Vulnerability And Hazard Classifications Of Dams Regulations, 2024
[05th
December 2024]
PREAMBLE
F. No.
TE-32/2/2023-NDSA-MOWR.-In exercise of the powers conferred by sub-section (1),
read with clause (b) of sub-section (2) of section 54 of the Dam Safety Act,
2021 (41 of 2021), the National Dam Safety Authority, on the recommendations of
the National Committee on Dam Safety, hereby makes the following regulations,
namely: -
Regulation 1. Short title and commencement.
(1)
This regulation may be called the
Vulnerability and Hazard Classifications of Dams Regulations, 2024.
(2)
This shall come into force on the date of its
publication in the Official Gazette.
Regulation 2. Definitions.
(1)
In this regulation, unless the context
otherwise requires, -
(a)
"Act" means the Dam Safety Act,
2021(41 of 2021);
(b)
"Authority" means the National Dam
Safety Authority established under section 8 of the Act;
(c)
"hazard" means a dangerous
condition or event, that threat or have the potential for causing injury to
life or damage to the property or the environment;
(d)
"hazard potential" means the
downstream damage that may result if the dam were to fail;
(e)
"risk" means a measure of the
likelihood and severity of adverse consequence, which is estimated by the
mathematical expectations of the consequence of an adverse event occurring i.e.
the product of the probability of occurrence and consequence;
(f)
"section" means a section of the
Act.
(g)
"vulnerability" means the relative
risk associated with the dams, on the basis of their condition, location and
damage;
(2)
Words and expressions used herein and not
defined in these regulations but defined in the Act, shall have the meanings
respectively, assigned to them in the Act.
Regulation 3. Criteria for vulnerability and hazard classification of a dam.
(1)
The State Dam Safety Organization shall
classify each dam under their jurisdiction as per the following, namely:-
(i)
Category-I: Deficiencies in dams which, if
left unattended, may lead to failure.
(ii)
Category-II: Major deficiencies requiring
prompt remedial measures.
(iii)
Category-III: None or minor deficiencies
which are rectifiable.
(2)
The vulnerability classification shall be
carried out in accordance with sub-para (c) of Part 2 of Schedule-I of the
Inspection, Instrumentation, Seismic Data, Risk Assessment and Evaluation of
Specified Dam Regulation, 2024.
(3)
The criteria of hazard classification shall
be as following, namely: -
(i)
Dam hazard classification shall be based on
an "additive weighting" and "point index" and aggregates
the assessment of the consequences in four major categories that is the capital
value of the project, potential for loss of life, the potential for property
damage and Potential for Environmental and Cultural impact.
(ii)
hazard potential is categorized into four
classes as specified in the Table-1 below based on the consequences category:
Table
1- Consequences Categories in the Dam Classification
|
Hazard Potential Class
|
Consequences Categories
|
|
Capital Valueof Project
|
Potential for Loss of Life
|
Potential for Property Damage
|
Potential for Environmental and Cultural
Impact
|
|
ClassI
|
Low
|
None. Occasional or no incremental population at risk,
no potential loss of life is expected. No inhabited structures.
|
Minimal. Limited economic and agricultural development.
|
None
|
|
ClassII
|
Average
|
Minimal or low population at risk. No potential loss of
life is expected even during the worstcase scenario of emergency management
|
Notable agriculture or economic activities. States
highways and rail lines
|
Minimal incremental damage. Short-Term or reversible
impact (less than 2 years).
|
|
ClassIII
|
Significant
|
Considerable. Several inhabited developments. Potential
for loss of life highly dependent of the adequacy of warning and rescue
operations.
|
Significant industry, commercial and economic
developments. National and state highways and rail lines.
|
Limited. Impact have a mid-term duration (less than 10
years) with high probability of total recovery after mitigation measures.
|
|
ClassIV
|
Critical
|
Extreme. High density populated areas. Potential for
loss of life is too high even during the best scenario of emergency
management.
|
Highly developed area in terms of industry, property,
transportation and lifeline features.
|
Severe. Long-term impact/effects in the protected areas
or cultural heritage sites with low probability of recovery.
|
Regulation 4. Failure Scenario for the Dam Hazard Classification.
