Estimation of Probable Maximum Precipitation (PMP) over the Hiramandalam Reservoir Catchment Using GIS

Abstract

Reliable estimation of the Probable Maximum Precipitation (PMP) and the resulting Probable Maximum Flood (PMF) is essential for fixing the design flood of high-hazard hydraulic structures such as spillways and flood-control dams. This paper presents a GIS-based estimation of one-day PMP and the corresponding PMF for the catchment of the Hiramandalam Reservoir, located in Hiramandalam mandal of Srikakulam district, Andhra Pradesh. The catchment, delineated from SRTM DEM data using GIS tools, has an area of 85.922 sq.km, a longest flow path of 17.650 km, a centroidal flow path of 8.503 km and a catchment slope of 3.563 m/km. A Synthetic Unit Hydrograph (SUH) for the catchment was derived using the generalised relationships given by the Central Water Commission (CWC) for the East Coast Sub-zone 4(a), yielding a peak discharge of 196.63 cumecs at a time to peak of 2.5 hours and a time base of 14 hours. The catchment was overlaid on the isohyetal maps of the CWC PMP Atlas for the Mahanadi and Adjoining River Basins to identify the critical rainstorm (14 October 1931, centred at Pottangi, peak rainfall 546 mm). Using the storm transposition and maximisation approach, with Transposition and Maximisation Adjustment Factors extracted from the surrounding grid points, the one-day Standard Project Storm value was computed as 542.5 mm, which, after clock-hour correction, yielded a design one-day PMP of 592.5 mm. This depth was distributed into two 12-hour bells using CWC time-distribution coefficients, reduced by the design loss rate to obtain effective rainfall, arranged in the critical B2-B1 sequence, and convolved with the SUH ordinates to obtain the flood hydrograph. The resulting Probable Maximum Flood for the Hiramandalam Reservoir catchment was found to be 44,109.78 cumecs. The study illustrates how a GIS-integrated workflow can make PMP/PMF estimation reproducible and readily transferable to other small to medium ungauged catchments for spillway adequacy and dam-safety assessment.

Introduction

The text presents a GIS-based estimation of Probable Maximum Precipitation (PMP) and Probable Maximum Flood (PMF) for the Hiramandalam Reservoir catchment in Srikakulam district, Andhra Pradesh. PMP represents an extreme physically possible rainfall for a specified duration, while PMF is the corresponding extreme flood used for assessing spillway safety, particularly for high-hazard dams.

The study is motivated by the need to independently evaluate spillway adequacy, especially for reservoirs designed using older and limited hydrological data. It combines GIS-based catchment delineation, Central Water Commission (CWC) PMP procedures, storm transposition and maximisation, and a Synthetic Unit Hydrograph (SUH) to estimate the PMF.

Main Objectives

• Delineate the Hiramandalam Reservoir catchment using SRTM DEM and GIS.

• Determine important physiographic parameters such as catchment area, slope, and flow-path lengths.

• Develop a Synthetic Unit Hydrograph using CWC regional relationships for East Coast Sub-zone 4(a).

• Identify a critical historical storm from the CWC PMP Atlas.

• Estimate one-day PMP using storm transposition and maximisation.

• Apply rainfall time-distribution coefficients and design losses to obtain effective rainfall.

• Convolve the effective rainfall with the SUH to estimate the Probable Maximum Flood and assess its implications for spillway design.

Literature Review

The study reviews several PMP estimation methods. Hershfield's statistical method uses annual maximum rainfall records and approach through its PMP Atlas, together with regional Synthetic Unit Hydrograph relationships. These frequency factors, while later studies have questioned interpreting its frequency factor as a fixed return period. Storm-based moisture maximisation and transposition methods have been widely used for Indian river basins.

The Central Water Commission (CWC) uses a hybrid storm-transposition and maximisation approach through its PMP Atlas, together with regional Synthetic Unit Hydrograph relationships. These CWC procedures form the methodological basis of the present study.

Study Area

The Hiramandalam Reservoir is located in Hiramandalam mandal, Srikakulam district, Andhra Pradesh, near the Odisha border. The reservoir has a gross storage capacity of approximately 10 MCM.

The GIS-delineated catchment has:

• Area: 85.922 km²

• Hydro-meteorological zone: East Coast Sub-zone 4(a)

• Base terrain data: SRTM DEM from the Bhuvan/ISRO geoportal

• Main rainfall influence: Southwest and northeast monsoons, with potential for intense rainfall from depressions and cyclonic storms.

Important Input Data

The study uses:

• SRTM DEM for watershed and stream delineation.

• GIS-derived catchment and physiographic parameters.

• CWC PMP Atlas for the Mahanadi and Adjoining River Basins.

• A critical historical storm of 14 October 1931 at Pottangi.

• Weighted historical rainfall of 500 mm.

• Average Transposition Adjustment Factor (TAF): 1.09.

• Average Maximisation Adjustment Factor (MAF): 1.33. regional SUH relationships, and rainfall–runoff transformation, the research aims to provide a conservative design flood estimate for

• 24-hour rainfall distribution of 76.8% in the first rainfall bell and 23.2% in the second.

• Design loss rate of 0.75 cm/hr for the first 9 hours.

• CWC regional SUH with a peak discharge of 196.63 m³/s and base time of 14 hours.

Methodology

The methodology consists of four major stages:

1. GIS Catchment Delineation
   The SRTM DEM is processed to determine the reservoir catchment, stream network, longest flow path, centroidal flow path, and catchment slope.

2. Synthetic Unit Hydrograph Development
   The catchment's physiographic characteristics are used with CWC's regional relationships for East Coast Sub-zone 4(a) to derive the SUH.

3. PMP Estimation
   A critical historical storm is selected from the CWC PMP Atlas. Its rainfall is adjusted using storm transposition and maximisation factors to estimate the design one-day PMP.

4. PMF Estimation
   The PMP is distributed over time using CWC coefficients. Design losses are deducted to obtain effective rainfall, which is then convolved with the SUH to produce the PMF hydrograph.

Conclusion

A GIS-based procedure was used to estimate the one-day PMP and the corresponding PMF for the catchment of the Hiramandalam Reservoir in Srikakulam district, Andhra Pradesh. The study leads to the following conclusions: 1) The Hiramandalam Reservoir catchment, delineated from SRTM DEM data, has an area of 85.922 sq.km, a longest flow path of 17.650 km, a centroidal flow path of 8.503 km and a catchment slope of 3.563 m/km. 2) The Synthetic Unit Hydrograph derived from CWC\'s generalised relationships for East Coast Sub-zone 4(a) has a peak discharge of 196.63 cumecs at a time to peak of 2.5 hours and a time base of 14 hours. 3) Overlay of the catchment on the CWC PMP Atlas for the Mahanadi and Adjoining River Basins identified the storm of 14 October 1931 at Pottangi as critical; storm transposition and maximisation using grid-based TAF and MAF values gave a design one-day PMP of 592.50 mm after clock-hour correction. 4) Distribution of the PMP into two 12-hour bells, reduction by the design loss rate, critical B2-B1 sequencing and convolution with the SUH together gave a Probable Maximum Flood of 44,109.78 cumecs for the catchment. 5) The magnitude of the estimated PMF underlines the importance of re-checking existing and proposed spillway capacities at the Hiramandalam Reservoir, and demonstrates a GIS-integrated workflow that can be extended to other ungauged catchments within the same sub-zone.

References

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Copyright

Copyright © 2026 Dodla Santhosh Kumar, Dr. Vemu Sreenivasulu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.