Low-water Crossings for Livestock and Equipment: Planning for Aquatic Organism Passage


[ Silence ]>>Operator: Welcome and thanks for standing by. At this time all participants
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may disconnect at this time. I would now like to turn today’s
meeting over to Holli Kuykendall. Thank you. You may begin.>>Holli Kuykendall: Thank you very much,
and thank you everyone for joining us today. I’m looking forward to an exciting
topic presented by Mr. Kale Gullett, who is our fisheries biologist here at the
East National Technology Support Center in Greensboro. His topic today is “Low-water
Crossings for Livestock and Equipment. Planning for Aquatic Organism Passage.” There are no CCA credits
today, but we welcome all of your questions at the
end of our webinar today. So if you would like, you can enter your
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do our best to get you an answer. Or if you want to hold your
questions to the end of the webinar, you can ask those verbally through the operator. And with that, we will get
started with Mr. Kale Gullett.>>Kale Gullett: All right. Hello everybody out there and
thanks for the introduction, Holli, and thanks to all of you folks for
dialing in and logging on today. All right. So over the next 45 minutes
or so I’ll provide an overview of aquatic organism passage
at low-water crossings. I won’t have a lot of time to go in great detail
about much of this material, so please feel free to contact me at any time to further discuss or get additional information
on anything I talk about today. In addition, you’ll see a number of citations
from handbooks and journal articles on some of these slides, and please let me
know if you’d like copies or web links so that you can get them on your own. This net meeting is the fourth
associated with the center presented over the last four years
concerning aquatic organism passage. The first part of the series sponsored
by NRCS biologists was moderated by me and included Scott Jackson, Kozmo Bates,
and Dick Quinn [assumed spelling]. These national experts discussed
passage ecology, culverts and tide gates and fish ladders. The second and third delivered by me during
other monthly net meetings covered largely the same material with special emphasis on passage
ecology and tools relevant to the east region. I encourage you to consult these three
presentations for additional basic information on passage ecology and techniques. They’re archived online at
the URL shown at the bottom of this slide, or I can send any by request. Today, I’ll focus on low-water crossings and
how to plan and design these structures to fit within landowner operations and minimize
crossing effects on river function and ecology. In addition, I hope to highlight
some elements of planning and design that will enhance structural longevity and may
minimize maintenance and replacement costs. Low-water crossings, or fords, at
least in the context of today’s talks, are sites where farm access roads or other generally unimproved
roadways cross water courses. Their primary use is to allow land owners
to access other parts of their operations with equipment and to provide
livestock with safe crossing points. The primary goal of a low-water crossing is
to provide safe and stable stream crossings that don’t negatively impact water and ecological quality while remaining
stable across a wide range of flows. So a recurring theme that I’ll
repeat a few times today is this; road-stream crossings are
meant to be static sections in the most dynamic part of the landscape. They often pose passage problems
and impair stream stability. Although the theme of today’s
talk is aquatic organism passage, I’ll spend much of my time talking about
how streams interact with the landscape. My reasons for this approach are, first, that
understanding how a river is supposed to look at a crossing is key to diagnosing the reasons
passage and site stability might be impaired. Secondly, you’ll need to know
a bit about river mechanics to know why a crossing is a
barrier and how to fix it. Third, using a geomorphic approach
to assess passage condition is useful because we don’t know the locomotive ability
of a whole bunch of aquatic organisms. And finally, using a geomorphic
approach to design and build crossings will usually increase
longevity and decrease maintenance activities and lost access time to parts
of a landowner’s operation. In the administrative context, I’d guess that many projects involving road-stream
crossings would include the first two practice standards listed on this slide. The first, Standard 578, or Stream Crossing,
is presently out for public review and comment as part of the standard revision process. The first criterion in Standard 578 calls
for us to design the stream crossing to accommodate passage of all
life stages of aquatic organisms in accordance with species requirements. In addition, 578 refers the practitioner to
Standard 396, or aquatic organism passage, for additional design guidance on projects
where passage is a resource concern, or a stated desire of the land owner. Although 578 contains a number of
criteria that dictate the type, appearance and design specifics of a stream crossing,
I’d submit that using a geomorphic approach that is trying to place the site in its
watershed context and designing a crossing to fit within that context to the fullest
extent possible will provide the best mix of passage conditions for all the
aquatic critters we may encounter and long-term stability in
the face of a full range of streamflow and the materials it carries. Road-stream crossings are a common
feature across the United States. For example, recent forest service estimates for
forested landscapes in the eastern U.S. suggests that there are about two crossings
per mile of improved forest roads. Although I’m unaware of similar
data for private roads and trails, I would guess that crossing densities are
very similar and may actually be higher. Most existing crossings– whether
they’re culverts under an improved road or low-water crossings on
unimproved private trails– were likely not designed and
constructed with passage considerations as a central element of the design process. For the most part, the idea of building for
passage at stream crossings is relatively new, only coming to the forefront
in the last 25 years or so. Because stream crossings are
intended to be stationary elements of a decidedly dynamic landscape, they affect
a number of aquatic and riverine functions. Broadly, they affect many aspects of aquatic
ecology including species distribution and population health and often impede
sediment transport, woody material cycling, and elements of surface and
shallow groundwater flow. From a management perspective, stream
crossings need to provide safe, reliable access for land owners and
