Debris Flow and Erosion 

Control Problems Caused 

by the Ash Eruptions of 

[razii Volcano, Costa Rica

3y HOWARD H. WALDRON

ONTRIBUTIONS TO GENERAL GEOLOGY

EOLOGICAL SURVEY BULLETIN 1241-1

\epared in cooperation with the Oficina 

de Defensa Civil and Oficina de Planifiacion de Costa Rica under the auspices of 

he Agency for International Development, U.S. Department of State

INITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 55 cents (paper cover)
CONTENTS

Abstract..-.____________________________________________________ II

Introduction. _____________________________________________________ 2

Study area______________________________________________________ 3

Eruption and ashfall.__________________________________________ 3

Meteorology and hydrology___________________________________ 5

Rfo Reventado basin.. ______________:.____________-____--______ 6

Problems caused by eruption....____________________________________ 8

Ash accumulation._________'___________________________________ 8

Accelerated erosion.___________________________________________ 10

Landslides.___________________________________________________ 12

Debris flows.-_--__--_________---------____-_____---_____-__-_ 16

Generalfeatures_____-______---__--________-_-__.__________ 16

Physical characteristics.___---__-_____-______-_-__________ 20

Movement__ ___________________________________________ 26

Control measures._________________________________________________ 29

Summary_______________________________________________________ 34

References-.._ __________________________________________________ 35

ILLUSTRATIONS

Page 

PLATE 1. Map of the southwest flank of Irazii Volcano. _______ In pocket

FIGURE 1. Photograph showing eruption of Irazu Volcano__--_.___ 12

2. Index map of Costa Rica, showing location of Irazti

Volcano.---_____------------________-_--___.____ 4

3. Profile of the Rio Reventado______________________ 7

4-10. Photographs showing:

4. Destruction of the vegetation by ashf all. _______ 9

5. Sheet and rill erosion of ash from upper slopes

of Irazii Volcano._-_-_--__--_.-_--_-_-_---- 11

6. Accelerated channel erosion in the upper Rio

Reventado Valley..----.___-______-__-___-_ 12

7. Parts of the Prusia landslide--__-------_-__-___ 14

8. Part of the Llano Grande landslide___________ 15

9. Highway bridge destroyed by the Llano Grande

landslide..-!------------------------------ 16

10. Effects of the debris flows near Cartago.-_-_-___ 18

11. Hydrographs of some of the larger Rio Reventado debris

flowsin 1964-_--__-_._____._._..._..._.____.. 19

12. Graph showing the size distribution of particles from

samples of some of the Rfo Reventado debris flows_ _ 21

13. Photograph of massive andesite boulder transported by a

Rfo Reventado debris flow._-_-_______---_-__----- 23

ni
IV CONTENTS

Pago 

FIGURE 14-16. Graphs showing relation of:

14. Velocity and sediment concentration for some of

the Rio Reventado debris flows_________ 124

15. Velocity of some of the Rio Reventado debris

flows to river stage.-______________________ 25

16. Sediment concentration of some of the Rio

Reventado debris flows to the river stage____ 27

TABLES

Page 

TABLE 1. Physical characteristics of the Rio Reventado watershed. ._._ 17

2. Summary of some of the physical data on the Rio Reventado

debris flows._________________________________________ 22

3. Comparison of sorting indices of the Rio Reventado debris 

flows in Costa Rica with those of similar deposits in the 

United States______-_-____________.--___.--_-__.___ 28
CONTRIBUTIONS TO GENERAL GEOLOGY

DEBRIS FLOW AND EROSION CONTROL PROBLEMS 

CAUSED BY THE ASH ERUPTIONS OF IRAZfi VOLCANO,

COSTA RICA

By HOWARD H. WALDRON

ABSTRACT

Irazu Volcano, in central Oosta Rica, erupted ash almost continuously from 

March 1963 through February 1965. The ash created and caused widespread 

damage to property, crops, and livestock. Much of the damage to property was 

caused by accelerated erosion and repeated floods of water, mud, and rock debris 

from the slopes of the volcano. Debris flows and erosion were especially severe in 

the Rio Reventado watershed and valley on the southwest slope of Irazu. In 

December 1963 a large debris flow in the Rio Reventado valley destroyed more 

than 300 homes and killed more than 20 persons' at Cartago. During 1964, more 

than 90 debris flows occurred in valleys on the isouth and west slopes of the 

volcano; about 40 of these flows were in the Rio Reventado valley.

All the debris flow and erosion problems resulted from profound changes in the 

hydrologic regimen of the streams brought about by the accumulation of ash on 

the upper flanks of the volcano. The mantle of ash not only gradually 'buried and 

destroyed much of the natural vegetation, but the surface of the ash mantle 

rapidly and continuously formed a thin, compact, impermeable crust that very 

drastically reduced infiltration of rainfall into the soil. Runoff soon became so 

greatly accelerated that even moderate precipitation caused flash floods. Each 

flash flood, then, produced torrents of mud and rock debris, most of which was 

derived from the slumping and caving of valley walls. The scouring and undercutting of valley walls by the debris flows created many unstable slopes, reactivated 'several old large landslides, and started many new ones. The debris flows 

had high densities as much as 1.98 grams per cubic centimeter and reached 

velocities as great as 10 meters per second or more. As the flows left the mountain 

front, their velocity suddenly diminished, and much of the coarser sediment of 

the flows came to rest on a broad alluvial fan near the city of Cartago.

Emergency measures developed in 1964 for the control of erosion and debris 

flows included (1) downstream flood control and protection by means of channel 

improvements and the construction of large levees and (2) headwater flood and 

erosion control "by means of watershed rehabilitation of the devastated areas 

through drainage diversions, contour ditching, small check dams, and artificial 

revegetation. The effects of these measures are being evaluated concurrently with 

the development of long-range plans for flood and erosion control.

II
12 CONTRIBUTIONS TO GENERAL GEOLOGY

INTRODUCTION

The prolonged eruption of ash from Irazu Volcano (fig. 1) in central Costa Rica from March 1963 to March 1965 caused severe and 

widespread damage to crops, livestock, and other property in the vicinity of the volcano. Much of the damage to property was caused by 

accelerated erosion from the upper slopes of the volcano and by repeated floods of mud and rock debris in the valleys that drain the 

upper slopes; one of these floods destroyed more than 300 homes and 

killed more than 20 persons. The purpose of this report is to describe 

the problems of debris flows and erosion their causes and effects 

and to discuss some of the emergency measures applied to their control.

This report is based on work done by the author in Costa Rica from 

May through October 1964, under the auspices of the U.S. Agency for 

International Development (AID) in cooperation with the Oficina 

de Defensa Civil (Office of Civil Defense) and Oficina de Planificacion (Planning Office) of the Government of Costa Rica. The author 

is indebted to R. L. Telles, U.S. Ambassador to Costa Rica, and to 

W. R. Stone, then mission director of AID, for their support and 

cooperation. Grateful acknowledgment is made for the data and reports made available to the author by the Servicio Meteorologico 

Nacional (SMN) (National Meteorological Service), the Institute 

Geografico de Costa Rica (Geographic Institute of Costa Rica), the

FIGURE 1. Eruption of Irazu Volcano, showing the plume of ash being carried

westward.
13

Ministerio de Agricultura y Ganaderia (MAG) (Ministry of Agriculture and Animal Husbandry), and the Institute Costarricense de 

Electricidad (ICE) (Costa Rican Institute of Electricity), and to 

members of their staffs, especially Ing. Manual Corrales, head of 

Oficina de Control de Rios (River Control Section) of ICE. Acknowledgment is also due Ing. Jorge Dengo, then director, Oficina de Planificacion, Ing. Rodolfo Dobles, director, Comite Ejecutivo de Irazu 

(Executive Committee of Irazu), and Gregorio Escalante, then geologist, Oficina de Defensa Civil, for their friendly cooperation and 

assistance.

STUDY AREA

Irazu Volcano is the highest (3,432 meters) of several composite 

volcanic cones which make up the Cordillera Central in central Costa 

Rica and which are located near the population and economic centers 

of this small Republic (fig 2). The summit of Irazu lies only about 

24 km (kilometers) east of San Jose, the largest city and capital of 

the Republic, and only about 15 km north-northeast of Cartago, the 

second largest city of the Republic and the capital of the Province of 

Cartago.

