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Papers by andrew simon

Lab on A Chip - LAB CHIP, 2010

Cell-free protein synthesis (CFPS) is an alternative approach to cell-based recombinant protein p... more Cell-free protein synthesis (CFPS) is an alternative approach to cell-based recombinant protein production. It involves in vitro transcription and translation in a cell-free medium. In this work, we implemented CFPS in a plastic array device. Each unit in the array consisted of an inner well and an outer well. Two synthesis steps, gene transcription and protein translation, took place in the inner well, in which a cell-free medium was used to provide ribosomes and additional components necessary for protein synthesis. The outer well was concentric to the inner well and it functioned as a nutrient reservoir. A nanoporous membrane was sandwiched between the inner and outer wells for retaining the synthesized proteins and removing the reaction byproducts. A microfluidic channel was employed to connect these two wells for supplying fresh nutrients for longer reaction time and higher expression yield. Synthesis of luciferase was shown to last 8 times longer and yield 10 times more proteins than in a conventional container. The device also enables more than 2 orders of magnitude reduction in reagent consumption compared to a bench-top instrument. The effects of the membrane pore size and microfluidic channel on the protein production yield were also studied. The array device has potential to become a platform for parallel protein expression for proteomics applications, matching high-throughput gene discovery.

Wetlands Engineering & River Restoration 2001, 2001

Riverbanks are usually stabilized by reinforcing the soil, installing horizontal drains or by reg... more 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.

Geomorphology, 1997

A deterministic numerical model of bed deformation and channel widening, which accounts for speci... more A deterministic numerical model of bed deformation and channel widening, which accounts for specific mechanisms of bank erosion and collapse, is used to analyze morphological and flow-energy parameters in adjusting sand-bed channels, for a range of simulated fluvial environments. The model is based on a set of conservation and process equations solved in conjunction with a set of specific initial and boundary conditions. Simulated channels with a range of assumed boundary-material characteristics were subjected to identical disturbances imposed by reducing the supply of sediment from upstream. Asymptotic reductions of the rate of energy dissipation (energy slope) and boundary shear stress were found to be unifying characteristics of channel adjustment in all simulations. Morphologic responses to an identical disturbance (reduction of sediment supply from upstream), with identical bed material (d50 = 1 mm) and channel gradient (Sb = 0.001), were diverse. The relative magnitude of simulated widening versus simulated bed-level change was greatest for sand-bank channels and least for clay-bank channels. Stable-channel dimensions were attained most rapidly for silt-bank channels, where simulated adjustments of channel width were similar to simulated vertical adjustments, because all components of total-mechanical energy (flow depth or pressure head; bed elevation and channel gradient or datum head; and flow velocity or velocity head) decrease simultaneously. Each simulated channel subjected to the baseline disturbance had an initial width-to-depth ratio (F) of 13.5. Asymptotic values of energy dissipation rate, adjusted F-values, and time taken to reach equilibrium after the disturbance are shown to vary as a function of simulated environmental boundary conditions. The importance of channel widening in controlling and reducing the flow depth of a given discharge and average boundary shear stress is highlighted for the silt- and sand-bank cases. Channel widening causes a shift from degradation to aggradation during adjustment, because of delivery of sand-sized sediments from failed bank material to the simulated channel.

Geomorphology, 1998

Two distinct phases of bed-level adjustment over the last 150 years are identified for the princi... more Two distinct phases of bed-level adjustment over the last 150 years are identified for the principal alluvial reaches of the Arno River (Upper Valdarno and Lower Valdarno). The planimetric configuration of the river in these reaches is the result of a series of hydraulic works (canalization, rectification, artificial cut-offs, etc.) carried out particularly between the 18th and the 19th centuries. Subsequently, a series of interventions at basin level (construction of weirs, variations in land use), intense instream gravel-mining after World War II, and the construction of two dams on the Arno River, caused widespread degradation of the streambed. Since about 1900, total lowering of the channel bed is typically between 2 and 4 m in the Upper Valdarno Reach and between 5 and 8 m in some areas of the Lower Valdarno Reach. Bed-level adjustments with time are analyzed for a large number of cross-sections and described by an exponential-decay function. This analysis identified the existence of two main phases of lowering: the first, triggered at the end of the past century; the second, triggered in the interval 1945–1960 and characterized by more intense degradation of the streambed. The first phase derived from changes in land-use and land-management practices. The second phase is the result of the superimposition of two factors: intense instream mining of gravel, and the construction of the Levane and La Penna dams.

Earth Surface Processes and Landforms, 2002

As a response to channelization projects undertaken near the turn of the 20th century and in the ... more As a response to channelization projects undertaken near the turn of the 20th century and in the late 1960s, upstream reaches and tributaries of the Yalobusha River, Mississippi, USA, have been rejuvenated by upstream-migrating knickpoints. Sediment and woody vegetation delivered to the channels by mass failure of streambanks has been transported downstream to form a large sediment/debris plug where the downstream end of the channelized reach joins an unmodified sinuous reach. Classification within a model of channel evolution and analysis of thalweg elevations and channel slopes indicates that downstream reaches have equilibrated but that upstream reaches are actively degrading.

Earth Surface Processes and Landforms, 1989

Dredging and straightening of alluvial channels between 1959 and 1978 in West Tennessee caused a ... more Dredging and straightening of alluvial channels between 1959 and 1978 in West Tennessee caused a series of morphologic changes along modified reaches and tributary streams. Degradation occurred for 10 to 15 years at sites upstream of the area of maximum disturbance and lowered bed-levels by as much as 6·1 m. Following degradation, reaches upstream of the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and gradient reduction. Aggradation downstream of the area of maximum disturbance reached 0·12 m per year with the greatest rates occurring near the stream mouths.The adjustment of channel geometry and phases of channel evolution are characterized by six process-oriented stages of morphologic development—premodified, constructed, degradation, threshold, aggradation, and restabilization. Down-cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and angle of the bank material is exceeded (threshold stage). Channel widening continues through the aggradation stage as the ‘slough line’ develops as an initial site of lower-bank stability. The bank profile develops three dynamic elements (1) vertical face (70° to 90°), (2) upper bank (25° to 50°), and (3) slough line (20° to 25°). Alternate channel bars form during the restabilization stage and represent incipient meandering of the channel.

