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TSHO ROLPA GLOF WARNING SYSTEM PROJECT
W.W. BELL1),
T. DONICH2), K.L. GROVES3), D. SYTSMA4)
1)Principal
Engineer, B.C. Hydro,
2)Vice
President Engineering, Meteor Communications, Corp.
3)Senior
Engineer, B.C. Hydro
4)President,
Meteor Communications, Corp.
ABSTRACT
Tsho Rolpa, the largest glacial
lake in Nepal is located in the Rolwaling Valley, approximately 30 km Southwest of Mt. Everest.
It has formed over the last forty years as the Trakarding glacier has
stagnated, melted and retreated. The lake, which is at an elevation of 4580 m,
is approximately 3 km long, 0.5 km wide and up to 130 m deep. It is retained by
a natural moraine dam that is unstable and threatens to burst. If the dam is
breached, the resulting flood of approximately 80 million cubic metres of water, would cause serious damage for 100 km or more
downstream, threatening
as many as 6000 lives, the construction site for a 60 MW hydroelectric project
and other infrastructure. The damage would have a serious impact on the economy
of Nepal. The current risk of a failure is considered to be high and increasing
rapidly.
This paper describes the design,
and installation of a warning system downstream of Tsho Rolpa. The warning
system is based on Extended Line of Sight VHF radio technology and will relay
any alarm from sensors located immediately downstream of Tsho Rolpa to the
seventeen warning stations along the Rolwaling and Tama Koshi Valleys. Warning
is issued by air horns backed up by electronic sirens. The system is fully
redundant so that failure of two successive stations would have to occur in
order to prevent the warning signal from being received by the most downstream
station. The sensing system is also fully redundant and multiple sensor
failures would have to occur before a false alarm or missed event could
occur.
Several of the GLOF warning
stations, as well as a GLOF sensing station, will also transmit and receive signals from a
Meteor Burst master station installed in Western Nepal, thus providing further
redundancy to the system in the event two or more successive stations should fail.
The system was designed, supplied
and installed in an accelerated program in order to have it operational by late
May for the 1998 monsoon season. It has operated continuously since then
through a severe monsoon season.
Keywords: warning, GLOF, moraine
failure, Nepal
INTRODUCTION
The design, supply, and
installation of the Glacial Lake Outburst
Flood (GLOF) Warning System had to overcome many unique problems. First was the
remoteness of the majority of the seventeen villages at risk. All the equipment
for the system had to be portered to the sites - some as much as six days walk.
The potential for extremely low winter temperatures combined with snow and
possible avalanches meant that standard water level switches would not provide
a reliable means of sensing the occurrence of a GLOF, instead a sensor would
have to be found that would be both secure (no false alarms) and reliable
(signal an alarm when required) under conditions of ice, snow, rain on snow,
dust etc. At the villages, the warning horn had to be loud enough to be heard
during monsoon conditions over distances in excess of 100 m. Finally, the
system had to be designed, supplied and installed in a period of less than five
months.
This paper describes the GLOF
warning system installed down stream of Tsho Rolpa and how the above problems
were overcome. The joint venture partners, BC Hydro International Limited of
Vancouver, B.C., Canada (BCHIL) and Meteor Communications Corp. of Kent, Washington,
USA (MCC), also required the efforts of BPC Hydroconsult, of Nepal, COMTEC of
Nepal and the client, the Department of Hydrology and Meteorology of His
Majesty's Government of Nepal (DHM).
RECONNAISSANCE
Based on information provided by
the client and the available contour maps, a preliminary layout for the warning
system was developed. Initially it appeared that there were only seven villages
which required warning stations along the 100 km reach of the flood path.
Several communication technologies were considered including Meteor Burst,
standard VHF, and satellite applications. The potential devastation of
hydroelectric facilities, villages and the loss of lives, required a robust
communication system that could reliably activate all warning horns under all
conditions. Meteor Burst communication was selected as the most appropriate
technology to meet these requirements. A preliminary review of warning devices
and sensors was also conducted. Prior to final design, a field reconnaissance
was conducted in November 1996 to confirm the feasibility of the proposed
layout and technologies.
