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									   APPLICATION OF SPOT 5/DEM AND ORTHOIMAGES TO ASSES THE SUCHIATE
    RIVER CONDITIONS AFTER HEAVY RAINS AT THE SOUTH PACIF COAST OF
                                MEXICO

                                         J.G. Ramos*, J.E. Mendoza, A.A. Jiménez, J. Gracia

              Dept. of Hydraulic, Engineering Institute, UNAM, Av. Insurgentes 3000, Mexico City, 04510
                                               jramosh@iingen.unam.mx



KEY WORDS: change detection, river divagation, floodplain, Spot DEM, satellite images, Chiapas


ABSTRACT:

The 2005 year was characterised by high precipitation records that in the case of the South Pacific Coast of Mexico signified the
floodplain of several rivers being one of the most important the Suchiate River that is the natural Border between Mexico and
Guatemala. This paper describes the work carried out using a Digital Elevation Model (DEM) and orthoimages from Spot 5/HRS
(High Resolution Stereo) to map the Suchiate River Basin conditions and topographic features before and after the hurricane Stan
and a tropical Depression in 2005. In addition, it was used medium resolution imagery (Landsat) in order to identify the river
movement under a temporal analysis, thus sediment deposits and previous flood plain areas could be established. The methodology
followed considered firstly to test the accuracy of the Spot DEM using Ground Control Points (GCP) at flat and mountain terrain.
Secondly, level curves were produced using the Spot DEM to establish the spatial distribution of topographic variables and the
hydraulic drainage in the basin. Thirdly, the land cover mapping was achieved for all the available satellite imagery, this
demonstrated that floodplain areas were gradually occupied by urban settlements or agricultural lands making these new areas very
vulnerable to damage. Finally, the integration of the available, high and medium, remotely sensed information before and after the
events during 2005 into a GIS permitted to address rapidly measures to reduce the negative impacts and to prevent them for the next
rainy season such as the case in 2006.


                     1. INTRODUCTION                                  satellite images. (ASFPM, 2007; Gracia and Maza, 1997). The
                                                                      advantage using satellite imagery is the capability to address
Several studies have been focused to determine the depth of           issues that difficult can be analysed using traditional methods,
flood waters and width of floodplains, in order to prevent events     for instance the river movements; topographic map update, map
with high socio-economic effects. To aboard these topics, the         currency auditing and land cover mapping, among others
study of the water movement in the watershed is basic to              (Holland et al., 2006).
determine the amount of water arriving to the river channel and
to be transported during the flood event. For a detailed              Today, there are different ways to generate a Digital Elevation
knowledge of the dynamics of the river, there is required a           Model (DEM) from topographic maps, stereopairs satellite
spatial-temporal distribution of flow in floodwater areas. This       images and RADAR sensors. The selection of the source of
kind of studies considers the role of the terrain, since flat areas   information used is function of the DEMs accuracy and of
are exposed to a slow-water movement that can cover large             course the associated cost. Satellite platforms such as SPOT 5
areas for days or weeks, while in deep areas, water moves faster      allow accurate DEM from High Resolution Stereoscopic sensor
having a destructive effect but the flooding conditions disappear     (HRS, 10 m) in track stereo-images and High Resolution
in a short time. In the dynamics of the river, the river channel      Geometric (HRG, 5m resolution) across-track stereo-images.
change as water move downstream, however during a flood               Using the DEM from SPOT-5, it is possible to derive
condition the movement force on the channel banks and the             orthoimages without the use of Ground Control Points (GCPs).
bottom increase affecting the original features of the river.         According with Toutin (2006), SPOT-5 HRS/HRG
Some of the changes observed are the creation of meanders,            stereoimages are correlated for radiometric distortions and the
which outside of them generates the erosion of the banks              image orientation is function of the satellite inclination. Toutin
making water scours against them, and inside makes sand               (2006) validated the DEM obtained from SPOT-5 imagery
deposits and sediments transferred from eroded sites. One             against LIDAR data, the results showed for 68% and 90 % level
important consideration is that meanders are in a constant            of confidence, an error of 5.2m with a bias of 2.0 m for HRS
movement, thus they migrate slowly downstream and across the          images and 6.5m with a bias of 2.0 for HRG images. These
flood plain, redrawing the river shape (ASFPM, 2007; Gracia           results indicate a good correlation mainly for bare surfaces
and Maza, 1997). Understanding, how the river channel has             without surface height, and some problems to represent the real
been moved thought the time and throughout the flooding areas,        elevation of forest canopy.
it is required to have accurate data related to the river shape and
changes in the floodplain, as well as the determination of            This paper describes the work       carried out using a Digital
ground elevations and obstructions to the flow (i.e. vegetation,      Elevation Model (DEM) and            orthoimages from Spot-5
sediments, debris). Part of this information can be obtained          HRS/HRG to map the Suchiate         River Basin conditions and
from ground surveys, aerial photographs, topographic maps and         topographic features of the river   channel before and after the
Hurracan Stan and Tropical Depression in 2005. In addition, it     produced a heavy and constant rain during four days (3-6
was used medium resolution imagery (Landsat) in order to           October) generating that the Suchiate River flooded and
identify the river movement under a temporal analysis, thus        covering large flood plain areas. In consequence, human and
sediment deposits and previous flood plain areas could be          economic losses as well as the destruction of hydraulic systems
established.                                                       to protect the cities were present due the dragging of several
                                                                   materials as result of the erosion process (natural and
                 2. SITE AND MATERIAL                              anthropogenic) that took place.

