Applications in geomorphology

GEOMOR-06729; No of Pages 19 Geomorphology xxx (xxxx) xxx

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Applications in geomorphology Edward Keller a,⁎, Chandler Adamaitis a, Paul Alessio b, Sarah Anderson c, Erica Goto d, Summer Gray e, Larry Gurrola f, Kristin Morell b a

Environmental Studies Program and Department of Earth Science, University of California, Santa Barbara, CA 93106, USA Department of Earth Science, University of California, Santa Barbara, CA 93106, USA Bren School of Environmental Science & Management, University of California, Santa Barbara, CA 93106, USA d Department of Geography, University of California, Santa Barbara, CA 93106, USA e Environmental Studies Program, University of California, Santa Barbara, CA 93106, USA f 308 Hayes Ave., Ventura, CA 93003, USA b c

a r t i c l e

i n f o

Article history: Received 28 September 2018 Received in revised form 31 March 2019 Accepted 1 April 2019 Available online xxxx Keywords: Applications in geomorphology Wildfire-Debris Flow Cycle Hazard reduction, assessment, and perception Boulder geomorphology

a b s t r a c t Geomorphology is a pure science with the goal of understanding Earth surface processes and landscape evolution, and it is also an applied science with the goal of addressing the needs of society. With many new highresolution methods of depicting topography and much improved numerical dating, geomorphologists are working on problems involving rates of surface processes, landscape evolution, and applications to areas of concern to society that were impossible to address a few decades ago. Some of the areas of enquiry where geomorphology has been applied include: natural hazards (landslides, floods, earthquakes, and tsunamis), ecosystem management, site anthropology, land-use planning, engineering geology, expert witness testimony, and hazard reduction, assessment, and perception. How people perceive and respond to potential hazards, how their vulnerability can increase their risk, and how preparedness and response can be improved depends as much on the social sciences as on physical science. © 2019 Published by Elsevier B.V.

1. Introduction The study of geomorphology is both a basic and applied science that is devoted to understanding Earth surface processes, system response, and landscape evolution. Geomorphology may also be linked to the needs of society. Until the past few decades, with a few notable exceptions that include Gilbert (1917), Bagnold (1941), Strahler (1952a, 1952b), Leopold et al. (1964), and Dunne and Leopold (1978), geomorphology was largely a descriptive science. This was a time of using mostly qualitative evidence from reasoning and fieldwork to ask questions such as “How old is the sediment, landform, or artifact?”, or “What's the rate of uplift and erosion?” In the 1950s, Alan Strahler and other geomorphologists and groups of graduate students began to quantify geomorphic processes (Strahler, 1952a, 1952b), and the so-called “quantitative revolution” in geomorphology was born. When numerical dating techniques that geomorphologists use today became available, the revolution in geochronology blossomed. It is a truth that if there are no dates, there are no rates. Geomorphology changed when Willard Libby and colleagues introduced radiocarbon dating in the 1940s (Arnold and Libby, 1949). We no longer correlate ⁎ Corresponding author. E-mail address: [email protected] (E. Keller).

river and marine terraces by topographic position but by their numerical date obtained from well-known methods that include: radiocarbon, optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL), cosmogenic nuclides, and Uranium series (Rhodes, 2011; Bierman and Montgomery, 2014). Older relative dating methods that utilize soil chronology from relative profile development, amino acid rasterization, and fossil assemblages are still used but have steadily moved to positions of less importance in the chronology of Quaternary systems. Geomorphology today is experiencing a remarkable growth in our capacity to produce high-resolution topographic data linked to new dating techniques, thereby producing a more comprehensive understanding of the landscape. Since the second Binghamton Geomorphology Symposium on quantitative geomorphology in 1971, geomorphology has undergone a revolution in the understanding of geomorphic processes over time and space that are quantified by ever-increasing high technological methods, including remote sensing, photogrammetry, and structure from motion and LiDAR. The research uses very highresolution topographic data linked to Geographic Information Systems (GIS) that we could only dream about in the mid-twentieth century (Van Westen, 2013). For example, research with LiDAR is now used to produce digital terrain models and to help recognize landscape characteristics to calculate a suite of geomorphic indices (including drainage density, the stream-gradient index, the steepness index, and valley

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width-to-height ratio) to study landscape and tectonic processes (Keller and Pinter, 2002). LiDAR is extensively used in geomorphic mapping, modeling, and quantification of Earth surface processes (Hofle and Rutzinger, 2011; Van Westen, 2013). Erosion rates when linked to estimated, in-situ cosmologic nuclides provide geomorphologists a tool to study transience changes in rates of uplift and erosion. This constitutes a revolutionary development of the study of landscape evolution. Previously, landscape change at varying scales of time and space was all but unknown. We now understand, from the study of convection of mantle plumes, isostatic compensation linked to growth and decline of continental glaciers, and uplift and subsidence linked to plate motions that cause folding and faulting, that the surface of Earth is constantly in motion (Mudd, 2017). 1.1. Objective The objective of this paper is to discuss examples of selected applications in geomorphology to demonstrate how geomorphology is being applied to contribute to the understanding of physical, chemical, and social questions that are of importance to science and society. 2. Applications in geomorphology Applications in geomorphology include the use of geomorphologic principles and methods to solve a variety of problems that are of particular interest to society, as well as to provide solutions to a variety of other research questions in many branches of the physical and biological sciences. A discussion of how applications of geomorphology are linked to society is presented in Meitzen (2017), and such applications are only briefly mentioned here. Knowing how landforms are produced and maintained helps answer questions concerning human use and interest in the land studied by engineers, consultants, land-use planners, environmental managers, and decision makers involved with mitigating hazard. Here we briefly illustrate the application of geomorphology in understanding natural hazards. Floods, landslides (including debris flows), earthquakes, volcanic eruptions, wildfire, tsunamis, and coastal erosion are examples of natural hazards that are intimately linked to surface processes. Natural hazards are of particular importance to society, and geomorphology is applied to many aspects of risk reduction. Hazardous events tend to be repetitive, and the study of the history of natural hazards provides useful information for disaster risk reduction plans (UNISDR, 2018). Understanding the nature and extent of a natural hazard requires knowing the historic occurrence, as well as any geomorphic features that it may produce. These features may be landforms, such as river channels, or structures, such as geologic faults, cracks, or folded rock (Keller and De Vecchio, 2019). For example, if we wish to evaluate the landslide history of a particular valley, an important task is to identify landslides that have occurred over historic time as well as the prehistoric past. We examine the landscape for evidence of past landslides, identify landslide deposits, and categorize the type of landslides. Landslide deposits often contain organic material such as wood that may be radiocarbon dated to provide a chronology of prehistoric landslides. This chronology could then be linked with the historic record of landslides to estimate how often landslides have recurred (see Section 2.1 of this paper). Moreover, understanding of geomorphological processes can be applied in land-use planning, engineering geology, expert testimony, and communication and education in communities so they can better respond to and prepare for hazardous events. The first application we will discuss is landslides with emphasis on the 2018 Montecito debris flow. Research on the debris flow is ongoing by our research team at the University of California, Santa Barbara, and specific research results are preliminary but are presented here because the research centers on several important applications of geomorphology.

