Full Paper
7th Australian Stream Management Conference - Full Paper
The Shifting Sands of Stockyard Creek: the geomorphic response to large wood reintroduction in a sand-bed stream 1
2
Hughes, R.M. , Cohen, T.J. and Brooks, A.P.
3
1. School of Earth and Environmental Sciences, The University of Wollongong, New South Wales Email: [email protected] 2. GeoQuest Research Centre, School of Earth and Environmental Sciences, The University of Wollongong, New South Wales, Australia 3. Australian Rivers Institute, Griffith University, Nathan, QLD 4111, Australia
Key Points • the reintroduction of wood through the construction of Engineered Log Jams (ELJs), can successfully increase geomorphic diversity and stability in an incised, partly confined sand-bed stream • ELJs can provide a stable pool habitat that would otherwise have not existed without intervention. • ELJ design and frequency can significantly affect patterns of pool and bar development • The increased habitat provided by constructed ELJs is further enhanced by natural wood recruitment and the colonisation of in-channel vegetation within sand-bed streams
Abstract In April 2002, 26 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. The experiment was set up as a standard Before-After Control- Impact (BACI) design, with a control reach situated in the upstream limit of the study site and the incorporation of an external control reach in an adjacent valley. This paper 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, and a number of smaller flow events. Four detailed topographic surveys of the study reach were completed during the 11 year study period and have been used to construct Digital Elevation Models (DEMs) of the in-channel bed topography. Geomorphic change detection analysis suggests the magnitude of change within the treatment reaches was much greater than that of the control, with the most pronounced response to ELJ introduction being the development of persistent pool habitat.
Keywords Large woody debris; Engineered log jam; Geomorphic change detection; Sand-bed stream, rehabilitation
Introduction In south-eastern Australia’s “degraded” fluvial landscape, changing environmental perspectives are significantly influencing river management strategy eliciting an increased use of bioengineered solutions to solve river management issues. Early European agricultural practices, such as land clearing and grazing, have altered the landscape of southeastern Australia, resulting in negative implications for the riverine environment (Erskine & Webb, 2003) including a loss of habitat and aquatic species diversity. The reinstatement of geomorphic diversity through the addition of bed roughness elements, such as vegetation and Large Woody Debris (LWD), has become a key focus of river rehabilitation, as morphological complexity is considered to act as a surrogate for species diversity (Brierley et al., 1999). Throughout the past few decades wood reintroduction has been widely implemented through the construction of Engineered Log Jam’s (ELJs) in order to fulfil specific rehabilitation goals, such as bank protection, habitat enhancement or increased bed stability (Brooks et al., 2004; Shields et al., 2004). However, few ELJ initiatives have been rigorously evaluated, with the majority of projects occurring within gravel-bed rivers in North America (Brooks et al., 2006). The application of ELJs within a sand-bed environment is considered to pose a differing set of challenges due to a smaller grain size, higher potential erosion rates and lower bed-slope. Therefore, Shields et al. (2004) suggests different design requirements are necessary for wood re-introduction. This paper presents a GIS based analysis of the geomorphic response to ELJ construction within an ephemeral sand-bed stream. The rehabilitation strategy aimed to: (1) increase the geomorphic diversity of the treatment reaches; (2) further stabilise the bed of an already incised stream; and (3) assess the suitability of utilising ELJs in a sand-bed context.
Hughes et al. (2014). The Shifting Sands of Stockyard Creek, in Vietz, G; Rutherfurd, I.D, and Hughes, R.M (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages 200-205. 200
7ASM Full Paper Hughes et.al. – The Shifting Sands of Stockyard Creek Field site and methods The study reach is situated on Stockyard Creek, a tributary of the Wollombi Brook and is located in the Hunter Valley, NSW. The river reach has a partly-confined river style (Brierley & Fryirs, 2005) with a catchment area of ~ 35 km² and a total length of 2.2 km. The ELJ project involved the installation of 26 ELJs as a large scale Before-After-Control-Impact (BACI) design field experiment, with two separate treatment reaches (treatment A and treatment B), an upstream control reach and an external control in an adjacent valley. ELJ designs were based on structures implemented by Brooks et al. (2004), and also included the construction of experimental V-weir log sills (Brooks, 2006). The site was chosen for the project, as the observed morphological and land use history was considered to be representative of the post-European riverine landscape and the site has undergone a continuous native revegetation strategy (~ 20 years) since the late 1970’s (Woodward B. pers. comm. 2013).