the things they need to make a living, specifically livestock in various
sizes and kinds of equipment, a way to get across a water course. I’m sure many of you have seen stream crossings
that present barriers to migratory organisms. All animals, both aquatic and
terrestrial, need to move for one reason or another throughout their lives. Whether it’s to reproduce, colonize new habitat,
seek refuge from adverse flow conditions, or forage for food, movement is a necessity. Daily, weekly, or yearly migrations are common and somewhat predictable
for a number of organisms. Although most commonly known
as fish passage in years past, the business of getting aquatic
animals from one point to another is now being increasingly
referred to ‘aquatic organism passage’ because of the wide range of animals for
which passage projects are being completed. The title of Standard 396 was changed
this year to reflect the broader purpose of many instream construction projects. Passage barriers can have persistent and
significant effects to aquatic species. For example, a recent study in mountain streams of northern Japan provides an
interesting example of the effects of fragmentation in riverine systems. Sabo, or check dams, have been
built by the thousands in an effort to control sediment discharge and
mitigate major damage from debris flows and landslides generated by
precipitation, earthquakes, or volcanoes. These dams have been built for hundreds
of years, but the biggest structures and greatest numbers have been
completed in the last 50 years or so. They provide some protection
from major sediment events but only have a finite amount
of storage capacity. They range in height from more
than 100 to less than 5 feet, but their effects on white-spotted char– resident salmonids related to
brook trout– have been dramatic. These fish exists as larger
migratory individuals and small resident fish that
normally interbreed. But sabo dams are complete
barriers that have isolated habitats and populations into many unconnected fragments. Two researchers have shown that the
probability of occurrence– or just survival– is related to watershed size, and that genetic
and population problems can occur in as little as 30 to 35 years following isolation. In addition, the researchers saw physical
deformities on fish during fieldwork that are usually associated
with small population genetics. They’ve speculated that about
one-third of the population they studied with disappear in the next 50 years. Other studies closer to home in the Midwestern
and eastern United States highlight the effects of stream crossings on aquatic ecology as well. Warren and Pardew looked at a number of road
crossings in the Ouachita Mountains in Arkansas and found marked population
differences above and below crossings. They also suggest that undersized culverts and concrete slab low-water crossings
presented the most significant passage barriers to all species. Ben Letcher and his colleagues looked at a
number of crossings in western Massachusetts and found that barriers fragmented
local populations. They also completed a modeling
exercise that suggested that local extinction could occur
relatively quickly without replacement from downstream sources, in this
case, between two and six generations. Bouska and Paukert looked at crossings in the
Flint Hills eco-region of east central Kansas and found that crossing types strongly influence
species passability and that crossing slope, length and width– especially
at low-water crossings– had a profound influence on passage success. And finally, Keith Nislow and his colleagues
in a paper published a few weeks ago, looked at a number of crossings on the
Monongahela National Forest in West Virginia and found that species composition and abundance
above impassable crossings was half that found above reaches with passable crossings. So restricting animals to small
or isolated habitat patches by impairing migration has
a number of consequences. Complete barriers can, over years, extirpate
migratory animals in upstream reaches. Fragmented populations are more
susceptible to serious decline or extinction and reduced gene flow can lead to genetic
problems that limit population fitness. Impassable crossings can
affect population demographics, specifically diversity, abundance
and spatial structure. And finally, populations
that are poorly distributed across a river basin have little
or no buffer to catastrophe. Evaluating the effects a road-stream
crossing has on passage can be difficult. In a few cases, it’s obvious– like these
kokanee blocked by a culvert in Idaho. But for the rest of the cases– those where there aren’t any bright red
fish blocked below a culvert– gathering information about the stream system
will help you understand how a crossing affects stream and passage condition. Again, using a geomorphic approach will
allow you to assess passage condition without a detailed knowledge
of individual stream organisms, their mobility or migratory habits. Quantifying the departure of a stream from its “natural state” will help you describe the
relative passability of a stream crossing and provide insight into designing
for aquatic organism passage. Climate, physiography, and geography conspire
to create watersheds which is where we work. Identifying the factors that govern landscape
condition is key to many of the types of work we do, and it’s the same
thing for aquatic organism passage. Placing a crossing within the larger context of its watershed will help you quantify the
effect a site has had on stream morphology. We know that watersheds can be
segregated into three component parts — headwaters, transfer and depositional. Knowing where a given road-stream crossing
fits within this continuum will lend insight into a whole bunch of things, like average
substrate size, channel gradient and planform, the relative influence of
vegetation on channel stability, and the important species and habitats. Since the basic function of a river is to move
water, sediment and organic material downstream, we can classify the stream
segment affected by a crossing as either a transport or response reach. Now, here’s a good question to ask
yourself the first time you visit a site. Is the main function of this
reach to move material through it or to respond to the material delivered? So here are some examples of transport
reaches in North Carolina and New Hampshire. Transport reaches are characterized usually by
steep gradients, boulder and cobble substrates with pockets of gravel and smaller material,
and narrow or nonexistent floodplains. Channel morphology is often cascade or step-pool
types, and the banks and beds are durable because they’re made up of big rock
and well-embedded large woody debris. Bankfull channel indicators can be hard to find because some reaches aren’t