(1)
The hazard potential classification assigned
to a dam is based on consideration of the effects of failure or mis-operation
during both normal and flood flow conditions.
(2)
The classification assigned shall be based on
the worst-case scenario of failure or mis-operation of the dam, i.e., the
assigned classification shall be based on incremental failure consequences that
will result in the assignment of the highest hazard potential classification of
all probable failure and mis operation scenarios.
(3)
Each element of a project shall be evaluated
to determine the proper hazard potential classification for the project.
(4)
Only one hazard potential classification
shall be assigned to the entire project and individual elements are not
assigned separate classifications.
Regulation 5. Vulnerability or severity assessment.
(1)
The degree of hazard shall vary with the
severity of flooding and is affected by the flood and its hydraulic behaviour
(extent, depth, velocity, duration and rate of rising of the floodwaters), the
topography, population at risk and emergency management.
(2)
Once the flood hazard of the dam event
(failure scenarios) has been quantified, the potential of the flood flows to
cause damage or danger shall be indexed against vulnerability curves linked to
meaningful hazard thresholds.
(3)
The vulnerability of the downstream community
and its assets shall be described by using thresholds related to the stability
of people as they walk or drive through flood waters, or shelter in a building
during a flood.
(4)
The vulnerability to hazard shall also be
influenced by the primary consideration of strategic land-use planning, which
is aimed at ensuring land use is compatible with the flood risk or assessing
development proposals or emergency management planning, which is aimed at
addressing residual flood risks.
(5)
A flood severity assessment conducted as part
of a dam hazard classification process shall provide information to identify those
consequences with the highest significance among the entire floodplain.
(6)
A combined set of hazard curves
(vulnerability of people, vehicles and buildings) are given at Table 2 and
Figure 1 below: -
Table
2- Vulnerability thresholds classification limits a
|
Hazard Vulnerability Classification
|
Description
|
Classifi cation Limit (Depth*
Velocity)
|
Limiting Water Depth (m)
|
Limiting Velocity (m/s)
|
|
|
H1
|
Generally safe for vehicles, people and buildings
|
D*V<0.3
|
0.3
|
2.0
|
|
|
H2
|
Unsafe for small vehicles.
|
D*V<0.6
|
0.5
|
2.0
|
|
|
H3
|
Unsafe for vehicles, children and the elderly
|
D*V<0.6
|
1.2
|
2.0
|
|
|
H4
|
Unsafe for vehicles and people.
|
D*V<1.0
|
2.0
|
2.0
|
|
|
H5
|
Unsafe for vehicles and people. All buildings are
vulnerable to structural damage. Some less robust buildings subject to
failure.
|
D*V<4.0
|
4.0
|
4.0
|
|
|
H6
|
Unsafe for vehicles and people. All building types
considered vulnerable to failure.
|
D*V>4.0
|
-
|
-
|
|
a Combined Hazard – Vulnerability Classification (Smith
et al., 2014)
|
xxx
Figure 1 - Flood Hazard
Vulnerability Curves (Smith et al. 2014)
Regulation 6. Limits of the Study Area.
