"CAUSES AND CONTROL OF SEDIMENTATION IN THE PARAPETI
RIVER BASIN, BOLIVIA, SOUTH AMERICA"

Prepared for the Organization of American States
By Law Environmental-Caribe, San Juan, Puerto Rico
November, 1987





1.0 INTRODUCTION

1.1 Objective

The objective of this project is the analysis and

delineation (using existing maps) of the areas where

sedimentation is produced in the Parapeti River Basin

upstream of San Antonio del Parapeti, Bolivia, South

America, to discuss the causes of sedimentation and define

the areas where sedimentation is deposited. Among the

topics included in this study is the delineation of the

actual uses of the land and a discussion of possible control

measures to minimize erosion from the basin.

This study is based on existing reports and data,

including maps, aerial photographs and satellite images. In

addition, visits were made to the site and people

knowledgeable of the geographic and hydrologic conditions of

the Parapeti River Basin were interviewed.

A list of the reports that were revised as part of this

study is included in the Bibliography.

1.2 Description of the Study Area
The Parapeti River Basin presents variable hydrologic

characteristics. It has a drainage area that consists in
part of mountains with heights exceeding 2,500 meters.





1O
.To













Between mountains there are plains used for

agricultural and residential activities. In the mountains a

large number of rivers and creeks are formed that during the

rainy season have a high potential to cause erosion of

mountains soil. In the valleys the rivers reduce their

slope significantly becoming wider and deeper. Apparently,

during floods part of the sediment is deposited in the flood

plain of the rivers due to the reduction in the velocities

and dragging forces of the run-off water. The majority of

the sediments, however, are carried over to the "Bafados de

Izozog."

The overflows of the Parapeti River supply the "Baiados

del Abapo-Izozog", a vast area of sandy soil that is very

permeable, allowing water to completely infiltrate even

during heavy rainfall events. The sediment load carried out

by the run-off water is deposited in the "Baiados de Izozog"

downstream of the town San Antonio del Parapeti creating a

sand-bank of more than a kilometer wide by 130 kilometers in

length.

The climate in the watershed is dry with an average

annual rainfall of 800 mm. Seventy per cent of the annual

rainfall occurs between the months of December and April.

The greatest amount of rainfall usually occurs during the

month of January and some years in December (Reference 1).




2












The average rainfall from May to November is notably low.

The average monthly rainfall in July, August and September

is less than 5 mm. During the heavy rainfall events mean

daily flows have been as high as 1400 m3/seg. with peak

instantaneous flows estimated at 2000 m3/seg. (Reference 2).

During the heavy rainfall season waves up to various meters

in height have been reported in the canyons formed by the

Parapeti River.

The average annual temperature in the study area is
approximately 230 C with the hottest months being November

and December. The lowest temperatures are registered in the

months of June and July with an average of 170C.

Natural vegetation in the area is not dense due to the

lack and variability of rainfall. There are large portions

of the watershed with very little or no vegetation. The

vegetation consists of shrubs, wild grasses and trees.

Among the variety of trees found are the "Cuchi", "Soto",

"Cedro", "Algarrobo", and others typical of the region. The

most common shrubs are the "Piquillin" and "Tartago".

In the valleys the vegetation is dense, but not enough

to protect the soils against erosion during heavy rainfall.

Along the river margins the valleys there exists
accumulation of sandy sediments that reach a width of up to
0.5 km in some sections, specially along the Parapeti River

upstream of San Antonio and up to 2 km. downstream of San


3
7./o













Antonio. Once the Parapeti River leaves the mountain

ridges, its course becomes unstable and is constantly

changing.

The geological materials found in some areas of the

mountain ridges is poorly consolidated and easily eroded.

1.3 Available Information

1.3.1 Reports

There are various reports containing valuable

information pertaining to the sedimentation phenomenon in

in the Parapeti River Basin. The most complete of these

reports was prepared by Mr. Hugo Benito, P.E., in August of

1987. He concluded that the results of the sediment load

calculations conducted by him, utilizing existing data are

only approximations of the sedimentation problem of the

Parapeti River Watershed.