Figure 1. (a) Map of the study reach at Stockyard Creek indicating the layout of the BACI experimental design with ELJ locations. (b) Study site located within the Wollombi Valley.
In-channel morphological response to ELJ intervention Within an 11 year period, four detailed topographic surveys of the in-channel topography were completed using a Trimble M3 Total Station and a consistent network of benchmarks established in 2001. Elevation data points were collected semi-randomly, focused within the active channel, but concentrated on depicting key breaks in slope to ensure the geomorphic diversity was accurately captured. The survey data was later processed using Delauney Triangulation in ArcGIS 10.0, to create a Triangular Irregular Network (TIN) for each survey date. TIN surfaces were processed into concurrent raster DEMs with a cell size of 1m and natural neighbour interpolation. To restrict the analysis to the active channel, the DEMs were masked by test reach to the bottom of bank line.
Hughes et al. (2014). The Shifting Sands of Stockyard Creek, in Vietz, G; Rutherfurd, I.D, and Hughes, R.M (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages 200-205. 201
7ASM Full Paper Hughes et.al. – The Shifting Sands of Stockyard Creek
Table 1 Survey attributes for each point data set used for the development of DEMs. The time period of the survey in relation to ELJ construction is indicated by ELJ Stage with “pre” representing pre-construction and “post” representing post construction period. Date of Survey April, 2002 (Baseline) May, 2002 19th July 2007 2nd August 2013
ELJ Stage pre post post post
Number points within channel 1940 2404 1343 4562
Point Density (pt/m²) 0.1 0.1 0.1 0.2
Geomorphic Change Detection 5.0 (GCD 5.0) tool for ArcGIS 10.0 (Wheaton et al., 2013) was used to perform a DEM of Difference (DoD) analysis (see Wheaton et al. (2013) for details) with a minimum level of detection (minLOD) of ± 0.1m. Net sediment change and sediment turnover volume estimates were calculated at the reach-scale for each DoD using the methods of Brooks et al. (2006).
Assessment of Geomorphic Diversity To quantify the changing complexity and stability of the three study reaches, multiple GIS analyses were performed using the DoD outputs including: the calculation of percentage new pool and bar surface area (see Brooks et al. (2006) for method) and the classification of pools by dominant scour mechanism. ‘Scour mechanisms’ were identified based on field observation and established using the pool classifications developed by Webb and Erskine (2005) and included: engineered log jam (elj), natural wood recruitment (nwr), compound (cop), bedrock forced (bep) and free-form alluvial pools (po). Variability indices including Standard Deviation of Depth (Bartley & Rutherford, 2005) and 3D morphological complexity index (Brooks et al., 2006) were quantified for each study reach through time. Reach-scale temporal longitudinal profiles were extracted from each DEMs using the April 2002 thalweg elevation data points.
Results and discussion The influence of ELJs in promoting geomorphic diversity The geomorphic diversity of the three long-term study reaches increased with time, with all reaches having a greater elevation distribution, increased pool/bar surface area as well as an increase in the variability of the long profile compared to the preconstruction in-channel topography. However, 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 spatial distribution of scour (Figure 2) indicates a high proportion of morphological change occurred in direct association with the ELJs, with 46-50% of new scour volume estimated to have been associated with this mechanism, which is in contrast to the dominance of bedrock in the control reach. This outcome affirms the findings of Brooks et al. (2006), whereby the treatment reach was found to be consistently more complex than the control throughout the 5 year study period. Anecdotal evidence suggests bedrock pools were previously an ephemeral component of Stockyard Creek, (Woodward B. pers. comm. 2013), therefore, the variability of the control, can be partially attributed to the natural dynamics of a partly-confined sand-bed stream (Brierley & Fryirs, 2005).