(1)
In order to establish the limits of the study
area for a consequences assessment the following aspects shall be considered
and verified in the dam break analysis, which may help to delimit the
boundaries of the model. -
(a)
no downstream presence of dwellings, services
and other infrastructures;
(b)
no future developments are expected
downstream of the boundaries of the model;
(c)
full attenuation of the breach outflow
hydrograph along the main river;
(d)
channel-conveyance capacity of the mainstream
(within the riverbanks) receiving the total outflow in the downstream end of
the model;
(e)
The existence of a downstream dam having a
reservoir that lies within the downstream limits of the hydraulic model of the
upstream dam, which is either able or unable to absorb the total outflow
hydrograph due to the dam breach upstream and in case the reservoir located
downstream is not able to absorb the entire volume received from the upstream
dams breach flood, a cascade failure effect shall be scrutinized and if a high
likelihood of failure is corroborated for the downstream dam the downstream
limits shall be extended;
(2)
The investigation of the impact of failure or
mis-operation of a dam on downstream human life, property damage, lifeline
disruption, and environmental concerns shall be sufficient to determine the
appropriate hazard potential classification:
Provided that if failure or
mis-operation of a dam leads to the failure of a downstream dam, the hazard
potential classification of the dam shall be at least as high as the
classification of the downstream dam and shall consider the adverse incremental
consequences of the domino failures.
Regulation 7. Data requirements.
(1)
The assessment of consequences requires information
on the effects of a potential dam break, to provide the basis for the level as
considered appropriate which includes data on dam and reservoir, topography,
flood characteristics and inundation maps, information about the community
residing in the downstream area, implications for service and business, and
implications to objects of strategic or national importance.
(2)
Dam and reservoir data shall include
information on dam type, dimensions of the dam such as height, width, and
length, spillway characteristics including gates and secondary spillways,
relationship showing the reservoir elevation versus storage volume, together
with spillway discharge capacity up to the dam crest and comments on design,
foundations and unusual conditions as well as available reports on the design,
construction, and management of the dam and information on past incidents.
(3)
(i) Topographic data shall cover the river
sections where the depth and velocities of the flow may cause harm to life or
damage the property and the information shall indicate the shape and slope of
the valley and, when possible, the structures or surfaces influencing on the
downstream flow (e.g., culverts, weirs, bridges, levees, embankments, temporary
storage, floodplains, vegetation and other objects affecting the water surface
elevations);
(ii) Topographic data shall
also determine the boundary limits of the hydraulic model of the dam break
analysis which is enough to represent the entire downstream consequences and
shall also include the location of major downstream tributaries that may cause
concurrent flooding;
(iii) Channel cross-sections
shall be taken at regular intervals along the flood channel and additional
sections may be obtained for areas with a change in the channel profile such as
restriction in section profile, like a bridge, confluence of tributaries to the
main channel and places with a significant change in the grade or cross-section
of the channel;
(iv) For the dam
classification process, the level of details of the topography or corresponding
digital elevation model (DEM) shall be consistent, as a minimum, to a tier-I
assessment.
(4)
Information on flood characteristics and
inundation maps shall include details about historical flood levels and flood
studies based on hydrographic data, as well as results from rainfall run-off
modelling pertaining to different return periods which also contain results of
dam-break flooding under overtopping and non-overtopping conditions and shall
include travel times of flood waves to downstream locations of habitations.
(5)
Information about the community residing in
the downstream area shall include.-
(i)
location, size and type of areas with human
settlements within the possible area of inundation, the vulnerability of
various elements of the downstream community and the warning time available;
(ii)
details about awareness and flood
preparedness of the downstream community and their temporal variation pattern
concerning days of the week and months and seasons;
(iii)
description of the areas identified for
future development along with the land use classes and areas with toxic
substances, and the information on the infrastructure that may be affected
(e.g., transport, power lines, water supply and sewerage lines, gas pipes
etc.).
(6)
Implications of the dam break consequences
for service and business shall include the importance of the water storage to
the business (e.g., municipal water supply, irrigation or hydropower
generation), financial overburden, to meet the costs of failure and the value
of water in the storage reservoir.
(7)
Implications to objects of strategic or
national importance include inundations of areas which shall have consequences
of national significance like an inundation of a nuclear power plant or a
thermal power plant, or places which if inundated will pose a threat to the
national security, and may also include important historical structures or
biodiversity reserves or both.
Regulation 8. Tiered assessment of Inundated Area and Flood Water Levels.
