Journal of Natural Disaster Science, Volume 23, Number 2, 2001, pp65-82

Flood and Sediment Disasters Triggered by 1999 Rainfall in Venezuela; A River
Restoration Plan for an Alluvial Fan
Tamotsu TAKAHASHI*, Hajime NAKAGAWA*, Yoshifumi SATOFUKA* and Kenji KAWAIKE***Disaster Prevention Research Institute, Kyoto University**Graduate Student, Graduate School of Engineering, Kyoto University(Received for 31 Aug., 2001)

ABSTRACT
Severe rainfall during December 1999 triggered numerous landslides, debris flows, and flash floods on the
Caribbean coast of Venezuela, resulting in a great number of deaths estimated as between 25,000 and 50,000.
The natural as well as social conditions that led to this severe disaster were analyzed. A combination of bad conditions was seen as the cause. A numerical simulation method for reproducing phenomena that occur on an alluvial fan is introduced. It was verified by a comparison of the calculated results with actual situations. This simulation method was used to assess the efficacy of possible countermeasure plans. A combination of sabo dams andchannel works was found effective on the fan of the Camuri Grande River.

1. INTRODUCTION
Continuous severe rainfall from 8 to 19 December 1999
caused disasters in the eight states of northern Venezuela. The
state of Vargas, located on the northern slopes of the Cordillera dela Costa mountain range, was the most severely damaged by landslides, debris flows, and flash floods induced by concentrated rainfall from 15 to 16 December.
The official statistics of damage compiled by the Civil
Defense Agency of the state of Vargas at the time of our reconnaissance in March 2000 were as follows (Maki, 2001): Affectedhouses, 40,160; completely collapsed houses, 20,000; affected persons, 214,000; injured persons, 2,700; dead persons, 248; missing
persons, 2,850; refugees, 43,569. The media and local governments, however, estimate the lives lost as 25,000 to 50,000, andthe International Red Cross makes it as high as 30,000. The reason
for the wide range in the death toll is due to the non-availability ofcensus data and difficulties in recovering bodies from under thickrocky debris or deep in the sea. This disaster is said to be theworst to have occurred in Latin America in the 20th century.
A reconnaissance team consisting of five specialists from various fields was dispatched to Venezuela in March 2000 by theJapanese Ministry of Education, Science, Sports and Culture. Its
official report has been published (Takahashi et al., 2001). This
paper is not a comprehensive summary of the official report, rather
it focuses on the natural and social conditions that resulted in thedisasters, makes a comparison with Japanese disasters that suggest
effective countermeasures, gives a theoretical reproduction offlooding on the fan of the Camuri Grande River, and theoreticalexaminations of several river restoration plans on that fan. Of thefour authors of this paper, the first two carried out the field investigations and the latter two collaborated with them in making thenumerical simulations.

2.

NATURAL CONDITIONS THAT PRODUCED HAZARDSThe state of Vargas is long and narrow and faces the
Caribbean Sea on the north. It is separated from Caracas to thesouth by the high Cordillera de la Costa mountain range. The central part, the most severely devastated area, is shown in Fig. 1. The
distance from the coast to the ridge is about ten kilometers, and thealtitude of the ridge is between 2,000 and 2,700m, therefore thenorthern slopes of the mountain range have an average steepnessof 10 to 15 degrees. The major rivers shown in the figure haveformed alluvial fans adjacent to the coast, but the rest of thiscoastal zone area consists of steep mountain fronts rising directlyfrom the shoreline. These alluvial fans provide the only relatively
flat areas, at first glance suitable to live on, but they are by nomeans safe. Towns, however, have been constructed on these fans,
and a road hugging the steep mountain fronts along the coast connects these towns.
The geological setting of this area has the general trend of abelt-like formation parallel to the Caribbean coastline (Chigira,2001). The zone, about 1.5km wide on the coast, is "mélange"
consisting mainly of calcareous phyllite. The phyllite has beenheavily weathered to clayey materials and forms steep slopes of 25
to 40 degrees that face the sea. Because this zone contains muchclay, water permeability is low. Vegetation in this zone is, perhaps
due to human activity, poor, mostly sparse shrubs and cactuses.
The roots of these types of vegetation are only 20 to 30cm deep.
To the south of this zone, schist and granitic gneiss zones form themain parts of the mountains.
In December 1999, because of a cold front that lingered forabout twenty days along the Caribbean coast, rainfall at theMaiquetia airport totalled 1,207mm. For the three days from the14th to 16th 914mm of rainfall was recorded. Rainfall records
from 1951 to 1997 at Macuto about 10km east of Maiquetia, show

KEY WORDS: sediment disaster, numerical simulation, Venezuela, countermeasures

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T. TAKAHASHI, H. NAKAGAWA, Y. SATOFUKA AND K. KAWAIKE

an annual mean rainfall of 589mm.
Although the ground level rainfall record is available only for
Maiquetia airport, the homepage of the USGS-CINDI (Center for
Integration of Natural Disaster Information) gives a two-dimensional distribution of rainfall from 15 to 17 December based on
automatic estimation of GOES-8 satellite data (USGS-CINDI
team, 1999). According to the record at Maiquetia, the rainfall on
17 December amounted to only 2.9mm. In practice, therefore, thedistribution map of CINDI shows the amount from 15th to 16th.
The rainfall estimated by CINDI at Maiquetia is 300 to 480mm.
The rain gauge record at Maiquetia for this period gives 774mm.
To cancel out the difference between these data, the rainfall estimated by CINDI was modified by multiplying by a factor of 1.9,

Fig. 1and the distribution map was enlarged, giving the isohyetal map in
Fig. 2. Although the accuracy of this isohyetal map is poor due tothe extreme enlargement and marked modification of the estimatedvalues, the general rainfall distribution tendency can be understood.
Figure 2 shows that the most severe rainfall was in the Macuto
area, decreasing to the east. It also shows that the strongest rainfall
was along the coast, weaker in the higher mountain areas. This situation is indirectly supported by the fact that, in the Caracas areaon the opposite of the mountain ridge, no severe flooding occurred.
The characteristic geological and rainfall distributions
described above therefore must be the main factors that determinedthe distributions of the numbers and positions of landslides andCentral part of the state of Vargas and metropolitan Caracas.

Fig. 2Major river systems and isohyetal lines of the rainfall

FLOOD AND SEDIMENT DISASTERS TRIGGERED BY 1999 RAINFALL IN VENEZUELA; A RIVERRESTORATION PLAN FOR AN ALLUVIAL FAN

debris flows. As seen in Photo 1, landslide density is very high inthe mainly weathered phyllite hilly area along the coast. Althoughlandslide density is very high, the scale of the respective landslides
is small; about 20m wide, less than 100m long and shallow. The
main road constructed along the coast was littered with thedeposits of landslides, and its function destroyed. The very dense
landslide area, however, is almost completely to the west of the
Naiguata River. This situation correlates well with the decrease in
the rainfall to the east.
Landslides that occurred on schist and gneiss slopes were akind of rockslides, and many of their scars are connected withriverbeds. Little debris remains on these scars or the riverbeds.
The landslide density on these slopes is much lower than in theformer area. Presumably this is because these areas are covered
with good vegetation and the rainfall was less than in the coastal
area.
Dark colored sands, derived from landslides in the phillitezone, cover the narrow beaches at the foot of the steep mountainfront, whereas white sands, gravel, and boulders derived from
granitic gneiss areas cover the alluvial fans of the major rivers(Fig. 2). They were transported to the fan area by debris flows or
flash floods. The shorelines of the alluvial fans have advanced offshore about 200m.

