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Doug Thompson

Connecticut College, Physics and Astronomy, Faculty Member

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Phone: 860-439-5016
Address: Department of Physics, Astronomy and Geophysics
Olin Science Center, Room 203
Connecticut College, Box 5585
270 Mohegan Avenue
New London, CT 06320

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Papers by Doug Thompson

Velocity reversals and sediment sorting in pools and riffles controlled by channel constrictions

Geomorphology, 1999

Keller Keller, E.A., 1971. Areal sorting of bed-load material: the hypothesis of velocity reversa... more Keller Keller, E.A., 1971. Areal sorting of bed-load material: the hypothesis of velocity reversal. Geological Society of x America Bulletin 82, 753-756 hypothesized that at high flow, near-bed velocities in pools exceed velocities in riffles and create pool scour. Pools, however, typically have larger cross-sectional areas of flow at bankfull discharge. This condition raises an inconsistency with Keller's velocity reversal hypothesis and the one-dimensional continuity of mass equation. To address this problem, a model of pool maintenance and sediment sorting is proposed that relies on constriction of flow by recirculating eddies and flow divergence over the exit-slopes of pools. According to the model, a narrow zone of high velocity occurs in the center of pools, creating scour. Along the downstream end of pools, an uphill climb of particles up the pool exit-slope promotes sediment deposition. The model is tested with field and flume measurements of velocity, water-surface elevation, and size of bed sediments in recirculating-eddy influenced pools. Local reversals of the water-surface gradient were measured in the field and a velocity reversal was created in the flume. The reversals that were measured indicate higher gradients of the water surface over the upstream portions of pools and higher velocities in pools at high flow. The distribution of bed sediments collected in the field also support the proposed model of pool maintenance.

The Long-Term Stability and Morphologic Influence of the Use of Instream Structures in Channel-Restoration Design

Widespread use of instream structures in the United States occurred as early as the 1930s under t... more Widespread use of instream structures in the United States occurred as early as the 1930s under the direction of the Civilian Conservation Corp. Specific designs of instream structures were well developed in the 1930s with prototype versions of most currently employed designs. The Civilian Conservation Corp and U.S. Forest Service helped to standardize the designs in the 1930s and subsequent decades, and promote their use throughout the country. Many modern designs of instream structures show little or no variation in materials, construction methods and placement specifications relative to the 1930s prototypes. Consequently, surviving instream structures from projects in the last seventy years provide an opportunity to assess the long-term stability and morphologic impact of these devices. Field studies in the northeastern U.S. show that decay of structures can create site conditions that adversely impact aquatic and riparian habitat. Loss of streamside trees and reduced bank cover ...

Formation and maintenance of a forced pool-riffle couplet following loading of large wood

Geomorphology, 2017

Pool-riffle maintenance has been documented in numerous studies, but it has been almost impossibl... more Pool-riffle maintenance has been documented in numerous studies, but it has been almost impossible to characterize detailed natural pool-riffle formation mechanisms because of the lack of baseline data prior to pool establishment. In 2013, a study was conducted on the Blackledge River in Connecticut to document the formation of a new pool-riffle couplet on a section of river that had previously been studied from 1999-2001. In 2001, the study reach contained a scour hole with a residual depth of 0.08 ±0.09 m downstream of a 1930s paired deflector with no identifiable riffle immediately downstream. At this time, a large, severely undercut, hemlock tree was noted along the left bank. Sometime between fall 2001 and 2004, the tree fell perpendicular to flow across the channel and formed a large wood (LW) jam and new pool-riffle couplet several meters downstream of the old scour hole. Pool spacing along the reach decreased from 4.47 bankfull widths (BFW) in 1999 to 3.83 BFW after the new pool-riffle couplet formed. The new pool has a residual depth, the water depth of the streambed depression below the elevation of the immediate downstream hydraulic control, of 1.36 ±0.075 to 1.59 ±0.075 m, which resulted from a combination of 1.32 ±0.

