Flood risk forecsting and
disaster countermeasures information system in
the Dongting Lake
region
Lu
Guonian
Laboratory of Resources and Environment Information
System,
Chinese Academy of Sciences, Beijing, China
Wan Qing
Dai, Tiefu
Hunan Remote Sensing Centre, Beijing, China
Abstract
In conjunction with the results of
remote sensing application experiment to flood risk and disaster
development monitoring in the Dongting Lake Region over the years, the
paper discusses the design, establishment and application of the Flood
Risk Forecasting and Disaster Countermeasures Information System in the
Dongting Lake Region (DLIS) as well as other many-sided problems. By means
of the analyses of the peculiarities of the regional landscape, the
authors have put forward the viewpoint that the spatial logic structure of
a geographic database is the combination of the river system in which the
lakes form the main body with the polygon of dykes, explored some methods
for DEM updating and developed many application analysis models. Practice
has demonstrated that the application of the geographic information system
(GIS) in regions can on one hand provide new technical method for flood
prevention and disaster relief, and on other hand promote the development
of GIS itself.
Introduction
Dongting Lake, the second
largest fresh water lake in China, is situated in the middle reaches of
Changjiang River. However, for a quite long time, siltation of mud and
sand in addition to the building of dykes for flood prevention purpose and
the reclaiming land from the lowland has led to rapid reduction of the
lake area and lake storage capacity and to the ever increasing
deterioration of the flood diversion and flood storage function, hence the
frequent occurrence of flood disasters and the tendency of the increased
frequency. With flood passing the region during each flood season, serious
losses result from the floods outside the dyke surrounded area (DSA) and
the water-logging inside it. Take the special big flood in 1954 for
example, the dykes broke in 356 spots within the Region with 3, 849,000 mu
of the cultivated land submersed and the area of the cultivated land
suffering water- logging being 1, 533,000 mu;at that time, 3300 people
drowned, and 30,000 people died of the following pestilence. In the year
of 1983, which was expected to be a normal water year, a rare flood
occurred with the flood water stage even higher than that of 1954 in many
places of the Region. Consequently, the broken dykes amounted to 145 spots
with 288, 000 mu of the cultivated land submerged and the area of the
cultivated land suffering the disaster effects being 1, 380,000 mu. It can
be seen that the regional planning for flood prevention purpose in the
Region has become an urgent task for the present. Owing to the
considerable investment and long cycle needed by building engineering
projects for flood prevention as well as some adverse effects which might
be produced, the execution of the non-engineering measures for flood
prevention is being sped up. The establishment of the DLIS is an important
aspect of the job, which bears important significance to the timely
grasping of information of flood risk and disaster development by the
flood control departments, to the important decision making with relation
to flood control and disaster relief work as well as to the guarantee of
the safety of the people's lives and properties, and will also produce
profound influences on the environmental management of the Region,
maintenance of ecological balance and creation of benefits to future
generations.
Design of DLIS
DLIS is project supported by
the central and local governments as well as sponsored by the
International Development Research Centre (IDRC) of Canada. The
development of the System aims at providing information to the decision
making departments in charge of flood control and disaster relief in a
speedy, accurate and timely way, effective processing and utilization of
the provided information, application of the GIS methods to the process of
decision making through the development of analysis models, meanwhile,
exploration of the function, role and potentialities of GIS as well as
appraisal of its application effect. Accordingly, the experience gained
from the design and establishment of the system will be extended to the
regions with similar geographical environments in China and Southeast
Asia.
- Regional Features: The relief of the Region is shaped like
the centre of a palm, and the Lake gathers water and sand from all sides
with the exception of the northeastern Chenglingji where the Lake pours
into the Changjiang River. The Region has a crisscrossed network of
water body with small lakes scattered all over like stars in the sky;
and through the links among the streams and lakes an network system of
water body has resulted. The Dongting Plain is an alluvial and littoral
deposit plain formed by the Four Rivers and the Four Distributaries. On
the Plain the landscape feature is characterized by the overall flatness
mixed with local unevenness. As a result of frequent dyke braking and
flood inundation, the sand spits closed in to form land with the
dish-like depressions appearing everywhere. In particular, with the
agricultural development , the DSAs have emerged for the purpose of
reducing the flood threats from either the rivers or the lakes, thus
forming the micro-geomorphology of which the land plots with dykes
intersect with one another and the dish-like depressions, big and small,
overlap, from which it can be seen that Region exhibits the
characteristics of special spatial structure with the intermixed
overlapping of island like dykes with the river system.
