sediment deposition mapping in aswan high …iwtc.info/2005_pdf/04-2.pdfmapping the reservoir...

Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt

239

SEDIMENT DEPOSITION MAPPING IN ASWAN HIGH DAM RESERVOIR USING GEOGRAPHIC INFORMATION

SYSTEM (GIS)

Hossam El-Sersawy Nile Research Institute (NRI), National Water Research Center (NWRC), Egypt.

E-mail: [email protected]

ABSTRACT

Reservoirs are a vital source of water supply, provide hydroelectric power, support diverse aquatic habitat, and provide flood protection. Sediment deposition has gradually reduced the effectiveness of reservoir operation over the years by decreasing the storage capacity. This research presents geographic information systems (GIS) based application for investigating sediment deposits in Aswan High Dam Reservoir (AHDR). It is the second largest man-made reservoir in the world, extending from the southern part of Egypt to the northern part of Sudan, about 500 km length. The Nile Research Institute (NRI) conducts annually bathymetric survey of the reservoir using Differential Global Positioning System (DGPS). Spatial data were collected from aerial photographs, bathymetric data, and satellite images corresponding to the study area. This research was performed in many stages as survey planning, survey execution and storage, data preparation and pre-processing, spatial data and attributes data creation, database building, and the results presentation and analysis.

The results included a detailed GIS database from which contour maps, color-by-depth hill shade maps, and surface difference maps of the reservoir bed elevations were generated. Surface difference maps were produced with each subsequent survey to illustrate the seasonal sediment transport characteristics of the system. Based on the results of the GIS applications, it was identified the major zones ranging from low to high sediment deposits. The accumulation of sediment deposits over the years, associated depths, and their geographical distributions was recognized by GIS capabilities. It was concluded that the GIS application, based on widely available and easily developed digital data, provides realistic and valuable information in a short time and is a powerful tool which could be used to develop sedimentation maps for the Aswan high Dam Reservoir.

KEY WORDS Aswan High Dam Reservoir, Geographic Information Systems (GIS), Sediment Deposition, Bathymetric Survey, Differential Global Positioning System (DGPS).


240

INTRODUCTION Reservoirs offer many benefits to the communities including flood control, water supply, fish, and hydropower. Determining the impacts of sediment on the reservoir operations is critical to maintaining current operations and planning for future needs. Sedimentation within the reservoirs is the main problem that could reduce the reservoir capacity and therefore affecting its economic life. Proper management of the reservoir requires that current reservoir volumes and sedimentation rates must be determined. Current trend towards a more efficient management of reservoir is using the application of Geographical Information System (GIS). The geographic information system (GIS) is used for importing, analyzing, modeling, visualizing, and reporting information for the reservoir and gives functions of spatial data management, mapping and analysis to assist decision-making. Mapping the reservoir bathymetry has been used to define reservoir bed characteristics. Repetitive bathymetric mapping can help in determining the sedimentation rates, scour and deposition of bed material, and the effectiveness of dredging.

The Nile Research Institute (NRI) annually performs the sedimentation survey for the Aswan High Dam Reservoir to determine the amount and distribution of sediment deposition through the reservoir. Since year, 1973 the bathymetric surveys were conducted for only few known cross sections because of the reservoir size which caused a difficulty to investigate the reservoir sedimentation progress by using the traditional survey method. However, from year, 1999, it was proposed an alternate method of mapping the reservoir bottoms by using a hydro acoustics system with a Differential Global Positioning System (DGPS) and Echo sounder to collect depth measurements and locations. The new technology provides the bathymetry data in suitable format that can be used to create digital maps. This paper will focus on using the geographic information system (GIS) technique to perform the analysis and process the bathymetric data to produce sedimentation maps for the bottom of the reservoir in different years. In addition, it was discussed the benefits of using GIS approach over the traditional methods for the determination of sedimentation locations and thickness in the reservoir.

OBJECTIVES

This research will utilize the Aswan High Dam Reservoir-related data to develop GIS application in order to meet the following three objectives:

- A comparison between the traditional and the GIS approaches for sedimentation analysis in the reservoir.