fundamentally alluvial. Many of these channels are composed of lag
deposits left by landslides, debris flows or glaciers as in the New Hampshire photo. So stream crossings in these systems
need to be steeply sloped and able to pass basically constant and periodically
high bedload rates and large pieces of wood. Limited interaction between the river and its floodplain confines most high
flow events to the channel margins. Channel changes associated with
crossings are usually localized to the area near the crossing itself, extending
up and downstream only a few channel widths. Virtually, all other stream segments in a
watershed can be classified as response reaches. Slopes can be locally steep, but
the major differences are seen in overall smaller substrate sizes, more complex
channel morphology, and the common presence of a well-defined floodplain
bracketing the river. Plane bed and pool ripple architecture– as seen in these streams in
Tennessee and Maryland– is common. Substrates are mostly cobbles and gravels and riparian vegetation helps govern
channel stability, location and geometry. Lower elevation response reaches usually have
pronounced meanders and complex morphology, like this river in South Carolina, and include
dune-ripple channels such as this stream in Alabama just a few feet from Mobile Bay. Substrates are generally
small and in large supply and large woody material
loads can be substantial. Response reaches are where the
majority of our customers live and work and where we are most often asked to help. These reaches are sensitive to landscape
and hydrologic changes within a watershed, and channel adjustments can
be significant and fast. Consequently, crossings in response reaches must
be designed with consideration for a wide range of channel changes with the potential to extend
for significant distances up and downstream. These sites can be complex, but,
knowing the general tendency of a reach will help you identify
the general trajectory of that reach with respect to geomorphic change. But taken as a whole, road-stream crossings
often impair physical and chemical processes that create and maintain habitat. They also interrupt sediment and wood
transport cycles, alter shallow ground water and surface water flow patterns
and can shift riverine habitat into areas more similar to lakes and wetlands. Road crossings are meant be static in a changing
environment– a really tough standard to meet. And in many settings, especially where
sediment and large woody debris loads are high, maintaining low-water crossings
can be difficult over time. However, looking at a few watershed
factors during the planning and design phase can improve structural
longevity and decrease the amount of maintenance necessary over time. Determining the watershed context
of a crossing site helps you begin to quantify the potential streamflow, sediment and debris transport
regime that may occur at that site. Site geomorphology helps you understand a
number of the design challenges you might meet and also provides insight into the type of
structure that may best fit the crossing. In addition to knowing where you are
within the watershed and its transport or response potential, it’s also important
to identify conditions that’ll affect channel and floodplain processes at your crossing site. For example, landslides can introduce huge
sediment and debris loads into a channel. Active head cuts adjacent to your crossing
site can migrate and endanger longevity. Analyzing the flow regime will
help you characterize flow extremes that can affect passage and crossing dynamics. Further, the effects of a current crossing
on stream morphology can be significant and will require consideration
during the design process. Taken as parts of a bigger picture, considering
these factors can help you diagnose present condition and put some error bars on the
range of changes that may occur in the future under alternatives proposed
for a stream crossing. At site scale, the relationship
between the active channel and floodplain is an important determinant
in the suitability of the cross section for a low-water crossing, and also
influences the type of structure installed. One useful descriptor of this channel-floodplain
relationship is the entrenchment ratio, defined as the floodprone width
divided the bankfull width. In areas where floodprone width is
difficult to determine from site factors but bankfull is relatively well expressed,
the floodprone width can be estimated by extending the water surface elevation
across the floodplain at an elevation equal to twice the maximum bankfull depth. The entrenchment ratio is a good index
to the degree of incision of a channel, either as a natural component of
the watershed or as influenced by past management activities
or changes in the flow regime. As far as low-water crossing siting and
design goes, the more entrenched a channel is, the less suitable it is for
a low-water crossing. Although, exactly quantifying the
entrenchment ratio isn’t necessarily critical for designing a low-water crossing, it’s useful
to index because the degree of entrenchment at a site is an important factor affecting
how steep the road approaches need to be, how sharp the vertical curve
of the crossing will be, and how well the stream can be protected from sediment produced along
the road surface and ditches. In general, more entrenched settings
are better suited for culverts, bridges and vented low-water crossings. Less entrenched sites are well suited
for unimproved or improved fords. Another aspect of considering watershed factors
during the first phases of a passage program or individual project is to consider the use
of formalized inventory and analysis methods. Formalized methods can help
guide data collection and will provide some very useful information. In general, these additional details will help
you quantify passage condition across a range of flows, prioritize crossing replacements
within a watershed, initiate design and approximate replacement
or construction costs. Here’s a great resource to evaluate passage at road-stream crossings published
by the Forest Service in 2005. Most western states have some form of a protocol
for evaluating crossing and some eastern states like Massachusetts, Maine and Vermont have
produced documents in the recent past. By the way, if you’re interested
in using the national procedure, the Forest Service recently completed
a 30-minute online tutorial found at the website shown on this slide. It’s a video primer that covers
all of the data collection methods and techniques necessary
to complete the procedure. In addition, the tutorial stops after
each section and illustrates how to fill out the inventory worksheets. Although this primer is focused on culverts, many of the concepts are directly
suited to low crossings as well. Both inventory and assessment