Geomorphology, 1995

A modular procedure to assess the magnitude, distribution, and potential for channel instabilitie... more A modular procedure to assess the magnitude, distribution, and potential for channel instabilities at a large number of sites has been designed and implemented. The procedure, based on diagnostic interdisciplinary criteria of alluvial channel morphology and associated riparian vegetation, is presented. The modules include (1) initial site evaluations, (2) GIS-based data input and management, (3) ranking of relative channel stability, (4) identification of spatial trends, (5) ranking of socio-economic impacts and identification of most “critical” sites, and (6) collection of additional field data for more detailed evaluation of the magnitude and type of future instabilities and the effects of proposed mitigation measures. The procedure, using site evaluation forms as the fundamental means of data collection, takes a trained person 1 to 1.5 hours to complete. Site evaluation forms can be altered according to the specific environment being studied and the objectives of the study.An objective ranking scheme based on physical attributes extracted from the GIS data base permits the identification of the most unstable channel sites and, thereby, focuses attention on potentially “critical” sites. If a significant concern about a bridge or adjacent lands arises, or if mitigation measures are considered, various methods to estimate future channel changes are proposed. These include (1) numerical alluvial channel modeling, (2) empirical models of channel evolution, (3) regime equations, and (4) empirical relations based on process dominance in different fluvial environments. The methods require differing amounts of additional field data and provide results of varying detail and accuracy. The decision on which method to use must be based on the objectives and resources of the agency involved in the evaluation study.

Geomorphology, 2009

As part of a study to investigate the causes of channel narrowing and incision in Canyon de Chell... more As part of a study to investigate the causes of channel narrowing and incision in Canyon de Chelly National Monument, the effects of Tamarisk and Russian-olive on streambank stability were investigated. In this study, root tensile strengths and distributions in streambanks were measured and used in combination with a root-reinforcement model, RipRoot, to estimate the additional cohesion provided to layers of each streambank. The additional cohesion provided by the roots in each 0.1-m layer ranged from 0 to 6.9 kPa for Tamarisk and from 0 to 14.2 kPa for Russian-olive. Average root-reinforcement values over the entire bank profile were 2.5 and 3.2 kPa for Tamarisk and Russian-olive, respectively. The implications of vegetation removal on bank stability and failure frequency were evaluated in two incised reaches by modeling bank-toe erosion and bank stability with and without vegetation. The effects of a series of 1.0-and 1.5-m-deep flows on bank-toe erosion, pore-pressure distributions, and bank stability were evaluated first. In addition, bank stability model runs were conducted using iterative modeling of toe erosion and bank stability using a discretized flow and groundwater record for one year. Results showed that the effects of root-reinforcement provided by Tamarisk and Russian-olive have a significant impact on bankstability and bank-failure frequency. Because the bank materials are dominated by sands, cohesion provided by roots is significant to bank stability, providing an average 2.8 kPa of cohesion to otherwise cohesionless bank materials. Bank retreat rates at one site following vegetation removal have approximately doubled when compared to the control reach (from an approximate rate of 0.7-0.8 m/y between 2003 and 2006 to 1.85 m/y during the year modeled). Vegetation removal along the entire riparian corridor in Canyon de Chelly may lead to the introduction of significantly more sediment to the system through bank widening processes, although it is not known whether this change alone would be sufficient to cause a shift in channel morphology to the wide-braided channels that were once characteristic of this canyon.

Earth Surface Processes and Landforms, 2002

Riparian vegetation strips are widely used by river managers to increase streambank stability, am... more Riparian vegetation strips are widely used by river managers to increase streambank stability, among other purposes. However, though the effects of vegetation on bank stability are widely discussed they are rarely quantified, and generally underemphasize the importance of hydrologic processes, some of which may be detrimental. This paper presents results from an experiment in which the hydrologic and mechanical effects of four riparian tree species and two erosion-control grasses were quantified in relation to bank stability. Geotechnical and pore-water pressure data from streambank plots under three riparian covers (mature trees, clump grasses and bare/cropped turf grass) were used to drive the ARS bank stability model, and the resulting factor of safety (Fs) was broken down into its constituent parts to assess the contribution (beneficial or detrimental) of individual hydrologic and mechanical effects (soil moisture modification, root reinforcement and surcharge). Tree roots were found to increase soil strength by 2–8 kPa depending on species, while grass roots contributed 6–18 kPa. Slope stability analysis based on data collected during bank failures in spring 2000 (following a very dry antecedent period) shows that the mechanical effects of the tree cover increased Fs by 32 per cent, while the hydrologic effects increased Fs by 71 per cent. For grasses the figures were 70 per cent for mechanical effects and a reduction of Fs by 10 per cent for the hydrologic effects. However, analysis based on bank failures in spring 2001 (following a wetter than average antecedent period) showed the mechanical effects of the tree cover to increase Fs by 46 per cent, while hydrologic effects added 29 per cent. For grasses the figures were 49 per cent and −15 per cent respectively. During several periods in spring 2001 the hydrologic effects of the tree cover reduced bank stability, though this was always offset by the stabilizing mechanical effects. The results demonstrate the importance of hydrologic processes in controlling streambank stability, and highlight the need to select riparian vegetation based on hydrologic as well as mechanical and ecological criteria. Published in 2002 by John Wiley & Sons, Ltd.

