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3—1
Chapter 3—Selecting the Best Structure for the Site
Low-Water Crossing Structure Selection Process
When deciding whether to use a low-water crossing and which low-water crossing type to select, it is important to evaluate the following: the site, costs, streamflow patterns, channel characteristics, and aquatic organism passage (AOP) needs. The various factors can be complicated and interrelated, but the selection process is simplified by a two-step process. First, evaluate whether a low-water crossing structure is appropriate and preferable to a culvert or bridge. Second, decide on the appropriate type of low-water crossing based upon the site characteristics and AOP needs. Each decision can be reached by considering these basic questions:
■ Is the road a noncritical route or is there alternative access to the area?
■ Is the traffic use low and are occasional traffic delays acceptable?
■ Is the channel ephemeral or does it have relatively low baseflow?
■ Does the watershed have large flow fluctuations or a “flashy” response?
■ Does the channel carry a large amount of debris?
■ Is the channel unentrenched to moderately entrenched (broad and shallow)?
■ Is a low-water crossing the most cost effective or inexpensive structure?
If the answer to most or all of these questions is YES, then the site is likely a good candidate for a ford or low-water crossing.
■ Does the stream have low or zero baseflow and “flashy” high flows?
If YES, first consider a simple (at-grade), unimproved ford.
■ Are the channel bottom and streambank materials soft or erodable?
If YES, consider an improved ford with a hardened driving surface.
■ Is AOP or maintaining stream function an important issue at this crossing?
If YES, consider (1) an unimproved ford with a natural bottom; (2) an improved at-grade ford with a roughened driving surface, (3) a low-water bridge, or (4) a high-VAR ford.
Questions To Consider in Choosing Whether To Use a Culvert, Bridge, or Low-Water Crossing
Questions To Consider in Choosing The Type of Low-Water Crossing
3—2
Low-Water Crossings
■ Is driving through water frequently prohibited or are long traffic delays unacceptable?
If YES, consider only the vented structures and low-water bridges with an elevated driving surface.
■ Is the channel incised or entrenched?
If YES, consider a vented structure with boxes that match the channel’s shape.
■ Is the channel very broad or does it carry a considerable baseflow with high peak flows?
If YES, consider a relatively long span low-water bridge.
■ Does the channel carry a lot of large woody debris?
If YES, consider an unimproved or improved unvented ford.
■ Does the drainage pass periodic debris torrents through an incised channel?
If YES, consider rock-fill fords. Alternatively, massive concrete vented fords have been used with trash racks to pass the debris over the structure.
■ Is a barrier needed to exclude exotic species?
If YES, consider an improved, unvented ford with a raised platform or a raised vented ford with a perched outlet (consider, however, potential adverse channel effects).
■ Is a grade control structure needed?
If YES to promote aggradation, first consider an improved unvented ford with a raised platform (a low dam). A vented ford with perched vents may also work.
If YES to stop headcutting, consider using a structure with a solid, stable bottom and downstream cutoff wall.
3—3
This chapter will help determine (a) whether an overtoppable structure is appropriate at a given site and, if so, (b) which type is most appropriate.
First determine whether a low-water crossing should be considered at all. Section 3.1 outlines considerations that help distinguish sites conducive to low-water crossings from those where an overtoppable structure would be undesirable. If conditions are conducive to a low-water crossing, then consider what type of low-water crossing structure would best achieve the multiple objectives of resource protection, traffic access, and safety (section 3.2).
3.1 Is a Low-Water Crossing Appropriate?
Low-water crossings have substantial limitations and are most suited for roads with low traffic volumes, and trails. The foremost constraint is public safety. According to a review of National Weather Service reports from 1969 to 1981 (French et al. 1983), nearly half the flood-related deaths in the United States each year were occurring in vehicles. Many of these deaths occurred when people drove into flooded crossings. Drivers may underestimate how fast small streams can rise in some parts of the country during a flood, and they may ignore the possibility the crossing has already eroded. Even 2 feet of water can float and wash away an ordinary car or truck (fig. 3.1).
Low-water crossings—especially vented fords—can also pose significant hazards for boaters and other users who might accidentally approach too closely (Robertson, personal communication). During higher flows, floating objects can be trapped and pinned against the structure by the force of the water. When flows increase such that they submerge a culvert, powerful whirlpools can form that can suck objects on the surface into the culvert opening. For these reasons, vented fords and low-water bridges may not be appropriate on streams that are used for recreation during moderate-to-high flows.
Another safety concern is winter ice on the roadway, a condition hazardous even to slow-moving traffic and especially likely on fords at the low point of the dip. Low-water crossings with steep approaches, particularly unvented fords, may not be good choices for icy roads in winter. Table 3.1 lists the traffic and environmental conditions most and least conducive to selecting a low-water crossing structure.
Chapter 3—Selecting the Best Structure for the Site
More recent confirmation of the ongoing risks of low-water crossings can be found at http://www.floodsafety.com, managed by the nonprofit Flood Safety Education Project.
Figure 3.1—Do not drive through floodwaters. Redrawn from USGS Fact Sheet 024-00, March 2000.
3—4
Low-water Crossings
3—5
Table 3.1—General selection factors for low-water crossings
Most conducive Least conduciveAccess priority Low High
Alternative route Available Not available
Traffic speed Low High
Average daily traffic Low High
Flow variability High Low
High-flow duration Short (hours) Long (days)
High-flow frequency Seldom (rare closure) Often (frequent closure)
Debris loading High Low
Channel entrenchment Shallow Deep
Access priority/alternative route Low-water crossings are not appropriate on roads that access essential
public facilities or that serve as the only public route to an area. Many States restrict construction of low-water crossings on school bus routes and on roads required for national defense. Typically, low-water crossings are not desirable for accessing an area with permanent residences. Even alternate routes to isolated towns may qualify as priority access.
Traffic speed Low-water crossings are most suited for rural roads with low-to-moderate traffic speeds. Unimproved fords may only be driven over at low speeds, less than 10 to 20 miles per hour. Vented fords with a broad, smooth dip and gentle transitions may be suitable for speeds up to 20 to 40 miles per hour. If high-speed traffic is anticipated, then low-water crossings are likely unsuitable for that road.
Average daily traffic during season of use The lower the traffic volume on a road, the more suitable a low-
water crossing is likely to be. With only a few vehicles per day, the consequences of periodic delays are minimal. With increasing traffic volume, the impacts of periodic or occasional delays become more important. Many times, short access roads (i.e., less than half mile of road) see little traffic, making a low-water crossing a desirable option.