2.1. Applications to landslide hazards Applications in geomorphology have strong links with landslide processes and the hazards they produce. The classification of landslides, both shallow and deep, as well as debris flows and mudflows, depends to a significant degree on the geomorphology (Varnes, 1978). Geomorphology can be applied to understand prior landslides and to characterize the risk of future landslides. For example, the 1995 landslide and 2005 reactivation that claimed 10 lives in La Conchita, California (Fig. 1) is part of the much larger Rincon Mountain megaslide (Gurrola et al., 2009). The megaslide is ~1000 larger than the 1995 and 2005 events, and it might not have been identified without high resolution images, such as those provided by aerial photography, Google Earth, and LiDAR. The LiDAR data of the landslides reveals the 1995 and 2005 events in detail, as well as other scarps and smaller failures that otherwise would be difficult to recognize. The megaslide on Rincon Mountain also has distinctive geomorphology, including a rough horseshoe shaped scarp, lateral scarps, and fault scarps that would have been difficult to map without high resolution LiDAR. Geomorphologists refer to hummocky ground and topographic roughness as a method with which to differentiate debris flow fans from fluvial-dominated fans (Costa, 1984; Whipple and Dunne, 1992). Alluvial fans are formed below the mountain drainage basin areas, some of which may be composed almost entirely of debris flow deposits. As a second example of application of geomorphology to landslides, consider debris flows. Evaluation of the surface morphology of debris flow fans can be applied to deduce something about the rheology of the flows themselves (Whipple and Dunne, 1992). In other words, it is possible to estimate some of the physical properties of debris flows based on the surface morphology. The study by Whipple and Dunne (1992) on debris flow fans on the west side of the Owens Valley suggested that geomorphic attributes of the fans, including slope, roughness, drainage patterns, and levee heights were strongly influenced by the distribution of debris volumes and hydrographs that delivered and deposited debris on the fans. That study went on to speculate that there is a framework that is available to explain morphological differences between debris flow fans, derived from different source areas and even under different climates. For example, it was determined that the behavior of a debris flow is mostly controlled by variation in the bulk sediment concentration and its influence on flow rheology. Of particular importance in the Owens Valley, California study was the observation that where deposition occurs on a debris flow fan depends largely on the interaction of the debris flows with the channel system and is linked to flow volume, discharge, sediment concentration, and viscosity. Low sediment concentration debris flows were found to have a relatively smooth surface in the lower fan. 2.1.1. Montecito debris flow of January 9, 2018 The close relationship between fire and debris flows in Southern California has been a topic of research for about 100 yr since the 1934 debris flow in Montrose that killed over 40 people and produced extensive damage (Chawner, 1934; Troxell and Peterson, 1937; Wells, 1981; USGS, 2005; Kean et al., 2013). The Montecito debris flows of January 9, 2018 destroyed or damaged hundreds of homes and other structures and also claimed 21 lives. The proliferation of new airborne LiDAR technologies and the increasing frequency of their application (Lopez Saez et al., 2011; Bremer and Sass, 2012) allow unprecedented estimates of deposition and erosion produced by large debris flows such as the 2018 Montecito debris flow. For example, Fig. 2 shows the difference in LiDAR-derived elevation (Δz) before (2015) and after (2018) the Montecito flow for a portion of San Ysidro Creek that was heavily affected by the debris flow. This differencing map shows both channel erosion and the extent and depth distribution of debris-flow-generated mud and boulders to a scale of ~3 cm.

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Fig. 1. La Conchita megaslide and much smaller slides of 1995 and 2005 that represent reactivation of the Pleistocene Rincon Mountain megaslide on the uplifted ~45 ka marine terrace. Source: LiDAR from State of California.

2.1.2. Wildfire-Debris Flow Cycle The linkage between fire and debris flow may be depicted as a periodic cycle we call the “Wildfire-Debris Flow Cycle” (Fig. 3). It is a process-response model. The response may be a fluvial flushing of sediment in response to low to moderate intensity of rain, or a large debris flow or series of smaller flows, in response to rainfall of high intensity. Following a large debris flow, fluvial sediment flushing may occur or another large debris flow could occur. Predicting these events depends on the rainfall intensity, availability of sediment that could contribute to another debris flow, and how the hillslope hydrology and vegetation have changed over time. The first part of the Wildfire-Debris Flow Cycle is a wildfire that, in Southern California under pre-European settlement, recurs on time scales of 30–65 yrs. (Byrne, 1979; Minnich and Chou, 1997). With recent climate change and human interference with fire processes, the fire intensity is increasing, and droughts are longer, as is the dry season. In Southern California, the fire period is nearly the entire year (IPCC, 2015; Mann et al., 2016). Full recovery of the chaparral ecosystem following wildfire can take 30 yr. (Minnich and Chou, 1997). Wildfire plays an important role in that it removes vegetation and changes the

composition and texture of surficial materials, resulting in increased runoff of water and sediment. Precipitation is the second process of the Wildfire-Debris Flow Cycle. Should a significant rainstorm of high intensity occur, especially within the first year following the wildfire and within b1 to 5 yr. following wildfire prior to sufficient recovery of the vegetation to intercept intense precipitation and partially protect the soil, the likelihood of a debris flow is greatly increased. One study of debris flows in California concluded that most of the debris flows they recorded occurred when intense precipitation occurred over a short duration, b30 min, and, in fact, the best correlation was with 5-min rainfall intensity and when the debris flow lagged the peak in precipitation by only about 5 min (Kean et al., 2011). Erosion is the third process of the Wildfire-Debris Flow Cycle. Most debris flows in the Chaparral of Southern California are linked to surface erosion related to soils rendered more impervious following wildfire. The impervious nature of the soils has been attributed to development of a waxy layer near the surface called hydrophobic soil, infiltration of ash and fine particles into the soil that seals the surface, and the fact

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hillslope to surface materials and complex processes where changes can happen very quickly rather than by way of a gradual evolutionary path (Bak, 1996; Favis-Mortlock et al., 2000). Indeed, rills may form in a few minutes with intense precipitation on slopes following wildfire (Wells, 1985). 2. Spacing of rills decreases as slope angle decreases, and as slope angle increases, rills tend to become more parallel to each other and more elongated. Rills on steep, straight hillslopes with little or no vegetation tend to be spaced evenly (Schumm et al., 1984; Favis-Mortlock et al., 2000). Spacing of rills also depends on rock type, such that rills are spaced closer together on shale than those formed on sandstone. 3. Rills form when a critical shear stress, velocity of flow and erosivity of overland flow are greater than resistance of the slope particles to erosion. Average critical shear stress for silty-clay dominated slopes is about 2 Pa. Average velocity of flow at rill initiation is about 4 cm s−1 and slope length to rill initiation is ~2–7 m; with increasing slope angle and intensity of precipitation, slope length to rill initiation decreases (Yao et al., 2008). 4. Sediment yield increases as slope angle increases (Yao et al., 2009). One experimental study found a strong logarithmic (power) relationship (r2 = 0.93) between stream power and sediment load (Nearing et al., 1997).

