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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The Transport of Gravels in Boulder-Bed Streams
Alan F, Barta, 1 Peler R. Wilcock,2 and Conor C, C. Shea I

Abstract

In boulder-bed, streams, gravel is typically found in isolated pockets protec.oI
by local flow obstructions. The deposition and entrainment of these gmvels depend
strongly on the "ocaI geometry of boulders and banks, which control the flow and
!I'ansporl fields IR the stream. To investigate general features of sediment transpon
In bo1Jl~r-bed streams, we observed flow and gravel entrainment from 70 vel
pockets 10 ~our ~ulder-bed streams. Once pocket geometry is accountS for
general relationships between flow discharge. gravel size and gravel t '
be developed. ranspon may

Introduction

In steep boulder-bed streams, gravels are found in pockets sheltered b
boulders (Kondolf el ai" 1991). Although limited in quantity th I Y
most of th sed' d' , ese grave s represent
. e Iment l~c~arge from the channel and provide important habitat for
spawmng fish and aquatic lDvertebrates The flow and lranspon fi Id . h
d d . Ie SlOt estream
epen s~ngly on ,the local g~~etty of boulders and banks (Furbish, 1993), As a
:su~t of thiS local Influence, It IS not possible to estimate sediment transpon from
I~ch arge, c~annel geometry, and gravel properties alone. At the same time it is
Delt er posslbl.e or practical to model flow and transport for every gravel p~kel
The cha1l~nge IS t? find a practical method that accounts for the average affect of .h;
boulders In a relationship between discharge and sediment trans n
po.

IOraduale Sludent Depanment of 0 h dE'
Johns Hopkins University, Baltimore ~a~I~Ia; nVlronmental Engineering, The
2Associate Professor Department of 0 h d .
The Johns Hopkins University, Baltimo~~ 11 ~~ 8 EnVironmental Engineering,

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TRANSPORT OF GRAVELS

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Sludv Sites and Methods
The goal of our field observations was to determine the movement of gravel
for a wide range of stream discharges. Gravel entrainment and flow hydraulics were
investigated at five study sites in the Eastern Sierra Nevada, California (Table I).
All sites are near existing gaging stations. The bed slopes at the sites range from 2 to
ne-arly 12 percent and the channel widths range from 3 to 1 meters. Median grain
~iles for the gravels are between 13 and 56 mm. Three of the study sites are
unregulated; the two study reaches on Bishop Creek are downstream of run-of-river
hvdroelectric reservoirs. High flow events on the unregulated streams occur as a
~~uh of snowmelt or winter storms. On Bishop Creek, high flow events occur when
the: reservoirs spill over during spring snowmelt. Table I lists the characteristics of
the study sites.

Table I. Chamcteristics or boulder-bed studv sites

Bankfull Mean
Bed Channel Annual
Gradienl Width Discharge Flow
Site Name (%) (m) (m3/sec) Regulation

Blackwood Creek 2.0 6 1.1 Unregulated
General Creek 3.5 4 0.7 Unregulated
Bishop Creek above Power Plant 3 5.0 7 2.8 Run-of-River
RishopCreek above Power Plant 2 5.7 6 2.7 Run-of.RiY<<
Horton Creek 11.6 3 0.2 Unrenlated

We observed now and gravel movement from 10 gravel pockets over three

water years (91, 92, and 93). Gravel movement was monitored through the use of

tracer panicles and scour chains. During the 1991 and 1992 water years, we

monitored two to three gravel pockets per study site by painting the native particles.

For the 1993 water year, we expanded the number of pockets monitored by using

32mm marbles as tracers. We developed stage discharge rating curves for each study

reach using one-dimensional hydraulic modeling calibrated with water surface

surveys for a range of discharges.

Result\; and Discussion
The goal of this work is to develop general relationships between stream
discharge, gravel properties, and gravel movement. Because of the local influence
exerted by boulders, a practical means of accounting for local obsbUction geometry
was sought. As a first step, we classified each obstruction according to its size and
its position relative to the associated gravel pocket.
On these streams, gravel deposits located either immediately upstream or
downstream of boulders, or are surrounded by boulders. On rare occasions we also
observed small gravel deposits on mid~channel or side-channel bars not directly
associated with any particular obstruction or group of obstructions. On the basis of
rhese observations, we classified all pockets into one of four categories:

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