Flow variability Low-water crossings are commonly used in areas with highly variable flows, such as desert streams subject to flash floods and thunderstorm-prone areas. High, short-duration peaks followed by long intervals of very low or no flow are most conducive to low-water crossings as long as traffic interruptions during floods are tolerable. Because standard
Chapter 3—Selecting the Best Structure for the Site
3—6
Low-Water Crossings
crossings need to be very large to convey such high flows together with their debris loads, they may not be economically feasible for many low-volume roads. Streams with highly variable flows may also be less stable than streams in which steady baseflows support vigorous riparian vegetation. Putting a large expensive structure on a channel that may shift within the structure’s lifetime is even less desirable. Chapter 4, section 4.5 contains information about hydrologic data useful for evaluating flow variability.
High-flow duration The duration of an overtopping flow controls how long a crossing will be closed. Although weather patterns are the greatest influence (e.g., intense summer short-duration storms), watershed attributes also play a large role. Characteristics of ‘flashy’ watersheds (where flows rise and fall rapidly) can include the following:
■ Steep, short drainage basin (high basin relief).
■ Small basin area.
■ High drainage density (miles stream/basin area).
■ Thin and/or impermeable soils.
■ Little or no flood plain.
■ Low vegetative cover.
High-flow frequency The peak-flow frequency during the season that the road or trail is used is another variable affecting the probability of traffic interruptions and safety problems. Look for long-term stream gauge records. Alternatively, road maintenance records, local newspaper archives, or interviews with area residents can indicate the historical frequency of and damage sustained from large runoff events and flooding.
Debris loading Channels in areas prone to landslides and/or debris flows may be good candidates for low-profile crossings that allow debris to pass over the road.
Channel entrenchment Channels deeply incised below the adjacent ground surface and channels
closely bounded by steep slopes (confined) are generally difficult locations for low-water crossings (fig. 2.1). In both cases, the soil disturbance necessary to construct approaches creates the potential for sediment to impair water quality and aquatic habitat. Although some low-water crossings are successful in such locations (case study 6), mitigating potential erosion problems requires special measures, such as paving
3—7
the approaches and rocking the ditches. Shallow channels on wide flood plains may be good candidates for low-profile crossing structures because the road approaches do not need to ramp up to cross a high culvert or bridge.
Table 3.2 provides more quantitative—although still subjective—selection criteria from a survey of transportation engineers from several different states (Motayed 1982). Although these numbers may not be applicable to all USDA Forest Service locations, they are a starting point for forests to develop their own criteria.
Table 3.2—Quantitative selection criteria for low-water crossings (Motayed et al. 1982).
Criteria Most favorable Least favorable for LWC for LWC
Average daily traffic (ADT) Fewer than 5 vehicles 200 vehicles
Average annual flooding Less than 2 times 10 times
Average duration of traffic Less than 24 hours 3 days interruption per occurrence
Extra travel time for Less than 1 hour 2 hours alternate route
Possibility of danger to Less than 1 in 1 billion 1 in 100,000 human life (with excellent warning systems)
Property damage, dollars None 1 million
Frequency of use as an None Occasional- emergency route frequent
3.2 What Type of Low-Water Crossing Best Fits the Site?
If site and traffic conditions are conducive for a low-water crossing, decide what structure type is best suited to the specific field situation. Table 3.3 lists key site, road management, resource protection, and recreational use factors affecting the choice of structure type. It points out pros and cons of each general structure type for each factor. Keep in mind, however, functionality depends strongly on specific features designed to
Chapter 3—Selecting the Best Structure for the Site
3—8
Low-Water Crossings
tailor the structure to the individual site. Chapter 4 contains information on these design considerations.
Low-VAR fords are undesirable in virtually all situations. Table 3.3 mentions them only to warn practitioners about specific effects they can have (see sidebar low-VAR fords).
Why are low- VAR fords not recommended? In very stable streams, low-VAR fords may not have severe detrimental
effects on channel stability. It is common, however, to see at least a fair amount of channel instability associated with these structures. When flow begins to exceed the vents’ capacity, low-VAR fords begin to function like low dams. They backwater flow upstream of the structure, and where the stream is carrying a substantial bed sediment load, deposition reduces channel capacity and elevates the streambed. This frequently leads to bank erosion and channel widening. Sometimes, the aggraded stream may also shift its location across the valley floor when it seeks lower ground or a steeper grade (fig. 3.2). In channels that are already laterally unstable, low-VAR structures exacerbate the tendency for bank erosion and channel shift.
Compared to high-VAR fords, these dam-like structures overtop
relatively frequently, so scour protection is critical to avoid bed erosion downstream. Even with adequate scour protection, streambed composition can become coarser when the fines are winnowed away, which happens in channels downstream of dams that impound sediment. Also, depending on how well the ford matches channel shape, the backwatered pool upstream may flow out around the structure’s edges. When flow reenters the channel downstream, it can erode both the flood plain and streambanks.
Because the vents or pipes in low-VAR fords are small compared to the stream, they plug easily and tend to require frequent maintenance. They are also hazards to swimmers and boaters. In addition to being small, they may or may not be installed at stream grade and they are usually at least partial barriers to AOP.
For all these reasons, the authors’ recommend high-VAR fords be used when a vented ford is desired. Low-VAR fords are not included in table 3.3, which deals with considerations for selecting the type of structure best suited to a site.
3—9
Figure 3.2—Sediment deposition and channel widening caused by the “damming” effect of a low-VAR ford.
Table 3.3 occasionally makes reference to Rosgen’s channel types. For reference, appendix C includes two figures illustrating the classification system. Readers unfamiliar with this classification system should refer to either Rosgen’s paper (Rosgen 1994) or book (Rosgen 1996).
Chapter 3—Selecting the Best Structure for the Site
3—10
Low-Water Crossings
Typ
e o
f F
acto
r U
nve
nte
d F
ord
s V
ente
d F
ord
s
Lo
w-W
ater
Bri
dg
es
Fac
tor
(Hig
h-V
AR
)
Bas
eflo
w
Bas
eflow
is th
e co
mpo
nent
of
stre
amflo
w d
eriv
ed fr
om g
roun
d w
ater
. It i
s re
lativ
ely
cons
tant
. F
lood
flow
s, w
hich
are
der
ived
fr
om s
urfa
ce o
r ne
ar-s
urfa
ce
runo
ff, v
ary
muc
h m
ore
than
ba
seflo
w.