Fig. 2. DEM to DEM differencing of Lidar-derived topography for a portion of San Ysidro Creek in Montecito, California, USA. Lidar data used were collected in 2015 and 2018 and gridded to 1-m resolution bare-earth DEMs.

that the vegetation has burned and the surface does not have a protective cover from intense precipitation (Wells, 1981, 1985). The discussion of wildfire associated with erosion of rills on slopes in the source area of debris flows (often long, narrow, closely-spaced surface channels eroded into slopes on burned areas) has been the subject of intensive research (Schumm et al., 1984; Nearing et al., 1997; FavisMortlock et al., 2000; Yao et al., 2008). These studies have been dominated by flume studies, as rill formation in the field is difficult to observe and measure. These studies suggest: 1. Rill development may be thought of as output from a self-organized dynamic system; rill formation is the result of response of the

One of the earliest studies that related the formation of rills to runoff and debris flows comes from the early 1980s (Wells, 1981). Wells was working at the San Dimas Experimental Watershed in the mountains above Los Angeles, performing experiments on slopes by artificially raining on slopes that he had burned. He noticed that with intense precipitation, the prevalence of many rills developed quickly, and each rill had a small debris flow, some of which coalesced into larger ones. It should be pointed out that the study area was very, very small. In comparison, the processes that formed rills apparently occurred over large areas in the source area for the 2018 Montecito debris flow (Fig. 4). The density of rills and, thus, the amount of runoff and production of fine sediment that contributed to the generation of mud and total volume for the Montecito event is most apparent in the shale units, compared to sandstone. It appears that the intense precipitation preceded the debris flow by only a matter of minutes, and thus, consistent with Wells' experimental work, a very quick rilling response occurred. Details of hydrologic processes linked to sediment transport, rilling, and small debris flows are being studied following the 2018 Montecito debris flows, and it is apparent that this is a complex and detailed process. However, without the production of the fine sediment or mud, development of large debris flows may not occur. The process of the mud production and its role in the debris flow is the topic of detailed research. The fourth process of the Wildfire-Debris Flow Cycle is generation of a debris flow. The processes involved in relatively fine sediment (ash, silt, sand, and gravel) eroded from slopes to channels where boulders are in storage, in response to precipitation and mobilization of that sediment into a fast-moving debris flow, are conceptually understood. However, details of the transformation are poorly understood (Kean et al., 2013). Every chaparral wildfire with subsequent precipitation that forms rills produces small debris flows of M 1–2 (where M is the Log-base 10 of the volume of the flow in m3; Keaton et al., 1988) that flow down the rills (Wells, 1981). The occurrence of high magnitude M 5+ events are infrequent, and details of the processes that transform boulders several meters in diameter stored in the channel to a moving mass of boulders traveling at high speed that are deposited as boulder fields and levees are poorly understood (Fig. 5). The fifth stage of the Wildfire-Debris Flow Cycle may be a flushing by fluvial processes. This stage may occur with or without high magnitude debris flows or may occur during moderate precipitation after a high magnitude debris flow has occurred. Sediment flushing refers to the process of fluvial transport of relatively fine gravels from a drainage

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Fig. 3. Diagram with photographs of the Wildfire-Debris Flow Cycle. See text for explanation. Sources of photographs: Wildfire (both), Rilling (left), debris flow (both) - County of Santa Barbara Fire Department, 2018.

Fig. 4. High-resolution terrain modeling and rilling in the source areas above Montecito (A); 3D model created with high-resolution aerial photographs (B); DEM created from highresolution aerial photographs. Scales of A and B are the same; 3D model of rilling in shale (C); Aerial photograph of gullying on sandstone (D).

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Fig. 5. Boulder field along San Ysidro Creek from the 2018 debris flow: (a) ground level and (b) boulder deposited high above the ground on the porch of a home.

basin, either following wildfire without debris flows or after debris flows have occurred (Florsheim et al., 1991, 2017; Keller et al., 1997). Fig. 6 shows sediment flushing from two storms in the Romero Canyon area that produced significant sediment flushing following the 2018 Montecito debris flow. The precipitation events that produced the flushing were moderate, with intensities of about 15 mm/h. What we may conclude concerning the Wildfire-Debris Flow Cycle is that the fire and debris flow are intimately linked by intense precipitation. However, whether or not a high magnitude debris flow occurs is also a function of geomorphic instability. Here, the instability is the result of the accumulation of boulders and other sediment in source area channels following a debris flow. The time necessary to build up sufficient material to produce another large debris flow is largely unknown. The concept is generally idealized on Fig. 7; if a wildfire is followed by an intense storm that occurs when geomorphic instability

is high, the probability of a large debris flow is much greater. Occurrence of a second or third large debris flow from a canyon over two or four years after a fire depends in large part on how much sediment was removed in the first flow. Lacking the geomorphic instability, sediment flushing is more likely. Therefore, following wildfire and precipitation, we generally would expect sediment flushing to be a common process, with less frequent high magnitude debris flows occurring. 2.1.3. Boulder geomorphology and debris flows Boulder geomorphology is the study of boulder morphology, weathering, and chronology. A weathering rind (often a red-brown color) on old boulders is often present at the surface above the case hardened layer. As case hardening progresses, ongoing mineral dissolution underneath the case hardened layer may eventually lead to detachment, recognized by fracturing that is a form of surficial, spheroidal

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Fig. 6. Sediment flushing in Romero Canyon above Montecito following rainstorms in March and May of 2018.

weathering. The boulder coating process starts with mineral weathering and mechanical fracturing of the outer few millimeters of a boulder to create weathering rinds that, over 1000s of years, are transformed to a case hardening at the surface that eventually may be several cm thick (Fig. 8) (Dorn et al., 2017). Most boulders on the surface of a debris flow (as a landform, such as a debris flow fan) are concentrated in boulder fields or boulder levees. However, scattered boulders may be found anywhere on a flow surface, even near the distal parts of flows. Boulders deposited on the surface of the 2018 Montecito debris flow are present on lower parts of the flow near the ocean, where the main deposits are from the wet-tail of the flow (mostly mud flow). These young (2018) boulders are found in all parts of a deposit from boulder fields near the mountain front to the lower parts of the flow where boulders are not common (Fig. 9). The 2018 boulders are generally angular and lack signs of weathering. It is assumed the transport and deposition during a debris flow removes almost all evidence of past weathering. Fig. 10 shows a boulder on the penultimate event at Montecito Shores condominiums just west of the present mouth of Montecito Creek. Montecito Shores is at the distal part of the Montecito Creek debris fan. The flow came down one of the lower channels of Montecito Creek, shown on an 1869 topographic map (U.S. Coastal Survey, 1869). The lower debris fan is particularly likely to change channel location as the fan develops, and it is the widest part of the fan. When the

