Floods

GEOL 4093

Risk Assessment

Resources

• http://lists.uakron.edu/geology/natscigeo/lectures/streams/Miss_Flood.htm

• http://www.usgs.gov/themes/flood.html

• http://www.fema.gov/nfip

• Figures are from From Murck et al., Dangerous Earth unless otherwise noted

What is a flood?

• Occurs when the level of a body of water exceeds its natural or artificial confines

• Water then submerges land in surrounding areas

Rio de la Plata, Puerto Rico after Hurricane Hugo

Three Kings Day storm, 1992, Puerto Rico, 24-hour rainfall totals for selected areas.

Flash flood from 1992 Three Kings Day flood, PR

Some Historic FloodsRiver Type of Flood Date Fatalities Remarks

Huang Ho, China River flooding 1887 900,000 130,000 sq km flooded

Johnstown, PA Dam failure 1889 2,200 10-12 m high wave rushed down valley

Yangtze, China River flooding 1911 100,000 Formed lake 130 x 50 km

Yangtze, China River flooding 1931 200,000 Extended over 800 km, millions homeless

Vaiont, Italy Dam failure 1963 2,000 Landslide into lake caused wave overtopping dam and flooding village below

Human Development

• Often near water, so highly susceptible to flooding

• Especially true in developing countries

• Bangladesh—half of land less than 8 meters above sea level

Humans tend to live near rivers—Paris and the Seine River

Causes of Flooding

• Unbalance in hydrologic cycle• Very high precipitation gives floods• Very low precipitation gives droughts• Often combined effects:

– Snow melt– Inadequate drainage– Water-saturated ground– Dam failures– High tides

River Systems

• Channels—passageways• Discharge—quantity of water passing a

given point within some interval of time (for example cfs or cfm)

• Load—sediment and dissolved material carried by river

• Gradient—vertical drop over measured distance

These plots are called long profiles and clearly show river gradients. Steep mountain streams (Sacramento) may reach 60 m/km or more; gradients near mouths of very large rivers (Missouri) 0.1 m/km or less).

Channel Patterns

• Straight—rare, occur for brief stretches only

• Meandering—series of bends and curves, very common, especially in low gradients

• Braided—channels divide and rejoin, usually indicate variable discharge and easily erodible banks

Pools are areas along a channel where water is deepest. Arrows indicate direction of flow and trace path of deepest water.

Channel Patterns

Straight Channel Meandering Channel Braided Channel

Drainage Basins

• Area that contributes water to a river

• Can be entire river system basin down to small tributary basin

• Basins separated by divides

Mississippi River drainage basin. Width of rivers reflect discharge.

Drainage Patterns

• Arrangement of channels

• Controlled by geology

• We won’t get into this

Streamflow Dynamics

• Overland flow—not channelized• Streamflow—channelized• Runoff—streamflow plus overland flow• Main controls on flow:

– Gradient– Cross-sectional area (width x average depth)– Average velocity– Solid load

Discharge

• Q = V x A

• Discharge = average velocity times cross-sectional area

Change in cross-sectional area of the Colorado River at Lees Ferry, Arizona, during a 6-month period in 1956. Note scour of the river bottom during rising water and deposition during falling water.

River Flooding

• Stage—height of a river

• Bankfull stage (or flood stage)—when a river’s discharge increases to fill channel completely

• Flood—water exceeds river’s banks

Hydrograph

• Graph used to show discharge versus time

• Lag time—time between precipitation and peak flood stage

• Peak discharge—maximum discharge during time of study (could be single flood event or entire recorded history of river)

Hypothetical flood hydrograph: (a) preflood stage, (b) bankfull stage, (c) peak discharge.

Hydrograph for 1993 Mississippi River flood

Floodplain

• Area surrounding river influenced by flooding

• Typically broad and flat, built of fine silt and mud from floodwaters

• Usually very good agricultural land

• Mississippi River floodplain covers 80,000 square kilometers

Levees

• Ridge of sediment built up adjacent to river channel

• Boundary between channel and its floodplain• When river floods, coarsest sediments deposit first• Also, smaller floods are more frequent than larger• Because of extensive river engineering we are

now forced to use the term natural levee

Main features of a river valley.

Infiltration Capacity

• Rate at which precipitation can be absorbed by ground

• Depends on ground composition, vegetation, slope, rate of precipitation

• Major control on runoff, thus flooding

• Runoff = precipitation – infiltration – interception – evaporation

Rainfall Distribution and Flooding

• Distribution of rainfall over an area is commonly shown on a contour map

Rainfall in the upper Mississippi River basin, January-July 1993, expressed in terms of the percent by which it exceeded the 30-year average rainfall for the same period.

