Hydrogeomorphological method for flood risk mapping
The hydrogeomorphological cartographic method is a method for mapping alluvial floodplains using
geomorphological surveys and aerial photography. The methodology was developed over twenty
years ago in France and has since been validated using model outputs and recent flood events. It is
now one of three recommended methods by the French government to characterize flood risk. It has
been widely applied in France and abroad (Africa, Asia etc) over 20,000 km of river. Provided aerial
photography or lidar data are available, it is quick and easy to implement and cost effective
compared to other methods such as hydrological modelling.
The aim of the methodology is to delineate floodplain boundaries using morphological features (e.g.
existing and old terraces) so as to identify areas likely to be flooded by increasing flood events (from
frequent to rare and exceptional floods).
The analyses are performed in two stages:
- Stereoscopic aerial photography (recent or old) or Lidar
data taken outside major flood events are analysed by
experts to enable a first delineation of floodplain
boundaries.
- The resulting map is then refined during a field survey by
expert geomorphologists using additional morphological
elements such as the size of sediments, the vegetation
structure, land use and evidence of old and recent activity
(erosion, deposition).
The main advantage of this technique is that it provides an overall consistent view of floodplain
boundaries at local or river-valley scale, thus enabling the identification of vulnerable areas such as
urban dwellings or industrial development sites.
In urban areas where morphological structures are more difficult to assess, photo interpretation is
complemented by a diachronic analysis (comparison with old aerial photographs) and an in-depth
field survey taking account of runoff patterns and the influence of existing structures and
modifications on flow.
Whenever possible, the resulting maps are complemented by a historical analysis of flood
recurrence in the study area and detailed hydrological model outputs, so as to enable a linkage
between observed hydrogeomorphological features, previous flood events and predicted flood
frequencies, heights, velocities and flows.
Far from being contradictory, hydrological, hydraulic and hydrogeomorphological approaches are
complementary. It is thus desirable that studies of flood risk are managed in a multidisciplinary
framework involving all disciplines and experts in an integrated approach.
HYDROGEOMORPHOLOGY
Identification of
floodplain
terraces and
surface
drainage
network
Validation by
comparison to
historical data
Quantification
of hydraulic
parameters and
identification of
return periods
Assessment of
historical data
Hydrological analysis and model calibration
HYDROLOGY-HYDRAULIC
MODELLING
HISTORICAL DATA
Historical floods flow estimates and extent
Integrated flood risk approach