062-erosionControlBasicsInTropicalAreas

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EROSION CONTROL BASICS IN TROPICAL AREAS

Jaime Suarez-DiazSchool of Civil EngineeringIndustrial University of Santander. Calle 41 28-33 Bucaramanga, Colombia.

Phone + 57 76341255 Fax + 57 76457507E-mail: [email protected]

Biographical Sketch of author

Jaime Suarez-Diaz holds Bachelor and Master of Science degrees in CivilEngineering from Rutgers University, New Brunswick, New Jersey. He is aprofessor and researcher at the Industrial University of Santander in BucaramangaColombia. His areas of teaching, research and consulting include slope stabilityand erosion from a geotechnical point of view. He has written several books onthese topics. The most recent Erosion Control in Tropical Areas is used astextbook for erosion control in several Spanish-speaking countries.

Abstract

Soil erosion is very difficult to control in tropical areas because the climatic andgeologic conditions are of extraordinary complexity. Precipitation is generally verysuperior in magnitude and intensity to that of temperate areas and the climaticfluctuations as El nio are very well marked.

The velocity of water flows commonly reach values higher than 5.0 meter persecond with flows of high turbulence. This is due to the high volume of flow andvery steep slopes. Additionally, water currents transport large quantities of coarsesediments.

The layer of residual soil and weathered rock in soil profiles is generally very thickand soils possess in many cases a very high susceptibility to erosion. The veryhigh values of environmental temperature also have considerable impact indeterioration of soils accelerating the erosion rates.

On the other hand, a very important positive element is the vegetation bio-diversity.The hundreds of thousands of vegetation species found in the tropical areasbecome a very useful tool for the control of soil erosion. In a single hectare oftropical forest there can be more than 300 vegetation species. Some of thesetropical species have specific properties that make them very useful in erosioncontrol.

The technologies of erosion control developed in the United States and Europehave had a mixed success in the tropical countries. Although the fundamentalprinciples of erosion control are applicable, modifications are required in theapplication approaches. Hard structures such as gabions and deep revetments,


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have had generally a good behavior while the use of soft linings has not been assuccessful.

The work shows the principal challenges that the specialist in erosion control facesin tropical areas. The document also presents several historical cases of erosion

in tropical countries.Key words : Hydrology, tropical environments, residual soils, bio-engineering,slope stability.

1. Introduction

Erosion control in the tropical areas has much in common with the practice oferosion control in non tropical areas. The basic principles are the same, but theenvironment in which the erosion processes are developed is very different. Thepresent document summarizes some of the basic factors to keep in mind whenmanaging erosion problems in tropical countries.

The principal elements of the erosion behavior in tropical areas are the following:- The hot and humid environment. High temperature and humidity duringpractically all year affect the behavior of soils and vegetation.- The hydrology. The regime of rainfall is characterized by very strong intensitiesof precipitation and for severe climatic fluctuations such as El nio.- The residual soils. Tropical soils are characterized by high weathering levelsand the predominance of residual soils, which commonly present a highsusceptibility to physical weathering and erosion processes.

The result is an environment in which, much more severe erosion phenomena cantake place than in non tropical areas. Before these circumstances the solution tothe problems, requires of works able to resist more intense tractive forces and tosupport stronger scour levels. The lack of knowledge about the characteristics oftropical environments can bring large failures of erosion control structures.

2. Hydrological factors

The tropical hydrology is characterized by high precipitations due to the crossing ofclusters of clouds in the areas of wind convergence. The clusters are groups ofclouds in greater scale, with areas of high rainfall up to of 50000 square kilometers(Smith, 1992). These cloud clusters generate rains of great intensity and ofseveral days of duration. These high rainfall areas are located on a relativelyparallel fringe to the Equator. The fringe moves toward the south in the event ofEl nio and toward the north in La nia.

The result is a system of rains of great intensity on a tropical fringe semi-pallel tothe equator (figure 1). The tropical areas can be subdivided in two areas ofdifferent hydrology:


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Figure 1. Areas of permanent high rainfall and semi-dry areas in the tropics.

-Areas of permanent high precipitation . They correspond to the areas ofcrossing of the clusters of clouds. In this areas, rain generally takes place yearlywith an average from 1400 to 3500 milimeters (60 to 150 inches) and in someplaces up to 10000 millimeters (400 inches) of rain a year. The maximum rainfallin a 24-hours period varies between 100 and 500 millimeters (4 to 20 inches).

