NATURAL HAZARDS AND DISASTERS

LANDSLIDES IN PUERTO RICO

Introduction

The Commonwealth of Puerto Rico, a possession of the United States, lies about 1700 kilometers southeast of the U.S. mainland at the eastern extremity of the Greater Antilles. Puerto Rico has an area of 9103 square kilometers, about 70 to 80 percent of which is hilly or mountainous. Approximately 60 percent of the population of 3.35 million lives in the four largest cities, San Juan, Ponce, Mayaguez, and Arecibo (Rand McNally, 1986), which are located primarily on flat or gently sloping coastal areas. Continuing growth of these urban centers, however, is pushing development onto surrounding steep slopes. The overall population density of Puerto Rico is 368 persons per square kilometer; the density outside the four major urban areas, including the mountainous interior, is probably between 100 and 200 persons per square kilometer. Average annual precipitation in Puerto Rico ranges from less than 1000 millimeters, along the southern coast, to more than 4000 millimeters, in the rain forest of the Sierra de Luquillo on the northeastern part of the island (Fig. 16). The mountainous Cordillera Central, which forms the spine of Puerto Rico, also receives considerable rainfall. Rain in Puerto Rico falls throughout the year, but about twice as much rain falls each month from May to October—the hurricane season—as falls from November to April (Monroe, 1980). Brief, intense storms are common; almost one-half of the National Weather Service's 40 weather stations record more than 13 millimeters of rainfall on 30 to 50 days each year (Calvesbert, 1970; Monroe, 1980).

Four hurricanes have passed over Puerto Rico in this century, and several others have passed close enough to produce extraordinary rainfall (Calvesbert, 1970). In October 1985, a tropical wave, which later developed in Tropical Storm Isabel, struck the south-central coast of Puerto Rico, and produced extreme rainfalls. Some areas received more than 560 millimeters of rain—more than one-half the annual average—in 24 hours. Rainfall intensities reached 70 millimeters/hour for a few hours (Jibson, 1986a; 1987a; in press).

The temperature in Puerto Rico is generally between 20 and 30 C, and the relative humidity is about 80 percent throughout the year (Monroe, 1980). The warm, moist air combined with the heavy rainfall promotes rapid, deep weathering of surficial materials, which leads to severe erosion and slope-stability problems throughout the island.

Physiography and geology of Puerto Rico

Puerto Rico can be divided into three distinct physiographic provinces (Fig. 17)—the Upland province, the Northern Karst province, and the Coastal Plains province (Monroe, 1980). The physiography and the geology of these provinces as described by Monroe (1976; 1980) are discussed below.

The Upland province, which has a maximum elevation of 1338 meters, includes the three major mountain ranges—the Cordillera Central, which forms an east-west axis through most of the southern two-thirds of the island; the Sierra de Luquillo in the northeast; and the Sierra de Cayey in the southeast. The mountainous areas are covered by dense, tropical vegetation and have been deeply incised and sculpted by streams and landslides; slopes as steep as 45 degrees are common. Also included in the Upland Province are interior lowlands surrounded by mountains and foothills to the mountains, primarily along the western, southern, and eastern flanks.

The Cordillera Central and Sierra de Luquillo consist of Lower Cretaceous to Middle Eocene volcanic and sedimentary rocks (Fig. 18, TKv). The Lower Cretaceous rocks are primarily submarine volcanic ash and interlayered lava flows; reef limestones are locally present near the top of the Lower Cretaceous section. The Upper Cretaceous section includes volcanic rocks intrerbedded with volcaniclastic sandstone and conglomerate as well as with reef limestones. Overlying the Cretaceous rocks are Paleocene to Middle Eocene tuff and sedimentary rocks. The Cretaceous and Lower Tertiary rocks have been extensively folded and faulted.

The Cretaceous and Tertiary volcanic and sedimentary rocks in the interior of the island are intruded by granodiorite and diorite plutons (Fig. 18, Tki). The largest of these plutonic intrusions forms the Sierra de Cayey (Fig. 17). In west-central Puerto Rico, Cretaceous serpentinite and chert are exposed (Fig. 18, Ks).

