CHANNEL RESPONSE TO EXTREME FLOODS Nicola Surian Department of Geosciences, University of Padova Masaryk University Brno, 26 September 2019 INTRODUCTION Floods are natural processes, taking place with different magnitude and frequency in all fluvial systems Floods are one of the major natural hazard that affect highly populated countries Overall aim of this lecture: Giving a new perspective («geomorphic perspective») about floods and related hazard Which processes occur during floods? Lateral mobility Channel aggradation Wood transport Not only inundation! Why the focus on extreme floods? Extreme floods may have strong impact on channel morphology and floodplain The risk associated to such floods can be very high In several areas extreme floods are likely to become more frequent (climate change) Outline of the lecture 1. Analysis of geomorphic response to extreme floods 2. Understanding processes: linking geomorphic response to driving factors 3. Case study: the Magra River flood event Hazard assessment: do we have effective tools to predict geomorphic response to extreme floods? Which processes can be expected in a specific river reach? Channel Changes Before the flood Flow Flow After the flood • Deposition occurs on the bars and on the floodplains through gravel deposition. • Aggradation is very common immediately downstream the tributary junctions, where the channel bed presents lower slope or in areas were the valley widens up and channels are unconfined. • On the floodplain, the amount of aggraded sediments tends to decrease with distance from the channel FLOOD and CHANNEL VERTICAL CHANGES Channel Changes: bed level pre post Aggradation Incision pre post Aggradation: On channels (bars) and floodplains Incision: on channels Channel changes only occur when the flood power exceeds the channel boundary resistance threshold, which depends on river bed and bank cohesive forces along the channel reach. Stream power has widely been used as a measure of the geomorphic effectiveness of floods because its measures quantify river energy expenditure in fluvial systems. Stream power ‘the rate of energy supply at the channel bed that is available for overcoming friction and transporting sediments’. Stream power STREAM POWER Ω = ρg Q S where Q [m3 s-1] is the flood discharge UNIT STREAM POWER Stream power per unit-wetted area is termed unit stream power, ⍵ [W m-2] and expressed as: ⍵ = Ω / w where w [m] is the top channel width corresponding to the flood level (Costa & O’Connor, 1995) Role of unit stream power and flow duration on geomorphic change Unit Stream Power: ω = ῤgQS / w Additional factors, besides hydraulic variables, should be incorporated to explain channel and floodplain response: Bed-load supply (e.g. Dean & Schmidt, 2013, Geomorphology) Lateral confinement (e.g. Thompson & Croke, 2013, Geomorphology) Artificial structures (e.g. Langhammer, 2010, Natural Hazards) «Despite decades of work in geomorphology on flood effectiveness, we still generally lack ability to predict sites of major geomorphic changes during extreme flow events» (Buraas et al., 2014; ESPL) Sediment sources and delivery 2- Coupled Landslides 3- Sediment Supply 1-Landslides mapping Field surveys; remote sensing Connectivity index (Cavalli et al., 2013) Integrated approach for investigating geomorphic response to an extreme flood Rinaldi et al., 2016, ESPL Case studies Magra River October 2011 Nure River September 2015 Posada River November 2013 Dolomites October 2018 Lierza River August 2014 Extreme flood event in the Magra and Vara basins Trebbia - Nure Magra - Vara MAGRA and VARA October 25th 2011 Nardi and Rinaldi, 2015 - ESPL; Rinaldi et al., 2016 – ESPL; Surian et al., 2016 - Geomorphology MAIN CHANNELS MAGRA VARA Catchment area (km2) 1146 571 Catchment max elevation (m) 1901 1404 Channel length (km) 70 58 Main Catchment Geology Sandstones Mudstones The 25th October 2011 event in the Magra River catchment: spatial distribution of rainfall maxima corresponding to three-hours rainfall duration Maximum hourly rates: up to 149 mm/hr, Event-accumulation maxima were up 500 mm (RI up to 300 yr) (Surian et al., 2016, Geomorphology) Geomorphic effects in the Magra River catchment Teglia River (Rinaldi et al., 2016, ESPL) Pre-flood Post-flood Widening observed in the study channels Average Width before flood =17m Average Width after flood = 43m Average Wratio = 4.3 Average Widening = 27m Widening observed in the study channels • The narrower the channel, the largest the max widening = − − • Very large scatter for similar channel size Width ratio W pre Relation between widening («width ratio») and unit stream power (Comiti et al., 2016) width ratio = channel width after / channel width before the flood Sediment connectivity analysis Multiple regression models between width ratio and controlling factors for the sub-reaches characterized by no-steep slope ( < 4%) (Surian et al., in review) Multiple regression models between width ratio and controlling factors for the sub-reaches characterized by steep slope ( > 4%) (Surian et al., in review) Some remarks on the Magra flood magnitude of changes: very intense channel widening (in several reaches channel widening took up most of the alluvial plain) controlling factors: besides hydraulic variables (unit stream power), channel confinement, hillslope sediment supply, artificial structures are significant regression models as predictive tools of channel widening: more reliable in the steep channels, less in the no-steep channels Final remarks Integrated approach: crucial for a comprehensive analysis of extreme floods Complex channel response: hydraulic variables are not sufficient to explain geomorphic response (e.g. Costa & O’Connor, 1995; Dean & Schmidt, 2013; Thompson & Croke, 2013; Buraas et al., 2014); confinement is a key factors; unit stream power calculated on pre-flood channel width Hazard assessment: channel often takes up the whole valley floor, in small streams; widening and lateral confinement are well related; widening often coupled with aggradation Need to include geomorphic processes in hazard assessment and mapping: inundation is not everywhere a major issue, much less relevant than channel dynamics Credits Margherita Righini, Andrea Brenna, Marco Borga, William Amponsah (Univ. Padova) Lorenzo Marchi, Marco Cavalli, Stefano Crema (CNR-IRPI, Padova) Francesco Comiti, Vittoria Scorpio, Ana Lucia (Univ. Bolzano) Massimo Rinaldi, Laura Nardi, Marco Benvenuti (Univ. Firenze) Alessandro Corsini, G. Ciccarese (Univ. Modena & Reggio Emilia) Ellen Wohl (Colorado State Univ.) Francesco Marra (Hebrew Univ. of Jerusalem) Main pubblications: Scorpio V. et al. (2018), Science of the Total Environment, 640- 641, 337-351. Righini M. et al. (2017), Geomorphology, 290, 184-199. Rinaldi M. et al. (2016), Earth Surf. Process. Landf., 41, 835-846. Surian N. et al. (2016), Geomorphology, 272, 78-91.