3. Two ancillary lines were defined between the thalweg and the right and left banks to address cases such as the variation of the channel width (Fig. 4). It also enabled the modelling of a section with a secondary channel north of an islet, which would be ignored if only three of the five interpolation supporting lines were used (Fig. 5);

4. Exceptions were considered in cases where sandbanks or islets

were present, of morphological changes of the river or the availability of 2007 data from the bathymetric survey (Fig. 6 -8);

5. Auxiliary points were distributed along the described lines. For

these, height values were linearly interpolated based on cross-section data, for each river section.

Santarém

Ómnias

Tramagal

A methodology to generate a digital elevation model by combining topographic and bathymetric data in fluvial environments

M. P. Matias 1, A. P. Falcão 1, A. B. Gonçalves 1, R. Pestana 1, S. Heleno 1, E. Van Zeller 2, T. Alvares 2 , V. Rodrigues 2

1 – Instituto Superior Técnico - Universidade de Lisboa 2 – Portuguese Environment Agency

INTRODUCTION Hydrodynamic models have limitations, not only in considering physical and hydrodynamic conditions in the simulation of floods, but also in the maximum number of elements used to describe the channel geometry and roughness, requiring a combined elevation from the river bed (bathymetry) with the topography from adjacent areas covering the floodplain. Results depend on the properties of the elevation model and the channel bathymetry, namely the horizontal resolution, is known to affect the results of models. To create a coherent digital elevation model (DEM) as required in the simulation stages, a set of GIS techniques for merging the datasets of bathymetry and topography, and acquired by different techniques is described in this poster, but only focused on DEM production. A validation of a simulation of a specific flood using the merged continuous river and floodplain elevation model was performed by comparing the flood extension with the results of a segmentation of a SAR image of the same moment

STUDY CASE The study area is a section of 70 km of the Tagus River, between Tramagal and Ómnias (Fig. 1). The Tagus River is the longest stream in the Iberian Peninsula and is responsible for periodical floods in an important agricultural area in Portugal.

RESULTS

The generated surface was resampled to a grid with 50m spatial resolution and used in the hydrodynamic simulation of the January 5th, 2001 Tagus River flood, modelled by the TUFLOW package. Validation of the results was performed by comparing the extensions of the identified flooded area in SAR imagery via segmentation - acquired by the ERS-2 sensor on the same day - and the results of the simulation. It is possible to observe that the simulation generated with the new surface model is very close to the flooded area, recorded in SAR imagery (Fig. 10).

Cross-section 34

Cross-section 33

Right bank

Left bank

Ancillary line

Ancillary line

Thalweg

Figure 3 - Identification of the thalweg line, right and left banks and the two auxiliary lines.

Cross-section 34

Cross-section 34

Without ancillary line With ancillary line

Legend

TIN_no_ancillary_line

Edge type

Soft Edge

Hard Edge

Elevation

21.034 - 22.02

20.049 - 21.034

19.063 - 20.049

18.078 - 19.063

17.092 - 18.078

16.107 - 17.092

15.121 - 16.107

14.136 - 15.121

13.15 - 14.136

TIN_ancillary_line

Edge type

Soft Edge

Hard Edge

Elevation

21.39 - 22.42

20.36 - 21.39

19.33 - 20.36

18.3 - 19.33

17.27 - 18.3

16.24 - 17.27

15.21 - 16.24

14.18 - 15.21

13.15 - 14.18

Legend

TIN_no_ancillary_line

Edge type

Soft Edge

Hard Edge

Elevation

21.034 - 22.02

20.049 - 21.034

19.063 - 20.049

18.078 - 19.063

17.092 - 18.078

16.107 - 17.092

15.121 - 16.107

14.136 - 15.121

13.15 - 14.136

TIN_ancillary_line

Edge type

Soft Edge

Hard Edge

Elevation

21.39 - 22.42

20.36 - 21.39

19.33 - 20.36

18.3 - 19.33

17.27 - 18.3

16.24 - 17.27

15.21 - 16.24

14.18 - 15.21

13.15 - 14.18

Figure 4 - Results of a linear interpolation without (left) and with (right) the supplementary lines.