Hughes et al. (2014). The Shifting Sands of Stockyard Creek, in Vietz, G; Rutherfurd, I.D, and Hughes, R.M (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages 200-205. 202
7ASM Full Paper Hughes et.al. – The Shifting Sands of Stockyard Creek
Figure 2. DoD for the treatment A reach between August 2013 – April 2002. Regions of erosion are indicated in red while regions of deposition are indicated in blue. ELJ locations are shown to highlight the relationship between topographic change and structure location. Label codes indicate ELJ type: DFJ (Deflector Jam), LS (Log sill) and VW (Vweir sill). Images taken at the conclusion of the study in August 2013: a) LS7, with deep step-pool; b) eroded bank caused by the failure of DFJ10; c) experimental v-sill weir with significant scour downstream of ELJ.
Channel Stability and Sediment Storage Bed Profile Stability Temporal long profiles, confirm the observations of Montgomery and Abbe (2006), that LWD jams form consistent ‘hard’ points within the bed-profile, therefore providing bed stability and halting widespread channel incision. Although the treatment A reach has shown an increase in sediment storage, the structures at Stockyard Creek have successfully maintained and often enhanced scour features resulting in a spatially variable morphology which is persistent through time (Figure 3). The development of persistent in-channel morphology has been noted as a significant rehabilitation challenge within a sand-bed context, due to a small average grain size and the high sensitivity of sand-bed channels to Hughes et al. (2014). The Shifting Sands of Stockyard Creek, in Vietz, G; Rutherfurd, I.D, and Hughes, R.M (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages 200-205. 203
7ASM Full Paper Hughes et.al. – The Shifting Sands of Stockyard Creek change (Shields Jr. & Gippel, 1995). The development of a stable bed-profile is a significant outcome, as flume studies suggest scour associated with LWD is often short-lived in sand-bed streams (Wallerstein et al., 2001).
Figure 3. Temporal Long profile for the treatment A reach. Pool development is visually apparent as a significant drop in bed-level while distinct aggradational features are evident directly upstream of constructed ELJs. Pool features are represented in grey while selected ELJs are labelled for context. In contrast to the work of Shields et al. (2004), the majority of ELJs at Stockyard Creek have maintained structural integrity under the influence of two high magnitude flood events. Significantly, only one major structural failure (DFJ10) (Figure 2) has occurred at the study site, with the structures recorded to be in a state of moderate decay with minor log displacement primarily brought on by a large tree falling on DFJ10 in 2007 (Daley & Brooks, 2013)_ENREF_14. The presence of in-channel riparian vegetation is assumed to have played an important role in stabilising the geomorphic features within the study reach and is assumed to have further enhanced the stability of the structures. Sediment Storage 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. In contrast, the low frequency of V-sill weir structures within the Treatment B reach successfully induced scour (13%) by producing consistently deep, oval shaped pools, however, the structures had little impact upon new bar development (4.5%). The interconnected nature of the control and treatment reaches may have affected the redistribution and storage of sediment within the low-flow channel. Therefore, the establishment of a positive (wood and vegetation) and negative (without wood) external control reach from the commencement of the project, would have provided further insight into the complex patterns of scour and fill occurring in association with the ELJs. The external control was only incorporated at the final survey. Although the treatment reaches experienced a greater magnitude of sediment change compared to the control, a high degree of inter-reach variability was observed with no constant reach-scale trend of erosion or deposition. It can be speculated the ELJs within Stockyard Creek are working to redistribute sediment through scour and sediment retention, creating local-scale sediment sinks (Fryirs et al., 2007). However, the infrequency of flow events may have inhibited the development of a consistent pattern of sediment storage, and therefore the implementation of a long-term event based data survey may provide a better understanding of the temporal dynamics of the reach. Another possible explanation for the observed variability in sediment storage within the treatment reaches is the specific design attributes of the ELJs.
