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P 11.7 Exploring the geomorphic response after sequential floods in a partially regulated alpine river

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Maha Sheikh at University of Bern
Maha Sheikh
Virginia Ruiz-Villanueva
Virginia Ruiz-Villanueva
Virginia Ruiz-Villanueva
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Abstract

Floods impact river morphology in several ways, i.e., lateral shift of the channel (i.e., avulsions), changes in bars spatial distribution (Rusnák & Lehotské, 2014), channel widening (Ruiz-Villanueva et al., 2023), and localized erosion and deposition. These morphological changes may benefit aquatic and riparian ecology, as they create and sustain heterogeneous and diverse habitats (Stanford et al., 2005), but can lead to long-term negative impacts. For instance, lateral shifts of river channels can cause damages to nearby infrastructure, properties and flood protection structures (Environmental Agency, 2018). Additionally, flood-induced morphological changes can also alter channel conveyance capacity, thereby affecting flood hazard (Todd-Burley et al., 2021). Therefore, understanding the rivers´ response to floods is critical for understanding fluvial dynamics and assessing potential hazards and risks. Still, predicting these changes remains challenging. The current study aims to understand and quantify the major geomorphological changes after floods in a partially regulated alpine River, the Spöl River (Graubünden). The flow of the River Spöl is regulated by two dams, Punt dal Gall and Ova Spin. As part of a river restoration program, experimental floods are released from these dams once or twice annually, creating a unique opportunity to study and analyse river morphological changes in a natural laboratory setting (Consoli et al., 2022). Our study site is located in the lower part of the river, downstream from Ova Spin and until the confluence with the Inn River. In this river segment, the Spöl receives flow and sediment from unregulated tributaries. To quantify the geomorphic changes following a sequence of floods, in terms of sediment erosion and deposition and changes in morphology, we used high-resolution digital surface models (DSMs) obtained by structure from motion based on drone-acquired data. The Geomorphic Change Detection (GCD) 7.5.0 standalone software was used to examine the geomorphic changes between the Digital surface Models collected before and after floods since 2018. The preliminary analysis revealed several flood-related morphological changes, stressing areas affected by significant erosion in the uppermost reach, and a general depositional pattern in the downstream part. In addition, we explored changes between the experimental floods, to analyse the effect of the unregulated tributaries input. The current study represents a unique opportunity to explore and quantify geomorphic changes after sequential floods. And, in addition, the study findings will be useful to inform flood and river management.
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367
Symposium 11: Geomorphology
Swiss Geoscience Meeting 2024 · Swiss Academy of Sciences (SCNAT) · Plaform Geosciences
367
P 11.7
Exploringthegeomorphicresponseaftersequentialfloodsinapartially
regulatedalpineriver
Maha Sheikh1, Virginia Ruiz-Villanueva1
1 Geomorphology, Natural Hazards and Risks Research Unit, Institute of Geography, University of Bern,
Hallerstrasse 12, 3012 Bern
Floods impact river morphology in several ways, i.e., lateral shift of the channel (i.e., avulsions), changes in bars spatial
distribution (Rusnák & Lehotské, 2014), channel widening (Ruiz-Villanueva et al., 2023), and localized erosion and
deposition. These morphological changes may benefit aquatic and riparian ecology, as they create and sustain
heterogeneous and diverse habitats (Stanford et al., 2005), but can lead to long-term negative impacts. For instance, lateral
shifts of river channels can cause damages to nearby infrastructure, properties and flood protection structures
(Environmental Agency, 2018). Additionally, flood-induced morphological changes can also alter channel conveyance
capacity, thereby affecting flood hazard (Todd-Burley et al., 2021). Therefore, understanding the rivers´ response to floods is
critical for understanding fluvial dynamics and assessing potential hazards and risks. Still, predicting these changes remains
challenging.
The current study aims to understand and quantify the major geomorphological changes after floods in a partially regulated
alpine River, the Spöl River (Graubünden). The flow of the River Spöl is regulated by two dams, Punt dal Gall and Ova Spin.
As part of a river restoration program, experimental floods are released from these dams once or twice annually, creating a
unique opportunity to study and analyse river morphological changes in a natural laboratory setting (Consoli et al., 2022). Our
study site is located in the lower part of the river, downstream from Ova Spin and until the confluence with the Inn River. In
this river segment, the Spöl receives flow and sediment from unregulated tributaries.
