Flood/Wetland Mapping

9.4.3.1 Introduction

SAR imagery has proven to be very useful for flood mapping and for wetland vegetation classification. The dark returns from water contrast with the brighter ones from land and flooded vegetation thus making it possible to identify open flooded areas. This makes possible the determination of the extent of the flood although problems can occur when attempting to map flooded vegetation. Wetland vegetation also has a variety of shapes, sizes, and distributions, which can be used for discrimination between vegetation types. The use of multi-temporal SAR data has been shown to be useful for wetland classification due to the seasonal changes in vegetation and water levels which affect the microwave backscatter. Polarimetric SAR data can be used to improve extraction of information for these applications.

9.4.3.2 Flooded Areas

Did you Know?

Wetlands are a key part of the ecosystem for maintaining both water quality and quantity. They are also prime breeding locations and thus are critical components for maintaining ecosystem health.

 

Figures 9-35 to 9-37 show various C-band images acquired by SIR-C of the Red River flood in 1994. There is a significant improvement in the mapping of the flooded regions using the HV image on April 11 compared to the HH image where most of the flooded area is delineated and the VV image where identification of the flooded region is difficult. On April 12, there is little such difference between images in the different polarizations. On April 16, the HH and HV images are very similar. These relative changes between images in the different linear polarizations show the value in the use of imagery in the various polarizations for flood mapping.

Figure 9-35

Figure 9-35

 

Figure 9-35. Linearly Polarized SIR-C C-Band images acquired on April 11, 1994 of the Red River, Manitoba (from Sokol et al).

Figure 9-36

Figure 9-36

 

Figure 9-36. Linearly Polarized SIR-C C-Band images acquired on April 12, 1994 of the Red River, Manitoba. (from Sokol et al).

Figure 9-37

Figure 9-37

 

Figure 9-37. Linearly Polarized SIR-C C-Band images acquired on April 16, 1994 of the Red River, Manitoba (from Sokol et al).

9.4.3.3 Wetlands

The use of imagery in multiple polarizations has been found to improve wetland classification when compared to the use of single channel data alone. Two classes may be confused in imagery of a particular polarization but separable in imagery at other polarizations. This is especially true for wetlands which have a mix of vertically oriented plants such as sedges, rushes, and grasses interspersed with shrubs and trees, which have a more random distribution of vegetative components.

An example C-SAR image of an area in the valley of the St. Lawrence River showing several classified wetlands is shown in Figure 9-38.

Figure 9-38a

Figure 9-38a

Figure 9-38b

Figure 9-38b

 

A = Marsh
B = Woody Marsh
C = Shrubby / Herbaceous swamp
D = Marsh / shrubby Swamp
E = Wooded Bog

Figure 9-38. False colour composite of C-SAR imagery acquired September 1, 1997 showing several wetland classes along the St. Lawrence River, Ontario: Red: HH; Green:HV Blue: VV. Courtesy of CCRS.