Records indicate that although Irazu Volcano has erupted frequently, it has erupted only ash during historic time (McBirney, 1958, 

p. 141-144; Dondoli, 1965). The upper slopes of the volcano, therefore, 

are mantled by thick deposits of older, weathered ash which overlies 

a great pile of interbedded lava, volcanic breccia, agglomerate, ash, 

and lahar deposits. Several craters now occupy the summit area of the 

volcano (pi. 1). The geomorphic appearance of the craters suggests 

that the main conduit has shifted progressively westward along a 

major rift zone and that the youngest crater is now on the west.

ERUPTION AND ASHFALL

The most recent eruption of Irazu started with an explosion on 

March 13, 1963, after which the volcano erupted large quantities of 

ash almost continuously for about 20 months and intermittently for 

an additional 3 to 4 months. No new eruptions of ash have been reported since March 19, 1965. The eruption was confined to three distinct vents in the youngest, or west summit crater. Most of the erupted 

material is a fine to very fine, angular, lithic ash of basaltic andesite 

composition (Murata and others, 1966a, p. 114). At the summit about 

80 percent of the ash particles was reported to be in the range of 160 

to 340 microns, but at San Jose almost 90 percent was reported to be 

in the range of 100 to 150 microns (from data supplied by the Oficina 

Defensa Civil, San Jose, Costa Rica). Only a small fraction of the
14 CONTRIBUTIONS TO GENE.RAL GEOLOGY 

84°

FIGURE 2. Index map of Costa Rica, showing location of Irazu Volcano, major 

distribution of ashfall, and area of heaviest ashfall.

ash was fresh or juvenile material, but many slaggy incandescent 

bombs were blown out of one or more of the vents at various times. 

The newly erupted ash, with an average pH of slightly more than 4.0, 

was highly corrosive to both plants and metallic objects.

The prevailing southeast to east winds deposited the ash along a 

narrow zone that surrounded the summit crater and extended downwind for several tens of kilometers to the northwest and west (fig. 2).
EROSION, ASH ERUPTIONS, IRAZU VOLCANO, COSTA RICA 15

The Oficina de Planificacion estimated that an area of about 100 km2 

(square kilometers) was devastated, another 300 km2 was seriously 

affected, and another 2,000 to 3,000 km2 was moderately affected by the 

ashfall. The thickness of the ash on the south and southwest sides of 

the volcano ranges from a trace on the lower slopes to as much as 

2m(meters) at the summit (Murata and others, 1966b, p. 787) and 

averages- about 15 cm (centimeters) in the upper basin of the Kio 

Keventado. Greater thicknesses of ash were reported from the northwest flanks of the volcano but these were not confirmed. Ashfall measured by the Servicio Meteorologico Nacional in San Jose amounted 

to an accumulated total of more than 26 kg per m2 (kilograms per 

square meter) during the period March 1963 through March 1965. 

Maximum accumulations of 4825 g per m2 (grams per square meter) 

occurred during December 1963 and 3825 g per m2 during January 

1964; minimum accumulations of 39 g per m2 occurred during September 1963. The average bulk density of the ash was about 1.276 g 

per cm3 (grams per cubic centimeter); hence a total thickness of about 

2.1 cm of ash accumulated in the city during the period of eruption.

METEOROLOGY AND HYDROLOGY

Very little was known about the meteorology of the upper slopes of 

the volcano until several months after eruption began. Similarly, 

virtually nothing was known about the hydrology of the streams draining the slopes of the volcano. Of the four rain-gaging stations in the 

area, the oldest and highest is at Sanatorio Duran (pi. 1), about 6.5 

km southwest of the summit at an elevation of 2,337 m; it has been in 

operation since 1943. The other three stations are much lower on the 

flanks of the volcano; they have been in operation about 6 to 14 years. 

During a brief period in August 1963, Coen (1964) investigated the 

influence of meteorological factors on the eruption of the volcano. In 

April 1964, several flash-flood warning stations (observation posts) 

were established by the Costa Kican Government at various locations 

on the volcano (pi. 1). These stations were manned continuously in 

order to report on rainfall, river stage, and flood potential of the 

major watersheds draining the southwest flank of the volcano. Consequently, almost no basic data about the hydrologic or meteorological 

conditions that existed on the volcano were available for either planning or forecasting purposes until more than a year after the eruption 

began. During the 1964 rainy season 8 stream gages and 16 rain gages 

were installed in the area (pi. 1), chiefly under the direction of R. D. 

Tarble, hydrologist, U.S. Weather Bureau and United Nations. A 

radar unit also was put into operation in San Jose and proved to be 

extremely valuable for tracking the path and development of storms
I 6 CONTRIBUTIONS TO GENE.RAL GEOLOGY

over the volcano and for forecasting the probable extent and intensities of rainstorms in the various watersheds.

Precipitation in the area is extremely variable in both time and 

place. It ranges from less than 950 mm (millimeters) to more than 

2,000 mm annually, as indicated by the more than 22 years of record 

at Sanatorio Duran. About 90 percent of this precipitation falls during the months of May through November. Although Irazu Volcano 

is situated on the Atlantic slope, it is also very near the Continental 

Divide and is subject to the movement of storms from both the Pacific 

and Atlantic Oceans. From May through September the flow of humid 

air from the Pacific Ocean predominates, whereas from October into 

December the principal storms originate from the Atlantic side. Occasionally storms may originate nearly simultaneously from both 

oceanic areas, and when these storms merge in the vicinity of the volcano, violent and prolonged local storms may result. Daylight morning hours during the rainy season commonly are clear or accompanied 

by a high overcast. By noon, however, storm clouds usually have built 

up, and thundershowers and heavy rainstorms may occur at any time 

throughout the afternoon, evening, or the following morning. Fortunately most of the high-intensity storms are brief and confined to 

relatively small localized areas; the most disastrous floods occur when 

a high-density storm is either more widespread or prolonged.

BIO BEVENTADO BASIN

Because the problems related to debris flows and erosion were especially severe in the Rio Reventado drainage basin on the southwest 

slope of the volcano, most of this report is concerned with the problems of this stream and its watershed (pi. 1). For convenience of 

description the watershed is divided into three subbasins (pi. 1) : 

The upper basin includes all the watershed area upstream from the 

confluence of the Rio Reventado with the Quebrada Pavas; the 

middle basin extends from the mouth of the Quebrada Pavas down to 

the mountain front; and the lower basin extends from the mountain 

front down to the confluence of the Rio Reventado and the Rio Agua 

Caliente. Actually, the lower basin is very extensive, but for the purpose of this report it is arbitrarily restricted to a width of about 500 

m. The Rio Reventado, which originates high on the southwest flank 

of the volcano, is joined in its upper basin by two tributaries on the 

west, the Rio Retes and Quebrada Pavas. Normally the Rio Reventado 

is a small stream that rarely exceeds 1 to 2 m in width or about 15 cm 

in depth. On the slopes of the volcano the stream generally is confined within a deep, narrow, V-shaped canyon. More than 45 percent 

of the upper basin has slopes steeper than 25 percent, so that steep
EROSION, ASH ERUPTIONS, IRAZU VOLCANO, COSTA RICA I 7

gradients prevail (table 1 and fig. 3) in the upper basin, and falls and 

rapids are numerous throughout both the upper and middle basins. 

At the foot of the volcano, at an elevation of about 1,450 m, the stream 

emerges from the mountain front and flows out over the nearly flat 

floor of an intermontane basin that borders the volcano on the south. 

Here the stream has built a large alluvial fan whose gradient is only 

about 4 to 5 percent. Cartago and several smaller suburban communities are situated on this fan. Physiographic data on the Rio Reventado 

and its watershed are shown in figure 3, plate 1, and table 1.

