Floods & Warnings
Flood forecasting is the use of forecasted precipitation and stream flow data in rainfall-runoff and stream flow routing models to forecast flow rates and water levels for periods ranging from a few hours to days ahead, depending on the size of the watershed or river basin. Flood forecasting can also make use of forecasts of precipitation in an attempt to extend the lead-time available.
Flood forecasting is an important component of flood warning, where the distinction between the two is that the outcome of flood forecasting is a set of forecast time-profiles of channel flows or river levels at various locations, while "flood warning" is the task of making use of these forecasts to tell decisions on warnings of floods.
Real-time flood forecasting at regional area can be done within seconds by using the technology of artificial neural network. Effective real-time flood forecasting models could be useful for early warning and disaster prevention.
Flood forecasting (FF) is one the most challenging and difficult problems in hydrology. However, it is also one of the most important problems in hydrology due to its critical contribution in reducing economic and life losses. In many regions of the world, flood forecasting is one among the few feasible options to manage floods. Reliability of forecasts has increased in the recent years due to the integration of meteorological and hydrological modelling capabilities, improvements in data collection through satellite observations, and advancements in knowledge and algorithms for analysis and communication of uncertainties. The present paper reviews different aspects of flood forecasting, including the models being used, emerging techniques of collecting inputs and displaying results, uncertainties, and warnings. In the end, future directions for research and development are identified.
Pakistan has experienced floods for fourth consecutive year. In 2010, 2011, 2012 and 2013, the riverine/ flash floods and urban flooding were caused by torrential rains during the monsoon season and destroyed hundred thousands of houses and flooded millions acres of agricultural land, killing thousands of people, besides damage to other public & private infrastructure. In Pakistan, floods are generally caused by the heavy concentrated rainfall, which are sometimes augmented by snowmelt and generate exceptionally high flood flows in rivers. Occasionally destructive floods are also caused due to Monsoon currents originating in the Bay of Bengal and resultant depressions which often result in heavy downpour in the Himalayan foothills, which is sometime augmented by the weather systems i.e. Seasonal Low from Arabian Sea & Westerly Wave from Mediterranean Sea.
Floods in PUNJAB
In Punjab, the flood protection marginal bunds have been generally constructed either to protect Headwork’s and other irrigation structures, or to safeguard certain towns, villages & adjoining agricultural lands. Due to general topography of the area sloping towards the south-west, pre-determined breaching sections have been provided in the right marginal bunds for operation for safety of Headwork’s/ barrages in case of exceptional high flood flows i.e. likely to exceed the designed level. In order to protect areas from erosion, spurs have been constructed in critical reaches. These spurs have protected vast areas and in some cases even large tracks of eroded lands have been reclaimed.
FLOOD PROTECTION FACILITIES
The existing flood management strategy includes flood flows regulation by three major reservoirs (Tarbela, Chashma on Indus & Mangla on Jhelum), protection of important private & public infrastructure, urban/rural abadies and adjoining agricultural lands located along the rivers banks by flood embankments and spurs & other interventions, besides, Flood Forecasting & Early Warning System, Rescue & Relief measures in case of flooding situation. The Provincial Irrigation Departments (PIDs) maintain about 6,807km of flood protection embankments and around 1410 spurs along main and other rivers.
HISTORICAL FLOOD EVENTS IN PAKISTAN
Since its creation, Pakistan has faced 21 severe flood event i.e. 1950, 1955, 1956, 1957, 1959, 1973, 1975, 1976, 1977, 1978, 19981, 1983, 1984, 1988, 1992, 1994, 1995, 2010, 2011, 2012 & 2013, the 2010 floods were worst ever in the country. The floods of various magnitudes since 1950 to 2013 affected vast areas in the four provinces including Gilgit Baltistan, FATA & Azad Jammu & Kashmir. Flood damages are caused mainly due to riverine flooding in main rivers and flash floods in Secondary & Tertiary Rivers/Hill Torrents, Coastal flooding due to Cyclone & urban flooding due to torrential rains and inadequate storm drainage facilities, besides, GLOFs. The unprecedented floods of 2010 were the worst floods in history of the country in which about 1985 people lost their lives, 1,608,184 houses were damaged/ destroyed, 17,553 villages were affected and total area of 160,000 Km2 was affected. Owing to adverse impacts of climate change, in the recent years, vulnerabilities of communities to coastal & urban flooding have also increased. The Sindh province, particularly southeastern parts of the province was severely affected due to unprecedented rains and inadequate drainage facilities during Monsoon Season-2011. The torrential rains during 2012 rains/floods affected Southern Punjab, Sindh & Baluchistan provinces; about 571 people lost their lives, 636,438 houses were damaged/ destroyed, 14,159 villages were affected and a total area of 4,746 Sq.km was affected.
The 2013-rains/floods damaged cropped area of about 1.107 million acres affected 8,297 villages, claiming about 333 lives, fully damaging 33,763 houses and 46,180 houses partially, besides, population of about 1.489 million was affected. The historical flood events experienced in the past and their damages are given in the Table.