Complex processes of sediment dynamics over (relatively) short time scales
- *A.Ockelford, A.Cundy, J.Ebdon and J.Stead, Process controls on microplastic recontamination in fluvial sediments due to flooding
Microplastic burden in aquatic environments is now recognised as one of the largest threats to human and environmental health of the 21st century. However, although microplastic transfers to the ocean from the terrestrial river network contributes between 64 and 90% of the plastics in the oceans (Mehlart and Blepp, 2012) the pathways and mechanisms of transfers are largely unconstrained. Within river channels it is the sediment bed that plays a vital role in mediating transfers since microplastics are buried and stored within the sediments on the river bed. During flood events erosion of the bed surface means microplastics buried within the bed are exchanged with the surface across the active layer and are potentially re-mobilised. However, we have very little grasp on the magnitudes of floods required to mobilise the sediment bed such as to re-mobilise the microplastics and hence very little understanding of microplastic recontamination risk. In order to quantify the effect of flood events on microplastic remobilisation a series of experiments were run within a glass-sided, flow-recirculating flume (8.2m x 0.6m x 0.5m). During experiments, a sediment bed of unimodal distribution (σg = 1.3 and D50 of 9mm) was seeded with high density PVC microplastic nurdles (between 3-5mm) at a 1% concentration. The sediment bed was exposed to a period of steady flow for five hours to water-work the surface (initial τ* ≈ 0.041). After this beds were exposed to a flood wave with a 2 hour rising limb and either a 3 or 8 hour falling limb during which discharge was changed in discrete steps (τ* ranged between 0.033 and 0.049). High resolution laser scans were taken of the bed surface prior to the application of the flood wave, after the rising limb and at the end of the experiment. Surface grain size distributions together with the bed load and microplastic transport rates were measured every half an hour during the steady flow period and at every step of the flood wave. Microplastic contaminant profiles were measured using 3D CT scanning of cores buried within the bed and removed at the same time the bed was scanned. Discussion concentrates on linking the changes in bed surface topography to the character of the bedload and microplastic flux in response to differing flood wave characteristics. Data shows that as the bed surface develops during the passage of a flood wave a significant hysteretic response develops in both the bedload and microplastic flux but that the degree of microplastic hysteresis is much greater. The outcome of this research is pertinent to developing understanding surrounding the sensitivity of fluvial systems to microplastic recontamination during flood events.
- *#Andrew Gunn, Jad Diaf, Douglas Jerolmack, Experimental turbidity current onset: Breaching front rheology
Quasi-steady turbidity currents are a sediment flow that have been extensively studied in the lab, and fluid-mechanical theories describe them reasonably well. The generation of currents, however, is less examined. Van Rhee and Bezuijen (1998) first pointed out a curious mechanism for sustaining turbidity currents coined ‘breaching’. Breaching is the phenomena where a near-vertical sediment pack face retreats slowly by spalling off grains. The process requires specific conditions within the pack itself; in order to produce a turbidity current, a retreating granular front must maintain pore pressure across the face that is sufficient to prevent mass failure. Based on previous experimental work we posit such conditions can only be satisfied when the pack; permeability is low producing face-normal pore pressure gradients, compaction is high ensuring dilative shear failure drawing fluid inward. Without both, the front collapses rapidly, producing short run-out debris flows. Whilst past experimental work examined pore-pressure at the front, so far there are no measurements of granular friction or rheology in breaching fronts. We have built a unique facility to find the boundary of a parameter space in which breaching lives. A 9 m flume with acrylic walls has a pneumatic `trap door’ at one end, behind which a pack of submerged grains is held. When the door is opened rapidly, the pack fails and a turbidity current travels down the flume. We can tune sediment compaction in our experiments by forcing fluid up through the initial pack before the door opens. To measure the breaching front, a 200 Hz camera on a motorized trolley moves in a Lagrangian reference frame along the flume at the front’s speed (inferred from the images in real-time) during the entire failure event. With PIV and pixel intensity, we examine the transition of granular rheology from static well behind the front to suspension in-front, and the fluid flow in the front’s vicinity with neutrally-buoyant seed-particles. By controlling the initial packing and the grain size, sediment failure is viewed through the lens of granular phase transitions; debris flows ‘melt’ where the front rheologically transitions from solid to liquid whereas, remarkably, breaching corresponds to where grains ‘sublimate’ from solid to suspension at a sharp front.
