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Sensitivity Matrix

III(a) Precipitation - Summary

Asset Categories Precipitation - Summary
Mode Sub-Mode Important Impact-Asset Relationships Threshold Mobile-Specific Detail Potential Indicators of Sensitivity
Bridges Bridge (Superstructure) Scour can make bridge more susceptible to collisions, wave action, and other impacts. None. Mobile County bridges (non-coastal) are usually built to withstand a 50 year storm. None.
Bridge (Substructure) Heavy precipitation events can increase the flow velocity and flow depth of a stream or river, which affect local scour depth (the depth of erosion to the bridge support). Scour depth resulting from a particular event can be calculated based on local parameters using the equation presented in HEC-18. However, as a rule of thumb, the maximum scour depth for round nose piers aligned with the stream flow is 2.4 times the pier width when the Froude Number is less than or equal to 0.8 and 3.0 when the Froude Number is greater than 0.8. [74] During flood conditions, if the stream elevation reaches the low cord bridge elevation, the local scour depths could be increased by 200-300%. [74] AASHTO LRFD specifications require that scour at bridge foundations be designed for the 100-year flood storm surge tide or for the overtopping flood of lesser recurrence interval. The bridge should also be designed to withstand the "super flood" or the 500-year flood. The corresponding 100-year design scour depth at bridge foundations is estimated by using a procedure in HEC-18. [73] The Alabama Department of Transportation reported having minimal issues with scour at non-coastal bridges. [77] List of scour critical bridges is available from the National Bridge Inventory. - HEC-18 recommends that elevation of lower cord of the bridge should be increased a minimum of 3 feet above normal freeboard for the 100-year flood for stream carrying a large amount of debris.
- Simple span bridges likely more vulnerable to scour [74]
- Bed material characteristics such as size, gradation, and cohesion can affect local scour [74]
- Pier width [74]
- Pier or abutment shape [74]
- Scour susceptible bridge rating from the National Scour Evaluation Program (based on elements such as piers and abutments designed with spread footings or short pile foundations, superstructures with simple spans or non-redundant support systems, bridges with inadequate waterway openings or with designs that collect debris)
Operator Houses (movable bridges) and electrical parts Since the machinery and electrical parts of a movable bridge are generally protected from precipitation, damage does not occur unless wind or some other stressor causes the machinery and electrical parts of the bridge to be exposed. While low levels of flooding in the operator house of a movable bridge could quickly cause the bridge to become inoperable, these conditions do not generally occur except during extreme storm events.
Roads and Highways Paved road surface Flooding at waterway crossings (where water has velocity) can be expected to cause pavement and embankment failure beginning when the water is high enough to flow over the roadway surface. Heavy precipitation and flooding can erode paved road surface. Over time, precipitation can also worsen existing pavement damage (for example, from cracking due to temperature impacts). [18] While lower functional class roadways are typically designed for the 10-25 year storm, Mobile County roads do not tend to experience damage from flooding until around the 50-100 year storm.[43] Mobile County currently experiences flooding issues during storms and heavy rainfall events. Due to a variety of factors, certain roads and bridges flood more easily. For example, county bridges which are elevated above the roadway can cause rainwater to runoff and flood either side of the road during heavy rain events. Small culverts or culverts blocked by debris can also cause flooding problems. [43] - Elevated bridge relative to bridge approach may encourage water to pool at base of bridge
- Location near stream
- Size of nearby stormwater drains
- Wind speed, accumulated debris
Road substructure (gravel base, substructure) During heavy precipitation events, rain can leak in under the pavement and damage the subgrade, which is very sensitive to moisture levels. [18] Multiple instances of complete pavement submersion are likely to lead to pavement damage over time. Heavy precipitation works in concert with other variables, such as temperatures changes and traffic loading, to shorten the working life of pavement. [65] - Thickness of pavement layers
- Sloping shoulder to divert precipitation away from road.
- Silt or clay subgrade soil
- Absence of underdrains
- Inadequate surface drainage
unpaved roads Heavy precipitation events often wash out unpaved roads. [43] While unpaved roads are more susceptible to damage during flooding events, they are also cheaper to fix. [43] There are 400 miles of county-owned unpaved roads in Mobile and a significant portion of the County transportation budget goes to converting unpaved roads to paved roads. [43] unpaved surface is more sensitive than paved roads to low levels of damage resulting from flooding.
Stormwater drainage (culverts, side drains, etc) Debris accumulation, sedimentation, erosion, scour, piping, conduit structural damage. Culverts: 25 year storm capacity (Mobile County design standards)
Cross drains: 10 year storm capacity (Mobile County design standards)
Side drains: 10-25 year storm capacity (Mobile County design standards)

