Methodology - Response of Flood Events to Land Use and Climate Change - page 78

3.9.6 Output Data

A lot of information for hazard assessment is contained within the time series of
maps with water depth, flow velocity, duration, shear stress, etc. To analyze these
parameters an aggregation procedure has been developed to create the parameter
maps that describe the different aspects of the flood event in the Barcelonnette
area. These parameters are explained as follows [ 68 ]:
Hydraulic depth (unit: m): In order to create depth mapping, assumptions
must be made on the nature of flows on floodplains. The model results can be used
to map water elevations onto a DEMof the catchments. This map shows the
maximum depth that occurred during the inundation. The rationale behind this
parameter map is that areas with deep water are more dangerous to people and
potentially more damaging to objects like houses and cars. It identifies areas where
the second floor, or even the third and fourth floor of houses, are not safe refuges.
The maximum water depth map also serves as a possible means for model cali-
bration. Maximum water depth is one of the few flood parameters that can easily
be retrieved after a flood event because of the wetting marks in and on structures.
Flow velocity (unit: m/s): This map shows the maximum flow velocity that
occurred during the inundation. The rationale behind this parameter is that velocity
is a component of the floodwater that can sweep people off their feet and make cars
float away. This map shows where preferential flow paths may develop that could
be dangerous for children, adults, and cars.
Duration (unit: h): This map estimates the time the floodwater remains at a
certain location. It is based on several assumptions regarding the drainage of the
floodwater from the flooded area. The rationale behind this parameter is that it
gives a first, rough impression of how long the floodwater will stay in the area.
This is the minimum time period that people have to be relocated, that busi-
nesses and industries are closed, and that transportation in and throughout the area
might be impossible or hindered. It is a strong parameter with which to assess the
economic and social impact of the flood on the people living and working in the
area.
It is also an important parameter for estimating agricultural damage because
many crops, like fruit bearing trees and vineyards, can withstand inundation of
their stems for a short time (usually some days), but if the period becomes too
long, the roots will starve from oxygen depletion and the trees will die.
Shear stress (t): In the case of open channel flow, it is the force of moving
water against the bed of the channel. This force induces ''bed shear stress'' which
can be determined from the following equation [ 69 ]:

s 0 ¼ cRS 0 ð 3 : 32 Þ

where,
s 0 = bed shear stress; c = specific weight of water; R = hydraulic radius; and
S = bed slope.