Erosion, deposition and channel adjustment in Lockyer Creek, South East post the January 2011 .

Erosion and Deposition during the January 2011 Lockyer .

1 2 3 4 Jacky Croke , Fiona Watson , Chris Thompson and Peter Todd

1 Australian Rivers Institute, Griffith University Nathan Campus, Queensland 4111 Email: [email protected]

2 Department of Environment and Resource Management, Remote Sensing Centre, Ecosciences Precinct, Dutton Park Q 4102 3 Integrated centre for Catchment Management (ICaM) Australian national University, ACT.

4 Department of Environment and Resource Management, Land Centre, Woolloongabba QLD 4102

Key Points • The Lockyer Catchment lies in an area of recognized hydrological variability. • For the January 2011 event, average peak flood transmission speeds varied from 24 kmh-1 to 2.9 kmh-1 between the upper and middle reaches. • Net erosion occurred only in the inner channel and macro-channel banks. • Dominant form of channel adjustment was channel widening through bank slumping and removal of within-channel benches.

Abstract This paper presents a preliminary assessment of the nature and spatial extent of erosion and deposition that occurred as a result of the January 2011 flood event in the , Queensland. Research methods include analysis of both pre- and post-flood, high-resolution (LiDAR) digital elevation data, aerial photography, and field validation of selected sites. Results indicate that extensive channel widening (3xincrease) occurred in the steep bedrock confined river reaches of the upper valley. Dramatic examples of whole-scale re-organisation of channel morphology and geometry are apparent in these high-energy reaches. In the middle and downstream reaches, the dominant process of channel adjustment is bank slumping which has left significant and spatially widespread erosional scars. Processes such as floodplain erosion, channel avulsion and meander cut-offs were rarely apparent. Over the entire study area, the floodplain is a net sink with a mean elevation difference pre- and post-flood of 0.07m. Considerable variability in channel adjustment occurred along the energy gradient, between confined and unconfined settings, and with distance away from the source of precipitation in the headwater catchments.

Keywords Lockyer Valley, Queensland, January 2011 Flood, channel adjustment, bank erosion, slumping.

Introduction In January 2011, a cloud burst over the in the River Catchment, Queensland saw the development of an extreme flood event that resulted in the loss of human life and extensive damage to infrastructure and facilities estimated to cost in excess of $30 billion. The January 2011 flood event has been rated as the second highest flood of the past 100 years, after January 1974, with an event peak of 648mm (BOM, 2011). Gauging station hydrographs indicate a rapid and extreme rise in discharge in the upper part of the Lockyer Catchment which has been referred to as a ‘wall of water’, to describe both the speed and force of the resultant floodwaters.

Spectacular examples of large-scale channel and floodplain adjustment following extreme or catastrophic flood events have been described worldwide (Baker, 1973; Church, 1978; Costa, 1983; House et al., 2002). In

Editors names. (2011). Proceedings of the 6th Australian 1 Management Conference. Canberra, Australian Capital Territory, pages XX - XXX.

Erosion, deposition and channel adjustment in Lockyer Creek, post the January 2011 Floods.

Australia, the most well documented studies of channel adjustment processes come from NSW notably the Hunter River and its tributaries (Erskine, 1993, Erskine and Saynor, 1996, Erskine and Warner, 1988, Erskine 2011). Studies revealed that channel adjustment due to extreme events can include modifications to channel planform, sinuosity, slope, channel metamorphoses, and catastrophic floodplain stripping (Nanson, 1986). Existing research confirms, however, that catchment response cannot be considered as simply the product of the intensity and magnitude of the rainfall event but rather the interaction of other factors (topography, flood sequencing, catchment management history) that contribute to the ‘susceptibility’ of a given catchment. Landscape susceptibility to a given process response may occur at spatially distinct locations in a catchment and over a rapid or slow response time. Such differences have previously been described in terms of a ‘pulsed’ and a ‘ramped’ input (Brunsden and Thornes, 1979) and as ‘response gradients of adjustment’ (Fryirs et al., 2009). Pulsed disturbance can produce more catastrophic adjustments such as avulsions, channel shortening and cut-offs, whereas ramped disturbance produces incremental erosion and adjustment over time. The major aim of this paper, therefore, is to present a preliminary assessment of the broad spatial patterns of erosion, deposition and channel adjustment following the Lockyer Creek January 2011 flood.

Study Area

Background

The Lockyer Valley lies to the east of on the Great Dividing Range with a catchment area of 2600km2 accounting for approximately one-quarter of the catchment. The rich sub-coastal land and alluvial plains of the catchment are one of the state’s most important centres of diversified agriculture centred around the principal towns of Gatton (pop. 6,000) and Laidley (pop.3,500) (Fig 1). Lockyer Creek flows generally east for ~100km and enters the Brisbane River at Lowood (Fig 1). Climate in the region is classified as sub-tropical and sub- humid.

