Flood and Lahar Control

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Flood and Lahar Control

ALAS, NORVIE ALAS, NORVIEP.P. QUIJAN

QUIJANO, JAMESO, JAMESGOODWGOODWILLILL TEJAD

TEJADA, ALLANA, ALLANCEDRCEDRIC P.IC P. TOREJA, ROMARK TOREJA, ROMARK

CE-4202 CE-4202

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Flood Control

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Flood Control

Flood

-

an over

an overflowin

flowing of a larg

g of a large amount

e amount

of water

beyond its normal confines, especially over what is

normally dry land.

Flood Control

-

refer

refers to all

s to all metho

methods use

ds used to

d to

reduce or prevent the detrimental effects

of

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Types of flooding

Areal

Areal –– when floods happen on flat or low-lying areas and when when floods happen on flat or low-lying areas and when water is supplied by rainfall or snowmelt more rapidly than it can water is supplied by rainfall or snowmelt more rapidly than it can

ei

eiththerer ininfifiltltraratete oror rurun n ofoff.f. Riverine

Riverine -- occurs occurs when rwhen rivers bivers burst thurst their baneir banks as a rks as a result esult ofof sustained or

sustained or intense rainfall.intense rainfall. Flooding can be Flooding can be worsened by meltingworsened by melting snow.

snow.

Estuarine and Coastal

Estuarine and Coastal -- is commonis commonly causely caused by a combinad by a combination oftion of sea

sea tidal tidal surges surges causecaused bd byy windswindsand and lowlow baromebarometric tric presspressure, ure, andand they may be exacerbated by high upstream river flow.

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Types of flooding

Urban Flooding

Urban Flooding -- is this the inune inundation dation of lanof land or pd or propertroperty in ay in abuiltbuilt environment, particularly in more densely populated areas, caused environment, particularly in more densely populated areas, caused by rainfall overwhelming the capacity of drainage systems

by rainfall overwhelming the capacity of drainage systems, such, such as

as stostorm serm sewerwers.s. Catastrophic

Catastrophic -- is usis usually ually assocassociated iated with mawith major injor infrastfrastructurructuree failures such as the collapse of a dam, but they may

failures such as the collapse of a dam, but they may also be caused byalso be caused by drainage channel modification from

drainage channel modification from a

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AREAL FLOODING

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R

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CATASTROPHIC FLOODING CATASTROPHIC FLOODING

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Lahar Control

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Introduction

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Definitions

Volcanic Debris Flow (Lahar) Volcanic Debris Flow (Lahar)

–

– A type of mudflow or debr A type of mudflow or debris flow composed of slurry ofis flow composed of slurry of pyroclastic material, rocky debris and water.

pyroclastic material, rocky debris and water. The materialThe material flows down from a volcano, typically along the river valley. flows down from a volcano, typically along the river valley. Lahars are extremely destructive: they can flow

Lahars are extremely destructive: they can flow tens oftens of meters per second, be 140 meters (460ft)

meters per second, be 140 meters (460ft) deep, anddeep, and destroy and structures in their path.

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Volcanic Debris Flow (Lahar)

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Definitions

Volcanic Debris Flow (Lahar)

–

– Any mass movement

Any mass movement involving volcani

involving volcani

cally derived,

sand-size material or finer.

–

This may be total mass moved.

–

More commonly this serves as a

matrix for the

movement of larger volcanic clasts.

–

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Definitions

Volcanic Debris Flow (Lahar)

•

• Thus range from slumped deposits of ash and

Thus range from slumped deposits of ash and

hyaloc

hyaloclasti

lasti

tes

to me

to megabrec

gabreccias.

cias.

• Deposits may be relatively local (few 100 m’s

in

length and few meters thick)

• To regional (10’s of km’s long and

> 100 m in

thickness)

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Formation

•

• During eruptionsDuring eruptions

•

• Reworking of volcanic debris during times of quiescence or Reworking of volcanic debris during times of quiescence or afterafter

volcano has become

volcano has become extinctextinct

• Ways of

• Ways of formationformation::

–

– Heavy rainsHeavy rains

–

– Melting snow or ice Melting snow or ice (Jokulhaups)(Jokulhaups)

–

– Walls of crater la Walls of crater lake collapseke collapse

–

– HydrothermHydrothermal al alteration-alteration- clay-rich clay-rich rocksrocks