- *Joe D. Collins, Mark W. Bowen, Tom Gill, Richard Langford, Using the lithostratigraphic record preserved within two playa wetlands to reconstruct paleoenvironmental change and geomorphic response, Harney Basin, Oregon (USA)
End-member mixing analysis (EMMA) was conducted on the grain-size distributions of 455 sediment samples collected from seven cores from the floors of Rimrock Lake (n=4) and Hay Lake (n=3), located along the western margin of the Harney Basin, Oregon (USA). The derived end-members (EM) were used in conjunction with GIS analyses and radiocarbon dating to investigate playa stratigraphic and geomorphic relationships. EMMA identified a 5-EM model that explains a combined 97.23% of the total grain-size variance across all sediment samples and successfully identified two major environments of deposition: lacustro-aeolian and fluvial-lacustrine. EM 1 and EM 5 explain a combined 70.5% of the total variation and are interpreted to represent lacustro-aeolian deposition. EM 2 and EM 4 explain a combined 19.28% and are interpreted to represent fluvio-lacustrine deposition. EM 3 represents 7.46% of the variance and is interpreted as a wet marsh or backshore environment receiving colluvial, fluvial, and aeolian sediments. The normalized difference between the finest and coarsest EMs also was calculated as a visual proxy for comparison of the change in energy within the playas. In Rimrock Lake playa, EM 1 and EM 5 are strongly associated with each other in the upper portions of the cores, while EM 2 and EM 4 are strongly associated with each other in the lower portion of the cores, forming two distinctive sedimentary facies separated by an abrupt and distinctive boundary. In Hay Lake playa, EM 2 is found in association with both EM 1 and EM 5 in the lower portion of the cores, while EM 4 forms a narrow interval across the upper portion of the cores. Eight radiocarbon dates suggest that the playas were established during the Late Pleistocene, ca. 19,500 – 18,500 cal yr BP, and experienced increased fluvial activity during the Early Holocene, between ca. 9000 and 8000 cal yr BP. A distinct and well-preserved portion of the relict Rimrock playa surface also was identified ~82 cm below the margin surface and is associated with a radiocarbon date of 6190 – 5990 cal yr BP just below the relict surface, suggesting increased aridity in the Harney Basin during the middle Holocene. Rimrock Lake playa experienced alternating lake levels well into the Late Holocene, even during times of increased aridity, while Hay Lake playa did not. GIS analysis indicates the two playas have similar morphology, with surface area, perimeter, width, length, and circularity differing < 4%, though Hay Lake playa has a much larger watershed and more extensive drainage network. At ~54 ha, both playas are relatively late compared to other wetlands within the Harney Basin. Based on examining DEMs and aerial imagery, and documented late Quaternary tectonic activity, regional tectonics may have played a significant role in diverting ephemeral channels towards Rimrock Lake and away from Hay Lake.
- *#Kathryn L Russell, Geoff J Vietz, Tim D Fletcher, Conceptualising and quantifying the urban coarse-grained sediment budget
Background The coarse-grained sediment budget of urban watersheds is of great interest to both stormwater and stream managers. Sediment in urban stormwater systems creates a significant maintenance burden, while a lack of coarse-grained bed sediment in streams limits their ecological value and geomorphic resilience. This problem of both ‘too much’ and ‘too little’ coarse-grained sediment indicates that the sediment budget of the whole watershed is profoundly altered. Very few catchment-scale sediment budgets have been developed for urban areas, and they rarely consider coarse-grained sediments. Methods We developed a conceptual urban coarse-grained (> 0.5 mm) sediment budget across three domains: hillslopes (urban land surfaces), the built stormwater network and stream channels. We then quantified key sources, sinks and storages for a suburban case study. Hillslope sources were computed from monitoring of sediment inputs to stormwater drains from nine street-scale watersheds with different land cover arrangements, over a period of one year. Sediment yield from the channel was estimated from bedload monitoring at the watershed outlet. Export by street sweeping and by clean-out of sediment ponds and gross pollutant traps (GPTs) was estimated from inspection of Council data. Results Hillslope sediment supply to the stormwater network was plentiful, furnished mostly by landscaped (lawn/garden/gravel) areas (54%) and lot-scale construction areas (35%). Most of the material eroded from hillslopes (~60%) was stored locally even in this highly-connected suburban watershed, and a further substantial proportion (5-33%) was exported by street sweeping. In the built stormwater network, around 11% of supplied sediment was captured in ponds and GPTs and exported. Additional reactive