Ashworth, Philip

Best, J.L.

Bridge, J.S.

Lane, S.N.

Lunt, I.A.

Parker, N.O.

Sambrook-Smith, G.H.

Simpson, C.J.

English

Integrated aerial photography and GPR profiling has been used to explore the surface and subsurface evolution of braid bars within the South Saskatchewan River, Canada 2004-2007. Following a large flood in 2005, areas of a kilometre-long braid bar experienced incision and the creation of new unit bars. Subsequent low-magnitude, high-frequency flows during 2006 produced a further new central unit bar. Analysis of GPR profiles shows the presence of bar margin deposits in the subsurface relating to the unit bars produced in flood conditions and low-magnitude high-frequency flows. Quantitative comparisons of deposits reveal that the bar margin deposits formed during the large flood are similar in scale to the low-magnitude, high-frequency deposits with respect to the height and width of individual strata. However, stratal angles appear steeper in bar margin deposits produced by the low-magnitude, high-frequency events

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12th International Conference on Ground Penetrating Radar, June 16-19, 2008, Birmingham, UKSedimentological impact of a high-magnitude, low-frequency flood in a braided river revealed from sequential GPR surveysN.O. Parker1, G.H. Sambrook Smith1, P.J. Ashworth2, J.L. Best3, J.S. Bridge4, S.N. Lane5, I.A. Lunt6, C.J. Simpson71School of Geography, Earth and Environmental Sciences, University of Birmingham, UK, B15 2TT Email: nop661@bham.ac.uk2School of Environment and Technology, University of Brighton, UK3Departments of Geology and Geography and Ven Te Chow Hydrosystems Laboratory, U. Illinois Urbana-Champaign, USA4Department of Geological Sciences, Binghamton University, USA5Department of Geography, University of Durham, UK6Norsk Hydro, Norway7Department of Geography, Simon Fraser University, CanadaIn submitting this paper for EuroGPR2008 I hereby assign the copyright in it to the University of Birmingham and confirm that I have had the permission of any third party for the inclusion of their copyright material in the paper.  The University of Birmingham will license EuroGPR to use this paper for non-commercial purposes.  This will be the sole use of this material.Abstract –Integrated aerial photography and GPR profiling has been used to explore the surface and subsurface evolution of braid bars within the South Saskatchewan River, Canada 2004-2007.  Following a large flood in 2005, areas of a kilometre-long braid bar experienced incision and the creation of new unit bars.  Subsequent low-magnitude, high-frequency flows during 2006 produced a further new central unit bar. Analysis of GPR profiles shows the presence of bar margin deposits in the subsurface relating to the unit bars produced in flood conditions and low-magnitude high-frequency flows.  Quantitative comparisons of deposits reveal that the bar margin deposits formed during the large flood are similar in scale to the low-magnitude, high-frequency deposits with respect to the height and width of individual strata.  However, stratal angles appear steeper in bar margin deposits produced by the low-magnitude, high-frequency events.1. INTRODUCTIONThe detailed three-dimensional visualisation of subsurface sedimentary deposits using Ground Penetrating Radar (GPR) has provided many new insights into the behaviour and evolution of braided rivers.  Previous work has quantified the internal structure of sandy braided river deposits[1,2,3,4] but to date no work has described the sedimentary deposits from different magnitude-frequency flow events. The relative importance of geomorphic events of differing magnitude and frequency is a subject of considerable ongoing debate in fluvial sedimentology[5,6,7,8]. Ongoing research since 2000 on the South Saskatchewan River, Canada, has integrated Ground Penetrating Radar, aerial imaging and differential global positioning system (DGPS) topographic surveys to investigate the processes and deposits of this sandy braided river. This paper develops this work by presenting a temporal sequence of GPR data collected over the same survey lines for four consecutive years (2004-2007).  