Relation Between Bedrock Geology, Topography and Lavaka Distribution in Madagascar

Abstract

The characteristic gullies of central Madagascar-lavakas-vary greatly in abundance over short distances, but existing understanding does not explain why some hillsides should have high concentrations of lavakas when nearby slopes have fewer. We present a GIS analysis of lavaka abundance in relation to bedrock geology and topography, covering two areas in the central highlands: the region near Anibatondrazaka and that around Tsaratanana. Both regions have similar average lavaka density (6 lavakas/km^ in Ambatondrazaka, and 5 lavakas/km^ in Tsaratanana, but local lavaka concentrations vary widely. Individual one-km^ squares can host up to 50 lavakas/km-in Tsaratanana and up to 150 lavakas/km^ in Ambatondrazaka. We find no predictive relationship between bedrock type and lavaka abundance. There is, however, a relationship between lavakas and slope such that lavakas increase in abundance as slopes get steeper, up to an optimum steepness, beyond which they become le.ss numerous. The optimum steepness for lavaka development is about 10 to 15° in Tsaratanana and 25 to 30° in Ambatondrazaka. Lavakas also seem to favour slopes where the gradient changes locally, with an optimum change in grade somewhere in the range 2 to 5°. Our results provide quantitative constraints on lavaka distribution that can be tested in other areas.

Landscape evolution and climate in Madagascar: lavakization in the light of archive precipitation data

“…In contrast to previously published studies (e.g. Mulder and Idoe, 2004;Voarintsoa et al, 2012), lavakas were found on lower elevation areas (the lowest lavaka occurs at 9 m a.s.l.) and on gentle slopes (minimum gradient: 0.6°) (Table 2; Fig.…”

“…3 to 6). The average lavaka density for Sites 1-4 is 1.7 km -2 (which is similar to data mentioned by Voarintsoa et al (2012) and Raveloson et al (2013): 2.2-2.9 km -2 ). Maevatanana was the most gullied area with a total of 427 lavakas and an average of 4.3 km -2 .…”

“…Lavakization occurs mainly in areas affected by strong contrasts between dry and wet seasons (Voarintsoa et al, 2012). This seasonality causes the degradation of the soils prone to lavakization.…”

“…(after ONE, 2008a) with the lavaka-prone area (red line; Cox et al, 2010) Numerous publications mention that climate is controlling the process of lavakization because of the occurrence of heavy rainfalls followed by dry seasons (e.g. Mietton, 2014;Voarintsoa et al, 2012). We particularly considered this aspect in our study, in order to understand to what extent the precipitation intensity contributes to the initiation and acceleration of gully formation and development.…”

Relationship of benggang number, area, and hypsometric integral values at different landform developmental stages

“…The similar erosional landforms are termed as crumbling in Japan, lavaka in Madagascar, and voçorocas in Brazil (Bacellar, Coelho Netto, & Lacerda, 2005;Lin, 2008;Voarintsoa, Cox, Razanatseheno, & Rakotondrazafy, 2012). Despite their similarity in dimension, lithology, erosion form and rainfall conditions (Cox, Zentner, Rakotondrazafy, & Rasoazanamparany, 2010;Voarintsoa et al, 2012;Wells & Andriamihaja, 1993), benggang is lower than lavaka in altitude, but higher in erosion intensity (Cox, Bierman, Jungers, & Rakotondrazafy, 2009;Xu, 1996). Comparatively, benggang poses more threat to the environment system and social economy due to its high erosion intensity and sudden occurrence.…”