Road-Stream Crossing Failure in an Extreme Flood Event: Mutual Benefits of Appropriate Design for Human Infrastructure and Stream Fish Populations

Improperly designed road-stream crossings can fragment stream networks by restricting or preventing aquatic organism passage.  These crossings may also be more vulnerable to high flow events, putting critical human infrastructure at risk.  In theory, properly designed road-stream crossings could achieve both of these goals. To test the relationship between risk of culvert failure, culvert dimensions, and stream connectivity, we used culvert and channel dimension and damage assessment data from two watersheds in southern New England that experienced extreme flooding in August 2011.  The large majority of culverts in our sample were narrower than the stream bankful width.  Constriction ratio (culvert width/bankful width) was a significant predictor of road-crossing failure, with all failures in both watersheds associated with constriction ratios >1.  Constriction ratio was positively correlated with aquatic passage score, indicating that undersized culverts were also more likely to be barriers to stream fish passage.  Our results reinforce the contention that undersized road-stream crossings represent a risk to both stream fish populations and human infrastructure, particularly under the more extreme and more frequent floods predicted by a number of climate scenarios.  Right-sizing road-stream infrastructure is a measurable step towards this aspect of climate change adaptation.