channel moved west in 2018, Interstate Highway 101 was filled with debris flow deposits that carried boulders to the ocean. A mud flow with a few boulders from the 2018 event filled the lower parking structures of the Montecito Shores condominiums. The thin weathering rind layers of the penultimate event are a millimeter or so thick. This thin surface boulder weathering rind and case hardening is similar to the site of a large debris flow (several millions of m3) near the historic Old Mission at Rocky Nook Park, dated by 14C at 1–1.5 ky (Urban, 2004; Keller, 2011; Gurrola et al., 2016). One hypothesis is that the penultimate debris flow at Montecito Shores at the housing area is 1–2 ka. Simple conditional probability supports this interpretation. If the average return period (RI) of a wildfire (for one particular debris flow-prone drainage) is assumed to be 25 yr. (annual p = 0.04, a conservative estimate) and a 75 yr. rainstorm (intense rainfall over 15–30 min) in the first year following the fire (annual probability of p = 0.013) is also assumed, then the conditional probability (product of the two probabilities) for the first year after the fire is 0.0005 (5 in 10,000), and the average RI of flows is 2000 yrs. If the required storm is to occur in the first two years following fire, then the conditional probability increases to 0.001, with an average recurrence interval of 1000 yr. Of course, two storms of RI of 75 yr. can occur in the same year or in consecutive years. However, our estimate of conditional probabilities of fire followed by intense rain, suggests the average RI for the first year or so after fire is at least 1–2 ky.

Fig. 7. Idealized diagram of timing of wildfires; flooding and basin instability combining to produce high magnitude debris flows. Lacking the basin instability, fluvial flushing of sediment is a common post-fire response. Modified after Schumm (1977).

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Uplift rates at the coast also support the hypothesis of an age of 1–2 ky for the penultimate flow at Montecito Shores. A Holocene uplifted wave cut platform just east of Montecito Shores is about 2 m above sea level. The rate of uplift is ~0.5 to 1.0 m/ky (Gurrola et al., 2014). The wet-tail of the flow that appears (based on boulder weathering) to be the same flow that is found at Montecito Shores is the surface the platform is cut on. Using this uplift rate suggests the age of the platform is 1 to 2 ky. It should be noted that small, moderate but potentially damaging debris flows occur in the first few years following wildfire, depending

in part on the intensity and duration of rainfall. In early February of 2019, small debris flows occurred in Romero Canyon (Fig. 11.) The flow was confined to the debris basin constructed decades ago and periodically cleaned out by the County of Santa Barbara (Tom Fayram, oral communication of February 2, 2019). The first few months following wildfire are of most concern. A fall fire followed a month or two later by intense precipitation is the most likely path to disaster. For example, a large wildfire occurred above Montecito in 1964, and a month later a light rain on parts of the burned

Fig. 8. Weathering of boulders has changed boulder geomorphology through the past 125 ka. Boulders from the 2018 event are not weathered, and corners of boulders are angular. Over time, boulders have increasing thickness of weathering rinds and case hardening: (a) Boulder from the 2018 event lacks weathering rind and has more angular corners; (b) Thin rinds (a mm or so thick) from a debris flow at Rocky Nook park in Santa Barbara with 14C dates of 1 to 1.5 ka; (c) A boulder on a debris flow surface at the Santa Barbara Botanic Garden. This flow is older than that at Rocky Nook but is of unknown age. Weathering rind is thick; (d) Boulder on a debris flow near Skofield Park in Santa Barbara. The weathering rinds are thicker than at the botanic gardens, and from a soil date (Best, 1989) may be as old as 20–30 ky; (e) Thick weathering rind on a debris flow above Skofield Park that is folded over the Mission Ridge Anticline; and Mission Ridge 80–130 ky, based by an exposure date (Gurrola, 2005); and (f) Very thick weathering rind on a debris flow on the southern limb of the Mission Creek Debris flow. The sequence a–f defines a relative chronology with limited calibration.

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Fig. 8 (continued).

Montecito and San Ysidro watersheds caused flash floods and debris flows that damaged homes and bridges. Fast moving flows of debris wood and boulders were reported as walls of water and debris up to 6 m high (U.S. Army Corps of Engineers, 1974). Severe flooding and smaller debris flows also occurred in 1969 (5 yr. following the Coyote Fire) as part of widespread flooding in California. Debris flows and debris floods from past wildfires followed by rain, while causing significant damages, were evidently much smaller than Montecito in 2018. The oldest debris flows in the Santa Barbara and Montecito areas, based on limited exposure dating using 21Ne, are 90–130 ky (Gurrola, 2005). These old fans are often over 100 m above local streams and are folded and faulted (Melosh and Keller, 2013). At another site, estimates of the age of two debris flows in Rattlesnake Canyon, based on soil chronology correlated with a chronosequence in the Ventura Basin, are b500 yr and ~8–30 ky (Best, 1989). The youngest flow has a very young soil and very thin weathering rinds, and boulders of the older

one has case hardening that is intermediate between flows estimated at ~1 ky and 90–130 ky (Fig. 8). Our working hypothesis is that there is a relative chronology present in the debris flows of Montecito - Santa Barbara. We await additional 14C dating of flows less than ~40 ky to produce a calibrated chronology to estimate the RI of past events. One criticism has been that we will miss some large flows that have been removed by erosion or buried by younger flows. Given the high rate of uplift and incision, this does not seem likely. In the canyons above Montecito, we have identified older flows that are well above the modern channel. Fig. 12 shows a large boulder on one of these flows. The large boulder (part of a debris flow) on bedrock clearly has a weathering rind, and rock incision has isolated the boulder. Boulder geomorphology involves linking boulder chronology to geomorphic maps of debris flow fans. The map of debris flow fans in Montecito is shown in Fig. 13. The base map is 2018 LiDAR from the County of Santa Barbara and Gurrola (2005). The mapping is based on

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Fig. 8 (continued).

debris flow morphology and cross-cutting relationships where one fan segment truncates another. Application of the boulder morphology and chronology is helping to understand the history of the flows with implications for understanding the future hazard. 2.1.4. Applications to hazard reduction, assessment, and perception Fast-flowing large debris flows triggered by intense precipitation that occurs shortly following wildfire are a serious hazard in Southern California (USGS, 2005). Estimating rainfall intensity-duration thresholds that result in generation of post-fire debris flows has been a primary objective of debris flow hazard assessment. The research has developed and tested models that assist early warning for debris flow occurrence following wildfire (Cannon et al., 2010; Staley et al., 2017; Staley et al., 2018). The models apply several geomorphic and soil parameters, such as drainage basin area, channel slope, and burn intensity,

along with intensity of rainfall over time intervals from 15 min to 1 h, to estimate threshold values of storm intensity. The objective is to identify those drainage basins with the highest probability of experiencing a large debris flow (Cannon et al., 2010). To this end, the U.S. Geological Survey (USGS) has developed a simple method of estimating the likelihood of a debris flow developing in specific basins in response to a peak 15-min rainfall intensity of 24 mm/h. The empirical method uses readily obtainable precipitation and geomorphic data as input into a GIS with logistic regression analysis. The likelihood model predicted that several basins above Montecito had a 60–80% likelihood of experiencing a debris flow if the threshold precipitation was exceeded. The threshold was exceeded, and the January 9, 2018 debris flow caused catastrophic damage and loss of life in Montecito. A second model estimates possible volume of the flow at the basin outlet (USGS, 2016). We will now explore some of the social science related to the debris flow hazard.