Geology

The Lockyer Valley is part of the Moreton Basin which was a large shallow fresh water lake during the Mesozoic Era. During the Jurassic, Figure 1. Lockyer Valley catchment the sandstones, shales and conglomerates of the Helidon and Marburg in SE Queensland. formations were deposited in beds that now dip in a southerly direction (Powell, 1987). The main stem Lockyer Creek drains west to east into the Brisbane River and is aligned with the Gatton Sandstone beds.

Geomorphology

Lockyer Creek has undergone several phases of incision and aggradation since the Tertiary with the river system cutting through the Tertiary volcanics and into the underlying Mesozoic sedimentary rocks to reveal the present day exposures (Powell, 1987). Several erosional surfaces have been identified including an early Miocene Upper Erosion Surface (Helidon sandstone 450-600 m above sea level (a.s.l.) and an early Pliocene Middle Erosion Surface (Lower Marburg at 120-210 m a.s.l.) (McTaggart, 1963). A later phase of renewed incision is thought to have occurred at some time close to the last glaciation (18,000-20,000 yrs) and again around c.3,000 B.P. that resulted in the development of the present-day entrenched river and its tributaries (Powell, 1987).

The present-day main stem of Lockyer Creek is a single, meandering channel of medium-high sinuosity (1.3- 1.5) with some tightly curving, locally incised meander bends. The upper reaches are bedrock confined high- Editors names. (2011). Proceedings of the 6th Australian Stream 2 Management Conference. Canberra, Australian Capital Territory, pages XX - XXX.

Erosion, deposition and channel adjustment in Lockyer Creek, South East Queensland post the January 2011 Floods. energy channels (see Thompson et al., this volume) but once the river emerges onto the unconfined alluvial plain around Helidon, the present channel is inset within a large (~100m wide and 20m deep) macro-channel. Some alluvial cutoffs are preserved on the valley floor but there is little topographical evidence of recent lateral migration of the river in the form of remnant scroll bars or extensive point-bar development. Preliminary investigations of some exposed floodplain stratigraphy reveal the dominance of vertical accretion processes with horizontally-bedded alternating sands and fine silts suggesting very slow rates of lateral migration. Levees are also notable features of the current entrenched Lockyer Creek with floodplain surfaces which slope steeply away from the present channel. Floodplain width varies with distance downstream from the confined, high energy reaches of the headwater tributaries where floodplain pockets occur on alternating sides of the valley floor, through to the wide alluvial plain at the lower end of the catchment.

Regional Hydrological Characteristics. The general region of the Brisbane and Lockyer River systems is characterized by patterns of floods and droughts which have been linked to the inter-annual rainfall variations of the El Nino-Southern Oscillation (ENSO) and the inter-decadal Pacific Oscillation (IPO) (Kiem et al., 2003, Rustomji et al., 2009). In NSW, these patterns of floods and droughts have been used to explain associated adjustments in channel morphology (Erskine and Warner, 1988). In a recent analysis of hydrological variability in all basins east of the Dividing range, Rustomji et al.,(2009) concluded that flood variability increased southwards through Queensland to reach a maximum in the Burnett Catchment, north of the study area. Magnitude Indices (FFMI) for the four gauging stations along the Lockyer range from 0.68-1.07 (Rustomji et al., 2009). While this index does not capture all components of hydrological variability, such high values indicate a propensity of alternating extremes. Erskine (1999), for example, found that a value of at least 0.6 characterised Australian rivers with a known high flood potential.

Hydrology of the January 2011 Flood. A detailed account of the event’s meteorological and hydrological characteristics is provided in Jordan (2011). A strong La Nina event and elevated sea surface temperatures (SSTs) lead to a wet summer in 2010-2011 and in the days leading to the January event, 20 to 30 mm had fallen across the catchment resulting in near- saturated soils and high runoff. On January 10th 2011, a number of massive storm cells converged and moved across the top of the Lockyer catchment and intensified due to the orographic effect of the Ranges. Recorded rainfall intensities in Toowoomba had annual exceedance probabilities of 18 years for 10 minutes duration, 200 years for 30 minutes duration and 370 years for 60 minutes duration (Jordan, 2011).

