Thad A. Wasklewicz

Evolution of a natural debris flow: In situ measurements of flow dynamics, video imagery, and terrestrial laser scanning

Many theoretical and laboratory studies have been undertaken to understand debris-flow processes and their associated hazards. However, complete and quantitative data sets from natural debris flows needed for confirmation of these results are limited. We used a novel combination of in situ measurements of debris-flow dynamics, video imagery, and pre- and postflow 2-cm-resolution digital terrain models to study a natural debris-flow event. Our field data constrain the initial and final reach morphology and key flow dynamics. The observed event consisted of multiple surges, each with clear variation of flow properties along the length of the surge. Steep, highly resistant, surge fronts of coarse-grained material without measurable pore-fluid pressure were pushed along by relatively fine-grained and water-rich tails that had a wide range of pore-fluid pressures (some two times greater than hydrostatic). Surges with larger nonequilibrium pore-fluid pressures had longer travel distances. A wide range of travel distances from different surges of similar size indicates that dynamic flow properties are of equal or greater importance than channel properties in determining where a particular surge will stop. Progressive vertical accretion of multiple surges generated the total thickness of mapped debris-flow deposits; nevertheless, deposits had massive, vertically unstratified sedimentological textures.

20th Century stage trends along the Mississippi River

River regulation has systematically increased along much of the Mississippi River throughout the 20th century. There is only a cursory understanding of changing hydrological processes along the entire length of the Mississippi River over this same time period. This study compared four measures of river hydrology, at the beginning (1910-1930) and at the end of the 20th century (1980-2000). River-stage data were statistically analyzed from 15 equidistant gauges along the main stem of the Mississippi River. The findings revealed (1) significant changes in components of river hydrology between both time periods and (2) varying patterns of change between the different river segments. The Upper Mississippi River (UMR) experienced significant increases in peak, mean, and minimum monthly stages between the periods, while variance of these same stage conditions declined. The Middle Mississippi River (MMR) exhibited significant increases in the magnitude and variance of river stages. The frequency and duration of flood stages increased between the two periods on the MMR. The Lower Mississippi River (LMR) demonstrated a mixed response during this time period. Gauges at the upper and lower end of the LMR changed similarly to the gauges on the UMR. However, gauges on the central part of the LMR showed decreases in peak, mean, and minimum river stages.

High-Resolution Analysis of Debris Flow–Induced Channel Changes in a Headwater Stream, Ashio Mountains, Japan∗

Coupled hillslope and channel processes in headwater streams (HWS) lead to rapid changes in channel dimensions. Changes in channel size and shape caused by a debris flow event along the length of a headwater stream in the Ashio Mountains, Japan, were captured with the aid of repeat high-definition surveys using terrestrial laser scanning (TLS) techniques. The HWS was classified into three distinct reaches below the debris flow initiation zone. A large knickpoint separated an upper bedrock reach from a colluvial reach along the midsection of the drainage. The colluvial reach transitioned to a lower bedrock reach that terminated at the master stream. Cross-sectional and morphometric analyses revealed no statistically significant changes in channel size or shape along the upper bedrock reach. Debris flow erosion generated significant differences in channel size and shape along a colluvial reach. Sediment bulking associated with erosion along the colluvial reach led to increases in channel size along the lower bedrock reach, but no statistical differences in channel shape. Morphometric analyses from the TLS point cloud revealed that debris flow erosion produced a distinct nonlinear change in channel dimensions in the downstream direction within the HWS. Variations in channel substrate along the length of HWS contributed directly to this nonlinear response. The episodic nature and nonlinearity of erosion associated with the current debris flow event highlights the importance of debris flows in general in understanding the transport of sediment, coarse to fine particulate organic material, and large woody debris, which are critical to the long-term management of riverine environments. TLS sampling methods show promise as one component of a multianalytical approach needed to continuously monitor and manage the dynamics of HWS.

OBSERVATIONS OF DEBRIS FLOWS AT CHALK CLIFFS, COLORADO, USA: PART 2, CHANGES IN SURFACE MORPHOMETRY FROM TERRE- STRIAL LASER SCANNING IN THE SUMMER OF 2009

High resolution topographic data that quantify changes in channel form caused by sequential debris flows in natural channels are rare at the reach scale. Terrestrial laser scanning (TLS) techniques are utilized to capture morphological changes brought about by a high-frequency of debris-flow events at Chalk Cliffs, Colorado. The purpose of this paper is to compare and contrast the topographic response of a natural channel to the documented debris-flow events. TLS sur vey data allowed for the generation of high-resolution (2-cm) digital terrain models (DTM) of the channel. A robust network of twelve permanent control points permitted repeat scanning sessions that provided multiple DTM to evaluate fine-scale topographic change associated with three debris-flow events. Difference surfaces from the DTM permit the interpretations of spatial variations in channel morphometry and net volume of material deposited and eroded within and between a series of channel reaches. Each channel reach experienced erosion, deposition, and both net volumetric gains and losses were measured. Analysis of potential relationships between erosion and deposition magnitudes yielded no strong correlations with measures of channel-reach morphometry, suggesting that channel reach-specific predictions of potential erosion or deposition locations or rates cannot be adequately derived from statistical analyses of pre-event channel-reach morphometry.