TABLE 1. Physical characteristics of the Rio Reventado watershed

[Data from Istaru topographic map, scale 1:50,000, sheet 3445 IV, Institute Oeograflco de Costa Rica, San

Jos6,1961]

Subbasin

Upper. _________ _-__-

Middle-----_--_------

TotaL---------

Drainage

Quebrada Pavas -._..__

Rfo Retes. -____.._____-

Rfo Reventado __ _--_.-

_---_do____ __..__.. _____

___-.do--_ __:___-___._.-

Area of 

basin 

(km«)

3.6

4.8

6.0

7.1

3.2

24.7

Length of 

stream 

(km)

4.4

3.8

6.6

7.0

6.4

20.0

Average 

stream 

gradient 

(percent)

18.0

17.1

17.4

8.5

4.5

9.8

ALTITUDE,, .c E

(METERS) £ 8 

ro o 12 

3600-,

3200-

2800-

2400-

2000-

1600-

1200

23 20 15 10 5 

DISTANCE FROM MOUTH, IN KILOMETERS

FIGURE 3. Profile of the Rio Reventado, showing limits of the upper, middle, and 

lower watershed subbasins. From Istaru topographic map, scale 1: 50,000, 

sheet 3445 IV, Institute Geografico de Costa Rica, San Jose", 1961.
I 8 CONTRIBUTIONS TO GENE.RAL GEOLOGY

PROBLEMS CAUSED BY ERUPTION

ASH ACCUMULATION

All the debris flow and erosion problems that have plagued the 

country since the eruption can be attributed to the accumulation of 

ash on the upper slopes of the volcano and to its effect on the hydrologic regimen of the streams draining these slopes. Prior to the eruption these slopes were covered with a luxuriant growth of vegetation, 

chiefly shrubs, but interspersed with open pasture areas that were 

mantled with thick grasses and a scattering of oak trees. Two very 

significant changes in the ground surface resulted from the ashfall. 

First, the mantle of ash rapidly destroyed much of the vegetation 

and buried the normal protective surficial mat of vegetal litter, so 

that a large area surrounding the summit was almost completely denuded of vegetation (pi. 1, fig. 4). Second, the 'ash continuously 

formed a thin, hard, and comparatively smooth crust that was 

virtually impervious.

The origin of this crust, which was only 10 to 15 mm thick yet strong 

enough to support a man's weight, is not fully understood. Segerstrom 

(1950, p. 56) described a similar crust that formed on the surface of 

the ash at Paricutin Volcano in Mexico. He considered it to be largely 

the result of compaction and sorting of the ash by raindrop impact 

and erosion. At Irazu, however, the continued formation of this crust 

only at the surface, despite the almost continuous deposition of new 

ash, suggests that the crust is primarily a chemical phenomenon rather 

than the result of raindrop impact and erosion. Because of the large 

quantities of halogens, especially fluorine, that are discharged in the 

gases of erupting volcanoes, D. E. White (oral commun., 1965) suggested that one of the soluble salts in the ash is possibly a halide. The 

halide is brought by capillary action to the surface, where it is precipitated by evaporation in the upper few millimeters of the ash to 

form the crust. When this crust is buried by newly deposited ash, the 

precipitated salts are redissolved and again carried upward by capillary moisture to the surface to be redeposited and to form a new crust. 

By this method, then, the formation of the crust is an almost continuous process that accompanies the deposition of the ash. The formation of the crust by such a process, however, cannot be a longlived phenomenon because it will be dependent on the replenishment 

of soluble salts through additions of halogens from successive ashfalls. Without a renewed source of halogens, the volatility and solubility of these salts are such that the crust probably could not be 

maintained for any great length of time.

As a result of the ash deposition and the destruction and burial 

of vegetation in the upper watersheds of the Rio Reventado streams,
EROSION, ASH ERUPTIONS, IRAZU VOLCANO, COSTA RICA I 9

FIGURE 4. Destruction of vegetation by ashf all. Upper: Along summit road near 

eastern summit crater. Lower: In Laguna area ; in center foreground, beginnings of deep gully erosion by the Rio Reventado.

some drastic changes also occurred in the relative rates of infiltration 

and runoff of precipitation. The capacity of the soil to absorb water 

was greatly reduced and runoff of rainwater was correspondingly 

increased. R. D. Tarble (oral commun., 1964) and the author estimated that runoff in the upper basins of the Rio Reventado exceeded 

80 percent of precipitation during any sustained low-intensity rain264-855 O 67 2
I 1.0 CONTRIBUTIONS TO GENERAL GEOLOGY

fall and that it may have been as much as 95 to 100 percent during 

periods of high-intensity rainfall. As a consequence, flash floods became frequent during the rainy seasons, and erosion was so accelerated 

that each flash flood became a debris flow.

ACCELERATED EROSION

Accelerated erosion on the slopes of the volcano was directly related 

to the increase in rate of both overland and stream runoff accompanying precipitation. Overland runoff caused sheet, rill, and gully erosion, 

and stream runoff caused channel erosion. Gully erosion, as used here, 

refers to erosion of new channels; channel erosion refers to erosion of 

preexisting channels. The accelerated channel erosion created many 

unstable slopes and resulted in an increase in landsliding.

The eroding capacity of overland flow is a function of several variables, namely: The rate of precipitation, infiltration capacity of the 

soil, length and steepness of the slope, and the natural resistance of 

the soil to erosion. The resistance of a soil to erosion is primarily a 

function of the degree of vegetal cover, but it is also related, in part, 

to the physical structure of the soil in the surface layers. According 

to Horton (1945, p. 318), the resistance of soils to erosion generally will 

tend to increase with the fineness of the soil particles or soil texture 

and with the degree of cementation or compaction of the soil.

In many ways the effects of the ashfall can be compared to the effects 

on erosion that result from the destruction of vegetation by fire in areas 

in the Western United States. Soil erosion studies in these areas by 

the U.S. Forest Service (Anderson, 1962; Copeland, 1965; Krammes, 

1960,1965; Sartz, 1953) indicate that overland flow and soil erosion can 

increase very rapidly when the vegetative cover is reduced below 60 to 

70 percent. Studies in Utah indicate that runoff may increase as much 

as 36 times and soil erosion as much as 110 times when the vegetative 

cover is reduced to 10 percent from 60 to 70 percent (Copeland, 1965, 

p. 76-79; Noble, 1965, p. 116-118).

The erodibility of the newly deposited ash tended to be variable 

despite its apparent uniformity. The resistance of the ash was initially 

very high because of the hard crust that formed on its surface. However, beneath the crust the ash remained soft and loose. Consequently, 

once the crust was broken or destroyed, sheet and rill erosion soon began to remove the underlying loose ash. Because erosion of this loose 

ash in turn also undermined the crust it collapsed. Destruction of the 

crust thereby was accelerated and the erodibility of the ash-covered 

slopes was correspondingly increased.

Erosion on a slope will not occur until the available energy produced by the overland flow exceeds the resistance of the soil to erosion
EROSION, ASH ERUPTIONS, IRAZTJ VOLCANO, COSTA RICA 111

(Horton, 1945, p. 320). Hence, the eroding force will be increased progressively downslope from a divide or watershed, and a zone of little 

or no erosion will be preserved near the crest of the divide. (See fig. 

5.) Observations by Renner (1936, p. 12-13) and computations by 

Horton (1945, p. 321) indicate that the amount of erosion tends to 

increase to a maximum on a slope of about TO to 84 percent (35° to 

40°) and thereafter to decrease to zero on a slope that approaches 

vertical.

During the 1964 rainy season most of the erosion of the fresh ash 

from the upper slopes of the volcano resulted from the headward and 

lateral growth of rills and gullies. Neither mass movement nor minor 

landslide failure of the new ash was observed. The stability of the 

new ash probably can be attributed to the high degree of internal 

friction and shearing resistance that is inherent in a mass composed 

almost entirely of fresh, angular, rock particles. From measurements 

made of ash removal and from a study of photographs taken periodically of the erosion features, it was estimated that one-third to onehalf of the newly fallen ash had been removed from the upper slopes 

of the volcano by the end of the 1964 rainy season. Some of this 

material was redeposited on valley floors and other areas of low 

gradient, such as roadways and ditches; the remainder was carried on 

downstream.

Channel erosion by streamnow (fig. 6) was many times more severe

FIGURE 5. Sheet and rill erosion of ash from upper slopes of Irazii Volcano.
112 CONTRIBUTIONS TO GENE.RAL GEOLOGY

than the sheet, rill, and gully erosion that occurred on the ash-covered 

slopes. The eroding capacity of streamflow depends largely on velocity 

and discharge of the stream. These factors are determined chiefly by 

the depth, width, and gradient of the channel, character and quantity 

of sediment load, and the material forming the channel. Much of the 

Rio Reventado channel consists of poorly consolidated, altered and 

weathered, clayey 1'ahar and ash deposits. Shear strength of these 

deposits is probably very low, particularly when saturated, owing to 

their high clay content; much of the original volcanic glass in the 

ash and lahar has been altered to a clay mineral. Thus, a combination 

of easily eroded channel material of low shear strength, high discharge, and high-velocity flow resulted in a very high eroding capacity 

for the Rio Reventado and other streams draining the volcano.

