Water and Floods

Groundwater and Floods The Hydrologic Cycle & Earth’s water Drainage systems — Drainage basins & Rivers Floodplains and levees Floods –Flash Floods –Regional Floods Controlling floods

Where is the fresh water? 97.5% of the earth’s water is in its oceans

The Hydrologic Cycle 15% Fresh water on land Oceans 85%

Drainages/Drainage Basins The land area that contributes water to a drainageRivers/Streams

Rivers and Streams Rivers are just large streams Floodplains Longitudinal (along-stream) profiles Stream cross sections and map views

Fast-moving water, steep gradient narrow channel Slower-moving water, low gradient, wider channel A C Stream profiles

Base Level Consistent Profile = Equilibrium Conditions Local base levels can be a stream, pond, or lake

Key stream variables that can change to preserve equilibrium 1.Discharge = volume of water a stream carries 2.Load = Available sediment/material available to be moved by the stream 3.Gradient = slope of the stream 4.Channel pattern = the sinuosity of the stream path (how straight or how curved a stream is)

Discharge The amount of water flowing past a certain point for a given amount of time Depends upon the amount of rainfall and the size of the drainage basin Usually reported as: Cubic feet per second (cfs) Cubic meters per second (cms)

Discharge of specific rivers Sacramento River ~ 40,000 cfs American River ~ 6,000 cfs Napa River ~ 700 cfs Local rivers for comparison:

Changes in Discharge Discharge rates change regularly (it’s seasonal) Changes in discharge require other stream changes Velocity/speed of the water flowing Size (width and/or depth) of the stream channel Or some combination of the above two

The greater the discharge, the more sediment (greater load) a stream can carry…

Stream channel development

Meandering Streams

Available sediment load The amount of material available to be transported. If the stream can carry more sediment than it has, then it will preferentially erode away at the surrounding rocks. If a stream has too much sediment, the stream bed/channel will build up (sediment falls out of the water column).

Mississippi River delta

Braided streams High sediment load

Gradient Streams and rivers can have highly variable gradients Gradients are a function of both elevation change and stream length, so variations in either affect stream gradient Higher gradients lead to higher water velocities

Channel Sinuosity

Headwaters Headwaters are characterized by large boulders and rapids — Narrow, straight channels form with high stream gradients

Dendritic streams Dendritic streams are found on steeper gradients

Form with low stream gradients Meandering streams

Stream pattern change with distance from headwaters and gradient

Stream patterns change with time

Floodplains

Levees Artificial levees are very common and have been built for thousands of years Are they cost effective? Give a false sense of security…

Artificial channels & levees

What happens during a flood with an artificial channel?

BeforeAfter

Levees and Flood Hazards

Flood Plains

100 year floods? 10 yr flood = 10% (10 in 100 years) 100 yr flood = 1% (1 in 100 years) 1000 yr flood = 0.1% (1 in 1000 years) These are probabilities, not schedules “Nature has neither a memory nor a conscience”

Types of Floods 1.Severe local thunderstorms that last for minutes to hours — Flash floods 2.Large regional rainstorms that last for days or weeks — Regional floods 3.Tropical storm surges (think hurricanes…) 4.Natural dam failures 5.Artificial dam and levee failures (think New Orleans after Katrina…)

Regional Floods Different from flash floods High water may inundate a large area for weeks Occur in low river valleys over broad regions Caused by large rain storms over prolonged periods (days to weeks) In the U.S., floodplains make up 2.5% of land and 6.5% of the population live on a floodplain Not as deadly as flash floods, but can be much more expensive

Mississippi River, 1993

Big floods in the midwest have several factors in common (an example — the 1993 flood) Preceding winters were wet — ground was saturated Jet stream brought low pressure systems further south Warm moist air from the Gulf of Mexico Some towns flooded for 160 days (over 5 months!) Largest flood in 140 years

Mississippi River, 1993

Mississippi River Floods Common Many steps have been taken to control flooding Levees Dams Diversions Meander removal Have they helped?

Highways 54 and 63 in Missouri

End Result 48 people dead 75 towns flooded 50,000 homes damaged or destroyed 4 interstates damaged and closed 12 commercial airports closed Damages around $12 billion dollars

Flash Floods Most flood-related deaths in the U.S. are from flash floods About half of these are vehicle-related Flash floods are aided by –Steep topography –Sparse vegetation –Thin soil horizons

Fifty percent of all flash-flood fatalities are vehicle related

sfsu2 Flash flood of SFSU campus in February 2004 Thornton Hall

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Controlling Floods Levees Dams Forecasting Zoning

Executive order (1977) Used by FEMA since 1982 Bans building within the 100 year flood plain (1% chance per year)

This will be on the final Once a floodplain, always a floodplain

Flood Insurance Not always mandatory Where it is optional, it is usually obtained by few people Why would you need it when you can get the rest of us to pay for disaster relief?

Urbanization and Water Hydrograph of a natural stream

Natural-state run-off Urbanized area run-off

Changes in Urban Areas Pavement and buildings replace dirt and trees Sewers Concrete channels replace natural streams

Houston, TX