Mr. Benito recommends the establishment of a sampling

station in Choreti and the collection of reliable data in

this station and in San Antonio, in order to conduct the

necessary analysis to be used in the feasibility study and

final designs of any water control facility proposed for the

Basin.

At present, additional samples are being analyzed, but

the results are not yet available.





4
'7-













1.3.2 Maps and Satellite Images


The work conducted in this study was realized using

available topographic maps and satellite images. Topographic

maps were provided at a scale of 1:50000 and 1:250000.

Satellite images were provided at a scale of 1:250000.

Aerial photographs of the Parapeti River Basin taken in 1956

were also provided.


1.3.3 Flow and Data Sedimentation

Two hydrometric stations known as Choreti and San

Antonio have been operating in different time periods in the

Parapeti River Basin. Also, flow measurements have been

conducted in different river locations. The Choreti station

was operated between 1942 and 1954 and San Antonio station

has been in operation since 1971. Although there are plenty

of daily flow measurements for both stations, in the study

conducted by Mr. Benito, P.E. it was found that the specific

flow yield for the basin between Choreti and San Antonio

was very high. According to Mr. Benito this responds to a

systematic error in the flow measurements procedure. The

existing data in both stations does not include measurements

of peak instantaneous flows.

The existing sedimentation data is limited to the San

Antonio station. The available data, however, represents

normal conditions in the Parapeti River, and not intense

flooding conditions. In addition, the samples taken were

5













not integral, but superficial which does not give a clear

idea of the sediment profile carried by the river (reference

2). These two conditions tend to produce sediment loads and

concentrations estimates lower than the real values. In

studies conducted for the Pirai River it was found that

integral sampling procedures produced higher sediment

concentrations than superficial sampling techniques.

Differences of more than 100% between both methods were

found.

It was found that apparently the sediment load

increases significantly between the towns of Camiri and San

Antonio. According to our observations this could be

caused by the sediment contribution of the mountain ridge

between Camiri and San Antonio, which shows a large number

of landsides exposing highly erosive material to the action

of the rain and the runoff. Unfortunately, the available

information does not permit a comparable evaluation of the

sediment load between these two sections of the watershed.

It is paramount, as recommended by Mr. Benito, P.E. that the

Choreti station be re-established as soon as possible, and

to conduct reliable sampling especially during heavy

rainfall events when the higher sediment loads may be

expected.






6
7./3












1.3.4 Field Visits
As part of this project an aerial reconnaissance of the

Parapeti River basin was conducted where a lot of

photographs were taken. These illustrate the most important

characteristics of the study area. Some of the photos taken

during the aerial reconnaissance are included on Appendix A.

Map 1 presents the approximate locations depicted by the

photographs. This aerial reconnaissance permitted the
verification of available maps and photographs provided for

the study, and the verification of data obtained during the
interviews. In addition to the Law Caribe personnel,

members of the Hydroelectric and Irrigation project for the

Parapeti River participated in this aerial reconnaissance.

Their comments and observations were very useful for the

evaluations made in this study.

1.3.5 Proposed Projects
There are several dam projects proposed for the

Parapeti River basin which need reliable sediment yield and

annual load data for their feasibility studies and final

designs. According to our observations, it appears that the

natural production of sediment in the basin is very high,
which is evident along the rivers flooded plains. Erosion
of the natural channels is evident in many rivers and
streams which shows a deep and rugged stream bed.



7
7p7.












In our opinion, the natural sediment production of the

watershed could impact significantly the design concept to

be utilized for the proposed projects. Unfortunately, this

condition can not be evaluated until reliable data is

obtained to compute sediment yields expected at several

locations in the basin.

At the present, a "pre-feasibility" study is being

prepared for a dam between Camiri and San Antonio (Oquitas),

and a derivation dam upstream of San Antonio. The purpose

of the project is irrigation and hydroelectric production.

