Another type is to reserve a comparable area on-site designated for mitigation alternatives like constructed or revitalized wetlands. 7 In some cases, the cost of mitigation can end up being even more expensive than the culvert itself. This might be a good time to compare what the cost of a bridge might be. Permitting authorities can be hesitant to approve a culvert because of its environmental impact, which includes waterway constraints and possible flood risk. These culvert impacts can include: clearing out the immediate natural surroundings, changing the flow of water, and filling in the surrounding landscape with dirt to buffer the culvert while supporting the above roadway.
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Obtaining a permit for culvert installation can result in high mitigation costs. The price of mitigation can push the costs of a culvert over the price of a bridge. Mitigation is the act of 'replacing' one environmental impact with an equivalent investment of another to offset the environmental impact of your project. 7,8 There are two main types of mitigation. One type is to pay impact fees that are typically associated with the square footage or acreage of the affected area.
There are three interrelated things to consider when evaluating the functional longevity of a bridge or culvert solution: design, maintenance, and risk.
What is it about design that affects resiliency in a bridge versus a culvert? A culvert is often a cookie-cutter element that can significantly alter the natural processes. In addition, a culvert is sometimes designed to accommodate flow but not necessarily clear the 100-year flood elevation; meaning that in some cases the 100-year flood elevation can exceed the designed roadway elevation and during extreme flooding, excess water is then expected to cross the roadway.10 Alternatively, bridges are typically designed with a bridge's low chord above the 100-year Elevation. A YBC bridge design is always recommended to be a minimum of 12 inches above the 100-year Flood Elevation. This element of bridge design helps to increase flood flow area, clearance for debris, and reduces the overall risk of adverse impacts during flood events.
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What is typical for responsible maintenance of a bridge versus a culvert? It's important to note that all roadways require a general level of maintenance, including keeping the path of travel clear, maintaining the surrounding areas, and performing routine inspections. For a YBC timber bridge, the low-maintenance recommendations include every 2 to 4 years general tightening of fasteners and updating the coatings. In contrast, culverts may require frequent cleaning out of debris to prevent clogging. Additionally, all crossings typically require an inspection about every two years. The frequency of inspection depends on the legal requirements per state.
Now that the design and maintenance roles have been evaluated for bridges versus culverts; how do they influence risks? As far as risks are concerned, there are two levels of risk: risk of improper maintenance and risk of design failure. An improperly maintained bridge or culvert of any type runs the risk of a reduced life span and capacity that's attributed to negligence. However, in the case of a YBC bridge, if low-maintenance design elements and regular inspection recommendations are followed, then it is unlikely to pose risks of hidden threats. Alternatively, an improperly maintained or clogged culvert can collapse and cause flooding, roadway failures, or other harmful environmental impacts.6 In the event of flooding, which is one of the greatest risks of nature that a bridge or culvert design will be subjected to, the weather intensities are greater than average conditions. When exposed to flooding, a bridge design has a reduced risk of failure because it can traditionally withstand flooding better than a culvert due to design requirements and increased clearances.7 In contrast, a culvert design inundated with increased flow and accompanying debris can be adversely impacted and potentially fail under the heightened storm conditions.10
The embankment soils are too steep and were not compacted, which has led to filling in the orifice.Backfill of soils under, adjacent to, and atop culverts is extremely important in realizing their full service life. Pipes should be separated enough so that the soil can be worked into a tight bond with a compactor. Where fill is loosely placed or voids occur in the backfill next to the culvert, a phenomenon called piping can occur. Piping means that water enters the space between the culvert and the backfill, saturates the soil, and slowly washes away fine particles. Over time this action creates larger voids and erodes the soil away until a failure occurs, either when a storm washes out the remaining soil and causes a blowout or when a vehicle sinks into a hole and crushes the culvert. Rule 4. Compact Clean Soil Tightly in and Around Culverts and the Cover MaterialYou can rent a 'wacker' for a few dollars at most rental centers. A wacker is a gas-operated, vibratory, single-operator compactor that can be brought to the site in a pickup truck. You can build a great road and design an incredible culvert crossing, but without compaction of the backfill, you will get to do it all over again. Poor compaction and debris-laced fill are the most commonly diagnosed failures in culvert crossings.Careful placement and compaction of clean fill is needed to install a culvert properly. Installing a granular backfill of pea gravel, often called pipe bedding, in the trench to a depth of 6 in. under the pipe and on both sides up to the midway point aids in draining groundwater and seepage away from the outer walls of the pipe and reliably assists in preventing piping failures.Rule 5. Construct the Road Section Low or Allow for Overtopping to One SideThe sizing of the culvert crossing is for an average year storm. Blockage, larger storms, rapid snowmelt, and debris flows all endanger the roadway and crossing. A prudent designer purposely will depress the road grade to allow for a storm-flow section equal to twice the cross-sectional area developed in Rule 2 to pass over the roadway. The site geometry will determine whether the overtopping section should be directly over the culvert or whether placing it to one side will be adequate. If possible, the overtop section should be placed to one side to prevent the erosive forces of the overtopping mixture of water and debris from destroying the downstream embankment face directly over the culvert. Overtopping to one side often causes the water to flow on a thinner fill section or, even better, over stable native ground. If the overtopping section is strategically placed and sufficiently armored, the access road will still be in place once the floodwaters have passed. Rule 6. Use Maximum Sideslopes of 2:1 (H:V) and a Road Surface Width of at Least 12 Ft. to Calculate the Pipe LengthA common problem with culvert installations is trying to get by with a short pipe length. The design needs to take into account the sideslopes, depth of cover, and roadway width to calculate the necessary length of pipe. Sideslopes steeper than 2:1 generally are not stable and erode quickly in the first few years. Steep slopes are difficult to establish vegetation on and hard to maintain. The slope should meet the bottom of the pipe and not the top; the latter causes a very steep point over the pipe that easily is eroded. Add together the diameter of the pipe twice, the slope length (cover times slope) twice, and the roadway width to determine the pipe width. For pipes set in steeply sloped channels, more length is needed, but in general this approach works for a majority of the crossings. If possible, always install a factory-made flared end section on the pipe, which aids in outfall dispersal of the stormwater and makes road embankment sideslopes above the pipe more stable.
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