INTEGRATED FLOOD MANAGEMENT IN SAMOA 3A... · 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Time (hours) Fl ow (m 3 /s) 1 hour storm storm duration 2 hour storm duration 3 hour

Pacific Islands Applied Geoscience Commission www.sopac.org

INTEGRATED FLOOD MANAGEMENT

IN SAMOAFrom Science to Policy

Amataga Penaia1, Ausetalia Titimaea1, Jude Kohlhase1, Filomena Nelson1, Netatua Pelesikoti2, Michael Bonte-Grapentin2, Litea Biukoto2, Allison Woodruff2& Darren Lumbroso3

1 MNRE – Government of Samoa, 2 SOPAC, 3 HR Wallingford


Why Flood Management?• Apia the capital of Samoa is partly

located on the floodplains of 5 streams• High intensity rainfall causes severe

flooding 1939, 1974, 1975, 1982, 1990, 1991, 2001, 2006

• Extreme events likely more frequent in future (Climate Risk Profile Samoa)

• Costs are high e.g. 2001 direct damage of 4 Million US$, not counting frequent disruption of business and basic services

ADB 2006


Why……issues• Health risks (waterborne diseases)• Lack of coordination and integrated planning

among agencies that should be responsible for flood risk reduction

• Lack of town planning in Apia - many commercial and residential properties are developed on floodplain

• Weak monitoring and early warnings to communities at risk

• Constraints in budgetary allocation to mitigation activities

• Limited capacity (staff number & skills)






Process Applied• Development part of SOPAC/EDF8 Reducing

Vulnerability Project• Capacity Building in Flood Hydrology, River

Modelling and Flood Mapping (from data capture to analysis and modelling)

• Development of Rainfall/Runoff and Flood Inundation Models

• Production of Flood Hazard Maps• Evaluation of Flood Management Options • Development of Policy Documents

– Flood Management Action Plan 2007-12– Floodplain Management Guidelines

• Benefit-Cost Analysis of Mitigation Options


Geographic Scope• Flood management

process is countrywide• Pilot: Vaisigano River

– flow records available – drains into Apia, – largest basin on Upolu– 35 km2

– IWRM Hotspot

ApiaHarbour

0 km

5 km

Vaisiganocatchmentboundary

ApiaHarbour

0 km

5 km

Vaisiganocatchmentboundary

Savai’i

UpoluApia

Pacific Ocean

Vaisigano catchment


PART IScience


Data Constraints• Only 22 years of Annual Max Flow• Many gaps due to equipment failure

or lost records (e.g. TC Ofa)• Available flow gauging only up to

1/10 of peak flood flows• Only limited intensity data available

in 100+ year rainfall record• High degree of uncertainty in

estimated flood flows ~ ± 20%


Estimation of Peak FlowMethod 1 in 100 yr

(m3/s)

Statistical single site analysis (22 yrs)Revised rating curve 656Weir equation 181Regional flood frequency analysisMAF based on revised rating 411MAF assuming higher 2001 flood flow 461MAF based on weir equation 253MAF based on regression for Pacific basins 636Rainfall – runoff modellingHEC-HMS kinematic wave model 560HEC-HMS SCS lumped model 564


Hydrograph and Design Storm• Critical storm duration

for flooding is 2-3 hrs • Estimated rainfall

intensities (mm/hr):

• Sparse calibration data available from 2001 and 1974 events

0

100

200

300

400

500

600

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Time (hours)

Flow

(m3 /s

)

1 hour storm storm duration 2 hour storm duration 3 hour storm duration 4.5 hour storm duration

Duration(hr)

1 in 10years

1 in 50years

1 in 100years

2 66 80 87

3 47 57 62

0

50

100

150

200

250

300

350

400

15/04/200119:12

15/04/200120:24

15/04/200121:36

15/04/200122:48

16/04/200100:00

16/04/200101:12

16/04/200102:24

16/04/200103:36

16/04/200104:48

16/04/200106:00

16/04/200107:12

Date and time

Flow

(m3 /s

)

HEC-HMS hydrograph with Yeo rainfall HEC-HMS hydrograph with Meteorology Division rainfallUncorrected flow at Alaoa East using revised rating Corrected flow at Alaoa East using revised rating