Conclusions Since the late 1970’s the study site at Stockyard Creek, has been successfully transformed from an incised river system with actively eroding river banks, to a stable system with near permanent pools and continuous riparian vegetation. This Hughes et al. (2014). The Shifting Sands of Stockyard Creek, in Vietz, G; Rutherfurd, I.D, and Hughes, R.M (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages 200-205. 204
7ASM Full Paper Hughes et.al. – The Shifting Sands of Stockyard Creek paper suggests the reintroduction of wood through the construction of ELJs, can successfully increase geomorphic diversity and stability in a partly confined sand-bed stream. Temporal long profiles of the three study reaches, suggest the most pronounced geomorphic change to have occurred at the site is the development of near permanent pools. Despite the fact all three study reaches experienced a significant increase in geomorphic diversity, the magnitude and persistence of change within the treatment reaches was not observed in the control. Such an outcome implies ELJs can potentially provide a stable pool habitat for the promotion of species diversity, which has been identified as a challenge in degraded sand-bed streams. Wood reintroduction using ELJs is seen as a short-term method of enhancing geomorphic diversity and channel stability; however, few wood reintroduction initiatives have been rigorously monitored through time. The detailed study presented in this paper is unique, as it was carried out over a long monitoring period (11 years) and coupled with a ~ 20 year riparian rehabilitation strategy. This study suggests in-channel wood plays a vital role in the development and stability of geomorphic features within sand-bed streams, and it provides a realistic sense of the timescale involved to achieve effective river rehabilitation.
Acknowledgments We would like to thank Brian and Sally Woodward for allowing access to their property at ‘Earthways’; your passion for the environment and support of the project is gratefully appreciated. Thanks to Rod Gleeson, Peter Coffey and Daniel Seddon-Powell for assisting with elevation data collection and to the School of Earth and Environmental Sciences at the University of Wollongong for providing field equipment. We would like to thank Heidi Brown, Chris Owers and Joe Wheaton for assistance with GIS analysis and for providing GIS data for this study.
References Bartley, R., & Rutherford, I. (2005). Measuring the Reach-scale Geomorphic Diversity of Streams: Application to a Stream Disturbed by a Sediment Slug. River Research and Applications, 21, 39-59. Brierley, G. J., Cohen, T., Fryirs, K., & Brooks, A. (1999). Post-European changes to the fluvial geomorphology of Bega catchment, Australia: implications for river ecology. Freshwater Biology, 41(4), 839-848. doi: 10.1046/j.1365-2427.1999.00397.x Brierley, G. J., & Fryirs, K. (2005). Geomorphology and River Management: Applications of the River Styles Framework. UK: Blackwell Publishing. Brooks, A. (2006). Design guideline for the reintroduction of wood into Australian Streams. Canberra: Land & Water Australia. Brooks, A. P., Gehrke, P. C., Jansen, J. D., & Abbe, T. B. (2004). Experimental reintroduction of woody debris on the Williams River, NSW: geomorphic and ecological responses. River Research and Applications, 20(5), 513-536. doi: 10.1002/rra.764 Brooks, A. P., Howell, T., Abbe, T. B., & Arthington, A. H. (2006). Confronting hysteresis: Wood based river rehabilitation in highly altered riverine landscapes of south-eastern Australia. Geomorphology, 79(3–4), 395-422. doi: http://dx.doi.org/10.1016/j.geomorph.2006.06.035 Daley, J., & Brooks, A. (2013). A Performance Evaluation of Engineered Log Jams in the Hunter Valley. Erskine, W. D., & Webb, A. A. (2003). Desnagging to resnagging: new directions in river