During a 10-25 year storm, small amounts of flooding occur. However, damage generally does not occur until around a 50-100 year storm. [43]
Highway, road and street signs and traffic lights Heavy rainfall can impact visibility of signs. Unknown. None documented. None documented.
Highway and road traffic and service Even light rain slows traffic and decreases the capacity of a road to handle traffic. Rain also increases safety risk on the road by impairing visibility and mobility and increasing the likelihood of hydroplaning. [49] Under very light rain conditions, road capacity may be reduced by 1-3%. Under light rain conditions, capacity may be reduced by 5-10%. Similarly, measured roadway speed decreases 1-2% under very light rain, 2-4% under light rain, and 4-7% under heavy rain. [1, 15]
Railroads Electrical Equipment (gates/flashers and signal bungalows) Electrical damages due to inundation of equipment (from precipitation-related flooding or SLR). [2] Heavy precipitation or any flooding.
Railroad Tracks, Ties, and Ballast Immersion of wooden ties in water softens/expands the wood, weakening its ability to support tracks. [57] Erosion of supporting systems (such as ballast and other nearby ground) can threaten track stability. [2, 49, 57] Heavy precipitation or any flooding.
Railroad services (i.e., operations) Flooded tracks, affecting the electrical systems on the tracks themselves or able to potentially short of locomotive motors threaten operations.
Additionally, heavy precipitation conditions can result in train speed reductions. Missed signals due to limited visibility from rain also of concern. Employees may have difficulty getting to work in heavy precipitation conditions. [2, 49, 57]
Heavy precipitation or any flooding. Just a couple of inches of flooding can short out locomotive motors. [2, 49]
Airports/ Heliports Runway and navigational aids Precipitation can impede aircraft traffic and flight departure/arrivals. [12, 16] Heavy rain of 1 to 2 inches per hour.
When runway becomes wet from precipitation. [12, 49]
Mobile Downtown Airport: Mobile Downtown Airport closes for two days at a time during hurricanes, typically two times per year
Mobile Regional Airport: Mobile Regional Airport closes for two days at a time during hurricanes, typically two times per year. [64]
- Insufficient or restricted stormwater drainage capacity
Aircraft Hail can damage aircraft that are not in flight.
Freezing precipitation can affect aircraft operation. [49]
Thunderstorms with hail within 5 nautical miles of airfield, with hail greater than 1 inch in diameter.
Any form of freezing precipitation. [49]
No documented relationship.
Airfield buildings and structures (e.g., terminal buildings, hangars, air traffic control tower)
Services and airport/ heliport operations (e.g., flight departures and arrivals, baggage/cargo transfers, ground transportation) Heavy precipitation and/or hail during storms can cause flight delays and/or cancellations, and delays in other airport operations. [49] Thunderstorms with lightning within 3, 5, or 17 nautical miles of the airfield (escalating impacts at shorter distances).
Thunderstorms with hail within 5 nautical miles of airfield.
Heavy rain of 1 to 2 inches per hour.
When runway becomes wet from precipitation. [49, 12]
Mobile Downtown Airport: Mobile Downtown Airport closes for two days at a time during hurricanes, typically two times per year
Mobile Regional Airport: Mobile Regional Airport closes for two days at a time during hurricanes, typically two times per year. [64]
Natural Oil and Gas Pipelines Pipelines, aboveground Damage caused by weakened soil structure due to precipitation or inundation from storms.
Pipeline roadbed can be scoured by heavy precipitation.
Possible disruption of inspection, construction, or maintenance cycles. [60]
No documented relationship.
Pipelines, underground Damage caused by weakened soil structure due to precipitation or inundation from storms.
Pipeline may be unearthed from flooding. [60]
Detailed analysis of geology and pipeline-specific conditions is required to draw precise conclusions about potential impacts. [60] No documented relationship. May be most pronounced in lowland and marsh areas. In lowland and marsh areas, primarily in Louisiana, soil is being washed away by storm activity.
Pipelines, offshore No documented relationship. No documented relationship. No documented relationship. No documented relationship.
Aboveground infrastructure (e.g., compressor stations, metering stations, other buildings, structures)
Utilities for pipelines - electricity
Electric Power Systems Electric Power Systems Soil moisture effects electrical power system components, including poles. [35, 45] - Soil moisture has been found to be positively correlated to power outages, but relatively small effect.
- Precipitation and soil moisture the day before hurricane landfall was found to be negatively correlated to power outages, suggesting that drier conditions increase the likelihood that trees will snap and break in storms, damaging power lines.
- Based on statistical analysis, clay-rich soils were found to be correlated with longer power outages; interviews with repair crews confirmed that repair trucks are more likely to get stuck in wet clay soils. [35, 45]
Marine Ports, Terminals, and Waterways Electrical Equipment Appears to be the highest risk of flooding. [2] River flooding and storm surge flooding are both a risk to Mobile port facilities.
Terminal Buildings Damages to structures and equipment in buildings. [2]
Channels - Erosion can cause weakening of supports but most of these marine structures appear be built to withstand water.
- Lower water levels due to drought decrease the cargo limits for shipping, especially for barges. To maintain efficiency, additional dredging of channels may be required in times of drought. [42, 54, 10]
Piers, wharves, and berths Channels/routes blocked
Port services (i.e., operations) - During heavy precipitation, electrical damages to equipment can result and restrict operations.
- Visibility/operations restricted by heavy rain. Risk of collisions also increased in heavy precipitation events.
- Lack of precipitation (drought) can cause lower water levels in shipping channels and therefore pose a threat to shipping operations. [42, 54, 10]

*Note dollar amounts are not adjusted for inflation.

Updated: 11/17/2016
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