8256

• UCREFRP Catalog Number: 8256
• Author: Holly, F. M. and R. Ettema
• Title: Sediment Imbalance in Rivers
• USFW Year: n.d.
• USFW - Doc Type: Simulation Possibilities and Problems
• Copyright Material: YES

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FORREST M. HOLLY. JR. AND RoBERTETIEMA 417

Fig. L Schematic of the Lane relation-
ship for qualitative analysis (Simons
and Li Associates 1982).

(Qs 050) is proportional 10 (Q S)

Where Qs. sediment discharge
o . median sediment size
50
Q . water discharge
S . slope

all sediment movement on the bed (Harrison 1950). effectively adjust. its length and thereby its local
This so-called armoring or paving phenomenon can slope as implied on the water side of Fig. 1. Mean-
be an essential mechanism for return toward equi- dering, though, can only occur through bank ero-
librium front a perlurbed state. , siop., which in turn.depends on the erodibility of the
A subtlety not explicitly shown in Fig. 1 is the banks. This erodibility depends strongly on the
macro-texture of the bed, that is, the existence of mechanical nature of the bank material, and also
bedforms such as dunes, ripples, and antidunes. on its vegetative cover. In addition, local or massive
These features, whose existence reflects a balance rock outcrops can limit bank erodibility, and thus
among shear stress, sediment size, and free-sur- constrain a river reach.
face conditions (Simons and Richardson 1966), ef- Figure 1 is implicitly limited to consideration of
fectively allow a channel bed to adjust its large- so-called bed-material sediment load, that is, to
scale roughness to the prevailing hydraulic and the situation in which any sediment in active
sediment conditions. Sediment-transport capacity transporl is represented in the bed, either on the
and large-scale channel roughness are intimately surface or immediately below it. However, there
:related (Karim and Kennedy 1982). may be additional sediment in transport that has
A channel cannot grossly adjust its overall lon- no interaction with the bed and is not represented
gitudinal valley slope, as this is essentially fIXed by in it; this is often called washload. A typical situ-
the geologic landform topography and sediment ation is one in which an upstream. event, such as
Provenance. In altering its channel form, for exam- flushing of a reservoir or a storm on a watershed,
pie by meandering around the flood plain, it can causes a large amount of fme material-material