protocols require various field methods and this guide covers the most common techniques
including basic topographic surveying, measuring channel cross sections and
longitudinal profile measurement. I highly recommend this guide because
it’s clear and concise and it covers many of the field methods and techniques necessary
to analyze and design stream crossings. So, with some of those larger landscape factors
in mind, and maybe a few of those field guides in hand, let’s move in to a general discussion
about low-water crossings and then talk more about types and designs with
reference to watershed location. Low-water crossings or fords are often
associated with historic sites and trails used by native peoples and early explorers. Generally, they’re found at sites created
by hydraulic controls like riffles or bedrock outcroppings that offer a uniform, relatively shallow spot that
remains stable over time. This site, Few’s Ford on the Eno
River near Raleigh, North Carolina, was associated with a mill and trading
trail dating back to colonial times. Archeological evidence from nearby sites
suggests that this crossing was probably used by native people long before
Europeans landed on the continent. Improperly designed and built low-water
crossings block aquatic organism passage such as shown in this picture. However, like other road-stream
crossings such as culverts or bridges, low-water crossings can also pose
a risk to livestock and equipment and cause chronic water quality problems. Low-water crossings can be built with many
types of equipment commonly found on farms. Front-end loaders or a blade on a three-point
hitch can easily be used to push material around and into place across the stream. However, unless a ford is placed at a site that
offers natural stability and built according to a few design principles
that work with instead of against the stream, it
may not persist over time. So before we get into some of the specifics
regarding types and design elements of crossings, I’d like to mention what I think
is another great Forest Service reference for designing low-water crossings
according to biological, geomorphic, and engineering considerations. This manual provides a comprehensive review
and approach to methods used at a range of different low-water crossing
types with extensive design guidance and a great set of case histories. Although much of the information presented
is geared toward road crossings larger than the types of routes NRCS is involves with,
the guidance on site and structure selection, design and construction considerations,
and fitting a crossing within a larger set of watershed factors is very
applicable to what we do. So in general, low-water crossings are built on
low volume or unimproved roads on private lands. They’re attractive because
they’re generally cost efficient and can be relatively easy to maintain. In addition, they can be designed
and built to pass large sediment and debris loads with some efficiency. However, because they are a static
structure in a dynamic system, they affect river mechanics
and aquatic organism passage. Planning for passage on a given
operation could be easily incorporated into our overall planning process. The nature and extent of a landowner’s
operations are key pieces of information needed to determine use and locations
of crossings on a property. For example, an essential part of designing
crossings involves knowing the types of equipment used on a farm and
the number and kinds of livestock that might be using a crossing structure. The type of equipment used on a farm
has a direct influence on the type of and design of a low-water crossing. We deal with a large range of
different operations that use implements that vary in size from big to small. In addition to considering the size
of individual pieces of equipment, we also need to account for
various other equipment and implements pulled by tractors and trucks. The physical dimensions, weight, turning
requirements and clearance of single vehicles and towing combinations are important factors
that dictate crossing design requirements. One critical consideration I would like to
propose during the planning process is a review of all road-stream crossings on
a property to determine whether or not crossings can be removed and
consolidated to the fullest extent practicable. This is very much site specific and requires a
lot of cooperation on the part of the landowner, but removing as many crossings as possible
while preserving the daily operations of a producer goes a long way toward
improving stream health and function and preserving good passage
conditions on a parcel of land. For example, here is an aerial image of a small farming operation south
of Greensboro, North Carolina. The landowners have a mixture of
pastures, hay meadows and crops that all require seasonal movement of equipment
and livestock — in this case, cattle — between different parts of the
farm, primarily from the barns and equipment sheds near the house. They presently use four stream crossings
to access all parts of the property from the farm headquarters and spend some time
on each one every year performing maintenance to keep the crossings clear and operational. Since the farm is bounded by
county roads on all sides, at least three of these crossings could
be removed if the landowner is willing to make changes to present operations
by using existing roads to move cattle and equipment to different parcels. Outlying parcels where access via other means
isn’t available or feasible could be reached by consolidating three total crossings
into one near the house because it’s a site that offers better stability
and less maintenance primarily because of the physical condition
of the stream — it’s located above a bedrock
outcrop at a natural gravel section and requires little maintenance over time. So this sort of planning obviously requires
the input and agreement of a landowner, which is part of what we do, and may be affected
by county ordinances concerning the use of roads for moving equipment and livestock. Granted, it’s a simplified example with other
workable outcomes but the gist is to remove as many of the water crossings as possible
within the context of the watershed and the daily operations of the landowner. So once you’ve done what you can to remove
and consolidate crossings, it becomes a matter of selecting the type of crossings suitable
for both the landowner’s needs and the type and location within the watershed
of the streams on the property. Here are some common types and categories
of low-water crossings in use today. The top two consist of simple fords located
at naturally stable features or fords at similar control sections improved with
materials that harden the travel surface. The middle two are known as vented fords and
consist of a hardened road surface underlain by a corrugated metal or concrete
