hazard maps in GIS for flood risk management is integral to local level
preparation. They increase the awareness of the local people for disaster risk
management and offer many additional benefits to conventional mapping (Tran et al., 2008). Figure 1 displays a clear
flood risk map for the impact of a 1:100 year flood, in Prudhoe. Overlaying
modern infrastructure onto an historic base map clearly shows how the
susceptibility of properties and infrastructure has changed. For example, the
construction of new terraced housing on Tyne Gardens, the development of
Ovingham School and the demolished Water Works. This information has clearly
influence the town to utilize flood plain zoning; creating parks and leaving
open spaces near the river banks and building residential properties at higher
elevations, away from the river. This map can be updated with ease to changes
in the landscape and used by investors for appropriate developments.The
data from Figure 1 can be manipulated to produce a graph assessing the change in
risk over time, as shown in Figure 2. This graph shows a clear increased risk
of flooding to all types of infrastructure in Prudhoe. However, the graph
offers no identification nor positioning or extent of damage and is therefore
impractical without the accompaniment of maps such as figure 1.As
shown below, Manning’s equation is used to estimate the hydraulic properties of
the 2005 River Tyne floods. The peak discharge measured at the nearest
monitoring station, Bywell, was 1374m3 s-1. Table 1 shows that this figure is
six times greater than even the largest estimate of discharge using Manning’s
equation. As the equation adopts the assumption that the ground slope is the
same as the water surface slope it is rarely applicable to flood conditions. To
overcome this obstacle the water surface slope (during the flood event) can be
surveyed and incorporated into the equation, producing more accurate results.
This technique of counteraction was used by Hessel et al. (2003) when studying Loess in China.
From the data available
the flood outlines fulfil 54.9% of the flood risk, therefore the Environment
Agency is over compensating the risk of a flood. However, the overlapping of
recorded flooding and flood risk is consistent throughout demonstrating accuracy
in flood prediction. The Environment Agency should place more emphasis on the
route of water influenced by man-made features, for example, following a road.
Furthermore, this data has no information relating to the depth of the water.
The field observations above demonstrate the depth of the flooding, however the
map shows no indication of this. This vital information must be incorporated
into the flood risk for precautionary actions to be taken. The high resolution
DEM for this area can be used to estimate a fairly accurate flood depth (Rahman
and Tahkur, 2017).
Taking aerial images
immediately after a disaster event can provide an historical record of the
event and facilitate preparedness of future events. Furthermore, this type of
remote sensing reduces the risk of danger for researchers.
Using a DoD has proved advantageous as a mapping mechanism for
geomorphic hazards. To overcome the difficulty of differentiating the erosional
and depositional features the DoD distinctly shows the direction of volumetric
change. However, DEMs do not provide information concerning river bathymetry
(Laks et al., 2017). This can lead to
confusion when assessing the volumetric change of the channel. To achieve this
data, a cross-section of the River Derwent channel must be determined. However,
this is time consuming and expensive to do before and after a flood.
A systematic offset as large as 20m is worrying for the precision of
this investigation. Inaccurate generalisations regarding lahar deposit
thickness could result.
Assessing the risk of lahar
flows requires an extensive a priori knowledge of the volcano and observations
from previous lahar flows. In this case, Calbuco is a glacier capped
stratovolcano with pyroclastic flows (Russell et al., 2016). Figure 7 shows the lahar routways, initial
observations include the influence of river channels on lahar extent. The
pre-exsisting channels act as motorways for lahar flow, reaching the furthest
distances in these efficient routes to lower eleations. Figure 7 also shows
lahars deviating fom this path to follow the roads as the deforrested areas
provide less resistance to flow. According to Franco et al. (2010), the infrastructure most at risk are those closest to
the centre of the river channel, here a house would be ‘wiped out’ by the
lahar. Destruction or burial is significantly less on the channel margins, however
these properties are still affected. These finding are in concurrance with the
findings in figure 10. The individuals living in
close proximity of the river channels are the most vunerable to the lahar
impacts. Therefore, relocation seems the pragmatic solution, however the broad distribution of tributaries
multiplies the risk of disaster and vulnerability, especially when lahars occur
simultaneously, this could lead to unnecessary relocations (Thouret et al.,
2013). An alternative solution of reducing the risk is the construction of
diversion channels before they reach critical areas. These are equivalent to
river levees in an attempt to reduce flood risk. Using this method ensures that
communities can survive small to medium events with little economic impact. Conversely,
this may lull the population into a false sense of security, lowering their
perception of the risk (Pierson et al.
2014). The use of a lahar warning systems is already in practice in the area
and seems very effective as 6500 people were evacuated to safety. For a
conclusive lahar risk assessment, a more precise delimitation of the areas
previously affected is required to accurately predict future extents (Leone and Lesales, 2009).
Using DAN3D to calculate the maximum extent and thickness of
landslides is advantageous over real-life modelling as it is inexpensive, less
time consuming and parameters may be varied to the slightest degree. However,
computer programmes cannot predict anomalous nuclei that dictate the mass
movement, resulting in inaccurate outcomes (Schraml et al., 2015).
Figure 12 uses DAN3D as a modelling techniques for geomorphic
hazards following the 1991 Mount Cook rock avalanche. The extent and depth of
the landslide were the known data, however the parameters controlling these
were not. While the modelled result may be similar to the actual event, this
does not equate to the parameters having any similarities. For example, the
mass doubled before reaching its final extent, this cannot be replicated in the
computer programme (Evans and DeGraff, 2002). Therefore, these results must be
generalised with caution.