Shobha Nandargi

A study of floods in the Brahmaputra basin in India.

During the summer monsoon months June‐September, the Brahmaputra River experiences severe floods that occur year after year causing death and destruction in the Assam Valley and neighbouring Bangladesh. During the period 1987‐1998, flood data of 21 gauge:discharge sites on the main Brahmaputra River and its 12 major tributaries have been studied with a view to discovering which sites and tributaries experience frequent floods in different monsoon months and how high flood magnitudes are. It is hoped that this study may be useful to planners and water resources engineers for taking the remedial measures against recurring floods in this region. Copyright

Extreme rainfall events over the Himalayas between 1871 and 2007

Abstract Currently there is much discussion regarding the impact of climate change and the vagaries of the weather, in particular extreme weather events. The Himalayas form the main natural water resource of the major river systems of the Indian region. We present a brief review of the available information and data for extreme rainfall events that were experienced in different sectors of the Himalayas during the last 137 years (1871–2007). Across the entire Himalayas, from east to west, there are now 822 rainfall stations. There was an increase in the rainfall station network from 1947 onwards, especially in the Nepal and Bhutan Himalayas. Extreme one-day rainfall has been picked out for each station irrespective of the period for which data are available. The decadal distribution of these extreme one-day rainfalls shows that there is a considerable increase in the frequencies during the decades 1951–1960 to 1991–2000, whereas there is a sudden decrease in the frequencies in the present decade during 2001–2007, indicating the need to understand the response of the systems to global change and the associated physical and climatological changes. This is essential in terms of preserving this natural resource and to encourage environmental management and sustainable development of mountain regions. Citation Nandargi, S. & Dhar, O. N. (2011) Extreme rainfall events over the Himalayas between 1871 and 2007. Hydrol. Sci. J. 56(6), 930–945.

On some characteristics of severe rainstorms of India

SummaryDetailed examination of daily rainfall data for India (and other relevant publications regarding severe rainstorms) show that during the last 110 years, the Indian region recorded 231 severe rainstorms. Of these, 27 were of 2-day duration and the rest (204) were of 3-day duration. Within a rainstorm, maximum rain fell on the 2nd day on 63% of occasions. It was also observed that the average raindepths obtained from slow moving rainstorms were greater in magnitude than those of fast moving rainstorms. All these rainstorms, during their movement through the country, had their rain centres at about 308 locations. This study has shown that only 19 locations recorded severe rainstorm centres on five or more occasions. Maximum number of rain centres recorded for one location was 13.

Relationships between rainy days, mean daily intensity, and seasonal rainfall over the Koyna catchment during 1961-2005.

There are limitations in using monthly rainfall totals in studies of rainfall climatology as well as in hydrological and agricultural investigations. Variations in rainfall may be considered to result from frequency changes in the daily rainfall of the respective regime. In the present study, daily rainfall data of the stations inside the Koyna catchment has been analysed for the period of 1961–2005 to understand the relationship between the rain and rainy days, mean daily intensity (MDI) and seasonal rainfall over the catchment on monthly as well as seasonal scale. Considering the topographical location of the catchment, analysis of seasonal rainfall data of 8 stations suggests that a linear relationship fits better than the logarithmic relationship in the case of seasonal rainfall versus mean daily intensity. So far as seasonal rainfall versus number of rainy days is considered, the logarithmic relationship is found to be better.

Flood study of the Himalayan tributaries of the Ganga river

A flood study of the Himalayan rivers has been attempted on the basis of gauge/discharge data from 27 sites on major tributaries of the Ganga between 1986 and 1999. This study has shown that the Ghaghra, Gandak, and Kosi are the most important tributaries of the Ganga and their flood waters should be harnessed for developmental purposes to save the Indo-Gangetic plains from the recurrence of yearly floods. It was also found that some of the sites on these rivers have recorded floods more than 100 to 150 times during the 14-year period. Copyright

Zonal estimation of probable maximum precipitation rain depths over the Krishna basin in peninsular India

