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Topic 16: Human Effects on Flooding

Types of Human Effects

Preventing ground soaking, which forces rapid/more water run-off into streams. Increasing sediment loads, which choke rivers and increase stream gradients. Constrict river channels and access to floodplains. Result: a smaller amount of water (rainfall) will cause flooding.

Effects of Urbanization (development) on Flooding

Logging, paving and building prevents water from soaking into the ground. Water must immediately move by over-ground flow, rapidly filling streams. Result: a smaller amount of water will cause flooding.

Dams

Water is stored in the lake to allow controlled, gradual release during dry times Dams are used for hydroelectric power, recreation, and as water reservoirs. However, dams interfere with fish and other habitat, and can reduce water

availability downstream.

Fires, Logging and Overgrazing

Removes natural erosion prevention. Rains erode more material into rivers. Excess sediment load chokes rivers Result: a smaller amount of water will cause flooding.

Bridges

Locally constrict river flow into narrower cross section Raises the level of the river Increases channel scour

Levees

Artificially raises the normal flow Constricts flow during large volume events: Result: Faster flow Result: Higher water level in levee and non-levee regions of the river Result: More channel scour AND bank erosion

Levee Construction (older levees) Usually built on natural levees Originally built using fine-grained river sediment Easily eroded Susceptible to earthquake damage Failure can be caused by animal activity (beavers, gophers)

Modern, well engineered levee Design

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Designed with deep barriers to water seepage LESS THAN 10-20% of the levees around Sacramento and the San Joaquin Delta

are new construction.

Most built by original farmers settling in the early 1900s. Many levees are privately owned

How do Levees Fail?

Bank erosion Underseepage and boils Piping to seeps (animal burrows) Over-topping/breaching flow

Land Use on Flood Plains

Floodplain land is cheap for developers They do not have to pay any cost of future floods Often argue that theres been no historic flooding OR levees will protect homes.

Flood Insurance (US)

NFIP: National Flood Insurance Program Currently $18 billion in debt (post Hurricane Katrina) Superstorm Sandy will add billions more to this debt Heavily subsidized by tax-payers Individuals only pay 38% of real risk rates. Rebuilding is allowed even after multiple floods.

Flood Damage Mitigation Strategies

Require notification of floodplain risks

Require floodplain insurance Make insurance cost reflect actual risk Do not allow rebuilding of homes/business

Flood Damage Mitigation Example

Napa Valley Flood: 21 major floods since 1862 Example: 2005 flood Rained 5 - 8 inches in 24 hours Crested 5 feet above flood stage Similar flooding occurred in 1986 and 1997 years

Napa Flood Mitigation

Restored marshes to the floodplain Removed bridges that constricted flow Removed buildings from oxbow bypass that fills during flooding To be completed in 2015

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1862 California Flood

45 days of extreme storms Sacramento Valley and San Joaquin Valley became an inland sea: an area 250

miles by 20 miles wide under water

State capitol was forced to move out of Sacramento temporarily This was the worst series of storm in the historic record but not the worst in thegeologic record!

Yolo ByPass as flood mitigation

Mimics Flood Plain: No development in the area that is planned to flood Effective flood protection: Stay out of the flood plain