The history of the use and effectiveness ofinstream structures in the United States

The use of instream structures, devices designed to improve fi sh habitat, began as early as 1880... more The use of instream structures, devices designed to improve fi sh habitat, began as early as 1880 in the United States and continues today. The practice of stream improvement was partially motivated by the desire to compensate for overfi shing problems. Many of the practices that involve the use of instream structures emerged during a time period when scientifi c-management principles offered the hope that humans could eliminate perceived ineffi ciencies and increase biological productivity in natural systems. Decades later, modern criteria of instream structures trace many of their details of design to experimental devices employed in the 1920s and 1930s. However, problems with the use of many styles were noted soon after they were fi rst deployed, and many of these troubles persist today. Dams can be undermined and outfl anked by fl ows. Defl ectors disrupt the bed and hamper the development of food organisms. Finally, cover structures suffer from siltation problems and longterm decay, which renders the devices useless. The best possible long-term solution to improved health of riverine fi sheries may be to avoid the use of static engineering structures when possible and focus on reforestation and erosion control in the watersheds. Even this recommendation dates back over 65 years to the period when the use of instream structures fi rst began to fl ourish in the United States.

A Comparison of A Seawall-Constrained and Unconstrained Beach in Groton, Connecticut

Changes in pool size in response to a reduction in discharge: a flume experiment

River Research and Applications, 2006

Management of river systems can create a reduction in flow with associated potential impacts on c... more Management of river systems can create a reduction in flow with associated potential impacts on channel morphology. A total of 30 flume runs were conducted with two different sediment sizes to determine the effect of a reduction in flow on pool size and depth in confined channels. The experiments were conducted in a 6 m long, 0.5 m wide recirculating flume. Pools were scoured adjacent to a triangular-shaped obstruction to flow from an initially flat bed. After 3 h of high flow, bed topography was measured and the flow was changed to a level of 50, 67, 80, 85 or 100% of the original flow. Changes in bed topography were then measured after 3 h under the modified flow regime. Results indicate that a reduction in flow creates smaller volume pools. With well sorted, fine-sand substrate, changes in pool volume, volume of deep habitat and depth were significant for both 50 and 80% reductions in flow level. However, development of an armor level in experiments with poorly sorted, very coarse sand limited sedimentation at lower flows. Consequently, no changes in pool size were statistically significant with this sediment. The results suggest that data on the dimensions of pools are needed prior to any flow modifications in order to detect degradation of the physical pool habitat. Monitoring programs initiated after flow regulation may only identify changes in a limited number of cases where sediments are mobile at low flows and the reduction in flow level is severe.

Channel-Bed Scour with High versus Low Deflectors

Journal of Hydraulic Engineering, 2002

... Res., 34(12), 36733681. Thompson, DM, Wohl, EE, and Jarrett, RD (1999). Pool sediment sorti... more

The influence of lee sediment behind large bed elements on bedload transport rates in supply-limited channels

Geomorphology, 2008

A coarse surface layer can help to limit bedload transport rates in channels with cobble and grav... more A coarse surface layer can help to limit bedload transport rates in channels with cobble and gravel beds. In these systems, periodic bouldersized clasts often exist with small deposits of fine material in the lee of these large bed elements. A combined field and flume study was conducted to investigate the potential impact of lee deposits with distinctly finer sediment-sizes behind boulders on bedload transport rates. Detailed sediment characterizations were performed on surface, subsurface, and lee sediments in two coarse-bedded Connecticut channels. Bedload measurements also were conducted in a series of flows that approached the bankfull level in these two systems to determine transport rates and the size distribution of bedload material. A 6-m long, 0.5-m wide flume was used to model these systems with fine sediment passing over a fixed bed of sediment particles with uniform-sized, large bed elements. Sediment distributions of the lee deposits in the two Connecticut channels indicate that lee deposits may be produced from winnowing of sediments from the surface layer. Lee deposits also exhibit sediment distributions similar to bedload sediment distributions from low to near-bankfull flow in one of the two channels. Bedload sediments in the second channel were finer than lee deposits, presumably from selective entrainment of fines. Flume experiments demonstrate that bedload transport rates are lower for periods of steady flow relative to periods that include either an increase or decrease in discharge. The results show that lee sediments establish a metastable deposit behind each obstruction for a given discharge. Either increases or decreases in discharge disrupt this temporary stability and increase sediment delivery to the main flow. The study suggests that the influence of the rate of change in discharge may be as important as the absolute magnitude of discharge on sediment transport rates at moderate and low discharges in sediment-limited systems with large bed elements.