box culverts, for example. The bottom structure is a low-water bridge
where the road surface is supported by piers or spread footings with a natural
channel bottom and the structure is meant to be overtopped by larger flow events. As you may imagine, these things range
in cost, construction requirements, and their effects on stream
and passage continuity. I showed a couple of vented fords
in the previous slide and one metric that describes the effect
of a low-water crossing on steam function is the
vent-to-area ratio or VAR. VAR is the ratio of the constructed openings
of a crossing to the bankfull area of a stream. Low VAR crossings are those where
the area of the vents is much smaller than the bankfull flow area of the stream. High VAR crossings are those where the
area of the vents is greater than or equal to the bankfull flow area of the stream. So in general, fords, high VAR crossings and low-water bridges all support
good river function and passage. Low VAR crossings and other
structures that affect sedimentation, debris dynamics impair passage
in much the same way as culverts. They create sections with shallow depths,
high velocities or elevation drops. The magnitude of these barriers often
scales with stream size and flow. Often, the more improved the crossing
is, the bigger a barrier it creates– assuming of course that the crossing
wasn’t designed specifically using passage considerations. So here are a couple of examples
of low VAR structures. These crossings in Illinois
and Texas are both jump, depth and velocity barriers to
migratory aquatic organisms. Further, they basically operate as low
head dams and force a slack water reach above the crossing that affects stream dynamics. By comparison, simple unimproved
or improved fords such as this one in northwest Arkansas provide year-round
passage and can remain stable for years. Granted, they won’t support
the same equipment load as the two crossings shown
in the previous slide. But, fords like this represent a good
number of the crossings that NRCS installs. Simple fords offer a good mix of construction
ease, stability, safety for livestock and equipment, and generally work
well with the stream over time, which equates to good passage conditions,
good stream function, and minimal maintenance. Low-water crossing design and analysis should
roughly follow the same procedures I’ve outlined for stream simulation culverts in years past. In fact, the low-water crossing document
I mentioned earlier suggests the use of stream simulation techniques
for low-water crossings at high VAR structures as
well as improved fords. Consequently, it’s best to characterize
the nature of the reach as either transport or response and account for the
flux of watershed materials. Generally, crossings tend to function best
when they are designed and built to conform to existing channel geometry and
slope, constructed to match the shape of the existing channel and oriented
across the stream at a 90-degree angle. Cross-sectional geometry has a big effect on the
type of crossing structure that’s appropriate, will provide safe and reliable
access for livestock and equipment, and will persist over time
with minimal maintenance. The concept of channel entrenchment
is useful here because entrenched channels
require different structures than a relatively unentrenched channel. Entrenched systems focus flow, sediment
and woody debris in a small area. Stream crossings have to account for
these factors in a design process to maintain stability and passage quality. So this table highlights some
of the major site factors– usually attributable to larger
watershed processes– that influence the suitability of four
general categories of crossing structures which are shown at the top as column headings. The four general categories are taken
from the previous slide that I showed — unimproved fords at naturally rocky sections,
improved fords outfitted with material that hardens the travel surface,
high VAR fords with metal or concrete box culverts and low-water bridges. The table highlights the applicability
of structures at locations affected by common watershed or site factors. All four categories provide some major
of passage and stream continuity– some better than others– and each is
better suited for certain sites affected by watershed factors, channel
geometry or flow regime. For example, unimproved or — excuse me,
unimproved and improved fords are better suited for sites with flashy streamflow or very low
base flows because they usually minimize changes to a channel cross-section as compared
to high VAR fords or low-water bridges. Conversely, using an unimproved or improved ford
is usually not advised in entrenched channels or broad channels that move
a lot of water at base flow. So I’ll go over some examples of
improved and improved fords, vented fords, and low-water bridges in the following slides. Rock fords– whether unimproved or improved by
the addition of rock fill or other materials that harden the crossing and
support equipment loads– are commonly used in the
eastern U.S. As many of you know, one of the most important design
considerations associated with this type of stream crossing is placement–
the best mix of passage and stability conditions will be
found when the crossing is sited at a naturally stable section of stream. Bedrock outcroppings, riffles, or steps composed of coarse material are great
locations for crossings. If these sites are found within the
landowner’s property, we can go in and improve the approaches, add material
to the stream adjacent to or on top of the naturally stable section, and use
fencing and gates to control livestock access. Design and construction costs can
generally be managed to small figures and sometimes on-site equipment and materials
can be used to complete the crossing. Improved fords are generally
constructed by opening up the stream above or atop a natural control and
backfilling the cut with a well-graded mix of rock back to the existing bed elevation. Sometime it’s necessary to augment or improve
the natural control with larger material. Depending on sediment texture and composition,
it may be necessary to line excavated sites with geotextile to control seepage or
cutoff subsurface flow that might flow under instead of on top of the backfill. When rock is added to a crossing
to improve the travel surface, there’s a danger of building a sediment
surface that creates passage problems because of turbulence or
depth and velocity problems. In addition, if rock backfill isn’t well graded, water can flow through instead
of on top of the travel surface. So the best conditions for improved fords are
usually realized when the backfill is composed of a mixture that mimics bed material as
evaluated from a reference reach adjacent to the crossing– preferably upstream and away
