EP031-0006
Drainage Reorganization Affects Valley Width-Drainage Area Scaling

Thursday, 10 December 2020
Poster
Elhanan Harel, Ben-Gurion University of the Negev, Geological & Environmental Sciences, Beer Sheva, Israel, Eitan Shelef, University of Pittsburgh, Pittsburgh, PA, United States, Liran Goren, Ben Gurion University of the Negev, Department of Earth and Environmental Sciences, Beer-Sheva, Israel, Onn Crouvi, Geological Survey of Israel, Jerusalem, Israel and Hanan Ginat, Dead-Sea & Arava Science Center, Tamra, Israel
Abstract:
The width of fluvial channels and valleys controls the bed and banks shear stress and affects erosion rate, sediment transport capacity, and channel hydrodynamics. Global observations show a power law relation between channel width, W, and drainage area, A, with a positive exponent, 'b': W=aAbSc , where the dependency on the slope, S, emerges under transient conditions. So far, however, the applicability of this relation was not explored under dynamic drainage area scenarios associated with reorganization of fluvial networks. As a consequence, our understanding of erosion and sediment transport following reorganization is critically hampered.

We hypothesize that following changes in drainage area via drainage reorganization, the exponent 'b' could significantly deviate with respect to globally observed values, because of the long adjustment timescale of the channel width to the new hydrologic conditions. The deviation could become even more substantial when W is substituted to valley width, which is expected to adjust even slower.

In the current study, we explored the relation between valley width, drainage area, and slope along fluvial basins in the hyper arid southeastern Negev Desert, Israel. The valleys in our study area could be classified into three groups: (1) Valleys not affected by reorganization, (2) Beheaded valleys, whose channel head was truncated by a migrating divide, and (3) Reversed valleys, that experienced a 180 degrees change in flow direction toward a new base level. To extract valley width, we developed a GIS-based algorithm that delineates the valley bottom and identifies points for reliable width measurements. Subsequently, the coefficient and exponents 'a','b' and 'c' were fitted using multivariate regressions.

Our results show that beheaded valleys are characterized by reduced 'b' values compared to non-reorganized valleys. In the reversed valleys, the 'b' values were consistently negative, namely, the valleys narrow downstream. Our results indicate that the W-A-S scaling in reorganized valley systems deviates from globally observed relations, which could significantly impact predictions of erosion, sediment transport and landscape evolution in these areas. We further propose that deviations in the value of the exponent 'b' could help to constrain the mode of reorganization.

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