The foothills along the southern flank of the Upland province extend locally to the Caribbean Sea; they consist of Oligocene and Miocene sediments unconformably overlying the Cretaceous and lower Tertiary rocks of the mountainous interior. The Oligocene sediments (Fig. 18, To) include conglomerate, sand, and clay derived from weathering, erosion, and marine reworking of the older igneous rocks. The clastic sediments are capped by Oligocene limestone, which is overlain by several hundred meters of Miocene limestone (Fig. 18, Tm). Near-vertical limestone cliffs above steep colluvium and residuum-covered slopes are common.

The Northern Karst province (Fig. 17) includes most of north-central and northwestern Puerto Rico north of the Upland province; locally, this province extends to the Atlantic Ocean. The maximum elevation in the Northern Karst province is 530 meters along the escarpment formed by differential weathering between the igneous rocks of the Upland province and the limestone of the karst area. The Northern province displays a wide variety of karst features (Monroe, 1976). The terrain, locally very rugged, ranges from steep slopes and shear cliffs to gently rolling hills. Most drainage is underground, which results in widespread collapse features.

Rock in the Northern Karst province are Oligocene and Miocene in age. The Oligocene section (Fig. 18, To) consists of clays, sands, and gravels (all derived from the igneous rock of the mountainous areas) capped by limestone. The Miocene section (Fig. 18, Tm) consists primarily of as much as 1400 m of limestone. The weak, saturated clays of the Oligocene San Sebastian Formation underlie much of the limestone in the area, and large landslide blocks of limestone have moved downward and outward along these north-dipping clays (Monroe, 1964).

The Coastal Plains province (Fig. 17) is a discontinuous, gently sloping area covered by Quaternary alluvial, estuarine, and beach deposits, which have been reworked locally by wind and wave action (Fig. 18, Qs). Inselbergs of older rocks project through the coastal plain sediments in some places. The only consolidated rock among these sediments is eolianite forming from cementation of beach sediments. Puerto Rico's major cities are built primarily in the Coastal Plain province, although population growth has pushed development onto adjacent slopes of the Upland and Northern Karst provinces.

Available literature on landslides in Puerto Rico

Geologic maps at 1:20,000 scale exist for most of Puerto Rico, and most of these maps include at least some reference to landslides. Several of the maps include landslide deposits as surficial map units.

Monroe (1979) produced a map of landslide susceptibility of Puerto Rico at 1:240,000 scale. Four categories were used to depict susceptibility—highest susceptibility was assigned to areas of past or current landslide activity, high susceptibility to areas that have slopes greater than 27 degrees and to areas that contain "slide-prone" rock and soil, moderate susceptibility to all other sloping areas, and low susceptibility to flat-lying areas. Judgement of high and highest susceptibility was based on landslide incidence, so the map actually is a generalization of landslide incidence. Susceptibility to specific types of landslides is among the most common and damaging in Puerto Rico. The map is generalized and shows large areas that has similar susceptibility to landsliding; its scale precludes the detailed quantification of susceptibility needed by the planning and engineering communities.

Monroe (1964) described large, retrogressive block slides along the northern limestone escarpment that separates the mountainous interior of the island from the northern karst area. Other landslide features associated with the San Sabastian Formation in northwestern Puerto Rico are described by Briggs and others (1970). Sowers (1971) described slope stability problems encountered in construction projects in the weathered volcanic rocks in the rain forests of northeastern Puerto Rico. Deere and Patton (1971) discussed the relation of weathering profiles to landslide formation in Puerto Rico. Brief discussions of other types of landslide processes have been presented by Farquhar (1978), Lewis (1975), and Ortiz (1974).

A major tropical storm in October 1985 triggered thousands of debris flows as well as the disastrous rock block slide that destroyed the Mameyes district of Ponce. Landslide activity triggered by this storm was documented by Campbell and others (1985), Campbell and others (1986), and Jibson (1986a, 1986b, 1986c, 1987a, 1987b) also briefly summarized landslide hazards throughout Puerto Rico.

