3.2 Impact and Restoration Modelling
39
• Restoration time (t res ), the re-growth time from restoration starts until the VEC is
recovered.
• Recovery time (t rec ), the sum of the three time-factors is the total time from spill
to recovered VEC.
In nature, there is no clear distinction between the time phases, as inhibition
of growth and re-growth can happen simultaneously, depending on the resource in
question. Much of the researched literature on restoration following historic spills
do not discriminate between lag- and restoration phase (see reference lists in the
background reports). However, ERA Acute offers the possibility if more knowledge
exists, for the user to make an expert judgement of the division between these parameters in the input, for example if there is a known threshold for recovery. For the four
compartments, different parameters and sub-models are used to calculate restoration
times.
3.2.3 The Two Steps Together and the Resource Damage
Factor
Figure 3.2 builds on Fig. 1.2 and illustrates how impact (population/community loss)
and recovery modelling in ERA Acute have been implemented, and where within
the framework the formulas are used. Impact-, lag- and restoration times are defined
along the time axis. The curve illustrates the initial steep decline in impacted resource
from pre-spill status, until full impact (Imp) is reached after t imp . Impact magnitude
is at its maximum until restoration can start after t imp + t lag , which is illustrated by
a re-growth curve to restored status of the VEC. The area formed by the curve and
timeline is the total of the impact extent and duration, as also proposed by Lein et al.
(1992). Restoration modelling to determine the time factors in ERA Acute reflects
different restoration mechanisms in individual compartments and/or resource groups.
For sea surface and water column, restoration modelling enables an integral
function for the calculation of the geometrical area that represents the combined
expression of damage extent and duration. This combined expression is called the
Resource Damage Factor (RDF) in ERA Acute (Eq. 3.5 (for surface) and Eq. 3.17
(water column)). This factor is in line with the approach used in the NRDA for the
Deepwater Horizon incident to calculate “cetacean-loss-years” (Deepwater Horizon
Natural Resource Damage Assessment Trustees 2016). A simpler approach has been
proposed and implemented for seafloor and shoreline to calculate the RDF. Based on
the total impact to a community, and including the duration of the impact, lag and
restoration times, the RDF for shoreline and seafloor is calculated using linearized
expressions of decline and re-growth, given in the compartment-specific sections
below (Eq. 3.9 (shoreline and seafloor)). The different formulas for calculating RDF
are summarized in Fig. 3.2).
39
• Restoration time (t res ), the re-growth time from restoration starts until the VEC is
recovered.
• Recovery time (t rec ), the sum of the three time-factors is the total time from spill
to recovered VEC.
In nature, there is no clear distinction between the time phases, as inhibition
of growth and re-growth can happen simultaneously, depending on the resource in
question. Much of the researched literature on restoration following historic spills
do not discriminate between lag- and restoration phase (see reference lists in the
background reports). However, ERA Acute offers the possibility if more knowledge
exists, for the user to make an expert judgement of the division between these parameters in the input, for example if there is a known threshold for recovery. For the four
compartments, different parameters and sub-models are used to calculate restoration
times.
3.2.3 The Two Steps Together and the Resource Damage
Factor
Figure 3.2 builds on Fig. 1.2 and illustrates how impact (population/community loss)
and recovery modelling in ERA Acute have been implemented, and where within
the framework the formulas are used. Impact-, lag- and restoration times are defined
along the time axis. The curve illustrates the initial steep decline in impacted resource
from pre-spill status, until full impact (Imp) is reached after t imp . Impact magnitude
is at its maximum until restoration can start after t imp + t lag , which is illustrated by
a re-growth curve to restored status of the VEC. The area formed by the curve and
timeline is the total of the impact extent and duration, as also proposed by Lein et al.
(1992). Restoration modelling to determine the time factors in ERA Acute reflects
different restoration mechanisms in individual compartments and/or resource groups.
For sea surface and water column, restoration modelling enables an integral
function for the calculation of the geometrical area that represents the combined
expression of damage extent and duration. This combined expression is called the
Resource Damage Factor (RDF) in ERA Acute (Eq. 3.5 (for surface) and Eq. 3.17
(water column)). This factor is in line with the approach used in the NRDA for the
Deepwater Horizon incident to calculate “cetacean-loss-years” (Deepwater Horizon
Natural Resource Damage Assessment Trustees 2016). A simpler approach has been
proposed and implemented for seafloor and shoreline to calculate the RDF. Based on
the total impact to a community, and including the duration of the impact, lag and
restoration times, the RDF for shoreline and seafloor is calculated using linearized
expressions of decline and re-growth, given in the compartment-specific sections
below (Eq. 3.9 (shoreline and seafloor)). The different formulas for calculating RDF
are summarized in Fig. 3.2).