- System Design: With relation to the special spatial structure
which features the combination of multi-layer island-like data with
net-like data, the System can be divided into three sections in
accordance with the application targets of the System, i.e. the database
system, database updating system and model base system. The interface
between the first two systems is responsible for providing timely useful
information to the database system and the continuous updating of the
database so as to ensure the current ness of the database ; while the
one between the database system and the model base system is responsible
for providing the required data and the feedback of the valuable results
of computation or the updating information to the
database.
Establishment of the database system
- Composition of the Database: The main target for the
establishment of the database system rests with providing basic data for
the estimation of the flood inundation losses, simulation and dynamic
display of flood process, the synthetic analysis of the formation,
developing conditions and dynamic process of flood risk and the and the
estimation of the benefits of flood preventive projects. The database
system so established to meet these targets is composed of the six
sections as follows: 1) Environmental background database, which
includes traffic network, distribution of the settlements, existing
landuse, etc. ; 2) Database of flood preventive projects, which includes
dyke types, distribution f the spots of potential danger, arrangement of
hydrologic stations, distribution of the storehouses with materials and
equipment used for flood control and disaster relief ;3) Database of
DEM, for which the entire Region employs the unified geographic
coordinates, the sizes of which are rated as 25X25 m, 50X50 m and 60X60
m; 4) Place name base, which includes place names, kilometer network
coordinates, longitude -latitude coordinates , etc.;5) Socio-economic
statistic database, which consists of to subdatabases representing the
database with the DSA as statistic units and the one with the natural
villages as the statistic units respectively , both covering population,
number of households, output values of industry and agriculture, annual
profits and taxes, etc. ; 6) Hydrologic Database, which mainly includes
water stages, discharges, sand contents, etc.
- Spatial Structure of Graphic Data: The basic graphic
components are points, lines and areas. The structure of graphic data
generally employs the grid structure or the polygon structure. As to the
ARC/INFO software which we use, its basic data structure belongs to
topological structure of ARC/INFO software which carries out effective
storage in terms of the structural grid of line segments and arcs
exhibited by the administrative boundaries or geographic elements
themselves. In addition to the recording of the recording of the
characteristic codes of the elements, it will digitize all the points
and arcs and then store them into the database, make topological
definitions of the points and arcs and define the polygons by means of
the arcs composing the boundaries and internal marking points . Using
this kind of data structure of arc-nodal points can rather accurately
express their profiles, locations, interrelationship between different
geographic elements, being capable of overlaying, jointing and mapping
and simple arithmetic operation, resulting in clear indication of
retrieving. If complex statistic operation, appraisal and analyses as
well as database updating using the remote sensing data are to be
performed, it is possible to proceed with the conversion of the data
formats so that vector data can be converted into grid data so as to
facilitate computation. This makes the system flexible and practical.
- Logic Structure of Attribute Data: All the data of statistics
and investigation are expressed in such units as the points, lines and
polygons. The relational data model of the INFO relational database
expresses the data in the form of a series of logic tables, the links of
which are established through a common item possessed by every table.
The tables are the sets of various attributes of the similar entities.
One table forms one file; and the method of data expression is unified,
simple and easy for expansion, adjustment and reorganization at that.
- Spatial Logic Structure of the Data Base System: Owing to the
complexity of spatial data structure, the great amount of data and the
always limited availability of economic but efficient computer
resources, the design of the data base's spatial logic structure becomes
the key to the success or failure of GIS. Starting from the
effectiveness of the system, we separate the study Region into some DSAs
and a river network so as to facilitate the storage of the related
information; that is to say, the spatial logic structure of the database
is mainly the combination of the data related to the island-like DSA
with those of the river system.
