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Types of Human Effects

Human activities can increase flooding in several ways. Preventing ground absorption of water and adding sediment to rivers from activities like logging, urban development, and overgrazing means water runs off faster into streams, allowing smaller rain events to cause flooding. Dams control flooding but also disrupt habitats, while bridges and levees constrict river channels and artificially raise water levels, increasing current speed and scouring. Old levees often fail due to erosion or seepage, worsening floods, but modern engineered levees are more stable if maintained. Allowing development on floodplains increases flooding risks and costs, though mitigation strategies like flood insurance programs and limiting rebuilding can help reduce damage.

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April Martínez
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59 views3 pages

Types of Human Effects

Human activities can increase flooding in several ways. Preventing ground absorption of water and adding sediment to rivers from activities like logging, urban development, and overgrazing means water runs off faster into streams, allowing smaller rain events to cause flooding. Dams control flooding but also disrupt habitats, while bridges and levees constrict river channels and artificially raise water levels, increasing current speed and scouring. Old levees often fail due to erosion or seepage, worsening floods, but modern engineered levees are more stable if maintained. Allowing development on floodplains increases flooding risks and costs, though mitigation strategies like flood insurance programs and limiting rebuilding can help reduce damage.

Uploaded by

April Martínez
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Topic 16: Human Effects on Flooding Types of Human Effects Preventing ground soaking, which forces rapid/more water

r 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

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

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 the geologic 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

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