The Oquitas dam will have an approximate height of 80 meters

and a normal reservoir storage volume of approximately 1100

Mm

1.4 Terms of Reference

This project is part of the project for the Development

of the Chaco Boliviano and its been realized under a

contract signed in September 1987, by Mr. Jesus Alberto

Fernandez representing the Organization of the American

States, and Mr. Alton F. Robertson representing Law Caribe.












8
I7/












2.0 Methodology

The analyses conducted in this project were based upon

information available at the time of the study. The

available data was supplemented with an overflight of the

Parapeti basin in order to have a general overview of

existing conditions within the study area.

2.1 Erosion and Sedimentation-Technical Aspects

The sedimentation process requires three elements:

erosion, transport and deposition. These are natural

geologic processes that have been occurring for millions of

years. Human activities such as agriculture, construction

and mining, alter the natural sedimentation process

increasing and accelerating the erosion-sedimentation

process.

The sedimentation process requires the detachment of

soils by rainfall, soil transport by runoff, and eventual

deposition of sediments due to reduction in runoff velocity

and dragging forces. The soil is detached by the direct

action of the rainfall drops and by the dragging forces of

the runoff water. Areas that have been exposed to human

actions usually are exposed to a greater degree to the

effect of the runoff waters.

Runoff is not produced until the infiltration capacity

of the soils is exceeded. Thereafter, the quantity and size

of the carried sediments will depend on the turbulence and

9
I ,4












velocity of the runoff. Sediments are deposited when the

runoff velocities and turbulence decrease significantly.

Sediment deposition is a selective process where bigger and

heavier particles are deposited first and fine particles are

carried out longer distances sometimes requiring long

periods of time to be deposited. Therefore if an estimate

of the sediment loads that will be deposited at a particular

location (i.e. a reservoir) is desired a size distribution

of the sediments must be obtained.

Runoff waters move initially as laminar flow but are

rapidly concentrated in swales, ditches and natural channels

starting a dynamic erosive process that increases as the

discharge flows increase. Excessive sediment loads are

usually produced by high flows corresponding to the rising

limb of the runoff hydrograph. As the runoff discharges

decrease the sediment deposition usually increases due to

the reduction in flow velocities. In this manner sediments

are carried out downstream from their sources in an

intermittent and progressive manner.
There are only a few methods available to estimate

sediment loads from areas exposed to erosion. From these

methods the most advanced and documented is the Universal

Soil Loss Equation (USLE) method developed by the U.S.

Department of Agriculture-Soil Conservation Service (SCS).




10
7/7












The USLE equation can be used to estimate the annual

erosion loads in tons/km2 that could be expected from areas

exposed to the erosion process. The total sediment load at
a particular location is computed utilizing the erosion load
computed by the USLE equation, and reducing it by factors
that take into consideration the size of the Basin, slopes,
hydraulic characteristics of the natural channels, and the
precipitation effects.
Eventhough this equation can be applied to the Parapeti

River watershed the parameters needed in its solution have
not been developed for this basin. However, using the
information generated by the SCS in the U.S. and the
Caribbean area and utilizing reported values for the
parameters needed in the solution of the equation, the
relative sediment contribution of some of the areas exposed
to erosion effects within the Parapeti basin could be
estimated. The computed values, even though not
representative of the watershed conditions, will give an
estimate of the areas with the highest erosive potential in
the Parapeti River basin.











