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Building Principles: Bridges, Culverts, and Fords: A Ditch in Time — Reprised!
In my November/December 2011 column, we looked at roadway drainage, focusing mostly on how water gets from the road surface and then how it’s conveyed in ditches parallel to the roadway. Since we only brushed the surface on how water gets from one side of the road to the other, a more detailed look at that is in order.
There’s an old joke that goes like this: An optimist looks at a glass and declares that it’s half full. The pessimist looks at the glass and announces that it’s half empty. The engineer observes simply that you’ve used too much glass. The hallmark of engineering is the economy of materials and money by using the minimum required to get the job done. When things seem oversized, chances are that either the engineer’s design accounted for conditions that seem very unlikely to the casual observer, or that the system in question was never actually engineered. Where wet areas and traveled paths intersect, this sense of “too big” can be especially true. For example, people in the usually arid Southwest know that the dry arroyo may carry torrents of runoff during an unusually heavy rainfall. Roads that must remain open are subject to different design consideration than the road that might be under water once yearly, or every five years. The type and size of the crossing depends almost entirely on everyday, commonsense parameters. The engineering art here is blending all of the needs into a single, economical design at the least material and labor cost.
Types of Crossings
There are three types of crossings: bridges, culverts, and fords. From a design standpoint, the defining feature of each is how it will perform during expected flows. A bridge allows passage over a body of water without the travel surface becoming subject to the forces of the moving water. For example, the heavy steel beams under an overpass aren’t particularly well anchored to prevent sideways movement because they’re only intended to handle the vertical loads of the traffic on top. Culverts allow flows to pass from one side to the other ranging from a trickle in the bottom of the pipe to both ends being fully submerged. In the most extreme rainfall or runoff events, the excess water that the culvert can’t carry might cross the roadbed itself. Fords allow the stream to flow over the road all the time without pipe, extra earth for an embankment, or any structure to be constructed or maintained. Vehicles simply cross through the shallow water and continue on their way. In many rural areas, the nature, severity, frequency, and impacts of a closed road make fords the most cost-effective way to deal with a short-term heavy runoff situation.
But how do you know which type of crossing you need? From an engineering standpoint, it’s determined by the client’s stated acceptable “level of service.” At one location, it may be alright for a homeowner to let a driveway flood once a year, but at the same time, municipal managers (and camp executives) probably face a stiffer test of liability for keeping the road that serves that driveway open because they’re usually expected to ensure access by emergency services personnel and property owners alike.
Just now, I qualified the discussion about crossing choices by referring to the desired level of service. That’s because increasingly, government agencies are setting standards for crossings that have little to do with the needs of the users. Instead, their focus is on the environment and biology of the watercourse. Imagine a ford which usually flows four or six inches deep. The stream water essentially washes the underside and wheels of fording vehicles, carrying oils, lubricants, and other automotive byproducts downstream. Despite their economy, some regulatory agencies prohibit “at grade” stream crossings. As in almost every other aspect of property infrastructure, keep in mind that regulatory requirements can completely change the design criteria from what will serve your need to what authorities will permit.
On one project, we spent months negotiating with the governing body to permit the replacement of a failing main entrance bridge into camp to overcome state biologists’ concerns for the fish. Originally we had proposed replacing the bridge with a culvert considering the hydraulic and geometric features of the site and the camp’s budget. In the end, the camp got a much more complex and expensive crossing (a precast concrete bridge) than the engineering analysis dictated. Although it was the simplest, least expensive solution we could develop that met the requirements both of the camp and the regulators, it cost about three times the original estimate and budget. Clearly, regional regulations and conventions can complicate the situation. So for the sake of this discussion, let’s focus on the more discrete design elements of bridges and culverts.
