Risk of Failure/ Vulnerability
Overall Risk of Failure is a combination of three component scores (structural, hydraulic and geomorphic risk) into a single score representing the overall potential for crossing failure. It is important to recognize one key difference between structural risk on one hand, and hydraulic and geomorphic risk on the other. Both hydraulic and geomorphic risk scores are based on the likelihood that a crossing will fail during or after a significant storm event, and it is assumed that there is little risk of hydraulic or geomorphic failure in the near future without a storm. The more severe the storm, the more likely that a particular structure will fail, and during a severe storm those structures with the highest risk scores are the ones most likely to fail. In contrast, structural risk is based on an assessment of bridge or culvert condition at each crossing. Although storms may increase the risk of structural failure, crossings rated as being at high risk are at risk under all weather conditions.
The Overall Risk of Failure score is derived from the three component risk scores (structural, hydraulic, and geomorphic):
Overall Risk of Failure = max [structural risk, hydraulic risk, geomorphic risk]
The rationale for this approach is that the mechanism of failure with the highest score is the most likely mechanism to cause a crossing failure. The metaphor of the weak link in the chain seems appropriate. Certain characteristics of a crossing may make it more vulnerable to failure from multiple mechanisms. For example, an undersized crossing would be more vulnerable to failure for hydraulic reasons, and may make it more likely to be plugged by sediment or woody debris and fail for geomorphic reasons. However, whichever mechanism reaches a critical stage first will determine whether a crossing fails under specified conditions. Therefore, the highest score among the three components was used as the Overall Risk of Failure for the crossing.
The critical factor in evaluating the geomorphic condition of a specific structure along a channel is determining in which direction the channel is changing (aggrading vs. incising) and how the channel may respond under annual flooding and extreme events. The key is placing each structure in the context of the entire watershed as well as understanding river dynamics at each specific crossing. What happens within the river system upstream and downstream of a structure can have an influence on a structure that is just as important as the impact that the structure may have on the river.
The geomorphic assessment is completed in two phases. Phase 1 involves a desktop, watershed scale analysis that calculates the specific stream power (SSP) for every distinct stream reach in the watershed. The SSP map produced can be used to identify areas of exceptionally high energy where scour is likely and areas of low energy where deposition can occur. This map provides the regional context for each crossing and can be used as a screening tool.
Phase 2 is a more detailed local-scale assessment of the geomorphic conditions at each individual crossing. The data from this assessment are used, along with SSP, to develop a scoring system that evaluates the propensity for woody debris production at the crossing, and susceptibility of the crossing to scour or sedimentation. Other factors included in the scoring are evidence of scouring of footings, existence of a downstream scour pool, and evidence of blockage. These are combined to determine a risk of failure score due to geomorphic stress.
Hydraulic Risk refers to the ability of a road-stream crossing to accommodate streamflow without overtopping and flooding of the road surface. Culvert “blowouts” are not explicitly accounted for, however the failure criteria for corrugated metal culverts is set to avoid conditions which have been observed to cause blowouts. Hydraulic risk is evaluated based on the perceived ability of a road stream crossing to pass a critical flow, defined as the maximum flow a road-stream crossing can accommodate before potentially damaging the road subsurface or causing road overtopping. Based on these criteria for defining critical flow, the allowable headwater elevation for the project is defined as 1 foot below the road surface. Hydraulic risk determination consists of two parts: calculation of the critical flow for a given location and determination of the likelihood, relative to the other crossings in the watershed, that the critical flow will be exceeded. Both elements of hydraulic risk determination are subject to uncertainty and as such need to be well defined in the context of stakeholder needs and expectations.
We have developed a protocol to assess culverted crossings and small bridges. (link to culvert condition assessment coming soon).
It is important to keep in mind when using Structural Risk scores that the culvert condition assessment is a non-technical, rapid assessment protocol intended for use by trained lay volunteers and technicians. This assessment protocol is intended as a coarse screening tool to draw attention to crossings that should be assessed by qualified engineers and/or highway personnel to determine if further action is required.