Eph
emer
al s
trea
m c
hann
els
are
abov
e th
e gr
ound
wat
er ta
ble,
an
d th
eref
ore
have
no
base
flow
. In
term
itten
t cha
nnel
s in
terc
ept
grou
nd w
ater
onl
y w
hen
the
wat
er ta
ble
is h
igh
afte
r a
runo
ff ev
ent.
Per
enni
al c
hann
el b
eds
are
belo
w th
e gr
ound
wat
er ta
ble
year
rou
nd. N
orm
al lo
w fl
ows
are
gene
rally
bas
eflow
s.
Hig
h-fl
ow
du
rati
on
du
rin
g
seas
on
of
use
See
dis
cuss
ion
in s
ectio
n 3.
1
Wo
od
y d
ebri
s an
d ic
e b
lock
age
haz
ard
Str
uctu
res
desi
gned
for
over
topp
ing
redu
ce th
e ris
k of
cro
ssin
g fa
ilure
and
str
eam
di
vers
ion
whe
re ic
e or
deb
ris
may
plu
g cu
lver
ts.
See
cas
e st
udie
s 16
, 17,
18,
19.
App
ropr
iate
for
ephe
mer
al
stre
ams
and
inte
rmitt
ent s
trea
ms
whe
re th
e ro
ad is
clo
sed
durin
g th
e flo
w s
easo
n. W
here
ba
seflo
ws
are
high
and
ste
ady,
ve
hicl
es m
ust d
rive
thro
ugh
wat
er th
at w
ill p
roba
bly
be a
ha
bita
t for
aqu
atic
spe
cies
. In
som
e pa
rts
of th
e co
untr
y,
unve
nted
ford
s ar
e st
ill c
omm
on
on p
eren
nial
str
eam
s w
here
tr
affic
vol
umes
are
low
, but
they
ar
e lik
ely
caus
ing
at le
ast s
ome
dam
age
to a
quat
ic o
rgan
ism
s an
d th
eir
habi
tats
.
Mos
t use
ful w
here
hig
h flo
ws
are
shor
t (so
that
traf
fic is
onl
y br
iefly
in
terr
upte
d).
Com
mon
ly u
sed
in
flash
y de
sert
str
eam
s.
The
ope
n cr
oss
sect
ion
pres
ents
le
ast r
isk
of d
ebris
- or
ice-
bloc
kage
.
Tra
ctio
n ca
n be
a s
ever
e pr
oble
m fo
r w
inte
r tr
affic
whe
n th
e fo
rd s
urfa
ce is
icy.
App
ropr
iate
for
inte
rmitt
ent a
nd
pere
nnia
l str
eam
s.
Con
side
r th
e ne
ed to
pro
vide
aq
uatic
org
anis
m p
assa
ge.
Con
side
r hi
gh-V
AR
ford
s w
here
ic
e or
deb
ris m
ay p
lug
a cu
lver
t. H
owev
er, e
ven
ford
s w
ith la
rge
open
ings
can
plu
g an
d be
m
aint
enan
ce h
eada
ches
if th
e ch
anne
l car
ries
a lo
t of l
arge
w
oody
deb
ris a
nd tr
ee tr
unks
.
Can
be
desi
gned
with
re
mov
able
driv
ing
surf
ace
(e.g
., ca
ttleg
uard
s) fo
r ea
sy c
lean
-out
.R
emov
able
met
al g
rate
s, c
an
be m
ore
slip
pery
than
gra
vel o
r pa
ved
surf
aces
whe
n ic
y.
The
se s
truc
ture
s us
ually
con
vey
a re
lativ
ely
larg
e vo
lum
e of
flow
un
der
the
deck
, so
they
are
use
ful w
here
hig
h flo
ws
last
long
er.
Whe
re th
e st
ruct
ure
will
be
subm
erge
d fo
r lo
ng p
erio
ds o
f tim
e, a
n al
tern
ate
rout
e sh
ould
be
avai
labl
e
App
ropr
iate
for
all fl
ow r
egim
es.
Con
side
r lo
w-w
ater
brid
ges
whe
re d
ebris
load
s ar
e ve
ry
high
and
traf
fic v
olum
e do
es n
ot
just
ify c
onst
ruct
ion
of a
brid
ge
high
eno
ugh
to a
void
deb
ris
jam
s. T
he h
azar
d of
deb
ris-
or ic
e-ja
mm
ing
is h
ighe
r th
an fo
r or
dina
ry b
ridge
s be
caus
e of
the
low
er c
lear
ance
.
Tra
ctio
n is
sues
are
sim
ilar
to th
ose
enco
unte
red
with
co
nven
tiona
l brid
ges.
Site
Tabl
e 3.
3—C
onsi
dera
tions
for
low
-wat
er c
ross
ing
stru
ctur
e ty
pe s
elec
tion.
3—11
Ty
pe
of
Fac
tor
Un
ven
ted
Fo
rds
Ven
ted
Fo
rds
L
ow
-Wat
er B
rid
ges
F
acto
r
(H
igh
-VA
R)
Chapter 3—Selecting the Best Structure for the SiteTa
ble
3.3—
Con
side
ratio
ns fo
r lo
w-w
ater
cro
ssin
g st
ruct
ure
type
sel
ectio
n—continued
Slig
htl
y en
tren
ched
(R
osge
n E
, D
A, s
tabl
e C
) S
ee c
ase
stud
ies
18, 1
9, 2
0.
Ove
rban
k flo
od p
lain
flow
s ar
e fa
irly
freq
uent
on
thes
e st
ream
s,
and
can
be o
f lon
g du
ratio
n.
Roa
d ap
proa
ches
are
ofte
n ra
ised
abo
ve th
e flo
od p
lain
su
rfac
e, b
ackw
ater
ing
over
bank
flo
ws
upst
ream
of t
he r
oad,
and
al
terin
g sc
our
and
depo
sitio
n pr
oces
ses
that
mai
ntai
n flo
od
plai
n ha
bita
ts a
nd g
roun
d w
ater
ta
bles
.
Une
ntre
nche
d ch
anne
ls in
wid
e flo
od p
lain
s ar
e of
ten
high
ly
sinu
ous
and
may
shi
ft th
eir
loca
tion
on th
e flo
od p
lain
ove
r tim
e.