Flood Extent 2001

1.1 m

1.3 m

0.7 m

500 m

Apia Harbour

20 m

conto

ur

Leone

0.7 m0.8 m

0.8 m

Vaisigano River

Faatoia

1.2 m0.8 m

0.4 m

Mat

afag

atel

eSt

reet

Cross Island Road

Lelata Bridge

Major roads

The approximate area inundated during the April2001 flood

1.1 m The depth of water above ground level during the April2001 flood

20 m contour lineRiver

Legend

Direction of flow

0 m Scale

N

1.1 m

1.3 m

0.7 m

500 m

Apia Harbour

20 m

conto

ur

Leone

0.7 m0.8 m

0.8 m

Vaisigano River

Faatoia

1.2 m0.8 m

0.4 m

Mat

afag

atel

eSt

reet

Cross Island Road

Lelata Bridge

Major roads

The approximate area inundated during the April2001 flood

1.1 m The depth of water above ground level during the April2001 flood

20 m contour lineRiver

Legend

Direction of flow

0 m Scale

Nafter S. Yeo 2001


1D HEC-RAS Model

24.00

23.50

23.00 21.50

21.00

20.00

19.50 18.00

17.00

15.00 13.00

12.00 10.00

8.00

7.00

5.00 4.00 3.00

2.00 0.50

Vaisig 3bridges + 4weirs Plan: 3brdge+4weir 03/08/2006 Legend

WS 500 m3/s

Ground

Bank Sta

Ineff

Estimated locationand height ofrock weirs

Leone Bridge

Vaisigano Bridge

24.00

23.50

23.00 21.50

21.00

20.00

19.50 18.00

17.00

15.00 13.00

12.00 10.00

8.00

7.00

5.00 4.00 3.00

2.00 0.50

Vaisig 3bridges + 4weirs Plan: 3brdge+4weir 03/08/2006 Legend

WS 500 m3/s

Ground

Bank Sta

Ineff

Estimated locationand height ofrock weirs

Leone Bridge

Vaisigano Bridge


Critical River Sections


Flood Depth Maps1 in 100years1 in 20

years


Assessing Flood HazardFloodwater depth

(m) xwater velocity (m/s)

(m2/s)

Degree of

hazard

Description

Less than 0.75 Low CautionFlood zone with shallow flowing water or deep

standing water0.75 to 1.25 Moderate Dangerous for some (i.e. children)

Danger: Flood zone with deep or fast flowing water

1.25 to 2.5 Significant Dangerous for most peopleDanger: Flood zone with deep fast flowing water

Greater than 2.5 Extreme Dangerous for allExtreme danger: Flood zone with deep fast

flowing water


Flood Hazard Maps1 in 100years1 in 20

years


Assessing Flood Risks

Return period

People @ risk Buildings @ risk

1 in 2 1139 244

1 in 5 1382 296

1 in 20 1536 329

1 in 50 1596 342

1 in 100 1634 350

• Estimate impacts to people and infrastructure

• Benefit-Cost Analysis


Assessing Mitigation Options• Structural :

– Flood walls and embankments.

– By-pass channel.– Upstream flood

storage.– Increasing channel

conveyance.– Flood proofing of

houses– Improving channel

maintenance.– Pumping.

• Non-Structural– Floodplain zoning &

Development control– Flood forecasting and

warning– Flood insurance– Flood preparedness

and response plans– Public Awareness– Land use change


Flood walls

Location of floodembankmentsLocation of floodembankments

Leone bridge


Flood embankments or walls

2 m2 m

12

12

12

12

2 m2 m

12

12

12

12

Average height of the embankment (m)

Return period (years)

Left bank Right bank 1 in 2 0.96 0.61 1 in 5 1.72 1.35 1 in 20 2.54 2.13 1 in 50 2.83 2.40

1 in 100 3.24 2.81


Diversion channel


Diversion channel 2

30 m

2

1

2

1

Top width of channel varies depending on the flow to be carried

30 m

2

1

2

1

Top width of channel varies depending on the flow to be carried


Capacity of the channel (m3/s)

Depth of channel (m)

Top width of channel (m)

1 in 2 74 0.9 33.6 1 in 5 183 2.0 38.0 1 in 20 318 2.1 38.4 1 in 50 370 2.4 39.6

1 in 100 450 2.7 40.8


Flood proofing of buildings

•The design and construction of buildings with appropriate water-resistant materials. •Raised floor levels above a flood with a specified return period (annual probability of occurrence).

1in 50 year flood level (2% annual probability ofoccurrence)


Flood Forecasting• Forecasting flood levels several hours in

advance is difficult owing to nature of catchment

• Possible to issue flood warnings based on flash flood guidance estimates and scenarios.

• Requires access to a weather prediction model (Australian) that provides hourly rainfall estimates


Flood Warnings• Establish a mechanism to warn

communities in the lower Vaisigano River even if a flood occurs in the middle of the night

• Develop a series of staged flood warnings

• Establish a process (e.g. an advertising campaign in the media) to ensure that communities understand what each different warning means.


Conclusions - Mitigation Measures

• Pumping, Catchment Management and Channel Maintenance have only limited effects on flood flows in Samoa

• Reduce uncertainty in data (hydrological, topographical etc) for more accurate assessments

• Update models, maps and Action Plan as more accurate data becomes available


PART IIPolicy


Recall the Processflood hydrology

river modelling

flood plain mapping

policy review

consultation

mitigation options

Capacity building and information development and management

Flood Plain Guidelines and Action Plan

Cost Benefit Analysis of flood plain mitigation options


Floodplain management guidelineFloodplain management is concerned both with land use planning and flood management planning

Guideline Objectives:1. Objectives of guidelines is to:

2. Raise awareness of floodplain management issues

3. Provide guidance on floodplain management to reduce flood risks within the present legislative context for land use planning and flood management in Samoa.