The reconnaissance included a
field trip, on foot, covering most of the study area and a visit to Tsho Rolpa
Lake. During this field trip, the villages to be warned were visited and
potential station locations were identified which satisfied communication and
warning requirements. At Tsho Rolpa, the moraine dam and near vicinity were
inspected to determine suitable locations for the sensors which would have to
detect the onset of GLOF conditions resulting from all potential failure modes.
The information obtained from the
reconnaissance trip resulted in several changes to the warning system design.
Most importantly, the field trip revealed that there were seventeen villages
located on the flood plain. Some of these villages had as few as forty
inhabitants, however, the separation between villages was such that these
villagers would have no means of receiving warning without a station located directly
in their village. Figure 1 shows the project area.
OVERALL
DESIGN CONCEPT
The overall objective for the
design of the GLOF warning system was to provide a safe, secure, reliable
warning to the downstream villages in as short a time as possible. Prior to the
field reconnaissance described above, the intent had been to only use Meteor
Burst to relay the warning signal from the sensors to a Master Station and then
back to the warning stations in the flood plain. After the reconnaissance it
was concluded that there were many more villages to warn than originally
assumed. It was then decided to use the faster signal transmission capabilities
of Extended Line of Sight (ELOS) embedded within Meteor Burst. A Meteor Burst Communication (MBC) network
provides long range communications, up to 1,000 miles, by reflecting radio
signals off the ionized trails left by micrometeors as they enter the earth's
atmosphere. At the shorter ranges (30-50 miles) communication is by ELOS using
groundwave. These two modes of operation are seamless and automatic within an
MBC network, thereby providing an additional level of redundancy for
communicating a warning message to all villages within the Tsho Rolpa
network. This is described further in
Section 6.0. The remainder of the design was then broken into four types of
components. GLOF sensing stations located just below Tsho Rolpa; GLOF warning
stations located at the villages; data management centres to monitor system
performance; and a Meteor Burst master station.
The GLOF sensing stations
consisted of three water level sensors located at different elevations above
the high water mark in a narrow section of the stream flowing out of Tsho
Rolpa. These were connected through shielded, armoured cables to a data logger,
remote transmitter, and battery mounted in a weather proof enclosure on a pole, located above the
estimated flood level. The solar panel, and antenna were also mounted on the
pole. Two sensing stations were installed as described in Section 4.0.
The warning stations consist of a
143 dB . air powered horn, a remote Meteor Burst
transceiver, a back up electric horn, antenna, solar panel, air bottle, pole
and equipment box (see photo 1). The air horns are described in Section 5.0
Photo 1
GLOF Warning Station
To ensure that the system would provide a reliable warning, even in the event of equipment failures, redundancy was built into the design wherever possible, including the sensing system, radio links, and warning horns. Project cost limitations prevented the installation of more than one warning station at each village, except for the major villages. Automatic station self-tests and alert messages were built in to the radio system, providing further system security and reliability.
Redundant ELOS links were provided
between all villages and an MBC link was also provided by the master station
situated in western Nepal, 500 miles away.
SENSING
SYSTEM
There is no reliable means to
instrument the moraine dam to determine where and when it might fail/has
failed. The field reconnaissance indicated that regardless of failure mode
outflow, the flood water would be channelled down a confined section of channel
downstream of Tsho Rolpa. This resulted in the design of a multi sensor system
that would sense sequentially a rise in water level above the maximum high
water level in this channel. As the sensing system must operate in the event of
a GLOF but must not give false alarms, two main problems had to be solved in
the design: the selection of a suitable sensor and the design of the control
logic that would determine if a GLOF was occurring without giving false alarms.
The original sensor design concept assumed the use of standard water level
switches; however, closer examination of this concept revealed two significant problems.
First, there appeared to be no way to ensure that the water level switches
would not freeze. Any protective
covering only seemed to compound the problems.