rehabilitation in southeastern Australia. River Research and Applications, 19(3), 233-249. doi: 10.1002/rra.750 Fryirs, K. A., Brierley, G. J., Preston, N. J., & Spencer, J. (2007). Catchment-scale (dis)connectivity in sediment flux in the upper Hunter catchment, New South Wales, Australia. Geomorphology, 84(3–4), 297-316. doi: http://dx.doi.org/10.1016/j.geomorph.2006.01.044 Montgomery, D. R., & Abbe, T. B. (2006). Influence of logjam-formed hard points on the formation of valley-bottom landforms in an old-growth forest valley, Queets River, Washington, USA. Quaternary Research, 65(1), 147-155. doi: http://dx.doi.org/10.1016/j.yqres.2005.10.003 Shields, F., Morin, N., & Cooper, C. (2004). Large Woody Debris Structures for Sand-Bed Channels. Journal of Hydraulic Engineering, 130(3), 208-217. doi: doi:10.1061/(ASCE)0733-9429(2004)130:3(208) Shields Jr., F., & Gippel, C. (1995). Prediction of Effects of Woody Debris Removal on Flow Resistance. Journal of Hydraulic Engineering, 121(4), 341-354. doi: doi:10.1061/(ASCE)0733-9429(1995)121:4(341) Wallerstein, N. P., Alonso, C. V., Bennett, S. J., & Thorne, C. R. (2001). Distorted Froude-scaled flume analysis of large woody debris. Earth Surface Processes and Landforms, 26(12), 1265-1283. doi: 10.1002/esp.271 Webb, A. A., & Erskine, W. D. (2005). Natural variability in the distribution, loading and induced scour of large wood in sand-bed forest streams. River Research and Applications, 21(2-3), 169-185. doi: 10.1002/rra.839 Wheaton, J. M., Brasington, J., Darby, S. E., Kasprak, A., Sear, D., & Vericat, D. (2013). Morphodynamic signatures of braiding mechanisms as expressed through change in sediment storage in a gravel-bed river. Journal of Geophysical Research: Earth Surface, 118(2), 759-779. doi: 10.1002/jgrf.20060
Hughes et al. (2014). The Shifting Sands of Stockyard Creek, in Vietz, G; Rutherfurd, I.D, and Hughes, R.M (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages 200-205. 205
The Shifting Sands of Stockyard Creek: the geomorphic response to large wood reintroduction in a sand-bed stream 1
2
Hughes, R.M. , Cohen, T.J. and Brooks, A.P.
3
1. School of Earth and Environmental Sciences, The University of Wollongong, New South Wales Email: [email protected] 2. GeoQuest Research Centre, School of Earth and Environmental Sciences, The University of Wollongong, New South Wales, Australia 3. Australian Rivers Institute, Griffith University, Nathan, QLD 4111, Australia
Key Points • the reintroduction of wood through the construction of Engineered Log Jams (ELJs), can successfully increase geomorphic diversity and stability in an incised, partly confined sand-bed stream • ELJs can provide a stable pool habitat that would otherwise have not existed without intervention. • ELJ design and frequency can significantly affect patterns of pool and bar development • The increased habitat provided by constructed ELJs is further enhanced by natural wood recruitment and the colonisation of in-channel vegetation within sand-bed streams
Abstract In April 2002, 26 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. The experiment was set up as a standard Before-After Control- Impact (BACI) design, with a control reach situated in the upstream limit of the study site and the incorporation of an external control reach in an adjacent valley. This paper 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, and a number of smaller flow events. Four detailed topographic surveys of the study reach were completed during the 11 year study period and have been used to construct Digital Elevation Models (DEMs) of the in-channel bed topography. Geomorphic change detection analysis suggests the magnitude of change within the treatment reaches was much greater than that of the control, with the most pronounced response to ELJ introduction being the development of persistent pool habitat.
