By Si Balch and Eric Walberg
(click to download a one-page synopsis of this article)
The August 2014 Climate Smart Land Network Bulletin provided an introduction to stream crossings and climate change. The September Bulletin provides additional detail on using watershed and stream corridor characteristics to design and size stream crossings that perform well in a changing climate. Stream crossings that maintain the slope, structure and dimensions of the natural streambed are proving to be robust in large flood events and have the added benefit of maintaining full ecological function and connectivity of streams. Figure 1 shows a hierarchy of stream connectivity that occurs as increasingly large portion of the stream channel and surrounding floodplain are spanned by a crossing.1 Moving up this hierarchy also increases the size of storm event that can be accommodated without damage to the structure.
Figure 1. Crossing Width and Stream Connectivity1
Determine the Minimum Opening Size
Stream channels are a manifestation of the water flow volumes and frequencies in the surrounding watershed. Identification of the normal high water mark provides a reference for identifying the minimum opening size needed to accommodate a given design storm. The normal high water mark is evidence of a frequent surface water elevation below bankfull that can usually be identified by physical scarring along the bank. Bankfull is typically defined as the point at which the stream channel transitions into the floodplain. Indicators of normal high water may include erosion, shelving, changes in soil characteristics, destruction of terrestrial vegetation, the presence of litter or debris, or other distinctive physical characteristics.1 The 10-year peak is approximately 2.5 times larger than the normal high water flow, the 25 year is approximately 3.5 times larger and the 50 year peak is approximately 4.5 times larger.
The first step in determining the opening size needed to accommodate a selected design storm is calculating the area of the stream bed (stream width in feet at the normal high water mark X average depth in feet). Multiply the area by 2.5 if the goal is to accommodate a 10-year storm, 3.5 if the goal is to accommodate a 25 year storm, and 4.5 if the goal is to accommodate a 50-year storm. This calculation will result in the opening size in square feet. If a culvert is to be used for the crossing it can be selected from the chart below. Note that if peak flows increase to 15% larger than historic norms going up one culvert size will address that. Embedding a culvert to match stream elevation and allow for inclusion of natural substrate may reduce volume by up to 35% and require sizing up to maintain desired flow capacity. The green section of the chart highlights the most commonly used culvert sizes. For opening sizes above 20 square feet structures other than culverts are typically used. If a pipe arch is to be used, double the opening size from the previous calculation and select the corresponding diameter from the chart.
Design the Crossing
As precipitation patterns continue to change in response to a warming climate stream crossings will be subjected to an increase in heavy downpours. The linkage between the increase in heavy downpours and flooding will be most direct in those watersheds that are vulnerable to flash flooding. Taking the regional trends in both extreme and total precipitation into account in designing and installing crossings will minimize the likelihood of having to spend time and money repairing or replacing the structures. The USGS StreamStats Program (http://water.usgs.gov/osw/streamstats/) provides a GIS-based interface to stream gage data and includes tools to estimate flows at non-gaged locations.
Temporary Stream Crossings: Temporary crossings have several advantages over permanent crossings including limiting exposure of the structure to flood events, limiting unauthorized property access and avoiding permanent stream corridor alteration. Temporary bridges have the added advantage of accommodating a larger design storm as compared to typically sized temporary culverts. In addition, a recent evaluation of cost benefit tradeoffs of temporary bridges ranked them low on cost and high on protection of water quality.2 Temporary bridges perform best when installed on abutments on both sides of a stream. This practice will both stabilize the bridge and facilitate removal in frozen conditions.3
Temporary culverts are typically sized to accommodate at 10-year storm if they will be in place during spring runoff or located in watersheds that are prone to flash flooding. Recommended minimum diameter is 12 inches as smaller sizes difficult to clear if they become clogged.3
Permanent Stream Crossings: Permanent crossings merit detailed design due to cost, greater potential for exposure to large flood events and potential for long-term stream impacts. Matching the width, slope and materials of a similar reference stream reach will improve the resiliency of the crossing to large storm events, maximize stream connectivity and preserve habitat value of the stream. Rules of thumb for a robust permanent crossing include:
· Locate a reference stream reach that is similar to the stream segment to be spanned and pattern the reconstructed stream segment on the reference segment,
· Identify stable endpoints for the crossing structure and span the bankfull width of the channel,
· Match the slope and elevation of the stream, embed culverts or use open bottom structures to maximize stream connectivity,
· Use substrate in the crossing that matches upstream and downstream reaches.
Installing crossings that maintain stream width and substrate may require openings that are larger than the size needed to handle stream volume. An 18” culvert may handle the estimate water flow, but to maintain stream width it might take a 24” culvert. For this reason many crossings are now being installed as arches or bridges, because they are less expensive and easier to install than huge culverts. Building crossings that meet these additional considerations normally result in openings that meet the 50 or more year peak flow estimates.
Recommendations for Permanent Crossings: Calculate the area of the stream bed as previously described. Multiple by 3.5 to calculate the minimum opening size needed to accommodate a 25-year storm. Increase the structure opening size as needed to ensure that the bankfull width of the stream channel is spanned.1 Install a structure that maintains stream width, slope and substrate and you will have a crossing that will likely withstand all but the most extreme flows.
Note on Regulatory Compliance: Regulatory requirements for stream crossings vary by location and are beyond the scope of this Bulletin. Please be sure you are familiar with the regulatory requirements for the location where you are working.
1. Clarkin, K. Stream Simulation: An Ecological Approach to Providing Passage for Aquatic Organisms at Road-Stream Crossings. (2008). at <http://www.stream.fs.fed.us/fishxing/publications/PDFs/AOP_PDFs/Cover_TOC.pdf>
2. Wilkerson, E. & Gunn, J. Quantifying Benefits and Costs of Applying Improved Forest Management Practices for Protecting Water Quality in the Northeast U.S. (Manomet Center for Conservation Sciences, 2012).
3. Blinn, C., Dahlman, R., Hislop, L. & Thompson, M. Temporary Stream and Wetland Crossing Options for Forest Management. (U.D. Forest Service, North Central Research Station, 1998).
Keith Kanoti – Maine Forest Service – Personal correspondence
John Magee – NH Fish & Game – Personal correspondence
Scott Olson – USGS – NH – Personal correspondence
Charles Hebson – Maine DOT – Chief Hydrologist – Personal correspondence
Adam Cates – Dirigo Timberlands – Personal correspondence
Stream Smart Crossing Principles – Maine Forest Service – 2013
Maine Audubon – http://maineaudubon.org/wp-content/uploads/2012/04/StreamSmart-How-To-TechnicalGuidance.pdf
Modeled Future Peak Streamflows in Four Maine Coastal Rivers – Hopkins & Dudley – USGS 2013
Urban Hydrology for Small Watersheds – TR55 – 1986
Culvert Material Cost Comparison – New England Environmental Finance Center for the Maine Dept. of Transportation Office of Environmental Planning – 2010
Trends in Extreme Precipitation Events for the Northeastern United States – 1948 – 2007 – Spierre and Wake – Univ of NH – 2010
Estimation of Flood Discharge at Selected Recurrence Intervals for Streams in New Hampshire – Olson – 2010 – USGS Scientific Investigations Report 2008-5206
The USDA Soil Conservation Service (SCS) Methods; specifically: “Urban Hydrology for Small Watersheds,” June 1986 Soil Conservation Service Technical Release #55.
The United States Geological Survey (USGS) Methods; specifically: U.S. Geological Survey. 1975. “A Technique for Estimating the Magnitude and Frequency of Floods in Maine.” Open- file Report 75-292.