91LOW-VOLUME ROADS BMPS :

Chapter 9

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LOW WATER CROSSINGS, fords, or drifts, as theyare commonly called, can offer a desirablealternative to culverts and bridges for streamcrossings on low-volume roads where road use andstream flow conditions are appropriate. Like otherhydraulic structures for stream crossings, they requirespecific site considerations and specific hydrologic,hydraulic, and biotic analyses.Ideally, they should beconstructed at a relativelynarrow, shallow streamlocation and should be in anarea of bedrock or coarse soilfor good foundationconditions. A ford can benarrow or broad, but shouldnot be used in deeply inciseddrainages that require a highfill or excessively steep roadapproaches.

Low-water crossings mayhave a simple rock reinforced(armored) driving surface oran improved surface such asgabions or a concrete slab, asseen in Figure 9.1a and Photo9.1. Vented fords combine the

“Keep the ford profile low, armor the driving surface,and protect against scour.”

Photo 9.1Photo 9.1Photo 9.1Photo 9.1Photo 9.1 Use armored low-water crossings, fords, or drifts as often aspossible to cross low flows in broad, shallow natural drainages, thusavoiding the use of pipes. Note that part of this reinforced driving sur-face needs repair.

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use of culvert pipes or box culverts to pass low flowsand a reinforced driving surface over the culverts tosupport traffic and keep traffic out of the water mostof the time, as seen in Figures 9.1b and c. The rein-forced driving surface over the pipes also resists ero-sion during overtopping at high water flows (Photo9.2). The entire wetted perimeter of the structure

LOW-VOLUME ROADS BMPS:92

Photo 9.2Photo 9.2Photo 9.2Photo 9.2Photo 9.2 A vented ford, using multiple culvert pipes to handle lowflows through the pipes, yet allow major flows and debris to overtopthe entire structure.

should be protected to a levelabove the anticipated high waterelevation.

Key factors to consider for thedesign and location of a ford in-clude the following: low and highwater levels; foundation condi-tions; scour potential; allowabletraffic delays; channel cross-sec-tion shape and confinement; pro-tection of the downstream edgeof the structure against localscour; stream channel and bankstability; locally available con-struction materials; and gradecontrol for fish passage.

commodate larger flows.

• Vented fords can be used topass low flows and keepvehicles out of the water,avoiding water qualitydegradation.

• The structure can bedesigned as a broad-crested weir that can passa large flow volume overthe top of the ford. It is

not very sensitive tospecific flow volumessince a small increase inflow depth greatly in-creases capacity. They canbe more “forgiving” andaccommodate moreuncertainties in the designflow and thus are ideal fordrainages with unknownor variable flow character-istics.

• The major advantage is thata ford is usually not suscep-tible to plugging by debris orvegetation the way a culvertpipe may plug.

• Fords are typically lessexpensive structures thanlarge culverts or bridges.They may be initially moreexpensive than culverts, butthey require less fill in thechannel, and they can ac-

ADVANTAGES OF LOW-WATER CROSSINGS

DISADVANTAGES OF LOW-WATER CROSSINGS

• Ford-type structures implysome periodic or occasionaltraffic delays during periodsof high flow.

• The shape is not easilysuitable to deeply inciseddrainages that would requirehigh fills.

• Since the shape of the struc-ture involves a dip andperiodic delays, they aretypically not desirable forhigh use or high-speed roads.

• Vented fords may back up thebedload in a stream channel,causing culvert plugging,requiring maintenance, and

causing other channeladjustments.

• Fish passage may bedifficult to incorporate intothe design.

• Crossing the structure canbe dangerous duringperiods of high flow(Figure 9.2).

93LOW-VOLUME ROADS BMPS :

a. Simple Low-Water Crossing with Reinforced Roadbed of Rock or Concrete

b. Improved (Vented) Ford with Culvert Pipes in a Broad Channel

c. Vented Ford with Pipes or Box Culverts in an Incised Channel

Maximum ExpectedHigh Water Level

Maximum ExpectedHigh Water Level

Reinforced Roadbedwith Rock orConcrete Slab

Original Ground Reinforced Road

Surface

Normal Water Level

FFFFFigurigurigurigurigure 9.1 e 9.1 e 9.1 e 9.1 e 9.1 Basic low-water crossing (fords or drifts) options. Note: Armor the road surface (withrock, concrete reinforcement, etc.) to an elevation aaaaabobobobobovvvvveeeee the high water level!

