Andrew Simon

Estimating the mechanical effects of riparian vegetation on stream bank stability using a fiber bundle model

[1]xa0Recent research has suggested that the roots of riparian vegetation dramatically increase the geomechanical stability (i.e., factor of safety) of stream banks. Past research has used a perpendicular root reinforcement model that assumes that all of the tensile strength of the roots is mobilized instantaneously at the moment of bank failure. In reality, as a soil-root matrix shears, the roots contained within the soil have different tensile strengths and thus break progressively, with an associated redistribution of stress as each root breaks. This mode of progressive failure is well described by fiber bundle models in material science. In this paper, we apply a fiber bundle approach to tensile strength data collected from 12 riparian species and compare the root reinforcement estimates against direct shear tests with root-permeated and non-root-permeated samples. The results were then input to a stream bank stability model to assess the impact of the differences between the root models on stream bank factor of safety values. The new fiber bundle model, RipRoot, provided more accurate estimates of root reinforcement through its inclusion of progressive root breaking during mass failure of a stream bank. In cases where bank driving forces were great enough to break all of the roots, the perpendicular root model overestimated root reinforcement by up to 50%, with overestimation increasing an order of magnitude in model runs where stream bank driving forces did not exceed root strength. For the highest bank modeled (3 m) the difference in factor of safety values between runs with the two models varied from 0.13 to 2.39 depending on the riparian species considered. Thus recent work has almost certainly overestimated the effect of vegetation roots on mass stability of stream banks.

Reservoir effects on downstream river channel migration

Summary Human occupation and development of alluvial river floodplains are adversely affected by river channel lateral migration, which may range as high as several hundred metres per year. Reservoirs that reduce the frequency and duration of high flows typically reduce lateral migration rates by factors of 3 to 6. The ecology of riverine corridors is dependent upon the processes of erosion and sedimentation, which lead to lateral migration. Multiple-objective use of floodplains adjacent to active rivers therefore requires tools for assessing the probability and magnitude of channel movements. Existing approaches for predicting river channel movement may be classified as empirical or mechanistic, and are inadequate for widespread application. The Missouri River downstream from Fort Peck Dam in Montana, a major alluvial river with flow highly perturbed by regulation, was selected for case study. Maps and aerial photographs were available before and after dam construction. This imagery was analysed by digitizing channel centrelines at successive coverages under pre-dam and post-dam conditions, and mean migration rates were computed by bend and by reach. The mean rate of channel centreline migration fell from 6.6 m yr -1 to 1.8 m yr -1 after impoundment. Bend-mean channel activity rates were only weakly correlated with variables describing channel form and geometry. Results indicate that flow regulation for flood control and hydropower production typical of the study reach had profound effects on river corridor dynamism, with implications for habitat type distribution and ecosystem integrity.

Effectiveness of grade-control structures in reducing erosion along incised river channels: the case of Hotophia Creek, Mississippi

Abstract Herein, we undertake a geomorphological analysis in which spatial and temporal trends of bed and bank erosion along an 18-km length of Hotophia Creek, Mississippi, are estimated for the period between 1961 and 2050. The evaluation was undertaken for two scenarios of channel response to channelization during 1961–1963. One scenario represents the ‘actual’ response of the channel and includes the effects of installing a series of grade-control structures (GCS) between 1980 and 1996, while the other represents a hypothetical scenario in which the channel is left to adjust naturally. This allows the effectiveness of GCS in reducing in-channel erosion to be assessed. The analysis relies on the availability of channel survey data to develop empirical bed and bank response models for each adjustment scenario, supplemented by bank stability modelling to predict future rates of bank erosion. Results indicate that channel erosion rates decline nonlinearly with respect to time since 1961, for both adjustment scenarios. However, by the year 2050, the “with” GCS adjustment scenario results in the cumulative removal of some 663,000 (9%) extra tonnes of sediment relative to the “without” GCS scenario. Most (63%) of this excess is derived from enhanced bed erosion during 1976–1985 and 1985–1992, with the remainder derived from increased bank erosion during 1985–1992. Detailed analysis of the patterns of erosion and deposition, and their association with the GCS, provides evidence to support the view that GCS installed along Hotophia Creek have, for the most part, been ineffective in reducing channel erosion rates. This is because the GCS were installed too late to prevent bed degradation, caused by the 1961–1963 channelization, migrating upstream. In addition, some structures have disrupted the downstream transmission of bed material from eroded reaches upstream, exacerbating bed degradation and bank erosion in incised reaches downstream.