Mo
der
atel
y en
tren
ched
(R
osge
n B
)
See
cas
e st
udie
s 4,
8, 1
2, 1
7.
En
tren
ched
(R
osge
n F,
A, G
)
See
cas
e st
udie
s 3,
5, 6
, 16.
E
ntre
nche
d ch
anne
ls a
re
eith
er in
cise
d be
twee
n hi
gh
bank
s or
are
clo
sely
bou
nded
by
val
ley
wal
ls. A
ppro
ache
s to
th
e lo
w-w
ater
cro
ssin
g ha
ve to
sl
ope
stee
ply
into
the
chan
nel,
crea
ting
the
risk
of s
edim
ent
intr
oduc
tion
from
roa
d su
rfac
e an
d di
tch.
Be
care
ful t
o ca
rry
chan
nel p
rote
ctio
n fr
om s
cour
hi
gh e
noug
h in
thes
e ch
anne
ls,
sinc
e th
e w
ater
sur
face
doe
s no
t spr
ead
very
muc
h (b
ut r
ises
ve
rtic
ally
) as
flow
incr
ease
s.
App
ropr
iate
whe
re c
hann
el w
idth
is
muc
h gr
eate
r tha
n de
pth,
so
that
a
ford
mat
chin
g ch
anne
l sha
pe
has
a ve
rtica
l cur
ve a
dequ
ate
for
pass
age
of th
e de
sign
veh
icle
.
Man
y ch
anne
ls in
wid
e flo
od p
lain
s ha
ve s
teep
, den
sely
veg
etat
ed
stre
amba
nks
(eve
n th
ough
th
ey m
ay b
e lo
w).
App
roac
hes
to u
nven
ted
ford
s cu
t thr
ough
th
e ba
nks
and
wid
en th
e hi
gh-
flow
cro
ss s
ectio
n, in
duci
ng
sedi
men
t dep
ositi
on. A
dditi
onal
ly,
man
y m
eado
w s
oils
are
sof
t an
d co
mpr
essi
ble,
qua
litie
s th
at
cont
ribut
e to
this
pro
blem
if
appr
oach
es a
re n
ot a
dequ
atel
y re
info
rced
or h
arde
ned
(cas
e st
udy
7).
Bec
ause
thes
e st
ream
s ha
ve
rela
tivel
y m
ild a
ppro
ach
grad
es
and
are
mod
erat
ely
wid
e re
lativ
e to
thei
r de
pth,
unv
ente
d fo
rds
can
be a
ppro
pria
te.
Gen
eral
ly n
ot a
ppro
pria
te a
t de
eply
ent
renc
hed
chan
nels
w
here
the
appr
oach
es r
equi
re
cutti
ng th
roug
h th
e ad
jace
nt
slop
es to
ach
ieve
a d
rivab
le
grad
e. A
ppro
ache
s m
ay
also
req
uire
ext
raor
dina
ry
stab
iliza
tion
mea
sure
s, s
uch
as p
avin
g an
d ro
ckin
g di
tche
s.
The
re m
ay b
e no
roo
m fo
r di
tch
relie
f or
a se
dim
ent t
rap.
If
aqua
tic o
rgan
ism
pas
sage
is n
ot
an is
sue,
roc
kfill
ford
s (e
.g.,
case
st
udy
3) c
an b
e ap
prop
riate
in
entr
ench
ed c
hann
els.
Goo
d op
tions
if p
oten
tial p
robl
ems
with
out
flank
ing
durin
g hi
gh fl
ows
can
be a
void
ed. O
verb
ank
flow
s th
at g
o ar
ound
the
stru
ctur
e ca
n ca
use
bed
and
bank
ero
sion
dow
nstr
eam
.
Roa
d ap
proa
ches
sho
uld
be k
ept l
ow a
cros
s th
e flo
od p
lain
, or
shou
ld
span
it to
avo
id in
terr
uptin
g flo
od fl
ows
and
sedi
men
t and
deb
ris
tran
spor
t on
the
flood
pla
in. (
See
cas
e st
udy
20 fo
r an
exa
mpl
e of
a
very
low
pro
file
low
-wat
er b
ridge
in a
flat
E c
hann
el in
Flo
rida.
)
Eve
n st
able
mea
nder
ing
stre
ams
expe
rienc
e m
eand
er s
hift.
At
bend
s, e
rosi
on o
n th
e ou
ter
bank
and
dep
ositi
on o
n th
e in
ner
bank
ca
use
the
chan
nel t
o ch
ange
loca
tion
over
tim
e. T
his
proc
ess
ofte
n m
odifi
es th
e ch
anne
l’s a
ngle
of a
ppro
ach
to a
cro
ssin
g. A
n ac
ute
angl
e of
app
roac
h ca
n ca
use
sedi
men
t and
deb
ris d
epos
ition
abo
ve th
e st
ruct
ure.
Cul
vert
s ca
n pl
ug a
nd b
ank
eros
ion
can
beco
me
seve
re.
Des
igne
rs s
houl
d co
nsid
er s
pann
ing
the
entir
e m
eand
er b
elt (
the
zone
of
act
ive
mea
nder
shi
ft), o
r th
ey m
ight
pla
n fo
r re
gula
r m
onito
ring
and
mai
nten
ance
.
Goo
d op
tions
.
Ban
k er
osio
n is
less
of a
n is
sue
than
for
slig
htly
ent
renc
hed
stre
ams
beca
use
the
chan
nel i
s m
ore
cons
trai
ned
and
less
abl
e to
shi
ft lo
catio
n.
App
licat
ion
limite
d to
sm
all
entr
ench
ed c
hann
els
whe
re th
e dr
ivin
g pl
atfo
rm c
an b
e ra
ised
hi
gh e
noug
h to
hav
e a
suita
ble
road
alig
nmen
t and
ent
ail
min
imal
ear
thw
ork.
Use
ful i
n st
eep
chan
nels
that
are
su
bjec
t to
freq
uent
deb
ris fl
ows
(cas
e st
udy
16).
Suc
cess
ful
vent
ed fo
rds
are
larg
e co
ncre
te
stru
ctur
es th
at a
llow
deb
ris to
rid
e ov
er th
e dr
ivin
g su
rfac
e an
d ha
ve r
emov
able
tops
for
clea
ning
out
trap
ped
debr
is.
