Do Debris Flows Pose a Hazard to Mountain-Front Property in Metropolitan Phoenix, Arizona?∗ Ronald I. Dorn Arizona State University Remnants of old debris flows occur on mountains that have been surrounded by growing suburbs of metropoli- tan Phoenix, Arizona, including on steep slopes above large single-family dwellings. Varnish microlamination and lead-profile dating techniques measured ages of 34 debris-flow levees in nine randomly selected catch- ments above home sites. Four catchments experienced debris flows in the last century, and four others had events in the last 350 years. Debris flows likely pose a modern hazard in Phoenix and perhaps other grow- ing cities in the desert Southwest. Key Words: debris flow, desert, geomorphology, hazards, urban sprawl. Los restos de antiguos flujos de detritos se presentan en las montanas˜ que estan´ siendo rodeadas por suburbios del area´ metropolitana de Phoenix, Arizona, incluso en las laderas inclinadas situadas mas´ arriba de residencias unifamiliares. Con tecnicas´ de datacion´ por barniz de microlaminacion´ y perfil de plomo se midio´ la edad de 34 elevamientos de flujo de detritos para nueve captaciones situadas arriba de las viviendas, seleccionadas aleatoriamente. Cuatro de esas captaciones experimentaron flujos de detritos durante el pasado siglo, y otras cuatro tuvieron eventos de tal naturaleza durante los pasados 350 anos.˜ Los flujos de detritos sin duda representan un riesgo moderno en Phoenix y quizas´ en otras ciudades en expansion´ en el Sudoeste desertico.´ Palabras clave: flujo de detritos, desierto, geomorfologıa,´ riesgos, expansion´ urbana descontrolada. ebris flows are fast-moving mass wasting Debris flows are a recognized hazard in D events where torrents of rock and mud a host of urban settings (McCall 1997), launch down steep mountain slopes, sometimes including beneath burned hillsides of South- disturbing only wild lands (Innes 1983; Iverson ern California (Cooke 1984; Keeley 2002), 1997; Bovis and Jakob 1999). Where the built within urban steeplands of humid tropical environment abuts steep slopes, however, de- settings (Gupta and Ahmad 1999), and where bris flows can impact homes and urban infras- structures are built under steep slopes in cold, Downloaded by [Arizona State University] at 19:52 24 April 2012 tructure. Debris flows are not floods. They are wet environments (Decaulne 2002; Glassey not giant rockslides. They are slurries that of- et al. 2002). Despite this general awareness, ten contain large amounts of rock material that debris flows are not recognized as a hazard start on steep slopes, continue down channels, for development in metropolitan Phoenix and and can eventually reach buildings and roads. other arid southwestern cities. More than fifty The Federal Emergency Management Agency different mountain masses are contiguous with (FEMA) treats debris flows as a distinct type of homes in metropolitan Phoenix (Figure 1), and natural hazard (FEMA 2009). housing developments now abut twenty-three ∗This article was supported, in part, by an Arizona State sabbatical. I thank anonymous reviewers for suggested improvements to the article, Google Earth for use of images for noncommercial purposes (http://www.google.com/permissions/geoguidelines.html), and the Maricopa County Tax Assessor’s Office for access to high-resolution aerial photography. C The Professional Geographer, 64(2) 2012, pages 197–210 " Copyright 2012 by Association of American Geographers. Initial submission, February 2010; revised submission, May 2010; final acceptance, July 2010. Published by Taylor & Francis Group, LLC. 198 Volume 64, Number 2, May 2012 Figure 1 Locations of randomly selected study sites, where numbers identify catchments that gen- erated debris flows now abutting housing. Study sites are superimposed on a Landsat 5 image of metropolitan Phoenix acquired 29 October 2009, courtesy of NASA Earth Observatory Image of the Day 42252. At the time, metropolitan Phoenix, Arizona, hosted a population of more than 4 million in an area of more than 37,000 km2.Urbangrowthhasbroughthousingintocontactwithfiftydifferentmountain masses, twenty-three of which show evidence of debris flows. mountainous areas with slopes containing ev- sider debris flows coming from steep moun- Downloaded by [Arizona State University] at 19:52 24 April 2012 idence of debris flows. Metropolitan Phoenix, tain slopes (City of Phoenix 2009). The Mari- Arizona, represents a world-class example copa County Flood Control District (Maricopa of urban sprawl (Helm 2003; Gober 2005), County 2010) exists to identify hazards related where expensive single-family dwellings occur to flooding, but debris flows are a type of mass beneath the steepest mountain slopes with wasting event. Although the U.S. Geological spectacular views (Figure 2). Building homes Survey sometimes mentions desert debris flows next to mountain slopes facilitates the addi- in reports about flooding in southwest urban tional amenity of being able to walk from a contexts (Norman and Wallace 2008), little at- backyard into mountain preserves designed tention has been paid to the potential for