Dam Break Analysis and Flood Inundation Mapping of Polavaram Dam Using HEC-RAS

Abstract

Failure of a dam can release a large volume of stored water suddenly and endanger the population and property downstream. This study presents a two-dimensional dam break analysis and flood inundation mapping of the Polavaram Dam, a multipurpose project on the Godavari River in Eluru District, Andhra Pradesh, India, using HEC-RAS 6.6. The reservoir was represented by a storage area and the downstream floodplain by a 2-D flow area with a 50 m mesh, built on SRTM 30 m terrain data and Esri 10 m land use/land cover data. Breach of the earth-cum-rock-fill (ECRF) dam section was simulated for overtopping and piping failure using five empirical breach methods, namely Froehlich (2008), Froehlich (1995), Von Thun and Gillette, Xu and Zhang, and MacDonald et al., for a simulation period of 24 hours. The Froehlich (2008) overtopping scenario gave the highest peak breach discharge of 174,862 m³/s and the largest inundation area of 717.224 km²; the corresponding piping scenario gave 120,453 m³/s and 559.097 km². Across the ten scenarios the inundation area ranged from 172.863 km² to 717.224 km². Maximum flood depth, velocity and water surface elevation were extracted at three downstream locations, where the maximum depth reached 17.138 m at Area 1. The results show that the choice of breach method strongly affects the peak discharge and flood extent, and the maps and values obtained can support emergency action planning and flood-hazard assessment downstream of the dam.

Introduction

The text presents a two-dimensional dam-break flood analysis of the Polavaram Dam in Andhra Pradesh using HEC-RAS. The study aims to understand the consequences of potential dam failure, estimate breach flood characteristics, and identify downstream inundation risks.

Background

Dam failure can release a large volume of and a gross storage stored water as a rapidly moving flood wave, potentially causing severe damage to lives, infrastructure, agriculture, and downstream settlements. Embankment dams such as the Earth-Cum-Rock-Fill (ECRF) section of Polavaram Dam can primarily fail through:

• Piping/internal erosion, where seepage progressively creates a breach.

• Overtopping, where water flows over the dam crest and erodes the embankment.

Breach size and development time strongly influence the resulting flood hydrograph and peak discharge. Because different empirical breach equations can produce different breach characteristics, comparing several methods is important.

Objectives

The study aims to:

1. Conduct a 2-D dam-break analysis of Polavaram Dam using HEC-RAS.

2. Simulate both piping and overtopping failure scenarios.

3. Estimate breach dimensions and breach-development characteristics using five empirical methods.

4. Generate breach flood hydrographs and determine peak discharges.

5. Produce downstream flood inundation maps.

6. Determine water depth, water-surface elevation, and flow velocity.

7. Compare the results of different empirical breach methods under both failure modes.

Study area

The Indira Sagar Polavaram Project is a large multipurpose project on the Godavari River in Andhra Pradesh. It has a catchment area of approximately 306,643 km² and a gross storage capacity of 194.604 TMC at full reservoir level.

The project consists of a concrete spillway and an ECRF dam section. The ECRF section is approximately 2,454 m long, while the spillway contains 48 gates. The study considers failure only of the ECRF section; the concrete spillway is assumed not to fail.

Data used

The main datasets include:

• SRTM 30-m Digital Elevation Model (DEM) for downstream terrain.

• Esri 10-m land-use/land-cover data for assigning surface roughness.

• Reservoir elevation–area data from the Water Resources Department.

• An observed 16 July 2022 reservoir inflow hydrograph, with a peak inflow of approximately 57,231 m³/s.

• Manning's roughness coefficients assigned according to different land-cover classes.

HEC-RAS methodology

The study uses HEC-RAS 6.6 with 2-D unsteady-flow modelling. The downstream floodplain is represented by a 2-D flow area with a 50 m × 50 m computational mesh. Manning's roughness values are assigned according to land use.

The ECRF dam is represented as an SA/2D connection between the reservoir storage area and downstream floodplain. The simulation covers 24 hours, corresponding to the duration of the input inflow hydrograph.

Reference points downstream are used to evaluate important flood characteristics such as:

• Flow velocity

• Flood depth

• Water-surface elevation

Breach scenarios

Ten breach scenarios are considered:

• 5 empirical breach methods for piping failure

• 5 empirical breach methods for overtopping failure

The five methods are:

• Froehlich (2008)

• Froehlich (1995)

• Von Thun and Gillette

• Xu and Zhang

• MacDonald and Langridge-Monopolis

These methods estimate parameters such as breach width, breach depth, side slopes, and breach formation time, which are then entered into HEC-RAS.

For piping, By comparing multiple breach formulations under both piping and overtopping conditions, it seeks to improve the breach initially develops as an opening caused by internal erosion and is represented using orifice flow. As the breach enlarges, the flow transitions toward weir-type behaviour. For overtopping, erosion begins at the dam crest and develops into a trapezoidal breach, with discharge represented using a weir-flow equation.

Importance of the study

The central focus is to determine how failure mode and empirical breach method influence peak discharge, flood-wave propagation, inundation extent, depth, velocity, and water-surface elevation.

Conclusion

This study carried out a two-dimensional dam break analysis and flood inundation mapping of the Polavaram Dam using HEC-RAS 6.6. Breach of the ECRF dam section was simulated for overtopping and piping failure using five empirical breach methods, and the resulting flood was mapped over the downstream floodplain. The main conclusions are as follows. 1) The breach method has a strong influence: the peak breach discharge ranged from 32,434 m³/s to 174,862 m³/s, a difference of more than five times, and the inundation area ranged from 172.863 km² to 717.224 km² among the ten scenarios. 2) The most severe scenario: the Froehlich (2008) overtopping scenario gave the largest breach width (753.04 m), the highest peak discharge (174,862 m³/s) and the largest inundation area (717.224 km²). The corresponding piping scenario gave 120,453 m³/s and 559.097 km². 3) Overtopping versus piping: for every method, overtopping produced a larger inundation area than piping. 4) Downstream locations: Area 1 experienced the highest flood-wave velocities (1.865 to 3.937 m/s). The maximum flood depths were 17.138 m at Area 1, 10.682 m at Area 2 and 15.772 m at Area 3, and overtopping generally produced higher depths and water surface elevations than piping. 5) Use of the results: the inundation maps and the depth, velocity and water surface elevation values can support dam safety assessment, emergency action planning, evacuation planning and flood-hazard assessment downstream of the Polavaram Dam, and can serve as a basis for detailed flood-risk, vulnerability and damage assessment.

References

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Copyright

Copyright © 2026 Thota Tejaswi, 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.