Sui
tabl
e fo
r sp
anni
ng s
mal
l ch
anne
ls.
A b
ridge
cro
ssin
g a
wid
e en
tren
ched
cha
nnel
wou
ld li
kely
be
a s
tand
ard
high
-cle
aran
ce
stru
ctur
e.
Site
3—12
Low-Water Crossings
Typ
e o
f F
acto
r U
nve
nte
d F
ord
s V
ente
d F
ord
s
Lo
w-W
ater
Bri
dg
es
Fac
tor
(Hig
h-V
AR
)
Lat
eral
ly u
nst
able
(es
peci
ally
R
osge
n D
, uns
tabl
e C
)
In d
epos
ition
al c
hann
els
such
as
brai
ded
or a
lluvi
al fa
n ch
anne
ls,
or in
str
eam
s un
derg
oing
rap
id
bank
ero
sion
, the
cha
nnel
may
sh
ift a
brup
tly o
r pr
ogre
ssiv
ely.
If
at a
ll po
ssib
le, a
void
loca
ting
a cr
ossi
ng in
suc
h a
spot
. If y
ou
cann
ot a
void
it, t
hen
low
-pro
file
or fl
ood
plai
n-sp
anni
ng s
truc
-tu
res
are
less
like
ly to
obs
truc
t flo
w a
nd e
xace
rbat
e la
tera
l shi
ft.
Late
rally
uns
tabl
e ch
anne
ls
freq
uent
ly h
ave
high
sed
imen
t lo
ads.
If a
cro
ssin
g st
ruct
ure
ob-
stru
cts
sedi
men
t tra
nspo
rt—
es-
peci
ally
if it
plu
gs—
it ca
n in
duce
ev
en m
ore
chan
nel e
rosi
on.
See
cas
e st
udie
s 2,
21.
Ver
tica
lly u
nst
able
(ag
grad
ing
or in
cisi
ng)
If at
all
poss
ible
, avo
id c
ross
ing
any
unst
able
str
eam
.
In a
ggra
ding
cha
nnel
s at
val
-le
y m
argi
ns o
r on
allu
vial
fans
, ob
stru
ctin
g se
dim
ent t
rans
port
ca
n le
ad to
plu
ggin
g or
cha
nnel
sh
ift. A
n in
cisi
ng c
hann
el c
an
caus
e an
y cr
ossi
ng s
truc
ture
w
ith a
floo
r to
bec
ome
perc
hed,
th
ereb
y ei
ther
und
erm
inin
g th
e st
ruct
ure
or c
ausi
ng th
e st
ruct
ure
to b
ecom
e a
grad
e co
ntro
l (i.e
., a
stru
ctur
e th
at c
ontr
ols
upst
ream
ch
anne
l ele
vatio
n an
d gr
ade)
. If
a gr
ade
cont
rol i
s de
sire
d,
pred
ict t
he d
epth
of f
utur
e do
wn-
cutti
ng a
nd p
rovi
de d
owns
trea
m
cuto
ff w
alls
to p
rote
ct th
e st
ruc-
ture
(se
ctio
n 4.
7).
Uni
mpr
oved
at-
grad
e ro
ck o
r na
tive
mat
eria
l for
ds a
re th
e ch
eape
st a
nd e
asie
st s
truc
ture
s to
rep
lace
whe
n th
e ch
anne
l sh
ifts
loca
tion,
and
are
com
mon
in
des
ert b
raid
ed s
trea
m
envi
ronm
ents
. Im
prov
ed fo
rds
in
thes
e en
viro
nmen
ts a
re u
sual
ly
pave
d or
con
cret
e di
ps.
Whe
re th
e ap
proa
ch r
oadfi
ll co
ncen
trat
es d
ispe
rsed
flo
odpl
ain
flow
s th
roug
h th
e fo
rd,
the
adde
d er
osiv
e po
wer
put
s th
e ch
anne
l dow
nstr
eam
of t
he
road
at r
isk
of lo
cal d
egra
datio
n.
In a
n ag
grad
ing
reac
h, u
nven
ted
ford
s ha
ve th
e be
nefit
of n
ot
plug
ging
, and
they
hav
e no
ro
adfil
l to
fail
in a
floo
d ev
ent.
Giv
en th
e po
tent
ial f
or la
tera
l sh
ift in
this
set
ting,
con
side
r us
ing
an u
nim
prov
ed fo
rd to
m
inim
ize
the
inve
stm
ent i
n th
e st
ruct
ure.
In a
deg
radi
ng r
each
, lik
e ot
her
cros
sing
str
uctu
res,
unv
ente
d fo
rds
may
be
unde
rmin
ed w
hen
an a
dvan
cing
hea
dcut
rea
ches
th
em. I
f the
str
uctu
re is
wel
l pr
otec
ted
from
und
erm
inin
g,
it w
ill in
adve
rten
tly b
ecom
e a
grad
e co
ntro
l, pr
even
ting
the
head
cut f
rom
mig
ratin
g up
stre
am.
See
cas
e st
udy
9.
Gen
eral
ly w
ould
not
be
used
be
caus
e of
the
risk
that
the
chan
nel w
ould
mov
e aw
ay
from
the
stru
ctur
e. If
use
d,
VA
R s
houl
d be
hig
h an
d ro
ad
appr
oach
es s
houl
d be
low
to
min
imiz
e ob
stru
ctio
n of
flow
and
se
dim
ent t
rans
port
.
App
ropr
iate
at d
epos
ition
al s
ites
such
as
whe
re a
trib
utar
y en
ters
th
e va
lley
floor
(ca
se s
tudy
13)
. U
se r
emov
able
tops
to fa
cilit
ate
debr
is r
emov
al.
Can
be
desi
gned
to fu
nctio
n as
gra
de c
ontr
ols
in in
cisi
ng
chan
nels
(ca
se s
tudy
15)
.
Brid
ges
are
less
like
ly th
an o
ther
cr
ossi
ng s
truc
ture
s to
exa
cerb
ate
chan
nel s
hifti
ng b
y ob
stru
ctin
g flo
w o
r se
dim
ent t
rans
port
.
Non
ethe
less
, the
pot
entia
l for
the
chan
nel t
o sh
ift a
nd is
olat
e th
e br
idge
sho
uld
be c
onsi
dere
d. A
lo
ng s
truc
ture
may
be
need
ed to
sp
an th
e flo
od p
lain
.
