Bank erosion and channel width change in a tropical catchment

Abstract

Catchment sediment budget models are used to predict the location and rates of bank erosion in tropical catchments draining to the Great Barrier Reef lagoon, yet the reliability of these predictions has not been tested due to a lack of measured bank erosion data. This paper presents the results of a 3 year field study examining bank erosion and channel change on the Daintree River, Australia. Three different methods were employed: (1) erosion pins were used to assess the influence of riparian vegetation on bank erosion, (2) bench-marked cross-sections were used to evaluate annual changes in channel width and (3) historical aerial photos were used to place the short term data into a longer temporal perspective of channel change . The erosion pin data suggest that the mean erosion rate of banks with riparian vegetation is 6·5 times (or 85%) lower than that of banks without riparian vegetation. The changes measured from cross-section surveys suggest that channel width has increased by an average of 0·74 (± ± ± ± ±0·47) m a − − − − −1 over the study period (or ~0·8% yr − − − − −1 ). The aerial photo results suggest that over the last 30 years the Daintree River has undergone channel contraction of the order of 0·25 m a − − − − −1 . The cross-section data were compared against modelled SedNet bank erosion rates, and it was found that the model underestimated bank erosion and was unable to represent the variable erosion and accretion processes that were observed in the field data. The reach averaged bank erosion rates were improved by the inclusion of locally derived bed slope and discharge estimates; however, the results suggest that it will be difficult for catchment scale sediment budget models to ever accurately predict the location and rate of bank erosion due to the variation in bank erosion rates in both space and time. Figure 1. (A) Daintree River catchment, showing the location of the main stream gauge, broad vegetation types and major tributaries. (B) Study reach between the stream gauge and Daintree Village, showing the location of field monitoring sites and additional aerial photo cross-sections and pin sites. This figure is available in colour online at

Little boats and big concerns? Seeking a balance between ecotourism and motorboat use in Tortuguero, Costa Rica.

“…Other more suitable options for the area include the use o f erosion pins to measure the physical loss of shoreline and aerial photography for longer term channel changes (Bartley, et al, 2008). Erosion pins can be checked weekly and aerial photography can be taken yearly, allowing time for measuring additional baseline catchment characteristics necessary to better understand boat influence on shoreline erosion in Tortuguero Lagoon.…”

“…23 At these four reference sites no BOTEs were conducted because they were identified by local people and Park employees as having 'low to no' boat traffic. Each collection site had relatively unique characteristics with respect to comparing erosion measurements across field sites (Bartley et al, 2008;Bauer, et al, 2002;Petticrew, 1991;Petticrew & Rex, 2006;Walling, 2007) The CM site is off limits to motorboat use, and therefore receives only canoe and kayak…”

“…A lack of shoreline vegetation has been shown to increase mean erosion rates by upwards of 85% when compared to areas with shoreline vegetation present (Bartley, et al, 2008). Loss of shoreline and near-shore in-water vegetation has also been attributed to motorboat use and the proximity of boat use to the impacted shoreline in tropical river and lagoon systems in Australia and Venezuela, as well as a river in Alaska (Bartley, et al, 2008;Bradbury, et al, 1995;Conde, 1996;Dorava, 2001) Decreased vegetation and stability of the shoreline are accompanied by increased use o f retention walls in Tortuguero. The construction of these retention walls is an attempt to mitigate loss of land and property due to increased boat-caused waves (participant observation and personal communications, 2011).…”

“…This could include using erosion pins and aerial photography25 to measure and document bank loss and related changes (Bartley, et al, 2008).…”

“…Using erosion pins is a relatively simple, low cost option that has been used to effectively measure shoreline erosion (Bartley, et al, 2008;Gabet, 1998).…”