67

3. SOCIAL CONDITIONS THAT RESULTED IN
CATASTROPHIESThe Venezuelan population is estimated to be 23.5 million,
and the land area 912 thousand square kilometers. About 75% of
the land consists of the tropical rain forest of the Orinoco Riverbasin, and most of the population lives in the northern highlands
and valleys. The capital city, Caracas, is on a 900m-high basin andhas about 10% of the country’s population. Congested slums
occupy the mountain slopes that surround the Caracas basin.
The central part of the state of Vargas is separated from themetropolitan Caracas by a very high mountain range (Fig.1), but iseasily accessible from Caracas via a pass. Maiquetia, the principal
international airport of Venezuela, and La Guaira, a major trading
seaport, are located in this state. Due to these advantageous geographical conditions and the state’s beautiful coastal and mountainsettings, the alluvial fans of the major rivers located between thefoot of mountain range and the coast have been highly developed
as resort towns. Development of the area has caused an inflow ofpeople for employment. These newcomers could not find flat land
suitable for building, so many shantytowns have been constructed
on very steep slopes highly vulnerable to landslides near the developed alluvial fans (Photo 2) or on canyon bottoms clearly vulnerable to floods (Photo 3). Luxurious high-rise hotels and condominiums, yacht harbors, and slums constitute the strange, characteristic
scenery of this area. These towns were the main sites of the disasters.
4. PRINCIPAL TYPES OF DAMAGE

Photo 1. Densely distributed landslides on steep phyllite slopes
near the coastline.

Photo 2.

A shantytown on a very steep, weak slope.

4.1 Destruction due to direct landslide strikes
As stated, low-income people are obliged to live on highly
hazardous land, typically on the very steeply sloping areas of themountain fronts that rise directly from the Caribbean Sea and sideslopes of developed alluvial fans. Because the landslide density inthese areas is very high, as is the density of the houses constructed,
these houses had high probability to be struck by falling earth
blocks or to undergo the sliding of the ground on which they werebuilt. Some houses were destroyed by the collapse of houses
above them. Poor drainage systems in these shantytowns also
could trigger landslides.

Photo 3.

A shantytown on a canyon bottom.

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T. TAKAHASHI, H. NAKAGAWA, Y. SATOFUKA AND K. KAWAIKE

Because the speed of weathering in this phyllite area is rapid
and unstable slopes remain around the scars of landslides, theseareas are highly vulnerable to similar future disasters.
4.2

Obliteration due to debris flow flooding on canyon bottoms
Narrow canyon bottoms between steep walls were filled withnumerous low-income houses which were obliterated by debris
flows exceeding the entire canyon’s width. Some houses were hit
directly and destroyed by landslides that occurred on the canyon
walls. This type of damage occurred along the Cerro Grande(Photo 4) and Uria rivers (Photo 5).

In the former, about a 800m long, 200m wide reach immediately upstream of the fan-top, forms a canyon plain, which was
crowded with numerous houses, except for a narrow channel thatdrained away the normal river water. Almost all these houses were
swept away by a debris flow.
In the latter, a long stretch of the coastal cliffs had been
abruptly hollowed out, forming a canyon 700m long and 220m
wide at the mouth of the Uria River. This plain was completely
covered by a shantytown, Carmen de Uria (Photo 5). Its two central blocks were obliterated by debris flows (Photo 5). The houses
remaining are by no means safe (Photo 6). More than five thousand people are said to have been killed in this town.

Photo 4.

Situation before and after the disaster at Cerro Grande

Photo 5.

Situation before and after the disaster at Carmen de Uria

FLOOD AND SEDIMENT DISASTERS TRIGGERED BY 1999 RAINFALL IN VENEZUELA; A RIVERRESTORATION PLAN FOR AN ALLUVIAL FAN

4.3 Damage due to flooding by debris flows/flash floods
Typical examples can be seen on the alluvial fans of the San
Julian (Caraballeda) and Camuri Grande (Camuri Grande) rivers.
The former fan was swept by several debris flows which left acover 1.2km2 wide and a few meters-thick deposit that included
boulders 2 to 3 meters in diameter. This fan, the largest in the central part of Vargas, has been highly developed as a resort town, andhas many high-rise hotels and condominiums with small individual
houses between them (Photo 7). Many lower situated houses were
flushed away or buried to the roof, and some high-rise concretebuildings partly collapsed because of being hit by big boulders(Photo 8).
Photo 9 shows the Camuri Grande alluvial fan. A tributary,
the Miguelena River, with the same comparative scale as the mainriver, joins it near the fan-top, but sediment runoff from that tributary was little. The sediment that covered the fan therefore almost
all was from the main Camuri Grande River. The main river channel changes direction perpendicularly eastward immediately
upstream of the fan-top, and, after joining the Miguelena River, itagain changes direction perpendicularly northward. Thus, the river
channel has a "crank" shape that creates an obstacle to flood flow.
On the left side of this crank channel (looking downstream) is thecampus of Simon Bolivar University.

Photo 6.

Destroyed houses at Carmen de Uria

Photo 7.

Alluvial fan of the San Julian River

69

A debris flow that carried many boulders more than 1m indiameter stopped in the upstream region of the confluence with theMiguelena River, and the subsequent flash flood spilled over thedeposit and separated in two. One overflowed the main channel
before entering the crank channel and continued to flow straight
into the university campus, destroying buildings or burying themto the roof. This flow bore much large woody debris.
Some of the flow that spilled over the debris flow deposit followed the crank channel and joined the flood flow from theMiguelena River. Part of that flow overflowed the right bank of
the channel in the neighborhood of the second change in direction.
It buried the first floors of a group of reinforced concrete apartments near the channel bend then continued its run down along themountain foot to the sea destroying and burying numerous small
houses.
The rest overflowed and continued along the crank channel
destroying houses along the main connecting road with othertowns then flowed out to the sea, extending the coastal line about
250m off shore.
The diameters of the sediment grains deposited on the fans of
the San Julian and the Camuri Grande rivers differed, and the damage aspects also differed. Very large boulders predominated, andthe destructive power of the flow was much greater on the fan of

Photo 8.

Building collapsed due to the direct strikes by boulders.

Photo 9.

