methods structures

7/28/2019 Methods Structures

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Boulder Weir/ One Rock Dam

Aboulder

weir

is

agrade

control

structure

that

is

constructed

with

asingle

layer

of

boulderslayingonthechannelbed. Thesinglelayerofbouldersisbuilttocovera

relativelylargesurfaceareaofthechannel. Thisallowsthestructuretoactasa

constructedriffle,addingroughnesstothechannel,slowingflowsandencouragingthe

depositionof

sediment

on

and

near

the

structure.

The

single

layer

of

boulders

also

actsasamulchandretainsmoistureinthebedofephemeralchannelsthatcan

supportvegetation,whichultimatelyaddsstrengthandlongevitytothestructure.

Boulderweirsareintendedtobebuiltlow,dropsrarelyexceed1.5 2inheight. This

minimizesthe

chance

of

scour

on

the

downstream

side

of

the

structure

and

ensures

thatnoonestructurecausesabruptchangesinthegeomorphologyofthechannel.A

OneRockDamisessentiallythesamestructurethatishandbuiltwithsmallerrocks.


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PlanView

Pool=ScourCrossover=Deposition

Profile

Crossover=Deposition

Boulder Weir

2010CS


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Cross Vane

Across

vane

is

used

to

provide

both

grade

control

and

bank

protection.

The

structure

isessentiallyshapedlikeaV. Thecenterofthestructureisknownastheinvert. The

invertsetstheintendedgradeofthechannelupstreamfromthestructure. Eacharm

oftheVisangledandslopedawayfromtheinvertandupintothebanks. The

slopedarms

steer

water

away

from

the

banks

and

toward

the

center

of

the

channel.

Theareaimmediatelydownstreamfromtheinvertisdesignedtoformascourpool.

Thispoolallowstheconcentrationofflowinthecenterofthechannelanddissipates

theenergyofthewaterasitfallsovertheinvertandarms. Thedegreeofscouris

controlledby

the

height

of

the

invert

and

the

relation

of

the

width

of

the

structure

to

thechannelformingflood. Crossvanesarebuiltwithdeepfooterstopreventthe

structurefrombeingundermined.


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Cross Vane

201

0CS


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Cross Vane with A-Bar

201

0CS


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Pool/standingwave


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Boulder Vane

Bouldervanes

are

used

to

stabilize

bank

erosion.

A

boulder

vane

is

essentially

one

armofacrossvanepositionedingroupsof2 5onerodingoutsidemeanderbends.

Theupstreamtipofthearmisthelowestpointofthestructureandispositionedas

closetobedelevationaspossible. Thedownstreambaseofthestructureisanchored

intothe

bank

and

the

elevation

of

this

point

is

determined

by

the

height

of

the

channelformingflood. Thestructuregraduallyslopesuphill,intothebankfrom

upstreamtodownstream. Thegradualslopeslowswaterthatflowsacrossthe

structurebecausethewaterhastoflowagainsttheslopeasitapproachesthebank.

Thiscauses

aslow

moving

cushion

of

water

to

form

near

the

bank

that

steers

the

fastermovingcurrenttowardthecenterofthechannel. Theslowmovingwaternear

thebankdepositssedimentandgraduallyallowsastablebanktoforminplaceofthe

onceerodingoutsidebend.


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Boulder Vane

201

0CS


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BoulderVanedeflectingfloodflowsawayfrombank


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J-Hook Vane

201

0CS


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Typical Vane Layout

2010CS


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Erodingoutsidemeanderbend

Thalwag current

PointBar


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Thefollowing

images

show

awetland

restoration

project

located

in

an

ephemeral

channelnearSantaFe,NM. Thisprojectcombinestheuseofboulderweirsforgrade

control,crossvanesforgradecontrolandbankstabilizationandbouldervanesfor

bankstabilization. Thewetlandwasactivelydowncuttingandinadegradedcondition

dueto

aseries

of

headcuts

moving

upstream

through

the

project

site.

The

wetland

driedrapidlyfollowingthepassageoftheheadcutsandwetlandplantspeciesbegan

todeclineimmediately. Thegoaloftherestorationwastorewetthewetlandby

liftingthebedofthechannelascloseaspossibletoitspreincisionlevel. Crossvanes

andboulder

weirs

were

used

to

reconnect

flood

flows

with

the

adjacent

floodplain.

Bouldervaneswereusedtostabilizeerodingoutsidemeanderbendscausedby

excessivepointbargrowth. Theprojecthasbeeninplaceforalmosttwoyearsnow

andhasexperiencedseveralfloodevents. Evenatthisearlystagethewetland

vegetationis

responding

to

additional

moisture

storage

and

reduced

erosion.

There

is

atleastoneperennialpoolthatwasnotpresentpriortoimplementationofthe

project. Weexpectconditionstoimproverapidlyfollowingfuturefloods. Itislikely

thatthisephemeralchannelwillsoonholdenoughmoisturetobeconsidered

intermittentor

even

perennial

interrupted.

Natural Channel Restoration Example


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