Keywords Large woody debris; Engineered log jam; Geomorphic change detection; Sand-bed stream, rehabilitation
Introduction In south-eastern Australia’s “degraded” fluvial landscape, changing environmental perspectives are significantly influencing river management strategy eliciting an increased use of bioengineered solutions to solve river management issues. Early European agricultural practices, such as land clearing and grazing, have altered the landscape of southeastern Australia, resulting in negative implications for the riverine environment (Erskine & Webb, 2003) including a loss of habitat and aquatic species diversity. The reinstatement of geomorphic diversity through the addition of bed roughness elements, such as vegetation and Large Woody Debris (LWD), has become a key focus of river rehabilitation, as morphological complexity is considered to act as a surrogate for species diversity (Brierley et al., 1999). Throughout the past few decades wood reintroduction has been widely implemented through the construction of Engineered Log Jam’s (ELJs) in order to fulfil specific rehabilitation goals, such as bank protection, habitat enhancement or increased bed stability (Brooks et al., 2004; Shields et al., 2004). However, few ELJ initiatives have been rigorously evaluated, with the majority of projects occurring within gravel-bed rivers in North America (Brooks et al., 2006). The application of ELJs within a sand-bed environment is considered to pose a differing set of challenges due to a smaller grain size, higher potential erosion rates and lower bed-slope. Therefore, Shields et al. (2004) suggests different design requirements are necessary for wood re-introduction. This paper presents a GIS based analysis of the geomorphic response to ELJ construction within an ephemeral sand-bed stream. The rehabilitation strategy aimed to: (1) increase the geomorphic diversity of the treatment reaches; (2) further stabilise the bed of an already incised stream; and (3) assess the suitability of utilising ELJs in a sand-bed context.
Hughes et al. (2014). The Shifting Sands of Stockyard Creek, in Vietz, G; Rutherfurd, I.D, and Hughes, R.M (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages 200-205. 200
7ASM Full Paper Hughes et.al. – The Shifting Sands of Stockyard Creek Field site and methods The study reach is situated on Stockyard Creek, a tributary of the Wollombi Brook and is located in the Hunter Valley, NSW. The river reach has a partly-confined river style (Brierley & Fryirs, 2005) with a catchment area of ~ 35 km² and a total length of 2.2 km. The ELJ project involved the installation of 26 ELJs as a large scale Before-After-Control-Impact (BACI) design field experiment, with two separate treatment reaches (treatment A and treatment B), an upstream control reach and an external control in an adjacent valley. ELJ designs were based on structures implemented by Brooks et al. (2004), and also included the construction of experimental V-weir log sills (Brooks, 2006). The site was chosen for the project, as the observed morphological and land use history was considered to be representative of the post-European riverine landscape and the site has undergone a continuous native revegetation strategy (~ 20 years) since the late 1970’s (Woodward B. pers. comm. 2013).
Figure 1. (a) Map of the study reach at Stockyard Creek indicating the layout of the BACI experimental design with ELJ locations. (b) Study site located within the Wollombi Valley.
In-channel morphological response to ELJ intervention Within an 11 year period, four detailed topographic surveys of the in-channel topography were completed using a Trimble M3 Total Station and a consistent network of benchmarks established in 2001. Elevation data points were collected semi-randomly, focused within the active channel, but concentrated on depicting key breaks in slope to ensure the geomorphic diversity was accurately captured. The survey data was later processed using Delauney Triangulation in ArcGIS 10.0, to create a Triangular Irregular Network (TIN) for each survey date. TIN surfaces were processed into concurrent raster DEMs with a cell size of 1m and natural neighbour interpolation. To restrict the analysis to the active channel, the DEMs were masked by test reach to the bottom of bank line.
Hughes et al. (2014). The Shifting Sands of Stockyard Creek, in Vietz, G; Rutherfurd, I.D, and Hughes, R.M (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages 200-205. 201
7ASM Full Paper Hughes et.al. – The Shifting Sands of Stockyard Creek
Table 1 Survey attributes for each point data set used for the development of DEMs. The time period of the survey in relation to ELJ construction is indicated by ELJ Stage with “pre” representing pre-construction and “post” representing post construction period. Date of Survey April, 2002 (Baseline) May, 2002 19th July 2007 2nd August 2013
ELJ Stage pre post post post
Number points within channel 1940 2404 1343 4562
Point Density (pt/m²) 0.1 0.1 0.1 0.2
Geomorphic Change Detection 5.0 (GCD 5.0) tool for ArcGIS 10.0 (Wheaton et al., 2013) was used to perform a DEM of Difference (DoD) analysis (see Wheaton et al. (2013) for details) with a minimum level of detection (minLOD) of ± 0.1m. Net sediment change and sediment turnover volume estimates were calculated at the reach-scale for each DoD using the methods of Brooks et al. (2006).
