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Chapter 16: Water Resources

16.41: Flooding in the Assiniboine Valley Near Virden

This radar image of part of the Assiniboine Valley near Virden was obtained in the spring of 1995, the same year as the photo in figure 16.40. Radar is an active form of remote sensing in that signals are sent out from a source, hit the ground, and are partly reflected to a recording apparatus. In the case of side-looking, airborne radar pulses are sent out from the side of the plane and the imagery is consequently “side-lighted.” Several factors affect the strength of the return signal: 1) Terrain orientation—slopes that face the incident radar beam give a strong return signal and are consequently light-toned on the final image; 2) Surface roughness—rough surfaces, termed diffuse reflectors, scatter the incident signal and some of the backscatter is picked up by the sensor resulting in a light tone on the image (figure 16.41a). In contrast, smooth surfaces, termed specular reflectors, allow the incident signal to bounce off them, away from the sensor; the result is a dark tone on the image (figure 16.41b). A bright image is also produced by “corner reflectors” in situations in which a lot of right angles occur, for example, buildings in a town (figure 16.41c).

16.41a) diffuse reflector   16.41b) specular reflector   16.41c) corner reflector[i]


3) Electrical characteristics of terrain features—“one measure of an object’s electrical character is the complex dialectric constant…. In the microwave part of the spectrum (which radar uses), most natural materials have a dialectric constant in the range 3 to 8 when dry. On the other hand, water has a dialectric constant of 80. Thus, the presence of moisture in either soil or vegetation can significantly increase radar reflectivity…. Metal objects also give high returns”[ii] and consequently appear light-toned on radar images.

“In the early spring (1995) a combination of above normal snow precipitation and unseasonably cool temperatures in the south-western part of Manitoba resulted in a large snow melt causing the Assiniboine to flood.”[iii] Although the Shellmouth Dam completed in 1967 partially controls flow along the Assiniboine, the Qu’Appelle and other tributaries enter the Assiniboine River below the dam and upstream from the location show here. Spring flooding is still common. “The 1995 peak flows for the Assiniboine were 360 m3/s, 566 m3/s, and 300 m3/s as compared to mean flows of 32 m3/s, 81 m3/s, 115 m3/s recorded for April at the three stations respectively since 1913.”[iv]

The very dark tones on the image delimit the flooded area 1. These result from specular reflectance from the smooth floodwater surface. The river channel 2 and three abandoned channels 3 are also dark-toned for the same reason. The light tones in the flood plain result from diffuse reflectance from rough treetop surfaces 4. The east wall of the Assiniboine Valley, facing the direction from which the incident rays came, is light-toned 5 whereas the west wall is darker 6. Also light-toned are the tree covered valley sides of Bosshill Creek 7 and two small creek valleys 8 on the east wall of the spillway.

Also of note are the following:

1) Virden 9 which is mainly light toned because of the existence of corner reflectors; and 2) a light-toned metal railway line 10 compared with a dark-toned, smooth surfaced road 11 (the Trans Canada Highway) 11. Section lines can be identified 12, and note that the correction north of township 10 in this western location is over one mile (1.6 km) 13. 3) the orange rectangle indicates the area covered in figure 16.42.

Figure 16.41: Flooding in the Assiniboine Valley Near Virden

Figure 16.41: Flooding in the Assiniboine Valley Near Virden

Figure 16.41

Platform CCRS (Canada Centre for Remote Sensing) Convair 580.

Imaging system: Side Looking Airborne Radar C-HH (C band horizontal transmit/receive polarization image configuration)

Scale: 1:90,000 (approx.)

Date: April 22, 1995

Location: Township 10 and 11; Ranges 25 and 26WI

Map sheets: 1:250,000 62F Virden

1:50,000 62F/15 Virden


[i] Lillesand, T. M. and Kiefer, R. W. Remote Sensing and Image Interpretation (Third Edition). New York: John Wiley and Sons. 1994, figure 8.18, 621.

[ii] Lillesand, T. M. and Kiefer, R. W. op. cit. 1994, 673.

[iii] Information from Canada Centre for Remote Sensing (CCRS) web site

[iv] Informatin from Canada Centre for Remote Sensing (CCRS) web site