Journal of The American Water Resources Association, 2000

ABSTRACT: The bess area of the midwestern United States contains thousands of miles of unstable s... more ABSTRACT: The bess area of the midwestern United States contains thousands of miles of unstable stream channels that are undergoing system-wide channel-adjustment processes as a result of (1) modifications to drainage basins dating back to the turn of the 20th century, including land clearing and poor soil-conservation practices, which caused the filling of stream channels, and consequently (2) direct, human modifications to stream channels such as dredging and straightening to improve drainage conditions and reduce the frequency of out-of-bank flows. Today, many of these channels are still highly unstable and threaten bridges, other structures, and land adjacent to the channels. The most severe, widespread instabilities are in western Iowa where a thick cap of bess and the lack of sand-and gravel-sized bed sediments in many channels hinders downstream aggradation, bed-level recovery and the consequent reduction of bank heights, and renewed bank stability. In contrast, streams draining west-central Illinois, east-central Iowa, and other areas, where the bess cap is relatively thin and there are ample supplies of sand-and gravel-sized material, are closer to recovery. Throughout the region, however, channel widening by mass-wasting processes is the dominant adjustment process.

Geomorphology, 2000

Gravitational forces acting on in situ bank material act in concert with hydraulic forces at the ... more Gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion. The interaction of these forces control streambank mechanics. Hydraulic forces exerted by flowing water on in situ bank-toe material and failed cohesive material at the bank toe are often sufficient to entrain materials at relatively frequent flows and to maintain steep lower-bank profiles. Seepage forces exerted on in situ bank material by groundwater, downward infiltration of rainwater and lateral seepage of streamflow into and out of the bank are critical in determining bank strength. Data from a study site on Goodwin Creek, MS, USA clearly show the temporal variability of seepage forces and the lag time inherent in reductions in shear strength due to losses of matric suction and generation of positive pore-water pressures. Negative pore-water pressures (matric suction) have also been shown to increase the resistance of failed cohesive blocks to entrainment by fluid shear. A stable bank can be transformed into an unstable bank during periods of prolonged rainfall through:increase in soil bulk unit (specific) weight,decrease or complete loss of matric suction, and, therefore, apparent cohesion,generation of positive pore-water pressures, and, therefore, reduction or loss of frictional strength,entrainment of in situ and failed material at the bank toe, andloss of confining pressure during recession of stormflow hydrographs.

Journal of Hydraulic Engineering, 1996

Geomorphology, 2004

Historical flow and suspended-sediment transport data from more than 2900 sites across the United... more Historical flow and suspended-sediment transport data from more than 2900 sites across the United States have been analyzed in the context of estimating flow and suspended-sediment transport conditions at the 1.5-year recurrence interval flow (Q1.5). This is particularly relevant with the renewed focus on stream restoration activities and the urgency in developing water-quality criteria for sediment. Data were sorted into the 84 Level III ecoregions to identify spatial trends in suspended-sediment concentrations and yields to meaningfully describe suspended-sediment transport rates across the United States. Arguments are developed that in lieu of form-based estimates of say the bankfull level, a flow of a given recurrence interval (Q1.5) is more appropriate to integrate suspended-sediment transport ratings for the purpose of defining long-term transport conditions at a site (the “effective discharge”). The use of the Q1.5 as a measure of the effective discharge for suspended-sediment transport is justified on the basis of literature reports and analytic results from hundreds of sites in 17 ecoregions that span a diverse range of hydrologic and topographic conditions (i.e., Coast Range, Arizona/New Mexico Plateau, Mississippi Valley Loess Plains, Middle Atlantic Coastal Plain). There is sufficient data to also develop regional curves for the Q1.5 in all but eight of the ecoregions. At the Q1.5 the highest median suspended-sediment concentrations occur in semiarid environments (Southwest Tablelands, Arizona/New Mexico Plateau and the Mojave Basin and Range); the highest yields occur in humid regions with erodible soils and steep slopes or channel gradients (Mississippi Valley Loess Plains [MVLP] and the Coast Range). Suspended-sediment yields for stable streams are used to determine “background” or “reference” sediment transport conditions in eight ecoregions where there is sufficient field data. The median value for stable sites within a given ecoregion are generally an order of magnitude lower than for nonstable sites.

Journal of Hydraulic Engineering-asce, 2003

Stream restoration, or more properly rehabilitation, is the return of a degraded stream ecosystem... more Stream restoration, or more properly rehabilitation, is the return of a degraded stream ecosystem to a close approximation of its remaining natural potential. Many types of practices ͑dam removal, levee breaching, modified flow control, vegetative methods for streambank erosion control, etc.͒ are useful, but this paper focuses on channel reconstruction. A tension exists between restoring natural fluvial processes and ensuring stability of the completed project. Sedimentation analyses are a key aspect of design since many projects fail due to erosion or sedimentation. Existing design approaches range from relatively simple ones based on stream classification and regional hydraulic geometry relations to more complex two-and three-dimensional numerical models. Herein an intermediate approach featuring application of hydraulic engineering tools for assessment of watershed geomorphology, channel-forming discharge analysis, and hydraulic analysis in the form of one-dimensional flow and sediment transport computations is described.

Hydrological Processes, 2009

... F. Douglas Shields Jr,* Andrew Simon and Seth M. Dabney USDA-Agricultural Research Service, N... more ... F. Douglas Shields Jr,* Andrew Simon and Seth M. Dabney USDA-Agricultural Research Service, National Sedimentation Laboratory Oxford ... Limited measurements following storm events documented dis-charge rates from individual seeps ranging from 0Ð068 to 0Ð931 m3 ...

