Water Resources Systems—Hydrological Risk, Management and Development (Proceedings of symposium 90 IIS02b held during IUGG2003 at Sapporo, July 2003). IAHS Publ. no. 281, 2003.
Flood forecasting and management in Pakistan
SHAUKAT ALI AWAN Flood Forecasting Division, 46 Jail Road, Lahore 54000, Pakistan [email protected]
Abstract Meteorologically, there are two situations which may cause three types of floods in Pakistan: (i) meteorological situation for Category-I floods, the seasonal low is a semi-permanent weather system situated over south eastern Baluchistan, southwestern Punjab, adjoining parts of Sindh, which intensifies and causes moisture from the Arabian Sea to be brought up to the upper catchments of the Chenab and Jhelum rivers; (ii) the situation for the more severe Category-II and Category-Ill floods, is linked with the monsoon low/depression. Such monsoon systems originate in the Bay of Bengal region and then move across India in a general west/northwesterly direction to arrive over Rajasthan or the adjoining states of India. Flood management in Pakistan is a multi-functional process involving a number of different organizations. The first step in the process is issuance of a flood forecast/warning which is prepared by the Pakistan Meteorological Department (PMD) utilizing satellite cloud pictures and quantitative precipitation measurements using radar data, in addition to the conventional weather forecasting facilities. For quantitative flood forecasting, hydrological data is obtained through the Provincial Irrigation Department and WAPDA. Furthermore, improved rainfall-runoff and flood routing models have been developed to provide more reliable and explicit flood information to a flood prone population.
Key words Category I, II and III floods; monsoon low/depression; Pakistan
INTRODUCTION Floods in Pakistan are mainly caused by the heavy monsoon rains during the summer monsoon period, from July to September (Thompson & Perry, 1997). Meteorologically there are two situations which cause flood producing rains in the upper catchments of the rivers (National Engineering Services, 1998). These two situations are related to different conditions of intensity and movement of lows/depressions, and produce three categories of floods: (i) Meteorological situation for Category-I floods: the seasonal low is a semi-permanent weather system situated over southeastern Baluchistan, south western Punjab, adjoining parts of Sindh, which gets intensified (due to a westerly wave) causing moisture from the Arabian Sea to be brought up to the upper catchments of the Chenab and Jhelum rivers resulting in heavy rainfalls along windward slopes of the mountain ranges due to orographic uplift of the moist air masses, (ii) Situation for Category-II and Category-III floods: this generating situation is linked with monsoon lows/depressions. Such monsoon systems originate in the Bay of Bengal region and then move across India in a west/northwesterly direction and arrive over Rajasthan or any of the neighbouring states of India. After the monsoon, the depression may take any of following three courses: (a) continue moving straight west causing heavy widespread rains over Sindh/Baluchistan, in which case no river flooding will occur; (b) turn to the northeast towards the upper catchments of the Sutlej, Ravi and Chenab Flood forecasting and management in Pakistan 91 rivers causing extremely heavy rainfall and consequently floods, first across the border in India and then (within hours) at the Rim Stations in Pakistan. This is a Category-II flood situation; (c) continue moving in a northerly direction under the effect of strong westerly winds over the plains of the Lahore/Gujranwala Divisions, to finally end up over the Rawalpindi/Hazara Divisions. The upper catchments of the Chenab, Jhelum and Indus rivers then come under its influence. The flood forecasting system in Pakistan comprises three components: (i) a meteorological component which relates to the forecasting of precipitation; (ii) a hydrometeorological component which relates to the generation of a flood wave at the Rim Stations; and (iii) a hydrological component which relates to routing the flood wave and forecasting flood peaks at downstream sites.
METEOROLOGICAL COMPONENT
The meteorological component is an extremely important part of any flood forecasting system, especially in tropical countries where floods are caused by rainfall only (with little or no snowmelt contribution). Flood producing rain is caused by the condensation of the moisture-laden monsoon air mass due to convection in a low-pressure area or due to orographic lifting across a mountain barrier, or both. The resulting super saturation of the cloud mass triggers precipitation, which depends upon the continued availability of a moist airflow into the precipitation region. In Pakistan, minor floods are caused by the penetration of the southeast and southwest monsoon currents, along the region of the active monsoon which includes the upper catchment of the Sutlej, Ravi, Chenab and Jhelum rivers. Major floods are, however, caused by the monsoon lows/depressions which have a long track stretching from the Bay of Bengal to the catchment areas of the rivers. The above two meteorological situations need to be closely monitored in order to develop the capability of predicting the timing and amount of the flood producing rains. The following meteorological methods are used for this purpose.