(1)
With due considerations of the time
requirement and cost of detailed studies required to delineate areas which
shall be inundated by a dam breach flood, tiered approach shall be taken to
produce dam-breach inundation zone maps.
(2)
The level of analysis for the tiered approach
shall correlate with the sophistication and accuracy of the analysis with the
scale and complexity of the dam and downstream area under investigation, as per
Table 3.
(3)
For Dam Hazard Classification process, a
Tier-I analysis as specified in Table 3 shall be considered and adopted in the
following cases, namely: -
(a)
dams which, due to their particular location
and own characteristics, may be directly inferred as "low hazard" and
a Tier- I analysis shall confirm the "low hazard" hypothesis;
(b)
as the first estimation for any high hazard
dam under analysis and if the results obtained through a Tier-I analysis are
enough to classify the specified dam under the highest hazard class, no further
refinement would be required in the classification process;
(c)
for any specified dam, irrespective of its "hazard",
only if the results from a Tier-I analysis are reliable enough to conclude that
further refinements shall not introduce changes in the hazard classification.
(4)
A higher level of complexity in the
consequences estimation (i.e. Tier - II and III) shall be introduced for the
classification process in the following cases, namely: -
(a)
consequences index (scoring points) obtained
through a Tier-I analysis set the Hazard Class near the boundary or threshold
of a higher or lower hazard category;
(b)
better estimate in the population at risk
assessment shall be ensured since it is considered as the critical indicator in
the hazard classification process of the specified dam;
(c)
clearer estimates in the flood severity
assessment are desired for the consequences evaluation, which shall make the
use of a two-dimensional model.
Regulation 9. Estimating population at risk.
(1)
The population at risk shall include persons
directly exposed to flood waters if they are not evacuated, and after the
delineation of the area inundated due to dam breach, estimation of the
population at risk may be carried out.
(2)
It shall be estimated using demographic data
with occupancy rates for residences, number of students at schools, number of
persons in industrial, hospital, commercial and retail areas.
(3)
The population at risk estimates may vary
according to the time of day, the day of the week and month or season and it is
necessary to prepare more than one estimate and select the highest of these for
determination of the consequence category of the dam.
(4)
It is important to consider the visitors to
the recreational sports, camps, concert halls, parks and gardens as also those
who are driving through the roads or taking a railroad journey which gets
inundated due to a dam break flood.
(5)
The incremental analysis shall be taken into
consideration for the estimation of population at risk and the rest of the
consequences.
Regulation 10. Assessing potential damages and losses.
(1)
The potential damages and losses due to a dam
break may be grouped into classes, including consequences of similar nature for
the purpose of risk assessment which includes total infrastructure costs,
losses accrued due to the dam not being able to serve the purposes it is meant
to, health and social impacts, and the environmental impacts.
(2)
Damages and losses may be classified as
direct or indirect and direct damages and losses comprise, those losses, which
result from contact with the floodwaters and indirect damages and losses
encompass all other damages and losses.
(3)
The severity of each of these damages and
losses may be either minor, medium, major or catastrophic which may be
established for each group of damage or loss, and summarised to the overall
level of severity, with due consideration to the regional or national economic
perspective.
Regulation 11. Assessment of health and social impacts.
The assessment of health and
social impact shall be assessed on the following parameters, namely: -
(a)
the effects of dam break on health and social
affairs shall depend on the nature, location, and extent of the area affected
by the dam failure, with regards to the distribution of the human habitation;
(b)
consumption of polluted drinking water or
food due to contamination of the source or supply network which may be due to
failure or shortage of water, sewage, power supplies and uncontrolled release
of sewage, industrial or toxic waste as a result of a dam break which may lead
to widespread contamination;
(c)
social impacts of dam break shall depend on
demographic characteristics, social and community values, needs and networks,
the extent of community support services, the capacity of responding
institutions as well as the degree of disaster preparedness and warning time
available.
Regulation 12. Consideration of Future Developments.
The consequence
classification for a dam shall reflect the current downstream development as
well as the future development plans.