clean-out of pipes and pits is anecdotally substantial. We estimate 40% of the sediment supplied to the stormwater network reaches the catchment outlet. Unknown sources and sinks in the channel domain include channel erosion, storage in and export from channels, and floodplain deposition. Stable, rock-lined channels indicate channel erosion is a minor source. Additionally, no substantial sedimentation or clean-out was observed during our monitoring. Floodplain deposition was observed multiple times during the monitoring period, and may be an important sink in the channel domain. Discussion An understanding of the sediment budget of urban headwater watersheds can provide stormwater and waterway managers with the information they need to better manage sediments in stormwater systems, and encourage geomorphic recovery of urban streams. Reductions in stormwater runoff from urban areas are required to reduce sediment transport and allow bed sediments to accumulate and persist in stream channels. However, measures which reduce stormwater runoff (e.g. rain gardens) tend to also reduce sediment supply from hillslopes, resulting in persistent sediment supply limitations. We suggest that stormwater controls on roof runoff (e.g. rainwater tanks) and fully impervious watersheds will produce the greatest benefits, reducing flows without blocking key coarse sediment sources. Controls on watersheds with landscaped surfaces may require sediment bypass arrangements. If stormwater runoff is managed and hydraulic conditions in the stream are appropriate, direct sediment replenishment may be a complementary measure to stormwater control.
- *#Kyle Stark, Daniel Cadol, Jonathan B. Laronne, David Varyu, Madeline Richards, Novel bedload monitoring technologies applied during flash floods in ephemeral tributaries of the desert Southwest
Arid regions of the world are home to more than 2 billion people – a populace that faces many obstacles, from water scarcity to land degradation. While they often rely on through-going perennial rivers for water supply, the more numerous ephemeral tributaries control sediment supply and connectivity to hillslopes. Hence, understanding how ephemeral channels transport sediment is key to managing water in dryland areas. The Rio Grande is a perennial river in the center of the largest semi-arid region in the United States, and is a crucial part of life in the Southwest U.S. Constant maintenance along such rivers is required to ensure that communities have consistent access to water. Numerical models for these rivers are often constructed to predict changes and allow for effective management. The largest source of uncertainty in modeling these rivers is associated with the sediment influx from ephemeral tributaries. Studies of ephemeral channels in arid environments are limited: few point measurements of sediment flux, let alone continuous datasets, exist. A new sediment monitoring facility has been in operation on the Arroyo de los Piños since early 2018. Flash floods carry sand-rich gravel directly into the Rio Grande causing a localized influx at the point of confluence. Sediment is easily transported through the Piños channel network due to the lack of armoring and of vegetation. Runoff production is high because of the intense nature of monsoonal storms, the sparse hillslope vegetation, and the thin soils. At this monitoring site, bedload and suspended load are sampled automatically and continuously using no less than ten different methods. At the heart of the system are three Reid-type slot samplers continuously monitoring bedload. Bedload is also measured by various surrogate methods: plate and pipe microphones, hydrophones, seismometers, and a plate geophone. These instruments record the acoustic and seismic response of bedload transport. We present first results from what is intended to be the premier sediment monitoring station in the American Southwest. It combines both proven and novel technologies to continuously monitor bedload flux by a variety of different methods. Data collected from the Piños will fill important gaps in our understanding of bedload dynamics in flash flood ephemeral channels. Four flood events, ranging in maximum flow depth from 16 – 160 cm, were recorded at the Piños sediment research facility during the 2018 monsoon season. Maximum calculated bedload flux was very high (6.5 – 16.5 kg/sm) by global standards. Sediment accumulates quickly in the Reid samplers at these fluxes; a single sampler can fill in as little as 20 minutes. Signals from one pipe microphone show similar peaks when compared to bedload flux. Unlike the Reid slot samplers, which are very time consuming to maintain and fill rapidly during the beginning of each flash flood event, every surrogate method records continuously for the duration of a flood. Successful calibration of these instruments will provide event-long understanding of the dynamics of bedload flux and volumetric sediment discharge during these relatively short-lived, high intensity flash floods.
