MAJOR FLOOD DISASTERS

Huang He River, China 1887 estimated 900,000 fatalities

Johnstown, Pennsylvania 1889 Dam failed, 2200 fatalities

Yangtze, China 1911 estimated 100,000 fatalities

Los Angeles, CA 1928 St. Francis Dam fails,

Yangtze, China 1931 estimated 200,000 fatalities

Huang He, China 1938 estimated 900,000 fatalities

NATURAL STREAM PROCESSES

Floods are natural behavior of streams

Size of an unmodified stream channel is directly related to the quantity of water that it usually carries (more frequent, moderate flows)

Most of the time river stage is below the channel banks

Times of higher discharge – overflow banks è flood

Bankfull Stage – water fills the channel to the level of the bank top

Bankfull Discharge – water discharge when the water level is at the tops of the stream banks

Recurrence interval for bankfull discharge has been found to be uniformly 1.5 years

Higher discharge flood events occur much less frequently

Sediment Transport

· stream competence = maximum sediment size stream can transport

· during flood stage, stream’s competence is high and can result in transport of boulders

Sediment Transport

· greatest erosion and sediment transport will occur during floods

· most of bed load carried at flows ranging from 90% of bankfull discharge to 2X bankfull discharge

· Increased water discharge during floods increases the stream’s capacity for carrying sediment

· Increased water velocities during floods increase the stream’s competence or the maximum size sediment eroded and moved

· However, if stream is in a dynamic equilibrium state, overall channel size and configuration does not change after high flows è channel built to accommodate both high and low flows with their associated sediment load without long term changes in morphology (erosion and deposition balanced)

STREAM HYDROGRAPH

Storm Event – see handout for hydrograph

· Rainfall produces runoff

· Runoff moves into stream channel and increases stream discharge

· Flood crest reached when peak flow is reached – usually a lag time from end of rainfall to flood crest

· Can be a lengthy time period before all storm runoff moves through channel and discharge returns to nonflood levels

FLOOD PREDICTION

· Government agencies attempting to reduce risk from flooding use estimates of the probability that large floods will be equaled or exceeded in any given year

· Flood Frequency Analysis – assumes recorded annual peak discharges (highest flows of the year) represent a random sample

· Produce a flood frequency curve

Flood Frequency Curve

· Plot occurrence of past maximum discharges on a probability graph

· Used to get probability of particular maximum discharge occurring

· Also referred to as recurrence intervals which are measure of how likely it is of particular flood magnitude occurring

Calculation of Recurrence Interval

1. Annual peak discharges (largest discharge of the year) are ranked according to size (1 = highest)

2. Each of the annual peak discharges are given a rank (m) of 1 (highest), 2 (second highest), …. To last rank which will be for the lowest peak discharge and will equal the total number of years of record

3. Total number of years with discharge record = n

4. Recurrence interval (R) for the particular annual peak discharge is calculated by adding one to the number of years of record and dividing by its rank: R = n+1/m

Flood Frequency Curve

· 1 in 50 probability of occurring

· 2% probability = (1/50)*100

· 50-yr flood

Source of Confusion

· Not the same as earthquake recurrence intervals

· Water is not building up somewhere waiting to be released (as strain is on earthquake faults)

· 100-yr flood simply means (1/100)*100 = 1.0 % chance or probability in each year of occurring

· Chicago area experienced two 100-yr floods within a 10 month time period in 1986-1987

Recurrence Intervals

· 1-yr flood = 100 % probability

· 2-yr flood = 50 % probability

· 3-yr flood = 30% probability

· 5-yr flood = 20% probability

· 10-yr flood = 10 % probability

· 20-yr flood = 5% probability

· 50-yr flood = 2 % probability

· 100-yr flood = 1 % probability

· 500-yr flood = 0.2 % probability

100-Yr Floods

· Flood control systems and management plans are generally designed to protect against a 100-year flood

· Flood insurance subsidized by the Federal Emergency Management Agency (FEMA) provided for communities designed to protect from a 100-yr flood or greater

· 100-yr floodplain is the area that would be covered by water in a 100-yr flood

Problems with Flood-Frequency Curve

· Based only on historic record which might be short

· Large floods are rare events, high flow part of curve is ill-defined

· If only have 50 years of record, will only obtain the R = n+1/m = 51/1 = 51-year flood maximum discharge

· Large floods which occur in the present can change curve drastically

· An occurrence of an extreme flood (such as those seen in 1993 and 1997) adds a point to the graph

· Addition of the extreme flood will increase the slope of the line on the graph

· This causes the estimated 100-yr flood discharge magnitude to increase

· What happens to all the flood control systems and buildings designed from the previous 100-yr floodplain?

· Human activities can affect flood frequency. Stream probably responds much differently today than 100 yrs ago.