2.1 Study site                                                     Although, the natural movement of rivers implies its divagation
                                                                   along the main floodplain areas, particularly where the slope of
The Suchiate River has its origin in Guatemala, in the Tacana      the river decreases, this movement is expected to take place in
Volcano. The River Basin of the Suchiate (RBSuch) has total        several years. However, the events described above among
area of 1,400 Km² of which 76% (1,064 Km2) are in the              previous events (hurricanes or tropical and winter storms since
Guatemalan territory and the rest 24% (336 Km2) are in the         1950`s) changed the morphology of the river dramatically. The
Mexican territory (Figure 1). The Suchiate River has a total       duration and intensity of the rainfall in 2005 generated a strong
longitude of 79.3 Km2, with a drained area until the               erosion in the mountain areas. Part of the material eroded was
hydrometric station “Talisman II” of 314.81 Km2, from which        sediment close to the place where was the landslide, but other
76.2 Km2 are in Mexico and 238.6 Km2 are in Guatemala. The         part was transported by the fast flow towards the lower part of
Hydrometric Station Suchiate II drainage 1,153.5 Km2 of            the basin, thus the damage produced was considerable. The
which 131.6 Km2 correspond to the Mexican territory and            change detection of the Suchiate River after the flooding in
1,021.9 Km2 to the Guatemalan territory. The climate is very       2005 was a difficult task using traditional technologies since in
humid forest and humid montano under subtropical in the high       some points the river margin need to be approximated. This
river basin (Guatemala), tropical very humid forest in the         uncertainty constrained the actions needed to improve the river
average river basin (Guatemala) and humid forest and               conditions before the next rain period. In order, to improve the
subtropical dry forest in the low river basin (Mexico). The        quality of the topography data, high and medium resolution
RBSuch has its maximum elevation at the Tacana Volcano with        satellite sensor imagery was used.
4,200 msl and the minimum in the Mexican Coast (0-10 msl).
Water in the basin is used in different ways, in Guatemala for     2.2 Materials
domestic uses, whereas in Mexico water is split between
irrigation and human consumption, 54% of superficial waters in     The requirements of the project made available to acquire high
the irrigation district of Suchiate-Cacahotán, 26% in human        resolution satellite information, in particular after the hurricane
consumption and a 10% in agro-industrial activities (Esquinca,     Stan. As the RBSuch is quite large, it was considered the
2005; Jiménez et al., 2006).                                       SPOT-5 imagery as appropriate since it gives a 2.5 m pixel
                                                                   resolution and covers an area of 60Km×60 Km. In addition, in
                                                                   order to understand the river features changes along time,
                                                                   Landsat imagery was also acquired. Also, orthorectified aerial
                                                                   photographs and a reference DEMs produced by Spot-5
                                                                   HRS/HRG stereoimages were available. Detailed information is
                                                                   as follows:

                                                                   •     Orthorectified SPOT-5 HRG images were acquired on
                                                                         15th and 20th November 2005, 6th and 16th February
                                                                         2006. The pixel size of the images was 2.5×2.5m per
                                                                         pixel as result of the fusion between the Panchromatic
                                                                         (2.5m) and, visible and infrared (10m) images.