Equilibrium pool dimensions and sediment-sorting patterns in coarse-grained, New England channels

Geomorphology, 2001

Pools and riffles are natural bedforms that produce variations in width and depth along channels ... more Pools and riffles are natural bedforms that produce variations in width and depth along channels deemed critical for aquatic habitat. An understanding of this morphology is necessary for the successful design of a reconstructed or restored channel. To better understand pool geometry and sorting characteristics of New England channels, characterizations of pool geometry were performed on 145 different pools strongly influenced by channel constrictions. Characterizations of channel-bed sediments were also performed at 34 of these locations. Data from 120 pools were used in least-squares, multiple-regression analysis with 19 different independent variables to develop prediction equations for pool depth, length, and exit-slope gradient. These equations were then tested using the 25 remaining pools. Eighteen different geometric variables were then used to predict sediment f values in pool centers, pool exit slopes, and riffles for the subset of 34 50 samples. The regression analyses for pool geometry indicate that pool depth is significantly influenced to a decreasing degree by pool exit-slope width, constriction gradient, constriction width, drainage area, upstream channel width, and the exit-slope expansion ratio, with an R 2 value of 0.53. Similarly, pool length is influenced by channel gradient, location of the channel constriction, pool width, drainage area and constriction length, with an R 2 value of 0.72. The gradient of pool exit slope is influenced by drainage area, channel gradient, and constriction gradient, with an R 2 value of 0.34. However, R 2 values for exit slope increased to 0.65 when sediment d data were utilized. Using the resulting regression equations to 50 predict dimensions for the 25 verification pools yields average residuals near zero, although moderate variations do exist. A Tukey-Kramer HSD comparison of the means for sediment f values indicates pool sediments are significantly coarser 50 than pool exit-slope and riffle sediments for the subset of 34 pools. Regression analyses also suggest significant relations between the size of sediments in pools and riffles and a combination of geometric variables with R 2 values that range from 0.45 to 0.62. The results suggest that pool dimensions are related to basin-wide variability and local hydraulic conditions. The findings also suggest that the characteristics of constrictions and the downstream section of pools provide an important

The characteristics of turbulence in a shear zone downstream of a channel constriction in a coarse-grained forced pool

Geomorphology, 2007

A field-based project was initiated to characterize the influence of varying discharge and bed to... more A field-based project was initiated to characterize the influence of varying discharge and bed topography along a pool exit-slope on the strength of turbulence generated by vortex shedding. Velocities were measured with an ADV downstream of a boulder constriction within a shear zone of a large pool. Measurements were repeated for four flows that varied from 20% to 50% of bankfull discharge. An additional longitudinal profile was conducted along the pool thalweg. Plots of velocity demonstrate high near-bed velocities and turbulence levels in the pools. Turbulence levels were highest closer to the constriction in the shear zone. Turbulent kinetic energy decayed in the downstream direction in response to the bed topography and at lower discharges. Instantaneous velocities are large enough to temporarily lift pebbles at this depth. It appears that instantaneous forces created by vortex shedding may play an important role in scouring pools downstream of constrictions. The turbulence may also respond to changes in bed topography in a manner that encourages feedbacks among bed topography, characteristics of turbulence, and sediment transport.

The effects of large organic debris on sediment processes and stream morphology in Vermont

Geomorphology, 1995

Research conducted on a small third-order stream in the Green Mountains of Vermont suggests that ... more Research conducted on a small third-order stream in the Green Mountains of Vermont suggests that large organic debris (LOD) has an important influence on areal sorting and storage of sediment, spacing of pool-riffle sequences, and channel geometry. Large organic debris forms local base-levels that trap over 26 cubic meters of sediment and cause over 14 percent of the vertical drop along a 412 meter study reach on Tiger Brook, Vermont. The local base-levels created by large organic debris accumulations are the most important control on sediment storage between large bedrock knickpoints along the study reach, focusing fine-grained sediment deposition in areas stable at low and moderate discharges. The micro-steps created by these LOD local base-levels dominate gradient and sediment storage at short timescales between the slowly evolving large bedrock macrosteps. The relation that exists among the volumes of standing timber adjacent to the channel, LOD and sediment stored behind LOD seems to indicate that large organic debris causes a negative feedback-type mechanism where channel degradation leads to increased standing timber recruitment and large organic debris sediment-storage sites.