Environmental Earth Sciences, 1987

Channel modifications from 1968 to 1969 on the South Fork Forked Deer River in western Tennessee ... more Channel modifications from 1968 to 1969 on the South Fork Forked Deer River in western Tennessee have caused upstream degradation, downstream aggradation, and bank failures along the altered channels, adjacent reaches, and tributaries. The result of these adjustments is a general decrease in gradient as the channel attempts to absorb the imposed increase in energy conditions created by channelization. Headward degradation at a rate of approximately 2.57 km/yr on the South Fork Forked Deer River caused from 1.52 m to about 3.14 m of incision over a 13.5 km reach from 1969 to 1981. As a consequence of substantially increased sediment supply, approximately 2.13 m of aggradation was induced downstream of this reach during the same period. This accumulation represents a 60% recovery of bed level at the downstream site since the completion of channel work in 1969. Gradient adjustment with time is described by exponential decay functions. The length of time required for adjustment to some new quasi-equilibrium condition is computed by these decay functions and is about 20 years from the completion of channel work. Adjusted slopes are less than predisturbed values, probably because straightened channels dissipate less energy by friction, allowing more energy for sediment transport. An equivalent sediment load, therefore, can be transported at a considerably gentler slope. The predisturbed slope exceeds the adjusted slope by an order of magnitude on the downstream reach of the South Fork Forked Deer River.

Earth Surface Processes and Landforms, 1996

Channel adjustments in the North Fork Toutle River and the Toutle River main stem were initiated ... more Channel adjustments in the North Fork Toutle River and the Toutle River main stem were initiated by deposition of a 2.5 km 3 debris avalanche and associated lahars that accompanied the catastrophic eruption of Mount St. Helens, Washington on 18 May 1980. Channel widening was the ...

Environmental Conservation, 2000

Human occupation and development of alluvial river floodplains are adversely affected by river ch... more Human occupation and development of alluvial river floodplains are adversely affected by river channel lat-eral 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 ...

Geomorphology, 2001

Ž . The management of large woody debris LWD should be based on a rational assessment of its recr... more Ž . The management of large woody debris LWD should be based on a rational assessment of its recruitment rate relative to its natural decay and removal. LWD recruitment may be controlled by 'natural' episodic terrestrial factors or by in-channel geomorphological controls related to the rate of bank erosion. The geomorphological controls are hard to quantify in laterally migrating channels, but in incising channels, a conceptual model may be developed based on the density of riparian trees relative to the knickpoint migration rate and bank stability analyses that predict the post-knickpoint width of the channel. The Yalobusha river network in Central Mississippi, USA, has twice been destabilised by channel straightening for flood defence and land drainage, most recently in 1967. System-wide rejuvenation has followed through a series of upstream migrating knickpoints several metres high that have caused mass failure of streambanks and the recruitment of large volumes of trees to the channel. LWD recruitment is maximised at the transition between stage III and stage IV channels, focusing attention on 11 sites in the network. The sites are upstream of knickzones ranging between 2.2 and 5.4 m high and migrating at rates of 0-13.8 m year y1 , based on 23-30 months of monitoring. Riparian conditions in 500 m 2 plots on each bank upstream of the knickpoints range from treeless to forested, containing 0-98 trees with an average diameter at breast height of 0.18 m and average maximum height of 14.0 m. The average volume of wood on each bank is 0.02 m 3 m y2 . Under rapid drawdown conditions, bank stability analyses suggest that the channels will widen in amounts ranging from 1.8 to 31.5 m. Combined with the knickpoint migration rates, riparian land losses are estimated to range from 8.0 to y1 3 ) Corresponding author. Philip front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 1 6 9 -5 5 5 X 0 0 0 0 0 6 3 -5 ( ) P.W. Downs, A. Simonr Geomorphology 37 2001 65-91 66 vegetation density in upstream and headwater riparian zones of each tributary. The 11 study sites are classified into groups with similar LWD management concerns based on these analyses. The models developed in this research provide the first precise quantification of LWD recruitment according to geomorphological controls and standing vegetation, and a rational assessment of its meaning, but further research is required to improve the accuracy of such estimates. q

Geomorphology, 1991

Hupp, C.R. and Simon, A., 1991. Bank accretion and the development of vegetated depositional surf... more Hupp, C.R. and Simon, A., 1991. Bank accretion and the development of vegetated depositional surfaces along modified alluvial channels. Geomorphology, 4:11 l-124. This paper describes the recovery of stable bank form and development of vegetated depositional surfaces along the banks of channelized West Tennessee streams. Most perennial streams in West Tennessee were straightened and dredged since the turn of the century. Patterns of fluvial ecological responses to channelization have previously been described by a six-stage model. Dendrogeomorphic (tree-ring) techniques allowed the determination of location, timing, amount, and rate of bank-sediment deposition. Channel cross sections and ecological analyses made at 101 locations along 12 streams, encompassing bends and straight reaches, show that channel and bank processes initially react vertically to channelization through downcutting. A depositional surface forms on banks once bed-degradation and heightened bank mass wasting processes have eased or slowed. The formation of this depositional surface marks the beginning of bank recovery from channelization. Dominating lateral processes, characteristic of stable or natural channels, return during the formation and expansion of the depositional surface, suggesting a relation with thalweg deflection, point-bar development, and meanderloop extension. Characteristic woody riparian vegetation begins to grow as this depositional surface develops and becomes part of the process and form of restabilizing banks. The depositional surface initially forms low on the bank and tends to maintain a slope of about 24 ° . Mean accretion rates ranges from 5.9 cm/yr on inside bends to 0 cm/yr on most outside bends; straight reaches have a mean-accretion rate of 4.2 cm/yr. The relatively stable, convex upward, depositional surface expands and ultimately attaches to the flood plain. The time required for the recovery process to reach equilibrium averaged about 50 years. Indicative pioneer species of woody riparian vegetation include black willow, river birch, silver maple, and boxelder. Stem densities generally decrease with time after and initial flush of about 160 stems per 100 m 2. Together bank accretion and vegetative regrowth appear to be the most important environmental processes involved in channel bank recovery from channelization or rejuvenation. 0169-555X/91/$03.50