Assessment of Geomorphic Diversity To quantify the changing complexity and stability of the three study reaches, multiple GIS analyses were performed using the DoD outputs including: the calculation of percentage new pool and bar surface area (see Brooks et al. (2006) for method) and the classification of pools by dominant scour mechanism. ‘Scour mechanisms’ were identified based on field observation and established using the pool classifications developed by Webb and Erskine (2005) and included: engineered log jam (elj), natural wood recruitment (nwr), compound (cop), bedrock forced (bep) and free-form alluvial pools (po). Variability indices including Standard Deviation of Depth (Bartley & Rutherford, 2005) and 3D morphological complexity index (Brooks et al., 2006) were quantified for each study reach through time. Reach-scale temporal longitudinal profiles were extracted from each DEMs using the April 2002 thalweg elevation data points.
Results and discussion The influence of ELJs in promoting geomorphic diversity The geomorphic diversity of the three long-term study reaches increased with time, with all reaches having a greater elevation distribution, increased pool/bar surface area as well as an increase in the variability of the long profile compared to the preconstruction in-channel topography. However, 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 spatial distribution of scour (Figure 2) indicates a high proportion of morphological change occurred in direct association with the ELJs, with 46-50% of new scour volume estimated to have been associated with this mechanism, which is in contrast to the dominance of bedrock in the control reach. This outcome affirms the findings of Brooks et al. (2006), whereby the treatment reach was found to be consistently more complex than the control throughout the 5 year study period. Anecdotal evidence suggests bedrock pools were previously an ephemeral component of Stockyard Creek, (Woodward B. pers. comm. 2013), therefore, the variability of the control, can be partially attributed to the natural dynamics of a partly-confined sand-bed stream (Brierley & Fryirs, 2005).
Hughes et al. (2014). The Shifting Sands of Stockyard Creek, in Vietz, G; Rutherfurd, I.D, and Hughes, R.M (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages 200-205. 202
7ASM Full Paper Hughes et.al. – The Shifting Sands of Stockyard Creek
Figure 2. DoD for the treatment A reach between August 2013 – April 2002. Regions of erosion are indicated in red while regions of deposition are indicated in blue. ELJ locations are shown to highlight the relationship between topographic change and structure location. Label codes indicate ELJ type: DFJ (Deflector Jam), LS (Log sill) and VW (Vweir sill). Images taken at the conclusion of the study in August 2013: a) LS7, with deep step-pool; b) eroded bank caused by the failure of DFJ10; c) experimental v-sill weir with significant scour downstream of ELJ.
Channel Stability and Sediment Storage Bed Profile Stability Temporal long profiles, confirm the observations of Montgomery and Abbe (2006), that LWD jams form consistent ‘hard’ points within the bed-profile, therefore providing bed stability and halting widespread channel incision. Although the treatment A reach has shown an increase in sediment storage, the structures at Stockyard Creek have successfully maintained and often enhanced scour features resulting in a spatially variable morphology which is persistent through time (Figure 3). The development of persistent in-channel morphology has been noted as a significant rehabilitation challenge within a sand-bed context, due to a small average grain size and the high sensitivity of sand-bed channels to Hughes et al. (2014). The Shifting Sands of Stockyard Creek, in Vietz, G; Rutherfurd, I.D, and Hughes, R.M (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages 200-205. 203
7ASM Full Paper Hughes et.al. – The Shifting Sands of Stockyard Creek change (Shields Jr. & Gippel, 1995). The development of a stable bed-profile is a significant outcome, as flume studies suggest scour associated with LWD is often short-lived in sand-bed streams (Wallerstein et al., 2001).