- A definition of sediment deposition patterns by taking the advantage of GIS capabilities to produce sediment deposition mapping for the reservoir bottom.


241

Study Reach

Figure 1. Hydrographic Survey Stations at Aswan High Dam Reservoir (AHDR)

- An evaluation of usage of GIS technique for estimating the lateral and longitudinal distribution of deposited sediment in reservoir.

STUDY AREA

The Aswan High Dam Reservoir (AHDR) is the second artificial lake in the world, extending from the southern part of Egypt to the northern part of Sudan, about 500 km length. It extends bout 340 km in Egypt and almost 160 km farther upstream in Sudan as shown in Figure 1. The reservoir was created after the construction of the High Aswan Dam (1964-1968). The reservoir width depends on the water levels, however the average width of the reservoir is about 12 Km. The water levels are oscillating between 152 to 182 meters over the sea level. The Aswan High Dam Reservoir (AHDR) is the water bank of Egypt, contains about 162 billion cubic of meters renewable fresh water. It was estimated that more than 134 million ton per year is deposit in the reservoir and has altered reservoir volume.

It was recognized that the potential ability for sedimentation has an effect on the capacity of the reservoir and the need for data to assess the status of sediment deposition. These management goals require a thorough knowledge of the reservoirs' characteristics including their volumes.

TRADITIONAL APPROACH FOR SEDIMENTATION ANALYSIS

Traditional methods of sediment deposition analysis in the Aswan High Dam Reservoir (AHDR) were based on the comparison of the surveyed cross sections in different years to estimate the sediment volumes of the reservoir.

The total amount of deposited sediment was evaluated by assuming gradual distribution to the amount of sediment between each two successive cross sections as shown in Figure 2. Reservoir volumes were calculated by measuring the area of the cross sections and


242

lengths between these sections. The volume was calculated as the sum product of the mean area of every two successive sections and the length between them. From a comparison of the year 2003 reservoir volume with the year 1964 (original volume), It was estimated that more than 5.2 Milliards tons of sediment deposit in the reservoir. In addition, the deposition thickness for each cross section of the reservoir was obtained from year 1964, to year 2003 as shown in Table 1. It was concluded that the traditional method has provided adequate results and accurately represents the sediment volume and it can represent the longitudinal distribution of sediment deposition along the reservoir as shown in Figure 3. However, the traditional method would never be used for mapping as it lacks a visual component to represent the extent of sediment deposition.

CROSS SEC.NO (D) KM 372 UPSTREAM ASWAN HIGH DAM

100110120130140150160170180190200

0 200 400 600 800 1000 1200 1400 1600 1800DISTANCE (M)

LEV

EL

(M)

2003

1998

1964

CROSS SEC.NO (3)KM 378.5 UPSTREAM ASWAN HIGH DAM

100110120130140150160170180190200

0 200 400 600 800 1000 1200DISTANCE (M)

LE

VE

L (M

)

200319981964

CROSS SEC.NO (28) KM 368 UPSTREAM ASWAN HIGH DAM

100110120130140150160170180190200

0 500 1000 1500 2000 2500DISTANCE (M)

LE

VE

L (M

)

2003

1998

1964

CROSS SEC.NO (24) KM 347 UPSTREAM ASWAN HIGH DAM

100110120130140150160170180190200

0 1000 2000 3000 4000 5000 6000

DISTANCE (M)

�L

EV

EL

(M)

200319981964

Figure 2. Cross Sections of Aswan High Dam Reservoir For Years 1964, 1998, and 2003


243

Table 1. Names and locations of the Hydrographic Survey Stations in AHDR

No. Cross Section

Name Cross Section

Number Distance

Upstream Aswan High Dam��(km)

Thickness of Sediment Deposition

(1964-2003) (m)