Lab on A Chip - LAB CHIP, 2010

Cell-free protein synthesis (CFPS) is an alternative approach to cell-based recombinant protein p... more Cell-free protein synthesis (CFPS) is an alternative approach to cell-based recombinant protein production. It involves in vitro transcription and translation in a cell-free medium. In this work, we implemented CFPS in a plastic array device. Each unit in the array consisted of an inner well and an outer well. Two synthesis steps, gene transcription and protein translation, took place in the inner well, in which a cell-free medium was used to provide ribosomes and additional components necessary for protein synthesis. The outer well was concentric to the inner well and it functioned as a nutrient reservoir. A nanoporous membrane was sandwiched between the inner and outer wells for retaining the synthesized proteins and removing the reaction byproducts. A microfluidic channel was employed to connect these two wells for supplying fresh nutrients for longer reaction time and higher expression yield. Synthesis of luciferase was shown to last 8 times longer and yield 10 times more proteins than in a conventional container. The device also enables more than 2 orders of magnitude reduction in reagent consumption compared to a bench-top instrument. The effects of the membrane pore size and microfluidic channel on the protein production yield were also studied. The array device has potential to become a platform for parallel protein expression for proteomics applications, matching high-throughput gene discovery.

Wetlands Engineering & River Restoration 2001, 2001

Riverbanks are usually stabilized by reinforcing the soil, installing horizontal drains or by reg... more 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.

Geomorphology, 1997

A deterministic numerical model of bed deformation and channel widening, which accounts for speci... more A deterministic numerical model of bed deformation and channel widening, which accounts for specific mechanisms of bank erosion and collapse, is used to analyze morphological and flow-energy parameters in adjusting sand-bed channels, for a range of simulated fluvial environments. The model is based on a set of conservation and process equations solved in conjunction with a set of specific initial and boundary conditions. Simulated channels with a range of assumed boundary-material characteristics were subjected to identical disturbances imposed by reducing the supply of sediment from upstream. Asymptotic reductions of the rate of energy dissipation (energy slope) and boundary shear stress were found to be unifying characteristics of channel adjustment in all simulations. Morphologic responses to an identical disturbance (reduction of sediment supply from upstream), with identical bed material (d50 = 1 mm) and channel gradient (Sb = 0.001), were diverse. The relative magnitude of simulated widening versus simulated bed-level change was greatest for sand-bank channels and least for clay-bank channels. Stable-channel dimensions were attained most rapidly for silt-bank channels, where simulated adjustments of channel width were similar to simulated vertical adjustments, because all components of total-mechanical energy (flow depth or pressure head; bed elevation and channel gradient or datum head; and flow velocity or velocity head) decrease simultaneously. Each simulated channel subjected to the baseline disturbance had an initial width-to-depth ratio (F) of 13.5. Asymptotic values of energy dissipation rate, adjusted F-values, and time taken to reach equilibrium after the disturbance are shown to vary as a function of simulated environmental boundary conditions. The importance of channel widening in controlling and reducing the flow depth of a given discharge and average boundary shear stress is highlighted for the silt- and sand-bank cases. Channel widening causes a shift from degradation to aggradation during adjustment, because of delivery of sand-sized sediments from failed bank material to the simulated channel.

Geomorphology, 1998

Two distinct phases of bed-level adjustment over the last 150 years are identified for the princi... more Two distinct phases of bed-level adjustment over the last 150 years are identified for the principal alluvial reaches of the Arno River (Upper Valdarno and Lower Valdarno). The planimetric configuration of the river in these reaches is the result of a series of hydraulic works (canalization, rectification, artificial cut-offs, etc.) carried out particularly between the 18th and the 19th centuries. Subsequently, a series of interventions at basin level (construction of weirs, variations in land use), intense instream gravel-mining after World War II, and the construction of two dams on the Arno River, caused widespread degradation of the streambed. Since about 1900, total lowering of the channel bed is typically between 2 and 4 m in the Upper Valdarno Reach and between 5 and 8 m in some areas of the Lower Valdarno Reach. Bed-level adjustments with time are analyzed for a large number of cross-sections and described by an exponential-decay function. This analysis identified the existence of two main phases of lowering: the first, triggered at the end of the past century; the second, triggered in the interval 1945–1960 and characterized by more intense degradation of the streambed. The first phase derived from changes in land-use and land-management practices. The second phase is the result of the superimposition of two factors: intense instream mining of gravel, and the construction of the Levane and La Penna dams.

Earth Surface Processes and Landforms, 2002

As a response to channelization projects undertaken near the turn of the 20th century and in the ... more As a response to channelization projects undertaken near the turn of the 20th century and in the late 1960s, upstream reaches and tributaries of the Yalobusha River, Mississippi, USA, have been rejuvenated by upstream-migrating knickpoints. Sediment and woody vegetation delivered to the channels by mass failure of streambanks has been transported downstream to form a large sediment/debris plug where the downstream end of the channelized reach joins an unmodified sinuous reach. Classification within a model of channel evolution and analysis of thalweg elevations and channel slopes indicates that downstream reaches have equilibrated but that upstream reaches are actively degrading.

Earth Surface Processes and Landforms, 1989

Dredging and straightening of alluvial channels between 1959 and 1978 in West Tennessee caused a ... more Dredging and straightening of alluvial channels between 1959 and 1978 in West Tennessee caused a series of morphologic changes along modified reaches and tributary streams. Degradation occurred for 10 to 15 years at sites upstream of the area of maximum disturbance and lowered bed-levels by as much as 6·1 m. Following degradation, reaches upstream of the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and gradient reduction. Aggradation downstream of the area of maximum disturbance reached 0·12 m per year with the greatest rates occurring near the stream mouths.The adjustment of channel geometry and phases of channel evolution are characterized by six process-oriented stages of morphologic development—premodified, constructed, degradation, threshold, aggradation, and restabilization. Down-cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and angle of the bank material is exceeded (threshold stage). Channel widening continues through the aggradation stage as the ‘slough line’ develops as an initial site of lower-bank stability. The bank profile develops three dynamic elements (1) vertical face (70° to 90°), (2) upper bank (25° to 50°), and (3) slough line (20° to 25°). Alternate channel bars form during the restabilization stage and represent incipient meandering of the channel.