Meteorological charts/maps
As members of the World Meteorological Organization (WMO) most countries of the world are committed to observe, collect and transmit meteorological data from their respective countries in accordance with the schedule laid down by WMO. The schedule provides that, commencing from 00:00 h GMT, the meteorological data at all surface observatories is observed every three hours, while that of the upper air observatories is recorded at a six-hourly interval. This makes it possible for Pakistan to obtain the much needed meteorological data which includes such elements as pressure, temperature, rainfall, clouds, surface and upper winds, etc. This data, along with that from Pakistan and the other surrounding countries is then plotted on the meteorological surface and upper air charts and analysed to identify the lows/depressions, etc., or the extent, and intensity of the monsoon incursion into the catchment areas. This, even though an age-old conventional process, is still the most important means of meteorological forecasting. 92 Shaukat Ali Awan
The depth of the low pressure for example, is an important indicator of the intensity and duration of the associated rainfall. In addition to the locally prepared meteorological maps/charts, those prepared at Tashkent (one of the WMO centres), and that prepared at Reading, UK (by the European Centre for Medium Range Forecasting) are also received as and when needed. A number of meteorological studies aimed at improving the methods of predicting rainfall from monsoon lows, and predicting the conditions of recurrence of monsoon lows in the catchment areas are in progress, while some are planned for the future.
Weather satellite images
There are two types of meteorological satellites in use for receiving the cloud pictures: the geostationary and the polar orbiting satellites (Donald, 2000). The geostationary satellites are more useful since they provide pictures at half hourly intervals (as they remain static relative to the earth and are thus continuously monitoring a very large region of the Earth under them). Perhaps Pakistan is the only country in the world today having no access to a geostationary satellites and thus has to rely on the use of polar orbiting satellites only. The orbits of polar orbiting satellites are continuously shifting with respect to the Earth. The amount of shift depends upon the time taken by the satellite in circling the Earth which is related to the orbital height. Presently, the two satellites in use are the NOAA and NOAA 14 satellites, which are orbiting the earth at a height of approx imately 850 km and thus take about 100 minutes to circle round the earth. This means that the Earth shifts by about 25.4 degrees eastward at about 25.4 degrees longitude, between each orbit, so progressively bringing new regions of the Earth under it. Polar orbiting satellites therefore have the advantage that a single satellite is able to cover all points on Earth. On the other hand, however, it has the disadvantage that long intervals of time occur between two passes over a particular location, providing only two to four usable passes in 24 hours. The two polar orbiting satellites together provide only two to six usable passes every 24 hours. Fortunately, the long awaited Russian satellite is about to be launched above the Indian Ocean at longitude 76°. This will be a geostationary satellite and will provide cloud pictures at half hourly intervals. The visible and the infrared (thermal) bands will be received to identify weather systems such as monsoon lows/depressions and the areas of extensive convective build-up within the monsoon air mass. It will be an excellent device for forecasting torrential rainfall and associated floods, particularly in connection with radar information.
Use of weather radars in flood forecasting
Radar is a remote sensing device capable of monitoring falling precipitation. It has a meteorological range up to which it can detect the existence of precipitation. Radar has a very special use in the context of flood forecasting in Pakistan in view of the fact that the upper catchments of most rivers lie across the border and are thus physically inaccessible for the installation of ground-based raingauges. Access to real-time Flood forecasting and management in Pakistan 93
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ÉêT...... t . '". " : Fig. 1 Areal coverage of weather radar network in Pakistan. rainfall data from the upper catchments in India is also not possible. The only alternative in this situation is to remotely sense the falling precipitation using quantitative precipitation radar. Consequently, a C-band 5.32-cm wavelength QPM radar was installed at Sialkot in 1977. The radar, despite its low specification (improper wavelength) has given useful service. But for the Sialkot radar it would not have been possible to detect the floods at Marala and Jassar. Since it was first installed, a lot of expertise has been developed in the use of the radar data. The radar catchments were retraced and the radar data suitably improved in relation to the prevailing weather conditions. The range circles of the Sialkot radar are shown in Fig. 1. A C-band radar has also been installed at Islamabad. It is a Japanese radar manufactured by Mitsubishi Co. The radar is linked to a computer with a number of terminals including one at Chaklala airport. Rainfall is indicated in levels shown on the screen in different colours relating to rainfall intensity. The Islamabad radar is used mainly for aeronautical forecasting. Recently, an S-band (10-cm) radar has been installed at Lahore. This is the much needed and long awaited equipment and provides a big step forward in flood forecasting in the country. It is a Doppler radar and is thus capable of monitoring the movement of the approaching weather system. The meteorological range of this radar is about 480 km, while the hydrological range is 240 km (Fig. 1). The radar, because of 94 Shaukat Ali Awan its location at Lahore, is able to cover the Sutlej and Beas catchments in India. It also covers the Ravi and Chenab catchments. Even though it cannot cover the Jhelum catchment as such, it will provide reliable information on the Mangla catchment during Category-II, and Category-Ill flood situations. The Lahore radar, on account of its longer wavelength, is subject to much less energy attenuation problems and so the accuracy of the data will be much greater. A rainfall-runoff model for each river has been developed based on the Sacramento watershed model (Federal Flood Commission, 1995). The radar will also provide invaluable rain data for the important nallahs in Lahore, Gujranwala, Kasur, Sialkot, Wazirabad and Gujrat districts, including the data from across the border. This will, therefore, make it quite possible to issue improved flood warnings for the nallahs where floods quite often cause great loss of life and property. More radars at locations such as Mangla and Pasni have been planned for the future to cover all the vital flood prone areas in the country. The radars would be linked with the FFD at Lahore. The range circles of these radars are shown in Fig. 1.