Table
3- Tiered approach to dam breach inundation mapping
|
Tier Level
|
Applications
|
Topography /Bathymetry
|
Breach Parameter Prediction
|
Handling of the Dam Breach Parameters’ Uncertainty
|
Peak Breach Discharge Prediction
|
Downstream Routing of Breach Outflow Hydrograph
|
Downstream Risk Evaluation
|
|
Tier 1 – Basic level screening and simple analysis
|
• Hazard Classification Process
• First level screening for significant or high hazard
dams
• Low hazard potential dams
|
Low resolution terrain data (e.g., SRTM, ASTER, or
ALOS), with maximum 30 m resolution. No bathymetry required
|
Empirical formulae
|
• Engineering Judgment
• Reasonableness of the peak breach discharge and
velocities
|
Empirical formulae if inflow design flood hydrograph is
not available, otherwise unsteady flow routing through modelled reach
|
HEC-RAS, MIKE or similar one dimensional (1D) or two
dimensional (2D) unsteady flow numerical models
|
Peak discharge, water surface elevation, depth*velocity
and flood wave travel time
|
|
Tier 2 – Intermediate level of analysis
|
• Large significant hazard dams
• All high hazard dams
|
Medium resolution terrain data (e.g., 10 - 15 m
resolution, CartoDEM1) and elevations adjusted through ground control points
(GCPs). Bathymetry required
|
Empirical formulae
|
• Reasonableness of the peak breach discharge
• Sensitivity analysis
|
Unsteady flow routing through modelled breach
|
HEC-RAS, MIKE or similar two dimensional (2D) unsteady
flow numerical models considering the bathymetry
|
Peak discharge, water surface elevation,
depth*velocity, flood wave travel time, and approximate PAR assessment
|
|
Tier 3 – Advanced level of analysis
|
• Significant hazard dams with complex downstream
flooding
• High hazard dams with large population at risk (PAR)
|
High resolution terrain data (Lidar, ALSO enhanced)
minimum 5 m resolution. Ground control points (GCPs) required. Bathymetry
required
|
Empirical equations, physically based models (one or
two dimensional)
|
• Probabilistic Analysis (Monte Carlo simulations or
similar methods)
|
Unsteady flow routing through modelled breach
|
Coupled one-two dimensional (1D2D) unsteady flow
numerical model. Bathymetry also considered
|
Peak discharge, water surface elevation,
depth*velocity, flood wave travel time, and detailed PAR assessment
|
Regulation 13. Hazard Classification for dams.
(1)
Hazard classification aggregates the
assessment of the consequences in following major categories, namely: -
(a)
the capital value of project shall include
the capital value of the projects elements which may be destroyed or damaged,
and the loss of benefits, services, revenues provided by the dam project.
(b)
the potential for loss of life be estimated
indirectly through the estimation of the total population at risk in the
downstream areas and this category shall also consider the severity of the
breach or failure flood if the quality or type of modelling meets some
standards, and the approximate arrival time of the flood wave (to the closest
and affected populated area), which is an indirect measure of the available
warning time;
(c)
the potential for property damage shall
include the amount of damage to residential and commercial property,
agricultural lands, transportation facilities such as roads and bridges, damage
and disruption of lifeline and community service facilities;
(d)
the potential for environmental and cultural
impact shall include the amount of damage to protected areas in the country
(wildlife sanctuaries, forest reserves, etc.) as well as potential impact to
infrastructures of cultural heritage or national importance.
(2)
The detailed descriptions of all categories
for each of the four hazard classes are specified in Table 4.
(3)
(a) the additive weighting or point index
scheme employs numerical ratings of the consequences which reflect the relative
importance of each consequence and the range of severity of the impacts;
(b) the summation of the
rating points from each consequence shall be used to establish the characteristics
of the consequences of failure of a given dam;
(c) the overall categories
and indicators used in the hazard potential classification for dams are
specified in Table 5.