The approximate magnitude of the January 2011 event was estimated by gauging stations along the course of the mainstem until they failed near the flood peak. In the headwaters, the Spring Bluff gauge (Station # 143219) measured 361.5 m3s-1 at 13:40 on 10/1/2011, Helidon (Station # 143203) measured 3642 m3s-1 at 15:10 on 10/1/2011 and downstream of Gatton, the Rifle Range Road gauge (Station # 143204) measured 1453.2 m3s-1 at 16:20 on 11/1/2011. Average transmission speeds of 24.2 kmh-1 for the flood peak between the Spring Bluff and Helidon gauges reduce considerably to ~2.9 kmh-1 between Helidon and Rifle Range Road in the middle-lower reaches.

Estimates of the average return interval (ARI) based on the Log Pearson type III (LP3) analysis of the annual flood series ranges from >>1000 years at the headwater gauge in Spring Bluff (~700m a.s.l), to ~80 years at Helidon and ~27 years at Riffle Range Road. These estimates produce similar values to the statistical metrics used by Rustomji et al. (2009) but are derived from relatively short time periods of gauging station data.

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Erosion, deposition and channel adjustment in Lockyer Creek, South East Queensland post the January 2011 Floods.

Methods

Data relating to channel erosion and floodplain erosion in the main stem of Lockyer Creek was obtained from three main sources;

Field Geomorphic Survey.

A geomorphic assessment of ~50 field sites throughout the main stem of Lockyer Creek was undertaken during May 2011. Sites extended along the long profile of the river from Spring Bluff (~700m a.s.l) through to the of the Lockyer with the mid-Brisbane River (Fig 2a) but did not include any of the major tributaries. Sites were grouped into 3 segments; Segment 1 (n=12) consisted of sites from the upper headwaters to Helidon (Fig 2b); Segment 2 (n=20) from Helidon to Rifle Range Road and Segment 3 (n=20) from Rifle Range Road to the junction with the mid Brisbane. Segment C sites currently lie outside the extent of LiDAR coverage but will be analysed when this data is made available later in 2011. Each site consisted of ~10 transects surveyed perpendicular to the main channel to assess the nature and degree of erosion along channel banks and the adjacent floodplain (Fig 3a).

Figure 2. (a) Location of sample sites with grey area representing the extent of LiDAR coverage. (b) Long profile of Lockyer Creek with field sites in Segments A and B

Field estimates of channel geometry (width, bank inclination and height) were obtained using a TruPulse™ hand-held laser scanner. The rapid re-growth of invasive weed vegetation in the period January-May 2011 obscured the accurate determination of some variables such as bank height and angle. Water level also remained high during this period, making accurate determinations of channel bed depth difficult. The field survey proved most useful in identifying areas of notable channel adjustment and in differentiating the likely causal process. The widespread occurrence of scalloped-shaped slumps on the banks of many sites was noted (Fig 3c). Field teams also recorded qualitative estimates of percentage ground cover, shrubs and trees at each transect.

Repeat high resolution LiDAR Surveys.

Capture of high resolution LiDAR (Light Detection and Ranging) data in August 2010 (pre-flood) and February/March 2011 (post-flood) provided the basis for comparison of pre- and post-flood alluvial landscapes. The extent of the LiDAR coverage is outlined in Fig 2a. High resolution Digital Elevation Models (DEMs) derived from both LiDAR captures were used to derive stream networks and extract stream channel characteristics, such as channel slope and sinuosity, as well as two dimensional cross sections.

Delaunay triangulation with linear interpolation was used to compute pre- and post-flood surfaces from the two LiDAR datasets. A change surface was then calculated by subtracting one from the other, and formed the

Editors names. (2011). Proceedings of the 6th Australian Stream 4 Management Conference. Canberra, Australian Capital Territory, pages XX - XXX.

Erosion, deposition and channel adjustment in Lockyer Creek, South East Queensland post the January 2011 Floods. basis of volumetric change estimates. Differences in sensor configuration within and between LiDAR captures, and environmental conditions (e.g. water levels) impacted the ability to detect and model the ground surface, and detect change between the two dates. Volumetric changes reported are therefore gross volumetric changes.

Figure 3. (a) Transects at a site. (b) Geomorphic Feature Classification. (c) Example of scalloped slump features. Geomorphic Form: Process Classification Five morphological classes were distinguished through a combination of 1-dimensional hydrological modeling (HecRas) and analysis of slope thresholds from the 2009 LiDAR data. The resultant classes include;

• Inner channel-defined as the present thalweg, inundated by Q2.33 flows, and with low slope. • Inner channel bank-inundated by Q2.33 flows but exhibiting steep slope. • Within-channel bench-defined as alluvial features within the main boundary of the larger macro- channel, exposed at Q2.33 but inundated by Qbf and with low slopes. • Macro channel banks-steep slopes falling within a range of flows ≥Qbf. • Floodplain-low slope zones beyond Qbf inundation within the valley margin as delineated using the valley bottom flatness index MrVbf (Gallant and Dowling, 2003)

Shapefiles for these features were constructed within a Geographic Information System package (ArcGIS). The upper and lower bounding transects for each field site were intersected with the feature classification to create a layer of geomorphic features at each site. Bounds were approximated where the configuration of transect and channel shape were unsuitable. Both sets of polygons were used to summarise the change surface and calculate volumetric change both by geomorphic feature and site, using a simple Simpsons rule.