Alluvial fan of the Camuri Grande River70

T. TAKAHASHI, H. NAKAGAWA, Y. SATOFUKA AND K. KAWAIKE

the San Julian than on the fan of Camuri Grande River. Many
houses on the San Julian fan were obliterated, and even concretebuildings collapsed, whereas on the Camuri Grande fan the majority of the houses were buried by sand. To clarify the reasons forthese differences, longitudinal profiles of the main channels ofthese rivers are shown in Fig. 3, in which altitudes and distancesfrom the river source are standardized to the height of the source of
the San Julian River, 2170m. Profiles of the Uria, Cerro Grande,
and Naiguata rivers are compared in the same figure. Damage on
the Naiguata fan was not slight, but it was the least in these fiveriver basins. Figure 3 shows that every river channel is steeper
than the slope of the debris flow generation limit (15 degrees) in
the upstream reaches. All these rivers actually produced debris
flows. Arrows show the positions of the upstream limits of residential areas in these river basins. In order, residential areas in theUria, Cerro Grande and San Julian basins are closer to the debris
flow generation reach and are on land steeper than 3 degrees
through which large scale debris flow can pass. In contrast, theresidential areas on the Camuri Grande and Naiguata fans are far
from the debris flow generating reach, flatter than 2 degrees which
debris flows do not reach (Takahashi, 1991).
5. COMPARISONS WITH JAPANESE SITUATIONSIs this Venezuelan disaster unparalleled in history in terms ofnatural phenomena? To answer this question and to evaluate
methods for restoration, the phenomena are compared withJapanese ones.
Figure 4 shows the relationship between specific sedimentyields and basin area for the seven rivers in Vargas and the same
relationship in conspicuous Japanese cases (Mizuyama, 1985).
Because the exact sediment yield volumes from the Vargas rivers
are not known, a rough approximation was made by multiplying
the area of sediment flooding obtained from aerial photographs by
2m. This figure shows that although the sediment yields from the
major rivers in Vargas are of the largest scale, they are not markedly larger than those in the Japanese examples. That is, this scale
sediment runoff has to be taken into account when planning countermeasures in such debris flow-prone river basins.

Fig. 3 Longitudinal profiles of the major rivers and positions of
upstream boundaries of residential areas

Resemblances in topography and the hazard areas recall thedisaster of 1938 in Kobe district, Japan, called the great Hanshin
flood disaster. Figure 5 shows the river networks originating in the
Rokko mountain range, the cumulative rainfall distribution from
July 3 to 5 1938, and the main sediment deposition areas.
Comparison of this figure with Fig. 2, shows the resemblance
between the conspicuous disaster range and distance from the coastto the mountain ridge. The highest peak of the Rokko Mountains
is only 931m, whereas that in Vargas is 2,740m. In Vargas, themountains are very close to the coast, whereas at Rokko the torrent
outlets are 2 to 3km upstream from the river mouths. The longitudinal inclinations of the torrents inside Rokko Mountain are similar
to those in Vargas, therefore sediment transportation in Rokko
should be similar to that in Vargas. The total sediment volume in
Rokko is estimated as 5 ~ 7.7×106 m3 and that from the major
Vargas rivers of the order of 6 ~ 7×106 m3. The sediment volumes were similar, but the damage in Rokko was much less: 616
died, 1,410 houses were obliterated, 854 houses were buried, and2,213 houses were partly destroyed. These differences presumably
are due to the fact that at that time developed areas in the Rokko
area were limited to flat zones distant from the outlets of thegorges.
After the 1938 disaster, as structural countermeasures, sabodams were intensively constructed. Totally, 174 sabo dams had
been finished when a matching rainfall event struck the same district in 1967. In 1967 the number of landslides was similar to thatin 1938, but due to the functioning of the sabo dams only 0.5 ×106
m3 of sediment flowed down to the alluvial fans, and other 0.17 ×
106 m3 overflowed river channels downstream. In the 1967 disaster, 98 people died and 363 houses were completely destroyed.
Whereas in 1938, the majority of injuries and deaths were downstream of the outlets of the gorges, in 1967, people died inside themountain area because by then residential areas extended well into
the mountain. One must take into account that the upgrading of
preparedness for disasters by the construction of structural countermeasures, sometimes, expecting too much of these structure’sfunctions, increases the number of people living in dangerous
zones.

Fig. 4 Relationship between specific sediment yields and basin
area

FLOOD AND SEDIMENT DISASTERS TRIGGERED BY 1999 RAINFALL IN VENEZUELA; A RIVERRESTORATION PLAN FOR AN ALLUVIAL FAN

6.

COUNTERMEASURES AGAINST FUTURE DISASTERS

Our reconnaissance showed that little sediment remains on the
upstream riverbeds of the major Vargas rivers. Until considerable
sediment is accumulated no episodic event comparable to that of1999 will occur. The sediment volume in the 1999 event is, however, not especially large when compared with other episodes. The
matching volume of sediment runoff therefore should be taken into
account when planning countermeasures.
A priority policy should be the removal of slums built on thevery steep slopes close to the coastal area. The president ofVenezuela announced as early as Dec. 20, 1999 that permanent
resettlement sites would be prepared in some remote states with
agricultural land and housing for the disaster victims (Maki, 2001).
Because many of those people had been living in these slums toescape the hardships of life in remote states, the success of thispolicy is questionable.
The canyon plains of the Uria and Cerro Grande rivers will
not be suitable residential areas unless sabo dams of sufficient
capacity are constructed.
The alluvial fan of the San Julian River has a slope that allows
large scale debris flow passage. Therefore, if a channel with sufficient conveyance capacity is constructed, most of the alluvial fan
area will mostly become safe. Debris flow material, however, will
necessarily be deposited at the river mouth and deposition willtend to go up the channel. A sand-pocket or sabo dam to store
excess debris will be necessary.
Debris flows in the Naiguata and Camuri Grande rivers willstop upstream of the fantop, and immature debris flows or flash
floods will run down to the fan area. Such flows will deposit
material within the flat reach of channel works before flowing into
the sea. Sabo dams of sufficient capacity therefore are needed
upstream of the fan-top.
The efficiency of such structural countermeasures must be
evaluated quantitatively by computer simulation as described

Fig. 571

below.
7. PROPOSED COUNTERMEASURE PLAN FOR THE
CAMURI GRANDE BASIN
7.1 Estimation of debris flow hydrograph in the river basin
For the design of countermeasures, a debris flow event equivalent to that of 1999 is needed. Because no quantitative data forthe debris flow that caused that disaster are available, the firstthing is to determine the design debris flow quantitatively; i.e.,
reproduction of the phenomena by computer. The simulation
model used combines flood runoff analysis by the kinematic wave
method and sediment runoff analysis based on various fundamental equations (Takahashi et al., 2001a).
7.1.1 Fundamental equations for routing highly sedimentladen flow
(1)
Equations used in flood runoff analysis by the kinematicwave method
Flood runoff analysis of a mountain slope can be made by thekinematic wave runoff model:
qs = αk h 5/3(1)

αk = sin θs /n e(2)∂h ∂qs+=r∂t ∂x e(3)

1/2

where qs = water discharge per unit width, h = water depth, θs =slope gradient, ne = equivalent roughness coefficient, re = effective
rainfall intensity, t = time, and x = distance from the slope’s top.
Neglecting the increase and decrease in discharge due to erosion and the deposition of sediment, flood runoff analysis in theriver channel is made by the following equations:

Sediment disaster of 1938 Kobe district, Japanq = 1 h5/3tan1/2θw
nm(4)∂h 1 ∂qB qs+=∂t B ∂x
B

(5)

