Insights into river bank erosion processes derived from analysis of negative erosion‐pin recordings: observations from three recent UK studies

Abstract

Recent studies of river bank erosion in three catchments in the UK have been characterized by the persistent occurrence of negative erosion-pin results. The cause of these negative recordings is considered with reference to field data from the Afon Trannon, Nant Tanllwyth and River Arrow, and to a laboratory study of freeze-thaw and desiccation processes. It seems that there is potential for, and in some cases evidence of, a number of different circumstances that generate negative results, but none of these alone is sufficient to explain all incidents. Factors considered include: deposition of sediment during high flows; soil fall from the upper parts of the bank on to lower erosion pins; loosening of the soil surface and expansion/contraction of the soil mass with fluctuations in temperature and moisture content; movement of the erosion-pin within the bank and human interference. Each has its own implications for the use of erosion pins.Further issues arise when including negative data in subsequent data analysis, and it is demonstrated that attempts to correlate erosion rates with hydro-meteorological data in order to ascertain causes of erosion will be influenced by the way in which negative data are handled. It is thus suggested that any study of river bank erosion using erosion pins should state whether or not negative data were obtained, and if so, how they were included in data analysis. Failure to include this information could lead to comparison of mean erosion rates that reflect bank processes very differently.The studies presented here offer a clear example of the value of 'anomalous' field data: results which do not appear to fit expected patterns can reveal as much about the processes in operation as those that do.

Magnitude and processes of bank erosion at a small stream in Denmark

“…Discussing erosion from streambanks is also made complicated by the fact that terms are defined differently in the literature. Whereas Lawler et al (1997) define fluvial erosion and subaerial processes as bank erosion and consider bank failure as a separate process, Yumoto et al (2006) and Couper et al (2002) describe all three processes as bank erosion. There is hence a need for agreement on how to define erosion on streambanks.…”

“…The observed net deposition of sediment (i.e. negative pin erosion readings) on the Harrested streambank supports observations by Couper et al (2002) in three UK catchments and may be attributed to a combination of bank failure and bank erosion. However, many studies do not report negative pin erosion readings and this makes it difficult to compare results and to obtain a proper description and understanding of the underlying dynamics of streambank erosion.…”

“…Furthermore, studies by Laubel et al (2003) indicate that most of the erosion processes take place on the lower part of the bank associated with fluvial erosion. However, Bradbury et al (1995) found that bank retreat could best be assessed looking at pins at the upper parts of the streambank as data from mid-and lower streambanks may be disturbed by deposition of sand, a pattern which is not supported by Couper et al (2002) and which could not be observed in this study either.…”

“…Negative pin readings are often excluded from the analysis, although a number of studies have found negative pin readings to be widespread (Bradbury et al, 1995;Prosser et al, 2000;Couper et al, 2002). Couper et al (2002) did for instance observe negative erosion pin data ranging from about 3 to 79% each time the experimental sites were monitored. In this study, the average percentage of negative readings over a year varied between 27 and 63% with no clear pattern when it comes to what years' negative readings were prevailing.…”

Potential controls of alluvial bench deposition and erosion in southern Piedmont streams, Alabama (USA)

“…This can be explored using erosion pins, clay pads, repeated topographic surveys, or one-, two-, or three-dimensional numerical or spatially explicit modeling (González-Díez et al, 2009). Both pins and clay pads take many years of monitoring to construct a data set that covers a large range of flows (Noe and Hupp, 2005) and is subject to many inconsistences and errors (Couper et al, 2002). Repeated topographic surveys also involve long monitoring periods and are very labor intensive (De Santisteban et al, 2006).…”