Shaded area is general area of flooding during the summer of 1993.

Upstream Flooding

• Intense, infrequent storms of short duration

• Cause flooding that is severe but local in extent

• Called “upstream flooding” because effects of the storm runoff usually do not extend to the larger streams further downstream

Upstream floods are generally local, with short lag times

Flash Floods

• Floods with exceptionally short lag time

• Peak discharge reached only hours or minutes after storm has passed

• Deadly

Downstream Flooding

• Usually from storms that last a long time and extend over large area

• Total discharge increases downstream as tributaries collect floodwaters

Downstream floods are regional in extent with longer lag times and higher peak discharges.

Examples of Flood Hazards from the 1993 Mississippi River Flood• Primary Effects

– Water damage to household items– Structural damage to buildings– Destruction of roads, rail lines, bridges, levees,

boats, barges– Historical sites destroyed– Crop loss– Cemeteries flooded, graves disrupted– Loss of life

1993 Miss. River Flood, cont.• Secondary and Teritiary Impacts

– Destruction of farmlands– Destructions of parklands and wildlife habitat– Health impacts

• Disease related to pollution• Injuries (back, electric shock, etc.)• Fatigue• Stress, depression

– Disruption of transportation/electrical services– Gas leaks– Lack of clean water

Secondary, Tertiary, continued– Impacts on crop prices; food shortages– Job loss and worker displacement– Economic impacts on industries

• Construction (beneficial impact)• Insurance (negative impact)• Legal (beneficial impact)• Farming (negative impact)

– Misuse of government relief funds– Changes in river channels– Collapse of whole community structures

Predicting River Flooding

• Statistical techniques to predict frequency of floods of a given magnitude

• Models and mapping to determine areal extent of flood hazards (paleohydrology)

• Monitoring a storm, forecasting warnings

Observing modern floods is a good way to learn what areas may be flooded in future events. This is the 1993 Mississippi River Flood near St. Louis, before and after.

Recurrence Interval

• Average interval between occurrences of two floods of equal magnitude

• Not actual recorded interval but statistical average

• 100-year floods don’t always occur 100 years apart

Flood Frequency Curve

• Plot of flood discharges versus recurrence interval for a flood of that magnitude

• Flood frequency curve for Skykomish River at Gold Bar, WA

Coastal Flooding

• Storm surge

• Recall causes, controls from previous lecture

Flood Hazard Mapping

• To depict the impact and areal extent of a flood

• Actual floods

• Modeling

• Paleohydrology

• FIRMs—Flood Insurance Rate Maps

Flood Zones

• A-zone = 100-year flood

• V-zone = 100-year flood with waves

• Old terms– B-zone = 100-500-year flood– C-zone = above 500-year flood

• New term– X-zone = outside 100-year flood zone

Coastal Flood Zones

FIRM for a coastal NC community

FIRM for a river system in Puerto Rico

USGS topographic map, Boca de Cangrejos area, Puerto Rico

Same area showing historical river flooding

Same area, this time showing storm surge flooding

Q3 Flood Layer

• FEMA Digital flood maps

• ArcInfo Coverages– “X” zone– “A” zone– “AE” zone– “X500” zone– “VE” and “UNDES” (undesignated) zone

SLOSH Maps

• Sea, Lake, and Overaland Surges from Hurricanes

• FEMA, Army Corps, National Hurricane Center, State Emergency Management Agencies

• Colors, shading show areas predicted to be inundated by various category hurricanes

Channel Modification

• Channelization (engineering)• For “safety” but has unintended consequences

– Interferes with natural habitats and ecosystems– Aesthetics– Groundwater problems– Pollution – May cause more severe flooding downstream

• See previous lectures for graphics of levee problems on the Mississippi River

Natural river system, undisturbed

Channelized river

River control structures built to maintain course of Mississippi River

Development and Floods

• Subsidence from groundwater removal (remember Baytown, TX)

• May decrease cross-sectional area of river, so flooding occurs with less discharge

• Runoff increases with more paving and with storm sewer drainage (less infiltration), called urban runoff

Development on a floodplain can alter cross-sectional area of channel

Development impacts shown on flood hydrographs—note decrease in lag time, increase in peak discharge.

Structural Responses to Flood Hazards

• Channelization

• Dams, retention ponds, and reservoirs – Controlled release of flood water

• Levees, dikes, flood walls– May worsen flooding upstream and

downstream

Nonstructural Approaches

• Reduction of Vulnerability• National Flood Insurance Act, late 1960s

started ball rolling– Floodplain zoning– Specialized building codes– Open-space planning in flood zones– Floodplain buyout programs

• FEMA plays big role