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Fig. 9. Large boulder deposited near the downstream end of the 2018 debris flow with mostly mud from the wet-tail (mudflow).

How people perceive their risk and respond to the debris flow hazard is at least as important to hazard reduction as the science of debris flows. We hypothesize that few people in Montecito prior to the 2018 event knew what a debris flow was. Some people living along the creeks had some past experience with flash floods that occurred decades ago (1969, for example) and were accompanied by smaller but damaging debris flows. Others in the community valued the large boulders in their gardens but did not know their origin. After the 2018 Montecito debris flow, people were desperate for information about the hazard and wondered if other flows would be likely in coming years. The answer can be framed as a conditional probability of the fire and rain.

Assuming the annual probability of a wildfire at a particular basin (following recovery from a previous fire rated at 0.10) and that the probability of a short duration high intensity storm is 0.02, then the conditional probability is 0.002 (one in 500). This would suggest that high magnitude debris flows emerging from any one canyon are relatively rare (see Section 2.1.2). Geomorphology is becoming increasingly applied to the social sciences and, in particular, the social study of disasters, hazard and risk reduction, and risk perception. The science of natural hazard events, including volcanic eruptions, landslides, and earthquakes capable of producing catastrophes, has expanded rapidly, but it is social science that

Fig. 10. Boulder with a thin weathering rind on the penultimate debris flow at Montecito Shores. The thickness of the rind compares with that of the young flow at Rocky Nook Park (Fig. 9b).

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Fig. 11. Relatively small debris flow in Romero Canyon that occurred on February 2, 2019 in response to intensive precipitation that was caught in a Santa Barbara County debris basin. Photograph by Jamila Rivera.

places these hazards in a human context with the goal of reducing hazards by identifying social vulnerabilities and patterns in human behavior that exacerbate risk. People live within socially constructed value systems and tend to engage or not engage with natural hazards and disasters that

affect their lives by way of both learned and shared perspectives linked to their belief systems (Santi et al., 2011) in ways that can vary greatly from place to place. Social science also goes one step further by offering alternatives that can reduce hazards and foster community resilience.

Fig. 12. Older debris flow with weathering rind well above Romero Canyon. Uplift and incision have been sufficient to preserve the flow.

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Fig. 13. Map of debris flow fans in Montecito, California. Large flows have occurred over the past ~100 ka, but timing and recurrence is poorly constrained.

With better understanding of the hazard, people may learn that the debris flows in Southern California occur when wildfire is followed by intense rainfall. And this can be used to produce a simple heuristic, akin to the existing “Duck, Cover, and Hold” for earthquakes. For example, we might use, “wildfire – intense rain – move uphill.” or “Ready (be prepared in advance to evacuate if necessary), Set (monitor fire data and post fire precipitation in preparation to evacuate), Go (evacuate when directed or if you are uncomfortable).” There is a Santa Barbara County hazard education program, and drills are currently being tested in elementary schools in Montecito as part of hazard education. During the Montecito debris flows, many people did not comply with the evacuation order. Understanding why people do not comply with evacuation orders concerning debris flows is key to knowing

how to better communicate the risks in ways that may lead to better disaster preparedness and response. For example, based on qualitative interviews, we have found that few in Montecito had ever heard of the term “debris flow” before January 9, 2018. Many felt more familiar with terms like “mudslide,” “landslide,” and “flash flooding,” and some even felt that these events were particular to places in the Global South. As one resident noted: “I know flash flooding and landslides happen … in foreign countries … but the mud and debris flow? This is new terminology for me.” In the aftermath of the disaster, the term “debris flow” itself worked to create a sense of confusion (and even discontent) between experts and victims. In an interview with one victim who lost a family member in Montecito, the term “debris flow” was a source of frustration on the grounds that it failed to capture the lived experiences

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of the trauma on the ground, including the stench of the disaster and other assaults on the body, thus devaluing the experiences of those most impacted. As this respondent explained: “for mudslide victims it makes them crazy [calling the debris flow a debris flow], they need reference to the mud.” Discussion, therefore, needs to take place concerning the shortcomings of the term “debris flow” and whether it can connote the visceral dangers of the hazard it aims to describe. As one respondent noted, “I've never seen anything like it. There were a thousand boulders, all sizes … I mean, talk about putting things in perspective. I think [the debris flow] was twenty, thirty feet high and moving fast, like thirty miles an hour.” Furthermore, with better understanding of the vulnerability of a given place, public officials and first responders may perceive the risk differently and learn that certain populations are more susceptible and therefore require targeted outreach. In the case of Montecito, nearly half of the 21 debris flow victims who perished were migrants from working class families. Based on our early interviews in Montecito, it is clear that divisions exist within and between renters, property owners, those who inherited property, those who are independently wealthy, those from the working class, those with celebrity status, the elderly, citizens, and non-citizens. How one is impacted and able to recover depends on a range of factors that determine social vulnerability, including income, political power, prestige, gender, race, age, education, and occupation, to name a few (Cutter et al., 2003). In order to better understand preparedness for and response to the debris flow hazard, surveys and interviews can probe such questions as: 1. What do residents think about the debris flow, the evacuation process, their own disaster preparedness, and teaching about future climate-related disasters? 2. What factors are part of residents' choices to evacuate or not? 3. Does explaining the link between debris flows and climate change affect people's support for disaster education and environmental policy or their pro-environmental and disaster preparedness behaviors? People who have experienced a recent event are psychologically closer to it, and psychological distance (a similar psychological bias to salience; Taylor and Thompson, 1982) shapes reasoning about events and behavior in major ways (Trope and Liberman, 2010). Salience also shapes governmental responses. For example, chemical treatments to mitigate fire are placed more often near communities that have experienced recent fires than near other communities that are likely to have higher fire risk exactly because they have not just experienced a wildfire (Wibbenmeyer et al., 2018). Psychological distance is also measured to see how it shapes residents' behaviors and attitudes. For example, we want to know whether the people for whom the debris flow was most salient were more likely to evacuate a second time should another potential debris forming precipitation event be predicted. 2.2. Applications to tectonic geomorphology and earthquake hazards The study of active tectonics is a relatively new research area, and tectonic geomorphology is part of that body of work. The development of tectonic geomorphology and earthquake hazards in the late twentieth century rejuvenated geomorphology, helping change it from a qualitative approach to landform development to a quantitative science. Tectonic geomorphology is the application of geomorphic principles and methods to solve tectonic problems, as well as the study of landforms produced by active tectonics (Keller and Pinter, 2002). The first paper with a title incorporating the words “tectonic geomorphology” of a particular area was in the 1980s. The application of geomorphology to tectonics developed rapidly in recent decades with the understanding that it is the very recent record of the magnitude and frequency of earthquakes that is necessary to define the earthquake hazard. As a result of increasing scientific and social interest in earthquake hazards, geomorphologists and structural geologists, particularly since