Experiment of the database
updating method
Both DEM and existing landuse information in the
database system vary with time. In order to ensure the currentness of the
system, it is imperative to updating the data in a speedy and timely way.
- DEM Updating: The relief of the Region changes very quickly,
being mainly expressed as the variatios in washing and silting of the
river courses and lakes, the extensive silting up of mud in the rivers
and lakes during the flood season every year, the washing of the river
courses during the dry season; whereas the topography within the DSA
exhibits no great change because of the protection offered by the dykes.
Consequently, the DEM updating in the Region is in essence the partial
updating. Three kinds of methods have been adopted with relation to the
different situations: 1) To collect elevation data from the air photos,
which means that, in accordance with the characteristics that in the
flood seson Dongting Lake always cover a vast expanse, and during the
dry season that becomes some flood channels, the air photos for the dry
season are to be selected for treatment with air line survey method or
regional mean interpolation for densifying the control points so as to
collect the elevation points according to certain standard on C-12,
plaincomp or Z2 Orthocomp; and the derived points are to be interpolated
as elevation data for obtaining the DEM data concerning rivers and
lakes. 2) To digitize the maps of topography ; with availability of
relatively new maps of topography, it is possible to read grid points or
digitize elevation points and contour lines so as to densify the
sampling points, carry out conversion of data and elevation
interpolation for obtaining partial DEM data. 3) To input the data
derived from radar depth survey; the development of radar depth survey
technique provides a speedy method for obtaining the underwater
topographical data ; with the input of the depth survey data obtained
during the flood season and the corresponding water stage data observed
by the regional hydrologic stations, it is possible to carry out
interpolation of elevations for obtaining the underwater DEM data. The
adoption of the method makes it possible to complete the updating of the
under water topographic data in the Region within half a year.
- Updating of the Existing Landuse Data: The remote sensing
information with relation to the data updating requires registration of
images with elevation data, classification of images, extraction of
information and thereafter the synthesis of information, format
conversion, entering into the system for comparison with the data in the
system so as to replace the old data by the new ones. Image registration
employs the mode of polynomial interpolation for the purpose of
correction, in which it is necessary to determine the polynomial
correction models through a certain amount of control points, using the
least square method for deriving the polynomial coefficient. The pixel
size is to be chosen in accordance with the characteristics of images
and the resampling is to be conducted using the method of the most
adjoining points. Geometrical corrections are to be made for both the
correction of geometrical error of images and the positioning of the
graphic coordinates into the geographical coordinates system. Image
classification employs the most extensively used method of the natural
supervision, classifying MSS, TM,SPOT images , air photos, sided-looking
radar images respectively. The extraction of the special information
uses the methods of threshold value, multiple layer analysis, structural
analysis, image analysis, logic sliding or repressive sliding, multiple
temporal analysis, etc. The database updating just means the regional
synthesis of the classified images, the connection of the classifed
images in terms of object types and then the conversion from grid to
vector for obtaining the vector information with relation to the objects
on the land, which are to be entered into the database for updating the
related data. Nevertheless, as not all the data in a certain layer shall
be updated, then artificial control is necessary. In connection with the
different conditions, we have provided protection for certain object
types such as the dykes, settlements, wood land and some other
information as slow change elements, which are not to be updated, or
chosen certain types such as the reeds outside the DSA, plots with water
weed and land on lake beach for the purpose of database updating. Such
method of data updating with relation to only certain object types and
local areas can greatly reduce the amount of work in volved by data
updating.
Model development and system
application
Development of application analysis models and expert
system is the key to the functioning of GIS. On one hand, the
scientificality of decision making is to be strengthened through the
application of the system so as to enable the methods of GIS to have a
share in the process of decision making; and on the other hand, it also
provided a new sphere of research for the GIS development.
- Analysis of the benefits of the DSA already chosen for the
purpose of flood storage: As to the selection of the DSA for flood
storage, it is imperative to take such factors as natural ecology,
social economy, psychology of the decision makers and so on into
account. The system model shall carry out appraisal of every DSA in
terms of flood storage in accordance with the areas of inundation
varying with different elevations of each DSA, storage capacity, amount
of mud and sand possibly taken in, inundation loses and regional
comprehensive function of flood prevention, raises the optimized schemes
and assess the schemes drawn by the decision makers.
