supply, And Riparian Vegetation On The Morphology Of The Braided Waitaki River, New Zealand
The Waitaki River is a large gravel-bed river (mean discharge $\sim$ 380 cumecs) draining the eastern slopes of the Southern Alps, South Island, New Zealand. The 60-km long coastal reach is broad and braided and has, since 1937, experienced a damped flood-flow regime and reduced sediment supply as a result of hydroelectric power development in the upper catchment. Sediment budget estimates indicate that the hydro dams have reduced the bed-material supply to the Lower Waitaki River by approximately 50%, while the mean annual flood discharge of the lower river has been reduced by 18%. Over the same time frame, the braidplain has become more vegetated, its active width has narrowed (from $\sim$ 2000 m to 500 m) and is now only prevented from further narrowing by human intervention. The invading vegetation species are largely exotic, and their establishment was catalysed by the regulated flow regime. Furthermore, the intensity of braiding has decreased, a dominant thalweg has appeared, channel locations have tended to become more stable, and surface armouring has generally increased. However, only the upper part of the reach has clearly degraded. The character of the change has varied downstream according to the relative effects of reduced flow and sediment supply as tributaries join the mainstem. Reduced gravel supply to the coast is also expected, although no clear signal has yet appeared in erosion rates of the adjacent coast. Potential for further flow regime change in the Waitaki catchment has led to the need to predict future river morphology, as this underpins the character of in-stream and riparian habitat. This is required over a 10-100 year time frame for the full span of lower river. Tools that offer a deterministic solution over these time and space scales are limited. A 2-d cellular model, that incorporates the influence of riparian vegetation on bank strength and flow resistance, has been used to simulate bedload transport and morphologic change in the river under historical and future scenarios of river flow regime and vegetation control. The model results are evaluated.
H42B-02 10:35h
Coupling Sediment Transport and Changing Channel Morphology in Response to Vegetation Forcing in a Laboratory Channel
We report results of an experiment lasting 22 weeks to study the long term evolution of a braided system continuously forced with vegetation and cycled high and low discharges. The duration and intensity of the high discharge, and the seeding density of the vegetation, were set so that channels typically migrated some 10% of their width during one high-discharge event. As a well-established vegetated floodplain develops, one of the braid channels emerges as the dominant channel and begins to widen and become more sinuous. This transition is associated with increases in mean channel depth, and decreases in total wetted width, velocity variability, and depth variability. A main characteristic of the single thread system is that once the channel becomes sufficiently sinuous the flow is no longer capable of removing clumps of vegetation that enter the channel. These debris jams lead to aggradation and flow separation upstream of the jam. This results in the channel eventually being abandoned and a new main channel establishing further upstream. The sediment output from the system was collected at 5-minute resolution throughout the run. The transition from unvegetated braided to vegetated meandering is characterized by sediment storage in the system, reflected in an imbalance between the amount of sediment being fed and the amount exiting the system. In addition it reflects finer scale processes such as the debris jams described above. Based on three tests, the level of recent bar-migration activity seems to have potential for predicting the eventual success of channel devegetation efforts.
http://www.geo.umn.edu/orgs/seds/
H42B-03 10:50h
Modelling the Effects of Coarse Vegetation on Braided Stream Pattern and Dynamics
Investigations using a 16m by 2m recirculating experimental flume model of an ephemeral braided river indicate that the presence of large and erosion resistant plants within the channel (e.g. trees or shrubs) can have a significant impact on channel pattern and plan form dynamics. Simulations show that these plants have two effects. First, they act as obstructions, in some cases forcing the flow to divide. This flow separation can allow the deposition of a small island immediately in the lee of the plant splitting the channel. The net result is a substantial increase in the number of channels and correspondingly the braid index. This is in direct contrast to previous studies, where increased levels of vegetation in perennial streams have decreased the braid index. Second, the plants stabilize braid bars and can form relatively stable islands in their lee, significantly reducing the longitudinal and lateral migration of islands typically associated with braided rivers. These results may have imporant implications for our understanding and management of braided rivers, as well as how vegetation interacts with flow and sediment transport.
