The Role of Groundwater in Geologic Hazards

W. Richard Laton, Ph.D., RG

Assistant Professor, Department of Geological Sciences, CSUF

Natural Hazards• Almost every imaginable natural

hazard exists in Southern California.

• Southern California is a complex area of differing geologic and hydrologic conditions representing many differing hazards.

• Hazard identification defines the magnitude and associated probabilities of natural hazards that may pose threats to human interests in specific areas.

Why do we need to study Natural Hazard Areas? Planning!

• Where (Identification)• Why (Understanding

the system)• What can we do

(mitigation)• Prevention

Who’s affected

• 35 million California citizens

• City, County and State Regulators

• Builders and Contractors

• Businesses

GIS and Natural Hazards

• Through the advanced utilization of GIS, maps of Engineering geology, shallow groundwater, surface water, liquefaction, subsidence, and landslide susceptibility can be produced.

Groundwater

• Groundwater is usually the lost component when it comes to defining hazards.

• Through GIS, groundwater becomes an important player in hazard management.

• Susceptibility maps of Liquefaction, Subsidence and Landslide prone areas

Example of Regional Groundwater Maps

• Riverside County shallow groundwater map was derived from over 12,000 water well records

• Used for Liquefaction and Subsidence maps

Problems with Regional Groundwater Maps

• Comprised of incomplete well records

• Incomplete well completion records

• Not enough coverage area

• Lack of aquifer information

Deep wells vs. Shallow wells

• Water levels less than 50-feet of most importance

• Most regional maps based on deeper wells

• Shallow well records come primarily from environmental investigations

Natural Hazards

• Liquefaction

• Subsidence

Groundwater and Hazard Identification

• Liquefaction – water within 50 feet of surface

• Liquefaction – over watering/irrigation

• Subsidence – groundwater withdrawal

Liquefaction Susceptibility

• Derived from Groundwater and Engineering Geology maps

• Identifies susceptible conditions in 4 general categories

• Used for geotechnical design-level studies

• Used for Zoning and Loss estimation analysis

Causes

• Shallow groundwater + fine grained materials + earthquakes

• Over watering - irrigation + fine grained materials + earthquakes

Liquefaction Flow Chart

Subsidence Susceptibility

• Derived from Groundwater and Engineering Geology maps

• Identifies susceptible conditions in 2 general categories

• Used for geotechnical design-level studies

• Used for Zoning and Loss estimation analysis

Engineering Geologic Materials Map of Temecula, California

Shallow Groundwater Map of Temecula, California

Subsidence Susceptibility Map, Temecula, California

Subsidence

• Un-regulated groundwater withdrawal

• Hydro compaction – Hydro consolidation

• Oil or gas withdrawal

What can we do better?

• Better well records• Better logging

techniques• Sharing of records and

data• Central housing of all

records• Use regional maps for

what they were intended

Conclusions• Regional maps should be

only used for gross planning purposes

• More detailed groundwater information is needed for less populated locations

• Need for better information gathering during soil and groundwater investigations