Table
4: Dam Classification based on the Additive weighting Scheme (Potential
Consequences Index)
|
Hazard Potential Potential Conseque Class
ncesIndex (PCI)
|
Consequences Categories
|
|
Capital Value of Project
|
Potential for Loss of Life
|
Potential for Property Damage
|
Potential for Environmental and Cultural
Impact
|
|
ClassI
|
300
|
Low
|
None. Occasional or no incremental population at risk,
no potential loss of life is expected. No inhabited structures.
|
Minimal. Limited economic and agricultural development.
|
None
|
|
ClassII
|
300
|
Average
|
Minimal or low population at risk. No potential loss of
life is expected even during the worst-case scenario of emergency management
|
Notable agriculture or economic activities. States
highways and/or rail lines.
|
Minimal incremental damage. Short-Term or reversible
impact (less than 2 years)
|
|
ClassIII
|
300 PCI 600
|
Significant
|
Considerable. several inhabited developments. Potential
for loss of life highly dependent of the adequacy of warning and rescue
operations.
|
Significant industry, commercial and economic
developments. National and state highways and rail lines.
|
Limited. Impact have a mid-term duration (less than 10
years) with high probability of total recovery after mitigation measures
|
|
ClassIV
|
600
|
Critical
|
Extreme. High density populated areas. Potential for
loss of life is too high even during the best scenario of emergency
management
|
Highly developed area in terms of industry, property,
transportation and lifeline features
|
Severe. long-term impact/effects in the protected areas
or cultural heritage sites with low probability of recovery
|
Table
5- Numerical Rating Points and Categories for Assessing Consequences
|
Conse quences Category
|
ndicat or Parame ter
|
Conse quences Rating Points
(min- max)
|
Conside rations
|
|
Capital Value of Project
|
Dam Height
|
20 -100
|
Revenue Generationor Value of Reservoir Content
depending upon the reservoir purpose (water supply, irrigation, hydropower,
etc.)
|
|
Project Benefits
|
0-300
|
|
Poten tialfor Loss of Life
|
Popula tionat Risk
|
20-600
|
Incremental Populationat Risk under higher flood
severity areas. Minimu marrival of the breach/ floodw aveto then earest
populated are adown stream of the dam
|
|
Critical Arrival Time
|
0- 100
|
|
Poten tialfor Prop erty Dam age
|
Infras tructures Dama ged
|
0- 330
|
Residential and Commercial Properties Roads, Bridges, Transportation
Facilities Lifeline Facilities and Community Services
|
|
Services Disrupted
|
0-185
|
|
Potential for Environ mental impact
|
Environ mental Impact
|
0 - 200
|
Protected Areas and Cultura linfras tructures of
National importance under higher flood severity areas
|
|
Cultural Heritage
|
0 - 140
|
(a)
Capital Value of Project
(1)
It shall include the capital value of the
projects elements which shall be destroyed or damaged, and the loss of
benefits, services, revenues provided by the dam project.
(2)
The two parameters namely:- Dam Height Index
and Project Benefits Index shall be used for assessment of capital value of
project,-
(A)
Dam height index
(i)
Dam height may be considered as indicative of
the capital value of a dam.
(ii)
Consequence rating points corresponding to
dam height index shall be calculated by the utility curve as specified in
Figure 2.
xxx
Figure 2 - Consequences
Rating Points for Dam Height Index (IDH)
Alternatively, the index can
be computed numerically according to the following formula:
xxxx
Where,-
IDH : Dam height index
points, and
h : is the height of the dam
in meters
(B)
Project benefits index. -
(i)
Project benefits index (IPB) shall be
estimated using the following subcategories, namely:- reservoir content or
water supply index (IRES), irrigation index (II), hydropower generation index
(IHP), and industrial use index (IIU), and accordingly the total project
benefits index can be calculated as follows:
IPB = IRES + II + IHP + IIU
(2)
(ii)
in case the cascade failure effect in a dam
series is ascertained, the rating points shall consider all the affected dams
in the system, regardless of the potential dams ownership conflicts or
administrative jurisdiction differences.