In 2005, a 1 in 38.5 year flood event occurred[9] that provided an unrivalled opportunity to place a large flood event into the context of data collected over the preceding and subsequent years.  This paper presents a quantitative comparison detailing deposition at the margin of braid bars formed by both the high-magnitude low-frequency (HMLF) flood event and subsequent low-magnitude high-frequency (LMHF) flow events.2. STUDY SITEThe study reach is located near the town of Outlook on the South Saskatchewan River, Saskatchewan, Canada. The river has a braided planform ~0.6 km wide.  Bed material is sand with average grain size 0.3mm.  The main channels are ~50-150 m wide and 2-5 m deep with an average bed slope of 0.0003.  Channels contain a range of bedforms including sand dunes and unit and compound bars. A bar is a depositional bedforms whose length is proportional to channel width, and whose height scales with flow depth[10].  Unit bars are relatively unmodified bars [1], typically with a lobate planform[11]. Typical dimensions of unit bars in the South Saskatchewan River range from 0.5-1.25 m height, 20 – 95 m length along stream and 12 – 58 m across stream.  Compound bars are an amalgamation of unit bars. Dunes scale with flow depth with heights ranging from 0.15 to 0.49 m. DATA COLLECTION	PARAMETERSStation spacing (m)	0.10Antenna separation (m)	0.5Stacks	8Sampling interval (ns)	0.4Antenna frequency (MHz)	200PROCESSING	Bandpass filter (MHz)	Trapezoidal with gates at 20-40-150-400Dewow (MHz)	14Set time-zero	YesDepth conversion velocity (m ns-1)	0.05AGC window (ns)	10The flood event that occurred in June 2005 peaked at 1830 m3s-1 on the 22nd June, with discharge remaining above 1000 m3s-1 for 19 consecutive days. The 2005 flood has a recurrence interval of 1 in 38.5 years[9]. Mean annual discharge of the South Saskatchewan River is approximately 200m3s-1. 3. METHODSA pulseEKKO PRO GPR system with 200MHz frequency antennae was used in this study, allowing penetration of 8 m at a decimetric vertical resolution.  Data were collected using a common offset (CO) method in continuous mode with transmitter and receiver fixed perpendicular to the survey line on a Smart Cart© (Table 1). A grid system was established for the survey lines using DGPS, which allowed resurvey of lines each year. The grid system is based on lines running downstream (south to north) and across stream (west to east) to centimetre spatial accuracy. Two reaches of the river are described herein; Reach A has a grid based on 50 m spacing whereas Bar B has a 25 m spacing.Data were post-processed using Ekko View (Sensors and Software) and Seismic Unix software (Table 1). DGPS surveys of the GPR lines were used to provide data for topographic correction.  GPR profiles were interpreted based on the principle that radar reflections represent sedimentary structures[12], with core data from the South Saskatchewan from previous years used as a ground truth to aid interpretation of reflections.In addition to the GPR surveys, aerial photographs were taken each year at a scale of 1:5000, allowing the surface evolution of bars to be mapped and quantified.4. RESULTS AND INTERPRETATION4.1	Surface EvolutionIn 2004, Reach A consisted of a bank-attached compound bar, occupying the east side of the reach. Surface morphological change due to the 2005 flood included dissection of the bank-attached bar through channel incision (see arrow, Figure 1, B), and the creation of a unit bar in the channel, which then migrated through the channel in 2006 (Figure 1, C). Over the majority of Reach A however, minor erosion and deposition occurred 2004-2006 (average net change <1.0 m).Bar B has formed entirely due to low-magnitude high-frequency flows after the 2005 flood. Bar B has migrated downstream between 2006 and 2007 by up to 100 m, dueto the accretion of two unit bars in front of the initial unit bar (Figure 1, D).GPR reflection(scale in metres)	Reflection type	Location	Formation	Facies 1. High angle inclined.Angle of dip from 6° to angle of repose.Commonly truncated at top and bottom by sub horizontal reflections	Active bar margin	Flow causes sediment  transport over bar migration into channel thalweg	Facies 2. Discontinuous undular or trough shaped reflections	Channel thalweg	Product of large sinuous crested dune migration in channel thalwegs	Facies 3. Low angle reflections (< 6°).Can be arranged in sets that dip up/downstream, laterally/horizontally.	