-Semi-dry or semi-arid areas . There are areas to the north and to the south ofthe permanent high precipitation areas (figure 2), with yearly average rainfall ofless than 1000 millimeters (40 inches). However, with a period of recurrencevariable between 5 and 50 years, rainy events with more than 500 millimeters (20inches) of rain in 24 hours can take place. These events are those that producecatastrophic problems of erosion and landslides.

The climatic disturbances The displacement of the fringe of high rainfall, in certain years, generates strongrains in the semi-dry areas, and these are the events that produce more seriousproblems of erosion. These catastrophic events are common in the semi-drytropical areas. Two worldwide well-know examples are Hong-Kong in May of 1982and Venezuela in December of 1999.

El Nio and La NiaThe climatic disturbances are generally related with El nio and La nia. Thesystem of fronts of clusters of clouds from east to west, manages most of thetropical climate. However in certain years anomalies of clusters take place in theopposite direction, with clouds coming from the north pole, or a displacement to thesouth or to the north of the normal fronts of clusters. These anomalies cangenerate very high precipitation in areas where the normal rain average is low, for


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example in the coast of Peru and Ecuador and in the coast of Venezuela. In Asiathe high rain areas can move north toward China, or south to the north of Australia.El Nio generates persistent rainfall toward Ecuador and Peru and toward Mexicoand Southern California and later on La nia locates these high rainfall areasover Central America and Colombia. These disturbances produce very intense

times of rain or drought, according to the location of the cloud clusters.

Figure 2. Average annual rainfall in the tropical area of America (Suarez 2001).

Hurricanes and Typhoons. Hurricanes and Typhoons are convective systems of great magnitude with highvelocity winds and very strong rains. Typhoons are characteristic of the Tropicalareas of Southwest and Southeast Asia. Hurricanes are characteristic of thetropical area of the Caribbean from Mexico down to the coast of Venezuela. Mostof the hurricanes are formed in the Atlantic ocean and they continue growing inforce as they advance toward the Caribbean. Finally they lose strength when theyenter the continent. The hurricanes are a very important source of erosion inCentral America and Mexico.

Wet and Drought SeasonsClimate changes along the year form very well defined seasons. Regions closest tothe equator may experience two rainy seasons and two dry seasons a year. Awayfrom the equator the year may split into single rainy and dry seasons. In someareas there is no rain most of the year, with a very long drougth season. When rainfinally comes there are events of very high erosion rates.

Case history 1: The rains of December 1999 in Venezuela. In only two days,there was a rainfall of 1,400 millimeters (56 inches) in an area where the averageprecipitation is less than 500 millimeters (20 inches) per year (Salcedo, 2002). Theresult was a catastrophic process of erosion, avalanches, sedimentation and more


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than 30,000 people dead. The erosion depth in beds and margins of waterways,reached values of up to 15 meters (50 feet).Some of the main characteristics of these torrential flows were the following:More than 15 basins presented torrential flows and processes of intense erosion insimultaneous form. The total volume of erosion in the event was of more than 20

million cubic meters. Flow velocities were higher than 7 m/sec (Chart 1). Thetorrential flows were able to transport rock blocks with weights between 250 and400 tons. It should be kept in mind that the event has a period of recurrence ofmore than 100 years.

Chart 1 Physical properties of torrential flows during the 1999 erosion event atVenezuela (Salcedo, 2002).

Creek Volume offlow

m 3/seg

Dynamicviscosity (kg.sec/m 2)

Unit weight ofwater soil mix

kg/cm 3

Gradient%

Velocitym/sec

Guanape 586 0.44 2.430 6.0 9.4San Julian 3192 0.47 2.450 8.0 15.0CerroGrande

1200 -- 2.000 7.0 8.0

Carmende Uria

1300 -- 2.200 6.7 7.0

3. Geotechnical factors.

The high weathering level of the rocks have produced residual soils of greatthickness, which present high susceptibility to erosion in most cases. According to

the type of parent rock the susceptibility to the erosion of the residual soil varies.The soils that present larger erosion rates are those produced by metamorphicrocks (schists) and those of carbonaceous or pirite shales and soils of volcanicorigin. Chart 2 shows Relative Erosion according to the rock type that producesthe residual soil.

Chart 2. Depth of weathering and relative erosion of residual soils.

