CULVERT OR BRIDGE: Five Points to Consider Before Choosing a Crossing Method

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Stop Defaulting to Culvert Solutions, Consider a Timber Bridge Alternative. 

Culverts can seem like a quick and easy fix to crossing water, wetlands, or floodways on a project. Although, sometimes the better option may be a bridge. Bridges can be more beneficial economically, ecologically, and aesthetically for a project. Consider how a bridge may satisfy your project needs using these five points before defaulting into a culvert solution.

Permitting: Bridges can take less time to permit than culverts

Culverts can be much more difficult to permit than bridges. This could mean an increased risk of delaying your schedule and project completion. Additionally, you could accrue time and expense for planning, permitting, or third-party verification for alternative solutions.

This is how it works: The Army Corps of Engineers (Army Corps) requires that you receive their approval before developing in areas with wetlands or other bodies of water of public interest.(source) For the Army Corps to approve a project, they review the environmental impacts that could be caused by your project. If the Army Corps thinks that the amount of environmental damage to be caused is too great, they will require you to revise the project proposal to mitigate the impacts.

Revising a project proposal could mean having to use a different type of culvert, engaging in environmental mitigation efforts, or utilizing something completely different, like a bridge. Typically, a bridge solution can significantly reduce, or in some cases eliminate the impacts that can be a cause for permit denial. Some projects are not subject to Army Corps approvals, but they will require an environmental assessment and approval by a similar local or regional authority. In either jurisdiction, these revisions can prolong a project and in turn cost more money.

Mitigation: Costs of culverts can skyrocket due to high levels of environmental impact

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.(source 1) (source 2)

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. Another type is to reserve a comparable area on-site designated to mitigation alternatives like constructed or revitalized wetlands.(source) 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 constraint 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. Additionally, culverts affect the flow of water by creating a bottleneck. This bottleneck changes how the water flows on either side of it and can affect aquatic life, as well as change the pattern of the river.(source) These environmental impacts can often contribute to increased mitigation costs.(source) Alternatively, bridges can allow for the natural flow of water, limiting their impact as compared to a culvert. This is why the Center for Environmental Excellence by AASHTO notes that “it is generally believed that culverts are more detrimental to streams than bridges”, in its Designing for Environmental Stewardship in Construction & Maintenance Chapter.(source)

Design, Maintenance, & Risk: The resiliency of a timber bridge may surprise you

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 vs. 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.(source) Alternatively, bridges are typically designed with 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. 

What is typical for responsible maintenance of a bridge vs. a culvert? It’s important to note that all roadways require a general level of maintenance, including keeping 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 update the coatings. In contrast, culverts may require a 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 vs. culverts; how do they influence risks? As far as risks are concerned, there are two levels of risk: the 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 inspections 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.(source) In the event of flooding, which 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.(source) In contrast, a culvert design inundated with increased flow and accompanying debris can be adversely impacted and potentially fail under the heightened storm conditions.(source) 

Aesthetics: A YBC timber bridge can create an iconic crossing for your project

A York Bridge enhances the style of your project which is a key element that most culverts can lack. At YBC, we synergize with you to customize your crossing. Culverts are common solutions and many times treated as a mundane or overlooked functional feature. In turn, a DeceroTM designed York Bridge is something that is guaranteed to catch people’s eye, while remaining equally if not more functional than the culvert. By incorporating a YBC timber bridge, you give your project something unique for people to talk about and enjoy while possibly reducing your costs.

In Project Planning, It’s Good to Have Options.

Feel free to visit www.ybc.com to see an abundance of comprehensive timber bridge galleries to assist with planning for your next project. Our bridges have been custom designed for our clients to meet their needs. We have enhanced the style and functionality of many residential, commercial, and mixed-use developments. A timber bridge might be your cost-competitive and environmentally-sensitive solution.