- Selection of the gate positions for flood diversion and flood
outflow: Whether the DSA' for flood storage will bring the maximum
benefits depends on the selection of the positions and sizes of the
gates for flood diversion and outflow. Their correct selection can
effectively lower flood water stages on one hand and bring in a great
amount of mud and sand so as to warp and to turn flood and silt into
resources on the other hand, thereby achieving the greatest benefits of
flood diversion. We have taken Huameiyuan as an experimental region, in
which the process of warping with relation to the variations of
environment within the DSA was simulated The result of simulation was
then compared with the actual warping result, thus arriving at the
conclusion that the number of the flood inflow gates should be as few as
possible , their position should be designed at the uppermost part of
the silt area so as to enable flood water to flow through the greatest
possible part of the silt area with the back water or dead areas being
as small as possible; and the outflow gates should be located at the
lower part of the silt area with a considerable distance between the
inflow and outflow gates so as to make a relatively great amount of mud
and sand taken in by flood water deposit in the silting area. As to some
small low-lying places with protective embankments within the Region ,
where flood prevention is poor in capacity and in need of a great amount
of engineering works and the production is unstable, it is possible to
warp to silt up the ground so as to find gradually a thorough solution
to the problem of flood prevention and drainage of flood-logged fields.
- Simulation of the paths for withdrawal of flood victims and the
network for reasonable distribution of relief materials and goods:
With support of the ARC/INFO system, we have ascertained those safe
areas without the possibility of inundation according to the DEM data
for various villages with protective embankments. The paths are to be
chosen with relation to each settlement versus each safety target with
the safe areas being the target and the shortest possible path and time
being the standard. The lengths of distance and required time are to be
compared for determining the shortest possible distance and time needed
to reach the targets. Considering the fact that the scope of each safe
area is limited and might not be able to admit all the people
withdrawing by the shortest possible paths within the shortest possible
time, the planning of the withdrawal routes are divided in terms of
different targets. The system model can also suppose some prohibited
area and then quickly provide new optimized schemes.
- Simulation of the potentially dangerous dyke sections: With
occurrence of dyke breaking or flood diversion, the erosion datum level
of the river upstream of the flood gates drops by a big margin, the
slope of water surface increases, the discharge and flow velocity in the
river downstream the flood gates reduce, thus inevitably resulting in
variation in washing and silting of the riverbed and possibly leading to
dyke breakings in both upstream and downstream sections due to flood
diversion at one place. This is the reason behind the chain reactions in
both upstream and downstream sections in the form of multiple dyke
breakings caused by dyke breaking at one place only. Through the use of
the data concerning the longitudinal river sections supplied by the
database and with aid of hydraulic equation and river correlation
equation, we have conducted simulation of the process of washing and
silting variations in river for the prediction of the potentially
dengerous dyke sections which might occur in different periods of time
following flood diversion; which follows that the variation process with
time is to be described through the retrogressive washing (or silting)
points.
- Analysis of flood conditions: Changjiang River can both
regard Dongting Lake as its flood channel and utilize the regulating
action of natural lakes to reduce the pressure of flood on Changjiang
River. However, owing to mud and sand siltation, the storage capacities
of the natural lakes are being reduce from time to time. In accordance
with the DEM data for the Region in the tow periods of 1950s and 1970s,
the variations of the lake capacities with the water stage of
Chenglingji at 32 m was computed ;and the result of computation
indicates that the lake capacity was reduced by 12, 600 million cu. m.,
of which the capacity of East Dongting Lake was reduced by 3, 250
million cu. m. and South Dongting Lake by 2, 050 million cu. m.,
markedly demonstrating the weakening of the flood storage capacity. With
dynamic simulation result of the future development of Dongting Lake,
the lake capacities for 1990 and 2000 have been estimated as 14,000
million and 1`0,550 million cu. m. respectively. However, with the use
of the system dynamic model for the simulation of the joint flood
conditions in Changjiang River and Dongting Lake, under the conditions
of reappearance of the 1954 flood, the control water stage at
Chenglingji is 32 m; and the total flood water volume in excess of the
river and lake capacities amounts to 45,000 million cu. m., ;even if the
Iingjiang Flood Diversion Area divert 16, 000 million cu. m. there will
be still 23, 200 million cu. m. of flood water which shall be undertaken
by Dongting lake. Though the simulation model also arrived at the
conclusion that the capacity of reducing the flood peak does not
necessary depend on the lake capacity entirely, but rests with the lake
area and the height of the pre-peak stage anyhow the increasing
reduction of the lake capacity will inevitably lead to the intensified
hazards caused by floods.