–

– Caldera collapse and mass wasting of wallsCaldera collapse and mass wasting of walls

–

– EarthquakesEarthquakes

–

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Mass Movement

•

• Once under way controlling factors for flow Once under way controlling factors for flow length andlength and speed are:

speed are: –

– Grain sizeGrain size –

– Grain size distribution-electrochemical forces-clayGrain size distribution-electrochemical forces-clay and silt-size

and silt-size –

– Solid Solid to wateto water ratir ratio-o- laminlaminar flowar flow Thicker near source

Thicker near source

Follows valleys and depressions, fans out away

Follows valleys and depressions, fans out away fromfrom valleys

valleys

Can cause erosion by undercutting banks of

Can cause erosion by undercutting banks of streams orstreams or those of s

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Since mid-June 1991, when

Since mid-June 1991, when violent eruptions triggered

violent eruptions triggered

Mount Pinatubo's first lahars in 500 years, a system to

monitor and warn of lahars has been in operation.

Radio-telemetered rain gauges provide data on rainfall

telemetered rain gauges provide data on rainfall

in lahar

source regions, acoustic flow monitors on stream banks

detect ground vibration as lahars pass,

and manned

watchpoints

further confirm that lahars are r

further confirm that lahars are rushing down

ushing down

Pinatubo's slopes. This system has enabled warnings to

be sounded for most but not all major laha

be sounded for most but not all major lahars at Pinatubo,

rs at Pinatubo,

saving hundreds of lives. Physical

preventative measures

by the Philippine government we

by the Philippine government were not adequate to stop

re not adequate to stop

over 20 feet (6.1 m) of mud from flooding many villages

around Mount Pinatubo from 1992 through 1998.

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TITAN2D

is a free software application developed by

the Geophysical Mass Flow Group at the

State

University of New York (SUNY) at Buffalo. TITAN2D

was developed for the purpose of sim

was developed for the purpose of simulating granu

ulating granular

lar

flows (primarily geological mass flows such as debris

avalanches and landslides) over digital elevation

models (DEM)s of natural terrain. The code is

designed to help scientists and civil protection

authorities assess the risk of, and

mitigate, hazards due

to dry debris flows and

to dry debris flows and avalanches. TITAN2D

avalanches. TITAN2D

combines numerical simulation

combines numerical simulations of

s of a flow with

a flow with digital

digital

elevation data of natural terrain supported through a

Geographical Information System (GIS) interface such

as GRASS.

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Types of Lahar & Flood

Control Structures

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Types of Lahar & Flood Control Structures

•

• Check DamsCheck Dams

•

• Retaining WallsRetaining Walls

•

• BundingBunding

•

• Sausage GroynesSausage Groynes

•

• Gabion/Mattress GroynesGabion/Mattress Groynes

•

• Paved DrainsPaved Drains

•

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Types of Lahar & Flood Control Structures

A dam is a barrier that imp

ounds water or

underground streams. Dams generally serve the

primary purpose of retaining water, while

other

structures

structures such

such

as floodgates

as floodgates or levees

or levees (also known

(also known as

as

dikes) are used to

manage or prevent water flow into

specific land regions.

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Types of Lahar & Flood Control Structures

Check Dams Check Dams

•

• These are small gravity dams, These are small gravity dams, usually constructed withusually constructed with

rocks and mortar or concrete, of variable height and rocks and mortar or concrete, of variable height and width.

width.

•

• This type of structures are located in small or mediumThis type of structures are located in small or medium

sized gullies to stabilize riverbed slopes and prevent soil sized gullies to stabilize riverbed slopes and prevent soil erosion.

erosion.

•

• Check dams, protect gullies from Check dams, protect gullies from being eroded by rainfallbeing eroded by rainfall

and runoff impact. and runoff impact.

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CHECK DAMS CHECK DAMS

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Types of Lahar & Flood Control Structures

Retaining Walls Retaining Walls

•

• These are rock/concrete block structures built on steepThese are rock/concrete block structures built on steep

slopes anywhere in the watershed, where the erosion of slopes anywhere in the watershed, where the erosion of the base foundation threatens lands and/or homes.

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RETAINING WALLS RETAINING WALLS

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Types of Lahar & Flood Control Structures

Bunding Bunding

•

• BundingBunding is the is the general name general name used in used in Jamaica for flexiJamaica for flexibleble

structures of variable thickness and length, composed of structures of variable thickness and length, composed of galvanized wire mesh, stone, wild-cane and riverbed

galvanized wire mesh, stone, wild-cane and riverbed materials.

materials.