debris to protect wild lands and open space (Ewan, flows to do serious damage in expanding urban Ewan, and Burke 2004). centers in the desert Southwest. Hazard assessment carried out prior to home The past perspective, in the Arizona Geo- construction in Arizona in general, and Phoenix logical Survey home owner’s guide, was that zoning ordinance in particular, does not con- “[m]ost of the debris flows that have occurred Debris Flows and Mountain-Front Property 199 Figure 2 Examples of homes in wealthy mountainside neighborhoods built under debris-flow- generating catchments. The upper left image shows a debris-flow fan near Taliesen West, Scotts- dale. The upper right image displays a large home and a future home site under debris-flow chan- nels at Mummy Mountain, Paradise Valley. The lower left image shows homes placed under bedrock channels at Camelback Mountain; the lower middle image from Ludden Mountain illustrates home placement on former debris-flow levees, and the lower right image from Hedgpeth Hills reveal homes placed on a debris-flow fan. These images follow permission guidelines for Google Earth Downloaded by [Arizona State University] at 19:52 24 April 2012 (http://www.google.com/permissions/geoguidelines.html). (Color figure available online.) in Arizona in the past several decades have been Survey then undertook mapping and a haz- restricted to mountain valleys and canyons” ard assessment (Pearthree and Young 2006; (Harris and Pearthree 2002, 16). However, Magirl et al. 2007; Webb et al. 2008; You- many of these mountain canyons have been berg et al. 2008). This event led to a change overcome by housing developments in just of thinking toward the view that desert de- the past half-decade since publication of this bris flows could potentially be an underappre- guide. An extreme July 2006 precipitation event ciated hazard (Pearthree, Youberg, and Cook near Tucson, Arizona (Griffiths et al. 2009), 2007). resulted in debris flows that impacted roads Flooding can be a form of social injustice and some structures. The Arizona Geological in urban Arizona where the poor often live 200 Volume 64, Number 2, May 2012 in the most dangerous settings in floodplains final section analyzes results with an attempt (McPherson and Saarinen 1977; Committee to answer the basic question of whether or not on Public Works and Transportation, Sub- these debris flows pose enough of a hazard to committee on Water Resources 1979; Saarinen warrant future restrictions. et al. 1984; Graf 1988; House and Hirshboeck 1997; Honker 2002). In contrast, the wealthy Study Area citizens in metropolitan Phoenix, who build homes next to desert mountains, are the ones Urbanization in metropolitan Phoenix has en- who might be at risk from debris flows. The veloped what were once isolated mountains question asked here is whether desert debris (Figure 1). Some of these mountains have been flows pose a significant hazard for the urban made into open space preserves (Ewan, Ewan, wealthy living next to steep mountains in and Burke 2004), but many others are privately metropolitan Phoenix. held. In circumstances where private land abuts For debris flows to pose a hazard in mountains, homes have been built at the very metropolitan Phoenix, as well as other expand- base of mountain slopes steeper than 20◦ and in ing desert cities, they must occur frequently some cases steeper than 30◦ (Figure 2). Some enough to put structures and people at risk. To homes have been built directly on fans com- understand occurrence rates of debris flows in posed of debris-flow deposits, and other homes a single mountain range, debris flows originat- have been built where slopes have been under- ing from 127 catchments on the north flank cut through material removal (Figure 3). of the Ma Ha Tuak Range were sampled for Debris flows can start in a number of dif- dating by the varnish microlaminations (VML) ferent ways: originating from large, impulsive and lead-profile chronometric methods; results loads derived from adjacent slopes; from move- yielded an estimate of fifty-six flows in the last ment of water through bedrock fractures; from century (Dorn 2010), leading to the conclusion a fire hose effect of bedrock funneling water to- that debris flows could pose a modern haz- ward colluvium; from wildfires; from mobiliza- ard. The question posed here is whether the tion of material in channels; from slumping of results from a single mountain range reflect channel banks; and from other processes (Innes hazards found on the other ranges that have 1983; Bovis and Dagg 1987; Webb, Pringle, been surrounded by the sprawl of metropolitan and Rink 1989; Coe, Cannon, and Santi 2008). Phoenix. The vast majority of metropolitan Phoenix de- This article explores the hypothesis that bris flows appear to have initiated from satu- rates of debris-flow occurrence above home ration of colluvium located in the lower center sites scattered throughout metropolitan of spoon-shaped catchments (e.g., Figure 4). Phoenix are similar to rates observed for These catchments are often small, most being the intensively sampled north side of the less than 20,000 m2.