Geometric adjustment of pools to changes in slope and discharge: a flume experiment

Geomorphology, 2002

To characterize the factors controlling pool shape, 30 different forced pools were created utiliz... more To characterize the factors controlling pool shape, 30 different forced pools were created utilizing a 50% triangular constriction in a 0.5-m wide, 6-m long recirculating flume. Pools were scoured from an initial plane bed of sand with a d 50 of 0.25 mm. Pool depth and length were measured and used as dependent variables in least-squares, multiple-regression analyses. Discharge, channel-bed gradient and energy slope were the independent variables. Additional linear-regression analyses were conducted with either pool depth or length and stream power. Results indicate that both pool depth and length are primarily a function of discharge. Channel-bed and energy slopes are also significantly related to pool length but are not significantly related to pool depth. Stream power is significantly related to both pool depth and length, but R 2 values for pool depth versus discharge indicate stronger relations than those between pool depth and stream power. Observations on the type of geometric adjustment indicate that pools may minimize their rate of energy expenditure primarily through elongation. In contrast, pool depth appears to be more sensitive to the characteristics of the constrictions that create the forced pools. The results suggest that many field studies may suffer from cross-correlation problems. In particular, channel erodibility may exert a more dominant influence on pool geometry than hydraulic controls in many constriction-influenced channels.

Geometric adjustment of pools to changes in slope and discharge: a flume experiment

Geomorphology, 2002

To characterize the factors controlling pool shape, 30 different forced pools were created utiliz... more To characterize the factors controlling pool shape, 30 different forced pools were created utilizing a 50% triangular constriction in a 0.5-m wide, 6-m long recirculating flume. Pools were scoured from an initial plane bed of sand with a d 50 of 0.25 mm. Pool depth and length were measured and used as dependent variables in least-squares, multiple-regression analyses. Discharge, channel-bed gradient and energy slope were the independent variables. Additional linear-regression analyses were conducted with either pool depth or length and stream power. Results indicate that both pool depth and length are primarily a function of discharge. Channel-bed and energy slopes are also significantly related to pool length but are not significantly related to pool depth. Stream power is significantly related to both pool depth and length, but R 2 values for pool depth versus discharge indicate stronger relations than those between pool depth and stream power. Observations on the type of geometric adjustment indicate that pools may minimize their rate of energy expenditure primarily through elongation. In contrast, pool depth appears to be more sensitive to the characteristics of the constrictions that create the forced pools. The results suggest that many field studies may suffer from cross-correlation problems. In particular, channel erodibility may exert a more dominant influence on pool geometry than hydraulic controls in many constriction-influenced channels.

The challenge of modeling pool-riffle morphologies in channels with different densities of large woody debris and boulders

Earth Surface Processes and Landforms, 2011

Studies on pool morphologies include reports of over 80% or 90% of pools being associated with st... more Studies on pool morphologies include reports of over 80% or 90% of pools being associated with structural controls and large obstructions that include boulders, bedrock outcrops and large woody debris (LWD). A Monte Carlo simulation approach and developmental computer model was created to predict pool formation, spacing and the percentage length covered by pools, riffles, scour holes and runs based on input data that include channel slope, width, the number of small and large boulders, and the number of 10-30 cm, 30-60 cm and >60 cm pieces of wood. The statistical-empirical model is founded on the idea that boulders, bedrock outcrops and large woody debris provide a physical framework that then controls local water-surface slopes, velocity patterns and the locations of pools and riffles. The spacing values of individual types and sizes of obstructions are modeled as log-normal distributions with separate distributions for each obstruction type. Pools are assigned different probabilities of development depending on the obstruction type. Pool and riffle lengths used to create the subsequent morphology follow their own slope-dependent, log-normal trends. A minimum distance develops between successive pools because of the backwater and turbulent conditions needed for pool formation. The total number and spacing of pools, riffles and scour holes thus reflects the number and locations of obstructions and characteristics of the pool-riffle couplet. The simulation model accurately captures the number of pools in the modeled data range at 65% of all the verification field sites, and 86% of the verification field sites with a more limited range of width and slope characteristics. Lower levels of prediction capabilities are associated with modeled numbers of scour holes and log jams. The model accurately mimics some statistical attributes of pool spacing, and future versions of the model could be developed to improve overall predictive capabilities.

