Investigation on the Factors Controlling the Response of Mountain Rivers to Extreme Flood Event (Case Study: Upstream Ilam Dam)

Document Type : Full length article


1 PhD Candidate in Geomorphology, Faculty of Geography, Univresity of Tehran, Tehran, Iran

2 Professor of Natural Geography, Faculty of Geography, Univresity of Tehran, Tehran, Iran

3 Associate Professor of Natural Geography, Faculty of Geography, Univresity of Tehran, Tehran, Iran

4 Professor of Remote Sensing and GIS, Faculty of Geography, Univresity of Tehran, Tehran, Iran


Large severe floods can have enormous influence on the fluvial system in comparison with the floods with lower magnitude and more frequencies. This work addresses the geomorphic response of mountainous rivers to extreme floods to explore the relationships between morphological changes and controlling factors. In October 2015, following the occurrence of a sudden extreme rainfall, a large and devastating flood occurred in Ilam province. The flood caused major changes in the morphology of Ilam's rivers. The rate of channel expansion is various in different sections of the studied rivers. Thus, we can examine the influential and controlling factors that led to diversity of river behavior. The hypothesis of this research is that explanation of geomorphic effects requires models that include other variables, e.g., lateral confinement, degree of sediment, besides hydraulic related variables (cross-sectional or unit stream power). The main purpose of this research is to explore the relationship between channel widening and arrange of controlling factors. We have addressed channel width (i.e: pre- or post-flood width) to calculate unit stream power in order to have a better explanation of channel response? Since few studies have been done in this field, this research was conducted with the aim of investigating the factors controlling the response of Mountain Rivers to extreme flood events in upstream of Ilam dam.
Materials and methods
The research has examined three tributaries of the Konjancham River (upstream of Ilam dam) whose catchments were affected by an extreme flood on 7th October, 2015. An integrated approach was taken to study this flood, including (i) Analysis of channel width changes by comparing aerial photographs before and after the flood, (ii) Estimation of peak discharges in studied reaches, and (iii) Determining the degree of sedimentation in studied reaches. Delineation of spatial units was carried out according to the approach proposed by Rinaldi et al. (2013), which is a modification of the approach by Brierley and Fryirs (2005). According to the approach, stream sectors were defined as macro reaches having similar characteristics in terms of lateral confinement, while the reaches are homogeneous in terms of channel morphology (channel pattern, width, and slope) and hydrology. We have used the reach scale (reach length was commonly from 200 to 1300 m) for an overall assessment of magnitude of channel changes and for a preliminary investigation of controlling factors. The dominant process observed in the study reaches was channel widening, which was analyzed by comparing aerial photographs taken before and after the flood. To assess the changes in channel width, channel banks, and islands, these features were digitized on pre- and post-flood orthophotos. The channel width was calculated by dividing channel area by the length of the reach, and changes in channel width were expressed as a width ratio (ratio of channel width after the flood to channel width before that flood). The estimation of peak discharges has been used to calculate cross-sectional stream power and unit stream power. The last part of the methodological section deals with statistical analysis carried out to explain channel response to the flood event by exploring the relationships between the changes in channel width and controlling factors.
Results and discussion
The relationships between the degree of channel widening and possible controlling factors were explored using multiple regression analysis. The analysis was carried out for the widening (width ratio) at reach scale. The entire data set includes 38 reaches. We analyzed seven controlling variables including confinement index, percentage of reach length with artificial structures, degree of sedimentation, channel slope, cross-sectional stream power, and unit stream power using pre-flood and post-flood channel width. Each regression model included only three to four variables. Each model included only one of the variables expressing potential or flood flow energy, e.g., channel slope, cross-sectional stream power, unit stream power. All four multiple regression models turned out to be significant (p< 0.001) and gave high coefficients of multiple determinations. The values of R2and adjusted R2 are ranged between 0.73 and 0.8 and between 0.69 and 0.77, respectively. The best model embraced unit stream power calculated based on pre flood channel width and confinement index as explanatory variables.
The results confirmed the main hypothesis of this work that hydraulic variables alone are not sufficient to explain channel response to an extreme flood event. The inclusion of other factors, specifically lateral confinement, degree of sedimentation, and percentage of reach length with artificial structures can lead to satisfactory models explaining the observed variability in the degree of channel widening. These results suggest that the widening process is essentially controlled by two factors: flood power and valley confinement. Flood duration exceeding a critical threshold was not included in our analysis, but it is a variable that very likely would increase the robustness of regression models in these reaches. The analysis carried out in the three subcatchments of the Konjancham River basin showed that unit stream power calculated based on pre-flood channel width has stronger relations with channel widening in comparison with that based on post-flood channel width and cross-sectional stream power. Because peak discharge was used for stream power calculation, we are aware that neither pre-flood nor post-flood channel width is actually appropriate for the estimation of unit stream power, as the most appropriate would be the (unknown) width at the flood-peak time. The pre-flood width has stronger relations with the degree of channel widening (width ratio). This could suggest the width changes occurred after the flood peak. 


