Identification of potential areas susceptible to flood inundation in the Lower Guayas River Basin Santiago Lopez Graduate Student Department of Geography

1.Introduction Evaluate the sensitivity of: DEM-based surface Landsat ETM image for characterizing flood inundation through statistical tests involving the comparison of flood areas extracted from geomorphology and soils maps.

Hypothesis There is no significant relationship between potential flood inundation areas obtained from geomorphology and soil maps and potential inundation zones determined with the aid of a DEM-based surface and a Landsat ETM satellite image.

2. Study Area Lower Guayas River Basin, Ecuador Ecuador Peru Colombia flythrough

3. Cartographic/conceptual model

4.Methods and Data a. Satellite image processing Tasseled cap transformation Wetness = TM TM TM TM4 – TM5 – TM7

The wetness index derived from the tasseled cap transformation

(1/d 1 n )V 1 +(1/d 2 n )V 2 +(1/d 4 n )V 4 +(1/d 5 n )V 5 +(1/d 6 n )V 6 +(1/d 7 n )V 7 +(1/d 8 n )V 8 (1/d 1 n )+(1/d 2 n )+(1/d 4 n )+(1/d 5 n )+(1/d 6 n )+(1/d 7 n )+(1/d 8 n ) A= b.Generation of the Digital Elevation Model Inverse Distance Weighting (IDW) i=1 Σ (1/di n ) i=1 n Σ (1/di n )V i n A= A = interpolated value di= distance to known value n= decay parameter

Elevation Distance DEM Mean DEM Convex areas c. Determination of convex areas based on the difference between the DEM and a mean DEM

4. Results a. Potential flood inundation areas based on wetness index

b. Potential flood inundation areas from DEM-based surface

c. Potential flood inundation areas derived from soil and geomorphologic units

Flood areas derived from the Landsat image vs. Map of Flooded Areas Chi-square 19.00, significant at a 0.05 level Reject null hypothesis Cramer’s Phi = 0.12 DEM areas vs. Map of Flooded Areas Chi-square 26.98, significant at a 0.05 level Reject null hypothesis Cramer’s Phi = 0.17 d. Statistical Analysis

5. Conclusions Integration of geomorphologic and soils data, simple DEM-based surfaces, and satellite images maybe a useful first approach to characterize flood inundation areas in regions where spatial data is scarce or non available. Important limitations for site-specific studies. For finer scale studies specific hydrologic data (e.g. profiles of flood elevation and in-situ flood measurements) could enable the analyses to be linked to real properties of the river flow regime.

6.Future research Compare results with a DEM generated with Arc Info's TOPOGRID model, which seems to generate surfaces more representative of the actual topography, specially for hydrologic modeling purposes. Derive a wetness index using the DEM and test its sensitivity in relation to the wetness index derived from the ETM scene. Wi = ln (A/tanB) Where Wi = wetness index; A = upslope contributing area; B = surface slope

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