Year

2014

Degree Name

International Bachelor of Science

ANZSRC / FoR Code

0406 PHYSICAL GEOGRAPHY AND ENVIRONMENTAL GEOSCIENCE

Department

School of Earth & Environmental Science

Advisor(s)

Tim Cohen

Abstract

In April 2002, 24 Engineered Log Jams (ELJs) were built within a 2 km treatment reach of what was a degraded ephemeral sand-bed stream at Stockyard Creek, Wollombi, NSW. Coupled with ~ 20 years of ongoing native revegetation this project aimed to increase the geomorphic diversity and ecological characteristics of the ephemeral stream. Commencing in 2002, the experiment was set up as a standard BACI design, with a control reach situated in the upstream limit of the study site. This thesis aims to assess the geomorphic response to the re-introduction of wood by comparing treatment and control reaches. Since construction, the ELJs have experienced a 5 year period of low or no flow conditions, as well as two major bed mobilising flood events which occurred in June 2007 and February 2013. Four detailed topographic surveys of the study reach were completed during the 12 year study period and have been used to construct Digital Elevation Models (DEMs) of the in-channel bed topography.

Differencing between successive DEMs and the development of longitudinal profiles for the study reach indicate the ELJs have been successful in promoting the colonisation of riparian vegetation, as well as increasing the geomorphic diversity of the bed topography. Significantly, the magnitude of change was much greater in the treatment reaches, with one of the treatment reaches (treatment A) having a depth range 1.5 times greater than that of the control, with a maximum scour depth of ~ 2.5 m. The most pronounced response to ELJ introduction was the development of persistent pools, with 46-50% of new pool volume estimated to have occurred in direct association with the ELJs. Patterns of scour and bar development were observed to vary in association with ELJ design, with the ELJs within the treatment A reach facilitating significant bar development (13.5%) with a net gain of (48.90 m³/1000m² of bed) compared to the preconstruction survey. Despite the influence of the ELJs, the control reach also experienced a significant level of change due to the presence of outcropping bedrock, native re-vegetation and the natural recruitment of woody debris. Both control and treatment reaches are excellent examples of riparian rehabilitation using native species. This thesis provides an assessment of the geomorphic response to the use of ELJs in an Australian sand-bed stream.

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