Main Subjects

رستمی، س. (1387). ارزیابی فرسایش و رسوب‏زایی حوضة آبریز سد ایلام با نگاه ویژه به نقش سازندهای زمین‏شناسی منطقه در تولید رسوب، پایان‏نامة کارشناسی ارشد.
شرکت آب منطقه‏ای استان ایلام.
مقیمی، ا. (1388)، اکوژئومورفولوژی و حقوق رودخانه، تهران: انتشارات دانشگاه تهران.
مهدوی، م. (1391). هیدرولوژی کاربردی، ج۲، تهران: انتشارات دانشگاه تهران.
Baker, V.R.; Kochel, R.C. and Patton, P.C. (1998). Flood Geomorghology, Jhon willey and sons, 528 P.
Brierley, G.J. and Fryirs, K.A. (2005). Geomorphology and River Management: Applications of the River Style Framework, Blackwell, Oxford, p. 398.
Buraas, E.M.; Renshaw, C.E.; Magilligan, F.J. and Dade, W.B. (2014). Impact of reach geometry on stream channel sensitivity to extreme floods, Earth Surf. Process. Landf., 39: 1778-1789.
Cenderelli, D.A. and Wohl, E.E. (2003). Flow hydraulics and geomorphic effects of glacial-lake outburst floods in the Mount Everest region, Nepal, Earth Surf. Process. Landf., 28: 385-407.
Costa, J.E. and O'Connor, J.E. (1995). Geomorphically effective floods. In: Costa, J.E., Miller, A.J., Potter, K..W. Wilcock, P. (Eds.), Natural and Anthropogenic Influences in Fluvial Geomorphology Monograph 89, American Geophysical Union, Washington, D.C., pp. 45-56.
Dean, D.J. and Schmidt, J.C. (2013). The geomorphic effectiveness of a largeood on the Rio Grande in the Big Bend region: insights on geomorphic controls and post-flood geomorphic response, Geomorphology, 201: 183-198.
Heritage, G.L.; Large, A.R.G.; Moon, B. P. and Jewitt, G. (2004). Channel hydraulics and geomorphic effects of an extreme flood event on the Sabie River, South Africa, Catena, 58, 151-181.
Hooke, J.M. and Mant, J.M. (2000). Geomorphological impacts of a flood event on ephemeral channels in SE Spain, Geomorphology, 34(3-4): 163-180.
Jansen, J.D. (2006). Flood magnitude–frequency and lithologic control on bedrock river incision in post-orogenic terrain, Geomorphology, 82: 39-57.
Johnson, R.M. and Warburton, J. (2002). Flooding and geomorphic impacts in a mountain torrent: Raise Beck, central Lake District, England. Earth Surf. Process. Landf., 27: 945-969.
Kale, V.S. (2007). Geomorphic effectiveness of extraordinary floods on three large rivers of the Indian Peninsula, Geomorphology, 85: 306-316.
Kale, V.S. and Hire, P.S. (2004). Effectiveness of monsoon floods on the Tapi River, India: role of channel geometry and hydrologic regime, Geomorphology, 57: 275-291.
Krapesch, G.; Hauer, C. and Habersack, H. (2011). Scale orientated analysis of river width changes due to extreme flood hazard, Nat. Hazards Earth Syst. Sci., 11: 2137-2147.
Kumar, S.; Ranta, M.J.; Praveen, T.V. and Kumar, V. (2010). Analysis of the Runoff for Watershed Using SCS-CN Method and Geographic Information Systems, International Journal of Engineering Science and Technology, 2: 3947-3654.
Langhammer, L. (2010). Analysis of the relationship between the stream regulations and the geomorphologic effects of floods, Nat. Hazards, 54: 121-139.