Random controls on semi-rhythmic spacing of pools and riffles in constriction-dominated rivers

Earth Surface Processes and Landforms, 2001

Average pool spacing between five and seven bankfull widths has been documented in environments t... more Average pool spacing between five and seven bankfull widths has been documented in environments throughout the world, but has limited theoretical justification in coarse-bedded and bedrock environments. Pool formation in coarse-bedded and bedrock channels has been attributed to bedrock and boulder constrictions. Because the spacing of these constrictions may be irregular in nature, it is difficult to reconcile pool-formation processes with the supposedly rhythmic spacing of pools and riffles. To address these issues, a simulation model for pool and riffle formation is used to demonstrate that semi-rhythmic spacing of pools with an approximate spacing of five to seven bankfull widths can be recreated from a random distribution of obstructions and minimum pool-and riffle-length criteria. It is assumed that a pool-riffle couplet will achieve a minimum length based on dominant-discharge conditions. Values for the minimum-length assumption are based on field data collected in New England and California, while the theoretical basis relies on the demonstrated hydraulic response of individual pools to elongation. Results from the simulations show that the location of pools can be primarily random in character, but still assume an average spacing between four and eight bankfull widths for a variety of conditions. Field verification data generally support the model but highlight a highly skewed distribution of pool-forming elements and pool spacing. The relation between pool spacing and bankfull widths is attributed to the common geometric response of these features to dominant-discharge conditions.

The Development and Historic Use of Habitat Structures in Channel Restoration in the United States: The Grand Experiment in Fisheries Management

Géographie physique et Quaternaire, 2002

The use of instream structures to modify aquatic habitat has a long history in the United States.... more The use of instream structures to modify aquatic habitat has a long history in the United States. Pioneering work by wealthy landowners in the Catskills region of New York produced a range of designs in the decades preceding the Great Depression in an effort to replenish fish populations depleted from overfishing. The scientific evaluation of structures began in 1930. Within two years, a Michigan research team claimed improved fish populations. Cheap labor and government-sponsored conservation projects spearheaded by the Civilian Conservation Corps allowed the widespread adoption of the techniques in the 1930s, before adequate testing of the long-term impact of the devices. The start of World War II temporarily ended the government conservation efforts and prevented the continued evaluation of structures. During the 1940s, 1950s and 1960s, designs of instream structures remained essentially unchanged. Meanwhile, the small number of evaluations of the impact of the structures often w...

Riparian Vegetation Influence on Channel Widths of Small Streams: Revisiting Sleepers River

Managing Watersheds for Human and Natural Impacts, 2005

Attempt has been made to obtain mullite as a double oxide compound of alumina and silica. Pure an... more Attempt has been made to obtain mullite as a double oxide compound of alumina and silica. Pure and fine mullite particle were prepared by supplying both aluminum acetate as corrosion product of aluminum in anhydride acetic acid solution containing hydrogen peroxide and silica as hydrolysis of tetraethoxysilane. One of new inorganic syntheses named corrosion synthesis is electrochemically explained in the case of making the mullite by applying corrosion phenomena and its product.