- Studies of the flood disaster countermeasures: The river -
harnessing ideology of 'Toprotect the north at he expense of the south'
adopted in the past has led to the deterioration of the relationship
between the River and the Lake. Then , what kind of influence will
various flood preventive projects which we are undertaking today produce
upon the regional environment ? This is a big problem which requires
solution. simulation of the flood conditions in 1983 with the use of the
hydraulic model has arrived at the conclusion that, if the discharge
capacity at Chenglingji is raised by 1 0 percent, it is possible to
reduce the bottom water between two flood peaks in Dongting Lake by 0.57
m, form which it can be seen that the increase of the discharge at
Chenglingji plays an important role in reducing the flood hazards in
Dongting lake . The river section from Chenglingji to wuhan belongs to a
silting type one; therefore , despite the conclusion arrising from the
simulation with the Hydraulic model that the increase of the discharge ,
blocking of the three distributaries for preventing mud and sand from
gathering into the Lake will only shift crisis onto other places,
intensify the raising of water stage at the entrance of the Lake and
deteriorate the flood conditions in the middle reaches of Changjiang
River . Moreover, the method of this type does not conform with the
regularity of the lake basin delta development in the mid-steam
inland.
The simulation model also demonstrates the fact that the
water stage at Chenglingji is not sensitive to various engineering
measures. it is possible to put some engineering project of considerable
scale within the Region so as to change the movement of water and sand
without affecting the river-lake relationship. Realignment of flood
channels is a measure yielding multiple benefits; to be specific , it is
possible to lower the water stages in most lakes, to prevent local
backwatering, to relieve the deterioration of flood conditions caused by
silting of mud and sand, to reduce the difference among the water stages
of Eastern , southern and western Dongting Lakes and to change the
situation in which Dongting Lake takes the lead in facing danger before
the specified stage for flood diversion is reached at Chenglingji.
Conclusion
The DLIS was designed and constructed in
connection with the special spatial structure of he Region. The
combination of the multi-layer island-like data with the river system
network data characterizes the spatial logic structure of the database. At
present, the construction of the system has been in the main completed;
and the preliminary operations have obtained comparatively ideal results.
In 1989 , the remote sensing experiment of flood prevention in the middle
reaches of Changjiang River conducted by the Ministry of Water resources
used our database for estimation of the disaster losses. This year , in
answer to the demand raised by Bangladesh, we have begun to undertake the
subject of remote sensing application for flood prevention. Through such
international channels, the experience gained from the design and
application of DLIS is being spread to the area with similar geographical
environments both in china and abroad, thus giving an impetus to the
application of the new technique in flood prevention and disaster relief.
This is just an important aspect of the ten-year calamity-reducing plan.
References
- Office of the National Territory Commission of Human Province and
Research Centre of Economy of Human Province: Special Report on the
synthetic survey and Study of the Renovation and Development of Dongting
Lake Region, 1988, pp 707-760
- National Laboratory of Resources and environment Information System
( LREIS) Institute of Geography , Chinese Academy of Sciences: Annual
Report of LERIS ( 1988-1989), Surveying and Mapping Press, 1989, pp
221-229.
- Hunan Provincial Centre of Scientific and Technological Consultancy,
Subject Group of Renovation and Development Planning for Dongting Lake
Region: Report on the Planning Model Technology for the Renovation and
Development of Dongting Lake Region in Hunan Province , 1988, pp. 60-68
- Xing Fulin, Journal of Sediment Research, No. 3, 1987, pp. 62-70
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