1 Daka 23 487 9.52 2 Akma 19 466 15.29 3 Malek Naser 16 448 27.38 4 Dewashiate 13 431 31.59 5 Ateri 10 415.5 35.26 6 Semna 8 403.5 43.52 7 Kagnarty 6 394 59.48 8 Morshed 3 378.5 53.93 9 Gomi D 372 61.13 10 Madik Amka 28 368 41.38 11 Amka 27 364 27.65 12 Gandal Sanii 26 357 48.66 13 Abdel Khader 25 352 43.36 14 Dogheem 24 347 33.37 15 Dabrossa 22 337.5 29.88

GIS Approach for sedimentation analysis The advantages of the new technologies such as GPS and GIS have been taken into consideration in order to create methodology of sedimentation analysis in reservoirs by offering a detailed characterization of sediment deposition distribution.

Figure 3. Longitudinal Section of the Aswan High Dam Reservoir

Longitudinal Bed Profile of High Aswan Dam Reservoir

5060708090

100110120130140150160170180

050100150200250300350400450500

Distance Upstream Of Aswan High Dam (km)

Eleva

tion (m

)

196419731977199519982003


244

Figure 4. Bathymetric Data of the Bed Elevations for the Study Area

1. Methodology

In this research, it was selected the bathymetry survey of years 1999, 2001, and 2003 only for the reservoir area between the cross section number (3) at Km 378.50 and the cross section number (24) Km 347.00 upstream Aswan High Dam which contain the major sediment deposition. In addition, this area has the availability of data in formats, which can be used to create digital surfaces of the reservoir bottom. A Geographic Information System utilizing the ArcView3.2 software was used in this application. The approach calculates the sediment volumes over entire reservoir area by comparing these digital surfaces. Through this comparison, sediment distribution can also be analyzed. The Universal Transverse Mercator (UTM) coordinate system was used in the application to allow for utilization of data from other resources. All data sources such as aerial photographs, satellite images, and other data have the same coordinate system and projection to create the base map for the reservoir. Sediment investigations will be not accurate if the baseline data have a poor quality.

2. Data Preparation

The process involves using Differential Global Positioning System (DGPS) to collect bathymetric reservoir data. The digital echo sounder were used to measure depths and an OMNISTAR 6300A receiver incorporating a Leica GPS systems was used to obtain real-time differential GPS data with an accuracy of 0.5 to two meters or less. The Coastal Oceanographic HYPACK MAX software was used for survey and control. The longitude and latitude of each sample point was recorded with its depth. Typically, the survey data has been collected along known cross sections (transects) along the reservoir area inside the Sudan border as shown in Figure 4. The data were analyzed by the depth sounding software, and then reviewed and cleaned in the post-processing stage. The data were then imported into a GIS system and analyzed in various ways. The data files were plotted and examined. Geographic coordinates of data were converted into decimal degrees with a precision of six decimal places. Depths were corrected for the submergence of the transducer and for the elevation of the reservoir surface below the actual water surface elevation measured in known cross stations on the reservoir. The data files containing a sequential identification number, longitude, latitude, and depth for each point were prepared for transfer to the GIS software.


245

Figure 5. ArcView interface application

3. GIS Data Processing The data files containing bathymetry survey data were used to create ArcView point coverage. Depths for each point were added as an attribute to the coverage. Then the loader module creates a point shape file based on the newly converted point’s readings by launching an Add-XY-Event function in ArcView software as shown in Figure 5. Because of errors, a relatively small number of points were located outside of the lake shoreline. An existing coverage of the shoreline which was created from the aerial photographs and the satellite images was used to delete these points from the coverage. The point coverage and coverage of the shoreline were applied to create a triangulated irregular network (TIN) surface model of the reservoir bottom. Contour maps were generated for Aswan High Dam Reservoir by using Spatial Analyst module in GIS software. The ArcView coverage of year 1999 bottom contours of the reservoir was obtained and edited in order to overlap it with the bathymetric data of years 2001 and 2003 as shown in Figure 6.