LANDSLIDES

The growth of old landslides and the formation of new landslides 

along the valley walls of streams that drain the upper slopes of the 

volcano were both extensive and very rapid. Prior to the eruption, 

the walls of most of these valleys commonly were oversteepened and 

barely stable even with a cover of vegetation. The importance of 

vegetation on slope stability was pointed out several years ago by 

Bailey (1941, p. 241), who found that densely vegetated slopes in

FIGURE 6. Accelerated channel erosion in the upper Rio Eeventado valley. Much 

of the upper basin of the Rio Reventado is underlain by soft, clayey lahar 

and ash deposits.
EROSION, ASH ERUPTIONS, IRAZU VOLCANO, COSTA RICA I 13

Idaho and Utah were stable at gradients up to 173 percent (60°), but 

the barren talus-covered slopes were stable only at gradients of 68 to 

80 percent (34° to 38°). The numerous flash floods that swept down 

the valleys during the 1963 rainy season, however, soon stripped the 

vegetation from the lower parts of valley walls, repeatedly scoured 

and progressively deepened stream channels, and severely undercut 

the oversteepened valley walls. Thus, the stage was set for the reactivation of numerous old landslides and the formation of many new ones.

The development and rapid growth of both old and new landslides 

was especially prevalent in the upper basin of the Rio Reventado. 

The upper basin is underlain chiefly by poorly consolidated, clayey 

lahar and ash deposits of low resistance to shearing stresses. Fewer 

landslides formed in the middle basin, where the slopes are not quite 

as steep as in the upper basin and where more of the valley walls are 

underlain by lava flows and well-cemented agglomerates and volcanic 

breccias. Although several small landslides had formed in the Rio 

Reventado valley by the end of the 1963 rainy season, only one large 

slide, the Prusia landslide, was evident at the beginning of tlie 1964 

rainy season (pi. 1, fig. Y). By the end of the season, however, undercutting of slopes by channel erosion was so severe that the Prusia 

slide had nearly doubled in area and volume, several other large slides 

had formed, and smaller slides not directly underlain by more resistant 

rock virtually lined all the valley walls in the upper basin. All the 

larger slides and many of the smaller ones appear to be old landslides 

that have been reactivated. Figure 8 shows a part of the large, reactivated Llano Grande landslide. The numerous old landslides along 

the Rio Reventado channel indicate that accelerated stream erosion 

and mass movements also must have accompanied some of the previous 

ash eruptions of the volcano.

Downslope movement along these unstable valley walls was almost 

continous during the rainy season, so that a nearly constant supply 

of earth and rock debris was contributed to the valley floors. The 

three largest slides Llano Grande, Prusia, and Retes (pi. 1) moved 

downslope at rates of 2 to 4 meters per week during the peak of the 

1964 rainy season. They continued'to move during the dry season but 

at a greatly reduced rate; movement on the smaller slides, however, 

virtually stopped during the dry season (Gregorio Escalante, written 

commun., 1965). R. D. Krushensky (written commun., 1965) estimated that by March 1965 these three large slides involved the mass 

movement of a total of more than 60 million cubic meters of earth and 

rock.

The numerous landslides not only contributed large quantities of 

material to the debris flows but destroyed roads and bridges (fig. 9)

264-855 O ,67 3
114 CONTRIBUTIONS TO GENERAL GEOLOGY

PIGUKE 7. Parts of the Prusia landslide. The average rate1 of movement of this 

landslide, which in October 1964 covered about 22 hectares, ranged from 12 

meters per month during the rainy season to less than 1 meter per month 

during the dry season (Gregorio Escalante, oral commun., April 1965). The 

Rio Reventado channel is in the foreground just out of both views. Upper: 

Southern part. Lower: Northern part.
EROSION, ASH ERUPTIONS, IRAZU VOLCANO, COSTA RICA I 15

FIGURE 8. Part of the Llano Grande landslide. Limits of the old landslide are 

evident in the steep scarp along the skyline (solid line). The head of the newly 

reactivated landslide is along this same scarp (dashed line). The average rate 

of movement of this large landslide, which in October 1064 covered about 41 

hectares, ranged from 6 meters per month during the rainy season to less than 

1 meter per month during the dry season (Gregorio Escalante, oral commun., 

April 1965).

and much valuable agricultural land. Many individuals also considered the larger landslides to be potentially very hazardous to Cartage 

and environs. If a more rapid or sudden movement of one or more of 

these slides occurred, it was feared that the slide debris would fill the 

valley and dam the stream. If such a dam is overtopped or breached 

in any way, the rapid release of ponded waters through the breach can 

produce a catastrophic flood of mud and debris downstream. Such 

an event occurred in the Gros Ventre River valley in Wyoming in 

1925 (Alden, 1928). Considerable damage certainly could be expected 

from any sudden release of floodwaters that might be temporarily 

dammed behind one of these large landslides; however, the possibility 

of a catastrophic flood occurring in the Rio Reventado valley appears 

remote. The steep gradients of the Rio Reventado combined with the 

narrowness of its valley preclude the formation of any large body of 

water behind such a dam.
I 16 CONTRIBUTIONS TO GENEBAL GEOLOGY

DEBRIS FLOWS 

GENERAL, FEATURES

Debris flows proved to be one of the most damaging of the many 

problems created by the eruption, and their control proved the most 

costly and difficult. Although some small floods and debris flows occurred in the Rio Reventado and other streams soon after the eruption began in March 1963, the most disastrous debris flow occurred 

in the Rio Reventado in December at the end of the 1963 rainy season. 

A violent rainstorm, which raged over much of the country for several 

hours, culminated in a torrent of water, mud, and debris that swept 

down the Rio Reventado valley with destructive force in the late 

evening hours of December 9. From detailed studies of the storm and 

the debris flow, it was estimated (Institute Costarricense de Electricidad, 1965, p. 56) that the discharge of this large flow was about 407 

cubic meters per second, or approximately 29 cubic meters per second 

per square kilometer, and that it was composed of about 65 percent 

water and 35 percent sediment by volume. This large flow removed 

most of the accumulated debris from the valley floor, stripped the 

vegetation from lower parts of valley walls, and severely eroded 

valley Avails and floors. In places the flood reached a height of more 

than 12 m. Near Cartago the debris flow spread out over the surface

FIGURE 9. Highway bridge destroyed by the Llano Grande landslide. View is 

looking upstream at observation post 2. Toe of the slide (on left) destroyed 

the abutment of the bridge.
EROSION, ASH ERUPTIONS, IRAZU VOLCANO, COSTA RICA I 17

of the alluvial fan in a tongue of destruction about 1 km wide, devastated an area of nearly 3 km2 , and inundated another several square 

kilometers with muddy waters. More than 20 persons were killed by 

this disastrous debris flow and more than 300 homes were destroyed. 

Damage sustained by this debris flow has been estimated by the 

Oficina de Planificacion to be as much as 25 million colones (or more 

than $31/2 million).

The floods of mud and debris started again with the first storm 

of the 1964 rainy season and they continued to be a hazard and menace 

to lives and property throughout the 1965 rainy season (Ulate and 

Corrales, 1966, p. 124). Although damage to property was considerable, no additional lives were lost, chiefly due to the efficiency of a 

warning system that was established early in 1964 by the Oficina de 

Defensa Civil. More than 90 debris flows occurred during the 1964 

rainy season in valleys draining the south and west slopes of the 

volcano; about 40 of these flows were in the Rio Reventado valley. 

Although none were as large as the flow in December 1963, nearly 

half of them were large enough (higher than 3 m at post 2) to constitute a hazard to Cartago and environs. Figure 10 shows some of the 

effects of these debris flows in the Cartago area. Hydrographs of 

some of the larger Rio Reventado flows of 1964 are shown in figure 11.

A survey of the literature reveals only meager data concerning mud 

and debris flows and their properties. Many individuals have studied 

the results but few have been privileged to observe and study the 

phenomena personally. For a review of the literature on mud and 

debris flows the reader is referred especially to Sharpe (1938) and 

to Sharp and Nobles (1953).