Figure 3. Temporal Long profile for the treatment A reach. Pool development is visually apparent as a significant drop in bed-level while distinct aggradational features are evident directly upstream of constructed ELJs. Pool features are represented in grey while selected ELJs are labelled for context. In contrast to the work of Shields et al. (2004), the majority of ELJs at Stockyard Creek have maintained structural integrity under the influence of two high magnitude flood events. Significantly, only one major structural failure (DFJ10) (Figure 2) has occurred at the study site, with the structures recorded to be in a state of moderate decay with minor log displacement primarily brought on by a large tree falling on DFJ10 in 2007 (Daley & Brooks, 2013)_ENREF_14. The presence of in-channel riparian vegetation is assumed to have played an important role in stabilising the geomorphic features within the study reach and is assumed to have further enhanced the stability of the structures. Sediment Storage 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. In contrast, the low frequency of V-sill weir structures within the Treatment B reach successfully induced scour (13%) by producing consistently deep, oval shaped pools, however, the structures had little impact upon new bar development (4.5%). The interconnected nature of the control and treatment reaches may have affected the redistribution and storage of sediment within the low-flow channel. Therefore, the establishment of a positive (wood and vegetation) and negative (without wood) external control reach from the commencement of the project, would have provided further insight into the complex patterns of scour and fill occurring in association with the ELJs. The external control was only incorporated at the final survey. Although the treatment reaches experienced a greater magnitude of sediment change compared to the control, a high degree of inter-reach variability was observed with no constant reach-scale trend of erosion or deposition. It can be speculated the ELJs within Stockyard Creek are working to redistribute sediment through scour and sediment retention, creating local-scale sediment sinks (Fryirs et al., 2007). However, the infrequency of flow events may have inhibited the development of a consistent pattern of sediment storage, and therefore the implementation of a long-term event based data survey may provide a better understanding of the temporal dynamics of the reach. Another possible explanation for the observed variability in sediment storage within the treatment reaches is the specific design attributes of the ELJs.
Conclusions Since the late 1970’s the study site at Stockyard Creek, has been successfully transformed from an incised river system with actively eroding river banks, to a stable system with near permanent pools and continuous riparian vegetation. This Hughes et al. (2014). The Shifting Sands of Stockyard Creek, in Vietz, G; Rutherfurd, I.D, and Hughes, R.M (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages 200-205. 204
7ASM Full Paper Hughes et.al. – The Shifting Sands of Stockyard Creek paper suggests the reintroduction of wood through the construction of ELJs, can successfully increase geomorphic diversity and stability in a partly confined sand-bed stream. Temporal long profiles of the three study reaches, suggest the most pronounced geomorphic change to have occurred at the site is the development of near permanent pools. Despite the fact all three study reaches experienced a significant increase in geomorphic diversity, the magnitude and persistence of change within the treatment reaches was not observed in the control. Such an outcome implies ELJs can potentially provide a stable pool habitat for the promotion of species diversity, which has been identified as a challenge in degraded sand-bed streams. Wood reintroduction using ELJs is seen as a short-term method of enhancing geomorphic diversity and channel stability; however, few wood reintroduction initiatives have been rigorously monitored through time. The detailed study presented in this paper is unique, as it was carried out over a long monitoring period (11 years) and coupled with a ~ 20 year riparian rehabilitation strategy. This study suggests in-channel wood plays a vital role in the development and stability of geomorphic features within sand-bed streams, and it provides a realistic sense of the timescale involved to achieve effective river rehabilitation.
Acknowledgments We would like to thank Brian and Sally Woodward for allowing access to their property at ‘Earthways’; your passion for the environment and support of the project is gratefully appreciated. Thanks to Rod Gleeson, Peter Coffey and Daniel Seddon-Powell for assisting with elevation data collection and to the School of Earth and Environmental Sciences at the University of Wollongong for providing field equipment. We would like to thank Heidi Brown, Chris Owers and Joe Wheaton for assistance with GIS analysis and for providing GIS data for this study.
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Hughes et al. (2014). The Shifting Sands of Stockyard Creek, in Vietz, G; Rutherfurd, I.D, and Hughes, R.M (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages 200-205. 205