11
I/S












3.0 Results


3.1 Existing Land Use
Based upon the information collected such as maps,

satellite images, aerial photos and the information

collected during the field visit and interviews, the

existing land uses within the Parapeti River basin were

defined. The principal land uses identified in this study

are presented in Map 2.
The existing land uses in the Parapeti River Basin were

divided into 5 groups:
1) Agricultural Land (A)
2) Grazing Land (P)
3) Petroleum Exploration Areas
4) Urban Areas (U)
5) Not developed areas (ND)

At present only a small percentage of the watershed has

been developed. As shown in Map 2 about 84% of the

watershed is in its natural conditions. The mountain ridges

with its abrupt topography and dry climate are not adequate

for agricultural or residential developments. Their only
future use could be for mining and petroleum exploration

activities.
Based upon the information provided for this study,

two areas dedicated to petroleum exploration were identified
within the Parapeti River watershed. One of them is being

conducted south-east of Camiri and the other south of the



12
I 19












Piraymiri community. These activities require the
construction of multiple access roads for which erosion
control methods are not implemented. These roads usually
expose large portions of highly sloped terrain to the
effects of rainfall and runoff. Even though these areas
have a tremendous potential for sediment production, at
present they only occupy about 1.5% of the watershed area.
Future mining and petroleum exploration activities could be
a significant sedimentation source if they are conducted
without implementing erosion control methods.
Urban areas within the watershed cover less than 0.5%.

The most important communities in the basin are Camiri, San
Antonio, Charagua, Vaca Guzman, Huacareta and Monteagudo.
In addition to these relatively large communities, there are

quite a few small communities all over the watershed. The
future development of these urban areas could constitute a
significant source of sedimentation if during the
construction of roads and building erosion control measures
are not applied.
The agricultural and grazing areas represent

approximately 14% of the watershed. The majority of the
valleys between the mountains are being cultivated or have
the potential for agricultural development. In Annex A,
photographs of various cultivated valleys are presented.




13
1












The principal products in the area are corn and citrus

fruits. At present, the total area being cultivated is very

small compared to the total area of the watershed. The

future development of other areas should be carefully

planned in order to avoid the high sedimentation loads that

usually are associated with these developments.

3.2 Sediment Producing Areas

Based on the information compiled during this study,

the areas with a great potential to produce significant

loads of sedimentation in the Parapeti River Basin have been

identified as follows:

1. Mountain Ridges Slopes whose geological formation
is not consolidated.

2. Areas between mountain ridges with a mild
inclination.

3. Agricultural areas.

4. Areas where petroleum explorations are being
conducted.

5. Grazing Areas.

Map 3 presents areas in the Parapeti River Basin where

sediment is produced. These areas can be divided into two

categories according to the phenomenon that causes them.

The first two areas produce sediment due to the natural

process of erosion. These areas could increment the total

production of sediments if preventive methods are not

applied. The most effective way to avoid increment of the




7a/l













sediment production in these areas is avoid human

development or to develop them only under strict erosion

control measures.

The last three areas enumerated as sources of

sedimentation are directly related to human developments.

These areas are of paramount importance for the control of

erosion in the basin, because their potential for producing

sediments is enormous if adequate measures are not taken.


3.2.1 Evaluation of Sediment Loads

As previously indicated it is possible to approximate,

using the USLE Equation, the specific sediment production

for each area exposed to the effects of erosion in the

Basin.

The USLE Equation established that:

A= RKLSCP

Where:

A=Annual Loss of Soil in Tons/Acre
R=Rainfall Erosion Factor
K=Soil-Erodobility Factor
LS=Slope-Length-Steepness Factor
L=Factor corresponding to the Length of the Slope
S=Factor corresponding to the Slope in Percentage
C=Cropping Management Factor
P=Factor for Control Measures

Using the "R" value, representative of the erosion

force of rainfall in the mountains, the "K" value for soils

with an intermediate capacity for erosion and assuming that

the Cropping Management Factor (C=1) has not been


15












implemented in the area and erosion control measures are not

applied (P=1), the relative values of specific sediment

production for different areas were computed as shown on

Table 1.
You can observe from the results of this Table that the

areas with a great potential for erosion are those exposed

to the natural effects of erosion, with the exception of

those areas used for exploration of petroleum. Although,

the values of annual soil erodibility presented in this

table are not representative of the Parapeti River Basin,

they indicate the areas in the basin with a greater

potential for erosion.

