Hydraulic and morphological impact of a closure dam in the Gulf of Khambhat

Abstract

Tidal basins are highly dynamic environments with a complex behavior that is often disturbed by human activities. Considering that tidal dynamics have a direct impact on surrounding engineering infrastructures, adjacent coastlines, nature environment and socio-economic human activities, it is crucial to know the impact of human interventions on these dynamics. This research focuses on the closure of the Gulf of Khambhat in India, which aims at the creation of a fresh water reservoir in the gulf by partly closing the current estuary with the Kalpasar dam. The most recent design of the closure concerns a 30 km dam from the eastern bank near Aladar to the western bank near Bhavnagar. This closure will significantly reduce the total basin area. Consequently, major and morphodynamic changes are expected in the basin.

The Kalpasar project has been on the Indian Government's agenda since 1986. Royal HaskoningDHV was involved in the pre-feasibility study, which was presented in the late 1990s. Since then, the existence of a detailed report study has not been confirmed and the status of the announced feasibility studies by the Indian government is unknown. The main objective of this research is to investigate the morphodynamic response at the seaward side of the dam after the closure.

A process-based morphodynamic model has been developed to study this response. Deltares and NIOT have provided a two-dimensional (2DH) numerical model of the Gulf of Khambhat to study the tidal propagation in the basin. After calibration and validation of the hydrodynamic predictions, the model has been extended to a morphodynamic model in order to perform morphological calculations. Several adaptations have been made to improve the hydrodynamic simulations of the model; the main contributing factor was the new initial bathymetry. An extensive spin-up simulation has been performed to gather this. To deal with introduced model artefacts, the model results are compared to the reference case. Therefore, two almost identical simulations are performed: the only difference is that one run contains the Kalpasar dam while the other does not. This way, the relative effect of the dam is determined. Moreover, the morphological results have to be interpreted qualitatively, since the predictive skill of the model has not yet been determined.

Model results show an overall increase of the tidal range in the basin after the closure, which will have effect up to 100 km from the dam. Close to the dam, the range will initially increase from 7.88 to 10.25 m, and up to 10.50 m after 96 years. Besides, the tidal signal switches from being ebb-dominant to flood-dominant. The velocities around the dam become negligible and the velocities at the main western channel significantly decrease. The eastern channel remains the main channel of the gulf, although its maximum flood- and ebb-velocities also decrease. As a consequence of the hydrodynamic changes, the basin will start importing sediment, directly becoming a sink. On the long term, the area up to 40 km southward of the dam partly fills in. The eastern side of the main channel will also accrete, although its western side will erode over the years.

The implementation of the dam will have negative impacts on several locations at the study area. Four vulnerable locations are identified, namely Dahej (India's largest LNG-terminal), Hazira (container terminal), Surat (> 6 million inhabitants) and Alang (largest ship wrecking worldwide). All of these locations will become prone to flooding because of the increased maximum water levels. Moreover, sedimentation at the approach channels of the ports of Dahej and Hazira may hinder their accessibility. The increased water levels at Alang may lead to the suspension of heavily contaminated sediments from these beaches. Furthermore, coastal erosion might be a problem for the entire study area, if the ebb-tidal delta is not large enough to balance the sand hunger of the basin.

Relocating the dam is not an effective measure to prevent the negative impacts. Maintenance dredging activities will probably be required to maintain the accessibility of the ports. Moreover, major infrastructural changes will be needed to prevent floods. It is recommended to execute follow up studies with more detailed tools to determine the exact response at these locations. More accurate and up-to-date hydraulic and bathymetric data is required to develop these tools. To this end, it is highly recommended to partner with local parties like the Government of Gujarat, the EAG and local universities.