Geomorphology, 1995

A modular procedure to assess the magnitude, distribution, and potential for channel instabilitie... more A modular procedure to assess the magnitude, distribution, and potential for channel instabilities at a large number of sites has been designed and implemented. The procedure, based on diagnostic interdisciplinary criteria of alluvial channel morphology and associated riparian vegetation, is presented. The modules include (1) initial site evaluations, (2) GIS-based data input and management, (3) ranking of relative channel stability, (4) identification of spatial trends, (5) ranking of socio-economic impacts and identification of most “critical” sites, and (6) collection of additional field data for more detailed evaluation of the magnitude and type of future instabilities and the effects of proposed mitigation measures. The procedure, using site evaluation forms as the fundamental means of data collection, takes a trained person 1 to 1.5 hours to complete. Site evaluation forms can be altered according to the specific environment being studied and the objectives of the study.An objective ranking scheme based on physical attributes extracted from the GIS data base permits the identification of the most unstable channel sites and, thereby, focuses attention on potentially “critical” sites. If a significant concern about a bridge or adjacent lands arises, or if mitigation measures are considered, various methods to estimate future channel changes are proposed. These include (1) numerical alluvial channel modeling, (2) empirical models of channel evolution, (3) regime equations, and (4) empirical relations based on process dominance in different fluvial environments. The methods require differing amounts of additional field data and provide results of varying detail and accuracy. The decision on which method to use must be based on the objectives and resources of the agency involved in the evaluation study.

Geomorphology, 2009

As part of a study to investigate the causes of channel narrowing and incision in Canyon de Chell... more As part of a study to investigate the causes of channel narrowing and incision in Canyon de Chelly National Monument, the effects of Tamarisk and Russian-olive on streambank stability were investigated. In this study, root tensile strengths and distributions in streambanks were measured and used in combination with a root-reinforcement model, RipRoot, to estimate the additional cohesion provided to layers of each streambank. The additional cohesion provided by the roots in each 0.1-m layer ranged from 0 to 6.9 kPa for Tamarisk and from 0 to 14.2 kPa for Russian-olive. Average root-reinforcement values over the entire bank profile were 2.5 and 3.2 kPa for Tamarisk and Russian-olive, respectively. The implications of vegetation removal on bank stability and failure frequency were evaluated in two incised reaches by modeling bank-toe erosion and bank stability with and without vegetation. The effects of a series of 1.0-and 1.5-m-deep flows on bank-toe erosion, pore-pressure distributions, and bank stability were evaluated first. In addition, bank stability model runs were conducted using iterative modeling of toe erosion and bank stability using a discretized flow and groundwater record for one year. Results showed that the effects of root-reinforcement provided by Tamarisk and Russian-olive have a significant impact on bankstability and bank-failure frequency. Because the bank materials are dominated by sands, cohesion provided by roots is significant to bank stability, providing an average 2.8 kPa of cohesion to otherwise cohesionless bank materials. Bank retreat rates at one site following vegetation removal have approximately doubled when compared to the control reach (from an approximate rate of 0.7-0.8 m/y between 2003 and 2006 to 1.85 m/y during the year modeled). Vegetation removal along the entire riparian corridor in Canyon de Chelly may lead to the introduction of significantly more sediment to the system through bank widening processes, although it is not known whether this change alone would be sufficient to cause a shift in channel morphology to the wide-braided channels that were once characteristic of this canyon.

Earth Surface Processes and Landforms, 2002

Riparian vegetation strips are widely used by river managers to increase streambank stability, am... more Riparian vegetation strips are widely used by river managers to increase streambank stability, among other purposes. However, though the effects of vegetation on bank stability are widely discussed they are rarely quantified, and generally underemphasize the importance of hydrologic processes, some of which may be detrimental. This paper presents results from an experiment in which the hydrologic and mechanical effects of four riparian tree species and two erosion-control grasses were quantified in relation to bank stability. Geotechnical and pore-water pressure data from streambank plots under three riparian covers (mature trees, clump grasses and bare/cropped turf grass) were used to drive the ARS bank stability model, and the resulting factor of safety (Fs) was broken down into its constituent parts to assess the contribution (beneficial or detrimental) of individual hydrologic and mechanical effects (soil moisture modification, root reinforcement and surcharge). Tree roots were found to increase soil strength by 2–8 kPa depending on species, while grass roots contributed 6–18 kPa. Slope stability analysis based on data collected during bank failures in spring 2000 (following a very dry antecedent period) shows that the mechanical effects of the tree cover increased Fs by 32 per cent, while the hydrologic effects increased Fs by 71 per cent. For grasses the figures were 70 per cent for mechanical effects and a reduction of Fs by 10 per cent for the hydrologic effects. However, analysis based on bank failures in spring 2001 (following a wetter than average antecedent period) showed the mechanical effects of the tree cover to increase Fs by 46 per cent, while hydrologic effects added 29 per cent. For grasses the figures were 49 per cent and −15 per cent respectively. During several periods in spring 2001 the hydrologic effects of the tree cover reduced bank stability, though this was always offset by the stabilizing mechanical effects. The results demonstrate the importance of hydrologic processes in controlling streambank stability, and highlight the need to select riparian vegetation based on hydrologic as well as mechanical and ecological criteria. Published in 2002 by John Wiley & Sons, Ltd.