In some situations, a bridge may be the right solution to your water crossing quandary. These can be pretty simple, one-size-fits-all answers. I recently encountered one example on a trip to Maine. Logging operations there require stream crossings that remain open in the spring when snow melt makes even little streams run high, fast, and deep. Their “standard” crossing consists of steel beams placed on top of precast concrete roadway barriers, with a treated timber deck for the vehicles to drive on. It appears that the Maine regulators have developed standards by which the logging companies can “field design” the structures and stay above the water flowing below. I understand that these crossings are installed in a matter of days, requiring only a few simple permits. Within pretty broad limits these structures appear to be well suited to carry and pass the heavily loaded timber trucks, balancing the risk of failure with the economy of design and construction. These are an engineer’s dream: extraordinarily simple, relatively inexpensive, and very functional!
But what if these sorts of expedient bridges aren’t accepted by your regulators or your insurance carrier? What will the engineer need to know to deliver the best design for you? In the preliminary design phase, you’ll need to anticipate what sort of vehicles will need to cross it. For example, if you have cabins on the far side, you will need to decide whether just ambulances will be sufficient or if it will need to accommodate fire pumper trucks. Plainly, those two vehicles apply very different wheel loads on the bridge. That single design parameter could double the cost of the bridge. Maybe the smarter answer involves sprinklers or water storage on the other side of the creek instead of a bridge that can safely pass a very heavy, fully loaded tanker truck. To ensure that the final project delivers the smartest bang for the buck, these are the sorts of questions that need to be worked through at the earliest stages of design.
A culvert is simply a pipe intended to move water from one side of a road to the other. But despite their apparent simplicity, culverts behave in unexpected and complex ways once either end is submerged. You may never have seen some of these peculiar things, but they’re actually quite common. For example, the inlet end can be completely submerged, and the discharge end appears to be running only half full — which means that the pipe that was running completely full just before the inlet end was submerged is now only carrying about half of the water that it had been. Flooding will follow very quickly. This is just one of about six different behaviors of culverts where one or both ends are submerged. So there’s a tip: Design and plan for pipes that don’t submerge either end. Here are some considerations to help you accomplish that.
How deep is the culvert pipe buried below the road? Deeper soil between the road bed and the top of the pipe tends to lower the effective weight of vehicles passing overhead. Most pipe types require a minimum amount of soil on top (“cover”), or the pipe could be crushed. Since a circle provides the most area for a given circumference, anything that distorts the round shape reduces the available area for water and makes it structurally weaker. Always replace outof- round culverts as soon as possible.
How deep is water expected to gather at the inlet or outlet during certain runoff events? Even without submerging the end of the pipe, buildup at either end of the pipe will affect how much and how fast water can pass through.
Think about the slope, diameter, internal geometry, and the materials from which the pipe is manufactured. Intuitively, you know that diameter, material roughness, and slope have enormous effects on how a pipe works: A bigger pipe should carry more than a smaller pipe, and a smooth inside should pass more than a rough surface. For example, the common spiral-looking galvanized pipe passes low flows of water more slowly than similar-sized pipes with smooth interiors because the ridges slow the flow. That property may be helpful if your soils erode easily because slower moving water at the pipe exit will not be stirred up. On the other hand, that feature may be a problem during bigger storms if flow is held back to the point where the road floods.
Consider the condition and materials of the channel leading to and from the pipe. Where water has to turn corners or run over rough surfaces, it slows down, effectively stacking up. A straight, smooth channel leading to and from the pipe helps keep things moving as quickly as possible.
Are the ends of the pipe fit with funnel shapes (“flared end sections”) or headwalls? Unlike power windows or heated seats in your car, these aren’t “convenience options,” but are as critical to the pipe’s performance as properly inflated tires are to your car. In some cases, these elements can be installed at a trouble spot, correcting it without replacing a pipe or even digging it up. Known as “end treatments,” they act like funnels and can increase water-moving capacity by 30 percent under certain flow conditions.
Bridges, culverts, and fords each have benefits and drawbacks for each situation and location. Only you, the user of the property, can anticipate what may happen during heavy rains and melt. By understanding how these different options work to fill your runoff needs, your next discussion about drainage will be more productive, and your project will see better results. Your budget will thank you!
Rick Stryker is a professional engineer with a particular passion for helping camps with infrastructure, planning, and regulatory issues. He can always be reached at email@example.com or 570.828.4004.
Originally published in the 2012 January/February Camping Magazine.