72

T. TAKAHASHI, H. NAKAGAWA, Y. SATOFUKA AND K. KAWAIKE

where q = water discharge per unit width of river channel, nm =Manning’s riverbed roughness coefficient, B = channel width, andθw water surface gradient.
(2) Equations giving the competence of flow to carry sediment
Erosion and deposition are neglected in the flood runoff
analysis, but, actually, the riverbed material together with confinedwater within the bed will be entrained into flow, provided the load
is still below the competence of flow and erosion takes place. In
contrast, if the competence of flow is insufficient to transport theload, sediment will be deposited. Therefore, erosion and deposition will change the flow discharge and simultaneously the particle
composition in the flow as well as the bed.
To consider variation in particle size distributions in flow andbed, grain size is divided into ke grades, and the diameter of thegrade k grain is written dk . Particles from grades k = 1 to k = k1 aredefined fine and considered to constitute a fluid phase if containedin the flow. Particles from grades k = k1 +1 to k = ke are definedcoarse particles. The volumetric concentration of coarse and fine
fractions, density of the interstitial muddy fluid, and mean diameter of the coarse particles in the flow respectively are expressed byke

CL = Σ Ck(6)

k=k1+1
k1

CF = Σ Ck / ( 1 -CL )

(7)

k=1

ρm =ρ+σ-ρ k1
ΣCk =ρ+ (σ-ρ)CF
1-CL k=1(8)kedmL = Σ dkCk /CL(9)

k=k1+1

where ρ= water density, CL = volumetric concentration of coarse
particles in the flow, CF = volumetric concentration of the fine particle fraction in the interstitial fluid, Ck = volumetric concentrationof grade k particles in the total water and sediment volume.
The competence of the flow to transport sediment depends onthe flow surface gradient. Herein, the majority of particles in theflow are considered too large to be suspended by the turbulence inthe flow. Competence therefore is described by the following
equilibrium concentration formulae (Takahashi, 1991):
If tanθw > 0.138, a stony type debris flow occurs, and
CL∞=

(10)

If 0.03 < tanθw < 0.138, an immature type debris flow occurs, and

｛

｝

(11)

occurs, andττ(1+5 tanθw) tanθw
1-α02 *c 1-α02 *cτ*τ*σ/ρm -1(12)

where CL∞= equilibrium coarse particle concentration in the flow,
φ= internal friction angle of the sediment,σ= particle density,andτ*c = 0.04×101.72 tanθw｛
2 0.425-(σ/ρm)tanθw /(σ/ρm-1)｝
α02=
1-(σ/ρm ) tanθw / (σ/ρm-1)(15)

in whichτ*c = non-dimensional critical shear stress,τ* = nondimensional shear stress, dmL = mean diameter of movable coarse
particles on the riverbed.
In the case of bed load transport, whether particles of diameterdk are movable can be judged by a modification of Egiazaroff’s
critical shear stress concept:
When dk /dmL > 0.4,

2
u*ck2
dk
log 19=2
u*cmL
log(19dk /dmL) dmL(16)

When dk /dmL < 0.4,

u*ck2
= 0.85u*cmL2(17)｛｝

where dmL = mean diameter of coarse particles on the bed surface,u*ck = critical friction velocity of the k th grade particle, u*cmL = critical friction velocity of a particle whose diameter is dmL (u*cmL =τ*c (σ/ρm -1)gdmL ).
(3) Riverbed particle composition
Because the particle size composition of riverbed material is
not necessarily the same as the particle size composition in a flow
above the bed, composition in the flow will be changed by entrainment of bed material. To determine the particle size compositionof runoff sediment, the particle composition of the riverbed withwhich the flow exchanges particles must be known.
Assuming that the total volume of grade k particles on a bed isVk, the ratio of this grade’s particles to the total particles (coarse
plus fine ones) is
fbk =

Vk
VL+VF

(18)

where
ke

k1

k=k1+1

k=1

VL = Σ Vk , VF = Σ Vk

(19)

The ratio of grade k particles (grade k signifying coarse material) to the total coarse particles, fbLk , isf (V +V )ff
fbLk = bk L F = kebk = bk
VL
F
Σ fbk

(20)

If the ratio of the total coarse particles in the bed, F, satisfies
F>2

If tanθw < 0.03, a turbulent water flow with bed load transportCL∞=htanθw(σ/ρm-1)dmL

k=k1+1

ρm tanθw(σ-ρm)(tanφ-tanθw)

ρm tanθw
CL∞= 6.7(σ-ρm)(tanφ-tanθw)

τ* =

(21)

the coarse particles form a skeletal structure, and fine particles canbe stored in its void spaces. In equation (21), C*L is the volumeconcentration of all the coarse particles when the bed is composed
only of coarse particles, and C*F is that of fine particles when it iscomposed only of fine particles. For simplicity, both C*F and C*L
are assumed to be 0.65, and the smallest F that satisfies equation(21) is 0.74. In this case, the volume ratio of the fine particles
assumed to be stored only in the void space of the coarse particle
framework, C*k , is

(13)
C*k =
(14)

C*L
C*L+C*F -C*L C*F

C*L VkC= *L
1-C*L VL 1-C*L

fbk
ke

Σ fbk
k=k +11, k = 1〜k1

(22)

FLOOD AND SEDIMENT DISASTERS TRIGGERED BY 1999 RAINFALL IN VENEZUELA; A RIVERRESTORATION PLAN FOR AN ALLUVIAL FANWhen the ratio of the fine particles is greater than 26%, coarse
particles can no longer form a skeleton and will lie scatteredamong the accumulated fine particles. These fine particles will
form a skeletal structure with the volume concentration C*F , but its
void space will be too small to store coarse particles. In this case,the ratio of fine particles is(23)

Then, the volume concentration of the coarse particle fraction on
the bed is(24)(4) Erosion and deposition velocity
The erosion velocity equation that can be used regardless thetype of sediment transport; debris flow, immature debris flow, or
bed load transport (Takahashi, 1991) is
σ-ρm
tanφ
isbo
= K sin3/2θw 1C-1
ρm L tanθw
gh

｛

1/2

｝

tanφ
h
× tanθ -1 (CL∞- CL ) d
mL
w

(25)

where isbo = erosion velocity of a riverbed whose mean particle
diameter is dmL, K = a constant, and g = acceleration due to gravity.
In debris flow and immature debris flow types, the largest particle that can be moved due to the effect of flow is assumed to havea diameter that is the same as the depth of flow. Under thisassumption, if dk 2+1 > h > dk 2 is satisfied, the ratio of erodible
coarsekk
fbLk . The
sediment is Σ fbLk , and the bulk erosion velocity isbo k=kΣ+1
k=k +1
substantial volume of coarse particles belonging to grades k1 < k <k2 is C*L fbLk . The erosion velocity for each grade of particle whendk 2+1 > h > dk 2 is
2

2

1

1

k2

k1< k< k2 ; isbk = isbo fbLk C*L Σ fbLk
k=k1+1; isbk = 0

}

(26)

Under bed load transport type, the critical tractive force offlow determines the size of the erodible particles on the bed.
The erosion velocity of fine particles, k < k1, isk2

isbk = isbo (1-C*L)C*K Σ fbLk
k=k1+1(27)

When the volumetric concentration of coarse particles, CL, at a
certain position is larger than the equilibrium concentration, CL∞, atthat position, the coarse particles will be deposited on the riverbed.
The excess volume of particles per unit area is h (CL - CL∞). If thisamount of sediment is deposited within the period ( h /u )/δd, deposition velocity is expressed by equation (28). The deposition
velocity of each coarse particle grade is given by equation (29);
CL∞-CL q
C*L
h

(28)

C
isbk = isbo k C*Lmax (k > k1)
CL

(29)

isbo =δdwhereδd = a constant, C*Lmax = volume concentration of the coarse
particles in the maximum compacted state, and u = mean flow
velocity.

