Floods
#1 weather-related cause of property damage in U.S.
#1 weather-related cause of deaths in U.S. and #2 worldwide
Annual average $5 billion personal property & agricultural losses in U.S.
Flood Intensity & Duration:
Direct: rain events (number, intensity & duration); size of rainfall area and its orientation w.r.t river drainage basin.
Indirect: snowmelt from previous storms; ice jams on previously frozen rivers; saturation levels of soils.
Non-weather-related: land use, levees & dams (effects on drainage); topography along a drainage (wide plains to narrow canyons).
Flash Floods: localized, short-duration floods (small rivers & streams): 1) slow moving thunderstorms; high rainfall rates; summer months; 2) dam break (e.g., earthquake); 3) occur rapidly, with little or no warning; many injuries and fatalities.
Widespread Floods: large amount of rain over a large watershed for many days: 1) rivers rise slowly; can last a week or more; 2) large amount of property damage and economic losses; 3) minimal fatalities due to advance warning.
Coastal Floods: storm surge from tropical cyclones (June to Nov): 1) dependent on geography & topography of region; 2) also, from earthquake-generated tsunamis.
Land-falling Tropical Cyclones: slow-moving tropical cyclones can drop extreme amounts of rain: 1) T.S. "Allison" (2001) $5 billion & 22 fatalities; 5 days of rain in Houston, TX (2" to 3 feet); Southern LA over 2 feet of rain; 2) Hurricane "Floyd" (1999) 15-20" of rain in NC/VA; 6" in Albany in 24 hours; 3) Hurricane "Georges" (1998) 20" rain in Mobile, AL; 4) Hurricane "Agnes" (1972) Susquehanna River Basin PA/NY 16"+ rain; deadliest & costliest flooding on record in northeast U.S.; 122 deaths was more than Hurricanes Hugo & Andrew combined!
Squall Lines and MCSes: usually between Rockies & Appalachians; "training" of squall line along or near a stationary front; if lasting several days can caused widespread flooding; slow-moving MCSes can drop 6-12" of rain. [Great Flood of 1993.]
Flooding from Frontal Overrunning: during cold season, mT air overrides east-west oriented frontal boundary, leading to heavy rain and thunderstorms on north side of front. [Kentucky: 13-17 FEB 1989; >10" rain.]
Snowmelt: rapidly melting snowpack increases runoff (frozen ground) due to heavy late-winter/early-spring rains; ice jamming on previously frozen rivers. [Red River (MN/ND in April 1997): Grand Forks, ND Red River crested at 54 feet on April 18th; normal flood stage 28 feet; predicted to peak at 49 feet; levees were built to 52 feet; record flooding. Bismarck, ND had 117" in winter of 1996-97.]
Mountain Thunderstorms & Flash Floods of the Southwest: "Arroyos" are dry riverbeds that fill during flash floods; Southwest "monsoon" in summer has increased moisture & strong sunshine (temps > 100 F), which leads to thunderstorms. [Big Thompson Canyon (CO) 31 JUL 1976: high pressure over the ND/Canada border set up moist (Td > 60 F) easterly upslope flow; thunderstorms developed, with rain & downdrafts in higher elevations and updraft inflow in lower elevations; weak upper-level wind flow to 50,000 feet; stationary thunderstorms; 6" rain in 5 hours, created a wall of water down the canyon 10-15 feet high; 139 deaths; a "200 year event" in front range; erosion by this event equivalent to normal erosion over 100 years.]
West Coast Floods: winter-time event; Sierra Nevada, Cascades & Coastal Ranges; central CA " wetland" is a prime agricultural area of the world and has an intricate system of levees and dams; moist Pacific air flow called "Pineapple Express", "Hawaiian Fire Hose, "Tropical Connection", enhanced during "El Nino". [Recent major flooding in winters ending in 1997, 1995 & 1986; flash floods & mudslides, levee & dam breaks.]
Flood Forecasting & Safety: NEXRAD Doppler Radar Network; the NWS issues Flood Watches, Warnings and Advisories; 6" of water can sweep a person away; a foot of water can wash a car away; abandon any car that stalls in flood waters; "Stupid Motorist Law" in AZ.