Figure 5 - Detail of secondary channel which would be ignored if the supplementary lines were not used.

Figure 2- Cross-sections location under DEM2008, and an example of two of the cross-sections profiles.

With ancillary line

Legend

tin_44_45

Edge type

Soft Edge

Hard Edge

Elevation

12.079 - 13.589

10.57 - 12.079

9.06 - 10.57

7.55 - 9.06

6.04 - 7.55

4.53 - 6.04

3.02 - 4.53

1.51 - 3.02

0 - 1.51

Figure 9 - Comparison of the DEM2008 dataset and the combined surface model

Legend

tin_35_36

Edge type

Soft Edge

Hard Edge

Elevation

17.456 - 18.667

16.245 - 17.456

15.035 - 16.245

13.824 - 15.035

12.613 - 13.824

11.402 - 12.613

10.192 - 11.402

8.981 - 10.192

7.77 - 8.981

Legend

tin_45_46

Edge type

Soft Edge

Hard Edge

Elevation

11.811 - 13.288

10.335 - 11.811

8.859 - 10.335

7.382 - 8.859

5.906 - 7.382

4.429 - 5.906

2.953 - 4.429

1.476 - 2.953

0 - 1.476

Initial DEM Final DEM

SECOND PART In the second part of the methodology, the height values for an intermediate region are set up by combining the river channel height model with the floodplain surface model. Following the strategy of Merwade et al. (2008), two buffer areas, that hem the river banks, delimitate the area for a combination of the channel and floodplain height datasets: auxiliary points in the most external buffer perimeter take the height values from the DEM2008 dataset, while points along the most internal buffer limit take the interpolated channel height model obtained in the previous step. Height values on the area covered by this intermediate region are linearly interpolated and will have precedence over the other two datasets for the final combination step - a combination of the height values where the intermediate region surface takes precedence over the remaining datasets, followed by the river channel and finally DEM2008 (Figure 9).

Figure 10 - Comparison of the extensions of the flooded area.

METHODOLOGY Available data A DEM was available (DEM2008), acquired in 2008 by radar techniques with 5m of spatial resolution, accurate in the floodplain area but artificial in the thalweg channel. Also available were 24 cross-sections acquired by eco-sounder equipment in 2012, numbered 33-59. (Fig. 2). Around the hydrometric stations (Tramagal, Almourol and Ómnias), a bathymetric survey was conducted in 2007 where points were acquired with 5m space resolution.

FIRST PART The first part of the methodology intends to reconstruct the thalweg channel morphology by combining cross-sections acquired in 2012 and the DEM2008 dataset.

REFERENCES Merwade, V., Cook, A. & Coonrod, J. (2008). GIS techniques for creating river terrain models for hydrodynamic modeling and flood inundation mapping. Environmental Modelling and Software 23, 1300 –1311.

Figure 6 - Detail of islet traversed by a cross-section.

Figure 7 - Detail of a morphological evolution in the islets.

Figure 8 - Detail of a section with a complex outline of sandbanks/islets

Figure 1- Study area: a section of the Tagus River between Tramagal and Ómnias.

Legend

Flood_segmentation _2001.01.05

Flood_final_DTM_2001.01.05

Legend

Flood_segmentation _2001.01.05

Flood_final_DTM_2001.01.05

Legend

Flood_segmentation _2001.01.05

Flood_final_DTM_2001.01.05

1. Based on DEM2008, the limit of the right and left banks of the thalweg channel were identified;

2. The start and endpoints of the thalweg line, sectioned into

segments of 25m, were based on cross-sections data, while between the sequential profiles this line was defined halfway between the right and left banks (Fig.3);