INVESTIGATING THE ROLE OF BEDROCK GEOLOGY AND SLOPE IN CONTROLING LAVAKA (GULLY) DISTRIBUTION IN CENTRAL MADAGASCAR

Lavaka are groundwater-sapping gullies in Madagascar, the causes of which are still controversial. This study quantifies lavaka density in relation to underlying lithology and slope characteristics for two areas in Madagascar’s central highlands: a 4620 km² area around Ambatondrazaka (east of Lac Alaotra), at higher elevation (755-420m), wetter, and with frequent seismic activity; and a 8496 km² area farther west near Tsaratanana, lower (23-1362m), drier, and with less seismity.

GIS analysis of the lavaka-prone terrain in these areas (using ArcMap v.10 software, Google Earth v. 6 imagery, 90m/pixel USGS DEM and new 1:50,000 geologic maps from the PGRM) shows, somewhat unexpectedly, that bedrock geology is a poor predictor of lavaka density. We found no significant relationship between intensity of gullying and underlying lithology. In contrast, slope exerts a significant control. Lavaka increase in abundance as slopes get steeper, up to an optimum steepness beyond which gully density declines. This tendency is independent of bedrock composition. Optimum steepness is less in Tsaratanana (10-20­º) than in Ambatondrazaka (25-35º). Lavaka are also less likely to form on slopes of uniform steepness: mild gradient change (about 2-5º) in the immediate vicinity seems to promote lavaka formation.

Overall lavaka density is greater in Ambatondrazaka (up to 150 lavaka/km²) than in Tsaratanana (max. 50 lavaka/km²), and most of the extra gullies are accommodated on the steeper slopes. At moderate slopes the two areas have similar densities; but at steeper slopes lavaka density increases significantly in Ambatondrazaka. For slopes 25-45°, lavaka density in Ambatondrazaka (per 5° slope bin) is 2-3 times greater than that of Tsaratanana.

Lavaka distribution and abundance are a function of multiple factors which vary locally and differ from one place to another. The differences in overall density may be due to climatic and seismic differences between the two regions; slope characteristics also clearly play some role in determining where lavaka will nucleate. Full understanding of the processes controlling lavaka location, however, requires further quantitative GIS analysis and possibly detailed local field studies.