For fish or aquatic speciespassage, a natural or rough streamchannel bottom should be main-tained through the ford, and wa-ter velocities should not be accel-erated. Ideal structures are either

vented fords with box culverts anda natural stream bottom (seePhoto 9.5) or simple on-gradefords with a reinforced, roughdriving surface (Figure 9.1a).

Maximum ExpectedHigh Water Level

Reinforced Roadbed withRock or Concrete Slab

Freeboard0.3-0.5m

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b. Low-water crossing at high water – WAIT!

c. Crossing during high water can be dangerous!

a. Low-water crossing at low water.

FFFFFigurigurigurigurigure 9.2 e 9.2 e 9.2 e 9.2 e 9.2 Danger of crossing a ford at high water. Fords require occasional traffic delays duringperiods of high water. (Adapted from Martin Ochoa, 2000 and PIARC Road Maintenance Handbook,1994)

95LOW-VOLUME ROADS BMPS :

RECOMMENDED PRACTICES

• Use an adequately long slabor structure to protect the“wetted perimeter” of thenatural flow channel. Addprotection above the ex-pected level of the high flow(Photo 9.3). Allow for somefreeboard, typically 0.3 to0.5 meters in elevation,between the top of thereinforced driving surface(slab) and the expected highwater level (see Figure 9.1).The flow capacity of a ford,and thus the high waterlevel, can be estimated usinga “Broad-Crested Weir”formula.

• Protect the entire structurewith cutoff walls, riprap,gabions, concrete slabs, orother scour protection. Thedownstream edge of a fordis a particularly criticallocation for scour and needsenergy dissipators or riprapprotection because of thetypical drop in water level

off the structure and theaccelerated flows across theslab.

• For simple rock fords, uselarge graded rock in theroadbed through the creek,large enough to resist theflow of water. Use criteria asshown in Figure 6.1. Fill thevoids with clean, small rockor gravel to provide asmooth driving surface. Thissmall rock will have to beperiodically maintained andreplaced.

• Use fords for crossingseasonally dry streambedsor streams with low flowsduring most periods ofroad use. Use improved(vented) fords with pipesor concrete box culverts topass low water flows(Photo 9.4). Accommo-date fish passage whereneeded using box culvertswith a natural stream

channel bottom (Figure9.1c and Photo 9.5).

• Locate fords where streambanks are low and wherethe channel is well con-fined. For moderatelyincised drainages, useimproved fords with pipeor box culverts (Figure9.1c).

• Place foundations intoscour resistant material(bedrock or coarse rock)or below the expecteddepth of scour. Preventfoundation or channelscour with the use oflocally placed heavyriprap, gabion baskets,concrete reinforcement ordense vegetation.

• Use well placed, sturdydepth markers at fords toadvise traffic of dangerouswater depths (Figure 9.2).

PRACTICES TO AVOID

• Constructing sharp verticalcurves on fords that can traplong trucks or trailers.

• Placing approach fill materialin the drainage channel.

• Crossing fords during highwater flows.

• Placing low-water cross-ings on scour susceptible,fine grained soil deposits,or using designs withoutscour protection.

• Constructing fords thatblock upstream and down-stream passage of fish.

LOW-VOLUME ROADS BMPS:96

Photo 9.3Photo 9.3Photo 9.3Photo 9.3Photo 9.3 With low-watercrossings, the downstreamedge of the structure typicallymust be protected againstscour and the entire wettedperimeter (to a level above thehigh water level) should bereinforced.

Photo 9.4 Photo 9.4 Photo 9.4 Photo 9.4 Photo 9.4 Use vented fordswith pipes or openings to keeptraffic out of the water most ofthe time, minimize trafficdelays, and allow for fishpassage. Note the downstreamscour protection with gabionsand rock.

Photo 9.5 Photo 9.5 Photo 9.5 Photo 9.5 Photo 9.5 Some fords can bedesigned as “low-water bridge”structures. They must bedesigned to be occasionallyovertopped and have an erosionresistance deck and ap-proaches. This structure is idealfor fish passage.