Con
side
ratio
ns fo
r ag
grad
ing
chan
nel s
ites
are
the
sam
e as
for
late
rally
uns
tabl
e ch
anne
ls; i
.e.,
the
chan
nel m
ay s
hift
away
from
th
e br
idge
.
For
inci
sing
cha
nnel
s, th
e ris
k of
sco
ur u
nder
min
ing
the
pier
s w
ould
be
a m
ajor
site
lim
itatio
n.
Des
igne
rs c
an m
inim
ize
this
ris
k by
avo
idin
g m
id-c
hann
el p
iers
an
d us
ing
long
spa
ns to
kee
p ab
utm
ents
out
of t
he c
hann
el.
Site
Tabl
e 3.
3—C
onsi
dera
tions
for
low
-wat
er c
ross
ing
stru
ctur
e ty
pe s
elec
tion—
continued
3—13
Ty
pe
of
Fac
tor
Un
ven
ted
Fo
rds
Ven
ted
Fo
rds
L
ow
-Wat
er B
rid
ges
F
acto
r
(H
igh
-VA
R)
Acc
epta
ble
tra
ffic
del
ay
Acc
epta
ble
depe
nds
on tr
affic
vo
lum
e, tr
affic
type
, and
the
avai
labi
lity
of a
ltern
ativ
e ro
utes
. W
here
no
traf
fic c
ount
s ex
ist,
mai
nten
ance
leve
l can
be
used
as
a r
ough
sur
roga
te fo
r tr
affic
vo
lum
e an
d ty
pe fo
r th
e pu
rpos
e of
sel
ectin
g th
e ap
prop
riate
st
ruct
ure
type
.
Act
ual d
elay
tim
es d
epen
d on
the
flood
freq
uenc
ies
and
runo
ff pa
ttern
s of
the
wat
ersh
ed
(sec
tion
4.5)
.
Veh
icle
typ
e
Gro
und
clea
ranc
e af
fect
s th
e de
pth
of w
ater
a v
ehic
le c
an b
e dr
iven
thro
ugh.
Whe
el-b
ase
leng
th a
ffect
s th
e al
low
able
ver
tical
cur
ve th
roug
h a
cros
sing
.
Trac
tio
n
Wet
or
icy
surf
aces
are
a s
afet
y ha
zard
.
Mai
nte
nan
ce a
cces
s an
d
com
mit
men
t
All
stru
ctur
es r
equi
re in
spec
ting
and
som
etim
es c
lean
ing,
m
aint
enan
ce o
r re
pairs
afte
r hi
gh
flow
s.
Usu
ally
lim
ited
to th
ose
loca
tions
w
here
del
ays
may
occ
ur w
ithou
t en
dang
erin
g th
e pu
blic
.
The
low
er th
e tr
affic
vol
ume
and
the
shor
ter
the
high
flow
du
ratio
n, th
e m
ore
likel
y a
ford
is
acce
ptab
le.
Uni
mpr
oved
ford
s m
ay b
e ap
prop
riate
for
road
s th
at a
re
clos
ed o
r m
anag
ed fo
r hi
gh-
clea
ranc
e ve
hicl
es o
r of
f-hi
ghw
ay v
ehic
les
only
(tr
ails
an
d m
aint
enan
ce le
vel 1
and
2
road
s).
If th
e ch
anne
l is
narr
ow a
nd
deep
, an
unve
nted
ford
may
hav
e a
shar
p ve
rtic
al c
urve
that
can
sn
ag tr
aile
r hi
tche
s an
d ca
use
long
whe
el-b
ase
vehi
cles
to d
rag.
Tra
ctio
n ca
n be
a s
ever
e pr
oble
m fo
r w
inte
r tr
affic
whe
n th
e su
rfac
e is
icy.
A g
ood
choi
ce o
n cl
osed
roa
ds
whe
re a
cces
s fo
r in
spec
tion
and
mai
nten
ance
will
be
diffi
cult.
T
hey
usua
lly r
equi
re m
inim
al
mai
nten
ance
, dep
endi
ng o
n th
e st
ruct
ure
type
and
the
qual
ity
of it
s in
stal
latio
n. M
aint
enan
ce
need
s ty
pica
lly a
rise
afte
r a
flood
; the
y in
clud
e re
mov
ing
sedi
men
t and
deb
ris, r
egra
ding
ap
proa
ches
, and
rep
laci
ng
surf
acin
g m
ater
ials
.
App
ropr
iate
for
high
er tr
affic
vo
lum
e ro
ads
(rou
ghly
m
ain
ten
ance
leve
l 2-3
roa
ds).
F
low
s pa
ss th
roug
h th
e ve
nts
mos
t of t
he ti
me,
min
imiz
ing
safe
ty h
azar
ds a
nd tr
affic
del
ays.
Rai
sed
driv
ing
plat
form
s ge
nera
lly m
inim
ize
the
dept
h of
w
ater
driv
en th
roug
h an
d fla
tten
out t
he r
oadw
ay s
ag “
vert
ical
cu
rve.
”
Fea
sibi
lity
for
vario
us v
ehic
le
type
s de
pend
s on
app
roac
h gr
ades
and
the
resu
lting
ver
tical
cu
rve.
Som
e ve
nted
ford
s ha
ve
rem
ovab
le m
etal
gra
tes,
whi
ch
can
be m
ore
slip
pery
than
gra
vel
or p
aved
sur
face
s.
Ven
ted
ford
s in
str
eam
s th
at
carr
y la
rge
woo
dy d
ebris
ofte
n re
quire
cle
anin
g an
d ca
n be
a
mai
nten
ance
hea
dach
e.
Rem
ovab
le to
ps a
re g
ood
desi
gn
feat
ures
for
stre
ams
with
larg
e de
bris
load
s. T
he to
ps c
an b
e re
mov
ed b
efor
e a
larg
e st
orm
or
a se
ason
al r
oad
clos
ure.
Bec
ause
brid
ges
usua
lly p
rovi
de
the
mos
t ope
n ar
ea u
nder
the
deck
, the
y ar
e ap
prop
riate
for
high
er tr
affic
vol
ume
road
s an
d w
here
traf
fic in
terr
uptio
n is
leas
t ac
cept
able
(ro
ughl
y, m
aint
enan
ce
leve
l 2-4
).
Low
wat
er b
ridge
s ac
com
mod
ate
the
wid
est r
ange
of v
ehic
le ty
pes.