• Responsible for land use planning and flood plain planning

• Development assessment and consent before a building permit could be issued by the Ministry of Works, Transport and Infrastructure (MWTI)


Development assessment• Development control is concerned with

whether or not developments should be allowed/constructed in areas that can flood and, where developments are permitted, the conditions or controls attached to the development of defined areas.

Such controls are aimed at, for example:• reducing the risk of buildings and other

assets being flooded.• reducing the resulting damage when

above floor flooding occurs.• avoiding increases in flood risk

elsewhere.


Con’t..Typical development control requirements

may include for example:

• identification of areas where building may and may not be permitted.

• minimum floor levels.

The safety of people during a flood event is important in the development of areas that are at risk from flooding. Evacuation can be very hazardous if safe evacuation routes are available


Floodplain Management Guidelines

• Provide generic background into management of floodplains

• Planning Tool– Provide guidance/checklist for

development assessment in floodplains according to level of risk

– Guide for integrating flood risk assessment into EIA process


Defined floods and flood risk zonesFloodwater depth

(m) xwater velocity (m/s)

(m2/s)

Degree of hazard

(rate of rise of water, warning time and

flood duration)

Description

Less than 0.75 Low CautionFlood zone with shallow flowing

water or deep standing water0.75 to 1.25 Moderate Dangerous for some (i.e. children)

Danger: Flood zone with deep or fast flowing water

1.25 to 2.5 Significant Dangerous for most peopleDanger: Flood zone with deep fast

flowing waterGreater than 2.5 Extreme Dangerous for all

Extreme danger: Flood zone with deep fast flowing water

High risk

“Functional floodplain”

Floodway

“Non-functional floodplain”


Flood risk area

1 in 1,000 year annual probability flood

1 in 100 year annual probability flood

Low tomediumrisk

Low tomediumrisk

Little or noflood risk


(a) Defined floods

(b) Flood risk zones

High risk

“Functional floodplain”

Floodway



Flood risk area

1 in 1,000 year annual probability flood

1 in 100 year annual probability flood

Low tomediumrisk

Low tomediumrisk



(a) Defined floods

(b) Flood risk zones


1. Examples of landuse on flood zones

Degree of risk Little/ none

Medium High


Non-functional floodplain: Developed

Non-functional floodplain: Undeveloped/sparse

Functional floodplain

Land use

River flooding

<1 in 1000 1 in 1000 to 1 in 100

>1 in 100 >1 in 100 >1 in 100

Open space/recreation Essential transport and utilities

(1) (1) (1)

Residential (2) (3) Commercial (2) (3) Industrial (2) Public institutions Hospitals Schools (4) Police Emergency services

• Notes:• (1) Should be operational in a flood.

Compensation works needed to avoid an increase in flood risk elsewhere.

• (2) Assuming appropriate flood defenses are provided.

• (3) Limited developments permitted in certain circumstances.

• (4) Not main school buildings and access routes.


Action Plan• Provides Background of Flood Mitigation

Options (Vaisigano/Samoa)• Identifies 48 Actions for Integrated Flood

Management, incl. responsible agencies and target dates

• Goals:1. Flood Risk Reduction 2. Strengthen Flood Preparedness and

Early Warning Systems3. Capacity Building in Flood Management4. Technological Information Management 5. Sustainable Watershed Management 6. Flood Governance


Purpose of Economic Analysis

• Ensures ‘efficient’ use of resources allocated to disaster management sector by implementing flood management measures with greatest ‘net’ benefits

• Advocacy tool- demonstrates the long-term savings that result from being pro-active by investing in flood management measures in the short-term


Flood management measures

• Priority flood management measures identified by stakeholders in March 2007:

• Structural measuresFloodwalls/embankmentsBy-pass/diversion channel

• Non-structural measuresImproved flood forecasting systemDevelopment control- elevated floor heights


Damages Associated with FloodingFlooding imposes significant costs on households and businesses in the lower Vaisigano:

Direct DamagesDamage to housesDamage to household contentsDamage to shops/officesDamage to business stockDamage to infrastructure (roads, power, water)Damage churches and schools

Indirect DamagesLost household incomeHousehold clean-up costsBusiness clean-up costsLost revenues


Impact of Flooding

Sources of data-Household and business surveys- SOPAC flood maps- Stage-damage curves

Flood events- 1 in 5, 1 in 20, 1 in 50, 1 in 100

Annual average damage estimated to be approximately WST$620,000 per year


Economic Pay-off from Investing in Selected Flood Management Measures

Flood measure Best case

Floodwalls 0.64

Diversion channel 0.09

Improved forecasting system 1.92

8.07

44.38

Elevated floor heights:

Existing homesNew homes

E.g. Estimated for every tala invested in constructing homes with raised floor heights, a maximum of WST$44 is avoided in future flood damages


Conclusions and RecommendationsStructural flood management options generally not cost-

effective due to high construction and maintenance costs

Improved flood forecasting system should be implemented, however this must be done in conjunction with improved warning systems and public awareness campaigns

Economic pay-off from raised floor heights in new homes very high

Economic analysis can assist with prioritizing which flood measures should be implemented and assist with leveraging funding for implementing the most cost-effective measures