Landslides in Puerto Rico

All major types of landslides occur in Puerto Rico, and all physiographic provinces of the island have landslides. Most of the Upland province and the Northern Karst province (Fig. 17), by virtue of high relief, steep slopes, and abundant rainfall, have continuing landslide problems. The drier southwestern part of Puerto Rico (Fig. 16) normally experiences landslides only during exceptionally heavy rainfalls; few landslides form there during periods of normal precipitation. The coastal plain has only localized landslide problems, predominantly along incised stream channels or where steeper inselbergs project through the younger surficial sediments.

Debris slides and debris flows—rapid downslope sliding or flowing of disrupted surficial rock and soil—are the most prevalent types of landslides in Puerto Rico. These landslides are particularly hazardous because they form with little or no warning and can move very rapidly down steep slopes. Structures at the base of such slopes are inundated or destroyed by the impact of the rapidly moving mixture of soil, rock, and water.

A common, but less abundant, type of landslide is rock fall—rapid movement by free fall, bounding, or rolling of bedrock detached from steep slopes. Rock falls are common on very steep natural slopes and especially on the numerous steep road cuts on the island. These landslides can be very damaging if they impact structures or passing automobiles. Recent major storms have triggered many rock falls of different sizes that closed roads and temporarily isolated parts of the island.

Block slides and slumps—masses of bedrock and overlying soil that move downslope either as intact blocks or as a collection of slightly disrupted blocks—are less common than debris slides and debris flows, but their effects can be catastrophic. Such was the case during the October 1985 storm, when the Mameyes district of Ponce was destroyed by a block slide that killed at least 129 people (Jibson, 1986a, in press). Block slides and slumps can disrupt large areas of the ground surface and thus lead to destruction of overlying structures and burial of structures downslope.

Earth flows—normally slow-moving masses of moderately disrupted earth that can move down even very gentle slopes—also occur in Puerto Rico. This movement commonly causes sufficient deformation of the ground surface to damage or destroy overlying structures or roads.

The igneous rocks in the Upland province weather rapidly to form a deep, predominantly coarse-grained saprolitic soil mantle. When saturated, this saprolite produces debris slides, debris flows, and slumps ranging from a few to several hundred meters long. In May 1985, severe storms in west-central Puerto Rico triggered hundreds of such debris slides and debris flows, which choked streams, blocked roads, and destroyed homes and other structures. Hurricanes David and Frederic in 1979 produced extreme rainfalls in northeastern Puerto Rico that triggered several debris slides as song as 750 meters and as deep as 25 meters on slopes of deeply weathered intrusive igneous rock (Fig. 19). Rock falls from natural slopes and particularly from the ubiquitous steep road cuts also are common in the igneous and sedimentary rocks of the Upland province.

The sedimentary rocks flanking the igneous interior of the island produce rock falls from steep cliffs and road cuts, large rock and debris slumps, and rock block slides. The deeper landslides form where limestone or sandstone overlie weaker silts and clays that act as slip surfaces. The colluvium and residuum on steep slopes composed of mudstone and particularly limestone are susceptible to failure as debris slides and debris flows; such landslides generally fail at the interface between the weathered surface material, commonly 0.5 to 2 meters deep, and the unweathered bedrock.

During the storm of October 1985, numerous debris slides and debris flows (Fig. 20) destroyed several homes and buildings between Penuelas and Coamo along the southern coast of Puerto Rico (Jibson, 1986a). These landslides originated as failures of thin colluvial soil mats on steep slopes (Fig. 21); the failed material scoured deep channels as it moved downslope in preexisting gullies or depressions (Fig. 22). Most of the 1985 debris slides and debris flows formed on limestone slopes underlain by a wide range of geologic structures (dip slopes and reverse-dip slopes, fractured and unfractured rock, folded and homoclinal bedding, and so forth). The limestone might produce weak or permeable residuum and colluvium more susceptible to failure, and ground-water flow patterns in the limestone might favour landslide formation. Many mudstone slopes also produced debris slides and flows in 1985.

