Statewide Highway Flood Vulnerability and Risk

Statewide Highway Flood Vulnerability and Risk Map

The following narrative provides background information describing the data presented in the Map.

As part of the scope of work for the Transportation Resilience Planning Tool, metrics were developed to quantify the flood vulnerability and risk of bridges, culverts and road embankments throughout the state, even if not within one of the TRPT watersheds. Vulnerability assessments have been completed for:

  • Road/river embankments along all state and town highways in the state
  • All long structures (spans greater than 20 feet) on state and town highways
  • All culverts and short structures on the state highway system

Although less precise than the watershed scale analysis in the TRPT, the statewide vulnerability analysis provides a reliable estimate that can also support emergency preparedness, capital programming and hazard mitigation planning. The statewide assessment also provides a metric for use in the project selection and prioritization process.

LIMITATION: small bridges and culverts on town highways are not included in the statewide vulnerability assessment. Inventories, and bank-full width assessments of short bridges and culverts is available on www.vtculverts.org for most Vermont municipalities.

Difference between the Watershed and Statewide Scale Vulnerability Analyses

The ten-point scoring framework shown in Figure 1 is used for the vulnerability assessments at the watershed scale presented in the on-line TRPT application and at the statewide scale. However, the statewide vulnerability analysis is based on fewer data inputs and therefore the results are less certain. Like all remote sensing methods, the results should be verified in the field or with other detailed studies. The data sources and methods for the statewide vulnerability analysis are described in the April 2018 technical memorandum from Milone and MacBroom available here. The vulnerability related data fields for the GIS files in the map service are described here.

Vulnerability Score (MAXSCORE, ISCORE, ESCORE and DSCORE Data Fields)

Vulnerability of culverts, bridges and road embankments to inundation, erosion and deposition has been calculated and converted to a 10-point scale. The scale is summarized in the table below and relates to the extent of the failure possible, the amount of recovery time required to restore travel, and the typical size of the impact.

Figure 1: Vulnerability Levels and Scoring

Vulnerability Score Failure Mode Influence Distance Vulnerability Type
1, 2, 3 Partial Closure Singlelane closure, reduced capacity with some allowable travel, <24 hours <0.25 miles Inundation
4,5 Full Closure Multi-lane closure, detour required, 24 hours to several days 0.25 - 1 mile Inundation, Erosion, or Deposition
6, 7, 8 Temporary Operational Failure Partial destruction of facility. Several days to 1 week for recovery. 0.25 - 1 mile Inundation, Erosion, or Deposition
9, 10 Complete Failure Complete destruction of facility. 1 week to months for recovery. Varies Erosion or Deposition

Novak, David C. and Jam​es L. Sullivan, “A link-focused methodology for evaluating accessibility to emergency services”, Decision Support Systems, Volume 57, January 2014, Pages 309-319.

Vulnerability has been estimated for inundation (ISCORE), erosion (ESCORE) and deposition (DSCORE). MAXSCORE is the maximum of the inundation, erosion and deposition scores, and should be used to screen for locations that are most vulnerable. The ISCORE, ESCORE and DSCORE values provide an initial assessment of the critical processes driving the vulnerability.  To simplify the graphics on the Map Service, vulnerability has been grouped into the following categories.

Figure 2: Generalized Vulnerability Categories and Scores for Map

Vulnerability Category Level MAXSCORE Generalized Vulnerability Score*

High

9 - 10

10

Moderate

5 - 8

6

Low

1 - 4

2

Not Vulnerable

0

0

* Field name "Vul_Gen" in shapefile attribute table

Transportation Criticality and Flood Risk

Transportation criticality metrics are provided in the Map-Service that quantify the importance of a road segment in the network related to general travel and emergency services accessibility. The flood vulnerability and transportation criticality metrics are combined to develop a risk score, which can help prioritize the need for mitigation.

Transportation criticality is a combination of the modified version of the Network Robustness Index (NRI) and the Critical Closeness Access score. The NRI was developed by the University of Vermont Transportation Research Center (TRC) using the statewide travel demand model. The original NRI is a measure of the relative statewide travel time impacts associated with closing each unique road segment in the model.  The TRC latter created a modified NRI that considers the dollar value of different trip purposes in the calculation (Sullivan, James; “Travel Importance and Strategic Investment in Vermont’s Transportation Assets”, February 2004). The modified NRI option 2A (mNRI2A) is being used as one dimension of the transportation criticality score.  

The critical closeness accessibility (CCA) index was also developed by researchers at the UVM TRC. CCA quantifies the relative importance of each link in a roadway network with respect to its system-wide contribution to emergency service accessibility and is calculated on a link-by-link basis .

To create a combined transportation criticality score, the mNRI2A and CCA values were normalized to 1.0 and then added together. The combined criticality scores were grouped into the generalized categories shown in Figure 3.

Figure 3: Generalized Criticality Categories and Scores for Map

Criticality Category Level Percentile Groupings by Number of Road Segments Generalized Criticality Score for Map*

High

Top 2.5%

10

Moderate

85% to 97.5%

6

Low

50% to 85%

2

Not critical

Lowest 50%

0

* Field name "Crit_Gen" in shapefile attribute table

Flood Risk can be used to prioritize actions. In general terms, a high-risk location is a highly vulnerable bridge, culvert or road segment at a critical location on the highway network. There is a high probability that it will be damaged during a flood and the resulting impact to travel and emergency access will be significant. Flood risk is the average of the generalized vulnerability and criticality scores and is grouped into high, medium and low risk categories as shown in

Figure 4: Risk Score for Map

Risk Category Level Risk Score*

High

8 - 10

Moderate

4 - 7

Low

1 - 3

No Risk

0

Not Available

-99

* Field name “Risk” in shapefile attribute table

LIMITATION: Transportation criticality and flood risk scores are available only for the highways included in the statewide travel demand model which excludes most local roads.

The criticality and risk related data fields for the GIS files in the map service are described here.

The shapefiles used to create the Map Service are available for downloading here: link to shape files