H Y D RO M ET EC) RO LOG IC A L COMPONENT
The hydrometerological component of the flood forecasting system represents runoff generation. The component mainly pertains to the flood forecasting at the rim stations which are Ganda Sinsh Wala, Jassar, Marala, Mangla and Tarbela on the Sutlej, Ravi, Chenab, Jhelum and Indus rivers respectively. Flood forecasting at the rim stations is based upon the rainfall-runoff transformations which were previously done using a Constraint Linear System (CLS) model and statistical correlations, etc., and have now been replaced by the Sacramento watershed model. The model requires rainfall data for the catchments above the rim stations and the flow data of the upstream stations including that of the important nallahs which enter the main channel above the rim station. Catchments above the rim stations of the rivers Sultej, Ravi and Chenab are almost completely across the border and thus radar rainfall data and the flow data as received through the Commissioner of Indus Waters (CIW) are the two sources for the rain and discharge data respectively. The radar at Sialkot covers only a small portion of the Sutlej catchment. The Sialkot radar therefore is not effective for the Sutlej and Jhelum catchments. The Islamabad radar covers a significant portion of the Mangla catchments, but is not yet linked with the Flood Forecasting Division at Lahore. Consequently, as far as the Sutlej River is concerned, flood forecasting has until now primarily been dependent upon the Indian flow data as received through the CIW. However, with the recent installation of the 10-cm radar at Lahore, the entire flood generating rainfall area of the rivers Sutlej and Bias has come under radar cover. This has made it possible to run the rainfall-runoff model for the Sutlej River as well. Additionally, the flow data below Bhakra and Pong reservoirs and that below Harike and Ferozpur (as received through CIW) plays an important role in forecasting floods at Ganda Singh Wala. The Ravi and Chenab flood forecasting procedures make use of the radar data and that of the few Indian discharge stations obtained through CIW. Regarding the River Jhelum, it is only the ground-based rain and discharge data which form the basis of the flood forecasting. Discharge measurement stations include Garhi Habibullah and Muzaffarabad on the rivers Kunhar and Neelum, respectively, and Flood forecasting and management in Pakistan 95
Domel, Kohala and Azad Pattan for the Jhelum main. The Poonch River is gauged at Kotli while Khanshi is gauged at Palote. A number of WAPDA telemetric and manual stations exist at Kotli, Kohala, Muzaffarabad, SehrKakota and Palandri. However, because of the very short time-to-peak after the rainfall peak, very little lead time is available based upon the actual rain and discharge measurement data. Quantitative Precipitation Forecasting (QPF) is thus essential for getting the necessary lead time at Mangla to afford proper flood management at the reservoir.
HYDROLOGICAL COMPONENT AND FLOOD SITUATIONS
The hydrological component of the Flood Forecasting System relates to the routing of the flood wave below the rim stations. The input data consists of the discharge data at the measurement sites along the river and the discharge data of the tributaries wherever available (see Fig. 2 for a map of the catchment). A river flow model called SOBEK is used for routing the flood wave. Computations start from the rim stations where the output from the rainfall-runoff model forms the input to the routing model. Data pertain ing to the channel geometry is available for every reach. The flood levels are thus computed at various locations. Flood inundation maps for flood levels corresponding to various return periods are prepared using the model. Such maps form the permanent record to be placed in Volume-II of the Manual. Along with maps, the lists of three categories of villages are provided. Category-I consists of the list of those villages (along the river channel) which are unlikely to be affected by a particular flood. Category-II on both sides are villages associated with a large risk of getting inundated. Category- Ill are villages within the flood risk zone that may or may not be flooded. Consequently, whereas for villages falling within the Category-II complete evacuation would be ordered, the villages within Category-Ill would be asked to maintain a close contact with the flood relief department and be prepared for evacuation if the situation arises.
