FLOOD07353

• County: Statewide
• Basin: Statewide
• Title: Hydraulic Engineering volume 2
• Date: 1/1/1994
• Prepared By: American Society of Civil Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

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TRANSPORT OF GRAVELS .
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e c) Downslteam Pockels
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ObstructiOh Height (m)

182

HYDRAULIC ENGINEERING '94

Upstream Gravel deposit is immediately upstream of onc or more
obstructions (32% of pockets).
Downstream Gravel deposit is immediately downstream of one or more
obstructions (17% of pockets),
InImllIl Gravel deposit is surrounded by flow obslruclions (47% of
pockels)
IlJI Gravel deposit is a small channel bar with no identifiable
dominant obsl111Ction (5% of pockets).
Observations of gravel movement at individual monitoring sites are divided
into three categories (I) negligible movement, (2) partial movement, and (3) general
bed movement. The negligible movement category includes gravel monitoring sile~
for which no movement was detected. or sites at which only a few particles moved
and that movement was a meter or two at most. Panial movement describe!
monhoring sites at which some, but not all, grain sizes moved. General bed
movement represents the condition in which grains of all sizes moved.
Figure I relates flow observations to gravel movem..:nt for three cases: a) aL
pockets except bar deposits. b) internal pockets, and c) downstream pockets. Bar
pockets are not ploued because they are not direcdy linked ro any single obstructioo
or assemblage of obstructions. Upstream pockets are not shown because vinually aJ
of our observations from them were of general gravel movement.
The ordinate on Figure I plots flow strength as the ratio of the total bed shear
stress ( 1',,) to the critical bed shear stress (1'c ) for the gravels. here referred to~!
relative bed shear stress. Total bed shear stress ('fc ) is estimated by pghS (the
depth-slope product), where p is the fluid density. gis the acceleration of gravil~
his the local flow depth, and S is Ihe energy slope. Flow depths and energy slop"
were detennined from field surveys and calibrated stage-discharge rating CUCV(,
The critical shear stresses for gravels were estimated following the method \~
Wilcock (1992). The height of the dominanl flow obSlruclion is ploltcd on Ih,
abscissa. Where multiple flow ob:ill'Uctions formed a pocket, we selected the heigt'
of the obstruction that most influenced the local flow (usually the largest).
Approximate gravel entrainment thresholds for steep boulder-bed streams aH
shown on Figure I. The flow required ro transpon pocket gravels increases \\'w
obstruCtion height, as larger obstructions are more efficient at sheltering the pochl\
All gravel movement occurred when the relative shear stress ('r(Jt"c) was grealerlh:
unity because much of the IOtal boundary shear occurs as fonn drag on the boulder\

C..onclu~ions

Flow obstructions produced by boulders or banks may prohibit It.:
downstream movement of pocket gravels by Obstructing transport or by proteclJr.j
the gravel from high velocity current. To estimate gravel entrainment, the effc:ml"
local obstructions must be induded. We have included these local effects in Ill'
ways, by morphologically classifying gravel pockets based on obstruction geoll'lC~
and by relating gravel movement to obslrUction height. This permits estimalts I"~

. General Movement 0 Partial M
I ',ure I R ,. . ovemenl X NegliRible Movemenl
. e abve bed shear Slress versus obsLroc ' 10_' h .
11>1.11 l>htar (from d pth . , lJOO IKilg L Relative bed shear stress is the
ICr..,'c1s A su es~ -slope prod.ucl) diVided by the critical shear SlreSs for the pocket
rn.h 'AII ~e1S ( gravel.:tralnmenllhreshold is represented by a line with question
"n lb. dowJlSUCam v::: are ~kets) are. Plotted on Figure Ia; internal pockelS are ploued
fill' ubslruction size and P oued on Figure. Ie. The figllres slIggesllhal byaccountins
dlMlnels. pocket type, gravel entralRmem can be estimated for boulder-bed

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