Bar surface	Accretion surfaces on which  dunes of a low amplitude are migrating over, or unit bar migration over bar surface	Facies 4. Reflections of variable dip enclosed by concave reflection	Channel	Filling of channel scours and cross bar channels by unit bars (angle of repose) or dunes (low angle or undular)4.2	Subsurface EvolutionGPR profiles were taken over consecutive years during 2004-2007 for Reach A and during 2006 and 2007 for Bar B. Four main types of sedimentary deposits have beenidentified on this reach of the South Saskatchewan, based on GPR radar reflections (Table 2). In this paper, bar margin deposits (Table 2, Facies 1) produced by the low-magnitude high-frequency flows on Bar B will be compared to deposits produced by the June 2005 flood on Reach A. Bar margin accretion surfaces and their deposits are classified here as high angle inclined reflections in the radar profile, dipping from 6° to the angle of repose[4]. These deposits are interpreted as products of sediment transport over bar margins, either by dune or unit bar migration[4]. Internal structures are composed of centimetric thick downstream inclined strata that can fine upwards and are produced by grain flows or superimposed down-climbing bedforms such as dunes[11].Bar margin accretion surfaces and their deposits were identified in GPR profiles (e.g. Figures 2 &3) and quantified in terms of strata height, individual strata width, and angle (Table 3).  Deposits were interpreted as formed by unit bar and migration rather than dunes as the maximum strata heights (0.8-0.9m) are higher than dune heights for the South Saskatchewan (0.15-0.49m). Common to all bar margin deposits was an increase in strata height and angle with proximity to the bar margin (Table 3).  This suggests that the bar margins have become steeper with growth.Flood and low-magnitude high-frequency flow deposits show similar strata heights (proximal and distal) and individual strata widths (Table 3). This suggests similar bar heights under the different flow conditions.  However, strata angles are lower in deposits formed by the high-magnitude low-frequency flood (Table 3). High sediment transport rates can promote frequent grain flows of sediment, which may accumulate at the base of a bar margin and result in a low angle slope[13].  High sediment transport rates due to the flood discharge may therefore be the cause of the lower angled strata.    		2005 Flood deposits	Low-magnitude high-frequency flow depositsStrata height (m)	proximal to margin	0.9	0.8	distal to margin	0.3	0.5Strata angle (°)	proximal to margin	11.9	17.6	distal to margin	8.7	13.9Individual strata width (m)	0.7	0.8Overall, the low-magnitude, high-frequency flow deposits appear to be of a similar scale with respect to height and width, but of a smaller angle, compared with the high-magnitude low-frequency flood deposits. The increased stage height during the 2005 flood, and significant surface morphological change through channel incision and unit bar creation thus does not appear to have produced a larger scale of deposits than produced by low magnitude high-frequency flows.5. CONCLUSIONSA comparison of aerial photographs of a braided reach of the South Saskatchewan River between 2004 and 2007 has revealed channel incision and unit bar deposition caused by a large flood.  Aerial photographs also show the creation and emergence of a new unit bar (Bar B) in 2006 due to low-magnitude high-frequency flows.  