                                                                   •     DEM provided by SOPT-5 before the Hurricane Stan.
                                                                         The total area was the RBSuch and was created from HRS
                                                                         images with a point spacing of 20 m.
                                                                   •     DEM obtained from the National Institute of Statistics,
                                                                         Geography and Informatics (INEGI) with a grid spacing
                                                                         of 30 m. The DEM covers all the Mexican Republic, thus
         Figure 1. Figure Suchiate River and its basin                   it was generated a polygon covering the RBSuch.
                                                                   •     Landsat images included those obtained from MSS, TM
The Pacific Coast of Mexico is subject every year to hurricanes          and ETM+ sensors. Date acquisition was for ETM+ on
or tropical depressions that affect socio-economically the zone.         19th April 2000, 15th February 2000, for TM on 30th
During the last century, several events were present modifying           December 1988 and for MSS on 15th February 1974.
the river channel and the floodplains. Some of the hurricanes      •     Orthorectified aerial photograph provided by the INEGI
with devastating effects are: Adriane in 1960, Gilbert in 1988,          in UTM and Datum ITRF92. These aerial photographs
Diana in 1990, Roxanne in 1994, Cesar-Douglas in 1996, Mitch             only cover Mexican territory with a 1:75 000 scale for
in 1998, Keith in 2000 and Stan in 2005. The 2005 year was               January 1996.
characterised by high precipitation records that in the case of    •     Aerial photograph provided by the International
the South Pacific Coast of Mexico signified the floodplain of            Commission of Limits and Water (CILA) and by the
several rivers being one of the most important the Suchiate              National Water Commission (NWC) before (1977) and
River. The occurrence at the same time of the Stan Hurricane             after (October, 2005) the Hurricane Stan.
and a tropical depression at the South of the Tehuantepec Golf
The Landsat imagery and the aerial photographs were
geometrically corrected using the orthorectified photographs.
The aerial photographs were grouped in a mosaic using ERDAS
and they covered a longitude of 4,800 Km from the Coast to
above Ciudad Hidalgo.

                    3. METHODOLOGY

The methodology followed is divided into four sections. The
accuracy of the SPOT-5 DEM was tested first, then the
hydrological definition of the basin and the land classification
of the flooding river areas were addressed to establish the width
of the floodplain zones. In the last step, the river divagation and
the river channel movement throughout the time were
evaluated.

3.1 Testing the SPOT-5 DEM accuracy

To test the accuracy of the SPOT-5 DEM, it was used Ground            Figure 2. Level curves at 2m point spacing generated from the
Control Points (GCP) at flat and mountain terrain. The GCP                                   SPOT-5 DEM
were provided by the CILA-NWC, an correspond to
monuments allocated one in the delta of the Suchiate River, one       Despite a point spacing curves were generated for 2 m, the
close to the Guatemala border at the middle basin and other in        definition of the drainage system was worse in flat terrain. As
the mountain Guatemala border. It was observed that there is a        result, it was difficult to achieve an accurate topographic
major deviation in mountain areas with a difference of 3 to 6 m       delimitation of the river morphology and, in consequence, of
in altitude that in flat terrain with 0.5 m. Similar results have     the floodplains areas.
been obtained from authors testing the elevation accuracy of the
SPOT-5 DEM in flat and mountains areas (Buyuksalih et al.,            3.3 Land cover classification
2004; Kournus et al., 2006; Reinartz et al., 2006; Toutin, 2005).
Additionally, as the DEM from INEGI is free and the one from          The unsupervised classification was achieved from the SPOT-5
SPOT-5 was so expensive, this made necessary to compare               fusion images. Six classes were defined: river main flow or
which of them was more accurate since the point spacing is 20         water mirror, sea water, flooding, trees, mountain vegetation
m for SPOT-5 DEM and 30 m for the INEGI DEM. Results                  and crops (Figure 3).
showed an error of 5% for SPOT-5 DEM whereas the error
increase to 15% using the INEGI DEM. This justified the
acquisition of the SPOT-5 DEM.