from any influences of an existing crossing that might have made the
crossing section different from a reference reach or a self-formed reach. A good rule of thumb for getting away
from the influence of a stream crossing is to move upstream at least 20 bankfull channel
widths before analyzing channel geometry and bed material composition. Moving this far upstream helps you get
away from any existing crossing effects that alter channel geometry or
bedform composition and spacing. In addition, it helps gets you
outside of what might be considered as the crossing maintenance zone where equipment
operations have affected channel metrics. Developing a backfill mix that approximates
the natural bed material requires a bit of characterization with
respect to the streambed. More data is usually better but a
backfill mixture composed of at least three to four particle sizes usually provides
good bed stability and passage quality. There are an awful lot of substrate
analysis methods out there. The one you use is likely going to be a function
of familiarity, maybe regional calibration or ease of use; but whatever methods you choose,
I would suggest that it’s important to account for bed shape and particle diversity. You might consider developing your own roughness
information if extensive modeling will be used, and that also helps you to gain a bit
of insight into streambed mobility. In addition, it helps to note
features that force bed morphology. An important example of this would be
boulder-forced steps in mountain channels. It also helps to investigate any
watershed factors that control permeability or that may be a function of site construction
activities like compaction or excavation that may interfere with shallow
groundwater flow. For example, each of these
unvented improved fords in Oregon, Virginia and North Carolina match the channel
shape and slope, crossed at a right angle and the bed surface is composed of materials
that closely match the natural bed forms. Consequently, they’ve been relatively long lived and provide good passage
across a range of flows. The backfill mixtures aren’t complex and
can be created at a quarry or borrow site so that trucks deliver a prescribed fill
mixture for direct placement into the section. You can also stockpile each size class on site and create the backfill mix using
construction equipment at the site. Buckets on loaders or excavators can be used
for measuring out rough composition as well as on-site mixing before
placement into the stream crossing. So here’s a shot of an excavated
crossing being backfilled with rock layers composed
of about three size classes. Once rock is dumped in layers, it
can be difficult to mix the materials so that the resulting streambed
surface is well graded. At sites where passage is a primary project
purpose, it’s usually better to mix the backfill at a quarry or staging area, place it
into the cut, and blend it laterally to the road approaches and
up-and-downstream bed forms. Rock size and gradation should be the
smallest mix needed to remain stable under prevailing flow conditions– bigger
isn’t necessarily better for passage because of turbulence and can also
create problems for livestock. You may need to add sand or soil into
the mix to seal the section to ensure that the stream doesn’t percolate
into the crossing substrate. In addition, smaller material increases bed
diversity, chokes voids between bigger stones and helps preserve passage quality. It’s also important to extend the driving
surface of the crossing to an evaluation that exceeds the known high water level. This design element is a criterion
in the Stream Crossing standard. Keeping the crossing at grade,
providing surface roughness and ensuring that the downstream edge doesn’t produce a
sharp drop in water surface is the best way to keep material and animals
moving through a system. Cable concrete or articulating concrete
block fords are a cost effective means to improve the stream crossing in situations
where traffic volume, average load sizes or the energy regime might be higher. They’re usually made of one-foot square concrete
blocks connected to each other with light cable. Block mats are usually available
in pre-made sections in widths of 4 to 8 feet and lengths from 8 to 16 feet. The concrete blocks are usually
between a couple and 8 inches thick. Some come with a geotextile
backing already attached but if not, a layer of geotext should be placed on the
prepared subgrade before the blocks are placed. Scour protection around the mat area is
important and this can be accomplished by placing a single roll of blocks at least 6 inches below the finished
subgrade around the perimeter of the mat. Unless the subgrade is uniform, smooth and
well compacted, the blocks can shift over time. Each block is independent of its neighboring
blocks and can settle, rotate or tilt. If this occurs, it can result in damage
to the mats and a rough driving surface that may be dangerous to livestock. Once the blocks start to move, scour can
endanger the longevity of the crossing. So, subgrade preparation is a key element of
crossing longevity for cable concrete fords. So here’s a few shots of a crossing project
in Oregon showing a cable concrete mat and excavator prior to installation, a
juvenile Chinook salmon moving upstream between the blocks the day after it was built, and the crossing after the first
rain-induced high flow event in the fall. Voids between the blocks can be backfilled
with smaller material following construction, but they tend to in-fill naturally in streams with relatively intact sediment
transport regimes. Here’s a shot of a cable concrete crossing on
a relatively steep transport reach in Wyoming where the mat was extended up to
bank line as high as necessary. In addition, the subgrade at this
crossing wasn’t well compacted because the presence of large boulders. Erosion and settling at the site has
exposed some of the structural cables, which can be a hazard to passing vehicles. It’s rough to drive across, but
functional from a passage perspective. However, sites like this where the blocks have
begun to move around often fail over time. Another form of improved ford involves
the use of concrete as precast planks or reinforced poured-in-place
slabs across the stream. They can be relatively simple
but also more expensive than articulated mats or rock-hardened fords. They’re durable and well suited for systems
with flashy runoffs and high debris loads. Concrete slab fords offer a very stable,
smooth travel path, but if the surface of the slab gets covered in algae,
they can be very slippery to livestock. A smooth concrete surface can
also be a problem for passage, but if the crossing is fully submerged and set to match stream gradient,
passage can be satisfactory. Sometimes low flow passage slots
