Characteristics and controlling factors of old gullies under forest in a temperate humid climate: a case study from the Meerdaal Forest (Central Belgium)

Abstract

In many forests of Northwestern Europe old gullies can be found, but few studies have reported their genesis and characteristics. This study investigates these old gullies under forest in the large case-study area of Meerdaal Forest, in the Central Belgian loess belt. The objectives are (1) to determine the spatial distribution of these gullies, (2) to measure their morphological and topographical characteristics and (3) to reconstruct the factors that led to their development. In the 1329-ha study area, 252 channel-like incisions were mapped. Different types of incisions could be distinguished. Besides small and large gullies, many incisions were sunken lanes or road gullies. These road gullies are aligned along north–south oriented lines, whereas the concentration of old gullies is strongly related to the distribution of archaeological sites. Out of the 252 mapped incisions, 43 large gullies and 21 representative road gullies were selected for detailed morphological and topographical measurements. The characteristics of these two types of incisions were compared with ephemeral gullies formed under nearby cropland. Significant differences in morphology between the three types could be demonstrated. Ephemeral gullies under cropland and large gullies under forest differ significantly in all measured parameters, except bottom width. Both the old gullies and road gullies under forest have a significantly larger cross section and total eroded volume compared with the ephemeral gullies observed under cropland. This indicates that once formed, the old gullies were not ploughed in nor were they filled by sediment originating in their drainage areas, because of limited sediment production. Comparing topographical characteristics (i.e. slope at the gully head and runoff contributing area) of forest gullies and ephemeral gullies that formed under cropland yields important indications about their formation. The larger sedimentation slope of forest gullies, compared with ephemeral gullies and road gullies, suggests that the forest gullies incised on vegetated slopes as a consequence of runoff from the adjacent plateau, where the forest cover was disturbed. For the old gullies under forest, no relation between slope at the gully head and runoff contributing area is observed, probably because most gullies occur on very steep slopes. When simulating arable land-use in the study area, zones where ephemeral gullies are expected to develop can be predicted using published topographical threshold relationships. Comparing the zones where ephemeral gullies are predicted with the position of old gullies under forest leads to the conclusion that gully incision was most probably not triggered by extreme rainfall events and that they are not of periglacial origin. The observed gully pattern can best be explained by local, anthropogenically determined land-use changes.