Also, it was tested the positional reliability of the georeferenced
SPOT-5 HRG-PAN images (2.5 m resolution pixel) using
orthorectified aerial photographs, results showed an accurate
match of both of them in all cases. This perfect match was
obtained since diverse spectral transformations such as
rationing, Principal Components Analysis (PCA), and Tasseled
Cap transformation were performed on the images
(Darvishsefata et al., 2004). To generate effective multispectral
bands with better spatial resolution, the green, red and near
infrared bands, which lie in the spectral range of panchromatic
band, were fused with PAN.

3.2 Level curves

Once tested the accuracy of the SPOT-5 DEM, level curves
were generated to establish the spatial distribution of
topographic variables and the hydraulic drainage in the basin.
This allowed settling those areas very susceptible to the erosion,
flooding and the features along the river from its origin to its
empty into the Pacific sea. The level curves were obtained in a
grid of 20 m and then interpolated to a point spacing of 2 m
(Figure 2).

                                                                           Figure 3. Flooding areas defined from unsupervised
                                                                                              classification

                                                                      Some of the provided information by the CILA was the
                                                                      delimitation of the river divagation or with of the floodplain
areas. This delimitation was achieved following the water          3.4 GIS integration
marks recorded from events presented before 1977 (Figure 4).
However, this information was available as scanned aerial          The integration of the available, high and medium, remotely
photographs non-orthorectified. The line drawing of the river      sensed information before and after the events during 2005 into
divagation was done till the Ejido Zaragoza at the north of        a GIS base permitted to address rapidly measures to reduce the
Ciudad Hidalgo. Thus, the work was to draw the floodplain          negative impacts and to prevent them for the next rain season
areas for the whole river Suchiate in the Mexican side             such as the case in 2006. One of the main results of this
throughout the time using the georeferenced data available, in     integration was the delimitation of theoretical border lines. In
order to cover different events.                                   fact, the CILA provided part of the theoretical border lines from
                                                                   a study realised in 1969 (Maza and Springall, 1969). However,
                                                                   the CILA border lines only covering from the Delta till 6,411
                                                                   Km above Ciudad Hidalgo, thus the rest (7000 Km) of the
                                                                   Suchiate River border lines were made using the GIS system.

                                                                                             4. RESULS

                                                                   The GIS system based on remote sensing imagery helped to
                                                                   determine the amount of rainfall in the RBSuch (absorbed by
                                                                   the soil, trapped in puddles, among others) and the rate at which
                                                                   the remaining amount of rainfall reached the Suchiate River.
                                                                   Once in the main stream, the computation of the water levels
                                                                   was crucial during the maximum rate associated to a return
                                                                   period. In the Suchiate II hydrometric station, the annual
                                                                   maximum rates were 1,049.2 m3·s-1, for 36 years of records
                                                                   (from 1955 to 2005), but in 2005 the maximum rate reached
                                                                   was 2,800 m3·s-1 (CILA, 2006). As the rate in 2005 was twice
                                                                   the average in 36 years, the estimation of the hydraulic profiles
                                                                   was basic in order to compare the water levels with the
                                                                   elevation at both sides of the river, and the rate and site at
                                                                   which started the flooding. Thus, a cross section to describe
 Figure 4. River flooding and floodplain zones in 1977 in the      graphically the depiction of the river channel and the floodplain
                 Delta of the Suchiate River                       at a particular point along the river channel was easily obtained
                                                                   applying the curve level map generated from the SPOT-5 DEM.
Using the flooding and the river divagation delimitation maps,
as well as the Landsat images, it was demonstrated that            Once the hurricane Stan passed the CILA-NWC sent a
floodplain areas were gradually occupied by urban settlements      topographic group to obtain cross sections of the flooding areas.
or agricultural lands, making these new areas very vulnerable to   One of the first observations was that the morphology of the
damage during strong rainfall. Figure 5 shows the variation of     river channel was modified and a huge amount of material was
the river morphology before (1977) and after (2005) the            deposit in the main stream. Two topographic groups went to the
Hurricane Stan.                                                    field to take cross sections: CILA and NWC groups. The NWC
                                                                   took cross sections every 500 m, however different problems in
                                                                   the field, made these section uncertain. One problem of these
                                                                   cross sections was easily observed once they were downloaded
                                                                   in the GIS system -the datum was not correct. The application
                                                                   of the GIS system allows the correction of the orientation of the
                                                                   cross sections, thus the elevation levels were obtained as well as
                                                                   the hydraulic profiles. The CILA group took cross sections
                                                                   every 100 m before (2003) and after (2005) the Hurricane Stan.
                                                                   The comparison of the CILA cross sections made possible to
                                                                   compare the amount of deposits in the river channel and the
                                                                   features of the river in areas were the stream change
                                                                   significantly. The CILA cross sections were downloaded
                                                                   without any problem. In this way, the hydraulic profile of the
                                                                   river was obtained after the hurricane Stan (Figure 6).




