Floods 20 Lecture #21
What Is a Flood? Def: high discharge event along a river! Due to heavy rain or snow-melt During a flood, a river:! Erodes channel o Deeper & wider! Overflows channel o Deposits sediment on floodplain
Mississippi River Channel Floodplain Bluffs Normal Year Flood Year
Changes During Floods Higher discharge -->! Faster flow! Increase in total load! Increase in size of sediment carried
Floods As Natural Events Typical river:! Some flooding annually! Can be good: o Fertilizes floodplain o Builds up the land
Example: New Orleans Levees block water from FP! Land < sea level...! Subsides o Weight of city o Removal of GW! Not rebuilt by flood deposits
Example: Nile River, Egypt Flooded every year! FP fertile! Egyptian civilization prospered Aswan Dam (1950s)! Flood control! Crop yield fell! Delta eroded
If floods can be good, what s the problem? Most common geologic hazard:! Affect more people than others combined! > Earthquakes & volcanoes In US...! ~10% of people live on floodplains! ~20,000 communities! ~6 million homes
Cost of Floods in US ~ $ 4 billion annually 1993: Mississippi & Missouri Rivers! Floods in MO, IA, IL! > $12 billion in damages 2008: somewhat less extensive! > $11 billion Costliest flood
1993 Gauging station Mississippi River Missouri River
Flash Floods Def: brief, quick, severe! Sudden rise, rapid fall! Small area o Smashes structures! Typical for mountain streams Cause! Local, intense rainfall o Too fast to soak in > runs off into rivers
Floodplain Floods Def: persistant, widespread! Steady rise, gradual fall! Large area o Soil gets waterlogged & muddy! Typical for lowland streams Cause! Widespread rain/snow across basin o Too wet to soak in > runs off into rivers
Big Thomson R (1976) Examples Great Falls MT (1975)
Prediction of Floods Save lives, minimize damage Short-term (days to months)! Uses accurate weather forecasting! NWS issues flood projections & warnings
Long-term Prediction Community planning (years to decades) Uses flood-frequency analysis! Based on: o Historical records o For a given area
Flood-Frequency Analysis Data needed:! Size of flood o Maximum discharge o Water depth!recurrence interval: average number of years between occurrences of a certain size flood
Flood-Frequency Analysis (cont.) To find RI:! Let N = # of years of records! Rank (M): flood order o Largest: M = 1 o Smallest: M = N! Then RI = (N + 1) / M
Flood-Frequency Analysis In general (cont.)!large floods have a long recurrence interval!small floods have a short recurrence interval!example: a 5-year flood occurs more frequently than a 50-year flood.
Flood-Frequency Analysis (cont.) Probability: likelihood that a certain size flood will occur in any given year = reciprocal of recurrence interval P = 1 / R.I.
Probability of Floods Recurrence Size Frequency Probability 100 yrs very large rare 0.01 = 1% 25 yrs large 2 yrs small more common very common 0.04 = 4% 0.5 = 50%
People and Floods Urbanization! Intensifies flooding: o Pavement, buildings stop water from soaking in o More rapid runoff
Effect of Urbanization After Before Size Time
Flood Control Measures Developed floodplains need protection But flood control => more development => more vulnerability to floods
Channelization Def: straighten channel (eliminate meanders)! Example: Boneyard Creek! More water can move through, faster Problems! River returns to meandering! Increased erosion & flooding downstream! Loss of habitat & beauty
Dams Block flow, make reservoir for:! Flood control! Irrigation! Recreation Problems! Reservoir fills with sediment! Increased erosion downstream
Flood Control A channelized creek Ultimate channelization Dam & reservoir
Artificial Levees Dirt, sand, or concrete: raise height of river bank! More water, without flooding Problems! False sense of security! Catastrophic failure
How Levees Fail Overtopping - not high enough Slumping - sliding away Seepage - water reduces levee strength
Preservation of Wetlands Water soaks in, instead of running off to river Many wetlands are lost to development
More Flood Control Earthen levee Concrete levee Wetlands