A mud or debris flow represents a form of mass movement of sediment that is intermediate between a normal streamflow and an essentially dry landslide. However, it is very difficult to assign any precise 

limits of definition to a mud or debris flow because they are members 

of a broad gradational series in which the relative proportions of 

sediment to water may vary widely. Sharp and Nobles (1953, p. 550) 

suggested that "debris flow should be used as a general designation 

for all types of rapid flowage involving debris of various kinds and 

conditions" and that "mudflow is simply a variety of debris flow 

in which the mud, although not necessarily quantitatively predominant, endows the mass with specific properties and modes of behavior 

which distinguish it from flows of debris devoid of mud." Varnes 

(1958, p. 37) distinguished between mudflows and debris flows 

solely on the basis of particle sizes. He applied the term "debris flow" 

to mixtures of water and sediment that contain a relative high percentage of coarse fragments, and he restricted the term "mudflow"
118 CONTRIBUTIONS TO GENE.RAL GEOLOGY

FIGURE 10. Effects of the debris flows near Cartago. Upper: Some of the few 

homes that were damaged but not completely destroyed by the flows in the 

vicinity of the Pan-American Highway. Lower: Upstream from area shown 

above; bouldery content of the debris flows deposited on the upper part of the 

alluvial fan.
SEPTEMBER 2 SEPTEMBER 8 OCT

FIGURE 11. Hydrographs of some of the larger Rio Reventado debris flows in 1

post 2 by Oficina de Def ensa Civi
I 20 CONTRIBUTIONS TO GENERAL GEOLOGY

to mixtures that contain at least 50 percent sand, silt, and clay-sized 

particles. Thus, according to Varnes' classification, all the flows from 

Irazu are debris flows, as they contain a relatively high percentage of 

coarse fragments.

Recently Beverage and Culbertson (1964, p. 125-126) proposed an 

arbitrary limit of 80 percent concentration of sediment by weight to 

"differentiate between the commonly accepted concepts of 'streamflow' and 'mudflow.' " Such a limit of concentration may be applicable 

to mudflow, as defined by Varnes, but it cannot be applied to the Bio 

Reventado debris flows. Mackin (1948, p. 469) pointed out that the 

use of any such arbitrary limit of concentration has little value or 

significance unless it is also accompanied by some statement as to the 

grain sizes involved, or to the range in grain sizes.

Studies of alluvial fan deposits in California by Bull (1962; 1963; 

1964) indicate that certain sediment parameters, based on measurements obtained from grain-size analyses of sediments, might be utilized to distinguish three types of alluvial fan deposits: Mudflow 

deposits, water-laid sediments, and deposits intermediate between the 

two. The Rio Reventado debris flows appear to be more correlative 

with Bull's intermediate deposits than with his mudflow deposits. 

(See table 3.)

PHYSICAL, CHARACTERISTICS

A summary of some of the physical characteristics of the Rio 

Reventado debris flows is given in table 2, and the particle-size distribution of some of the flows is shown in figure 12. These data are based 

on velocity measurements and sediment samples obtained and analyzed 

by the Institute Costarricense de Electricidad (1965, p. 252-258) during the period September 8 through November 11, 1964. All the sediment samples were collected by means of an open-mouthed container, 

15 cm in diameter, dipped from the debris-flow surface.

The physical characteristics of Rio Reventado debris flows varied 

greatly from one flow to another and during the final phase of their 

fluid stage. Most flows became very thick and pasty during the final 

phase, much like a thick, flowing concrete, but some became thin and 

watery. These thin flows were potentially more dangerous, as they 

were capable of severly eroding levees and other manmade protective 

structures. The range in grain size and sediment concentration, of 

course, is much higher than indicated in either figure 12 or table 2, 

simply because it was impossible to obtain a sample of the total range 

in particle size. Boulders as much as 2 to 3 m in diameter were very 

common in all the flows, and several as large as 4 to 5 m in diameter 

were observed. The largest boulder known to have been transported
* ro oj -pi* en CT> -xj oo 

SIZE* INDICATED THAN* WEIGHT BY "PARTICLES'COARSER* OF PERCEN?AGE 
I 22 CONTRIBUTIONS TO GENERAL GEOLOGY

TABLE 2. Summary of some of the physical data on the Rio Reventado debris flows

Physical data

Density.... _________ _ _gpercm3 ..

Specific gravity (solids) ... _____

Concentration. . . ___________ __ppm__ 

Discharge _______________ m3 per sec. _

Grain-size distribution : Median (Md). ............ ____

Phi median (Md+} ... _______

1st quartile (Q2s) ----- -_-

3d quartile (Q7s) __ - - ----

16th percentile.--.----- ___

84th percentile._---_-_-____ _._

Sorting indices:

0 /^_- S °-VQ_; '

O/C 

07. Qi&Qi* 

T, ,j "

_

Md2

Q£*=J*(*26-*7«) -----------------

Number 

of 

samples

31

80

15

89 

17

10

10

10

10

10

10

10

10

10

10

Range

2. 86-10. 00

1. 13-1. 98

2. 50-2. 70

200, 000-788, 000 

57-407

. 07-. 60

3. 87-3. 35

. 013-. 10

. 19-3. 00

. 006- 040

. 32-6. 20

2. 58-7. 01

. 50-1. 28

2. 62-4. 04

1. 37-2. 94

Average

5. 27

1.61

2. 61

574; 000 

170

.28

1.88

.054

1. 15

. 022

2.06

4. 61

.84

3. 12

2.09

by one of the flows measured approximately 7.5 by 5.5 by more than 

3 m and must have weighed at least 200 tons (fig. 13).

Sediment concentrations and surface velocities of the debris flows 

varied considerably both in time and in place. Although no direct 

relation is apparent, a crude general trend of an increase in sediment 

concentration with an increase in velocity can be seen in figure 14. A 

total of 90 samples was taken from the Rio Reventado on 19 different 

days, and a total of eight samples was taken from the Rio Agua 

Caliente on 2 different days. All samples with a sediment concentration 

below 200,000 ppm (parts per million) were arbitrarily eliminated, so 

that a total of 89 samples remained (fig. 16). Of these, 82 were taken in 

the upper basin 65 at observation post 2, from 17 of the flows, and 

17 at post 3, from 7 of the flows. Another seven samples were taken 

in the lower basin from the Rio Agua Caliente at a gaging station 

about 2 km downstream from the confluence of the Rios Reventado 

and Purires (pi. 1). Sediment concentrations of the 89 samples ranged 

from 200,000 ppm (20.0 percent by weight) to 788,000 ppm (78.8 

percent by weight). The samples average about 574,000 ppm (57.4 

percent by weight), with a median of about 609,400 ppm (60.94 percent 

by weight). All the 82 sediment concentrations from the upper basin 

should be increased by an unknown, but certainly considerable, factor 

in order to account for the large number of unsampled sediment particles in the flows that are greater than 15 cm in diameter. Similar 

ranges in the sediment concentration of small ash-laden streams from
EROSION, ASH ERUPTIONS, IRAZTJ VOLCANO, COSTA RICA I 23

FIOTJEE 13. Massive andesite boulder transported by a Rio Reventado debris 

flow. Photographed at site of bridge that was later destroyed (fig. 9). Boulder 

measures about 7.5 by 5.5 by more than 3 m and is estimated to weigh at least 

200 tons. ' - .... . -..- , .. -
I 24 CONTRIBUTIONS TO GENE.RAL GEOLOGY

20.0

Q

§10.0 

O 9 

£ 8

tt 7

LlJ Q- 6

UJ 4

z 

- 3

O 

O 

-I 2*

1.0.

Average ®* * 

... .

 O Median 

200 400 600 800 

SEDIMENT CONCENTRATION, IN PARTS PER MILLION X 1000

FIGURE 14. Relation of velocity and sediment concentration for some of the Rio 

Reventado debris flows. From data provided by Servieio MeteorolCgico Nacional 

(unpub. data, 1964) and Inistituto Costarricense de Electricidad (1965, p. 

255-258).

Paricutin Volcano in Mexico from less than 20 percent to nearly 

80 percent by weight were reported by Segerstrom (1950, p. 96-107). 