Second, there was no way of knowing at the control centres, if a water
level switch would operate. All that could be known, was whether or not there
was an electric
current through the switch. The solution to this problem was provided by RST
Instruments, who proposed the use of a ground water reflectometer mounted in
air. This is a static device which produces an output frequency/period that is
a function of the dielectric surrounding its two probes. Laboratory and field
tests demonstrated that this device would have sufficient discrimination
between normal (dry) and alarm (wet) states so that immersion in saturated snow
or ice would not cause false alarms. In addition, its output would provide
sufficient information to determine whether it was in an operating or
failed state. Six of these sensors were located at elevations between 0.4 and 2
m above the maximum high water mark in the channel. The control logic that would determine
whether a GLOF is occurring is based on assigning weights to each of three
states (dry, wet, failed) and then summing these weights. If the sum exceeds a
pre-set threshold value, and a majority vote of the operating sensors occurs,
then an alarm will be issued. Failure of one or more sensors only results in an
alert being sent to the data management centre, notifying operations personnel
of this occurrence. The system therefore allows for the possibility of
component failure but minimizes the possibility of false alarms.
WARNING
SYSTEM
The homes in Nepali villages are typically
spread out. In order to provide warning
to as many villagers as possible under monsoon conditions it was necessary to
use a horn with a considerable sound power. A cost effective solution to this
problem was to use an air powered horn supplied from a compressed air bottle.
The horn supplier, Air Chime, provided a 143 dB (at one metre) low frequency
air horn (to maximize sound transmission during monsoon conditions) complete
with an actuator and controls that would open the valve on the air bottle to
supply the required air. Using the actuator to operate the valve on the bottle,
rather than leaving the valve open and controlling the horn with a solenoid
valve, essentially eliminates the possibility of loss of the air supply through
leakage in the system. Software in the remote transmitters were programmed to
operate the air horns for two minutes.
This leaves a further one minute supply of air in the bottle in the
remote event the horn should be operated by a false alarm. As a further
back-up, electric burglar alarm horns are used. The sound power of these horns
is 120-125 dB, significantly less than the air horns; however they should
wake some of the villagers in the event of a GLOF who could then alert their
neighbours.
RADIO
TRANSMISSION SYSTEM
An MBC network operates on a
single frequency in the low VHF band (40-50 MHz) and uses robust protocols, sensitive
receivers and short packetized messages. The specific frequency used in the
Tsho Rolpa network is 41.7 MHz. MCC's
MBNET 200 protocols and network software use both carrier sense multiple access
(CSMA) and time division multiple access (TDM) to achieve a channel utilization greater
than 90% on this single frequency. The link and network protocols provide
automatic transmission of the warning message to all nodes within the network,
error free and with end-to-end acknowledgement. With these efficiencies,
sufficient channel time is still available for two-way messaging between all
villages, performance monitoring of all network elements and various other data
transfers. The network is fully automated and can also be interactively
controlled from the Khimti Hydroelectric Facility and the DHM offices in
Kathmandu.
The network capability described
above is provided by both ELOS and MBC communication technologies. The
uniqueness of the MBNET-200 network software allows each of these technologies
to form independent networks, yet operate interactively, thereby providing
enhanced levels of reliability and redundancy.
EXTENDED LINE OF SIGHT SIGNAL
TRANSMISSION
The MCC-545A RF Modem is the VHF
transceiver used to provide real-time communication between all villages and
warning horns. It reliably transmits a warning message from Tsho Rolpa Lake to
all villages within 45 seconds of sensor activation. The network connectivity
is programmable into each RF Modem, either on-site or from the control centres
in Khimti and Kathmandu. For purposes of redundancy, each station (village) was
programmed to communicate with at least four of its adjacent neighbours, two
upstream and two downstream. Therefore, the loss of any one station would not
prevent a warning message from propagating along the entire string of 17
villages from Naa to Rajgaon as depicted in Figure 1.
Seven stations were also
programmed to communicate with the master station at Dhanghadi in Western
Nepal. This provides a further level of redundancy in the event two adjacent
ELOS stations should fail prior to a flood.