W
eigh
t lim
itatio
ns m
ay b
e a
desi
gn c
onsi
dera
tion.
Tra
ctio
n is
sues
are
the
sam
e as
thos
e en
coun
tere
d w
ith
conv
entio
nal b
ridge
s.
Gen
eral
ly th
e on
ly m
aint
enan
ce
requ
ired
is a
fter
larg
e flo
ods,
w
hen
debr
is tr
appe
d on
the
stru
ctur
e ne
eds
to b
e re
mov
ed
and
any
chan
nel o
r ba
nk e
rosi
on
arou
nd th
e st
ruct
ure
is r
epai
red.
Road Management
Tabl
e 3.
3—C
onsi
dera
tions
for
low
-wat
er c
ross
ing
stru
ctur
e ty
pe s
elec
tion—
continued
Chapter 3—Selecting the Best Structure for the Site
3—14
Low-Water Crossings
Typ
e o
f F
acto
r U
nve
nte
d F
ord
s V
ente
d F
ord
s
Lo
w-W
ater
Bri
dg
es
Fac
tor
(Hig
h-V
AR
)
Co
st
Cos
ts a
ssoc
iate
d w
ith c
ross
ing
stru
ctur
es in
clud
e in
stal
latio
n,
mai
nten
ance
, rep
lace
men
t, an
d (s
omet
imes
) ro
ad u
ser
cost
s.
Tota
l life
-cyc
le c
osts
als
o in
clud
e th
e da
mag
e to
str
eam
cha
nnel
s,
flood
pla
ins,
and
aqu
atic
hab
itats
ca
used
by
obst
ruct
ion
of n
atur
al
tran
spor
t pro
cess
es o
r by
st
ruct
ure
failu
re.
Aq
uat
ic O
rgan
ism
Pas
sag
e
Uno
bstr
ucte
d pa
ssag
e fo
r al
l aq
uatic
ani
mal
s is
pre
sum
ed
to e
xist
whe
n th
e st
ream
bed
is c
ontin
uous
thro
ugho
ut th
e cr
ossi
ng. W
hen
chan
nel w
idth
, de
pth,
and
str
uctu
re (
e.g.
, ste
ps,
pool
s, r
iffles
) in
the
cros
sing
are
si
mila
r to
thos
e of
the
natu
ral
chan
nel,
the
cros
sing
sho
uld
be n
early
indi
scer
nibl
e to
aq
uatic
spe
cies
. Des
igni
ng fo
r sw
imm
able
vel
ociti
es a
nd d
epth
s at
sel
ecte
d flo
ws
can
also
allo
w
som
e fis
h pa
ssag
e (s
ee s
ectio
n 4.
3).
Gen
eral
ly c
ost m
uch
less
to
inst
all t
han
othe
r st
ruct
ures
(W
arho
l 199
4) a
nd, w
hen
prop
erly
con
stru
cted
, the
y re
quire
littl
e m
aint
enan
ce.
Con
stru
ctio
n an
d m
aint
enan
ce
cost
s m
ay b
e hi
gher
whe
re
spec
ial e
ffort
s fo
r w
ater
qua
lity
or
habi
tat c
once
rns
are
nece
ssar
y;
in s
uch
case
s, a
noth
er s
truc
ture
ty
pe m
ay b
e pr
efer
able
any
way
(s
ee a
ppen
dix
D).
Due
to th
e po
tent
ial f
or h
abita
t di
stur
banc
e, fi
sh in
jury
, and
th
e w
ater
qua
lity
impa
cts
of
vehi
cles
pas
sing
thro
ugh
wat
er,
unve
nted
ford
s ar
e no
t gen
eral
ly
reco
mm
ende
d fo
r lo
catio
ns w
ith
fish
pass
age
conc
erns
. How
ever
, fo
rds
may
be
a go
od c
hoic
e w
here
•
traf
fic v
olum
e is
low
•
aqua
tic h
abita
t is
limite
d
•
aqua
tic h
abita
t is
poor
qua
lity
•
the
chan
nel i
s pr
one
to
shift
ing
In s
uch
case
s, th
e re
sour
ce o
r ac
cess
ben
efits
mig
ht n
ot ju
stify
th
e co
st o
f a c
ulve
rt o
r br
idge
th
at w
ould
pro
vide
bet
ter
fish
pass
age.
Goo
d de
sign
is c
ritic
al
for
ford
s w
here
aqu
atic
org
anis
m
pass
age
is d
esire
d (s
ee s
ectio
n 4.
3)
Mos
t for
ds p
rovi
de a
t lea
st
part
ial p
assa
ge w
hen
they
are
ba
ckw
ater
ed o
r su
bmer
ged.
Hig
h-V
AR
ford
s ar
e lik
ely
to h
ave
inst
alla
tion
cost
s si
mila
r to
thos
e of
ord
inar
y cu
lver
ts.
Sin
ce v
ente
d fo
rds
are
desi
gned
to
sus
tain
ove
rtop
ping
flow
s,
repl
acem
ent c
osts
sho
uld
be
low
er, e
spec
ially
whe
re p
lugg
ing
mak
es o
rdin
ary
culv
ert f
ailu
res
freq
uent
.
Can
pro
vide
pas
sage
by
mat
chin
g ch
anne
l wid
th a
nd
mai
ntai
ning
str
eam
bed
cont
inui
ty
thro
ugh
the
stru
ctur
e (c
ase
stud
ies
17, 1
8, 1
9). U
sual
ly,
thes
e st
ruct
ures
are
box
cul
vert
s th
at a
llow
the
cros
sing
str
uctu
re
to b
e sh
ort i
n th
e al
ong-
stre
am
dire
ctio
n.
Som
e co
ncre
te fo
rds
incl
ude
a sl
ot to
con
cent
rate
low
flow
s an
d pr
ovid
e pa
rtia
l fish
pas
sage
(c
ase
stud
ies
9, 1
2). T
his
type
of
ford
sho
uld
gene
rally
be
a la
st r
esor
t in
mos
t fish
-bea
ring
stre
ams;
it is
unl
ikel
y to
pro
vide
m
ore
than
par
tial p
assa
ge fo
r a
targ
et s
peci
es a
nd m
ay p
rovi
de
no p
assa
ge a
t all
for
othe
r aq
uatic
spe
cies
.
