NATURAL HAZARDS

William M. ShehataSaudi Geological Survey

Definition

Natural hazards are naturally occurring physical phenomena having atmospheric, geologic or hydrologic origin.

Natural hazards are not entirely natural for people are also agents of disaster.

Examples of Atmospheric Hazards

Hurricanes Tornadoes Drought Winter storms Dust storms

Examples of Geologic Hazards

Earthquakes Volcanic eruptions Landslides Subsidence

Examples of Hydrologic Hazards

Floods Tsunamis

Geological Hazardsin Saudi Arabia

Landslides, rock falls & debris flows.

Floods. Collapsible & swelling

soil. Sabkha soil. Volcanic hazards.

Dune migration & sand drifting.

Sinkholes & caves. Salt domes. Earth fissures. Earthquakes.

RE

D

SEA

ARABIAN

GULF

ARABIAN

SHIELD

ARABIAN

SHELF

Alwajh

Yanbu

Rabigh

Jeddah

Al-Lith

Tabuk

Hail

Ar-Riyadh

Jazan

Dhahran

Almadinah

42 54

28

20

Scale0 100 200

km

Great Nufud

Ad-Dahna

Elrub-Elkhali

17

Shield and Arabian Shelf

Sabkha

Loess & swelling soilEscarpmentMain valley courseSand sea

Volcanic area (Harrat)

Sinkhole or caves

Al-Jafu

rah

Salt domeEarth Fissures

Landslides, Rock Falls & Debris Flows

Landslides, Rock Falls & Debris Flow

Example of a slow failures

Landslide in California

Example of fast failures

Rock Fall Al Hada road

Al Hada road

Al Baha descend

Jabal Khandamah

Examples of debris flow

Al Baha Descent Bridge #8A failure

Rock Falls

Wedge failure at Al Jowah descent

Wedge failure at the road to Al Soda

Flood Hazards

Flood Hazards

Foods in Wadi

Bayish

Fatiha 1 (hr)

0

2

4

6

8

10

0 2 4 6 8 10 12 14 16 18 20 22 24 26

t (hr)

qp

(m

3 /s)

Sheet flood (Wadi Al Lith)

Collapsible & Swelling Soil

Collapsible & Swelling Soil

Swelling shale of Ra’an formation

(Tabuk)

Crystallization of salt in the shale

(Tabuk)5 cm

Gypsum crystallization

Heave in the shale bed

Collapse in a loess sample

Sabkha Soil

Sabkha Soil

Salt concentrations in sabkha

Sabkha Properties*

* Modified after Hodgson et al. (1985) & Dhowian et al. (1987)

Layer Average Thickness

(m)

Description SPT

(Soil class)

Crust 1.0-1.5 Fine sand-silt cemented with salts

9-16

(ML-SM)Compressible zone

8.0-10.0 Non plastic fine sand to highly plastic organic clay

1-6

(SM, CL, SC, OH)

Base ? Dense to very dense fine sand

Variable up to refusal

Corrosivity*

Sulfates Chlorides

Soil (%) 0.12 – 14.90 0.12 – 10.72

Groundwater (ppm)

1,098 – 3,876 14,200 – 97,625

*Sabkha soil and groundwater can generally be classified as very corrosive (Hodgson et al., 1985)

Effect of sabkha brines on structures

Volcanic Hazards

Volcanic Hazards

Fumarole

Volcanic vent

Harrat Al Madinah

Preliminary studies show an increase in the recurrence rate of volcanic activity in the past 3 million years.

The latest eruption in Harrat Al Madinah was in year 654 H. (1256 G.).

Dune Migration & Sand Drifting

Great Nufud

Elrub-Elkhali

Dune Migration and Sand Drifting

Dune migration

2 Years of Monitoring

Sand drifting erosion & deposition (Eastern Province)

Sinkholes & Caves

Sinkholes & Caves

Collapse sinkhole

(As Somman Plateau)

Collapse Sinkholes

(Al Khobar)

Cover Collapse Sinkholes

(Tabarjal)

Salt Domes

Jazan Salt Dome

Jazan 50 years ago

Jazan now

Salt dome 4 sq. km and 50 m

high

Salt mushrooms

Salt diapiric intrusion

Jazan salt dome

Salt dissolution leads to soil collapse

Rock Salt

EolianDeposits

MixedMaterial

SandstoneGypsum

Cavity

Beach sand

Eolian deposits

Cap rock

Sabkha

N

500 m

Damagedbuildings

Locations of affected buildings

(Modified after Erol, 1989)

Examples of affected buildings

Earth Fissures

Earth Fissures

Earth fissures due to groundwater withdrawal

(Tabah)

Earth fissures due to groundwater withdrawal

(Al Yutamah)

Earth fissures due to 1995 earthquake

(Haql)

Earth fissure associated with Red

Sea tectonics

(Jeddah).

(Al Quhtani, 1998)

Earthquakes

Earthquake Hazards

Earthquake Epicenters

Haql 1995 Earthquake

0 20 40 60

20

40

60

80

0

SPT (Blows/ft)

De

pth

(ft

)

Liquefactionvery unlikely

Liq

uef

acti

on

ver

y lik

ely

Liq

uef

acti

on

dep

end

s o

n s

oil

typ

e &

ear

thq

uak

e m

agn

itu

de

Maximum ground surface acceleration = 0.25 g

Groundwater level

0 20 40 60

20

40

60

80

0

SPT (Blows/ft)D

epth

(ft

)

Liquefactionvery unlikely

Liq

uef

acti

on

ver

y lik

ely

Liq

uef

acti

on

dep

end

s o

n s

oil

typ

e &

ear

thq

uak

e m

agn

itu

de

Maximum ground surface acceleration = 0.15 g

Groundwater level

Liquefaction Potential of Sabkha Soil

Hazard Mitigation

Any action taken to reduce or eliminate the long term risk to human life and property from natural hazards– Understanding of the geodynamic phenomena– Worldwide spread of scientific knowledge– Adoption of appropriate public and management

policies– Increase of awareness programs– Landuse restrictions– Adequate building construction– Wise environmental management

Role of theSaudi Geological Survey

Towards Hazard Mitigation

Define, analyze and recommend remedial measures through its rapid response teams and in-house projects.

Prepare comprehensive and analytical engineering geological maps for the major cities in the Kingdom

Natural Seismograph