 Figure 5. River channel and river divagation (yellow line) a)
    before (1977, blue line) and b) after (2005, red line) the
                         Hurricane Stan
                         30




                                              20 + 000 NWC
                                                                                                                                  LEFT MARGIN (GUAT EMALA)
                                                                                                                                  RIGHT MARGIN (MÉXICO




                                                                     16 + 700 NWC
                         25                                                                                                       HYDRAULIC PROFILE
                                                                                                                                  RIVER T HALWEG


                         20




                                                                                                                5 + 800 CILA
         ELEVATION (m)




                                   CIUDAD




                                                                                                                                         1 + 500 CILA
                         15
                                  HIDALGO




                                                                                                                                                        1 + 000 CILA
                         10
                                                                      JESUS CARRANZA

                                                                                      LÓPEZ RAYÓN
                          5
                                                                                                              LA LIBERT AD

                          0
                                                                                                                                              MIGUEL ALEMAN
                                                             CNA                                                 CILA                                                          CNA
                         -5
                              0        2000                  4000   6000            8000   10000    12000     14000            16000   18000                           20000   22000
                                                                                              DIST ANCE (m)




                                       Figure 6. Thalweg of the Suchiate River after Stan from the Delta until Ciudad Hidalgo.


                         30
                                                                                                                                  LEFT MARGIN (GUAT EMALA)
                                                                                                                                  RIGHT MARGIN (MEXICO
                         25                                                                                                       HYDRAULIC PROFILE
                                                                                                                                  T HALWEG DEL RÍO


                         20
         ELEVATION (m)




                         15



                         10



                          5

                                   B = 120 m
                                   Q = 1000 m3 / s
                          0



                         -5
                              0        2000                  4000   6000            8000   10000    12000     14000            16000   18000                       20000       22000
                                                                                              DIST ANCE (m)



                                              Figure 7. Hydraulic profile after returning the border to its original features.


One result expected from the evaluation of the Suchiate
River after the hurricane Stan was the return of the river to its                                      It was in the decision of the areas to be dredges, that the GIS
original theoretical border lines. This part was achieved                                              system played an important role, since it made obvious the
through the computation of the hydraulic profile using a 1000                                          areas with more damage through the use of the satellite
m3·s-1 rate. Results are shown in Figure 7, after considering                                          images. Also, helps to establish together with the hydraulic
that zones with huge deposits needed to be dredges, thus the                                           profile the reference marks and bench marks with a recorded
deep and width of the river will be as before of the event.
elevation to describe the changes in the ground levels or         the 3rd EARSeL Workshop Remote Sensing for Developing
stream characteristics.                                           Countries, September 26-29th, Cairo, Egyt.