Rock falls from steep limestone slopes, in the southern flank of the Upland province and in the Northern Karst province, have repeatedly blocked major and minor roads. Most of the limestone in Puerto Rico is porous and fractured, so infiltration of rainfall can build up pore pressure within the rock and trigger rock falls. Limestone rock falls have triggered debris flows on steep colluvial slopes below near-vertical bedrock faces (Fig. 23). At one site, large limestone boulders fell from a bedrock face and impacted the head of the colluvial slope; this either disrupted the saturated colluvium enough to cause it to flow downslope, or rapidly increased the pore pressure in the colluvium and caused it to mobilize.

Large, retrogressive block slides are present in the Northern Karst province where thick limestone formations overlay clay in northwestern Puerto Rico (Monroe, 1964). These generally slow-moving landslides occur where deep river valleys expose the underlying clay beds on which sliding occurs. The block slides leave deep, vertical sided valleys between the head scarp and the upper edge of the landslide block.

Some clays in the sedimentary belt produce earth flows. The San Sebastian area, in northwestern Puerto Rico, is particularly susceptible to earth flows that form in the clayey members of the Oligocene San Sebastian Formation. These earth flows commonly form on gentle slopes and create a subdued, hummocky topography. Several houses and roads have been damaged severely or destroyed by these landslides (Fig. 24).

The young sediments forming the coastal plain surrounding Puerto Rico have very gentle slopes. Landsliding on the coastal plain is generally limited to small riverbank failures that choke river channels and increase erosion, which endangers homes built on the coastal plain. Steep-sided inselbergs of Tertiary and Cretaceous sedimentary and igneous rocks, which project through the young sediments, also produce landslides. If these sediments are subjected to seismic shaking, then lateral-spread landslides could form in parts of the coastal plain underlain by liquefiable sediments.

Social and economic impact of landslides in Puerto Rico

Statistics on fatalities and damage from landslides in Puerto Rico are not systematically recorded; therefore, estimating losses is difficult. The Mameyes landslide of October 1985 is definitely the worst landslide disaster in Puerto Rico's history (and in North American history); at least 129 people were killed, and the death toll could have been between 200 and 300. More than 100 homes were destroyed, and about as many were later condemned and removed because of continuing risk from landsliding (Fig. 25).

Confirmed reports of fatalities from other landslides are sparse. Anthony Santos (local resident, personal comm., 1985) stated that two motorists were killed by a limestone rock fall in the early 1980"s near Penuelas, west of Ponce. Monroe (1979) reported that motorists have been killed by rock falls along the Rio de la Plata between Comerio and Bayamon; he also reported several instances when numerous houses were damaged or destroyed by landslides. Isolated landslides, particularly debris flows and debris slides, reportedly have caused fatalities and destroyed property. The greatest cost to public works is road maintenance, but no estimates of total cost due to landslides are available. The frequency of serious storms in Puerto Rico suggests that a long-term average of a few (less than 5) fatalities per year could occur. On average, perhaps tens of houses are destroyed, and hundreds are damaged by landslides each year.

The storms of May and October 1985 increased public and government awareness of landslide hazards and the need to mitigate those hazards. The Puerto Rico Department of Natural Resources and the Puerto Rico Planning Board are cooperating with the U.S. Geological Survey on projects to develop landslide susceptibility maps for parts of the island. In the wake of the Mameyes landslide disaster, the Commonwealth government, using partial funding by the U.S. Federal Emergency Management Agency, relocated hundreds of people to a new community built on stable ground. These relocated included those whose homes had been destroyed by the landslide of October 7 and those in adjacent areas where the level of landslide hazard was judged to be unacceptably high. The ongoing concern about landslide hazards within local and Commonwealth governments could lead to implementation of policies and programs to significantly reduce landslide risk.

Briggs, R.P., P.A. Gelbaert, D. Jordan, E. Aquilar, R.M. Alonso and R.M. Valentine, 1970. Engineering geology in Puerto Rico. Assoc. of Eng. Geol. Ann. Field Meeting Fieldtrip Guidebook 6:23.