Ck
1-CL

(30)

Erosion and deposition velocities in bulk that include void
space are given by equations;
isb =

FC*F
C*L =
C*FF+1-F

k> k2

If settling due to its own density is neglected, the fine particle
fraction mixed with water is considered to constitute a fluid phaseand trapped within the voids of the coarse particle’s skeleton
formed by the deposition of coarse particles. Then, the deposition
velocity for fine particles; k < k1, isisbk = isbo (1-C*Lmax)

f C
C*k = bk *F
1 -F

73

isb =

1 ke
Σ1+1isbk
C*L k=k
1

(31)

ke

Σ+1isbk
C*Lmax k=k

(32)

1

(5) Formulae describing flow discharge
The sediment concentration in a flow necessarily influences
resistance to that flow. According to Takahashi (1991), the unit
width discharge in stony debris flow isq=2
g σCL+(1-CL)ρm 1/2 C*L 1/3-1 h 5/2 tan1/2θw
σ5dmL 0.02
CL

｛

｝
｛

｝

(33)

For an immature debris flow it isq=

0.7 g 5/2 1/2
h tan θw
dmL

(34)

For turbulent water flow that transports bed load, it is given
by Manning’s formula;q=

1 5/3 1/2
h tan θw
nm

(35)

where q = the water plus sediment discharge per unit width.
Equation (35) also is used for a flow laden with highly concentrated sediment that satisfies the condition h /dmL > 30. This is becauseof the predominance of turbulence which suspends particles, inwhich case the resistance formula does not differ from that ofwater flow.
(6) Continuity equations
The continuity equation for the total volume of water plus
sediment is∂h 1 ∂qB
q+= isb K1+ s∂t B ∂xB(36)

where B = width of flow, isb = the erosion (isb > 0) or deposition ( isb< 0) velocity, qs = inflow discharge per unit length of channel, andK1 = a coefficient given as follows:
K1 = C*L + (1 - C*L ){C*F + (1- C*F ) sb} ; (isb > 0)
K1 = 1 ; (isb < 0)in which sb = degree of saturation of the bed.
The continuity equation for each particle grade is∂Ckh 1 ∂qCkB
q+= isbk + sk
∂t
B
B ∂x

(37)

where isbk = erosion or deposition velocity, and qsk = flow rate of kth grade particles from the side banks per unit length of channel.
Note that because isbk does not include void space,is not equal
to isb .

74

T. TAKAHASHI, H. NAKAGAWA, Y. SATOFUKA AND K. KAWAIKE

Generally, in a river reach in which the canyon between steepside banks is wide, the flow width, B, is narrower than the canyon
width, B0 . Even in such a reach, riverbed variation occurs laterally
flat along the entire canyon bottom due to the stream channel shifting. Riverbed variation therefore is
∂z B
+
i =0
∂t B0 sb

(38)

where z = riverbed elevation.
(7) Variation of riverbed grain sizekisbkin a flow is
Exchange of particles on the bed with thoseΣ
k=1
assumed to take place in the surface layer of a bed whose thickness
is δm . Variation in the ratio of grade k particles on the surface
layer of the bed ise

For deposition:
For erosion:∂fbk B isb J0 f0k - isbk - isb (J0 - J) fbk
=
∂t B0
δm J
∂fbk B - isbk + isbC* fbk
=
∂t B0
δm J

(39)
(40)

where J = solids concentration in the bed’s surface layer, J0 = J in
the lower layer beneath δm . They are given by the equations
J = C*L + (1 - C*L) C*F(41)

J0 = C*L0 + (1 - C*L0) C*F0(42)7.1.2 Application to the Camuri Grande River
Two comparable scale rivers, the Camuri Grande (23.8km2)
and Miguelena (19.0km2) form the Camuri Grande River basin(Fig. 6). This basin was divided into 525 sub-basins for a flood
runoff analysis, and the river channels divided into 1157 sections,Δx = 100 m, to calculate the various hydraulic quantities in eachsection. Canyon width, B0, determined from aerial photographs
and our reconnaissance data is distributed as in Fig. 6. Our fieldsurvey also found that before the flood the riverbed had stored sediment about 5m thick, whose particle size distribution is shown in
Fig. 7. In the calculation, particle size was divided into 15 grades.
The fraction smaller than 100μm (10% of the total particles) was
considered the fine fraction which constitutes the muddy interstitial water.
The estimated rainfall intensities are given in Fig. 8 (Onda andShibayama, 2001). Taking into account the amount of antecedent
rainfall, the intensities in Fig. 8 are considered entirely effective torunoff.
The spatial grid-size in the calculation was set at Δx = 100 m.
Time intervals were changed depending on the flow velocity; Δt ==0.1s for velocities of 60cm/s or more, Δt =0.2s for those of 4060cm/s, Δt =0.4s for 20-40 cm/s, Δt =1s for 10-20cm/s, Δt =2.5s
for 5-10cm/s, and Δt =5s for 0-5cm/s.
Values of the other quantities were set; C* = C*L =C*F = C*Lmax =C*F max = 0.65 ,σ= 2.65 g/cm3, ρ= 1.0 g/cm3, tanφ=0.7, and δm =0.5m. The degree of saturation in the bed and Manning’s roughness coefficient were set as sb = 1.0 and nm = 0.03 for B0 > 50 m; sb= 0.8 and nm = 0.04 for B0 = 20-40 m; and sb = 0.8 and nm = 0.05 for
B0 = 10 m. The equivalent roughness coefficient on the slope, ne ,is assumed to be 1.0, and the coefficient in the formulas for theerosion and deposition velocities, K andδd , respectively are
assumed to be 0.5 and 0.0002. As the downstream boundary condition, the sea surface level 500m off the river mouth was set as
constant.
Because the number of landslides was small, the sedimentsupply from the side banks was neglected, sediment runoff being
attributed only to bed erosion.
Figure 9 shows temporal variations in the rates of flow andsediment runoff and in the mean diameters calculated at the con-Fig. 7