Table 1. (Gross volumetric) Changes in erosion (-), deposition (+) and net volume (m3) for each geomorphic feature across the study area of the main stem of the Lockyer Creek following the 2011 event. Mass was estimated using an average sandy soil bulk density of 1.6 g/cm3. Values rounded to two decimal places.

Geomorphic Area Net Net Net Mass Mass/Unit Feature (ha) Volume Deposition Erosion Area change (m3) (m3) (m3) (t) t/ha Inner channel 131 +1,410,000 1,500,000 89,000 2,254,000 17,000 Inner channel banks 137 +675,000 874,000 198,000 1,080,000 8,000 Bench 246 -314,000 531,000 818,000 -500,000 -2,000 Macro channel banks 355 -587,000 525,000 1,112,000 -940,000 -2,600 Floodplain 27460* +2,232,000 2,786,000 554,000 3,571,000 130

* The areal estimate of floodplains presented here represents an envelope around the flood inundated extent and is an underestimate of the total floodplain area.

Editors names. (2011). Proceedings of the 6th Australian Stream 5 Management Conference. Canberra, Australian Capital Territory, pages XX - XXX.

Erosion, deposition and channel adjustment in Lockyer Creek, South East Queensland post the January 2011 Floods. Results and discussion

Net Catchment Erosion and Deposition

Table 1 outlines the results of erosion, deposition and net volume changes as derived from the pre- and post- flood LiDAR derived elevation surfaces and delineated by geomorphic feature. Of the five geomorphic features examined, only the benches and the macro channel banks experienced net erosion with an estimated mean volume loss of ~1,400,000 tones of sediment. The gross net gains recorded for the inner- channel zone are a function of a number of factors. As noted above, fluctuations in vegetation cover, flood debris and water levels will be manifested as surface change within this zone. We cannot differentiate the relative contributions of these various noise signals at this stage in our analysis. The floodplain is a net depositional feature with an estimated mean elevation gain of 0.07 m and net deposition of 2,232,000m3 of sediment. This analysis confirms that over the entire study area, net erosion was primarily confined to the macro-channel bank and bench surfaces which show an average loss per unit area of between 1800-2600 t/ha of sediment (Table 1).

When comparing multi temporal datasets in this manner, it must be acknowledged, however, that inherent noise within and between datasets is difficult to isolate, quantify and remove from estimates of change. Differences in terrain, sensor configuration and echo detection at high scan angles result in differences in the behaviour of LiDAR returns, within and between captures. This impacts the ability to reliably detect and model the ground surface. Furthermore, two LiDAR datasets captured and processed independently (in this case by two different providers) may also exhibit variability in horizontal and vertical registration that can introduce spurious change. Environmental variability introduces further complexity, with fluctuations in vegetation cover, crop cover and water levels, for example, also manifesting as surface changes. Thus, isolating a particular change process such as channel morphological change represents a considerable challenge. By focusing on specific geographic areas, we believe that a better understanding of channel scale processes is possible. Values presented here, therefore, are best interpreted as relative differences rather than absolute values.

Segment Differences. Spatial differences between the upstream Segment 1 and middle, Segment 2 are outlined in Table 2. The overall pattern of net erosion confined to the benches and macro channel banks is consistent between the two segments. The most notable gross difference between the two segments is in mean estimates of ‘deposition’ recorded for the inner channel and inner channel banks which is 3-4 times higher in Segment 2. Again the precise composition of this surface elevation gain is unclear but will reflect the higher water levels, debris, and flood vegetation in the larger channels downstream. Field observations, however, confirm that significant deposition occurred within this inner channel zone as stream power and sediment transport capacity reduce with distance downstream. More detailed analysis of changes in bed elevation is being undertaken.

Within Site Variability High degrees of variability are also noted within, and between sites especially in the downstream Segment 2. Figure 4 provides a representative example of changes in channel cross-sections for the two LiDAR data sets. Each cross-section displays differences in elevation between the 2010 (solid) and 2011 (dashed) cross- sections. Segment 2, Site 7 contained 6 transects (Seg 2 7/1-7/6) spaced ~ 100m apart and display major differences in channel adjustment ranging from small net deposition (Seg. 2 7/4) to considerable channel expansion through the removal of the within-channel bench feature (Seg. 2 7/3).