- *Nathaniel Bergman, Noam Greenbaum, Yaron Beeri-Shlevin, Hillel Glazman, Shulamit Nusboim, Alon Rimmer, Suspended and bedload sediment transport in a basaltic gravel-bed stream, Nahal Meshushim, Israel
Nahal Meshushim is the largest watershed draining the central Golan Heights into the Sea of Galilee. It originates from the vast basaltic Golan plateau as a shallow bedrock channel system, and further, cuts a deeply incised canyon close to the western flank where it drops steeply into the NE delta of the Sea of Galilee. After the Jordan River, it is the most important perennial, gravel-bed river in terms of water and sediment volumes which support an important ecological littoral zone of the lake as well as two large nature reserves of the delta. Yet, until recently, suspended sediment sampling was of low-resolution, sporadic and conducted during each flood season by Mekorot (Israel National Water Company) or during short-term studies, whereas data on bedload transport is almost absent. As part of a larger geomorphic study program in the delta, we present a new monitoring system in Nahal Meshushim which includes three optic turbidity sensors for tracking continuous suspended sediment transport (TSS – Total Suspended Solids) in addition to traditional hand-held bottle sampling and three geophones for measuring real-time bedload transport. Sediment net traps were placed downstream of the geophone sensors to capture some of the bedload in motion and get an idea of grain size distributions. The geophones are located downstream of the concrete weir of the hydrometric station of the Israel Hydrological Service with a calibrated pressure transducer which complements the system with the concurrent discharges. The system was first tested in the hydrological year of 2017-2018 which was the fifth consecutive drought year in northern Israel. Nevertheless, several small and medium flows, larger than baseflow, occurred and enabled to test the bedload monitoring system for the first time. During most flows, suspended sediment usually lags the quick changes in the hydrograph creating anti-clockwise hysteresis loops. As expected, the relatively sparse hand-held bottle sampling is inferior to the continuous turbidity measurements, doesn’t capture quick changes in TSS and requires attendance in the field. The geophones were able to capture bedload motion during all flows, from incipient motion of loose gravel at the beginning of the year up to significant movement of the bed in motion, when the discharge peaked at 42 m3/s. The bedload motion is also heterogeneous along the X-section where the highest instantaneous values are not necessarily at the channel center. Bedload sediment sizes were predominantly sand up to coarse gravel, but cobbles and small boulders were also in motion.
- *#Panshu Zhao and John R. Giardino, Object-oriented characterizations of supra-glacier surface structures over Glaciar Perito Moreno and Glaciar Ameghino, Southern Patagonia Icefield
Patagonian glaciers in Argentina have been exhibiting a mixed pattern of retreat for some and advance for others during the last decade. This contrasting activity has been suggested to be a response to the direct impact of a changing local climate. Unfortunately, we do not know the impact of change on a regional scale, as no accurate mapping of glacier boundaries to record glacier fluctuations has been carried out. Thus, we developed an innovative geomorphometry algorithm—divergence index — to characterize topographic change at a regional scale. With this index, we assume that topo-climate relationship characterized by topographic shielding index and topographic forcings have impact on the mode of glacial activity. Our results indicate that glacier-surface topographic patterns are spatially complex compared to the surrounding topography, and that spatial patterns vary significantly over time due to complex climate-glacier dynamics. Various patterns are associated with meltwater and ice flow, down-wasting, supraglacial lakes, and the build-up of supraglacial debris. Quantitative characterization of these patterns can be used to map various process-form relationships over time. The presence of unique glacier morphologies and their rates of change may enable a first-order approximation of glacier sensitivity to climate change, as glacier-surface topography integrates feedback mechanisms that account for microclimate, ablation and meltwater production, and sediment transport and debris depth. We report on the utility of these metrics and patterns for characterizing and mapping glacier-surface morphodynamic condition.