Thirty-one velocity measurements were made at post 2 on 12 of the 

flows (figs. 14, 15). Four others were made but were deleted three, 

because they were measured on flows whose concentration of sediment 

was less than 200,000 ppm, and one, because its velocity of 1.02 m 

per sec (meters per second) was so much lower than any of the others 

that it might be in error. The 31 measurements ranged from 2.86 to 

10 m per sec, with an average of about 5.27 m per sec and a median 

of about 4.46 m per sec. Peak velocities of 8 to 10 m per sec were 

measured on only two flows. The river stages of these two flows, as 

measured at post 2, ranged from 1.8 to 5.5 m (fig. 15) at the time of 

sampling, or from a low to a moderate flood stage. The sediment concentrations of these two high-velocity flows ranged from 606,000 to 

737,000 ppm (fig. 14). Unfortunately, the velocities of some of the 

larger flows were not measured; therefore the maximum velocities 

attained are not known, but it seems very likely that speeds in excess 

of 15 to 20 m per sec were attained by some of these larger flows. Flood 

stages of 10 to more than 12 m at post 2 were reached by these larger 

flows. Sharp and Nobles (1953, p. 552) reported surge-front velocities 

for the Wrightwood debris flows in southern California that ranged
EROSION, ASH ERUPTIONS, IRAZU VOLCANO, COSTA RICA 125

20.0

10.0

0 9

8 8

£7 

LJ 6

2 -

1.0,

 I 

..Average

Median 

O 

234 

RIVER STAGE, IN METERS

FIGURE 15. Relation of velocity of some of the Rio Revantado debris flows to 

river stage of the Rio Reventado at post 2 at the time of measurement. From 

data supplied by Servicio Meteorologico Nacional (unpub. data, 1964) and 

Institute Costarricense de Electricidad (1965, p. 255-258).

from less than 0.5 to 4.41 m per sec and averaged about 2.87 m per sec. 

The Wrightwood debris flows, however, were flowing on much lower 

gradients than the Rio Reventado. Lynch (1941, p. 216) reported 

probable velocities as high as 9 m per sec for debris flows that occurred in 1934 in Los Angeles County, Calif. Curry (1966, p. 772) 

reported surface velocities as high as 16.3 m per sec for alpine mudflows in Colorado. lida (1938, p. 681) estimated a velocity of 25 m 

per sec for a mudilow from Mount Bandai in Japan. Recently, however, Lang and Norris (1966, p. 589) pointed out that visual estimates 

of speed commonly tend to be too high, especially when violent motion 

is involved.

Although an increase in discharge of the Rio Reventado normally 

was accompanied by an increase in the velocities and sediment concentrations of the flows (figs. 15, 16), the amount of sediment in the 

flows varied widely (fig. 16). Despite numerous difficulties that were 

encountered both in sampling the flows and in measuring the river
I 26 'CONTRIBUTIONS TO GENERAL GEOLOGY

stages, data on a few of the debris flows suggested that the sediment 

concentration increased during a rising stage of the stream and decreased during a falling stage. The flash-flood and variable nature 

of these flows, however, made it difficult to ensure that a sample of 

the sediment load was obtained precisely at the time of maximum 

discharge. Both sediment concentration and discharge are very sensitive to the areal distribution and intensity of rainfall. Consequently, 

only minor differences in rainfall intensity, or slight shifts in the 

path of a storm, or both, can easily result in major differences in the 

relative proportions of sediment and water that are contributed to 

and carried by different segments of the stream.

The Eio Eeventado debris flows were poorly sorted, containing 

material that ranged in size from clay to large boulders. The flows 

also commonly carried abundant fragments of tree trunks and limbs 

and other vegetative matter that was derived from the caving of the 

channel walls. The clay-sized content of the debris flows was low, 

ranging from about 1 to 10 percent and averaging about 5 percent. 

The combined silt- and clay-sized content also was low, ranging from 

16 to 49 percent and averaging about 27 percent. Sorting of the Eio 

Eeventado debris flows is summarized in table 2 and compared with 

other mud and debris flow deposits in table 3. In order that comparisons could be made, the author used the same indices that were used 

by Bull (1964, p. A24) : The Trask sorting coefficient, S0 the phi 

standard deviation,^; and the quartile deviation, QD 0 . In table 3, 

the Eio Eeventado flows appear to be more comparable with the 

intermediate deposits (between mudflow deposits and water-laid sediments) than with the mudflow deposits of Bull (1964, p. A18). This 

may be because the Eio Eeventado sorting ratios do not include any 

particles larger than 15 cm in diameter and, hence, tend to be lower 

than would be expected. The very low range in the sorting indices of 

the Osceola Mudflow suggests that it may have some unique properties that distinguish it from the other deposits. The matrix of the 

Osceola, although poorly sorted, appears to have changed very little 

in composition from near its source on Mount Eainier to its distal 

end, some 20 miles down the White Eiver valley (Crandell and 

Waldron,1956,p.349).

MOVEMENT

Most observers report that mudflows move in successive waves or 

pulsations. Many of the Eio Eeventado flows, however, did not show 

this pulsating characteristic, but rather they appeared to flow down 

the channel in a single surge, one for each rainstorm or torrential 

downpour that occurred in the watershed or within parts of the water-
EROSION, ASH ERUPTIONS, IRAZU VOLCANO, COST A RICA 127

u 

00

O

81-1 o

X o

IO

z

0

_l _1

5

tr

UJ

Q.

w

H

cr

<

0.

0

-°

z

o

t-

<

cc1- 

z

UJ

o

z

o 

o

1-

5

UJ

w

o

1 0

0

o 2

o * OAQ

A

o

9 *

8 . |

O

o o

-

 .

i

-

 . . 

 A

0 1

1 1 1 ' 0 '

 

 

 o 

 

 i

9 

 A * 

.* " *

°' * .

 .

 

 

. A

-

EXPLANATION

Location of sampling

 Gaging station 2

O

, Gaging station 3

A

Agua Caliente

234567

I 1 I I 1

REVENTADO RIVER FLOOD STAGE, IN METERS

FIGURE 16. Relation of sediment concentration of some of the Rio Reventado 

debris flows to the river stage of the Rio Reventado at post 2 at the time of 

sampling. From data supplied by Servicio Meteorol6gico Nacional (unpub. 

data, 1964) and Institute Costarricense de Electricidad (1965, p. 255-258).

shed. Each flow was preceded by a slow to rapid rise in the level of the 

stream, depending on the rate or intensity of rainfall, and most flows 

were accompanied by an increase in the amount of sediment carried 

in the stream. Subsequently, the main flow of mud and rock debris 

would appear in the channel as a rapidly advancing wave or surge with 

a definite frontal lobe. It is estimated that the fronts of those lobes 

observed ranged in height from less than 30 cm to at least 2 m; the 

maximum surge height is not known, however, because none of the 

larger debris flows were seen by the author. After the initial surge,
128

TABLE 3. Comparison of sorting indices of the Rio Reventado debris flows in Costa 

Rica with those of similar deposits in the United States

Deposits and locality

Elo Reventado debris flows, 

Costa Rica.i 

Wrightwood debris flows, 

Califomia.2 

Mount St. Helens lahars, 

Washington.3 

Osceola Mudflow, Washing- ton.* 

Mudflow at Trout Lake, 

Klickitat County, Wash.s 

Mudflow deposits on alluvial 

fans, Fresno County, Calif. 6 

Intermediate deposits on 

alluvial fans, Fresno 

County, Calif.9

Age

1964 --------

1941------..-.

Recent about 

2,000 yr B. P. 

Recent about 

4,800 yr B. P.

 .do 

--.do. .

Number of 

samples

10 

10 

6 

8 

3 

50 

16

Sorting coefficient (So)

Range

2. 6-7. 0 

2. 7-5. 0 

4.9-10.2 

9. 0-16. 0 

5.8-11.0 

5.0-25.0 

2.6-5.0

Avg

4.6 

3.9

7.8 

11.7 

7.7 

9.7 

4.0

Phi standard 

deviation (<r$)

Range

2.6-4.0

3.3-4.1 

4.5-5.8 

3.8-4.3 

4. 1-6. 2 

3. 1-4. 7

Avg

3.1

3.8 

5.0 

3.1 

4.7 

3.9

Quartile deviation (QDj,)

Range

1.4-2.9

2.4-3.2 

3.2-4.0 

2. 1-4. 1 

2.3-4.7 

1.4-2.3

Avg 

2.1

2.9 

3.5 

2.9 

3.1 

2.0

1 Institute Costarricense de Electricidad (1965).

2 Sharp and Nobles (1953).

3 Mullineaux and Crandell (1962).

< D. R. Crandell (written commun., 1965). 

s F. O. Jones (written commun., 1959). 

