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J. Wesley Lauer University of Minnesota Gary Parker University of Illinois. Net Transfer of Sediment from Floodplain to Channel on Four U.S. Rivers. Problem.

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j wesley lauer university of minnesota gary parker university of illinois
J. Wesley Lauer

University of Minnesota

Gary Parker

University of Illinois

Net Transfer of Sediment from Floodplain to Channel on FourU.S. Rivers
problem
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
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
relation for floodplain sediment balance
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 balance7
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
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

net bank erosion comes in two flavors shaving and extension

Hbf

ct

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 extension10
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.

net bank erosion comes in two flavors shaving and extension11



Hbf

ct

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

ct

several processes might result in short term or local net erosion from banks
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
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.

slide16

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
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.
the important floodplain exchange processes associated with meander migration

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.

slide19

Backpack for scale

Typical Bank, Strickland River, Papua New Guinea

Silts and clays

Sand

slide20

Point Bar Deposit on Neuse River, North Carolina is mostly sand but with some layers of silt and clay mixed in.

measuring the exchange rates

At t1

At t2

Lake

Floodplain

Channel

Measuring the exchange rates

Conceptual Model of System

simplified 2 d representation
Simplified 2-D Representation

Floodplain

Floodplain

Channel + Lake

for a graded non subsiding valley in which bankfull elevation is not changing over time
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
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
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
Measuring Shaving
  • Get local migration rates from historic aerial photo analysis
  • Get bank elevations from LIDAR survey
slide28

Digitized 1952 Banks

Digitize Banks (Vegetation Line) By Hand

centerline interpolation

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
Correction for Downstream Translating Bends

Channel Centerline at t

D

l

di

Channel Centerline at t +Δt

, where

slide35

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
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
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
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
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
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
slide52

Pearl River

Reach 1

Insert Label Image

Reach 2

Bogue Chitto River

Reach 3

Reach 1

Reach 2

Reach 3

Reach 4

study areas where both shaving and extension will be computed
Study Areas Where Both Shaving and Extension will be Computed
  • Validation on Vermillion River, MN
  • Apply to 3 Southern US Rivers
    • Pearl River, LA/MS
    • Bogue Chitto River, LA
    • Neuse River, NC
neuse 1 10000
Neuse 1:10000

Neuse River

computation of extension term
Computation of Extension Term
  • Requires Cross Sectional Area Ac
    • Assume Ac ≈ BH
    • B from photo
    • H from USGS gauge
    • Assumes Ac remains relatively constant in time
channel characteristics
Channel Characteristics

Typical USGS Rating CurveUsed To Develop Table

results
Results

Extension

results residuals only
Results-Residuals Only

Extension

Shaving

Assume ρb = 1.9 g/cm³

a model for the attenuation of a contaminant by exchange with a clean floodplain

ερb C(x,t)

ερb Cbank(x,t)

QsC(x+Δx,t)

QsC(x,t)

A model for the attenuation of a contaminant by exchange with a clean floodplain

Control Volume Approach

Assume negligible

Where

C = the fraction of sediment in a size class of interest that is contaminated

Cbank = the fraction of contaminant in the eroding banks (assume negligible)

ε = lateral exchange flux with the floodplain, L²/T (i.e shaving rate E per unit channel length)

ρb = sediment bulk density

Qs = the mass sediment transport rate in the grain size of interest

the resulting mass conservation model at steady state
The resulting mass conservation model at steady state

represents an e-folding distance for the contaminant concentration, or

represents the distance it takes for contaminant concentration to be cut in half.

x1/2 can be computed easily for the shaving rate (assumed to primarily represent fine sediment cycling) or the gross bank erosion rate (assumed to primarily represent bed material cycling). It is a quantitative way of describing the effectiveness of a floodplain at capturing potentially contaminated sediment.

placing the results in context by computing x 1 2
Placing the Results in Context by Computing x1/2
  • Total suspended sediment load calculations performed on USGS gauge data
  • Assume 20% Sand for Pearl
  • Assume mud load corresponds with shaving, sand load corresponds with gross flux
take home points
Take Home Points
  • Net bank erosion is a small fraction of gross bank erosion
  • Both shaving (upper, finer material) and extension (lower, coarser material) play a role in setting net bank erosion
  • In a graded stream net erosion can be completely balanced by floodplain deposition (floodplain and lakes), so that banks need not be a net source of sediment at all.
  • Valley bluffs, as opposed to banks, can be a net source of sediment
  • Floodplain exchange distance x1/2 small for sand
  • x1/2 larger for finer material in upper banks, but still on order of channel length, so floodplain cycling appears important on these rivers
slide73

Reach 3

Reach 1

Reach 2

slide75

Reach 4

Reach 3

Reach 2

Reach 1

slide81

Reach 3

Reach 4

Reach 2

Reach 1

slide84

Reach 3

Reach 1

Reach 2

assume channel can adjust to constant bankfull shields stress
Assume Channel Can Adjust to Constant Bankfull Shields Stress

Dimensionless Discharge vs. Bankfull Shields Stress

Implies Channel Maintains Constant Cross Sectional Area if Qbf, D, Cf remain Constant

floodplain transfer categories

E1

DE

F2

OD3

E2

F4

OD2

OD1

C2

F1

F3

S2

C1

FP2

FP1

B1

S1

B2

B3

Floodplain Transfer Categories

“Shaving” will be used synonymously with E1

2 d representation of floodplain
2-D Representation of Floodplain

Lake

FP

Floodplain

Floodplain

Channel

other assumptions
Other assumptions
  • Channel extends continuously, so instantaneous extension rate is same as long-term rate, which is easily measured
  • Cross-sectional area conserved
  • Computation of Shaving Transfer E1:
    • E1 = Σ(ηouter- ηinner)cLouter
    • Units L3/T
  • Computation of Extension Transfer ∆VC+L/ ∆ t:
    • ∆VC+L=[(Lc(t+ ∆t)+Lcutoff) – Lc(t)]Ac
    • Units L3/T