Erodibility of loess depending on its weathering: field experiments

Piping is an erosion process in which cracks and macropores extend into channels with a diameter of cm or more. The study of loess erosion is important because loess covers about ten percent of the continents surface and is susceptible to piping, formation of gullies and intense erosion of agricultural soil. Study was done in Střeleč quarry (Czech Republic), where a several meters thick loess cover occurs in the upper part, sometimes with cracks and macropores. Rill erosion and piping conduits are formed in the loess cover and this makes it an ideal place for field experiments and observations. The erosion rate of the loess by water trickle at quarry face, erosion of the drill hole and erosion under the impact of the droplets were studied. The erosion rate of the rills was measured using long screws screwed directly into the rill. Rapid erosion occurred within first tens of centimeters from original ground surface in the zone where the loess structure was disintegrated by frost or wetting-drying cycles. Below this zone, the erosion rate was much lower, and it ceased with time as rill deepened. Small piping conduits developed rapidly by pouring water into small desiccation cracks on the loess surface. On the other hand, the dril hole did not expand into piping conduit in deeper zone of loess. Moisture content of small loess blocks have strong impact on final degree of erosion. While dry blocks began to disintegrate relatively quickly into incoherent material, the pre-wetted samples did not disintegrated and more or less kept their initial shape. This shows that slaking is responsible for disintegration of small dry blocks on loess surface. While the surface zone of the loess is highly erodible by flowing water, probably due to the loss of its original structure, the loess in the deeper zone is far less erodible in the quarry and even pre-formed conduit (dril hole) do not develop into larger conduit.

 

Many thanks to the management of Střeleč Quarry for enabling of the field documentation and experiments. The research was supported by Charles University Grant Agency (GAUK #1292119).