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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.
  1. 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.

  2. 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
  1. 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.

  2. 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.

  3. 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.

  4. 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.

  1. 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.

  2. 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.

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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