'Bull (1964).

either the stream level dropped off rapidly or, more commonly, the 

flow decreased to a lower level that approximated the level of the flow 

immediately behind the surge front. The duration of flood flow ranged 

from less than 1 hour to as much as several hours (fig. 11), during 

which time both river stage and sediment concentration varied. Most 

of the fluctuations shown in figure 11 reflect either successive rainstorms or the effects of differences in the peak flows from tributary 

watersheds during the passage of a single storm over the area. Only 

the flow of September 8 (fig. 11) suggests a pulsating character of 

flow.

The coming of the main flows always was heralded by a loud roaring 

and rumbling noise, and the passing of the flows always was accompanied by considerable noise and a noticeable shaking of the ground. 

This ground motion undoubtedly was due to the violent turbulence 

of the flow and especially to the bumping, rolling, and grinding of 

the numerous boulders in the fluid mass. Some of the many slope failures that occurred along the channel may be attributed, in part, to the 

effects of this ground motion on the unstable walls.

Superelevation or banking of the debris flow surfaces was common, 

especially in the upper basin where channel gradients are steep. Banking at sharp bends in the channel occurred whenever velocities of the 

flows were sufficiently high. Depending on the size and velocity of the 

flow, the surface of the flow at these localities could be as much as 

several meters higher on the outside than on the inside of bends. A 

maximum difference in elevation of approximately 7 m was noted 

between the high flow marks left on opposite sides of a 90° bend in
EROSION, ASH ERUPTIONS, IRAZU VOLCANO, COSTA RICA I 29

the channel of the upper Rio Reventado, which was about 15 m wide at 

the bend.

The actual mechanics of flowage of debris flows is not fully understood at present. The increased transport capacity of a stream having 

a high concentration of suspended sediment is apparently largely due 

to the increased density and viscosity of the mass and to the greatly 

reduced fall velocities of the sediment particles. Other factors, however, also are involved, such as the buoyant and hydrodynamical lift 

forces acting on the particles of sediment (Am. Soc. Civil Engineers, 

1966, p. 292), the chemical and physical properties of the fine sediment 

(Simons and others, 1963, p. G45), and the dispersive forces acting 

between the moving grains (Bagnold, 1956, p. 270). That loads of fine 

and coarse sediment in a stream tend to be largely independent of 

each other (Passega, 1957, p. 1953) is another factor that may affect 

the suspension capabilities and flowage characteristics of debris flows. 

According to Bagnold (1962, p. 316), the transport rate or capacity 

of the coarser grades is a function of streamflow, whereas that of the 

finer grades appears to be unlimited.

CONTROL MEASURES

The engineering problems involved in the control of erosion and of 

floods of mud and rock debris include several mutually related but 

diverse activities. Most flood-control measures are designed to reduce 

the hazards associated with flooding, and, consequently, the measures 

will provide flood protection rather than complete control; seldom, if 

ever, can absolute control of a river and its flood plain be attained. The 

principal methods of flood control are (1) downstream control, which 

is aimed primarily at flood protection and which is accomplished by 

means of improvement of channels and construction of dikes or levees 

and the construction of diversion channels and (2) headwater control, 

which is aimed primarily at decreasing the rate of runoff and retarding peak discharges and which is accomplished through the rehabilitation of watersheds and the construction of temporary reservoirs or 

detention basins. The principal methods of erosion control are (1) 

reduction of slope erosion by watershed rehabilitation, (2) reduction 

of channel erosion by channel stabilization, and (3) reduction of the 

quantity of debris by retention and temporary storage of sediment in 

debris basins. Each method of flood or erosion control has its advantages and also its limitations. Consequently each stream basin, and 

even each segment of the stream, must be analyzed individually in 

order to determine which method, or combination of methods, may 

prove to be the most effective and the most economical for that stream.

The control of mud or debris flows involves some special engineering
I 30 CONTRIBUTIONS TO GENE.RAL GEOLOGY

problems in that some of the measures commonly used for the control 

of flood- waters cannot be used effectively for the control of flows of 

mud and rock debris. One problem is due to the destructive forces that 

are generated by the flows as a result of their high velocities and 

densities and their bouldery contents. Another problem is due to a 

rapid reduction in velocity that occurs in areas where the stream channel is unconfined or where the stream gradient is suddenly diminished. 

As a consequence, when a layer of mud and rock debris spreads over 

a gently sloping area, such as the surface of an alluvial fan or a broad 

flood plain, it will gradually come to rest and congeal into a permanent 

deposit of earth and rock, whereas a layer of water spread over the 

same surface will eventually drain away. According to Bull (1964, 

p. A17), deposition on an alluvial fan occurs not only as the result'of 

a decrease in slope, which brings about a decrease in the velocity of 

the flow, but also because of the decrease in depth and velocity of the 

flow that results from the widening and thinning of the flow as its 

spreads over the fan. The thickness and extent of the resulting deposit 

will be dependent upon the 'viscosity of the flowing mass and on the 

relative proportions of sediment to water in the flow. Thus, although 

the bulk volume of two debris flows may be equal, a thicker mixture 

of sediment and water not only will come to rest more quickly than a 

thinner mixture, but it will produce a layer of greater depth of earth 

and rocks over a smaller area (Woolley, 1946, p. 81).

Initially, due to the almost complete lack of basic data; available, 

it was necessary to start some of the emergency control measures with 

little or no prior planning or preparation. As soon as possible, therefore, programs were developed for the collection and collation of basic 

data on the meteorology, hydrology, engineering geology, topography, 

volcanology, soils, and vegetation of the Irazu area, and especially 

of the Rio Eeventado watershed. A network of rain and river gaging 

stations was installed for collecting the meteorologic and hydrologic 

data. Detailed topographic maps and profiles were compiled of the Eio 

Reventado basin and of the larger landslides. Periodic measurements 

also were made of the larger slides in order to determine the magnitude and direction of their movements; some subsurface investigations 

of the large slides also were undertaken. Geologic mapping and investigations of the volcano, its products, and its history were started; 

and engineering geologic maps and studies were made of parts of the 

Reventado, Retes, and Pavas channels in order to determine the foundation conditions that existed in the channels and to determine the 

geologic settings of the landslides. The soils and vegetation of the area 

were studied; before the revegetation program began, test plots were 

established on the ash-covered slopes to determine which plants would
EROSION, ASH ERUPTIONS, IRAZU VOLCANO, OOSTA RICA 131

grow the fastest and thrive the best in the new ash. Concurrent with 

these investigations, consulting engineers began preparing plans, designs, and cost estimates based on the available data, both for implementing the emergency control measures and for developing longrange plans for the control of erosion and protection from flooding 

(Institute Costarricense de Electricidad, 1964, 1965).

Downstream control measures applied to the Rio Reventado in 1964 

were concerned chiefly with the urgent and vital need to protect life 

and property in the city of Cartago and its environs. All three methods of flood protection were considered stream diversion, channel 

improvements, and construction of levees.

Although diversion of some of a flood flow can frequently be the 

most direct and effective way to cope with the situation, the economic, 

political, and social aspects of the problem, particularly in a heavily 

populated area, often can offset obvious technical and engineering 

benefits. Implementation of proposals made for diversion of part of 

the Rio Reventado was not attempted during 1964 primarily for these 

very reasons. Any long-range plans for flood control of the area, however, should certainly investigate thoroughly the possibilities of diverting at least a part of any large discharge by means of an emergency 

spillway and auxiliary channel. Such an auxiliary channel is most 

effective when the diverted water can be taken away completely from 

the main channel. If the diverted water must be returned to the stream 

channel downstream, it should be returned as far downstream as 

economically feasible in order to reduce or eliminate any backwater 

effect on the main channel. If sufficient land can be made available, the 

auxiliary channel can be designed to provide storage capacity as well 

as discharge capacity. Temporary storage of debris in a debris basin, 

although costly to construct and maintain, is perhaps the best protection yet devised for the positive control of debris (Eaton, 1936, p. 

1324; Dodge, 1950, p. 343-345).

Channel improvements undertaken during the 1964 dry season included the widening, deepening, and straightening of parts of the Rio 

Reventado channel in its lower basin. Unfortunately these measures 

proved to be only of very limited value for controlling debris flows. 