the 1980s, have focused on what deformation of landforms can reveal about rates of the processes of folding and faulting. The important field of paleoseismology (study of prehistoric earthquakes) is based in part on application of geomorphic information in the near-surface geologic record to estimate the probability, magnitude, and return period of earthquakes that concern society. Studying processes in geomorphology is an example of applying geomorphology to the relative age of fault scarps (and, thus, the earthquakes that produced the scarps), known as fault scarp degradation (Wallace, 1977). This application of geomorphology uses a processresponse model that integrates materials, landforms, and chronology to assist in deriving rates and recurrence intervals of earthquakes (Keller, 1986). When the surface of the Earth is faulted, a fault scarp composed of alluvial material may occur. Initially, there may be a free face, which is a steep slope element. The free face may not last more than a few decades to a few hundred years, and, eventually, processes of degradation transform the free face into other slope elements, including a debris slope and wash slope. Along with this, the top of the free face may become more rounded. Eventually, the debris-controlled slope elements give way to wash-controlled elements, with further reduction in the slope of the fault scarp until the scarp itself resembles a lazy “S” shape. The time necessary for the transformation may be many thousands to tens of thousands of years or longer, depending upon the nature of the slope materials, climate, and slope processes. Once fault scarps are dated in an area and their morphology, based upon the distribution of slope elements such as free face and is determined, then the geomorphology may be directly applied to relatively date the time since the occurrence of past earthquakes (Wallace, 1977; Nash, 1980; Keller and Pinter, 2002). A more quantitative application of fault scarp degradation is to survey past scarps of known age, as for example the 1872 M 7+ event in the Owens Valley of California. The change in the fault scarp since 1872 can be used to estimate a slope diffusion constant that can then be applied to other scarps in the Owens Valley to estimate when earthquakes there occurred (Keller and Pinter, 2002). Uplift of several meters of Holocene wave cut platforms, possibly from very large M7+ earthquakes in Southern California near Ventura, is another example of applying geomorphology to help address important questions about the possible size of future earthquakes (Rockwell et al., 2016). That research uses LiDAR in combination with 14C dating and archeology (see Section 2.5 of this paper). 2.3. Applications to coastal erosion With 7% of the Earth's population living within 5 km of the coast and 27% living within 100 km of the coast, beaches and sea cliffs are features of geomorphic and societal significance (Kummu et al., 2016; Griggs, 2017). As the climate warms and dries, sea levels are rising and the potential for large storms and El Niño events appears to be increasing. The risk of coastal erosion and damage to homes and infrastructure is likely to increase and expand around the world, and this damage will cost billions of dollars in damages (Nicholls and Cazenave, 2010). Therefore, increased interest has been placed on understanding rates, patterns, and processes of sea cliff erosion, and the threat that these events pose on coastal communities and ecosystems has sparked the development of tools and computer models that aim to evaluate and forecast coastal hazard probability and occurrence. However, predicting the rate that the shoreline will recede over any time period is recognized as a notoriously difficult problem. While maps, imagery, and other geomorphic information have been available over the last century to evaluate the rates of erosion and fluctuations in beaches, the spatial and temporal resolution of data and techniques to detect change from two different topographic models was relatively poor, making it difficult to link individual processes with rates of change. Paramount to recent advances in linking processes with erosion has been the accumulation and analysis of high resolution data sets, which lend a more complete

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understanding of coastal processes (Warrick et al., 2016). Overall, quantifying change is essential for calculating trends in erosion and evaluating the individual processes that shape coastal cliffs, as well as verifying the predictive capability of models. With the recent use of terrestrial LiDAR, drone photography, high-resolution aerial photographs, and computer programs that produce topographic models, much more can be said about rates, patterns, and processes of coastal erosion (Young et al., 2011; Katz and Mushkin, 2013; Young, 2018). For example, with centimeter-scale data, various parts of a sea cliff may be individually analyzed and linked with wave energy, rainfall rates, and total water levels after individual storm events (Fig. 14). Understanding the natural variability of sea cliff erosion and the role of physical variables (season, timing of storms, beach sand supply, wave energy, and weathering) has advanced our understanding and are a direct result of the application of detailed geomorphic analyses. Using this information, predictive models that are used to predict the future location of the shoreline are becoming more reliable. The most comprehensive model for forecasting coastal change induced by extreme storms and sea-level rise is the CoSMoS model, developed by the USGS. CoSMoS is a process-based modeling system that has been designed to assess the physical impacts (e.g., flooding, beach erosion, and cliff failures) of severe storms both operationally and based on future sea-level rise and climate change scenarios (Barnard et al., 2014; Vitousek et al., 2017). This will be important in the coming decades for planning as well as areas that already contain development and infrastructure. In addition, engineering applications of highresolution data include the ability to perform rockfall stability analyses and develop a rockfall activity index along steep rock slopes (Dunham et al., 2017) that can be used along road cuts and other areas where steep rock cliff are a concern. Similar to our discussion of fault scarp degradation, the role of geomorphology in the sea cliff ecosystem is important because the variables that influence its form have significant geomorphic components (Komar, 1998). Emery and Kuhn (1982) suggested that profile shapes of sea cliffs are related to process, and that the cliff profile can indicate whether terrestrial and/or marine processes are driving change. In general, when marine processes dominate, the steep sea cliff often lacks talus or other materials accumulating at the base, and, as subaerial processes dominate, the top of the sea cliff tends to round, until the sea cliff approaches more of a lazy “S” profile. This has been studied further with the use of high-resolution data and differencing of topographic models and found to be accurate (Johnstone et al., 2016). Therefore, by using morphology alone, investigators interested in coastal zone management may observe the morphology of sea cliffs and relate them to relative intensities of marine and land erosion. 2.4. Applications to climate change Natural hazards are linked to climate change, which is perhaps the most important topic in applied geomorphology and science today. For example, as Earth warms, the frequency and intensity of wildfires are increasing, and wildfire in areas such as Southern California that once had a “fire season” now seem to occur nearly year-round. Intensity, duration, and type of precipitation is also changing as a result. Dry areas are getting drier, increasing the occurrence of drought, while more intense rainfall is increasing the occurrence of flash flooding in other areas. Geomorphology may be applied to climate change through: development of proxy records of climate from the study of the landscape, linked to soil processes over time; glaciers and glacial processes that demonstrate change over hundreds to many thousands of years; changes in patterns of landscape evolution over time; and, finally, changes in the magnitude and frequency of geomorphic processes, such as flooding and landslides, influenced by changing precipitation. Of particular importance has been the use of numerical dating methods of deposits forming geomorphic landforms and processes over relatively short periods (past few hundred years to decades) of time that