•

• BundingBunding is used is used to prevent bto prevent bank erosion ank erosion and landslipsand landslips

and to protect agricultural lands from being flooded. and to protect agricultural lands from being flooded.

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Types of Lahar & Flood Control Structures

Sausage Groy

Sausage Groynesnes

•

• These are long, cylindrical, slightly flexible structures ofThese are long, cylindrical, slightly flexible structures of

variable thickness, composed of wi

variable thickness, composed of wire and rocks.re and rocks.

•

• GroynGroyneses are placeare placed at the foot of banks alod at the foot of banks along small,ng small,

slow-moving streams, where there are signs of slow-moving streams, where there are signs of

undermining and threats to permanent structures. undermining and threats to permanent structures.

•

• SausaSausage groynege groyness are desiare designed to stop sucgned to stop such action byh action by

allowing the bank to collapse to an angle of repose in allowing the bank to collapse to an angle of repose in such a manner as to form a moderate slope, which such a manner as to form a moderate slope, which encourages the growth of vegetation.

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Types of Lahar & Flood Control Structures

Gabion/Mattress Groynes Gabion/Mattress Groynes

•

• These are long, flexible structures of variable thickness,These are long, flexible structures of variable thickness,

composed of wire and rocks. composed of wire and rocks.

•

• They are placed on the shaped banks of large, fastmovingThey are placed on the shaped banks of large, fastmoving

streams where severe erosion is occurring and many pose streams where severe erosion is occurring and many pose a danger to permanent structures.

a danger to permanent structures.

•

• Mattress groynesMattress groynes are designeare designed to d to prevent the prevent the furtherfurther

erosion of the riverbank. They trap soil particles to allow erosion of the riverbank. They trap soil particles to allow a build-up of soil; thereby encouraging the growth of

a build-up of soil; thereby encouraging the growth of vegetation.

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GABION WALLS GABION WALLS

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Types of Lahar & Flood Control Structures

Paved Drains Paved Drains 1.

1. LeveesLevees -- are mare made oade of claf clay or eary or earth fith fill mall materterial arial aree being used with som

being used with some structural modifications and havee structural modifications and have proven quite effective. The slopes of an earth

proven quite effective. The slopes of an earth fill leveefill levee should be no more than 2:1.

should be no more than 2:1.

2.

2. Flood WaFlood Walls of reinforlls of reinforced concrete -ced concrete - require veryrequire very little space and are often used

little space and are often used to protect cities andto protect cities and developed areas. They are costly to construct, but developed areas. They are costly to construct, but involve minimal maintenance costs.

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Types of Lahar & Flood Control Structures

Paved Drains Paved Drains 3. Channel Alter

3. Channel Alterations -ations - reduce floods by deepeningreduce floods by deepening and widening the channel by

and widening the channel by cutting meanders. Sometimescutting meanders. Sometimes these works can have undesirable effects, by aiding the

these works can have undesirable effects, by aiding the sediment transport process. Care must be taken when sediment transport process. Care must be taken when channel alternatives are considered.

channel alternatives are considered.

4. Detention Ponds

4. Detention Ponds -- ponds placed on and off-side, canponds placed on and off-side, can minimize the damage to downstream structures by

minimize the damage to downstream structures by reducing peak flows. They should be considered in the reducing peak flows. They should be considered in the design of downstream protective structures.

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PAVED DRAINS PAVED DRAINS

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Types of Lahar & Flood Control Structures

Reservoirs Reservoirs

•

• Reservoirs are one of the most Reservoirs are one of the most direct methods of flooddirect methods of flood

control through storing surface runoff; thus, attenuating control through storing surface runoff; thus, attenuating flood waves and storing flood water

flood waves and storing flood water to be redistributedto be redistributed without exceeding downstre

without exceeding downstream flood conditions.am flood conditions.

•

• For flood control, it is ideal to maintain the reservoir atFor flood control, it is ideal to maintain the reservoir at

the lowest level possible for storage. the lowest level possible for storage.

•

• On the other hand, keeping the reservoir at On the other hand, keeping the reservoir at a high levela high level

provides the ability to maintain low flows

provides the ability to maintain low flows andand hydropower production in droughts.

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RESERVOIR RESERVOIR