NON-STEADY EVOLUTION OF LONGITUDINAL PROFILES OF GRAVEL-BED CHANNELS IN RESPONSE TO CHANGES IN FLOOD REGIEME AND BASE LEVEL LOWERING IN DRY ENVIRONMENTS

Longitudinal profiles of alluvial channels may be altered rapidly in response to base level lowering or changes in streamflow regime. Previous models simulating the response to such changes assumed steady and uniform discharge, or used a calibrated diffusion coefficient as a proxy for discharge. Such models do not account for intra and inter annual variance of flash flood volume and peak discharge, which is typically high in channels of dry environments. We adopt a different approach: (a) A new model for gravel-bed channels combining kinematic wave flood routing with sediment transport based on the Meyer-Peter-Muller equation. This model predicts changes in channel longitudinal profile in response to changing streamflow regimes and base-level lowering rates; (b) A stochastic approach which produces a synthetic data series of floods based on the probability distribution of peak discharge and hydrograph properties in a specific basin. The properties of this “flood generator” can be altered to stimulate flood characteristics (hydrograph shape, flood frequency, magnitude) which mimic potential climate variability or changes. The model was applied to the lower reach of Nahal Darga gravel-bed channel, which drains into the Dead Sea lake and is located in a dry climate. During the last 40 years, the initial uniform (linear) profile of this reach has changed to a convex profile due to the drastic artificial lowering of the Dead Sea level at a rate of 1 m/y. Several measured channel profiles during this time interval were used for the model initial evaluation. Following this stage, the effect of different scenarios of lake level drop and flash flood regime has been examined. The modeling results delineate a wide range of potential channel profiles, which is strictly due to the natural flow variance under a given flow regime – thus emphasizing the key role of stochasticity in channel morphology. Although extreme floods had a major impact on channel profile evolution, the effective discharge at the Darga channel consisted of floods with medium peak discharge and a recurrence interval of ~10 years. Novel approaches, which combine flood generators and process-based modeling could help decipher the morphological signature of past and future climate scenarios.