Abstract Estimation of design storm rain depths of probable maximum precipitation (PMP) magnitudes over different zones of the Krishna basin, India, was attempted by using daily rainfall data of about 570 raingauge stations in and around the basin for a 102-year period (1901–2002). By considering the geographical location, orographic features and rainfall characteristics, the entire basin was divided into nine zones. Analysis of severe rainstorms over each zone was carried out and envelope rain depths (standard project storm, SPS, rain depths) over each zone were estimated. These SPS rain depths were then adjusted with moisture maximization factors (MMFs) of respective severe rainstorms to work out the PMP design rain depths. On the basis of available hourly data, average time distribution was also calculated. The study was carried out with the aim of estimating the requisite standard projects storms and probable maximum storms or PMP depths to derive the corresponding design flood estimates. This may serve as a convenient tool for design engineers, in the estimation of design storm rain depths for different sub-basins and the entire Krishna basin while constructing any hydraulic structure connected with irrigation, hydropower generation and flood control in the homogeneous region of the basin. Citation Kulkarni, B. D., Nandargi, S. & Mulye, S. S. (2010) Zonal estimation of probable maximum precipitation rain depths over the Krishna basin in peninsular India. Hydrol. Sci. J. 55(1), 93–103.

A comparative flood frequency study of Ganga and Brahmaputra river systems of north India - A brief appraisal

Abstract An attempt has been made to study the occurrence of floods in the two important river systems of north India, viz. Brahmaputra and Ganga. Both these river systems are located north of Lat. 22°N and lie in the longitudinal belt of north India from Long. 73°E to 97°E. Both these river systems are affected by floods during monsoon months of June–September. It has been seen after examining rainfall and floods of the period 1986–1999 that although variation of monsoon rainfall magnitudes received by these two basins differ considerably in each monsoon season, but by and large, the frequency of floods at their terminal gauge/discharge (G/D) sites at Dhubri and Farakka do not differ very much from each other.

Envelope Depth-Area-Duration raindepths for different homogeneous rainstorm zones of the Indian region

SummaryAbout one hundred severe rainstorms occurred in the Indian region during the period between 1880 and 1990. The centres of these severe rainstorms occur mostly in four preferred zones. The two major zones where the bulk of rainstorm centres are located are in north and central India. These two zones have been further sub-divided into four homogeneous subzones for the purpose of rainstorm transposition. For each of these homogeneous sub-zones and zones envelope Depth-Area-Duration (DAD) raindepths have been calculated. These are based on the DAD data of severe rainstorms that have occurred within each zone in order to obtain probable maximum precipitation (PMP) estimates.

Efficiency of Severe Recorded Rainstorms over Maharashtra State

Abstract12 severe rainstorms have been experienced by different meteorological sub-divisions of Maharashtra State during the last 100-year period from 1891–1990. For each of the rainstorms efficiency factors (i.e. P/ M ratios) were worked out for maximum one-day duration and for three standard areas of 1000, 5000, and 10,000 km2. Comparison of these ratios with the past has shown that the most efficient rainstorm over Maharashtra was the rainstorm of June, 1908 over the Vidarbha region whose full DAD data as well as isohyetal pattern have been given.

Hydrological analysis of extreme rainfall events and severe rainstorms over Uttarakhand, India

ABSTRACT Following the June 2013 disaster in the Uttarakhand Himalayas, many discussions are ongoing with regard to how climate change is seeking revenge on mankind by endowing us with disasters! The event was mostly linked with the occurrence of an extreme event due to climate change. In view of this, an attempt has been made in this paper to analyse the extreme rainfall events experienced by the Uttarakhand during 1901–2013 using more than 100 stations’ daily rainfall data. The study revealed that during the 113-year period, the highest numbers of extreme events were recorded during the decade 1961–1970, and to some extent in the decade 1981–1990. Thereafter, there is a decrease in extreme rainfall events. The comparative study of extreme events prior to 1901 showed that on 17–18 September 1880, a rainstorm which occurred in close vicinity to Uttarakhand caused serious floods and damage to lives and properties. The extreme rainfall recorded by some stations during this unprecedented rainstorm has not been surpassed to date.