Some Characteristics of Drainage Basin Realignment. Les Hasbargen Dept. of Geology University of Delaware

Transcription

1 Some Characteristics of Drainage Basin Realignment Les Hasbargen Dept. of Geology University of Delaware

2 Overview: Styles of Drainage Realignment Differential erosion drives migration Migratory drainage divides in experimental and numerical landscapes Diagnostic topographic form of migrating divides Initial results on testing erosion laws

3 The geometry of ridge migration Dz Dx Dz a Dz b a b x b From the following relations: tan a = ( Dz a - Dz) / Dx tan b = Dz / (x b + Dx) We determine the divide migration rate: Dx/Dt = (Dz a - Dz b ) / (tan a + tan b) Implications? Migration requires erosional variability Uniform erosion rates = no migration Low angle divides migrate faster

4 coefficient of variation coefficient of variation Erosion rate variability for experimental steady state landscapes decreases over longer times (power law) y = x run 1 run 2 run 4 run physical experiment numerical model time separation (s) eroded relief (h/h r ) Erosion rates computed by differencing gridded elevation data Eroded relief: eroded distance expressed as a fraction of the total relief

5 Modes of drainage realignment Ridge elongation and shortening Short-term divide offset often marked by deepseated hillslope failures Ensemble migration of divides involving valley annihilation or growth Ridge triple junctions at valley heads migrate Valley opening: triple junction migrates upstream Valley closure: triple junction migrates downstream

6 z (cm) Pre-failure topography Hillslope Failure Profiles Post-failure topography note: mass of post-failure profile is ~0.5% greater than pre-failure profile x (cm)

7 Numerical model Erosion rate Valley Closure Erosion rate Physical model Lateral movement of central divide

8 Characteristic Erosion Pattern Marks Central Divide Migration Erosion rate -Scaled to long-term average erosion rate -Whites and Blacks are ~1 s/m

9 Vertical shot of basin Tank is ~ 90 cm wide Note the narrow long drainage in center

10 elevation above datum (cm) Stream profiles: the center profile is being encroached 15 middle stream 10 5 Right hand side profile is advancing upon middle stream lhs stream rhs stream 3 per. Mov. Avg. (middle stream) 3 per. Mov. Avg. (rhs stream) 3 per. Mov. Avg. (lhs stream) distance from divide (cm)

11 A, cumulative area (cm 2 ) scav. stream adv. stream A adv = 3.5L A scav = 4.5L L, distance from divide (cm)

12 elevation (cm) Valley profiles over time downstream from a downstream migrating triple junction (closure) y = x y = x y = x Very minor change in slope!!! distance from divide (cm)

13 elevation (cm) Divide asymmetry Note: Peaks have been aligned to highlight asymmetry advancing side retreating side distance (cm) Migration direction

14 Conclusions Spatial variability of erosion will lead to drainage realignment This takes several forms Shorter term crest oscillations (slumping, out of phase knickpoints) Longer term persistent migration Valley opening/closure (triple junction migration) Ridge extension/shortening Initial test of stream power erosion law is not convincing very minor change in slope as drainage decreases in size (may imply weak dependence on discharge) Divide asymmetry and long narrow drainages appear to be good signs of migration

15 Title: Some aspects of drainage realignment Abstract: This paper investigates topography and erosion rates around migrating drainage divides in experimental, numerical, and natural landscapes. Lateral movement of a drainage divide demands differential erosion rates on opposing sides of the divide. Time series experimental and numerical data provide a way of measuring erosion rates around migratory divides, and offer a means of investigating the ways in which drainage basins can reconfigure themselves over time, even under steady uniform forcing conditions. Modes of drainage basin realignment include: ridge elongation and shortening; short-term divide offset often marked by deep-seated hillslope failures; and persistent ensemble migration of divides involving valley annihilation or growth. This paper will document the relative frequency of these modes of migration and erosion rate patterns associated with each mode in an experimental environment, and juxtapose them against longer term basin averaged erosion rates. In addition, the topographic form of migratory ridges will be documented. Two such configurations commonly mark divide migration: narrow perched sub-basins, and sharply asymmetric divides. Asymmetric divides in the vicinity of a ridge triple-junction set up conditions conducive to valley growth or annihilation, depending on divide velocities. Dr. Leslie E. Hasbargen Visiting Assistant Professor Geology Department 203B Penny Hall University of Delaware Newark, DE phone: lesh@udel.edu fax: H53A-04 Time: 1445, Friday, May 26, 2006; BC 324