Fig. 6

River channel system and channel widths used in the simulation

Fig. 8

Particle size distribution in the riverbed material

Estimated hourly rainfall intensity in the Camuri Grande basin

FLOOD AND SEDIMENT DISASTERS TRIGGERED BY 1999 RAINFALL IN VENEZUELA; A RIVERRESTORATION PLAN FOR AN ALLUVIAL FAN

fluence of the two rivers. A little before 6:00 a.m. on December
16, a debris flow ran out from the Camuri Grande River. It had apeak discharge of about 700m3/s and a sediment concentration
almost equal to C* . A little after that there was a debris flow from
the Miguelena River which had a peak discharge of about 200m3/s.
The durations of these debris flows were less than 30 minutes, andafter them flash floods with comparatively thin sediment concentrations occurred for more than 10 hours. Mean particle size in thefirst debris flow was calculated as larger than 20cm. Althoughoccasionally mean particle size in the after floods was large, generally it was 5 to 10cm, coinciding with the mean particle size of theriverbed.
Calculated longitudinal distributions of riverbed variations
and the mean diameter on the surface of the riverbed along themain channel of the Camuri Grande River are given in Fig. 10. In
this figure, t = 0 is 3:15 a.m. on 16 Dec. At t = 2h, severe erosion
took place within the upstream river reach, but neither erosion nor
deposition occurred in the downstream reach. The characteristic
saw-tooth pattern reflects scattered junctions with tributaries atwhich locations runoff sediment is temporarily stored. At t = 4h, arise in the riverbed of about 2m is seen from the river mouth toabout 4km upstream. This agrees with our field investigation findings. The tendency for particle mean diameter to become small inthe downstream reach also agrees with those findings. The calculated results, however, show that particles with a diameter larger
than 1 meter are deposited at the river mouth, and such a large particle deposit actually occur at the fan-top. This discrepancy must
be due to the neglect of particle segregation within the flow. Thatis, particle segregation within a debris flow concentrates the largest
boulders at the forefront, which part is deposited at the fan-top.

The calculated substantial volumes of sediment runoff fromboth rivers are shown in Fig. 11. The final substantial volume is
850 thousand cubic meters (bulk volume; 1,300 thousand cubic
meters). The estimated volume deposited on the fan is
1,618,000m3 , obtained by comparisons of the ground level of the
fan before and after the flood. The difference is attributable to various causes; the error included in the estimation that uses groundlevels, the premise of calculation that the thickness of the depositon the riverbed before the flood is 5m, and the effects of landslides
being neglected. Because the area of sediment deposition on the
fan is about 0.79km2, the calculated runoff sediment volume isabout 1.6m thick. This value has the feeling of reality as comparedwith impressions gained on the spot.
7.2

Reproduction of sediment flooding on the Camuri
Grande fan
7.2.1 Fundamental equations
For the reproduction of sediment flooding, the fundamental
equations are the depth-averaged two-dimensional momentum andcontinuity equations (Nakagawa and Takahashi, 1997):∂M∂(uM)∂(vM) ∂(z + h ) τbx
+β
+β
=
∂t
∂x
ρT
∂y
∂x

(43)

∂N
∂(uN)
∂(vN) ∂(z + h ) τby
+β
+β
=
∂t
∂x
∂y
∂y
ρT

(44)

Fig. 10

Fig. 9Calculated results for hydrographs, sediment graphs andmean particle diameters of the two rivers

75

Sequential time changes in longitudinal distributions ofriverbed variation and in mean diameter

Fig. 11 Substantial sediment volume yields from the two rivers andtheir totals.

76

T. TAKAHASHI, H. NAKAGAWA, Y. SATOFUKA AND K. KAWAIKE

∂h
∂M ∂N
+β
+
= i{C* +(1-C*) sb}
∂t
∂x ∂y

(45)

The continuity of the coarse particle fraction sustained in theflow by the action of particle encounter is∂(Ch) ∂(CM) ∂(CN)
+
+
=
∂t
∂x
∂y

iC*L

｛ iC

*Lmax

(i > 0)(i < 0)(46)

The continuity of the fine particle fraction suspended in theinterstitial fluid by turbulence is∂｛(1-CL)CFh｝ ∂｛(1-CL)CFM｝ ∂｛(1-CL)CFN｝
+
+
∂t
∂x
∂y
i(1-C*L)C*F(i > 0)*Lmax)CF (i < 0)

=
｛
i(1-C

(47)

where M (= uh) and N (= vh) are the x and y components of theflow flux, u and v the x and y components of the mean velocity, β
is the momentum correction coefficient, τbx and τby are the x and ycomponents of the resistance to flow, ρT =σCL + (1 - CL)ρm . Withreference to equations (33), (34) and (35), the resistance terms are
given by the following respective equations for stony debris,
immature debris, and turbulent water flows:τbx =u u2+ v2
ρT dmL 2(48)
2
8 h ｛CL + (1-CL)ρm /σ｛｝(C*Lmax / CL)1/3-1｝

τbx =v u2+ v2
ρT dmL 2(49)
2
8 h ｛CL + (1-CL)ρm /σ｛｝(C*Lmax / CL)1/3-1｝

τbx =ρT dmL 2
u u2+ v2
0.49 h

(50)

τby =

ρT dmLv u2+ v2
0.49 h

(51)

τbx =

ρgnm2 u u2+ v2
h1/3

(52)

τby =

ρgnm2 v u2+ v2
h1/3

(53)

2

7.2.2 Calculation method and the boundary conditions
The total area of the Camuri Grande fan was divided into arbitrary, unstructured mesh areas reflecting topographical conditions
and the arrangement of buildings. For the calculation of theunknown variables in each mesh area (Fig. 12), variables M, N, u,
and v were defined on the midpoints of the boundaries of the area,and variables h, CL , and z b defined on the centroid of the area. The
flow flux between the adjacent mesh areas of a particular area wascalculated at the time step n, n+2, … and flow depths in the respective areas were calculated at the time step n + 1, n + 3, … using discretized fundamental equations; a leap-frog method. Herein, thefinite volume expressions of fundamental equations are abbreviated (Takahashi et al., 2001b).
Figure 13 shows the adopted mesh division of 2,736 areas, aswell as the thickness distribution of the deposit on the fan obtainedby comparison of the two topographic maps (1/1,000), before andafter the flood. The upper boundary of the figure is the coastline,
the lower most two horn-like projections the inflow points of the
Camuri Grande and Migelena rivers. The old river channel in thecentral part of the figure bends like a bow. A very thick deposit ofmore than 5m is on the left side of the old channel a little upstreamof the river mouth. On the basis of our field observations, this is
too thick and should be considered an error in the reading of theaerial photos. A thick cover of woods could cause such an error.
The hydrograph, sediment graph, and temporal variation in
the mean particle diameter, obtained by the flood routing describedearlier, are given as the input boundary conditions at the inflow
points of each river.
A free-dropping type condition is used as the outflow boundary condition at the river mouth and at the shoreline;
M or N = ( 2 / 3 )3/2 h gh

The constant time interval, Δt = 0.2s was used for the entire
calculation period of 25 hours; 3:15 a.m., Dec. 16 to 4:15 a.m.,
Dec. 17. The numerical quantities in the calculation were tanφ=0.7, σ= 2.65 g/cm3, C*DL = 0.65, ξ= 1.0, ρ= 1.0 g/cm3, nm = 0.03,
and δd = 0.001.
7.2.3 Calculation results and discussion
Figure 14 shows the final calculation results for the deposit
thickness distribution. In comparison to Fig. 13, the calculation