- *#Raquel Granados-Aguilar, John R. Giardino, and John D. Vitek, Mechanics and Processes of the Alpine fluid-mass cascade system: Upper Camp Bird and Yankee Boy, San Juan Mountains, CO, USA
Geomorphic processes that form and shape alpine environments are exceedingly complex and operate as a trifecta through commencement, transfer and accumulation of debris. Because the fluid-mass cascade system has numerous pathways in these alpine watersheds, it has been fundamental to identify the range of pathways. Past research has demonstrated that rock glaciers are the major component of the fluid-mass cascade in these environments. Thus, this landform serves as transporter and storage for much of the debris movement, as well as a supplemental source and pathway for meltwater. Because rock glaciers consist of poorly sorted angular-rock fragments with matrices of a core of ice or interstitial ice and fines, water, ice, rock and debris are all part of periglacial landscape space-time continuum. Unfortunately, we still do not have a clear understanding of the processes and fluid-mass pathways on and within a rock glacier. To identify and study these pathways we used field mapping, drone imagery, water dying techniques and geophysical methods, including Ground-Penetrating Radar (GPR) and Electromagnetic Induction (EM) to model the fluid-mass cascade at different scales within the periglacial continuum. Tradition field mapping supplemented with drone imagery provided a detailed map of surface pathways, fluid entry and exit portals. EM and GPR methods were used to obtain a detailed picture of the internal structure and to determine boundaries between resistive (rocks, sediment, and ice) and conductive materials (water) of the rock glacier. The systems provided continuity across the data sets and validated the applicability of new EM techniques (G-TEM). Water tracing provided estimates of residence time and rates of flow for water. Rock glaciers in the study are tongue-shaped and active. The Upper Camp Bird rock glacier is 780m long and 215 m wide whereas the Yankee Boy rock glacier is 800 m long and 300 m wide. Interpretation of the GPR data (Yankee Boy rock glacier) suggest layers representing ice-supersaturated sediments and coarse blocky rockslide debris are present with numerous prominent reflections that show broad layers 10-15 m thick. These layers represent contacts between major depositional units. The preliminary G-TEM models for the Upper Camp Bird rock glacier suggest five layers of different resistivity. The thickest layer (50m-100m depth) with the highest resistivity is interpreted as the ice within the rock glacier. Dye tracing of fluid flow through the rock glaciers shows an average rate of 50 m hr-1, and a discharge of ~ 3.4 m3 min-1. This research characterizes the internal structure, including water storages, pathways, and thresholds, as well as estimation of the volume of ice stored within two rock glaciers and facilitates modeling the role of the fluid-mass cascade within the periglacial continuum.
- #Alexander Neely, Roman DiBiase, Controls on the slope and relief of headwater channels in steep landscapes: Field constraints from the San Gabriel and San Jacinto Mountains, CA
Headwater colluvial channels where sediment transport is thought to be dominated by mass-wasting events form a key but understudied link between hillslope and fluvial domains. Surface processes in headwater channels influence 1) drainage density responses to changes in climate or tectonics; 2) the grainsize distribution and flux of sediment delivered to downstream channels; and 3) the size, extent, and recurrence of hazardous debris flows. Here, we use field observations, cosmogenic nuclides, and high-resolution topographic and imagery data to study headwater channels in rocky landscapes of the Northern San Jacinto Mountains and Eastern San Gabriel Mountains in southern California. Both study sites have similar runoff regimes and lithology, but bedrock cliffs in the San Gabriel Mountains are highly fractured compared to massive bedrock exposed in the San Jacinto Mountains. Sparser fracture spacing in the Northern San Jacinto Mountains leads to steeper cliffs, coarser sediment, steeper fluvial channels, and a reduction of catchment erosion rates from ~0.6-2.2 mm/yr in the Eastern San Gabriel Mountains to ~0.1-0.3 mm/yr in the Northern San Jacinto Mountains. Colluvial channels show similar gradients (~35°) in both landscapes, but are 2-3 times longer in the Northern San Jacinto Mountains, increasing headwater channel relief in this mountain range. In both landscapes, field observations indicate that most colluvial channels are mantled with sediment, but the steepest headwater channels are bedrock-dominated, consistent with a critical slope for colluvial channels that appears to be independent of grain size. We hypothesize that the extended colluvial network in the San Jacinto Mountains could be a consequence of coarser sediment cover that requires a larger drainage area threshold for fluvial transport, but we also explore the potential controls of hillslope rock mass strength and sediment supply. Our findings highlight how changes in rock properties affect drainage density and the partitioning of relief between hillslope, fluvial, and colluvial domains.