To quantify the geomorphic changes following a sequence of floods, in terms of sediment erosion and deposition and
changes in morphology, we used high-resolution digital surface models (DSMs) obtained by structure from motion based on
drone-acquired data. The Geomorphic Change Detection (GCD) 7.5.0 standalone software was used to examine the
geomorphic changes between the Digital surface Models collected before and after floods since 2018. The preliminary
analysis revealed several flood-related morphological changes, stressing areas affected by significant erosion in the
uppermost reach, and a general depositional pattern in the downstream part. In addition, we explored changes between the
experimental floods, to analyse the effect of the unregulated tributaries input.
The current study represents a unique opportunity to explore and quantify geomorphic changes after sequential floods. And,
in addition, the study findings will be useful to inform flood and river management.
REFERENCES
Consoli, G., Haller, R. M., Doering, M., Hashemi, S., & Robinson, C. T. (2022). Tributary effects on the ecological responses of
a regulated river to experimental floods. Journal of Environmental Management, 303(October 2021), 114122. https://doi.
org/10.1016/j.jenvman.2021.114122
Environmental Agency. (2018). Estimating the economic costs of the 2015 to 2016 winter floods. www.gov.uk/environment-
agency
Ruiz-Villanueva, V., Piégay, H., Scorpio, V., Bachmann, A., Brousse, G., Cavalli, M., Comiti, F., Crema, S., Fernández, E.,
Furdada, G., Hajdukiewicz, H., Hunzinger, L., Lucía, A., Marchi, L., Moraru, A., Piton, G., Rickenmann, D., Righini, M.,
Surian, N., … Wyżga, B. (2023). River widening in mountain and foothill areas during floods: Insights from a meta-analysis
of 51 European Rivers. Science of the Total Environment, 903. https://doi.org/10.1016/j.scitotenv.2023.166103
Rusnák, M., & Lehotské, M. (2014). Time-focused investigation of river channel morphological changes due to extreme floods.
Zeitschrift Fur Geomorphologie, 58(2), 251–266. https://doi.org/10.1127/0372-8854/2013/0124
Stanford, J. A., Lorang, M. S., & Hauer, F. R. (2005). The shifting habitat mosaic of river ecosystems. SIL Proceedings, 1922-
2010, 29(1), 123–136. https://doi.org/10.1080/03680770.2005.11901979
Todd-Burley, N., Halwyn, A., Wem, C., Ing, R., & Hemsworth, M. (2021). Understanding river channel sensitivity to
geomorphological changes Literature review and understanding factors that influence river channel change. http://
evidence.environment-agency.gov.uk/FCERM/en/Default/FCRM.aspx
ResearchGate has not been able to resolve any citations for this publication.
Tributary effects on the ecological responses of a regulated river to experimental floods
Article
Full-text available
  • Nov 2021
Rivers regulated by dams display several ecosystem alterations due to modified flow and sediment regimes. Downstream from a dam, ecosystem degradation occurs because of reduced disturbance, mostly derived from limitations on flow variability and sediment supply. In the last decade, most flow restoration/dam impact mitigation was oriented towards the development of environmental flows. Flow variability (and consequent disturbance) can be reintroduced by releasing artificial high flows (experimental floods). Flow-sediment interactions during experimental floods represent strong ecosystem drivers, influencing nutrient dynamics, and metabolic and functional properties. In river networks, sediment and water inputs from tributaries generate points of discontinuity that can drive major changes in environmental conditions, affecting habitat structure and determining functional differences between upstream and downstream. However, despite the relevance for management, flow/sediment relations during environmental flows − and more importantly during experimental floods − remain poorly understood, mostly due to the lack of empirical evidence. In this study, we examined how a major tributary (source of water and sediments) modified the physical habitat template of a regulated river, thereby influencing ecological and geomorphological responses to experimental floods. Methods combined high-resolution drone mapping techniques with a wide range of biological samples collected in field surveys before, during, and after experimental floods in an alpine river. Data were used to quantify changes in relevant functional and structural ecosystem properties, relating ecological responses to geomorphological dynamics. Results highlight the importance of tributaries in restoring ecosystem properties lost after damming, enhancing the resilience of the system. In addition, we observed that disturbance legacy played a fundamental role in determining ecological conditions of a river prior to experimental floods, thus confirming that considering flow variability and sediment availability is crucial in adaptive dam management and environmental flows design.