3.3 Areas of Sediment Deposition
The deposition of sediments carried by floods generally

occurred in flat areas where the velocity and turbulence of

the runoff decrease significantly and is insufficient to

maintain in suspension the carried sediments. In most

parts, this phenomenon occurred in the abandoned meanders of
the river and in the river margins when the flows exceeded

the normal capacity. In the Parapeti River Basin, the
floods caused by heavy rainfall deposits part of the
sediments along the river margins, but the larger part of

the sediments produced in the basin are carried to the
"Bafados de Izozog." Some of these areas of sediment



16
.R3

















TABLE 1
Relative Sediment Loads
Parapeti River Basin


Sedimentation Source

1- Erosive Mountain Ridge

2- Areas between mountain Ridges

3- Agricultural Land

4- Petroleum Exploration Areas

5- Pasture Land


Factor Factor
R K

600 0.10

600 0.10

600 0.10

600 0.10

600 0.10


Slope
Average
Length
(FT)

1500

2000

2000

1500

2000


Average
Slope
(%)

50

20

4

44

10


Factor
LS

60.83

18.24

1.32

45.83

6.13


A A Erosive
tons/acre/year tons/km2/year Potential

3650 901550 I

1094 270218 II

79 19513 III

2750 679250 I

368 90896 III


Notes:l- Erosive Potential [>II>III
2- A = Annual soil loss
3- A values presented are not representative of real conditions in the
Parapeti River Basin. The only purposes of this analysis is to present
a relative idea of the erosive potential between sedimentation sources
within this watershed.












deposition observed during the field visit are included in

Photos 1,2,3,7,25 and 30 presented in Annex A.

3.4 Actual Causes of Erosion

The erosive patterns in the Parapeti River Basin are

caused by natural patterns as well as by the effects of

human activities. Most of the uninhabited part of the basin

is in its natural state. The geological material that

outcrops at the surface are being exposed, as they have been

for millions of years, to the erosive effects of rains, wind

and gravity. Generally, the natural channels are deep and

unstable canyons which are eroded by the effect of the flows

concentrated in the channel.

On the other hand, approximately 16% of the basin is

exposed to development activities like agriculture, road

construction and mining activities. These activities

although at present occupy a very small portion of the

basin, have the capability of producing a significant amount

of sediment.













17
7.5












4.0 Erosion Control Measures Applicable to the Parapeti

River Watershed

At present, a large portion of the Parapeti River

Watershed is being affected by natural erosive processes not

related to human developments. In these areas the

implementation of structural erosion control measures may

only produce a small reduction in the sedimentation load of

the watershed usually obtained on a very high cost. For

instance, in the study prepared by the German Mission in

1985, a total of more than 100 million dollars were

estimated for structural measures needed to reduce the

sediment load of the Pirai River upper basin. In areas

where the natural production of sediments is high the

approach of implementing basin wide preventive measures is

more effective and less expensive.

On the other hand, part f the watershed is

being utilized and has potenti 1 for future human

developments. In these areas unpl nned development could

cause significant sediment loads tha could be diminished if

proper erosion control methods are i plemented.

The erosion control measures pplicable to the study

area can be divided into four groups
Structural measures

Vegetative Practices




18

WZa













Agro-Forestation Measures

Preventive Measures

Structural measures include engineering structures with

the purpose of controlling runoff flows, minimize erosion

and sediment interception. The structural methods

considered for this study are:

Stabilization Dikes

Sediment Dams

Flood Control Dams

Perimeter Ditches

Drainage Ditches

Gabions or Riprap

Vegetative practices provide vegetative cover to

exposed areas utilizing typical vegetation from the study

area. The purpose of these measures are soil stabilization,

minimization of the erosive effects of runoff, and

increasing the natural beauty of the area treated. The

vegetative practices considered for this study include:

Mulching

Vegetation Berms

Re-Forestation

Agro-Forestation measures include integrated methods of

soil conservation and agricultural and forest production.