- #Brandon Dillon and Kyle Strom, Action Minimization Analysis of the Problem of Incipient Motion
The difficulties surrounding the identification of a threshold condition for the mobilization of alluvial material are well known in the study of geomorphology. Despite the detailed observation and careful analysis of hundreds of well executed field studies and laboratory experiments spanning the last century, the discipline still struggles to identify a clear, quantifiable mobilization threshold. Reported here are the results from a study in which, quite by accident, a new type of mobilization threshold has appeared. We analyze the problem of incipient motion by searching a large set of numerically generated single-particle entrainment test cases for a conserved quantity. We observe that, with respect to momentum, there exists a subtle but significant difference between the applied hydrodynamic force or force-time product and the momentum transferred from the flow to the particle. In noting this phenomena, we recast the dynamics of an entrainment event by viewing the problem not as an evolution of a system though time, but rather, as the trajectory of the system through a phase-space description of the test particle’s equation of motion. We note that, as a consequence of the Principle of Least Action, any pathway in this space that leads to entrainment can be described and separated from all other paths by an energy threshold criteria. And while this criteria is related in physical mechanism to previous criteria for incipient motion, the approach significantly increases the probability of correctly predicting entrainment events. Application of this method in the numerical experiments conducted in this study produced a model likelihood exceeding 0.99.
- #Chamil Perera, Weiming Wu, and Jarrell Smith, Erosion of sand and mud mixtures
Sand and mud mixtures are abundantly found in rivers, lakes, and estuarine and coastal waters. They play significant roles in hydrodynamic and morphological processes in surface water systems. When sand mixed with mud, they exhibit cohesive properties demonstrating complex erosion processes, i.e., from pure sand to pure mud, these sediment mixtures have different erosion mechanisms. In this study, erosion of sand and mud mixtures are investigated using well-sorted quartz sand mixed with one of the Kaoline, Kaoline-Bentonite, and Mississippi River muds with varying proportions from 0 to 100%. For each mixture sample, erosion flux is measured with increasing bed shear stress in SEDFlume erosion test apparatus. Using the measured data, three erosion flux models: segmented-linear, nonlinear, and exponential model, are validated. The model parameters of each model are correlated with physical properties of sediment mixtures, such as clay fraction, mud fraction, mixture dry density, and mud dry density.
- #Jonathon G. Chester, Douglas A. Miller, Jonathan M. Duncan, Brennan R. Holderman, Quantifying the temporal variability of terrestrial structure-from-motion photogrammetry: Evaluating the potential of a low-cost image-based method for monitoring geomorphic change
Structure-from-Motion (SfM) photogrammetry is an emerging method in the geosciences for acquiring high resolution digital surface models and 3D landscape reconstructions. Because SfM does not rely on precise camera parameters and calculated image overlap, as does traditional photogrammetry, it can be performed using an on-the-ground, point-and-shoot digital camera. With SfM, a series of pseudorandom images are captured at varied locations, orientations, and scales, whereby 3D “structure” is derived from the “motion” of the camera throughout a scene. A growing body of literature suggests that the SfM method can yield a dense point cloud with elevation accuracies comparable to those from other more expensive and skill-intensive survey methods (e.g. laser scanning and total station surveys). Applied to the geomorphology, SfM therefore appears to be a low-cost and easier-to-use alternative for monitoring geomorphic changes related, but not limited to, sediment erosion and deposition, fluvial processes, and hillslope dynamics. However, despite the increasing number of published SfM accuracy assessments, few studies have been found that discuss the reproducibility of SfM-derived elevation products across repeat surveys. If SfM is to be considered as a method for monitoring geomorphic changes, its capability to produce consistent and reliable information must first be understood. As with any change-detection study, researchers must be able to differentiate between real geomorphic change and error inherent to the survey method. Therefore, this study seeks to understand the temporal variability of terrestrially-acquired SfM data products. Using a simple point-and-shoot digital camera, a small controlled test area (~200m²) displaying geomorphic stability and discrete landscape geometry is photographed across multiple days using different control point distributions and image acquisition orientations. The resulting dense clouds are compared to assess the internal precision of the SfM method. External cloud accuracy is calculated by comparing SfM dense clouds with a point cloud obtained from a terrestrial laser scan. Field measurements are used to validate observed cloud accuracies. Results presented here suggest that when consistent image acquisition and processing methods are employed, terrestrial SfM photogrammetry can repeatedly yield dense clouds with sub-decimeter precision and accuracy. Despite the lurking variables inherent to the SfM process, these results indicate that a careful technician can obtain precise and accurate dense clouds from multi-temporal image surveys of a small study area. We caution, however, the extrapolation of these results to larger study areas given that the present study is based on a small test site lacking geomorphic complexity. Additional research is necessary to expand this study to a variety of landscapes to fully understand the spatiotemporal precision of terrestrial SfM. Nonetheless, we demonstrate with these preliminary results that SfM photogrammetry using a simple point-and-shoot digital camera appears to be a viable method to map and monitor geomorphic change in small areas over (relatively) short time scales.
