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Floodplain Mapping and Flood Risk ASsessment

Image credit: Hatfield Consultants

PRODUCT DESCRIPTION

Category

Component products

Integrated Product

  • § Building inventory
  • § Elevation
  • § Equipment mapping and status
  • § Flood extent
  • § Geomorphology map
  • § Land cover characterization
  • § Land use characterization
  • § River/lake ice (extent, timing, depth)
  • § Roads status
  • § Slope, curvature, aspect
  • § Soil sealing
  • § Surface deformation monitoring (environmental and production related)
  • § Surficial geology/soil type
  • § Transport network
  • § Urban area/settlement map
  • § Waterbody extent
  • § Wet areas (inc. ephemeral)
 

Uses

  • Surface geology mapping - structural interpretation
  • Surface geology mapping - terrain evaluation and geo-morphology characterization
  • Subsidence monitoring - land motion relating to fault lines or other causes
  • Environmental monitoring - natural hazard risk analysis
  • Logistics planning and operations - baseline mapping of terrain and infrastructure

Geo-information requirements

  • Land cover
  • Land use
  • Near surface geology
  • Terrain information
  • Topographic information
  • Water quantity & quality
  • Distribution and status of infrastructure
  • Distribution and status of assets

Description

Floodplain mapping provides a basis for evaluating flood hazard. When factored into field work or development planning, risks can be controlled. Characterisation of the floodplain can reduce costs and assist strategic planning related to equipment and operational risks. Industrial activity and changes to floodplain extent can also be monitored to address environmental concerns.

The floodplain is defined in terms of a hazard model, typically based on a high water return period such as 10 year or 100 year flood. Two floodplain limits are of interest: the floodway channel; and the flood fringe. The floodway is the area of deepest, fastest and most destructive flow for a given flood event. The fringe extends from the floodway limit to the limit of the hazard (flooded) area. For a suitable choice of flood return period, development within the floodway should be discouraged, and structures within the flood fringe should incorporate flood-proofing designs. Distinguishing these zones is possible using high resolution EO data, including optical imagery, radar imagery, and elevation data.

Floodplain limits from frequent periodic flooding, such as yearly high water, are usually visible in satellite or aerial imagery (e.g. through visual interpretation of lacustrine and fluvial soils, and by discrimination of typical vegetation types). Flooded land cover takes on different structure, texture and colour. It is possible to observe even in medium resolution images due to specific discolorations around the river/water body.  Evaluation of extreme event limits, such as 10 year or 100 year peak water, can be assessed from historic images and refined with historic records, hydrological modelling, and sediment core sampling.

Climatology and forecasting can assist in evaluating seasonal hazards based on modelling of peak flow. For example, in northern or high elevation regions, high snow pack levels before spring freshet would increase risks to downstream infrastructure and fieldwork. Factors such as snow pack extent (and even volume) can be estimated from EO data.

Detailed hydrological models depend on accurate elevation data. Depending of the size of a river and floodplain extent, different EO-derived elevation data can be used. Low resolution SRTM or ASTER datasets are useful for rivers with large flow and expansive floodplains. Higher resolution elevation data can be extracted from radar (using interferometric or radargrametry methods). The best quality and most accurate EO-derived elevation data is produced by optical systems (using stereo and tri-stereo images). See the Elevation product sheet for more details. Radar imagery is effective for deriving historic flood extent and can be combined with flow gauge records to calibrate hydrological models. See the Flood Extent product sheet for more details.

The floodplain product captures the extent of the floodway and flood fringe (vector coverage) and reports on hazards and risks based on knowledge of assets, infrastructure and historical flood patterns.

Known restrictions / limitations

  • Accuracy of mapping under forest canopy can be an issue.
  • Historic mapping is dependent on the completeness of image archives.
  • EO elevation products are not as detailed or accurate as LiDAR, but can be more cost effective and flexible. Precision elevation data requires ground control data.

Lifecycle stage and demand

Pre-license

Exploration

Development

Production

Decommission

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Pre-license: Access planning and safety risks.

Exploration: Reduces risks in development planning. Safer access for exploration and evaluation.

Development: Reduces development risks and down-time. Improves construction safety.

Production: Helps to mitigate environmental damage from improper land use, and reduces operating risks and down-time. Improves operation safety.

Decommissioning: Reduces safety risks associated with decommissioning clean-up and environmental monitoring.

Geographic coverage and demand

Demand is global, especially in areas with seasonal flooding. For example, monsoon areas or regions with significant seasonal snow pack melt.

Challenges Addressed

OTM:005  Monitoring natural fault movement

OTM:036  Geohazard exposure analysis

OTM:051  Identification of fault lines

OTM:065  Floodplain mapping

OTM:072  Monitoring flash floods

HC:2401  Identify geohazards and landscape change rates

HC:3101  Baseline and monitoring of areas with active faults and subsidence

HC:4302  Floodplain mapping and understanding flood extent and flood frequency

HC:4304  Situational awareness information on water levels and lake extents and potential flooding

HC:5102  Assess potential project site for historical use

PRODUCT SPECIFICATIONS

Input data sources

Optical:  HR1, HR2, MR1

Radar:  HR1, HR2, MR1

Supporting data:

  • Aerial imagery
  • Survey cross sections
  • Flow gauge data
  • Historical data such as maximum flood levels, disaster reports, etc.

Spatial resolution and coverage

10–100 m resolution is adequate for delineating floodplain extent from interpretation of soils. Vegetation interpretation may require higher resolution. Available data may cover a range of scales. High quality DEMs are preferred for accurate hydrological modelling.

Minimum Mapping Unit (MMU)

Variable, depending on the hydrological setting. The floodplain width for small watercourses to major rivers can be from tens of metres to tens of kilometres.

Accuracy / constraints

Thematic accuracy: 80-90%.

Spatial accuracy: Floodplain limits (floodway and flood fringe) derived from an elevation model will be accurate within plus or minus half the defined elevation contour interval.

Accuracy assessment approach & quality control measures

The limits of a floodplain are not discreet (the boundary is imprecise). A general, often stated accuracy limit for the elevation model used is indicated above. Sediment cores and flood extent derived from radar imagery can be used to evaluate historic inundation as a calibration and testing method for hydrological models.

Frequency / timeliness

Observation frequency: One baseline assessment is needed. Additional yearly snapshots may be needed for hazard assessment prior to the wet season or spring freshet in regions prone to seasonal flooding.

Timeliness of delivery: Imagery and elevation can be acquired quickly, but survey work and hydrological modelling can require significant lead time.

Availability

On-demand availability from commercial suppliers.

New acquisitions can be requested globally.

Delivery / output format

Data type:

  • § Vector polygon or double-line polyline features for floodway and flood fringe limits
  • § Reports of hazard and risk estimations

File format:

  • § Shapefile, or client-specified common spatial formats
  • § Common office document formats

 

Dowload product sheet.

 

Lead Author:

HC/Hatfield

Peer Reviewer:

OTM/GeoVille

Author(s):

Barry Pierce

Document Title:

Floodplain Mapping and Flood Risk Assessment

# of Pages:

4

Circulation:

Internal – Project consortium and science partners

 

External – ESA

 

 

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