are incorporated into slab fords but these can be pretty hard to maintain
and tend to fill up with bedload. Passage slots sometimes self-scour
at higher flows but problems generally arise during low flows
when deposition in the slots blocks access and organisms are forced to try
and negotiate the slab surface. Okay. Here are a couple of
examples of concrete slab fords. This crossing uses hog slats
on an ephemeral stream where passage may be possible at high flows. Here’s a poured-in-place slab crossing in
Arkansas that was submerged at the time of the photo, but has a small drop along the
downstream edge that might present a problem for a weak-swimming fish or animals that crawl. Slab fords can be a problem for passage again when the crossing surface is wide
and flows are shallow and fast. So it’s important to do all you can to
design these things to be underwater at all but the very lowest flows. Another way to improve a ford is to use geocells
or plastic cellular confinement structures. This technology is best suited
for low gradient streams with bed material consisting of sand and gravel. As with some of the other
approaches I’ve talked about today, the subgrade for a geocell crossing should
be smooth, uniform and well compacted. This requirement coupled with
the need to properly expand and place the geocells before filling them means that the crossing site should be completely
dewatered prior to and during construction. Geocells should be placed on geotextile and
then backfilled with a mixture of small gravel. Finally, the cells should be
covered with a mix of larger gravel, and the top of the crossing should be kept
to the level of the natural streambed. Keeping a cover layer on the cells is important. Tire damage can weaken them
and result in elevation changes that may affect sediment transport, crossing
stability and safety for livestock and equipment when uncovered geocells become
part of the travel surface. Here’s an example of a geocell crossing and you can see the cells at
various areas along the ford. Although not presently an issue for
passage, the long-term stability of this crossing is a problem unless the
geocells are covered with a surface layer. And I’d guess that a bit more excavation
at this site would’ve helped preserve cover at the crossing, improving its function
for farm operations and passage as well. All right. Vented fords are found in many locations
across the Midwest and eastern United States and most were built between the
50s up to about the mid-80s. These fords are commonly consist
of multiple small culverts set at or near the streambed level that were covered
with backfill and commonly capped with concrete or asphalt to harden the embankments
along the upstream and downstream sides. Vented fords pass low flows through the pipes
and are meant to be overtopped by high flows. Older-vented fords typically have a low
vent-to-area ratio and basically act like dams, causing upstream backwater and
aggradation and sometimes downstream scour. Passage is often a problem because
culvert outlets can be perched, and if not perched often
have high flow velocities. New approaches to vented fords that maximize
the vent-to-area ratio offer much better passage and perform better across a range of flow,
sediment transport and debris regimes. High VAR fords can consist of bottomless
arched culverts in smaller steams, and multiple concrete box
culverts countersunk and backfilled with streambed material in larger settings. So here are a couple of examples
of vented fords. This is an older ford in Missouri that illustrates a commonly
constructed low VAR crossing. Multiple culverts set at different
elevations are capped by a concrete layer that extends along both margins of the
crossing to armor it from overtopping flows. Passage conditions are suboptimal
and although one of the culverts is fully submerged,
velocities are fairly high. This example from the Ouachita Mountains
in Arkansas shows a new concrete box, high VAR ford that maximizes the
channel area across the section and is bedded with natural stream substrates. Both passage and stream continuity are
largely preserved in a crossing like this. The final example of a low-water crossing
that I’ll cover today are low-water bridges. These are low bridges, and the deck or
crossing surface is commonly supported by piers or spread footings. The elevation of the deck is set above bankfull, yet low enough to be overtopped
by larger floods. These structures are expensive but as you might
imagine, provide the best passage conditions and channel function of all the
crossings that I’ve covered today. Because they’re used in larger streams and
intended to be overtopped by large flows, low-water bridges can be fairly robust and
require a lot of excavation and concrete work. Given the costs involved, they’re
generally used on higher volume roads and are an attractive alternative
to conventional bridges. Low-water bridges need to be outfitted with
some form of railing or curbing for safe vehicle and equipment passage because
they’re usually lumped in with standard bridges
in the permitting context. However because they’re intended
to be overtopped by high flows, railing designs can be tricky
because of the need to balance regulatory height
with passing debris loads. Here are a couple of examples
of low-water bridges. This one is in Montgomery
County, North Carolina, and concrete footings support steel
eyebeams that support the single-lane road. This crossing on a national forest in
Minnesota spans the active channel and a portion of the floodplain and uses concrete piles to
support the concrete deck on a two-lane road. Each of these structures provides great passage
and preserves a good bit of channel function and stream continuity along
the road- stream crossing. Crossing a stream at right
angles is the best approach, and is a criterion of the
Stream Crossing standard. However, aligning a crossing and its attached
road at a right angle to the downstream axis of the channel also creates runoff
pathways that can pulse large amounts of sediment to the stream system. Controlling approach runoffs and
sediment is best done by creating roadside and leadoff ditches associated with water
bars or dips as far away from the channel and onto the floodplain as possible. In addition and depending on
site conditions like road, grade, soil type and expected rainfall, it also helps to extend road surface armor
upslope to the nearest cross-drain. Handling runoff and sediment before it reaches
the channel will help protect water quality and may promote crossing longevity. Another criterion under the Stream Crossing
standard calls for the use of fencing and gates to manage livestock use of the area. As many of you know, it can be difficult
to maintain fences that cross streams. Sometimes a fence that won’t get torn up by