4. GIS Data Gridding Gridded depths were used in all analysis and presentation for the research. The gridded data were chosen because the GIS process used to identify change from each subsequent survey required gridded data. ArcView 3.2 software was used to create the difference grids for the bathymetric survey of the reservoir. Difference grids were created using the Raster Calculator function in ArcView. Grid extent, cell size and the horizontal position of the grid nodes were consistent between surveys to ensure accuracy in the output grids. The values of the difference grid nodes were created by subtracting the grid of year 1999 data from the grid of years 2001, and 2003 data.

5. Sediment Deposition Mapping The sediment deposition maps from the different surveys years 1999, 2001, and 2003 covering the same geographic area were produced and compared to identify changes in the reservoir bed elevations and illustrate the sediment deposition in the reservoir values as shown in Figure 7. The presentation of this analysis was images color coded by the amount of change. Areas shaded with green indicate of the significance of sediment deposition. Areas of no overlapping data or erosion remained white. It was detected that more than 80 percent of the deposition thickness was more than 2.50 meters. The larger changes from year 1999 to year 2003 occurred in the wider entrance of the reservoir. These maps were generated volumes are based on a more accurate method that used data for the entire reservoir and not just data from a few cross sections. The accuracy of these maps may be affected by the density of the data coverage.


246

Bed Elevation (m) for Year 2003

142.0146.0150.0154.0158.0162.0166.0170.0


142.0146.0150.0154.0158.0162.0166.0170.0


142.0146.0150.0154.0158.0162.0166.0170.0 Sediment Deposition From Year 1999 to 2001

0.000.440.891.331.782.222.673.113.564.00

Sediment Deposition From Year 2001 to 2003

0.000.440.891.331.782.222.673.113.564.00

Sediment Deposition From Year 1999 to 2003

0.000.440.891.331.782.222.673.113.564.00

Figure 6. Reservoir Bed Elevations Maps For years 1999, 2001, and 2003

Figure 7. Sediment Deposition Maps For years (1999-2001), (2001-2003), and (1999-2003)


247

Ideally, data should be collected in a grid pattern dense enough to allow the maps to identify all of the bottom features. The results from the analysis of these maps are being used to track sediment migration pattern with high resolution than can be modeled.

CONCLUSION AND RECOMMENDATION

This study indicated that the combination of hydro acoustics, GPS, and GIS are capable of producing bed elevations maps comparable in accuracy and quality to traditional surveying method. A key difference between the traditional and GIS analysis approaches is that the GIS approach calculates sediment volumes over the entire reservoir area by comparing digital surfaces, whereas the traditional approach applies an average area method to calculate volumes based on a limited number of cross sections.

A future benefit of the GIS analysis approach will be the ability to view time perspective of sediment change and support automated sedimentation analysis. As additional surveys are performed for the Aswan High Dam Reservoir, new sediment depth grids can quickly be created to represent sediment depositions with respect to prior surveys. However, certain issues and problems were recognized during this study. The amount of the surveyed data of the reservoir is not enough to cover the entire reservoir due to the huge area. It is recommended to complete bathymetric survey for all the remaining areas of reservoir in future.

REFERENCES 1. Aziz, M.S. et al. (2003), “Study of Aswan High Dam Reservoir Sedimentation – July

2003)”, Nile Research Institute (NRI) Report.

2. Maguire, D.J., 1992, “Geographical Information Systems: Principles and Applications”, Long man, England

3. Mau, D.P., and Christensen, V.G., 2000, Comparison of Sediment Deposition in Reservoirs of Four Kansas Watersheds: U.S. Geological Survey Fact Sheet 102-00, 4 p.

4. Morris, L.M., and Fan, J., 1998, Reservoir Sedimentation Handbook: New York, McGraw-Hill, p. 4.17.

5. Mustafa, G. (1987). Reservoir Sedimentation, Post Graduate Course in Sediment Transport Technology, proceeding Vol. 2 Ankara, Turkey

6. Sheppard, S.R., 2000, “Visualization Software: Bringing GIS Applications to Life”, GeoEurope, Issue 8.

sediment deposition mapping in aswan high …iwtc.info/2005_pdf/04-2.pdfmapping the reservoir...

Documents