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Time-focused investigation of river channel morphological changes due to extreme floods
Article
Full-text available
  • Jul 2014
  • Z GEOMORPHOL
The typical morphological response of the meandering rivers to large floods is the lateral shift of their channel which triggers the formation of a new morphological structure from the initial destruction by erosion over deposition of new sediments and stabilization of vegetation. The article deals with the effect of extreme flood events on lateral channel shift and bar pattern with relation to changes of the riparian zone land cover structure by using multitemporal analyzes of aerial photographs (three time horizons – 1987, 2002 and 2009) in the GIS environment on the example of the 13.2 km long less regulated and laterally partly-confined meandering reach of the Ondava River (Eastern Slovakia). The photographs were chosen in a way to capture the morphological changes that occurred after floods. The average lateral channel shift per year was 1.17 m in 1987–2009, maximum 217 m. The river has eroded in total 35.6 ha and deposited 31.6 ha. Gravel bars in 1987, 2002 and 2009 spread a channel total area of 21.1 ha, 17.8 ha and 19.7 ha. The most eroded category is that of arable land, followed by grasslands and pastures and shrubs. We conclude that in case of the Ondava River, low magnitude high frequency floods, instead of causing destruction of the system, led to the stabilization of the channel, erosion of the concave bank and to the formation of the meandering planform. In contrast, the short recurrence interval of extreme floods led to an increased in tensity of erosion processes, a change of the meandering planform to slightly braided one, straightening of the channel and formation of gravel bars.
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River widening in mountain and foothills areas during floods: Insights from a meta-analysis of 51 central European Rivers
Article
  • Aug 2023
  • SCI TOTAL ENVIRON
River widening, defined as a lateral expansion of the channel, is a critical process that maintains fluvial ecosystems and is part of the regular functioning of rivers. However, in areas with high population density, channel widening can cause damage during floods. Therefore, for effective flood risk management it is essential to identify river reaches where abrupt channel widening may occur. Despite numerous efforts to predict channel widening, most studies have been limited to single rivers and single flood events, which may not be representative of other conditions. Moreover, a multi-catchment scale approach that covers various settings and flood magnitudes has been lacking. In this study, we fill this gap by compiling a large database comprising 1564 river reaches in several mountain regions in Europe affected by floods of varying magnitudes in the last six decades. By applying a meta-analysis, we aimed to identify the types of floods responsible for more extensive widening, the river reach types where intense widening is more likely to occur, and the hydraulic and morphological variables that explain widening and can aid in predicting widening. Our analysis revealed seven groups of reaches with significantly different responses to floods regarding width ratios (i.e., the ratio between channel width after and before a flood). Among these groups, the river reaches located in the Mediterranean region and affected by extreme floods triggered by short and intense precipitation events showed significantly larger widening than other river reaches in other regions. Additionally, the meta-analysis confirmed valley confinement as a critical morphological variable that controls channel widening but showed that it is not the only controlling factor. We proposed new statistical models to identify river reaches prone to widening, estimate potential channel width after a flood, and compute upper bound width ratios. These findings can inform flood hazard evaluations and the design of mitigation measures.
View
Estimating the economic costs of the 2015 to 2016 winter floods
  • Jan 2018
Environmental Agency. (2018). Estimating the economic costs of the 2015 to 2016 winter floods. www.gov.uk/environmentagency
Understanding river channel sensitivity to geomorphological changes Literature review and understanding factors that influence river channel change
  • Jan 2021
  • N Todd-Burley
  • A Halwyn
  • C Wem
  • R Ing
  • M Hemsworth
Todd-Burley, N., Halwyn, A., Wem, C., Ing, R., & Hemsworth, M. (2021). Understanding river channel sensitivity to geomorphological changes Literature review and understanding factors that influence river channel change. http:// evidence.environment-agency.gov.uk/FCERM/en/Default/FCRM.aspx