high flow and debris doesn’t do a good job of excluding livestock, and fences that
handle livestock well are often great at catching debris. These shots illustrate a cross-fencing
technique that I’ve used in the past on projects that seems to do a good job of handling
stream debris and excluding cattle. Sections of 1-1/2 to 2-inch PVC are hung
from a cable stretched across the channel, and turnbuckles at the cable ends help
tension it when it becomes stretched or if the posts sag or move a bit. Each pipe section is separated from the
next by rubber hose or smaller PVC slipped over the cable with spacers
and the pipes could be cut to various lengths to match the channel’s shape. These things swing when pushed by high flow
and even provide boat passage in larger rivers. Gates for livestock control are essential because given time cattle will figure
these things out and push on through. All right, one final consideration I’d
like to cover today is crossing safety. I think it’s important to emphasize
that many of the techniques I’ve talked about are crossings intended for
use under low-water conditions. Things can get dangerous as flow
depths increases at a crossing. Both lateral and buoyancy forces increase with
depth, and muddy water can hide any scour damage that may have occurred during high flows. So taken together these factors create
potentially life-threatening conditions at flooded fords. Consequently, we should inform landowners
about the dangers of flooded fords and include signage whenever we can. All right, that’s all I have today and I
really appreciate your time and attention, I know I’ve blasted through an awful lot
of stuff here; but before I turn this over to Holli, I’d like to
mention another upcoming webinar. I’m going to work on for delivery
at the end of January 2012. The working title is “Stream Habitat
Management, Assessing Stream Condition and Identifying Management Options.” And with that, I’ll now turn this back over to
Holli and would be pleased to hear any questions or comments that you have
regarding today’s webinar.>>Holli Kuykendall: Thank you very much, Kale. And Operator, can you please come
on and help us with our Q&A, please?>>Operator: Thank you. We will now begin the question-and-answer
session. If you’d like to ask a question, please press
‘star one,’ make sure your phone is unmated and you must record your first and last
name slowly and clearly for introduction. To withdraw your requests,
you may press ‘star two.’ Once again, for a question or a comment,
please press ‘star one’ at this time. One moment while we wait for questions. I would also remind our participants
that if they’d like to type in a question in the Q&A we can certainly
field those questions as well. And I’m currently showing no questions. Again, as a reminder, it is ‘star one.’ Un-mute your phone and record
your name for introduction, and it is ‘star two’ to withdraw that request. Again, if you have a question or a comment,
please press ‘star one’ at this time. One moment while we wait for questions.>>Holli Kuykendall: Well, we have one
question about printing the presentation and I would refer our participants to our
new science and technology training library. In about a week’s time we’ll have Kale’s
webinar and presentation up at the science and technology training library
where you’ll be able to access those files directly
and print or use as you need to.>>Operator: And I’m currently showing
no audio questions or comments.>>Holli Kuykendall: Okay, one
more logistical question kind of along the same lines about the replay. In just a few minutes I’ll show a
URL that refers you to the science and technology training library, plus if you visit the East National
Technology Support Center’s website through the NRCS website, you’ll
find our webinar’s page there. So you could either do a Google
search or you can contact me or Kale and we can help you get there. We’ve got one more question that was typed in. Kale, this says, “The North Carolina
DOT requires culverts be buried 0.2…>>Kale Gullett: …diameter.>>Holli Kuykendall: Okay, and what
is your recommendation for passage?>>Kale Gullett: Well, first of
all, thanks for the question. Countersinking culverts, you know,
thank goodness we’re starting to do that quite a bit more all over the place. My response to sort of a uniform applies
everywhere all the time embedment depth, or a countersinking depth, is — you
know, for me you’re really trying to maximize the pipe opening in
relation to the size of the stream. Other factors like sites,
slope — the crossing slope, bed material size obviously
affect cover inside of the pipe; but the biggest I guess factor I’ve seen that
promotes the best passage conditions and, you know, under which culvert stick around the
longest is when you can put a pipe on there that matches bankfull width
as close as possible. So I don’t know if that answers your question
but sometimes, for example, you know, you can get to a little bit wider portion of
the pipe by sinking it a little bit deeper. So it’s interesting that they’re
requiring culverts to be buried to — I guess, that’s two-tenths of the culvert
diameter, but for me, again, it comes to trying to match that culvert opening whether that’s the
span or riser width, excuse me [clears throat] to closely match the stream width, that’s
where you get the best bang for your buck and they seem to stick around the
longest and provide the best passage.>>Operator: And I’m still currently
showing no audio questions at this time.>>Holli Kuykendall: Okay. Well, I think that probably concludes
our Q&A and I’m going to pull up now the replay address that
people were interested in. Again, this is kind of a screenshot of our
new science and technology training library. You’ll see the URL to reach
the library below that image. Feel free to contact me or Kale if you’re
having difficulty accessing that site. We’ve mentioned that Kale’s previous
webinars are also uploaded to the library, you can go back through and view the
webinars and see the presentations there. For our non-NRCS participants,
you can get the replays by going to our public website that’s
associated with the NRCS website. We just mentioned in our regular monthly series
our next topic is Crop Diversity Rotations and Systems for Soil Health; that will
be presented on Wednesday September 28th at 2 o’clock Eastern, and
our speaker is Ray Archuleta who is a conservation agronomist
here at the tech center. And with that, then we will conclude this
presentation and thank you very much. We had excellent participation today. I saw a high number of 185. Thank you much, everybody. Good-bye.>>Kale Gullett: Yeah. Thanks, everybody. I really appreciate your attendance. Bye.>>Operator: That concludes
today’s conference call. Thank you for your participation. You may disconnect at this time.

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