(iii)
the following procedure shall be followed to
estimate the rating points for each of the sub- categories: -
(I)
Reservoir contents or water supply index(xx):
The value of the reservoir contents shall be calculated as a function of the
time that it would take to fill the reservoir and the time is computed in
months, by dividing the reservoir volume in million cubic metre (Mm3) by the
average river flow in m3/s (average of daily means, not daily peaks), and
converting the resulting number into months by multiplying it by 0.386.
Example; - If there is a reservoir
with volume of 1200 Mm3, and an average flow of 40 m3/s, the time to fill the
reservoirwith the average river flow shall then be:
TF = V Q/ 0.386 (3)
Where,
TF = Time to fill the
reservoir in months
V = Volume of the reservoir
in Mm3
Q = Average river flow in
m3/s
Applying the equation to the
values in the example, we get
TF= 1200/40*0.386 = 11.5
months
The time to fill is entered
into Figure 3, and for the 11.4 months, shall be read as 23.7 points, and an
alternative way to compute the number of points is by using the following
formula; namely;-
IRES {50
24 / TF , (for TF 24)
100 , (for TF 24)
Where,
IRES = Reservoir Content or
water supply Index
xxx
Figure 3 - Consequences
Rating Points for Reservoir Content Index (IRES)
(II)
Irrigation land index(xx): Consequences
rating points for irrigated land index shall be calculated as
specified in Figure 4.
The number of points for
irrigated land is also computed as:
II
{area , (for area 100)
100 , (for area 100)
Where,
= irrigated area in 1,000
ha.
= irrigated land index
(III)
Hydropower generation index(): Consequences
rating points for hydropower generation index shall be calculated as specified
in Figure 5.
The hydropower generation
index can also be computed by:
IHP {P10 / , (for P 1000)
100 , (for P 1000)
Where,
P = installed capacity in
MW.
IHP = Hydropower index
xxx
Figure 4 - Consequences
Rating Points for Irrigated Land Index (II)
xxx
Figure 5 - Consequences
Rating Points for Hydropower Generation Index (IHP)
(IV)
Industrial use index (xxx): The corresponding
rating points shall be calculated using the same methodology as explained for
waters supply index and specified in Figure 3 and equation (3).
(b) Potential for loss of life. - (1) The
potential for loss of life shall be estimated indirectly through the estimation
of the total population at risk in the downstream areas and this category shall
consider the severity of the breach or failure flood, and the approximate
arrival time of the flood wave, which shall be an indirect measure of the
available warning time.
(2) In this category, as in
the project benefits index (IPB), the evaluation of failure consequences of a
dam in a cascade system shall include the failure consequences of dams located
downstream if such failure be caused by the dam under scrutiny and if that
failure shall not otherwise have occurred in the scenario under study.
(3) The rating points shall
consider the consequences of all dams involved, regardless of the potential
dams ownership conflicts or administrative jurisdiction differences.
(4) In cases where the
failure of the downstream dam is ascertained, the highest hazard category may
be adopted without the need for further justifications.
(A)
Incremental population at risk index (IPAR).
- (i) An envelope curve for estimating the potential loss of life (PLL) when
there is greater than five minutes of warning, but less than ninety minutes,
may be expressed as a function of the population at risk (PAR) as
PLL = PAR0.6
Equation (7) has been used
to establish the general shape of the utility curve for the PAR (Figure 6).
Equation (8) below can also be used for numerical calculation
PAR0.2954, (for PAR 100,000)
600 , (for PAR 100,000)
Where,
PAR = incremental population
at risk
(i)
As the equation (8) specifies, the
incremental population at risk shall be used in this approach by subtracting
from the consequences of the dam failure, the ones that would have happened by
the natural flow anyway, that is, even if the dam had not failed, provided, for
a sunny-day failure scenario (normal operation), incremental and total
consequences are to be considered equal.