Incorporating Bank-Toe Erosion by Hydraulic Shear into the ARS Bank- Stability Model: Missouri River, Eastern Montana

Bank-stability concerns along the Missouri River, eastern Montana are heightened by a proposed change in flow releases from Ft Peck dam to improve habitat conditions for Pallid Sturgeon. The effects of the proposed flow releases on streambank pore-pressures and banktoe erosion needs to be evaluated to properly model bank-stability. The ARS Bank-Stability Model incorporates pore-water pressure distributions, layering, confining pressures, reinforcement effects of riparian vegetation and complex bank geometries to solve for the factor of safety (Fs). To increase the applicability and accuracy of the model for use in predicting critical conditions the hydraulic effects of bank-toe erosion have been added. Upper-bank stability is often a function of the degree of fluvial undercutting that occurs during rises in stage when the bank toe becomes submerged and steepened. This erosion, which is a function of the erodibility of previously failed materials and in situ sediments at the toe has been difficult to measure or estimate in the field. Recent field research on erosion of in situ cohesive streambeds and bank toes with a submerged jet-test device provides a means of calculating bank-toe erosion. Results of almost 200 tests at stream sites across the United States provide the following general relation: k =0 .1τc –0.5 ;w herek = erodibility coefficient in cm 3 /N-s and τc = critical shear stress in Pa. Critical shear stresses are obtained in situ with the jet-test device (92 tests) for bank-toe materials along the Missouri River, Montana to obtain k and to calculate an erosion rate based on an excess shear-stress relation: e = k (τ - τc); where e = erosion rate in m/s and τ = average boundary shear stress in Pa. Inputs for the bank-toe erosion routine are: (1) a rectangular-shaped hydrograph of specified height and duration, (2) bed or water-surface slope, (3) flow depth, (4) bank geometry, and (5) τc for all bank layers and failed debris. Erosion is simulated normal to the submerged bank surface and the resulting bank geometry serves as input into the bank-stability part of the model. According to the proposed flow-release plan, flows of 216 m 3 /s are increased by 38.3 m 3 /s/day for 12 days to 675 m 3 /s, held for 60 days and decreased for 12 days back to 216 m 3 /s according. As expected, results show the important contribution of bank-toe erodibility in controlling mass failure. All sites modeled indicate a destabilizing influence (reduction in Fs) during the lowering of stage after 60 days, much of it due to lateral seepage and losses of matric suction and confining pressure. Banks at river miles 1624, 1676 and 1716 attain Fs < 1.0 indicating imminent failure. These sites contain less resistant sandy-silt material at the bank toe, and experienced simulated undercutting up to 3m. More resistant cohesive, clay bank toes at river miles 1589 and 1762 were undercut only 0.2 m and remained stable.

Combined Geomorphic and Numerical-Modeling Analyses of Sediment Loads for Developing Water-Quality Targets for Sediment