http://www.coulthard.org.uk
H42B-04 INVITED 11:05h
Response of River Bars to Discharge Variations
Designing controlled hydrographs that produce desirable morphologic characteristics in rivers requires a detailed understanding of the linkage between flow, sediment tranport, and local erosion and deposition. For example, developing flow scenarios in managed rivers that optimize river bar forms (e.g., point bars, alternate bars, braid bars, and lateral separation bars) for habitat enhancement or other objectives requires the ability to predict how various bars respond morphologically to a given discharge or a time series of discharge. Using a variety of field and laboratory measurements in conjunction with computational models for predicting flow and bed evolution, we investigated the response of bar forms in natural rivers to discharge variations. In particular, our investigation concentrated on the effect of hydrograph stage and discharge on bar growth rate and amplitude, and on the important role of hydrograph ramping rates on bar morphology, especially during waning flow. Our results show that using similar quantities of water can produce drastically different morphologic results depending on the duration and amplitude of flow releases. Similarly, different post-flood ramping rates can result in much different quantities and spatial distributions of sub-aerial bar extent at lower flows. Taken together, our results indicate that predictive methods are useful for designing hydrographs to achieve management objectives regarding bar morphology. Furthermore, through comparison of measurement and computation, we confirm that even a relatively simple coupled model for flow and sediment transport provides good predictions of bar response to hypothetical hydrographs, and can be used to help design such flow scenarios.
H42B-05 11:20h
Interaction of Hydraulic and Geotechnical Processes, and Sediment Transport in Controlling Channel Morphology in an Active Meander Bend
Evolution of meanders in incised alluvial channels is controlled by interactions between hydraulic forces acting on the bed and bank toe, and gravitational forces acting on in situ bank material. Vertical and lateral accretion of point bars lead to re-direction of flows in a downvalley direction that impinge on bank-toe surfaces causing undercutting, steepening and ultimate failure of the bank mass by gravity. The processes and forms inherent in incised meanders have been studied at an actively evolving meander bend on Goodwin Creek, Mississippi since 1996. Periodic surveys dating from 1977 to 1996 coupled with the dating of woody vegetation growing on the channel banks and bars were used to determine a migration rate of about 0.5 m/y since the mid-1960s. Up to 28 repetitive surveys were conducted at each of 10 monumented cross sections between 1996 and 2003 over an 82 m-long reach. Over the seven years of monitoring, 894 m3 of bank materials have been eroded from the reach while 436 m3 of sediment have been deposited on the bed and bar. Mean annual sediment concentrations have been essentially stable since the percent of cultivated land in the basin stabilized in the early 1990s. Migration of the outside bend by mass-wasting and toe removal of failed material has been matched by commensurate lateral accretion of point bars. Existing point bars grow vertically at a rate of about 5 cm/yr and now support establishing woody vegetation. Aggradation on the channel bed and lateral channel migration by bar accretion and opposite-bank retreat results in net erosion of all sediment but a net deposition of hydraulically-controlled sands and gravels. The erosion represents series of bank-failure episodes with subsequent removal of failed material by stormflow. A direct correlation between volumes of bar accretion and opposite-side bank erosion was developed, thus establishing a link between hydraulic and geotechnical processes. Peak-flow water-surface slopes increase with increasing discharge from 0.0025 to 0.0035 at 15 m3/s (81.5 m) and then decrease as stage and discharge increase further, causing a flattening of the shear stress-discharge relation at 35 N/m2. This change coincides with the elevation of the top of the newly formed berm where the establishment and proliferation of woody, riparian vegetation occurred once vertical accretion reduced the frequency of bar inundation. This marks the initial development of an inner channel and new floodplain below the level of the previous floodplain surface which now exists as a terrace.
H42B-06 11:35h
Estimating Net Bank Erosion Rates From the Floodplains of Meandering Rivers
Eroding streambanks are sometimes cited as net sources of sediment for rivers. If generally true, this presents an obvious mass balance problem: If rivers continually remove material from active floodplains without replacing all of it, then the floodplains should eventually disappear. For graded river/floodplain systems, then, deposition on the floodplain must, over the long term, balance what is eroded. However, there are at least two reasons why the net erosion due to bank migration, defined as the volume eroded from cut banks minus the volume deposited on point bars, should usually be positive. First, rivers generally migrate into natural levees that are somewhat higher in elevation than the rest of the floodplain. Since point bars are not built as high as natural levees, this represents a net loss of material from the floodplain. Second, river bends tend to migrate outwards, expanding over time. Since the eroding bank is invariably longer than the depositional bank, more material is eroded than deposited, even if the elevation at the top of both banks is constant. This leads to a steady increase in channel sinuosity over time until a cutoff occurs. For a floodplain that is in equilibrium, the erosion caused by natural levee recycling should be balanced primarily by overbank deposition, while the erosion caused by the systematic sinuosity increase should be balanced primarily by depositional processes in abandoned stream courses or oxbow lakes. Until now, it has not been clear which of the two processes is generally more important. This study presents a comparison of their relative importance, as well as system-wide net erosion rates, for portions of three U.S. rivers: a 91 km reach of the Pearl River in Louisiana, a 62 km reach of the Bogue Chitto River in Louisiana, and a 35 km reach of the Neuse River in North Carolina. The study is made possible by high resolution LIDAR datasets along these systems that represent the topography of the natural levees and newly constructed point bars in some detail. Channel migration rates taken from sequences of historic aerial photographs, together with the detailed topography, allow the levee recycling rate to be approximated. The sequential photos also allow the rate of channel sinuosity increase to be computed, which, combined with a characteristic bankfull geometry, allows the flux associated with the sinuosity increase to be estimated. For the systems studied here, the flux due to the sinuosity increase is generally greater than the flux due to levee recycling.