Fig. 2 Indus Basin. 96 ShaukatAli Awan
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Fig. 3 Meteorological situation for the Category-I floods in Pakistan.
Category-I floods The meteorological condition for category-I floods in Mangla is depicted in Fig. 3; a strong southwesterly monsoon incursion reaches the catchment due to the accentuation of a seasonal low. Under this condition, heavy short-lived rainfall occurs, mostly late-at-night/early-morning lasting until about 10:00 h. Sharp peaks of short duration may arrive at the Chenab and Jhelum rivers at the rim stations Marala and Mangla during this situation. The peaks may occur daily as long as the seasonal low remains in the accentuated state, which may last 3-5 days at a time. The seasonal low is generally strong during the early monsoon period and therefore, Category-I floods are more common from the last week of June to the end of July, but may occur during August occasionally (Harza Engineering Company International, 1976). More than 80% of the flood peaks under these conditions may be smaller than 5660 m3 s"1 at the Mangla gauging station on the River Jhelum (a tributary of the Indus). Only 15% of the peaks range between 5660 to 11320 m3 s"1 while only 5% exceed 11320 m3 s"1. All floods have short durations and they carry a relatively small volume of water into the reservoir, posing no serious flood management problem.
Category-II floods The second condition (Fig. 4) under which floods in the Mangla catchment occur is when a monsoon low/depression approaches from east/southeast traversing across the Sutlej, Ravi and Chenab catchments. Under this situation, the rain producing capacity of the depression is considerably reduced (by the time it reaches the Mangla catchment) due to extensive moisture shedding (in the form of heavy rains) over the Sutlej, Ravi and Chenab catchments. Consequently, the floods Flood forecasting and management in Pakistan 97
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Fig. 4 Meteorological situation for the Category-II Floods in Pakistan. under this condition are generally less than 8490 m3 s'1. Only in an extremely rare case, when the depression happens to be extremely intense, a peak ranging from 16980 to 22640 m3 s"1 may occur. Again no serious flood management problem would arise in this case.
Category-Ill floods Third, the most critical situation is (Fig. 5) when the depression takes a direct northerly path from its position in Rajasthan and enters Pakistan along the Lahore Division and then continues to move towards Rawalpindi Division, skipping the catchment of Sutlej and Ravi to the east. Under this condition, the Chenab and Jhelum catchments will be severely affected because, firstly, it is the northeast sector of the depression which gets the heaviest precipitation and secondly because the depression reaches the vicinity of the Mangla catchment (along the plains) with most of its intensity still intact. Additionally, since the northwards movement of the depression is caused by the presence of a strong westerly wave to the north, heavy rains will always extend further north to cover the lower part of the Terbela catchment, Hazara Division and the whole of Kashmir. The heaviest floods on record are caused during this situation (the 1928, 1929 and 1992 flood situations). During the July-September 2001 flood season the radar images helped a lot in determining the rainfall over the catchment areas of the neighbouring country. This quantitative cross-border rainfall estimate was an extremely useful input before the maximum runoff was reached in the plain of the Sutlej River. The rainfall estimates assisted in both determining the peak discharge value and the time of its arrival. 98 Shaukat Ali Awan
A timely forecast was issued on 15 August 2001 at 10:30 h and the actual peak occurred on 18 August at 00:00 h, which enabled the District Management of Kasur to evacuate people and cattle who were on the river bed of the River Sutlej, and so a heavy loss of life and property was avoided.
CONCLUSIONS
Information on heavy rainfall generating meteorological situations assisted in accurately calculating peak discharges of river flows at designated sites. This contrib uted to saving life, cattle and property by issuing timely forecasts/warnings to Government Agencies responsible for flood relief evacuation and mitigation for the people living in the flood plains.
REFERENCES
Donald, A. C. (2000) Essentials of Meteorology: An Invitation of the Atmosphere. Brooks/Cole, Pacific Grove, California, USA Federal Flood Commission (1995) Development of the Indus Flood Forecasting System. Final Report, vol. 1. Federal Flood Commission, Pakistan. Harza Engineering Company International (1976) Existing Flood Control Structures and Recommendations For a Planning Programme. Harza Engineering Company International.' National Engineering Services (1998) Flood Forecasting Manual. National Engineering Services, Pakistan. Thompson, R. D. & Perry, A. (1997) Applied Climatology: Principles and Practice. Routledge, London, UK.