A flume experiment on the effect of constriction shape on the formation of forced pools

Hydrology and Earth System Sciences, 2010

A series of 18 flume runs were conducted in a 6-m long, 0.5-m wide recirculating flume with a bed... more A series of 18 flume runs were conducted in a 6-m long, 0.5-m wide recirculating flume with a bed gradient of 0.8% to determine the influence of obstruction shape on the formation and characteristics of forced pools. Six different-shaped obstructions were added to the flume with the maximum width of the obstruction held constant at 20 cm, which equaled a 40% constriction of flow. The obstruction shapes used included a square, a rectangle, a right triangle with the hypotenuse-facing upstream, a right triangle with the hypotenuse-facing downstream, a combination of a square and triangle with the hypotenuse-facing upstream, and a rectangle and semi-circle shape. Three flume runs were conducted with each obstruction shape. A profile of the flume bed was taken after each experiment and a grid measurement of bed elevations for the last run were conducted to create topographic maps of the flume bed to compare poolriffle morphologies. ANOVA results indicate pool depth, pool location, and the distance between the pool center and the riffle crest all vary with the obstruction shape. Obstructions with a more blunt upstream face created deeper pools, more total scour and longer pool-riffle sequence lengths than pools formed by obstructions with a more gradual narrowing of flow. The increased volume of scour associated with obstructions that rapid narrow flow also creates larger volume riffles that cover a greater extent of the channel bed. Some of the difference in pool shape highlight variations in helical flow development and vortex shedding location that result from different approach conditions of the flow.

A Geomorphic Explanation for a Meander Cutoff Following Channel Relocation of a Coarse-Bedded River

Environmental Management, 2003

The Veteran's Fishing section of the Blackledge River in central Connecticut was relocated in the... more The Veteran's Fishing section of the Blackledge River in central Connecticut was relocated in the late 1950s. The relocation resulted in an unstable channel despite extensive efforts to prevent erosion. Overbank erosion and meander cutoffs were investigated using detailed survey data, characterizations of sediment deposits, flow modeling, and a moment-stability analysis. Limited reworking of revetment boulders indicate that riprap bank material was immobile during a 1979 flood event responsi-ble for the formation of the cutoff channel. A moment-stability analysis factor-of-safety value of 1.1 supports the conclusion that riprap was not directly eroded from the banks. Alluvial particles with d 95 values ranging up to 120 mm were deposited along a bar downstream from the cutoff channel at flows estimated to be below a 1.5-year recurrence interval flow. Development of the bar deposit resulted in locally elevated water surfaces at high flow. The resulting overbank flow across the meander neck to the adjacent downstream bend led to the creation of an upstream migrating knickpoint, the erosion of approximately 16,000-year-old sediments, and the subsequent meander cutoff. The results of the study indicate that traditional erosion-control measures cannot prevent extreme channel adjustments if the geomorphic processes that control sediment continuity also are not considered.

Did The Pre-1980 Use Of In-Stream Structures Improve Streams? A Reanalysis Of Historical Data

Ecological Applications, 2006

In the 1930s, after only three years of scientific investigation at the University of Michigan In... more In the 1930s, after only three years of scientific investigation at the University of Michigan Institute for Fisheries Research, cheap labor and government-sponsored conservation projects spearheaded by the Civilian Conservation Corps allowed the widespread adoption of in-stream structures throughout the United States. From the 1940s through the 1970s, designs of in-stream structures remained essentially unchanged, and their use continued. Despite a large investment in the construction of in-stream structures over these four decades, very few studies were undertaken to evaluate the impacts of the structures on the channel and its aquatic populations. The studies that were undertaken to evaluate the impact of the structures were often flawed. The use of habitat structures became an ''accepted practice,'' however, and early evaluation studies were used as proof that the structures were beneficial to aquatic organisms. A review of the literature reveals that, despite published claims to the contrary, little evidence of the successful use of in-stream structures to improve fish populations exists prior to 1980. A total of 79 publications were checked, and 215 statistical analyses were performed. Only seven analyses provide evidence for a benefit of structures on fish populations, and five of these analyses are suspect because data were misclassified by the original authors. Many of the changes in population measures reported in early publications appear to result from changes in fishing pressure that often accompanied channel modifications. Modern evaluations of channel-restoration projects must consider the influence of fishing pressure to ensure that efforts to improve fish habitat achieve the benefits intended. My statistical results show that the traditional use of in-stream structures for channel restoration design does not ensure demonstrable benefits for fish communities, and their ability to increase fish populations should not be presumed.