Parent rock Depth of weathering(meters)

Relative Erosion 1

Schists and

metamorphic rocks

5 to 20 5.0

Volcanic rocks 5 to 20 4.5Carbonaceous or piriteshales

3 to 15 3.0

Granites 5 to 20 2.5Sandstones 1 to 5 1.0Claystones 1 to 5 0.51 Relative erosion 1.0 is the erosion of an alluvial medium size sand


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Case history 2: Hong-Kong 1982. During the rains that took place in Hong-Kongthe days 28 and 29 of May of 1982, the profiles of residual soil of the slopes ofHong-Kong, with thickness higher than 5 meters, were totally saturated and thesoils flowed as a liquid, generating a catastrophe. The levels of average annual

rainfall in Hong-Kong are high but this rainy event can be considered an anomaly.However, these events repeat with a period of recurrence of less than 10 years(Chart 3).

Chart 3. Approximate relationship between the intensity of rainfall and catastrophicevents of erosion and landslides in Hong-Kong (Brand, 1985).

EROSION AND

LANDSLIDE EVENT

FREQUENCY

DISASTROUS 1 IN 5 YEARSSEVERE 1 IN 2 YEARS

MINOR 3 IN 1 YEAR

NIL ---

4. Altitude and topographical factors

The presence of mountains, such as the Himalayas in Asia and the Andes inSouth-America, jointly with the hydrological and climatic factors form a combinationthat can generate erosion problems of great magnitude.- The hydrology and the characteristics of the vegetation change with the altitude.At certain altitudes rainfall reaches its maximum (Figure 3) making erosionproblems bigger.- The high volume of the discharges and the high erosion rate of the residual soilsconvert the mountain rivers, into torrents, characterized by high speed and thetransportation of great quantity of sediments of large size. Most of the rivers oftropical mountains during the rainy seasons reach flow velocities higher than 5

m/sec and transport large volumes of coarse sediments.- The formation of debris flows. When an event of anomalous rain occurs in atropical semi-dry area usually a large quantity of debris flows take place. It is achallenging to manage erosion problems produced by debris flows.

10070

40

0

300200

100

0 H O U R L Y R A I N

F A L L ( m m

)

2 4 - H

O U R R A I N

F A L L ( m m

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Figure 3. Differences in the historical maximum rainfall in 24 hours at differentaltitudes in the same area in the foot of the Andes in Colombia(Suarez 2001).

5. Some basic details to take into account in the design of erosion controlworks in tropical areas.

Hard structures are preferred-For the erosion control in tropical areas generally hard structures are preferred asthey are able to resist velocities higher than 5 m/sec, and important volumes ofsediments present in the flows. Some of the most common materials used forerosion control are: rock blocks, Rip-Rap, grouted rip-rap, gabions, concrete bagsconcrete blocks and sprayed concrete.

Soft structures have to be combined with tropical vegetation-Soft structures such as the TRMs or the ECRMs have presented problems inmany occasions. Some erosion control mats do not behave well in tropical climatesand become brittle a few years after installation. Although high strength productsare being developed, it is possible that stronger products will be needed in order tobe successful in tropical countries. An alternative is the combination of theseproducts with deep root tropical vegetation species, of high resistance to erosionsuch as the vetiver grass.

It is difficult to establish vegetation on steep slopes in the tropics-There have been some problems with very steep slopes where the establishmentof vegetation is very difficult.(Li et al, 1999). The erosion control mats becomeineffective if not securely fixed to the slope surface. Hydroseeding with erosioncontrol mats alone may not be effective in establishing a reliable, long termvegetation cover in steep slopes.Barker(1999) mentions some of the general problems in vegetation establishmenton steep slopes in the tropics:


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Climate: drought, exposure to wind, slope moisture burnt from sunrise onwards.Soil Conditions: acidity, low nutrient levels, desiccation, heavy leaching rates, higherosion rates from intense rainfall, very high or very low weathering.

Use of the great vegetation bio-diversity of the tropical areas

-In a single hectare of tropical forest there can be more than 300 different species.The bio-diversity is such that professionals in erosion control know of hundreds ofvegetable species that possess bioengineering properties required for erosioncontrol. Some of these plants have very deep roots and have a great capacity tocontrol soil erosion.Nevertheless there are only a few species where research knowledge has beendeveloped. People who live in rural areas generally know better the native speciesthan the professionals in erosion control. Most of the research on tropicalvegetation for erosion control and slope stability, has been carried out in HongKong (Greenway 1998, Webb 1991, Geotechnical Engineering Office, 1994).