Calversbert, R.J., 1970. Climate of Puerto Rico and U.S. Virgin Islands. Climatology of the United States, P. 50-52. Washington, D.C., U.S. Environmental Science Services Administration.

Campbell, R.H., D.G. Herd, and R.M. Alonso, 1985. Preliminary response activities and recommendations of the USGS landslide hazard research team to the Puerto Rico landslide disaster of October 7, 1985. U.S. Geological Survey Open-File Report 85-719.

Campbell, R.H., D.G. Herd, and R.W. Jibson, 1986. Preliminary review of the landslide disaster of October 7, 1985, near Ponce, Puerto Rico. Geol. Soc. Amer. Abs. With Program 18: 93.

Deere, D.U. and F.D. Patton, 1971. Slope stability in residual soils. In 4th Pan. Amer. Conf. Soil Mech. and Found. Eng. Proc. 1:87-170.

Farquhar, O.C., 1978. Landslides in Puerto Rico. Internat. Assoc. Eng. Geol. Bull. 16:44.

Jibson, R.W., 1986a. Evaluation of landslide hazards resulting from the 5-8 October 1985 storm in Puerto Rico. U.S. Geological Survey Open-File Report 86-26.

Jibson, R.W., 1986b. Landslides resulting from the October 5-8, 1985 storm in Puerto Rico. Assoc. Eng. Geol. Abs. and Program, 29:52.

Jibson, R.W., 1986c. The Puerto Rico landslide disaster of October 1985. Assoc. Eng. Geol. Newsletter, 29:17-20.

Jibson, R.W., 1987a. Debris flows triggered by a tropical storm in Puerto Rico. In A.P. Schultz and C.S. Southworth (eds.), Collected papers on Eastern North American Landslides, p. 9-10. U.S. Geological Survey Circular 1008.

Jibson, R.W., 1987b. Landslide hazards of Puerto Rico. In W.W. Hays and P. L. Gori (eds.), Proc. Assocessment of Geologic Hazards and Risk in Puerto Rico, p. 183-188, U.S. Geological Survey Open-File Report 87-008.

Jibson, R.W., (in press). The Mameyes, Puerto Rico landslide disaster of October 7, 1985. Engineering Geology Case Histories. Boulder, Geological Society of America.

Lewis, L.A., 1975. Slow slope movement in the dry torpics-La Paguera. Puerto Rico. Zeit. Fur Geomorphologie 19:334-339.

Monroe, W.H., 1964. Large retrogressive landslides in north-central Puerto Rico. U.S. Geological Survey Professional Paper 501-B.

Monroe, W.H., 1976. The karst landforms of Puerto Rico. U.S. Geological Survey Professional Paper 899.

Monroe, W.H., 1979. Map showing landslides and areas of susceptibility to landsliding in Puerto Rico. U.S. Geological Survey Miscellaneous Investigations Map I-1148.

Monroe, W.H., 1980. Some tropical landforms of Puerto Rico. U.S. Geological Survey Professional Paper 1159.

Ortiz, C.A., 1974. Relationship between engineering properties and topographical expression of the rocks in Rio de la Plata's valley walls, Comerio, Puerto Rico. Intern. Assoc. Eng. Geol., 2nd Intern. Conf. Proc. 6:1-8.

Rand McNally, 1986. The new international atlas. New York: Rand McNally.

Sowers, G.G., 1971. Landslides in weathered volcanic soil in Puerto Rico. Proc. 4th Pan. Amer. Conf. Soil Mech. and Found. Eng., 1:105-115.

(Taken from: DeGraff, J.V., Bryce, R., Jibson, R.W., Mora, S., and Rogers, C.T. 1989. Landslides: Their extent and significance in the Caribbean. In E.E. Brabb and B.L. Harrod (eds), Landslides: Extent and Economic Significance. p. 51-80. Rotterdam: A.A. Balkema. )

" Transcribed by Nicholas DeGraff, University of California"