Existing criteria for these flows are given by the gradient of
the flow surface as discussed earlier.
A conservation equation of particle numbers is needed toaccount for variation in particle diameter in debris flows.
iC*Lmax
∂ CLh
∂ CLM
∂ CLN
+
+
=
∂t ξdmL3 ∂x ξdmL3 ∂y ξdmL3
ξdmL3

(54)

where ξ= shape factor of sediment particles and ξdmL3 = volume
of a sediment particle.
Erosion and deposition velocities are given by equations (25)and (28), in which instead of θw , the energy slopeθis used; tanθ= τbx2+τby2 / (ρT gh) .
The equation of change in the bed surface elevation is
∂z
+i=0
∂t

(56)

(55)
Fig. 12

Arrangement of variables on amesh area

FLOOD AND SEDIMENT DISASTERS TRIGGERED BY 1999 RAINFALL IN VENEZUELA; A RIVERRESTORATION PLAN FOR AN ALLUVIAL FAN

results tend to concentrate the flow in a narrower area along theold river channel and the overflow from that channel begins further
upstream than it actually did. Flow and deposition along themountain’s foot adjacent to the western boundary of the fan andthe flooding and deposition situations along the outer bank at channel bends, however, are well reproduced by the calculations. A
conspicuous discrepancy is present in the northeast part of the fan.
In the calculations, no sediment was deposited here, whereas inreality (Fig. 13) the deposit was thick. This discrepancy was not
because of defective calculation, the actual deposition was due tosediment runoff from the basin that emerges from the mountainadjacent to this area.
7.3

Examination of the effectiveness of sabo dams and river
improvement
There are various plans for coping with possible future debris
flow hazards. The effectiveness of these plans can be assessed by77

the numerical simulation method discussed earlier. One particularplan, the combination of sabo dams and channel works is considered. The conceptual arrangement is shown in Fig. 15.
7.3.1 Effectiveness of sabo dams
(1) Method for taking into account in calculation the effect of asabo dam
The method used to calculate the hydrograph, sediment graph,and particle composition at the fan-top is one-dimensional routing,described in 7.1. The sabo dam is set at the calculation point offlow discharge per unit width, q (i) (Fig. 16). Elevation of the dam
crown, z1, is set equal to the sum of the mean bed elevation, {z (i 1) + z (i)} / 2, and the dam’s height. The flow surface gradient, θe,
and effective flow depth, h', are substituted into equations (33),(34) or (35) to calculate q (i) ;［z(i)+ h(i) -z1］
(Δx/2)｛

θe = tan-1

｛

h' =

｝

h(i) +z(i) -z1 ; h(i) +z(i) -z1 > 00; h(i) +z(i) -z1 < 0h(i);z(i) > z1

(57)

(58)

The notations in these equations appear in Fig. 16.
The gradient,θe', needed to calculate the equilibrium sedimentFig. 13

Arrangement of mesh areas and distribution of the deposit
thicknesses obtained from differences in elevation on maps
before and after the flood

Fig. 15
Fig. 14

Calculated results for the final stage

Location of sabo dams and plan view of the channel works

78

T. TAKAHASHI, H. NAKAGAWA, Y. SATOFUKA AND K. KAWAIKE

concentration, CL∞, is evaluated by［z(i) - z1］｛ (Δx/2) ｝

θe' = tan-1(59)

Whenθe' is less than zero, the equilibrium concentration is set
at zero.
(2) Calculation conditions and results
Three cases were examined. In case 1, no dam is set in the
Camuri Grande or Miguelena rivers. In case 2, both rivers have
10m high sabo dams, one per river. In case 3, the Camuri Grande
has a 20m high sabo dam, and the Miguelena a 10m high one.
In cases 1 and 2, canyon width is assumed to be 100m in thereaches 300m immediately upstream of each dam, whereas in caseFig. 16

Fig. 17

Definition of the gradient used in calculating
the discharge over the dam

Calculated hydrograph, sediment graph and mean diameterof the overflows of sabo dams on the two rivers

3 the 400m reach just upstream of the 20m high sabo dam in the
Camuri Grande River is assumed to be 100m wide. The flow
width in these reaches is assumed to be equal to the canyon width.
The same rainfall as in the reproduction of phenomena
described in 7.1 is given.
The calculated flow hydrograph, sediment graph, and mean
diameter of the runoff sediment at the dam sites are shown in Fig.
17 for case 3. A comparison of Fig. 17 with Fig. 9, which gives
the results for case 1, shows that a sabo dam can cut off the sharp,
large peak of the debris flow front. The peak flow rate in the
Camuri Grande River without the sabo dam (case 1) is about
700m3/s, whereas with construction of a 20m high sabo dam it isdecreased to about 300m3/s, and the debris flow front that comesout at about 6:00 a.m. is completely trapped by the dam. For case2, although the peak discharge is decreased to about 550 m3/s thedebris flow front can not be completely cut off.
The peak sediment discharge of the debris flow front with nodam in the Camuri Grande River is about 450 m3/s. Because thedebris flow front is completely cut off in case 3, the peak sediment
discharge is markedly decreased, to about 40 m3/s.
Table 1 shows the effects of sabo dams in terms of sedimentvolume yields on the fan area.
7.3.2 Effectiveness of the combination of sabo dams andchannel works
Although sabo dams are very effective for reducing the peakflow discharge as well as peak sediment discharge, sediment vol-Table 1.

Effectiveness of sediment control by sabo dams

Fig. 18

Channel cross-sections

FLOOD AND SEDIMENT DISASTERS TRIGGERED BY 1999 RAINFALL IN VENEZUELA; A RIVERRESTORATION PLAN FOR AN ALLUVIAL FAN

umes flowing out of the dams are still large. Unless larger dams
are built, countermeasures such as channel works must be introduced to prevent sediment flooding.
The combination of sabo dams and channel works therefore
was considered. The plan view of the channel works is shown in
Fig. 15. Cross-sections of the channels in the Miguelena and
Camuri Grande rivers and of the Camuri Grande River downstream of the confluence are shown in Fig. 18. Longitudinal profiles of the channel bed elevations are given in Fig. 19. The
unstructured mesh for the calculation of flooding and deposition on
the fan, as well as inside the channel, was altered from that in theprevious section to fit the new channel alignment.
Five different combinations were examined (Table 2). In case
1 there is no sabo dam but channel works 2m deep exist. Case 4
has the combination of 20m high and 10m high sabo dams in therespective rivers, and channel works 3m deep. Referring to cases 1
and 4, the conditions for other cases are clear in Table 2.
The inflow boundary conditions are those for cases 1 to 3 inthe previous section that correspond to the respective dam conditions.
An imaginary sea basin is provided as the outflow boundary
condition near the river mouth. Sedimentation due to the settlingof particles is considered when the settling velocity of each particle
diameter becomes larger than the local shear velocity in the basin.
Figure 20 shows the deposition area pattern and the deposit
thickness distributions in the cases at the final stage (t = 25 h). The
sediment deposit area and thickness of the deposit are muchgreater in case 1, in which no sabo dam exists and the depth ofchannel is shallow (2m). Because the channel can not store thetotal sediment volume of 1,309,000 m 3, the materials of bothrivers, water and sediment, overflow the channel, and there aresome places in the channel where the thickness of the deposit isFig. 19

Longitudinal channel work profiles

Table 2.

