Lecture 29 Subsidence Types Subsidence Less obvious than

Lecture 29 Subsidence Types

Subsidence • Less obvious than catastrophic sinkhole formation • Develops gradually and is typically widespread • You can’t tell by looking that the San Joaquin Valley floor was once 35 ft higher in some locations • Mapping is critical: In. SAR (inteferometric synthetic aperture radar) uses repeat-pass radar images from Earth-orbiting satellites to measure subsidence at sub-centimeter resolution

Ground subsidence • Can occur owing to the compaction of material as • Water is drained from the soil, often caused by a reliance on groundwater supplies. • Organic material decays • Oil is removed

• In the arid southwest, a visible sign may be fissuring of the earth.

Groundwater pumping and land subsidence

Amount of subsidence • West of Phoenix: 18 feet • Las Vegas: 6 feet • Albuquerque: 1 foot • California • Lancaster: 6 feet • Davis: 4 feet • Santa Clara Valley: 12 feet • Houston, TX: 9 feet

San Joaquin Valley

The future? • Increasing population in the southwest means more groundwater pumping, and continued subsidence, possibly spreading to metropolitan areas where damage will be great.

• Well head protrusion in Las Vegas.

Subsidence…free home remodeling. Las Vegas

• Some of the most spectacular examples of subsidence-related earth fissures occur in southcentral Arizona.

Subsidence in the LA Basin? Causes 1. Tectonic activity 2. Water and oil withdrawal • Examples include • Pomona (water); • Beverly Hills (oil) • San Pedro and Long Beach (probably oil)

Groundwater withdrawal, Lancaster

Texas: oil related subsidence. Bay waters intruded the oil field.

Historic Houston subsidence. Over 3, 200 mi 2 of Houston has sunk at least one foot. The Houston ship canal has sunk 10 ft. Cause: groundwater withdrawal.

Drainage of organic soils • Occurs when soils rich in organic carbon are drained for agriculture or other purposes. • The most important cause is microbial decomposition, which, under drained conditions, readily converts organic carbon to carbon-dioxide gas and water. • Compaction, desiccation, erosion by wind and water, and prescribed or accidental burning can also be significant factors.

Two important areas of subsidence: 1. 2. • • Sacramento-San Joaquin Delta Everglades Continuing organic-soil subsidence threatens agriculture Affects engineering infrastructure and water transfers Complicates ecosystem restoration Threatens the integrity of levees

Santa Clara Valley

Expansive soils • Soils which shrink in dry periods and swell during wet periods. • Causes differential movement which damages foundations. • Clay-rich

Expansive Soils

Expansive Soils • Check the soil survey for your area.

Tectonic Subsidence • Land subsidence due to earthquakes • Fault movement • Crustal deformation

Tectonic Subsidence Not to scale! Subsided region can be ~ 100 miles wide!

Permafrost • Melting of frozen soils • Common at higher latitudes • Why not lower latitudes? • Frost heaving (opposite of subsidence) also a hazard

Permafrost

Permafrost • Extent

Permafrost

Permafrost • Mine in permafrost

Permafrost

Deflation of Magma or Lava Reservoirs • Lava tubes may collapse much like caves • Common in Hawaii • ”Caldera collapse”

Toba Indonesia

Lava Tube Collapse

Links to other Natural Hazards • Earthquakes • How? • Volcanoes • How? • Floods • How?

Benefits • Karst provides unique and seldomly explored frontier • Caves contain many rare endemic species • Troglobites • New sinkholes create new lakes

Sinkhole lakes

• Florida has most sinkholes in nation

Caves • One of the last frontiers for exploration • Many provide fascinating irreplaceable features and rare species • Kentucky • New Mexico • Sierra Nevada – Sequoia/Kings Canyon NP • Hurricane Crawl Cave • Lilburn Cave • Ursa Minor Cave • Crystal Caverns

Hurricane Crawl Cave • One of Park’s most decorated

Lilburn Cave • Park’s longest (~20 miles)

Crystal Cave • Tourist Cave

Ursa Minor Cave • Park’s newest (non-alpine)

Mineral King • Best alpine karst in nation • Hundreds of sinkholes • ~130 cave entrances • Numerous springs and disappearing streams • Not fully mapped until last summer

Troglobites • Cave-adapted species