CUMULATIVE PROBABILITY

· Probability that a particular flood will happen in your lifetime (70 yrs)?

· = {1 – [1 – (1/recurrence interval)]⁷⁰ } *100

· 100-yr flood è 50.5% probability will occur within your lifetime

· 70-yr flood è 63% probability will occur within your lifetime

· 30-yr flood è 91% probability will occur within your lifetime

CUMULATIVE PROBABILITY

· Probability that a particular flood will happen over a 30-yr mortgage

· 100-yr flood è 26% probability

· 30-yr flood è 63% probability

LAND USE EFFECTS

· Urban development = streets, parking lots, and buildings

· ground surface covered by impermeable material

· ==> less infiltration ==> more runoff

Effect on stream hydrograph is higher peak discharge or stage and shorter lag time

· Less infiltration = less water into groundwater zone

· Dry season stream flows from groundwater seeping into stream bed are reduced

· Building on floodplain increases flood stage (water level) for a given discharge

· Buildings displace volume

· Filling in floodplain for building displaces volume

· Same amount of water will fill valley to a higher height

· Storm sewers and agricultural field drains funnel water quickly to rivers (instead of slow infiltration into subsurface) è more runoff arriving quickly to river

WHY BUILD ON FLOOD PLAINS?

· Rivers provide transportation routes to cities

· Central areas of US are broad, flat plains – cannot limit building for miles around river

· Flat surfaces for building

· Fertile topsoil for farming

· Scenic

SMART LAND USE

· Zone areas which are frequently inundated (such as 25-yr flood plain) for livestock grazing pasture, parks, golf courses

· Raised buildings in 100-yr flood plains

FLOOD CONTROL

Artificial Levees

· earthen mounds built on banks of river to increase the volume of water channel can hold

· most are not built for extreme flooding

· false sense of security can increase development on the other side ==> extreme floods will result in extreme damage

· if levees are overtopped, water is trapped in floodplain and cannot drain back into river after flood

Flood-control Dams

· store flood waters and then slowly release

· lowers flood crest by spreading volume of water flow over a longer period of time

Bypasses and Retention Ponds

· discharge from the main river channel is diverted into retention basins along the flood plain (bypasses)

· runoff collected into retention ponds and water slowly infiltrates (unless retention ponds flood)

Channelization

· modifications to increase velocity of water, volume of water in channel or both

· will increase rate at which excess water is carried away

GREAT FLOOD OF 1993

· Spring 1993, 1 m rain falls in Iowa during 3 spring months (equal to amount normally falling in 1 yr)

· At St. Louis, Missouri, river discharge was 6X normal and river crested 20 ft above flood stage

· Main period of flooding continued for 79 days (previous record was 77 days in 1973)

· 50 people killed

· 55,000 homes partially or completely destroyed

· $12 billion damages, mostly crops

· more than 40,000 km² bordering Mississippi and Missouri rivers under water

· 100-yr flood discharges exceeded on many rivers (Mississippi, Missouri, Iowa, Platte, Racoon)

RED RIVER FLOOD 1997

Red River of northern U.S. runs through North Dakota and Minnesota northward to empty into Lake Winnipeg, Canada

· Had a very snowy winter

· April snowmelts had started

· Final winter storm in early April rebuilt snowdrifts to 20 ft high

· Rapidly rising temperatures melted snow in days

· River discharge increased from 2500 cubic feet per second to 150,000 cubic feet per second

· floodwaters crested at 26 feet above flood stage

· Approximately 4.5 million acres of land under water

· Population (50,000) of Grand Forks, N.D. evacuated

· 500-year flood = 1/500 th (0.2 %) probability of occurring

· flood plain is broad and flat (stream gradient between 5 inches per mile and 1.5 inches per mile)

· ==> floods produce massive shallow lake

FLASH FLOODS

· Exception to slow-motion, long-lasting floods

· Local, sudden floods of large volume and short duration

1976 Big Thompson River of north-central Colorado

· Winds from east push moist air up front of Colorado Rockies è thunderstorms in steep mountainous terrain

· 30 cm rain in 2 days

· discharge in river 4X previously recorded maximum

· 139 fatalities, 5 missing, $35 million damages

1997 Fort Collins, Colorado

· 8 inches of rain in 5 hrs

· 12-ft wall of water through Spring Creek

· 15-ft high railroad embankment breeched

· trailer parks flooded

· 5 fatalities, 40 injured

FEDERAL AGENCIES

Federal Emergency Management Agency (FEMA)

· federal flood insurance available for communities with floodplain management plan

· must map 100-yr floodplain

· identify floodways = areas to control development as have potential to increase flooding

Other Rain-Triggered Disasters

· runoff down slopes

· saturated slopes slipping (landslide)

· movement of sediment and water downslope (natural process but homes in the way)