The principle objective of the study was to determine sediment loads for James Creek, Mississippi and for similar, but stable “reference” streams to develop water-quality targets for sediment. “Reference” sediment-transport loads were determined from stable streams with historical flow and sediment-transport data in the Southeastern Plains Ecoregion. Using the discharge that occurs, on average every 1.5 years (Q1.5) as the “effective discharge,” an initial “general reference” of 0.31 T/d/km was obtained. This value, however, is skewed towards streams with sand beds and does not accurately reflect conditions along James Creek. A refined “reference” condition was developed for stable silt/clay-bed streams in the Southeastern Plains resulting in a “reference” suspended-sediment yield of 3.23 T/d/km at the Q1.5. A weighted-reference condition based on the percentage of the drainage area encompassed by the various bed-material types results in a reference yield at the Q1.5 of 2.2 T/d/km. Similarly, a weightedreference concentration of 160 mg/l was obtained. “Actual” sediment-transport loads were obtained by: simulations of flow and sediment transport using the Simon is a Research Geologist at the USDAARS, National Sedimentation Laboratory, Oxford, MS 38655. E-mail: asimon@ars.usda.gov. Bingner is an Agricultural Engineer, Langendoen is a Research Hydraulic Engineer, Wells is a Postdoctoral Researcher , and Alonso is Research Leader, all at the USDAARS, National Sedimentation Laboratory, Oxford, MS 38655. . model AnnAGNPS and by simulations of channel flow and sediment transport by the channel-evolution model CONCEPTS. Average sediment loads at the mouth of James Creek over the 35-year period are about 250,000 T/y with 88% emanating from channels and 12% from upland sources. This loading value, however, is somewhat misleading in that severe channel erosion occurred between 1967-1968 following channel clearing and snagging over the lower 17 km. Since this time, sediment loads attenuated and the contribution from channels and uplands over the period 1970-2002 shifted to 70% and 30%, respectively. “Actual” simulated suspended-sediment loads at the Q1.5 show a 35-year average of 675 T/D/km; 155 T/D/km over the past 10 years. Following the installation of low-water crossings in 1999 loads decreased to about 39 T/D/km. This value is more than an order of magnitude greater than the “reference” yield.

Mechanisms and Rates of Knickpoint Migration in Cohesive Streambeds: Hydraulic Shear and Mass Failure.

This paper presents results from a three-year field study that is being conducted to determine the morphology, processes and migration rates of nine knickpoints in the cohesive clay beds of the Yalobusha River System, Mississippi. The roles of surface erosion by hydraulic shear stress and mass failure during the upstream migration of knickpoints are evaluated. Results of submerged jet tests reveal a wide variation in the erosion resistance of the streambeds. Values of critical shear stress, τc, span almost four orders of magnitude with a median of 31 Pa, while values of the erodibility coefficient, k, span about three orders of magnitude with a median of 0.022 cm 3 /N-s. To relate τc and k-values to the erosion potential of flows, an excess shear stress approach is used to estimate erosion rates using values of average boundary shear stress, τ0. Most flows are competent to erode streambeds composed of the Naheola formation (τc = 2.24 Pa), while only the deepest flows and steepest hydraulic gradients generate boundary-shear stresses great enough to erode the Porters Creek Clay formation (τc = 199 Pa). Repeated thalweg surveys indicate that knickpoints formed in Naheola clay have migrated at an average rate of 7.2 meters per year since 1997, while knickpoints in Porters Creek Clay have migrated at an average rate of around 1.2 meters per year over the same period. Comparison of calculated erosion rates and observed knickpoint retreat rates indicates a discrepancy between migration rate and available hydraulic shear stress. Additional mechanisms have hence been identified as contributing to the migration of cohesive knickpoints: a cyclical mass-failure mechanism, and detachment of aggregates by upward directed pore-water pressure on the falling limb of hydrographs. To assess the role of mass failure in knickpoint retreat a combination of finite-element hydrologic and limit equilibrium slope-stability modeling was carried out. Thalweg survey data were used to construct a series of finite-element meshes based on the geometries of the knickpoints during migration, while head and tail water elevations were taken from stage data logged every 30 minutes above and below the knickpoint. At one example location in Naheola clay, Big Creek, every observed event greater than 0.3 m has been extracted from the stage record and simulated. Where a mass failure was indicated by a factor of safety close to or below one, the predicted failed distance was compared with the observed knickpoint retreat distance at the next survey. Results at this location show a close agreement between modeled and observed retreat rates. Quantification of knickpoint migration rates and processes in the field enables river managers to accurately determine the locus of incision at a particular time, allowing estimates of sediment sources and yields. This study has also shown that knickpoints formed in resistant substrata may act as a form of natural grade control, while knickpoints formed in more erodible substrata may require direct intervention by management agencies.