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may be closely linked to climate change now impacting humans (IPCC, 2015; Mann et al., 2016; State of California, 2018; Swain et al., 2018; Keller and De Vecchio, 2019). 2.5. Applications to anthropology Geomorphology has been applied for decades to better understand physical anthropology. Of particular importance is providing context of archaeological sites in terms of what the land looked like, what its age was, and how the landscape was linked to the people being studied. For example, Rockwell et al. (2016) identified several Holocene uplifted marine terraces (uplift was several meters per event) near Ventura, California, that had been occupied by native peoples. Careful integration of the geomorphology and the anthropology suggested that several large earthquakes that uplifted the coastline occurred with the return period of about 1000 yr, and that, in each case, Native American villages were abandoned and then re-occupied in the coastal zone where the ocean and its resources were more available. Integral to this study was the recognition of the marine platforms, based on fieldwork, 14C dating, and analysis of topography from airborne LiDAR. In recent decades, significant advances have been made in identifying ancient cities and structures in the tropics. For example, 3dimensional color LiDAR images show in considerable detail the epicenters of Maya civilization in Belize and other areas. Of particular importance is the identification of human structures such as terraces for agriculture and irrigation systems, as well as cultural features such as walls and buildings, as shown in Fig. 15 (Chase et al., 2010; Carter et al., 2019). 2.6. Applications to ecosystem management Applications of geomorphology to ecosystem management are nowhere more apparent than in river restoration. Rivers are complex, open systems where the interactions of variables over time and space produce a variety of channel forms that have been modified by human use and interest. How best to restore degraded streams has been a major area of applied geomorphology. Approaches have varied from engineering to bioengineering and application of natural processes, but what seems to be clear is that geomorphic understanding of a stream is critical in development of restoration plans (Smith and Prestegaard, 2005; Wohl et al., 2005; Pasternack, 2013). Management of the sea cliff and beach ecosystems requires application of coastal and beach geomorphology. The NOAA ecosystem coastal classification includes natural features that are the existing coastal ecosystems that provide multiple benefits to communities such as storm protection through wave attenuation or flood storage capacity and enhanced water services and security and nature-based features. These are also called green/grey infrastructure that are engineered systems where natural features are combined with hard/structural engineering approaches to produce a hybrid system as in a living shoreline. The role of geomorphology in the sea cliff ecosystem is important because the ocean, beach sea cliff, and human variables that influence the form of beach and sea cliff have significant geomorphic components. 2.7. Applications to land-use planning The field of land-use planning is one of the important ways that geomorphology is applied to the needs of society. How can we decide the best use of land in terms of potential uses that may vary from urban development to agriculture, mining, and forestry, as well as siting of features such as highways and critical facilities including power plants and hospitals without considering landform processes and potential natural hazards? The application of geomorphology is particularly important in several aspects of land evaluation for site selection of facilities to support human activity such as power plants, dams, industrial complexes, and housing areas. Environmental impact analysis is often an

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Fig. 14. Sea cliff at Isla Vasta, CA that is being monitored with terrestrial LiDAR (A); Sea cliff erosion after a storm in 2016 created by differencing sequential scans. The capacity to make these measurements after individual storms allows for erosion rates, patterns, and processes to be identified (B).

integral part of land evaluation, and the evaluation includes geomorphic maps as well as hazard assessment. This is particularly important, given that humans have continued to dominate much of the environment, particularly near large urban regions (Keller, 2011; Meitzen, 2017). 2.8. Applications to engineering geology Engineering geology, because it is often concerned with surficial processes, involves the application of geomorphology. The engineering geologist does not design engineering structures to a problem but assists geological and civil engineers by identifying geologic and geomorphic constraints of site conditions that might impact a project. Many engineering geologists are well trained in both geology and geomorphology. Landforms at a site and surrounding areas may be analyzed using images from LiDAR (see Fig. 1). Identification of the possible megaslide would have been much harder without the clear geomorphology that shows both the much smaller 1995 and 2005 slides. Only a small portion of the 1995 slide was reactivated in 2005. The future slide hazard was possible to evaluate because of the clear geomorphology shown in the LiDAR. Notice the pattern of bluff erosion changes east of the megaslide compared to that along the landslide (Fig. 1). That pattern change is a red flag for slope stability. The smooth curve in the bluff is slope protection that involves benching the slope into seven cuts, like a flight of stairs. The benches reduce the high eroding slope into a series of more manageable slopes, each with drainage. The geomorphology assisted with slope evaluation in this landslide-prone section of coast. Likewise, coastal geomorphologists use coastal morphology extensively as part of coastal erosion and landslide studies (see Section 2.3 of this paper). Fluvial geomorphic processes related to floods and bank/slope erosion are commonly applied by engineering geologists as tools in evaluation of the flood hazard and bank stability. Recognition of channel pattern and location of existing or potential bank erosion problems would be difficult without geomorphic observations. 2.9. Applications to expert witness investigations As another example of how geomorphology may be applied to needs of society, consider the role of the expert witness within our court system. Natural hazard events such as the floods and landslides and debris flows, especially if there are deaths and financial damage, often generate legal activity where those injured hire lawyers to initiate lawsuits on those they think might be responsible for their losses. Geomorphologists are particularly well-suited for expert witness work in law cases

stemming from erosion, flooding, and landslides damages. The role of the geomorphologist is to work with lawyers and others involved with natural hazards and other events that are being settled through the courts by applying the best science to the subject at hand. It is important that geomorphologists acting as expert witness collect their own data and come to their own conclusions (Keller, 2014). The scientific process can be applied to provide evidence as to whether there are causal factors linked to hazardous events, such as landslides, floods, wildfire, or debris flows. Being an expert witness is an application of geomorphology where the expert provides advice to lawyers regarding what the science questions are and how they might be resolved. Working for a defendant accused of doing something thought by others as harmful, for example, modifying a floodplain or releasing flood water to adjacent property, involves evaluation of the client exposure and making recommendations on whether the client should settle or go to court. More importantly, the expert searches for the scientific basis of what occurred. Working with lawyers allows a geomorphologist to directly interact with society and the legal system. However, there is a difference between the results of scientific investigation based on facts and the decisions of judges and juries. One of the tasks of the expert is to educate the judge and jury about the facts of a case. Many geomorphologists avoid trial work as they fear cross examination by opposing lawyers. In truth, the opposing lawyers may fear the expert just as much and probably more. If the expert has gathered his/her data and come to his/her own decision, scientific results may be successfully and forcefully defended. The job of the expert goes well beyond winning or losing. People are generally sued because damage has been done. This is best illustrated with a brief discussion of two cases. In one case, the lower part of an alluvial fan was flooded when a stream overflowed its banks to damage orange trees on an adjacent property. A series of storms impacted such a channel, and property owners (those being sued, the defendants) opened a notch in the top of the channel to protect homes and water supply wells from being damaged. The defendants tried to clean out the channel after a storm that filled it with sediment before a second storm came, but time was short between storms and cleaning out the channel was not possible. One legal position is “the lesser of two evils,” i.e., better to flood fields than homes. The defense attorney spent a day with a small soil augur with a sample tube around a number of trees and announced that the trees were not buried (damaged) by sediment as the experts for the plaintiff contended. The expert in geomorphology was called to verify the results. There were clearly lobes of sediment on the flooded orchard. Taking the first small core, there appeared to be no flood layers. However,

