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Storms and Stream Crossings

There is clear evidence that precipitation in the Northeast is more intense than it was in the past. 

The increase in the Northeast has been greater than any other region in the U.S. (Figure 1). Between 1901 and 2014, total annual precipitation increased slightly (7%), but that arriving in extreme events (downpours) increased by 41% (Huang et al. 2017). Heavy downpours are occurring more frequently in the region, most often in the summer and fall. Recent investigations of regional weather records show that the biggest increases have occurred in the last 2-3 decades (Figure 2); a distinct change in the long-term record (Huang et al. 2017, Kunkel et al. 2013, Spierre and Wake 2010). This trend of more intense rainfall is projected to continue.  One report suggests that by the end of the century, the entire Northeast region may receive 21% more rainfall in events that drop greater than 1 inch of rain at a time (Kunkel et al. 2013).

Very heavy precipitation is increasing
Figure 1: The map shows percent increases in the amount of precipitation falling in very heavy events (defined as the heaviest 1% of all daily events) from 1958 to 2012. Trends are larger than natural variations for the Northeast. Figure source: U.S. National Climate Assessment, 2014.

More Water Can Stress Out Stream-Crossings

Flash floods, and seasonal flooding events can trigger catastrophic disturbances in forest ecosystems when floodwaters exceed the ability of a culvert to pass water (known as the ‘hydraulic capacity’ of the structure). When culverts or stream structures are overtopped or plugged by woody debris (like sticks and branches), this can lead to soil disturbances and streambank erosion that often deliver large volumes of sediment downstream. Flood events caused by undersized or poorly designed stream-crossings can impart major damage to wildlife habitat and adjacent properties.  Stream-crossings have a lower risk of failure if designed appropriately for the stream, and with consideration of current and future regional climate conditions. Careful evaluation of structure size considering all of the factors that influence potential flood-risk can minimize future risks. Managers may also seek additional consideration of the role of land-use change and wetland losses that may exacerbate flood hazards when evaluating stream crossings (Wisconsin Wetlands 2018).  

Figure 2: Percent changes in the annual amount of precipitation falling in very heavy events, defined as the heaviest 1% of all daily events from 1901 to 2012. The frequency of events is increasing over time. Note: the last bar on the graph is for the years 2001-2012 (figure source: NCA, 2014). 

Stream-Crossing Infrastructure, What Are the Options, How Much Will It Cost?

Stream-crossing installation, maintenance and retrofitting is expensive, there is no way around it. Of course, there are upfront installation costs, and maintenance costs to budget for when considering an infrastructure replacement project. Yet, there are also “hidden” long-term costs to be considered, like repeated replacements and repairs for undersized or poorly designed structures that fail after large rain or flood events. With precipitation inputs on the rise, the likelihood of infrastructure blowout and failure for inadequately designed structures increases over time. 

There are several crossing options to choose from, for example - bridge, round culvert, arch culvert, or plastic culverts. Ball parking average costs, a steel arch culvert designed for the natural features of the stream has a 50-75yr life span, whereas a standard round culvert (hydraulic designed) has a lower 25-50yr life span, and carries a higher replacement and repair risk (Gillespie et al. 2014). Crossings designed for the natural characteristics of the stream have an impressive record of withstanding large rain and flood events. Using design tools like the Forest Service Stream Simulation tool, specialists evaluate local stream characteristics, and natural variation of the stream channel during the design process. Although natural design is an ecologically sound and better alternative to other methods, this method can also increase costs by 9-22% (Gillespie et al. 2014).

There are key differences worth considering in the stream crossing design process such as the stream’s capacity to move floodwaters, aquatic organism passage effectiveness, installation and replacement costs, annual maintenance costs, and structure lifespans. In addition to structural considerations, there are several costs such as up-front installation costs, maintenance costs, and possible NRCS cost-share payments that might off set total costs. Based on a 2015 NRCS analysis using actual planning cost data and potential payments from Maine’s 2015 Practice Payment Schedule (Knight 2015), the results suggest bridges and arch culverts can be the lowest cost options with the least long-term maintenance and longest life-spans (Table 1, Table 2).

Note: Before entering into a design project the crossing and stream should be evaluated by a specialist to assess and evaluate local conditions. Contact your local NRCS field office to begin a design project today.