Editors names. (2011). Proceedings of the 6th Australian Stream 6 Management Conference. Canberra, Australian Capital Territory, pages XX - XXX.

Erosion, deposition and channel adjustment in Lockyer Creek, South East Queensland post the January 2011 Floods.

Table 2. Differences in erosion (-) and deposition (+) in across the geomorphic features between the upstream Segment 1 and middle Segment 2. Values rounded to two decimal places.

Segment 1 Segment 2 Feature Area Mean Depth Volume Area Mean Depth Volume (km2) change (m) (m3) (km2) change (m) (m3) Inner channel 0.79 +0.59 +468000 0.51 +1.86 +957000 Inner channel 0.91 +0.30 +278000 0.46 +0.91 +421000 banks Bench 1.62 -0.13 -218000 0.88 -0.17 -146000 Macro channel 2.11 -0.22 -464000 1.43 -0.21 -301000 banks Floodplain 16.52 +0.08 +1322000 10.87* +0.06 +729000

Dominant Channel Adjustment Processes. From the analysis completed so far, the dominant process of channel adjustment in the main stem of the Lockyer Creek is channel widening. Two specific processes dominate; (a) channel widening due largely to mass failure and bank collapse and (b) channel widening in the mid-lower cross-section through the removal of within-channel benches. Scalloped-shaped features indicative of bank slumping are spatially widespread and in some cases extreme. Spatially, the most extreme forms of channel adjustment occurred in the upper, confined high energy reaches where unit stream powers peaked at 4600 W m-2 (Thompson et al., this vol). The relatively high degree of channel change in this reach is due to the confined valley setting which effectively concentrated flood waters, high-energy and the proximity of these sites to the headwaters which were the source of the rapid onset and fast transmission of the flood peak. As these factors reduced with distance downstream, the degree of channel adjustment reduced. Overall, preliminary results indicate that there were relatively small adjustments to total channel length and sinuosity as a result of the January 2011 flood. Channel avulsion along the main stem of the Lockyer is also largely absent. Localised floodplain stripping was observed at several sites outside the current aerial extent of LiDAR.

Figure 4. Representative cross-sections (elevation by distance in metres) from Segment 2 Site 7 showing variable response to channel adjustment. Solid line is cross-section from 2010 LidAR and dashed line from 2011.

Conclusion A major storm event occurred in SEQ during January 2011 that resulted in major losses to human life, infrastructure and degraded water quality. This paper presents a preliminary assessment of erosion, deposition and channel adjustment at a number of spatial scales and across a range of geomorphic features. Features such as within-channel benches and the banks of the Lockyer macro-channel are the zones of net erosion. In contrast, both the inner channel and floodplains are zones of net deposition although it remains difficult to discern the relative contribution of water, sediment and debris from the LiDAR data. Further investigations into this aspect of the study are being undertaken. While the absolute values incorporate errors both due to the processing techniques and the complications of a higher water level in the post-flood imagery, relative differences are important. From these, sediment redistribution is seen as a dominant process with net gains and losses occurring across geomorphic feature and within and between sites. The high degree of variability in channel adjustment between channel cross-sections reflects the variable and often stochastic nature of channel bank erosion. Further analysis of variables is currently being undertaken.

Editors names. (2011). Proceedings of the 6th Australian Stream 7 Management Conference. Canberra, Australian Capital Territory, pages XX - XXX.

Erosion, deposition and channel adjustment in Lockyer Creek, South East Queensland post the January 2011 Floods. Acknowledgments This project was funded by Queensland DERM as part of the Flood Recovery Project 2011. We are particularly grateful to field crews from the Chemistry Centre (Ellie Britton, Joanne Burton, Kate Dolan, Bianca Scott, Ashneel Sharma), Land Resource Assessment (Ian Hall, Kate Hughes, Lauren O’Brien, Don Malcolm, Jeremy Manders Reanna Willis); and Remote Sensing (James Taylor, Grant Ross, Sel Counter). Thanks also to Ken Brook, Dan Brough, Rob Dehayr, Paul Lawrence and Christian Witte for facilitating access to personnel and resources. Bernie Powell and Cyril Ciesiolka provided informative discussions and shared their knowledge of the Lockyer Valley. Reviews by ****greatly improved a previous version of this manuscript.

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Editors names. (2011). Proceedings of the 6th Australian Stream 8 Management Conference. Canberra, Australian Capital Territory, pages XX - XXX.