- Matthew O’Connor, Jack Lewis, Gregory Andrew, Sediment Size Distribution Variation 2012 to 2017 in a Flow-regulated Gravel Bed Stream, Marin County, California
Lagunitas Creek is a 282 sq km watershed in coastal northern California that drains the north flank of Mt. Tamalpais. Municipal water supply reservoirs operated by Marin Municipal Water District (MMWD) reduce streamflow and sediment supply to most of Lagunitas Creek which hosts a run of endangered coho salmon. The State of California requires MMWD to reduce sedimentation and improve fish habitat conditions as a condition permitting water diversion. The Lagunitas Creek Sediment and Streambed Monitoring Plan (http://marinwater.org/DocumentCenter/View/4095/Lagunitas-Sed-Monitoring-Final—Sept-10-2015?bidId=) was developed by the authors to evaluate spatial and temporal variation of sediment. Streambed monitoring under the Plan began in 2012; here we summarize analyses of 6 years of monitoring data through 2017. The monitoring framework for the Plan incorporates large scale spatial variation in fluvial geomorphology. Three “reaches” of mainstem Lagunitas Creek about 5 to 9 km in length were differentiated. These reaches have mean slope gradients of 0.002, 0.003 and 0.004, and corresponding mean bankfull widths of about 12, 15 and 18 m respectively; all are affected by upstream reservoirs. Median diameter of sediment on the bed surface is about 10 to 15 mm. Two tributaries of about 2 to 7 km length have mean slopes of 0.02 and 0.007 and bankfull widths of 6 and 9 m respectively; these are unaffected by upstream reservoirs and median sediment diameter is 16 to 22 mm. Within each of these five reaches, we established by random selection four monitoring sites comprised of 30 transects spaced at intervals of about one-half bankfull width. On each transect, 10 sampling stations were established at uniform intervals from the left bank with a random start point. This sampling framework (n=1200 for each site) was used to establish baseline conditions. Baseline data was analyzed to evaluate spatial differences within and between reaches. To evaluate temporal variation, one site in each reach (n=5) is sampled annually. In 2016 we resampled all sites in three reaches (n=12) for comparison with baseline data. Data analysis focuses on statistical inference and estimation to detect and quantify differences among sites and reaches. Excessive quantities of fine gravel and sand (“fine” sediment) is detrimental to survival of embryonic and juvenile salmonids. In this study, sediment < 4 mm diameter is collectively regarded as fine sediment. A prior investigation of Lagunitas Creek (OEI, 2006) found fine sediment distributed in patches (referred to as fine sediment facies). Fine sediment facies were characterized and quantifed and a suite of data were collected at each sample station along the transects. Data include a sediment clast diameter at each sample station, a sediment facies classification, and a fish habitat classification (e.g. pool, glide, run, riffle). We also measured the depth of fine sediment deposits by inserting a thin rod into the bed to the depth of refusal. In addition, a bulk sample of sediment representative of spawning gravel was collected for size distribution analysis. We found significant spatial differences between some sites and reaches. Few significant temporal were detected until very wet conditions in 2017 produced a 20-fold increase in bedload sediment transport. O’Connor Environmental, Inc. (2006) Lagunitas Creek Fine Sediment Investigation. Prepared for Marin Municipal Water District. 74 p.