79

more than 5m. This suggests that, if the scale of a flood is comparable to that in 1999, even were very wide channel works constructed, flooding and deposition of sediment would be inevitablewithout an appropriate scale of sabo dams.
In case 2, the calculated sediment deposit area is very similar
to that in case 1, but the thickness of the deposit in the channel isless. In case 3, the sediment deposit area is decreased and thethickness of deposit as well. As a deposit area remains on the fan,the channel works need to be deepened. As a method to deepen
the channel, dredging would not be effective because of the limitation of downstream elevation imposed by the sea. The ground elevations of the mesh areas adjacent to the channel, raised asembankment, therefore were evaluated.
For a minimum channel depth of 3m (case 4), the sedimentdeposit area and thickness of the deposit are very similar to case 3.
A wide deposit area exists downstream of the channel, because
sedimentation near the river mouth is affected by the sea level.
In case 5, little flooding or sediment deposition occur on the
fan area. At the few places flooding does occur, local protection
by raising the embankment would be most suitable.
8.

CONCLUSIONS

The 1999 Venezuelan sediment disaster was caused neither by
the unprecedented severity of the rainfall nor the unexampled volume of sediment runoff in history. Comparable natural phenomena have been recorded in Japan; for example, the Great Hanshin
Flood of 1938. The resulting Venezuelan disaster, however, is oneof the most disastrous events on record in view of the number ofcasualties.
The natural and social factors that resulted in such numerous
casualties were analyzed. Poor natural conditions, deeply weathered geological formations and very steep topography, are combined with the chance to earn money in the central part of the stateof Vargas. Because of these conditions, numerous low-income
people live illegally in very hazardous areas in which even slightly
adverse conditions can cause disasters.
Japanese experiences suggest that intensive sabo works canremarkably reduce the chance of disaster. We consider that similarsabo works, a combination of sabo dams and channel works,
would be effective for mitigating disasters on the Camuri Grande
fan area.
We propose a numerical simulation method that reproduces
the hazards on that fan area that uses only the rainfall, topography,
and existing sediment conditions in the river reaches as a tool toassess the effectiveness of these sabo plans. This method was validated by a comparison of the calculated results and actual observations.
Possible sabo plans were assessed by this simulation method,
and an appropriate plan was proposed.

Calculation conditions

ACKNOWLEDGMENTS
This work was supported by a Grant-in-Aid of Scientific
Research, No.11800020, from the Japanese Ministry of Education,
Science, Sports and Culture. The field work was done by T.
Takahashi, H. Nakagawa, M. Chigira, Y. Onda, N. Maki, J.
Aguirre-Pe and E. Jauregui.

80

T. TAKAHASHI, H. NAKAGAWA, Y. SATOFUKA AND K. KAWAIKE(CASE-1)(CASE-4)(CASE-2)(CASE-5)(CASE-3)

Fig. 20

Calculated results for particular cases

FLOOD AND SEDIMENT DISASTERS TRIGGERED BY 1999 RAINFALL IN VENEZUELA─ A RIVER RESTORATION PLAN FOR AN ALLUVIAL FAN--

Special thanks are owed to I. Okano, a JICA expert attachedto the Venezuelan Ministry of Environment and Natural
Resources, for the arrangement of the necessities for doing thefield work and for the preparation of various data. Sincere thanks
are given to A. Carrillo of the Ministry of Environment andNatural Resources, Venezuela for his active cooperation and leadership in the field work.
R. Leonett, Vice-Minister and B. Norys, Advisor of the
Ministry of Environment and Natural Resources, Venezuela madeour investigation possible, and the Japanese Embassy in Caracas
provided the conveniences for the field survey.
NOMENCLATURE
B
B0
Ck: stream channel width: canyon width: volumetric concentration of grade k particles in the
total volume of water and sedimentCF: volumetric concentration of the fine particle fractionin the interstitial fluidCL: volumetric concentration of coarse particles in theflow
CL∞ : equilibrium coarse particle concentration in the flowC*: volumetric particle concentration in the static bed
C*F : volumetric concentration of all the fine particles when
the bed is composed only of the fine particles
C*F 0 : C*F in the lower bed layer
C*k: volumetric concentration of grade k particles in thebed
C*L : volumetric concentration of all the coarse particles
when the bed is composed only of coarse particles
C*L max : volumetric concentration of the coarse particles in the
maximum compacted state
C*L 0 : C*L in the lower bed layerdk: diameter of grade k particledmL: mean diameter of coarse particles in the flow
dmL' : mean diameter of movable coarse particles on theriverbed
fbk: volume ratio of grade k particles to all particles existing on the bed
fbLk: volume ratio of grade k particles to all coarse particles existing on the bed
F: volume ratio of the coarse particle fraction to all particles on the bedg: acceleration due to gravityh: flow depthh': effective flow depth
isb: bed erosion or deposition velocities in bulk
isbk
: erosion or deposition velocities of grade k particles
on the bed
isbo: erosion or deposition velocities on a riverbed comprised of particles with the mean diameter dmLJ: solids concentration on the surface layer of the bed
J0: J in the lower bed layer beneath δmk: grade of particle diameterk1: largest diameter grade of fine particleske: largest diameter grade of coarse particles
K: a numerical coefficient

M
ne
nm
N
q
qs
qsk
re
sb
t
u
u*ck
u*cmL
v
VF
Vk
VL
x
y
z
z1
αk
αo
β
δd
δm
ξ
θ
θs
θw
ρ
ρm
ρT
σ
τbx
τby
τ*
τ*c
φ

81

: x component of the flow flux
: equivalent roughness coefficient of the slope: Manning’s roughness coefficient of the riverbed: y component of the flow flux
: water or debris flow discharge per unit width of theriver channel: water flow discharge per unit width on the slope: inflow rate of grade k particles from side banks perunit length of channel: intensity of effective rainfall: degree of bed saturation: time: flow velocity: critical friction velocity of grade k particles: critical friction velocity of a particle whose diameteris dmL: y component of the flow velocity: total volume of fine particles on the bed: total volume of grade k particles on the bed: total volume of large particles on the bed: longitudinal distance: lateral distance: riverbed elevation: elevation of dam crown: a coefficient defined by eq.(2): a coefficient defined by eq.(14): momentum correction factor: a numerical constant: thickness of the particle exchange layer: shape factor of sediment particle: energy slope: slope gradient: water surface gradient: density of water: apparent density of the interstitial fluid: apparent density of the flowing materi: density of particles: x component of resistance to flow
: y component of resistance to flow
: non-dimensional shear stress: non-dimensional critical shear stress: internal friction angle
REFERENCES

Chigira, M., 2001. 1999 landslide disasters in Venezuela. In: Flood andSediment Disasters Caused by the 1999 Heavy Rainfall in Venezuela,
Research Report on Natural Disasters, DPRI, Kyoto University,
Kyoto, 7-14.
Maki, N., 2001. Damage and disaster response on the 1999 flood and sediment disasters in Venezuela. In: Flood and Sediment Disasters Causedby the 1999 Heavy Rainfall in Venezuela, Research Report on Natural
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