Use site and local specific species-A site may have some local vegetation species characteristic of the place. Theuse of specific species, instead of a standard design, has many advantagesincluding better chances of survival for the vegetation, especially for sites withadverse ground conditions. Semidry-areas generally do not accept vegetationfrom high rain areas. Most tropical vegetation species only adapt at certainaltitudes.

Use shrubs-In tropical areas, there is a great diversity of shrubs. The use of shrubs instead oftrees or grasses helps to solve erosion control problems in steep slopes. However,there is a limited knowledge about local shrubs in most places, and it isrecommended to carry out specific studies on local species before using them on alarge scale.

Use Live staking-The use of live stakes and live poles of trees and shrubs has been very successfulin many cases. A large quantity of tropical species reproduce using stakes. Theuse of bamboo stalks, or some other species such as Gliricida sepium, is verypopular in most tropical countries.

Proprietary products for erosion control are generally of high cost-The cost of the products for erosion control developed in the industrializedcountries is very high in most tropical countries. Their use is economically feasibleonly in large oil or mining projects. The communities and the entities of the statethat have to manage problems of erosion control generally appeal to solutions withlocal products such as non-industrialized gabions and empiric bioengineeringworks.However, products using natural local fibers have been developed. It is the case ofthe natural fiber known as fique in South-America. This tropical fiber similar to


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jute is used locally to produce erosion control blankets, at costs that areaffordable for the common infrastructure projects (Duque, 2002).

Case history 3: The Panama Canal . The soils along the Panama Canal aremainly residual soils of volcanic rocks and high weathered shales. These soils

have a very high susceptibility to erosion. The works of erosion control along thePanama Canal have consisted mainly of hard works including marine mattressesmade with high resistance fibers (Figure 4), sheet pile walls, concrete piles, rip-rapand reinforced concrete blocks (Fernndez, 2002)

Figure 4 . Marine mattresses and rock blocks in the Panama Canal (Fernandez2002).

6. ConclusionsWhen professionals in erosion control have to solve erosion problems in tropicalareas, they should be prepared to confront cases of very high velocities of water,with the presence of very high volumes of coarse sediments in the flow and soilswith very high susceptibility to erosion. It is recommended to investigate thehydrology of the site and the characteristics of the soils very well. The materials touse generally include hard works and tropical vegetation species with specialproperties in erosion control.

Acknowledgments

The author thanks Carlos Fernando Diaz for reviewing the text. He also thanksDaniel Salcedo for the data from the Venezuela case, and Luis Carlos Fernandezfor the information about the works in the Panama Canal.


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References

Barker D. H. 1999. The introduction of ground and water bioengineeringtechniques to the humid tropics First Asia-Pacific Conference on Bioingeneering

for Erosion Control and Slope Stabilization. Manila. p. 3-17.Brand E.W. 1985. Predicting the performance of residual soil slopes. EleventhInternational Conference on Soil Mechanics and Foundation Engineering. SanFrancisco. p. 2541-2578.

Duque J.G. 2002. Erosion control using products of natural fiber, fique. FirstLatin American Symposium on Erosion Control, Bucaramanga-Colombia.

Fernndez L.C. 2002. Erosion control in the Panama Canal. First LatinAmerican Symposium on Erosion Control, Bucaramanga-Colombia.

Geotechnical Engineering Office. 1994. Geotechnical manual for slopes. CivilEngineering Department. Hong Kong. p. 118-121.

Greenway D.R. 1998 Biotechnical slope protection in Hong Kong. IECA Soilstabilization series Volume 5. Methods and Techniques for Using Bioengineeringto Control Erosion. p. 399-411.

Li C. O. A., Watkins A. T.,Ho Y.K. 1999. Use of vegetation as surface protectionfor steep slopes in Hong Kong. First Asia-Pacific Conference on Bioingeneeringfor Erosion Control and Slope Stabilization. Manila p. 477-484.

Salcedo D. 2002. Evidences of deep erosion during the torrential flows ofDecember of 1999, in the state Vargas-Venezuela. First Latin AmericanSymposium of Erosion Control, Bucaramanga Colombia.

Smith J.A. 1992. Precipitation. Handboof of hidrology. Madiment D.R. editor.McGraw-Hill Inc. p. 3.1 - 3.47.

Surez J. 2001. Erosion control in tropical areas. Industrial University ofSantander. Bucaramanga Colombia.

Webb R. 1991. Tree planting and maintenance in Hong Kong. Landscapetechnical group. Hong Kong Government.

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