Monitoring of test gullies development from the Moldavian Tableland in 1986 - 2008 period

Abstract

 In a study area in Romania covering about 25,000 km 2 over 9000 gullies were mapped. The areal distribution of the gullies indicated some areas where a higher density occurs. These areas are dominated by hillslope orientation towards the northwest and, respectively, the southeast, slope inclination between 20 to 30 m/100 m, slope length between 300 and 500 m and by a dominantly sandy lithology. Detailed field surveys were made of 17 gullies. Each gully was marked with a network of stakes to show the location of cross sections. The distances between cross sections were around 3-4 times the gully width.

Topographical maps at scales of 1:200 to 1:500 were made for gully geomorphological surveys. In 2007-2008 we were interested in the evolution of the test gullies and if the evolution pattern we had proposed in 1999 was verified by the new measurements. In this
work we will focus on the monitoring methods of the test gullies from our work database. Finally, we intend to present the calibrated pattern of the gully evolution. The distribution of gullies indicates that the highest gully density reflects hillslope orientation, inclination and length, and a sandy lithology. In Moldavia the average number of
gullies per kms is between 2 to 4, with a maximum of 20 gullies per kms; average density is between 0.1 to 1 km per kms, with a maximum length value of over 3 km per km. Gully number and density per surface unit are considered as classification criteria of gully erosion.
The areas with the highest susceptibility of gully initiation in Moldavia are at the intersection of following factors: hillslope orientation towards NW and SE, respectively (hillslopes of consequent valleys), slope inclination between 20 to 30%, slope length between 300 and 500 m and a dominantly sandy lithology. These areas have a high density of points.
Detailed investigations on 17 gullies sampled from the different areas of Moldavia
between the Siret and Prut rivers in the period 1986-2008 show that some of the properties of gully geometry favour using discriminating factors to produce a diagnosis and a possible assessment of geomorphic development. Factor identification can be used to identify the most important combinations of geomorphic processes in gully development.
The surface deposits of gullies studied vary from silty-clays in the northern half of the territory, to silts in the middle to southern area and sands in the southern end of the region. For the cases studied, the sidewall processes contribute 1-5 times more to the gully evolution than the incision processes. The nature of the processes in a gully may be inferred by the grain size analysis of the surface material.
The rate of gully head cutting is over 1.5 m/year for gullies cut in sandy deposits and under 1 m/year for the gullies cut in marls and clays. A model of gully development is proposed which shows an accelerated rate of gully development immediately downstream after their initiation and a reduced and even cessation of advance on attaining an equilibrium length.