Observations of debris flows at Chalk Cliffs, Colorado, USA: Part 1, in-situ measurements of flow dynamics, tracer particle movement and video imagery from the summer of 2009

Debris flows initiated by surface-water runoff dur - ing short duration, moderate- to high-intensity rainfall are common in steep, rocky, and sparsely vegetated terrain. Yet large uncertainties remain about the po- tential for a flow to grow through entrainment of loose debris, which make formulation of accurate mechani- cal models of debris-flow routing difficult. Using a combination of in situ measurements of debris-flow dynamics, video imagery, tracer rocks implanted with passive integrated transponders (PIT) and pre- and post-flow 2-cm resolution digital terrain models (ter - rain data presented in a companion paper by staley et alii, 2011), we investigated the entrainment and trans- port response of debris flows at Chalk Cliffs, CO, USA. Four monitored events during the summer of 2009 all initiated from surface-water runoff, generally less than an hour after the first measurable rain. Despite reach- scale morphology that remained relatively constant, the four flow events displayed a range of responses, from long-runout flows that entrained significant amounts of channel sediment and dammed the main-stem river, to smaller, short-runout flows that were primarily depo - sitional in the upper basin. Tracer-rock travel-distance distributions for these events were bimodal; particles either remained immobile or they travelled the entire length of the catchment. The long-runout, large-entrain- ment flow differed from the other smaller flows by the following controlling factors: peak 10-minute rain in- tensity; duration of significant flow in the channel; and

Timing and rates of Holocene normal faulting along the Black Mountains fault zone, Death Valley, USA

Alluvial fans displaced by normal faults of the Black Mountains fault zone at Badwater and Mormon Point in Death Valley were mapped, surveyed, and dated using optically stimulated luminescence (OSL) and 10 Be terrestrial cosmogenic nuclide (TCN) methods. Applying TCN methods to Holocene geomorphic surfaces in Death Valley is challenging because sediment flux is slow and complex. However, OSL dating produces consistent surface ages, yielding ages for a regionally recognized surface (Qg3a) of 4.5 ± 1.2 ka at Badwater and 7.0 ± 1.0 ka at Mormon Point. Holocene faults offsetting Qg3a yield horizontal slip rates directed toward 323° of 0.8 +0.3/–0.2 mm/yr and 1.0 ± 0.2 mm/yr for Badwater and Mormon Point, respectively. These slip rates are slower than the ∼2 mm/yr dextral slip rate of the southern end of the northern Death Valley fault zone and are half as fast as NNW-oriented horizontal rates documented for the Panamint Valley fault zone. This indicates that additional strain is transferred southwestward from northern Death Valley and Black Mountains fault zones onto the oblique-normal dextral faults of the Panamint Valley fault zone, which is consistent with published geodetic modeling showing that current opening rates of central Death Valley along the Black Mountains fault zone are about three times slower than for Panamint Valley. This suggests that less than half of the geodetically determined ∼9–12 mm/yr of right-lateral shear across the region at the latitude of central Death Valley is accommodated by slip on well-defined faults and that distributed deformational processes take up the remainder of this slip transferred between the major faults north of the Garlock fault.

Surface Variability of Alluvial Fans Generated by Disparate Processes, Eastern Death Valley, CA

This study explores the surface variability of alluvial fans from digital elevations model (DEM) derivatives generated from 1-m planimetric resolution airborne laser swath mapping data. Channel and interfluve dimensions of debris flow (DF) fans and fans generated from predominantly fluvial flows and some older debris flows (mixed flow [MF]) are extracted with the aid of a planimetric curvature classification. Significant differences are identified between the fan surface topography of DF and MF fans. MF fans tend to have smaller channel and interfluve widths, have smaller elevation differences between the crest of the interfluve and channel, and are more dissected than DF fans. The morphometric differences between the two fan classes can be explained by differences in the primary processes that develop the surficial features, but also the preponderance for secondary erosional processes acting on the MF fans.

Chapter Seven – Data Sources

This chapter reviews various types of spatial data used for geomorphological mapping, with reference to their basic characteristics, historical background and some examples of mapping using the data. Data types include text descriptions; hand-drawn illustrations; data from ground surveying using triangulation (plane table, level, global navigation satellite systems, total station, laser range finder and terrestrial laser scanner); existing topographic maps; ground and aerial photographs and videos for visual interpretation; satellite and aerial imagery for visual interpretation; height data from analogue, analytical and digital photogrammetry; height data from airborne/satellite light detection and ranging and interferometric synthetic aperture radar; and compiled digital elevation models. This chapter also discusses recent trends, problems and future perspectives concerning the use of data for geomorphological mapping, with reference to various aspects of data including format (analogue versus digital), spatial and temporal resolution, error and availability. Technical and strategic issues related to data selection and conversion, combined use of various data, application of manual and automated methods for mapping and labour/cost of work are also discussed.

3.6 Digital Terrain Modeling

Geomorphologists require quantitative information about the land surface. New sensors can now measure elevation changes at a variety of scales and these data are used to generate digital terrain model (DTM) that accurately characterize topography. A variety of DTM analytics are used to support a multitude of geomorphological studies. However, there are numerous issues involving representation, sampling, interpolation, and error assessment and correction, which must be addressed before using the elevation data. Data reduction, filtering, and accuracy are key aspects to consider. Knowledge of these issues is critical for terrain analysis and the communication of information derived from a DTM.

PROVENANCE OF AEOLIAN SEDIMENT: THE UPPER COACHELLA VALLEY, CALIFORNIA

Rapid urbanization around Palm Springs, California has encroached upon the sand habitats of the endangered Coachella Valley fringetoed lizard (Uma inornata). The Nature Conservancy and the Coachella Valley Preserve System are concerned about maintaining an adequate sediment supply to the dune environments that urban growth might interrupt. Determining the source of the existing aeolian sediments is a critical first step toward ensuring a perpetual sediment supply. Geochemical links between the aeolian sediment sources and the Coachella Valley Preserve dunes were discovered by mapping the aeolian sand distribution and analyzing trace elements using multivariate statistical techniques.