- Rebecca Hodge, Hal Voepel, Julian Leyland, David Sear, Sharif Ahmed, X-Ray CT reveals that grain protrusion controls critical entrainment shear stress in fluvial gravels
The distribution of critical shear stresses (τc) for grains in a sediment river bed is a poorly constrained component control on sediment transport rates. Directly calculating values of τc has previously been hindered by the inability to measure the geometry of in-situ grains, i.e. the location of the grain relative to surrounding grains and the bed surface. We use X-Ray Computed Tomography (CT) scanning to measure the grain geometry of undisturbed water-worked sediment samples extracted from a prototype scale flume experiment. The data are used to parameterise a new 3D grain entrainment model, providing estimates of τc for 1092 grains. We find that parameters including pivot angle and proportional grain exposure do not vary systematically with relative grain size. τc is primarily controlled by grain exposure, not pivot angle. Our data suggests equal mobility, which is caused by larger grains experiencing larger forces as a result of projecting higher into the logarithmic flow profile. We suggest that grain protrusion might be a suitable proxy for assessing gravel bed stability.
- #Rebecca Owens, The Impact of Urbanization on River Dynamics and Sediment Transport in Four Rivers in Texas
By the year 2050, it is estimated that 6.3 million people will live in urban areas. The expansion of the built environment will affect all facets of the natural, and must be implemented with awareness of its role in the Earth system. For rivers in the Gulf Coast region such as those in this study, changes from urbanization will be compounded by effects of sea level rise and changes in weather patterns resulting from climate change. This research examines the influence of urbanization on the geomorphic stability of low-elevation river channels through quantitative and qualitative evaluation of four rivers impacted by urban centers in Texas and Louisiana. Qualitative characteristics evaluated include sediment size, bank angle, channel vegetative protection, bank cutting, channel debris, and presence of mass wasting events. Quantitative evaluation includes determination of critical shear stress ratios for each river. Preliminary results indicate localized reduction in geomorphic stability near urbanization followed by recovery, as indicated by changes in channel shape, sediment distribution, and level of vegetative cover. The changes in sediment distribution and resulting effects on channel stability for these areas has developed within the past 200 years, and have the potential to extend into the future as long as urban expansion continues.
- Yazidhi Bamutaze, Moses Tenywa, Majaliwa Mwanjalolo, Joy Obando and Petter Pilesjo, Drainage area and morphological effects on the magnitude of discharge, runoff and sediment yield in a small catchment on Mt. Elgon, Uganda
The relationship between drainage area and sediment yield is complex and not fully resolved. But more glaringly in tropical mountain catchments in Sub Saharan Africa (SSA) Uganda, a dearth of knowledge on the magnitude of discharge, runoff and sediment yield across spatial and temporal scales exists. Coupled with morphological effects, the gist of our study was to combinatively address these issues, cognizant of the terrestrial implications of sediment processes and livelihoods in the strongly coupled Mt. Elgon ecosystems in Uganda. The study was conducted in a small catchment of 320 hectares but collapsed into three sub-catchments measuring 1.3 km2, 0.5 km2 and 0.3 km2. Morphometric attributes were extracted from a 30m Digital Elevation Model (DEM) in a GIS environment. The catchment and sub-catchments were instrumented with H-flumes and automatic devices for determining total suspended solids, stream discharge and surface runoff. The catchments morphologically depict convex and S-curve hypsometry structure. Hypsometric integrals are low and vary from 0.49 to 0.51 signifying a relatively young catchment. Mean annual discharge varied from 52,790 m3 to 2, 073,238 m3, but a significant proportion (52%) is generated from 35% of the catchment. Mean surface runoff ranged from 758 to 2932 m3 ha-1 yr-1. Surface runoff constituted a small fraction of rainfall depth varying from 5.1 to 15%. The proportion of runoff to total stream water flow was strongly and significantly related to drainage area (R2=0.81, p<0.05). Mean annual suspended sediment load varied from 5 to 175 t yr-1 while sediment yield ranged from 15 to 55 t km2 yr-1. A spatially disproportionate contribution was detected with 54% of the sediment generated from 34% of the entire catchment. Sediment load and sediment yield linearly increased significantly with drainage area (p<0.05). A strong temporal sediment footprint was observed with 92% of the annual sediment generated in 6-7months. Observed sediment delivery ratios between 1.6 and 4.7% indicate that a significant proportion of eroded sediment is retained in catchment. Catchment hypsometry and mean slope gradient had a strong bearing on sediment generation and transmission. High variability in discharge, runoff and sediment loading were detected signified by coefficients of variations (CVs: 91-897%). Our study gives insights on niches for spatial and temporal targeting for combating soil erosion and sediment based on source-sink pathways in high energy environments.
(*Attendance supported by a grant from NSF; #PhD student)