The SPOT-5 fusion images showed clearly the flood of the          Darvishsefata A., Fatehia P., Khalil Pourb A. and Farzanehb
river in both border lines. Jiménez et al. (2005) explain the     A., 2004. Comparison of spot5 and landsat7 for forest area
loss of the borders as results of the deforestation in the        mapping. Proceedings of the XXth ISPRS Congress Geo-
mountain areas. This deforestation increases the runoff, thus     Imagery Bridging Continents", July 12-23, Commission III
the quantity of water arriving to the mean stream is              papers, Vol. XXXV, part B3, Istanbul, Turkey.
considerable as well as the landslide material. The material is
transported till find calm waters (lower slope) favouring its     Esquinca F., 2005. The Importance of the Ecoregión Sierra-
deposit and, in consequence, change the shape of the river        Costa of Chiapas and the Water and Nature Iniciative in The
generating meanders, which properties on the outside of           Tacana Volcano with the UICN. Proceedings of Water and
curves face a double threat of inundation and undercutting        Politics, World Water Council, September, 2004.
from riverside erosion during flood. The inundation produced
during flood affects crops areas, protection structures and       Gracia J. and Maza J.A. 1997. Morfología de los ríos.
human lives and wellbeing.                                        Capítulo 11 del Manual de Ingeniería de Ríos, Series del
                                                                  Instituto de Ingeniería, No. 590, Mexico (In Spanish)
                    5. CONCLUSIONS
                                                                  Jiménez A.A:, Gracia J., Ramos J.G., Mendoza J.E.,
The use of SPOT-5 DEM provided important information,             Domínguez R., López G., Osnaya J. and Franco V. 2006.
since it was demonstrated to give accurate terrain heights.       Anteproyecto de la Rectificación Integral del Río Suchiate y
This accuracy varied from 3 to 6 m in mountain areas, and         la Rehabilitación del Bordo de la Población de Hidalgo.
0.5 m in coastal areas. Although, the terrain elevation           Repot to the Comisión Nacional de Agua, by the Instituto de
accuracy was good, it was difficult to generate the level         Ingeniería, UNAM, México. (in Spanish)
curves in coastal areas due to the flat of the terrain. To have
appropriated level curves, it was required more work to           Kornus W., Alamús R., Ruiz A. and Talaya J., 2006. DEM
correct lines and interpolate to a less point spacing. The work   generation from SPOT-5 3-fold along track stereoscopic
required was achieved using the DEM and the fusion images;        imagery using autocalibration. ISPRS Journal of
the result was a good approximation in the definition of the      Photogrammetry and Remote Sensing, 60(3), pp. 147-159.
river shape in the coastal areas. The integration of the
available, high and medium, remotely sensed information           Maza J.A. and Springall R, 1969. Modelo del río Suchiate.
before and after the events during 2005 into a GIS permitted      Instituto de Ingeniería, UNAM. Proyecto patrocinado por la
to address rapidly measures to reduce the negative impacts        Report to the Comisión Internacional de Límites y Aguas
and to prevent these for the next rainy season such as the        (CILA), Mexico. (In Spanish)
case in 2006.
                                                                  Reinartz P., Müller R., Lehner M. and Schroeder M., 2006.
At different points during a flood, people are displaced, and     Accuracy analysis for DSM and orthoimages derived from
damage occurs for the inundation and the sediments                SPOT HRS stereo data using direct georeferencing. ISPRS
transported in the river. This situation can be extended          Journal of Photogrammetry and Remote Sensing, 60(3), pp.
depending of the severe of the flood from days to weeks,          160-169.
until a cleanup program begins. Also, the formation of
meanders and their undercutting can generated                     Toutin T., 2006. Generation of DMs from Spot-5 in track
socioeconomic conflicts, being increased when the river is an     HRS and across-track HRG stereo data using
international border. Thus, the use of information before and     spatiotriangulation and autocalibration. ISPRS Journal of
after this kind of events aid to define losses or gains of        Photogrammetry and Remote Sensing, 60(3), pp. 170-181.
terrain in both sides of the border and, as result, to prevent
political conflicts due to the movement of the border line.

                ACKNOWLEDGEMENTS

The authors thank to the National Water Commission and the
Comisión Internacional de Límites y Aguas (CILA), which
were the sponsor of the project. Also, they thank to Mr.
Javier Osnaya for his participation in the development of the
project.

                      REFERENCES

ASFPM, 2007. Managing Floodplain Development through
the National Flood Insurance Program, FEMA Home Study
Course IS-9. Association of State Floodplain Managers.
http://www.floods.org (accessed 5 Apr. 2007)

Buyuksalih G., Oruc M., Topan H. and Jacobsen K., 2004.
Geometric accuracy evaluation, DEM generation and
validation fro Spot-5 level 1B Stereo Scene. Proceedings of

								
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