Floodplain Mapping and Flood Risk ASsessment
Image credit: Hatfield Consultants
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
Lifecycle stage and demand
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.
OTM:036 Geohazard exposure analysis
OTM:051 Identification of fault lines
OTM:065 Floodplain mapping
OTM:072 Monitoring flash floods
Input data sources
Optical: HR1, HR2, MR1
Radar: HR1, HR2, MR1
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.
On-demand availability from commercial suppliers.
New acquisitions can be requested globally.
Delivery / output format
Floodplain Mapping and Flood Risk Assessment
# of Pages:
Internal – Project consortium and science partners
External – ESA
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