Application of a Deterministic Bank-Stability Model to Design a Reach-Scale Restoration Project

Sediment is one of the leading contributors to water-quality impairment in the United States and streambank erosion has been found to be the dominant source of sediment in many watersheds with heightened sediment loads. Goodwin Creek is a typical incised channel in northeastern Mississippi (4.7 m-deep) that yields about an order of magnitude more suspended sediment than stable, “reference” streams in the Mississippi Valley Loess Plains ecoregion. Periodic channel surveys in conjunction with dating of woody vegetation growing on the channel banks and bars in an actively eroding meander bend between 1977 and 1996 were used to determine an average migration rate of about 0.5 m/y over the period. Channel processes and sediment-transport rates were monitored in detail between 1996 and 2006 and showed a similar migration rate. Because of continued land loss in adjacent agricultural fields by mass failure of the streambanks, a restoration project was designed to stabilize the banks and to protect a road running parallel to the bendway. Bank retreat occurs by interactions between hydraulic forces acting at the bed and bank toe and gravitational forces acting on in situ bank material. To provide a stable alternative, analysis of the restored configuration needed to address both hydraulic erosion and geotechnical stability. This was accomplished using a Bank-Stability and Toe-Erosion model developed at the USDA-ARS National Sedimentation Laboratory. The proposed design was limited to 1:1 bank slopes due to the proximity of the road and included longitudinal stone-toe protection and bendway weirs to counter basal erosion by hydraulic shear. Worst-case geomorphic conditions under the proposed design were simulated by modeling (1) typical, annual high flows (3 m-deep) to evaluate the amount of bank-toe erosion that would occur, and (2) geotechnical stability where groundwater levels were high and flow had receded to low-flow conditions in the channel (drawdown case). Results showed that the bank would still be unstable at 1:1 under the drawdown case but that the addition of specific riparian vegetation on the slope would stabilize the bank even under worst-case conditions. The design was, therefore, implemented and constructed in a week.

Riverbank Stabilization Using Low Cost Submersible Pumps

Riverbanks are usually stabilized by reinforcing the soil, installing horizontal drains or by regrading the slope. Though well established, these techniques incur problems including ground disturbance, inability to drain deep within the bank, loss of land and expense. A potential alternative is to increase bank stability by actively lowering the water table using submerged pumps, reducing positive pore-water pressure and promoting the development of matric suction. This approach is suitable in critical locations such as bridge abutments, where rapid bank stabilization is required, or where deep drainage is needed. It also has potential as a medium term technique to stabilize banks until vegetation or other reinforcing measures have had time to take effect. A self contained and inexpensive submersible pump system has been developed and installed in a section of 6 m high incised streambank in Northern Mississippi, as part of the Demonstration Erosion Control (DEC) project. Pumps have been installed in pairs at 2 and 4 m depth, spaced 15 m apart. The de-watered site and an adjacent control site have been continuously monitored for one year, to evaluate rainfall, water volume extracted by the pumps and pore-water pressure. The data have been applied to the ARS Streambank Stability Model to calculate Factor of Safety (Fs), and rates of bank retreat at the two sites have been monitored and compared. Initial results show that the pumps lowered the water table by 0.5-1.0 m over a radius of 7.5-m, reducing pore-water pressures by 5-10 KPa during the critical winter wet period. When the resulting distribution of pore-water pressures was applied in the bank stability model, Fs increased by 15-20% for the site with pumps installed, and remained above the failure threshold throughout the year. By contrast Fs on the control site fell below the failure threshold on several occasions. During the winter period bank retreat was 11 cm for a section of the de-watered site situated above a large woody debris (LWD) structure, compared with 27 cm for a section of bank that was de-watered but not protected with LWD, and 43 cm for the control site. No mass failures were recorded at either portion of the de-watered plot, in contrast to the control plot. The cost of bank stabilization using submerged pumps was approximately

The Role of Pore-Water Pressures and Upward-Directed Seepage Forces in the Erosion of Cohesive Streambeds

40 per m, compared with

Comparison of Empirical and Analytical Physical Assessment Approaches for Stream Restoration: A Case Study on Abrams Creek, Great Smoky Mountains National Park, Tennessee

300 per m to stabilize similar banks using rip rap, suggesting that the approach is a viable and cost effective method. Introduction Riverbank failure and retreat is a significant threat to bridges and roads, causes loss of farm and residential land and is a major source of sediment in streams. It is a particular problem in the Southeast and Midwest USA where channel incision due to straightening and dredging has led to thousands of kilometers of unstable banks. As part of the US Army Corps of Engineers Demonstration Erosion Control (DEC) Project, the National Sedimentation Laboratory is