The deposits at the margin of these bars comprise steeply dipping strata. Bar margin deposits produced during these different magnitude-frequency events reveal a similarity in the scale of deposits at the margins of the bars with respect to height and width. Strata angles are, however, lower in deposits produced by the large flood event, which is thought to be caused by increased deposition on the lower bar margin. To assess fully the scale of deposits formed under different magnitude-frequency events, other depositional areas on this study reach will be quantified and compared. In order to assess the role of topography on the preservation of deposits formed by different magnitude-frequency events, ongoing work is producing digital elevation models of the study reach using photogrammetric methods. ACKNOWLEDGMENTSNOP thanks the School of Geography, Earth and Environmental Sciences, University of Birmingham, for PhD research funding, whilst the field data were collected under NERC grants NE/D005701/1 and NER/A/S/2003/00538, with support from Exxon Mobil (USA).REFERENCES[1]  Smith, N.D. (1974) Sedimentology and bar formation in the Upper Kicking Horse River, a braided outwash stream.  Journal of Geology 82, pp. 205-223.[2]  Bridge, J.S. and Lunt, I.A. (2006) Depositional models of braided rivers. In Sambrook Smith, G.H., Best, J.L., Bristow, C.S., and Petts, G.E. (eds) Braided Rivers: process, deposits, ecology and management, Special Publication Number 36 of the International Association of Sedimentologists. Blackwell Publishing, pp.11-50[3]  Skelly, R.L, Bristow, C.S., and Ethridge, F.G. (2003) Architecture of channel-belt deposits in an aggrading shallow sandbed braided river: the lower Niobrara River, northeast Nebraska.  Sedimentary Geology 158, pp. 249-270.[4] Sambrook Smith, G.H., Ashworth, P.J., Best, J.L. Woodward, J. and Simpson, C.J. (2006) The sedimentology and alluvial architecture of the sandy braided South Saskatchewan River, Canada.  Sedimentology 53, pp. 413-434.[5]  Wolman, M.G. and Miller, J.P. (1960) Magnitude and frequency of forces in geomorphic processes.  Journal of Geology 68, pp. 54-74 [6]  Gupta, A. (1983) High magnitude floods and stream channel response. In Collinson, J.D. and Lewin, J. (eds) Modern and Ancient Fluvial Systems, Special Publication Number 6 of the International Association of Sedimentologists. Blackwell Publishing, pp. 219-228.[7] Kochel, R.C. (1988) Geomorphic impacts of large floods: Review and new perspectives of magnitude and frequency.  In Baker, V.R., Kochel, R.C. and Patton, P.C. (eds) Flood geomorphology. John Wiley and Sons, pp. 169-188.[8]  Magilligan, F.J., Phillips, J.D., James, L.A. and Gomez, B. (1998) Geomorphic and sedimentological controls on the effectiveness of an extreme flood. Journal of Geology 106, pp. 87-95.[9] Thomas, R.E. (2006) Flow processes and channel change in sand-bedded braided rivers. Unpublished PhD thesis, University of Leeds.[10] Bridge, J.S. (2003) Rivers and Floodplains: Forms, processes and sedimentary record. Blackwell Publishing[11] Reesink, A.J.H. and Bridge, J.S. (in review) Distinguishing cross strata formed by dunes and unit bars. Journal of Sedimentary Research.[12]  Jol, H.M. and Smith, D.G. (1991) Ground penetrating radar of northern lacustrine deltas. Canadian Journal of Earth Science 28, pp. 1939-1947.[13] Reesink, A.J.H. and Bridge, J.S. (2007) Influence of superimposed bedforms and flow unsteadiness on formation of cross strata in dunes and unit bars.  Sedimentary Geology 202, pp. 281-196.Figure 1. Evolution of Reach A 2004-2007 and Bar B 2006-2007. Note dissection of Reach A due to flood in 2005 (B), and emergence of Bar B and a unit bar on Reach A in 2006 (C).Table 1. GPR data collection and post processing parametersFigure 2. 2007 GPR profiles of bar margin deposits produced by the 2005 flood on Reach A.  Bar has migrated westwards. Deposits increase in height and angle towards the west bar margin edge.Table 2. Deposits identified in the South Saskatchewan River 2004-2007, adapted from [4].Figure 3. GPR profiles of bar margin deposits on Bar B. Flow direction is up page. Migration direction is eastwards. Bar margin deposits increase in height and dip towards margin.Table 3. Mean dimensions for bar margin deposits on Reach A and Bar B. Deposits are distinguished by magnitude frequency formative flow and proximity to margin. n= 4 strata sets (2005 flood), n=11 strata sets (low-magnitude high-frequency flows).PAGE  

Sedimentological impact of a high magnitude, low-frequency flood in a braided river revealed from sequential GPR surveys

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Sedimentological impact of a high magnitude, low-frequency flood in a braided river revealed from sequential GPR surveys

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