Journal of The American Water Resources Association, 2000

ABSTRACT: The bess area of the midwestern United States contains thousands of miles of unstable s... more ABSTRACT: The bess area of the midwestern United States contains thousands of miles of unstable stream channels that are undergoing system-wide channel-adjustment processes as a result of (1) modifications to drainage basins dating back to the turn of the 20th century, including land clearing and poor soil-conservation practices, which caused the filling of stream channels, and consequently (2) direct, human modifications to stream channels such as dredging and straightening to improve drainage conditions and reduce the frequency of out-of-bank flows. Today, many of these channels are still highly unstable and threaten bridges, other structures, and land adjacent to the channels. The most severe, widespread instabilities are in western Iowa where a thick cap of bess and the lack of sand-and gravel-sized bed sediments in many channels hinders downstream aggradation, bed-level recovery and the consequent reduction of bank heights, and renewed bank stability. In contrast, streams draining west-central Illinois, east-central Iowa, and other areas, where the bess cap is relatively thin and there are ample supplies of sand-and gravel-sized material, are closer to recovery. Throughout the region, however, channel widening by mass-wasting processes is the dominant adjustment process.

Geomorphology, 2000

Gravitational forces acting on in situ bank material act in concert with hydraulic forces at the ... more Gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion. The interaction of these forces control streambank mechanics. Hydraulic forces exerted by flowing water on in situ bank-toe material and failed cohesive material at the bank toe are often sufficient to entrain materials at relatively frequent flows and to maintain steep lower-bank profiles. Seepage forces exerted on in situ bank material by groundwater, downward infiltration of rainwater and lateral seepage of streamflow into and out of the bank are critical in determining bank strength. Data from a study site on Goodwin Creek, MS, USA clearly show the temporal variability of seepage forces and the lag time inherent in reductions in shear strength due to losses of matric suction and generation of positive pore-water pressures. Negative pore-water pressures (matric suction) have also been shown to increase the resistance of failed cohesive blocks to entrainment by fluid shear. A stable bank can be transformed into an unstable bank during periods of prolonged rainfall through:increase in soil bulk unit (specific) weight,decrease or complete loss of matric suction, and, therefore, apparent cohesion,generation of positive pore-water pressures, and, therefore, reduction or loss of frictional strength,entrainment of in situ and failed material at the bank toe, andloss of confining pressure during recession of stormflow hydrographs.

Journal of Hydraulic Engineering, 1996

Geomorphology, 2004

Historical flow and suspended-sediment transport data from more than 2900 sites across the United... more Historical flow and suspended-sediment transport data from more than 2900 sites across the United States have been analyzed in the context of estimating flow and suspended-sediment transport conditions at the 1.5-year recurrence interval flow (Q1.5). This is particularly relevant with the renewed focus on stream restoration activities and the urgency in developing water-quality criteria for sediment. Data were sorted into the 84 Level III ecoregions to identify spatial trends in suspended-sediment concentrations and yields to meaningfully describe suspended-sediment transport rates across the United States. Arguments are developed that in lieu of form-based estimates of say the bankfull level, a flow of a given recurrence interval (Q1.5) is more appropriate to integrate suspended-sediment transport ratings for the purpose of defining long-term transport conditions at a site (the “effective discharge”). The use of the Q1.5 as a measure of the effective discharge for suspended-sediment transport is justified on the basis of literature reports and analytic results from hundreds of sites in 17 ecoregions that span a diverse range of hydrologic and topographic conditions (i.e., Coast Range, Arizona/New Mexico Plateau, Mississippi Valley Loess Plains, Middle Atlantic Coastal Plain). There is sufficient data to also develop regional curves for the Q1.5 in all but eight of the ecoregions. At the Q1.5 the highest median suspended-sediment concentrations occur in semiarid environments (Southwest Tablelands, Arizona/New Mexico Plateau and the Mojave Basin and Range); the highest yields occur in humid regions with erodible soils and steep slopes or channel gradients (Mississippi Valley Loess Plains [MVLP] and the Coast Range). Suspended-sediment yields for stable streams are used to determine “background” or “reference” sediment transport conditions in eight ecoregions where there is sufficient field data. The median value for stable sites within a given ecoregion are generally an order of magnitude lower than for nonstable sites.

Journal of Hydraulic Engineering-asce, 2003

Stream restoration, or more properly rehabilitation, is the return of a degraded stream ecosystem... more Stream restoration, or more properly rehabilitation, is the return of a degraded stream ecosystem to a close approximation of its remaining natural potential. Many types of practices ͑dam removal, levee breaching, modified flow control, vegetative methods for streambank erosion control, etc.͒ are useful, but this paper focuses on channel reconstruction. A tension exists between restoring natural fluvial processes and ensuring stability of the completed project. Sedimentation analyses are a key aspect of design since many projects fail due to erosion or sedimentation. Existing design approaches range from relatively simple ones based on stream classification and regional hydraulic geometry relations to more complex two-and three-dimensional numerical models. Herein an intermediate approach featuring application of hydraulic engineering tools for assessment of watershed geomorphology, channel-forming discharge analysis, and hydraulic analysis in the form of one-dimensional flow and sediment transport computations is described.

Hydrological Processes, 2009

... F. Douglas Shields Jr,* Andrew Simon and Seth M. Dabney USDA-Agricultural Research Service, N... more ... F. Douglas Shields Jr,* Andrew Simon and Seth M. Dabney USDA-Agricultural Research Service, National Sedimentation Laboratory Oxford ... Limited measurements following storm events documented dis-charge rates from individual seeps ranging from 0Ð068 to 0Ð931 m3 ...

