KNICKPOINT RETREAT AND DRAWDOWN REACH EVOLUTION ALONG THE ARID ESCARPMENTS OF THE DEAD SEA BASIN
Lithologically-controlled, sheer bedrock knickpoints, reaching up to 300 m in height, prevail along the Dead Sea arid escarpments. Although field observations suggest that rockfall rather than direct fluvial abrasion is the dominant retreat process, drainage area strongly controls retreat velocity. This points to a linkage between the frequency of mass wasting events and drainage area. Since no buttressing of the knickpoints faces or plunge pool erosion was observed another process should account for the observed dependency. A probable candidate is groundwater sapping whose intensity relies on infiltration of flood water through joints and potholes.
Cosmogenically-derived (36Cl) short-term retreat rates calculated in two channels with an order of magnitude difference in drainage area (17 km2 vs. 173 km2) were found to be 0.45 m/ka and 1.2 m/ka. These rates are remarkably similar to long-term retreat rates (0.25-0.5 m/ka vs. 1-2 m/ka) calculated using the estimated age of the escarpment (4-8 Ma) and the distance of the knickpoints from its base.
A free-fall of flowing water across a sheer knickpoint is expected to increase the shear stress above the free-fall lip and thus to accelerate incision and create an oversteepened drawdown reach (Gardner, 1983). Relying on hydraulic engineering equations (Rouse, 1936; Hager, 1983; Stein and Julien, 1993) we derive an expression predicting the increase in erosion rate (E) due to a free-fall effect: Efreefall/Enormal=(1+0.4/Fr2)3n, where Fr is the Froude number and n ranges between 0.6-2. This relation suggests that erosion rates above the lip can be more than 5 times higher than the expected erosion rates away from the knickpoint. Initial results of a numerical incision model comply well with measured profiles of several Dead Sea drawdown reaches. Though the uniqueness of the results, as well as complete sensitivity analysis, is yet to be tested, we suggest that landscape evolution models should explicitly account for drawdown effects.