H42B-07 11:50h
Geomorphic Adjustment of a Gravel-bed Stream Meander During a Large Flood
Detailed field measurements of channel topography and bed surface grain size were made in a gravel-bed meander near the headwaters of the Colorado River during the summer of 2002. The spring snowmelt discharge for that year peaked at 5.4 cms (34% of normal peak flow) and probably mobilized only the finest fraction of the bed materials. The following year (2003) snowmelt discharges peaked at 27 cms (170% of normal peak flow) and all available grain sizes within the study reach were mobilized. Additionally, during the 2003 peak flow the river avulsed immediately upstream of the study reach, thereby abandoning approximately one half kilometer of the former channel. The abandonment was rapid (probably within a few hours) leaving the bed texture essentially frozen in place at peak flow conditions. These events provided us with a unique opportunity to test the hypothesis that the bed surface texture of gravel-bed rivers becomes finer as the shear stress and sediment transport rate increase. All 70 previous sampling locations were resampled following the stream abandonment. In response to the high flow the surface median grain size (D50) coarsened slightly in the outer part of the bend while remaining nearly constant along the inner part of the bend, resulting in an overall increase from 18 to 21 mm for the study reach. Thus, the bed surface texture persisted despite shear stresses throughout the bend that were well above the critical entrainment value. These data are inconsistent with the hypothesis that bed surface textures become much finer during large floods. Also, the distribution of local depth (h') increases closely matched that of the surface D50 such that the reach-averaged ratio of h' to D50 remained approximately constant despite the differences in runoff from 2002 to 2003. Therefore, our results suggest that in meander bends the stream is able to adjust the surface grain sizes and pool depths to provide roughly constant local shear stress values over time despite being subjected to a range of sediment and water discharges.
H42B-08 12:05h
Autogenic variability and dynamic steady-state in sand-bedded rivers
In sand-bedded rivers, the local physics of sediment transport produces spatially varying topography that evolves unpredictably in time, even when the structure of the stream-bed varies little in a statistical sense. Understanding autogenic adjustments within trains of bedforms under conditions of steady and uniform flow is necessary before we can predict the response of channel morphology to changes in flow conditions, e.g. the stage-discharge relationship. Also, dunes may coalesce to form bars, which are capable of laterally deflecting flow and ultimately modifying the path and shape of a channel. Bedforms are the link between sediment transport and channel morphology in sandy rivers, and their collective interactions maintain a dynamic steady-state on the river bottom. We document the evolution of fields of dunes under steady flow in the N. Loup River, NE, using topographic maps generated from low-altitude aerial photography. The distributions of bedform height, length and migration rate are broad (coefficient of variation 0.5 for each), but remain stationary in time. Individual bedforms, however, undergo substantial deformation during migration, through interactions with neighboring bedforms and the associated spatially varying sediment flux. Cross-correlation techniques show that the spatial/temporal correlation coefficient of the sediment-fluid interface decays exponentially with migration distance and time. Hence, the dunes themselves are inherently unstable objects and become unrecognizable from their original form after migrating a few wavelengths, corresponding here to a distance of 2 m and a time of 1 hour. If bedload is the dominant style of sediment transport, then sediment flux may be treated as responding instantaneously to the flow field. We build a simple mathematical model in which instantaneous sediment flux is computed locally from a combination of bed elevation and slope, and we deduce the general form of a surface evolution equation for bedforms. The goal is to capture the styles and rates of bed adjustments under steady flow, such as splitting and merging of individual bedforms, and to reproduce the spatial variability of bedform shape and size. Many qualitative aspects of bedform geometry and kinematics are reproduced, however, after long model times, a uniform field of periodic bedforms emerges. Nonlocal and stochastic (turbulent) effects of fluid flow are neglected in this treatment, and their inclusion might produce a field of continuously varying bed topography similar to what is observed in the field.