Environmental Earth Sciences, 1987

Channel modifications from 1968 to 1969 on the South Fork Forked Deer River in western Tennessee ... more Channel modifications from 1968 to 1969 on the South Fork Forked Deer River in western Tennessee have caused upstream degradation, downstream aggradation, and bank failures along the altered channels, adjacent reaches, and tributaries. The result of these adjustments is a general decrease in gradient as the channel attempts to absorb the imposed increase in energy conditions created by channelization. Headward degradation at a rate of approximately 2.57 km/yr on the South Fork Forked Deer River caused from 1.52 m to about 3.14 m of incision over a 13.5 km reach from 1969 to 1981. As a consequence of substantially increased sediment supply, approximately 2.13 m of aggradation was induced downstream of this reach during the same period. This accumulation represents a 60% recovery of bed level at the downstream site since the completion of channel work in 1969. Gradient adjustment with time is described by exponential decay functions. The length of time required for adjustment to some new quasi-equilibrium condition is computed by these decay functions and is about 20 years from the completion of channel work. Adjusted slopes are less than predisturbed values, probably because straightened channels dissipate less energy by friction, allowing more energy for sediment transport. An equivalent sediment load, therefore, can be transported at a considerably gentler slope. The predisturbed slope exceeds the adjusted slope by an order of magnitude on the downstream reach of the South Fork Forked Deer River.

Earth Surface Processes and Landforms, 1996

Channel adjustments in the North Fork Toutle River and the Toutle River main stem were initiated ... more Channel adjustments in the North Fork Toutle River and the Toutle River main stem were initiated by deposition of a 2.5 km 3 debris avalanche and associated lahars that accompanied the catastrophic eruption of Mount St. Helens, Washington on 18 May 1980. Channel widening was the ...

Environmental Conservation, 2000

Human occupation and development of alluvial river floodplains are adversely affected by river ch... more Human occupation and development of alluvial river floodplains are adversely affected by river channel lat-eral 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 ...

Geomorphology, 2001

Ž . The management of large woody debris LWD should be based on a rational assessment of its recr... more Ž . The management of large woody debris LWD should be based on a rational assessment of its recruitment rate relative to its natural decay and removal. LWD recruitment may be controlled by 'natural' episodic terrestrial factors or by in-channel geomorphological controls related to the rate of bank erosion. The geomorphological controls are hard to quantify in laterally migrating channels, but in incising channels, a conceptual model may be developed based on the density of riparian trees relative to the knickpoint migration rate and bank stability analyses that predict the post-knickpoint width of the channel. The Yalobusha river network in Central Mississippi, USA, has twice been destabilised by channel straightening for flood defence and land drainage, most recently in 1967. System-wide rejuvenation has followed through a series of upstream migrating knickpoints several metres high that have caused mass failure of streambanks and the recruitment of large volumes of trees to the channel. LWD recruitment is maximised at the transition between stage III and stage IV channels, focusing attention on 11 sites in the network. The sites are upstream of knickzones ranging between 2.2 and 5.4 m high and migrating at rates of 0-13.8 m year y1 , based on 23-30 months of monitoring. Riparian conditions in 500 m 2 plots on each bank upstream of the knickpoints range from treeless to forested, containing 0-98 trees with an average diameter at breast height of 0.18 m and average maximum height of 14.0 m. The average volume of wood on each bank is 0.02 m 3 m y2 . Under rapid drawdown conditions, bank stability analyses suggest that the channels will widen in amounts ranging from 1.8 to 31.5 m. Combined with the knickpoint migration rates, riparian land losses are estimated to range from 8.0 to y1 3 ) Corresponding author. Philip front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 1 6 9 -5 5 5 X 0 0 0 0 0 6 3 -5 ( ) P.W. Downs, A. Simonr Geomorphology 37 2001 65-91 66 vegetation density in upstream and headwater riparian zones of each tributary. The 11 study sites are classified into groups with similar LWD management concerns based on these analyses. The models developed in this research provide the first precise quantification of LWD recruitment according to geomorphological controls and standing vegetation, and a rational assessment of its meaning, but further research is required to improve the accuracy of such estimates. q

Geomorphology, 1991

Hupp, C.R. and Simon, A., 1991. Bank accretion and the development of vegetated depositional surf... more Hupp, C.R. and Simon, A., 1991. Bank accretion and the development of vegetated depositional surfaces along modified alluvial channels. Geomorphology, 4:11 l-124. This paper describes the recovery of stable bank form and development of vegetated depositional surfaces along the banks of channelized West Tennessee streams. Most perennial streams in West Tennessee were straightened and dredged since the turn of the century. Patterns of fluvial ecological responses to channelization have previously been described by a six-stage model. Dendrogeomorphic (tree-ring) techniques allowed the determination of location, timing, amount, and rate of bank-sediment deposition. Channel cross sections and ecological analyses made at 101 locations along 12 streams, encompassing bends and straight reaches, show that channel and bank processes initially react vertically to channelization through downcutting. A depositional surface forms on banks once bed-degradation and heightened bank mass wasting processes have eased or slowed. The formation of this depositional surface marks the beginning of bank recovery from channelization. Dominating lateral processes, characteristic of stable or natural channels, return during the formation and expansion of the depositional surface, suggesting a relation with thalweg deflection, point-bar development, and meanderloop extension. Characteristic woody riparian vegetation begins to grow as this depositional surface develops and becomes part of the process and form of restabilizing banks. The depositional surface initially forms low on the bank and tends to maintain a slope of about 24 ° . Mean accretion rates ranges from 5.9 cm/yr on inside bends to 0 cm/yr on most outside bends; straight reaches have a mean-accretion rate of 4.2 cm/yr. The relatively stable, convex upward, depositional surface expands and ultimately attaches to the flood plain. The time required for the recovery process to reach equilibrium averaged about 50 years. Indicative pioneer species of woody riparian vegetation include black willow, river birch, silver maple, and boxelder. Stem densities generally decrease with time after and initial flush of about 160 stems per 100 m 2. Together bank accretion and vegetative regrowth appear to be the most important environmental processes involved in channel bank recovery from channelization or rejuvenation. 0169-555X/91/$03.50