On the other hand, preventive measures deal with the

planification and control of development activities within


19















TABLE 2


Erosion control Measures


Structural Measures







Vegetative Practices



Agro-forestation


Preventive Measures


Stabilization Dikes
Sedimentation Dams
Flood Control Dams
Perimeter Ditches
Drainage Ditches
Drainage Channels
Riprap or Gabions

Mulching
Vegetation Berms
Reforestation

"Silvo-pastoriles"
Soil conservation measures

Exclude grassing from high slope areas
Pasture Protection by livestock rotation
Increase productivity of small slope terrain
Separation of forest and Pasture areas
Determination and delineation of hazard areas
Prohibition of developments in hazard areas
Determination and delineation of conservation
areas
Education Program


Adapted from references 4 y 8


Cateaoru


Measure Name


Code


Code i ....














TRBLE 3

Applicable Erosion Control Measures


Code For Control Measures
(see Table 2)
Sedinentation source El E2 E3 E4 E5 E6 E7 V1 V2 V3 Al R2 P1 P2 P3 P4 PS PS P7 P8
Erosive Mountain Ridge (SE) X X X X X X X X X X

Area Betmeen Mountain Ridge (RES) X X X X X X X X X X X

Agricultural Land CRA) X X X X

Petroleum Exploration Area (REP) X X X X X X X X X X X X X X

Pasture Area (P) X X X X X X X X


Leyend : X= Applicable












5.0 Conclusions and Recommendations

At present, approximately 84% of the Parapeti River

Basin is undeveloped. Only about 16 % is exposed to human

development activities including agriculture, mining and

construction. In many undeveloped portions of the watershed

the surface geologic material observed is mainly composed by

unconsolidated sandstone, easily erodable. These areas have

a tremendous potential for sediment production which may be

effectively reduced by the implementation of a basinwide

erosion control plan.

For these areas preventive measures as discussed in

section 4, as well as vegetative practices and agro-

forestation measures should be implemented as the first line

of defense against erosion. The structural measures

previously discussed, even though applicable to the study

area, are not recommended due to the high cost usually

associated with them.
For areas presently affected by human developments, it

is recommended to implement, as soon as possible, the

erosion control measures discussed in Section 4. New

developments within the watershed should not be allowed

without the implementation of adequate erosion control

methods that could minimize the impact of the proposed

development.




21

130












In order to implement an effective and adequate

basinwide management of the Parapeti River Watershed it is

recommended to:
1. Conduct a detailed study of the major land
use of the watershed to define hazard and

conservation areas and the areas on which

reforestation could be implemented.

2. Develop specific and enforceable laws and
regulations that could be utilized to

implement a basinwide erosion control plan.

3. Develop an education program to teach the
population of the watershed about the

problems associated with sediment

contamination and the measures available to

control erosion and sedimentation.

Based upon the erosion patterns found in the study area

and the existing sedimentation data for watersheds adjacent

to the Parapeti Basin (i.e. Pilcomayo, Grande and Pirai) it

appears that the specific sediment yield as well as the
annual sediment loads estimated for the Parapeti River are a

lot less than those that may be expected. Unfortunately

there is no adequate data available to verify this
condition. We agree with Mr. Hugo Benito that the Choreti
Station must be installed as soon as possible and integrated

samples must be collected in Choreti and San Antonio.


22
17.3













Sedimentation sampling in these two stations must be

conducted for at least one year, including sampling during

heavy rainfall events, before reliable estimates of specific

sediment yields and annual loads could be computed for this

basin.

It is our opinion that, the design concept of any water

control structure proposed for this watershed must be

revised if it is found that the sediment loads on the basin

are high. The concept of passing sediments through the

reservoir instead of storing them could be the most

practical approach when sediment loads are expected to be

high.




By Alton F. Robertson and Raul Colon





















23

1 .59___