In addition to the GLOF sensing
and warning stations one additional station was located at Tsho Rolpa Lake for
meteorological monitoring (water level, wind speed, wind direction and temperature).
Additionally, three stations were also equipped with water level sensors for
monitoring the speed, duration and height of the flood wave. Three stations were installed as signal
relay stations.
METEOR
BURST COMMUNICATIONS
A dual redundant master station
was located at Dhanghadi in western Nepal. The master station is used as
another level of redundancy and monitors the entire network at all times. It
forwards all data to the DHM offices in Kathmandu. It consists of two MCC-520B
Master Stations, each equipped with a 15 kW diesel generator in the event of a
power failure. Switching between master stations and diesel generators is fully
automatic and no on-site operator is required.
The Meteor Burst master station
has a range of 1,000 miles and provides communication coverage for all of
Nepal. In addition, it has the capacity for communicating with hundreds of
remote stations located anywhere within the country. With this capability in
place, the government of Nepal has the option of using this as a nation-wide
communication service network for applications such as email, environmental
monitoring, trekking and tourism, remote village communications and monitoring
other potential GLOFs.
INSTALLATION
The installation program commenced
in April 1998 and required approximately six
weeks to complete. All of the major components of the system were bench
tested in Kathmandu before shipping to the field and as much of the
pre-assembly as feasible was also performed in Kathmandu.
Shipping the station components to
the sites was a monumental logistic challenge.
With the exception of a few road accessible locations, all of the
components had to be shipped by human porter from the road head to the
villages. For each station, this required more than 800 kg of station
components and civil materials to be packaged and labelled in manageable sized
pieces that could withstand fairly rough treatment. In addition to arriving in good condition, it was imperative that
the right equipment end up at the right village as it would take days of
walking along the route to locate any misplaced deliveries. With the superb
assistance of the Nepali "Sirdhar", not a single load went astray. Portering is
the traditional Nepali method of transport along these routes as helicopters are
costly and have limited high altitude load capacity. The employment of local labour provided a significant
contribution to the local economy.
The installation area was divided
amongst three installation crews and with very few mishaps, the equipment was
installed within three weeks. The erection of the main pole and fencing was
completed by civil crews working in advance of the installation crews. The
communication equipment and other electrical components had been pre-wired and
installed onto a back plate before shipping. This back plate was simply bolted
to the back of the instrument box in the field and then connected to the
antenna, solar panel, battery and horn. As each station was installed, its
individual components were tested including a test sounding of the air horn and
backup electric horn. Local villagers tended to gather around during the
installation and this was the prime time for the delivery of a village
awareness program. Discussion groups were held in each village to inform the
locals of the importance of the equipment, how it worked, and how they should
respond to a GLOF warning signal.
OPERATION
The Tsho Rolpa network is fully
automated, but it is also monitored and controlled from the client's office in
Kathmandu and the Khimti Hydroelectric facility. Data Management and Control
(DMC) Application Software is installed on PC workstations at each of these
locations at which all network functions and messages are monitored, displayed
and archived. This provides a full playback capability at any time for
post-analyses of all events that may have occurred in the network, including
self- test results, messages, flood alerts and the
status of the sensors at the moraine dam. In addition, commands may be sent to
each station individually or collectively to the entire network. These commands
include system ARM, system DISARM, flood test messages, reporting intervals and
network connectivity changes.
CONCLUSION
The successful implementation of
the Tsho Rolpa project was rooted in the robustness, reliability and proven
performance of all technologies, components and software that was used. The
case study described above demonstrates the feasibility of providing secure
reliable warning systems in remote, harsh environments. Moreover, similar warning networks can now
be rapidly replicated anywhere in the world to minimize loss of life due to
similar threats.
ACKNOWLEDGEMENTS
The authors are pleased to
acknowledge the assistance of all those who so ably participated in the design,
supply, installation and co-ordination of this project. In particular Mr. K.S.
Yogacharya and Mr. A.P. Pokhrel of DHM and Mr. W.C. Seyers of BCHIL.