Lenzi, M.A.; Mao, L. and Comiti, F. (2006). Effective discharge for sediment transport in a mountain river: computational approaches and geomorphic effectiveness, J. Hydrol., 326: 257-276.
Magilligan, F.J.; Buraas, E.M. and Renshaw, C.E. (2015). The efcacy of stream power andow duration on geomorphic responses to catastrophic flooding, Geomorphology, 228: 175-188.
Mahdavi, M. (2012). Applied Hydrology, Vol. 2, University of Tehran Press, 437 pages.
Mishra, S.K.; Tyagi, J.V.; Singh, V.P. and Singh, R. (2006). SCS-CN-based Modeling of Sediment Yield, Journal of Hydrology, 324: 301-322.
Moghimi, A. (2009). River Eco-Geomorphology and Rights, University of Tehran Press, 296 pages.
Moraru, A. (2017). Streambank erosion and channel widening: implications for flood hazard. Master thesis, MSc in Mineral Resources and Geological Hazards, University of Barcelona.
Nardi, L. and Rinaldi, M. (2015). Spatio-temporal patterns of channel changes in response to a major flood event: the case of the Magra River (central-northern Italy), Earth Surf. Process. Landf., 40: 326-339.
Phillips, J.D. (2002). Geomorphic impacts of flash flooding in a forested headwater basin, J. Hydrol., 269: 236-250.
Righini, M.; Surian, N.; Wohl, E.; Marchi, L.; Comiti, F.; Amponsah, W. and Borga, M. (2017). Geomorphic response to an extreme flood in two Mediterranean rivers (northeastern Sardinia, Italy): analysis of controlling factors, Accepted to J. Geomorphology.
Rinaldi, M.; Surian, N.; Comiti, F. and Bussettini, M. (2013). A method for the assessment and analysis of the hydromorphological condition of Italian streams: the Morphological Quality Index (MQI), Geomorphology, 180-181: 96-108.
Rinaldi, M.; Amponsah, W.; Benvenuti, M.; Borga, M.; Comiti, F.; Lucìa, A.; Marchi, L.; Nardi, L.; Righini, M. and Surian, N. (2016). An Integrated Approach for Investigating Geomorphic Response to Extreme Events: Methodological Framework and Application to the October 2011 Flood in the Magra River Catchment. Italy, Earth Surface Processes and Landforms (in press).
Rostami, S. (2008). Estimation of erosion and sedimentation of Ilam Dam basin with special attention to the role of geological formations of the region in sediment production. Master's thesis, Science and Research Unit of Tehran, 134 pages.
Surian, N.; Mao, L.; Giacomin, M. and Ziliani, L. (2009). Morphological effects of different channel forming discharges in a gravel-bed river, Earth Surf. Process. Landf., 34: 1093-1107.
Thompson, C. and Croke, J. (2013). Geomorphic effects, flood power, and channel competence of a catastrophic flood in confined and unconfined reaches of the upper Lockyer valley, southeast Queensland, Australia, Geomorphology, 197: 156-169.
Tongbi, T.u.; Kara, J.; Ercan, A.; Trinh, T.; Kavvas, M. and Nosacka, J. (2017). Assessment of the effects of multiple extreme floods on flow and transport processes under competing flood protection and environmental management strategies, Science of the Total Environment, 607-608: 613-622.
Wolman, M.G. and Miller, J.P. (1960). Magnitude and frequency of forces in geomorphic processes, J. Geol., 68: 54-74.