J. Wesley Lauer University of Minnesota Gary Parker University of Illinois Net Transfer of Sediment from Floodplain to Channel on FourU.S. Rivers
Problem • Bank erosion is often considered a source of sediment for stream systems. Rivers, however, must widen infinitely, and their floodplains must be destroyed, if bank erosion represents a net source of sediment to the stream. • Why do so many studies show banks being a net source of material? Are such studies even correct? • How continuous in time and space might we expect the erosion and replenishment processes to be?
Goals of Talk • Present measurements of cut bank erosion rates on long reaches of several U.S. rivers • Important for gross cycling of point bar material since most of what is eroded is replaced immediately in point bars. • Estimate the difference between cut bank erosion and point bar deposition on the same systems • Two processes lead to this difference. One is important for characterizing exchange of fine material between channel and floodplain and has contaminant transport implications. • Emphasize the importance of floodplains in the transport of material downstream through an alluvial valley
Is the sediment load of this river increasing downstream due to bank erosion?
Relation for floodplain sediment balance Consider the sediment budget of a reach of a river-floodplain complex containing a meandering river. /t(Sediment in floodplain) = a) Overbank deposition rate on the floodplain + b) Deposition rate in floodplain lakes (oxbows) – c) Rate of sediment loss to channel by bank erosion
Relation for floodplain sediment balance Define the following parameters: sv = valley length of reach under consideration s = sediment density b = density of sediment deposit = (1 - p) = volume rate per unit valley length of overbank deposition = volume rate per unit valley length of lake (oxbow) filling = volume rate per unit valley length of (net) bank erosion
In a graded stream, any net loss of sediment from the floodplain must vanish Erosion from the floodplain must be balanced by deposition on it: Any net source of sediment is from erosion into bluffs, not erosion into the floodplain
Hbf ct Net bank erosion comes in two flavors: shaving and extension Shaving: the top of the inner point bar tends to be somewhat lower than the opposite eroding cut bank. The difference drives a net erosion of mostly finer (higher) floodplain material into the channel Note that most of the eroded sediment is recycled in building point bars!
Net bank erosion comes in two flavors: shaving and extension Extension: as a channel migrates and elongates, it creates an ever-increasing volume of “hole” (channel) in the floodplain. This process of increasing arc length due to migration is balanced by cutoff. The oxbows, however, remain as “holes” until they are filled with sediment. Note that the surface area of the eroded zone on the outer bank is greater than that of the eroded zone on the inner part of the bank. Extension yields mostly coarser (lower) floodplain sediment to the channel.
Hbf ct Net bank erosion comes in two flavors: shaving and extension c = migration rate sc = centerline arc length Hbf = bankfull depth so = outer bank arc length Bbf = bankfull width si = inner bank arc length Rc = centerline radius of curvature Hbf ct
Hbf ct At the reach-averaged level:
The Bogue Chitto River, Louisiana: A typical actively migrating river system
Several processes might result in short term or local net erosion from banks • Type 1: Cut bank is higher than point bar • Type 2: Cut bank is longer than point bar EUB “Shaving” “Extension”
An example of typical bank geometry from the Bogue Chitto River, Louisiana Flow is near bankfull stage Left Bank (outside of bend) Right Bank (inside of bend) Since the inner bank is not built to the elevation of the higher outer bank, migration in effect “shaves” off the highest part of the floodplain.
Pearl River, Louisiana/Mississippi, near Bankfull Stage. Vegetation on point bars is submerged while eroding cut banks are exposed. Wild pigs provide scale.
Replenishment processes should depend on the type of erosion • Type 1 (Shaving): Should be balanced by overbank deposition • Type 2 (Extension): Should be balanced by filling of or migration through the oxbow lakes that eventually form • This talk makes an attempt to measure the relative magnitudes of the shaving and extension erosion processes for the purpose of characterizing their importance in real systems.
Mud & Sand (Shaving) Sand & Mud The important floodplain exchange processes associated with meander migration: Extremely simplified More realistic The point is that much of the cohesive material exchange occurs through the shaving process.
Backpack for scale Typical Bank, Strickland River, Papua New Guinea Silts and clays Sand
Point Bar Deposit on Neuse River, North Carolina is mostly sand but with some layers of silt and clay mixed in.
At t1 At t2 Lake Floodplain Channel Measuring the exchange rates Conceptual Model of System
Simplified 2-D Representation Floodplain Floodplain Channel + Lake
For a graded, non-subsiding valley in which bankfull elevation is not changing over time: Net volumeexported fromfloodplain EUB DO DO ELB DL+C
Measurement of Erosion Terms • It would be great to simply subtract two surfaces, but this is not possible • Only one topographic survey generally available • A few repeatedly surveyed cross sections do not provide ELB • Instead, estimate rate EUB=dEUB/dt based on bank geometry and local migration rate • Estimate rate ELB using long-term change in channel length, including newly formed lakes ;
Where are the banks (the border between channel and floodplain)? • Outer bank: Easy, since usually a cut bank on actively migrating streams • Inner bank: Boundary between … • Proximal and distal sources of sediment • Lateral and vertical accretion • Presence of material finer than available on bed of channel (sand vs silt) • Use first break in slope inside vegetation line
Measuring Shaving • Get local migration rates from historic aerial photo analysis • Get bank elevations from LIDAR survey
Digitized 1952 Banks Digitize Banks (Vegetation Line) By Hand
a a b q b q Centerline Interpolation Final Initial Iterate through theta until a = b where a and b are the shortest distances to the respective curves from a given point
Correction for Downstream Translating Bends Channel Centerline at t D l di Channel Centerline at t +Δt , where
An example of the correction procedure The procedure ensures that the method does not predict outward migration at downstream translating bend apices.
Characterize Bank Elevations Using LIDAR (Light Detection and Ranging) • Scanning Airborne Laser/Digital GPS Unit • Various returns recorded—useful for removing vegetation from final DEM, but smoothing also required Images from Harding, 2000
Sources of Error in LIDAR • Errors in laser rangefinder—generally small • Errors in angle of laser—important near edges, on steep slopes • Vegetation • Water • Post-Processing • Smoothing • Vegetation Removal • Result: LIDAR is not good at detecting edges, but we’ll try anyway
Lidar Data Sources • State or Local Floodplain Mapping Projects • Louisiana FEMA Project http://atlas.lsu.edu • North Carolina Floodplain Mapping Program http://www.ncfloodmaps.com • Dakota County, MN • Used ungridded data (i.e. bare earth returns) • Gridded to 5-m DEM (LA) or 5-ft DEM (NC, MN) • Define banks by hand based on point density and topography, buffer these banks, compute mean elevation from LIDAR in buffered region
Validation: Vermillion River, MN • Test measurement of shaving rate • Can banks be identified accurately enough from LIDAR alone? • Method: Compare shaving computed using previous method with shaving computed using Δη from field-surveyed banks
Study Areas Where Both Shaving and Extension Have Been Computed • Validation on Vermillion River, MN • Apply to 3 Southern US Rivers • Pearl River, LA/MS • Bogue Chitto River, LA • Neuse River, NC