1.
Flood Hazard Mapping in Small Island Developing States
using GIS-based geomorphological analysis techniques
and LiDAR datasets
Donal Neville, Marc Girona-Mata and Clive Carpenter
GWP Consultants LLP, Water Resources Department, United Kingdom
donaln@gwp.uk.com

2.
Outline
• Geomorphological approach to preliminary flood
hazard mapping using ArcGIS
• Piloted in Samoa as part of 2 projects
• Deliverable: national and district scales flood
hazards GIS products

3.
Stakeholders
Pilot Program for
Climate Resilience
(PPCR)
Enhancing Resilience of
Coastal Communities to
Climate Change
Government
of Samoa

4.
Samoa: a case study
• Small Island Developing State (SIDS)
• Volcanic island in the Pacific Ocean
• Tropical cyclones and tsunamis hazards
• Data availability dichotomy
– Lack of hydrometric data
– LiDAR coverage for the entire country

5.
Data & Methods
• High-resolution terrain morphology data
– 1m resolution LiDAR-derived DEM
– High-resolution (0.25cm) aerial imagery
– No other data (i.e. rainfall, flow, geology/soil) included
• Geomorphological GIS-based analysis
– Using ArcGIS terrain processing & hydrological tools
• Spatial Analyst and 3D Analyst
• Field activities (i.e. method & results validation)
– Collector App to:
• Aid topographic ground surveying
• Populate the Drainage Infrastructure Database
• Verify GIS products (ground truthing)

6.
Classified
LiDAR point
cloud

7.
Resulting LiDAR-
derived Digital
Elevation Model
(DEM)

8.
ArcGIS
modelling
routines

9.
Limitations
Many!
• LiDAR-derived DEM inaccuracies
• Areas with no LiDAR coverage (due to cloud cover)
• Disregard to many factors
– lack of data availability / reliability
– need for a simplistic approach
• Final results are difficult to validate

10.
Results
• GIS Deliverables – Flood Hazard Products
• Simple catchment metrics
• Fluvial flood hazard products
• Coastal flood hazard products
• Drainage Infrastructure Database
• Other interesting insights, e.g.:
• Identification of sub-surface cavities (i.e. lava tubes)
• Potential for risk mapping and communication approach

11.
Detailed catchment categorisation
using simple metrics

12.
Detailed fluvial geomorphological flood hazard products

13.
Elevation-based coastal
inundation flood hazard
products

14.
Zones of
uncertainty
Potential DEM
inaccuracies due to
cloud cover and/or
flat & densely
vegetated areas

15.
Ground truthing and product delivery
Infrastructure details logged using
Collector App (700+ infrastructure
objects in 5 weeks!)
ArcGIS Online – Web Maps and
Applications used to present and
deliver information to non GIS users

16.
Lava tubes
• Partially collapsed sub-
surface cavities
• Identifiable by delineation
of DEM ‘sinks’

17.
GIS-based Risk mapping
𝑅𝑖𝑠𝑘 =
𝐻𝑎𝑧𝑎𝑟𝑑 𝑥 𝑉𝑢𝑙𝑛𝑒𝑟𝑎𝑏𝑖𝑙𝑖𝑡𝑦 𝑥 𝐸𝑥𝑝𝑜𝑠𝑢𝑟𝑒
𝐴𝑑𝑎𝑝𝑡𝑖𝑣𝑒 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦
• Hazard refers in this case to the flood event
• Vulnerability refers primarily to reliance on other persons (e.g. children, elderly) and the
resilience of infrastructure (i.e. whether it was constructed with potential hazards in mind)
• Exposure refers primarily to location of any given receptor (but also proximity to escape routes
when considering persons) – spatial awareness
• Adaptive Capacity refers to ability to prepare, respond and/or recover from the event

18.
Lessons Learned
The piloted approach to GIS geomorphological flood hazard mapping
using ArcGIS:
• Cost-effective (when LiDAR is available!) and has led to
preliminary consistent results across the study region
• Enables the identification of high hazard areas requiring
further, more detailed, flood modelling efforts to better understand
risk
• Leads to easy-to-interpret results, and it therefore has potential
for improving risk communication strategies

19.
Recommendations / next steps
• Introduce rainfall data in a simplistic manner (e.g., using ArcGIS
routines to develop unit hydrographs)
• Further develop risk mapping – ArcGIS routines to identify areas
of high risk
• Risk communication approaches – develop user friendly ArcGIS
Online Applications
• Assess the role of sub-surface cavities (i.e. lava tubes) into
rainfall runoff routing – ArcGIS routines to assess their role in
flooding occurrence

20.
Thank you!
Donal Neville, Marc Girona-Mata and Clive Carpenter
GWP Consultants LLP, Water Resources Department, United Kingdom
donaln@gwp.uk.com