Instead of increasing the discharge capacity of the stream channel, as 

would be expected with an ordinary flood of water, the channel improvements actually functioned as miniature debris basins. The first 

storm of the rainy season promptly filled the enlarged channel with a 

deposit of mud and rock debris. The main flow was then forced to 

leave its old channel and seek a new one on a lower part of the alluvial 

fan. Thus, during a flood the stream might be diverted by as much as 

50 m from its previous channel. The gradient of the stream channel
I 32 CONTRIBUTIONS TO GENE.RAL GEOLOGY

on the alluvial fan was much too low and the volume of debris brought 

down by the flows was much too large for these minor channel improvements to be effective, particularly in the upper part of the lower 

basin where most of the coarser material was being deposited. Farther 

downstream, where only sediment of sand size and smaller was carried by the stream, some increase in the carrying capacity of the channel might be realized by means of channel improvements. Future work 

should include a compilation of data on discharge capacities of the 

channel throughout various stretches of the lower basin and an investigation of the probable effects on the channel of any improvements 

that might be made in the lower basin.

The construction of dikes or levees is one of the oldest and most 

extensively used methods of flood protection in the world. Often they 

also are the most rapid and economical means of obtaining this protection. These features, however, also can be potentially very hazardous in that, although they can provide complete protection up to a certain flood stage, a major disaster can result if the levees are breached 

or if a higher flood stage occurs and the levees are overtopped. The 

design and construction of levees, therefore, should conform to standard engineering practice, in which the height, width, side slopes, and 

construction should be such that the levees will not fail owing to 

seepage or piping, or to undercutting by the stream, or through failure 

of the side slopes.

Two large levees were constructed in 1964 along the Rio Reventado 

channel extending downstream for several kilometers from where the 

stream debouches from the mountain front. These levees involved 

the emplacement of more than l1/^ million cubic meters of material. 

Most of this construction was done by U.S. Navy Seabees in cooperation with the Oficina de Defensa Civil and other Costa Rican government agencies. Some minor levees farther downstream on the Rio 

Reventado and in other river basins also were constructed by the local 

government agencies. Work on the levees was hampered by numerous 

debris flows that occurred during the 1964 rainy season, but fortunately the size and spacing of the flows was such that no overtopping 

or breaching of the levees occurred during construction. Although 

considerable minor damage to property did occur from the debris 

flows, major damage was confined to secondary roads and bridges and 

to the frequent disruption of rail traffic between San Jose and the vital 

port of Limon on the east coast, which can be reached only by rail or 

air. These downstream control measures, which were extremely valuable and necessary for the safety of the Cartago area, however, did 

not solve the basic problems of excessive runoff and accelerated 

erosion.
EROSION, ASH ERUPTIONS, IRAZU VOLCANO, COSTA RICA I 33

The most effective means of alleviating the basic problems of runoff 

and erosion can be accomplished only through headwater-control 

measures. If runoff can be decreased, then erosion is lessened, peak 

discharges of the streams are reduced, and the undercutting of channel slopes is greatly diminished. Effective stabilization of the channel 

slopes and streambeds can be accomplished by a system of check dams 

only after the runoff has been decreased. Consequently, in 1964 a program of watershed rehabilitation was initiated which was aimed at 

reducing runoff and retarding erosion of those watershed slopes that 

were the most seriously affected by the ashfall. These rehabilitation 

measures included surface diversions of drainage, contour ditching of 

slopes, construction of brush-check dams in the smaller gullies, and the 

artificial revegetation of the ash-covered, devastated areas. The revegetation in 1964 consisted of planting a mixture of fertilizer, oats, 

and pasture grasses in hand-hoed rows. All this watershed rehabilitation work was done under the direction of the Ministerio de Agricultura y Ganaderia, utilizing funds provided, by the U.S. Agency for 

International Development. The revegetation program was revised 

and expanded in 1965 (Ulate and Corrales, 1966, p. 127).

The emergency control measures developed for Irazu in 1964, although developed independently, were almost identical to those used 

by the U.S. Forest Service after a fire in 1960 in the San Dimas Experimental Forest in southern California (Rice and others, 1965). 

The emergency measures used at San Dimas included the broadcast 

sowing of annual grasses, contour trenching, side-slope stabilization 

by contour furrow planting, and channel stabilization by check dams.

The effects of the Rio Reventado watershed control measures, in 

conjunction with some natural revegetation that took place as the 

ash was eroded from the slopes, began to show up in the rainfall-runoff 

pattern by the end of the 1964 rainy season. Early in the rainy season 

the duration of runoff was almost the same as the duration of rainfall, 

but by late October, R. D. Tarble (oral commun., 1964) noted that 

the duration of runoff was beginning to exceed the duration of rainfall by a considerable margin.

A flood-control reservoir also can be a very effective means of storing 

water during times of extreme flow because the water can then be 

released later when the critical flood conditions have passed. Such 

a measure, however, is not an emergency flood control measure and 

only can be considered in a long-range plan. .Unfortunately no suitable 

reservoir sites exist on the Rio Reventado. Throughout most of its 

length the channel is too narrow and the gradient is too steep to provide any appreciable volume of storage behind a low dam. The high 

cost and lack of storage also precluded any consideration of a large
I 34 CONTRIBUTIONS TO GENERAL GEOLOGY

dam for this purpose. Only at one locality is the gradient low enough 

and the; valley broad enough to bo considered a potential site for a 

storage reservoir. This site is in an area in the upper basin known 

locally as the Laguna (pi. 1). However, at this site it was determined 

that the foundation conditions for a dam were poor, and the storage 

available behind a low dam was not sufficient enough to make the 

site economically feasible or practical.

Check dams have been used successfully for channel stabilization 

in many countries, notably in the alpine regions of Europe, and in 

Japan, China, South America, and the United States (Baumann, 

1936; Ferrell, 1959; Ferrell and Barr, 1965; and Morum, 1936). 

Such a stabilization system, however, rarely proves to be an effective 

emergency measure, but it should be a major part of any long-range 

plans that are developed for the area. A series of low check dams 

installed in a mountain stream such as the Rio Reventado can be one 

of the most practical and effective means of controlling channel erosion and thus reducing debris production. Unlike high dams, low check 

dams cannot provide any immediate or direct protection against large 

floods, but if properly designed and installed they can rapidly reduce 

the production of debris from channel downcutting and from undercutting and slumping of channel slopes during minor floods. To attempt to utilize check dams for channel stabilization in a valley such 

as the Rio Reventado before headwater-control measures have become 

effective, however, can be disastrous and very costly. Six gabion check 

dams, which were built at the toe of the Llano Grande landslide in 

1965 by the Government of Costa Rica, were nearly completed when 

all were destroyed by two large successive debris flows in May 1965 

(Institute Costarricense de Electricidad, 1965, p. 286-312; Ulate and 

Corrales, 1966, p. 127).

SUMMARY

Large volumes of fine lithic ash were erupted from Irazu Volcano 

during the period March 1963 to March 1965. The continued deposition and accumulation of this ash on the slopes of the volcano created 

many problems in the area. Some of the most serious were due to 

changes that occurred in the hydrologic regimens of the streams that 

drained the upper slopes of the volcano, especially in the watershed 

of the Rio Reventado on the southwest slope of the volcano. The 

mantle of ash not only destroyed much of the original vegetative cover 

that existed on the slopes, but it also formed a hard crust that was 

virtually impervious. As a result, infiltration of rainfall was very 

greatly reduced and runoff was very greatly increased. The accelerated 

runoff increased slope erosion and produced frequent flash floods. The 

flash floods severely eroded channels and created many unstable slopes.
EROSION, ASH ERUPTIONS, IRAZU VOLCANO, OOSTA RICA I 35

As a result of the accelerated erosion of slopes and especially of stream 

channels, each flash flood became a debris flow, more than 90 of which 

were produced during the 1964 rainy season. Most of the sediment 

in these flows was derived from channel erosion in the upper parts of 

the watersheds where steep slopes and stream gradients produce highvelocity flows and where the channel floors and walls largely consist 

of easily eroded material of low shear strength, mainly deposits of 

poorly consolidated, clayey ash and lahar. Debris flows in the Rio 

Reventado overflowed streambanks and inundated lowland areas, destroyed much agricultural land and other private and public property, 

and seriously threatened the city of Cartago and environs.

Emergency measures that were applied to the problems included 

channel improvement and the construction -of levees in the lower Rio 

Reventado basin to promote flood protection for Cartago and environs, and watershed rehabilitation by terracing, contour trenching, 

drainage diversion, and artificial revegetation in the upper basin. 

The effects of all these measures are still being assessed, and data on 

these measures, along with other basic data, will be utilized in the final 

preparation of long-range plans for the control of erosion and debris 

flows from Irazu Volcano.

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