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Fig. 15. Geomorphology of a Maya Civilization city site in Belize showing buildings, agriculture terraces, and irrigation and drainage ditches revealed form LiDAR. Courtesy of Arian Chase.

knowing that the core might be smeared by the coring and collection, the expert took a pocket knife and split the core, exposing flood layering as pulses of sediment that accumulated around the fruit trees. Had the geomorphologist not been called in, the lawyer not only would have been embarrassed but would have lost credibility. The defendants clearly had caused the flooding and damages to the trees. The geomorphologist clarified that the deliberate breaching of the channel had flooded the lower orchard. The expert also addressed other flooding upstream of the breach on the orchard property (of the plaintiff), thus limiting the lowering of damages from the defendants' actions. The defendants lost the case, but damages were probably more equally divided between the parties involved. In this case, the geomorphic analysis involved understanding fan morphology, fluvial processes of alluvial fan flooding, and sediment accumulation from pulses of flood water and sediment. The second example (discussed in more detail in Keller, 2014) was the result of home owners suing the county for building a hard flood control levee on a river that protected existing development across about half of the floodplain, but concentrated erosion against a high bluff on the other side of the river where homes were sited on top of the bluff. Landslides developed on the bluff, which damaged property. The county stated they had never been successfully sued and pulled out all the strings in defense, but the county lost. The case was tried by a judge, and the case results turned on the geomorphology of the river. Questions investigated were: 1. What is the geomorphic and tectonic history of changes to the river valley over the past 100,000 yr, and is that history relevant to the situation of the river today? 2. What is the history of the river near the bluffs that eroded during the past half-century? 3. How have the eroding bluffs in the last half-century changed with respect to slope stability and riparian forest cover? A 1904 USGS map clearly showed the position of the river over 100 yr ago. The river was clearly much closer to where the levee was built, and it subsequently, as a result of the levee, moved to the base of the bluff. The judge found that compelling and ruled in favor of the

home owners (plaintiffs), based on the map interpretation over the more quantitative modeling that showed that the stream power against the bluff increased significantly after levee construction. 3. Discussion Applications in geomorphology discussed in this paper are a subset of the total applications being used today. Examples selected reflect the bias we as authors have through the geomorphology we are studying. Without geomorphology, physical anthropology of pre-historic and historic sites could be poorly interpreted. Without geomorphology, land-use planning and ecosystem management would flounder from lack of understanding of surface processes and landforms affecting planning and management. Hazard identification, mapping, analysis, and reduction would be handicapped without geomorphology. For example, mistakes from well-meaning emergency management teams, whose task it is to issue evacuation orders for areas of mandatory evacuation from flash floods and landslides may result in lives lost due to lack basic geomorphic understanding of the surficial processes linked to the hazard. Without geomorphology, engineering geology (application of geology to engineering problems and solutions), civil engineering, and geologic engineering (design of engineering solutions to slope stability, erosion control, flooding, and earthquake deformation) would be handicapped. The most extensive application in this paper is the Montecito debris flow of 2018. This event is emphasized (even as it is being studied in more detail) because it uses state-of-art methods and is strongly linked to the social aspects of the debris flow hazard. Implications of the research on debris flow return periods and areal extent (magnitude and frequency concept) will help the education of the public and informing agencies managing future risks. There are also applications of physical and social science for building sustainable, resilient communities. Consequences of a hazardous event include damage directly from the event, and those losses in property and lives may be very significant. However, losses from the event itself may not result in the greatest losses. Often, it is what happens before, during, and after a hazardous event occurs that determines most of the losses. Consider total losses

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1 + 2 = 3, where No. 3, is the total loss; No. 1 represents the direct losses from the event; and No. 2 consists of losses related to human actions including inadequate engineering; not wanting to or unable to spend funds for disaster planning that result in supplies, water, food, and medicine being quickly delivered after a disastrous event has occurred; not having public safety plans ready to be implemented; and incompetence, or just not doing the right thing or even knowing what to do (Marsa, 2013). Total losses from human processes (No. 2) is often much greater than No. 1 (Keller and De Vecchio, 2019). In the case of the Montecito debris flow as well as mud flows and flash flows (all may have higher magnitudes following wildfire), the building of more small debris basins or placing debris nets across channels may be effective for small to moderate events but will likely provide inadequate protection for large catastrophic events such as that of January 9, 2018. Only very large debris basins such as the engineered structure on Santa Monica Creek in Carpinteria can capture large events, and then only if it has been cleared out prior to an event. Debris basins and nets may also provide a false sense of security. The most effective response to the flash flood and debris flow hazard is to just say no to development along potential flow paths. That is difficult for home owners who want to rebuild following damage or loss, but it is an important option to provide for sustainability and resiliency of communities. Even with this, damage in lower parts of debris flow fans and alluvial fans will remain a hazard as channel position is uncertain and may change from flow to flow as the fan morphology develops. 4. Conclusions With many new high-resolution methods of depicting topography and much improved numerical dating, geomorphologists are tackling problems we could only think about a few decades ago. Entire new methods of studying the landscape, based on high resolution numerical dates and sequential high-resolution topography, are revolutionizing the study of surface processes. Applications in geomorphology use principles and methods to solve a variety of problems within geomorphology, as well as other branches of earth science. Another way to look at such applications is that the questions analyzed are often those of particular interest to society. These would include but not be limited to climate change and mitigation of natural hazards to better understand how people perceive and react to natural hazard events such as flash flooding and debris flows. The application of geomorphology to social issues is of particular importance to social scientists' research to relate hazardous events to what communities know about hazards, what they think the appropriate response of emergency management should be, and what their personal response to loss and damage should be. Social science is at least as important to hazard response and mitigation as physical science. Acknowledgments This paper is dedicated to the people of Montecito impacted by the 2018 Debris Flow and the First Responders who provided rescue and aid. We greatly appreciate the personal responses of those in the community who are sharing their experiences. From them, we hope to provide others with a better understanding of the debris flow hazard and response, so that we will be better prepared for future hazards. Critical comments and suggestions for improvement of this paper by Derek Booth, Mark Capelli, Tom Dunne, and an anonymous reviewer are appreciated. The UCSB Debris Flow Research Team consists of four professors in hydrology, seismology, and geomorphology; two professors in social science; three professional researchers; three graduate students; and four undergraduate student researchers. The work on the 2018 debris flow is ongoing and only mentioned briefly in this paper. The research is funded in part by The National Science Foundation Award Number 1830169 (USA), Wildfire Rain (USA), Stantec Consultants (USA), and Montecito residents (USA).

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