Like
ly to
be
som
ewha
t che
aper
th
an s
tand
ard
brid
ges,
bec
ause
th
ey d
o no
t nee
d to
be
as
high
abo
ve th
e w
ater
sur
face
. A
lthou
gh th
e ris
k of
deb
ris
bloc
kage
is h
ighe
r fo
r lo
w-w
ater
br
idge
s, th
ey a
re n
ot li
kely
to fa
il be
caus
e of
it. C
osts
ass
ocia
ted
with
mai
nten
ance
may
be
high
er
in a
reas
with
larg
e de
bris
load
s.
In b
road
, val
leys
whe
re d
ebris
pl
uggi
ng c
ould
cau
se c
hann
el
shift
aw
ay fr
om th
e br
idge
in a
m
ajor
floo
d, c
hann
el r
ehab
ilita
tion
coul
d ad
d to
the
lifet
ime
cost
.
Ass
umin
g th
ey a
re w
ide
enou
gh,
low
-wat
er b
ridge
s ar
e a
good
st
ruct
ure
type
bec
ause
the
stre
ambe
d re
mai
ns c
ontin
uous
th
roug
h th
e cr
ossi
ng.
Economic Resource Protection
Tabl
e 3.
3—C
onsi
dera
tions
for
low
-wat
er c
ross
ing
stru
ctur
e ty
pe s
elec
tion—
continued
3—15
Ty
pe
of
Fac
tor
Un
ven
ted
Fo
rds
Ven
ted
Fo
rds
L
ow
-Wat
er B
rid
ges
F
acto
r
(H
igh
-VA
R)
Chapter 3—Selecting the Best Structure for the Site
Wat
er q
ual
ity
(See
als
o ap
pend
ix D
: Low
-w
ater
cro
ssin
g ef
fect
s on
wat
er
qual
ity.)
The
impo
rtan
ce o
f pro
tect
ing
wat
er q
ualit
y at
a s
ite a
ffect
s th
e ch
oice
of s
truc
ture
type
bec
ause
st
ruct
ure
type
det
erm
ines
how
fr
eque
ntly
veh
icle
s dr
ive
thro
ugh
wat
er, a
nd th
e fe
asib
ility
of
dive
rtin
g se
dim
ent-
lade
n ro
ad
surf
ace
runo
ff be
fore
it r
each
es
the
stre
am.
Dep
endi
ng o
n th
e S
tate
and
the
proj
ect,
one
or m
ore
perm
its m
ay
be r
equi
red
befo
re u
nder
taki
ng
cons
truc
tion
in a
str
eam
cha
nnel
.
No
xio
us
wee
ds
Veh
icle
s m
ay c
arry
inva
sive
aq
uatic
and
oth
er w
eeds
(se
eds
and
plan
t par
ts)
and
deliv
er th
em
to w
ater
way
s.
Haz
ard
s fo
r re
crea
tio
nal
use
rs
Any
in-s
trea
m s
truc
ture
in
fluen
ces
wat
er v
eloc
ity a
nd
dire
ctio
n in
way
s th
at m
ay c
reat
e ris
ks fo
r sw
imm
ers
or b
oate
rs,
espe
cial
ly a
t hig
h flo
ws.
Con
stru
ctio
n m
ay a
ffect
wat
er
qual
ity te
mpo
raril
y de
pend
ing
on s
ite s
tabi
lity,
exc
avat
ion
requ
irem
ents
, and
flow
s du
ring
cons
truc
tion.
Tra
ffic
thro
ugh
wat
er is
mos
t lik
ely
to d
eliv
er p
ollu
tant
s to
w
ater
way
s an
d ca
use
som
e ch
anne
l ero
sion
, alth
ough
si
gnifi
cant
effe
cts
have
not
bee
n de
mon
stra
ted.
Bec
ause
they
dire
ctly
con
nect
so
me
port
ion
of th
e ro
ad to
the
stre
am, u
nven
ted
ford
s ha
ve th
e hi
ghes
t pot
entia
l for
sed
imen
t de
liver
y of
any
str
uctu
re ty
pe.
How
ever
, sin
ce th
ere
is n
o fil
l to
fail,
they
do
not p
rodu
ce th
e la
rge
volu
mes
of s
edim
ent t
hat
culv
ert p
lugg
ing
som
etim
es
prod
uces
dur
ing
cata
stro
phic
flo
ods.
Mos
t lik
ely
to p
erm
it w
eed
intr
oduc
tion
to n
ew w
ater
way
s.
Dep
endi
ng o
n de
pth
of fl
ow,
float
ers
mig
ht s
crap
e th
e fo
rd
surf
ace.
If th
e fo
rd is
ele
vate
d ab
ove
the
stre
am b
otto
m,
the
hydr
aulic
jum
p th
at fo
rms
dow
nstr
eam
dur
ing
over
topp
ing
flow
can
con
stitu
te a
ris
k to
bo
ater
s an
d sw
imm
ers.
Con
stru
ctio
n ca
n af
fect
wat
er q
ualit
y te
mpo
raril
y if
exca
vatio
n an
d fil
l ar
e no
t iso
late
d fr
om fl
owin
g w
ater
.
Tra
ffic
over
thes
e st
ruct
ures
is li
kely
to h
ave
little
or
no e
ffect
on
wat
er q
ualit
y, e
spec
ially
if b
est m
anag
emen
t pra
ctic
es a
re
impl
emen
ted
to is
olat
e ro
ad s
urfa
ce r
unof
f fro
m th
e st
ream
.
Whe
re th
e dr
ivin
g su
rfac
e ha
s ga
ps (
e.g.
, cat
tle g
uard
s), w
eed
seed
s co
uld
fall
off v
ehic
les
into
the
chan
nel.
No
info
rmat
ion
abou
t how
si
gnifi
cant
this
ris
k is
was
foun
d.
At h
igh
flow
s, a
s th
e ris
ing
wat
er s
urfa
ce n
ears
the
road
bed,
boa
ts
can
be tr
appe
d ag
ains
t the
str
uctu
re. W
hen
flow
ove
rtop
s a
culv
ert
open
ing,
raf
ts a
nd s
wim
mer
s ca
n be
suc
ked
unde
rwat
er. S
ectio
n 4.
9 in
clud
es d
esig
n id
eas
for
min
imiz
ing
thes
e ris
ks.
Resource Protection Recreation
Tabl
e 3.
3—C
onsi
dera
tions
for
low
-wat
er c
ross
ing
stru
ctur
e ty
pe s
elec
tion—
continued
