Closure of the Gulf of Khambhat

Abstract

This final thesis report describes the results of the feasibility analysis to the closure of the Gulf of Khambhat done by Erik Broos and Kees Wiersema, from Delft University of Technology. The study started with site visits to both Gujarat and La Rance where interviews were held to obtain insight in the desires of concerned parties as the Government and private investors, followed by a design study to the innovative methods of closure.The closure of the Gulf of Khambhat in the state of Gujarat, India, is the largest closure of a tidal estuary in the world so far. It is part of the multi-purpose Kalpasar project. This project solves the two main problems in Gujarat: the shortage of (irrigation) water and the shortage of electricity. The closed Gulf will be divided into two reservoirs: a fresh water basin for irrigational purposes and a tidal basin for generation of electricity. Secondary aspects of the Kalpasar project are a shorter road connection and the possibilities of port development and land reclamation. The dam alignment runs south of the Narmada river, to ensure inflow of fresh water into the basin. The total dam length is about 60 km, 30 km through deep water, 30 km through shallow water. Constructing a dam in this Gulf is very difficult, as the tidal difference is extremely large. The average difference between high and low water is about 8 meters, while during spring tide the difference can be more than 10 meters. Together with a water depth of sometimes more than 30 meters below Mean Sea Level and a 30 km wide closure gap, it is clear that this is a project unlike any other. A dam of 30 km closed off the Zuiderzee in the Netherlands, and the Tidal Power Station in La Rance (France) has a tidal difference of 13 meters at spring tide, but the Gulf of Khambhat combines these two problems.Although several designs have been made, it has become clear that, to become feasible in India, the design should be 'cheap', fast to build and using local material as much as possible.To reduce the scale of the problem without losing total overview, the closure dam is split into smaller components. These components each have similar design problems, but on a smaller scale. Integrating these in a total design while making use of each alternative's advantages creates an innovative design where problems of one component are reduced by the advantages of another. Interviews during the visit to India learned that a tidal power facility was one of the most desired components. The main components are the tidal power facility (TPF), the spillway of the reservoir (Narmada spillway), the final closure gap and the secondarydamsections.During an early stage the idea was born to incorporate the tidal power facility in the closure process. The tidal power facility requires a large orifice to fill and empty the tidal basin. This orifice proved to be very useful to reduce current velocities during final closure (from 8 rn/s to 6 rn/s). In fact, the tidal power plant is used as a sluice. It is consists of 45 concrete caissons, which are constructed in a construction dock and floated into position. This same concept is used for the Narmada spillway that is needed to regulate the reservoir level. As this placing takes place in an early phase of the project, the current velocities are low. For the closure of the final gap, between the tidal power facility and the Narmada Spillway, two alternatives have been developed. The first is a temporary railway bridge from where rock is dumped.The second is using sand-filled geotextile Superbags. Their analysis showed that the second one should be chosen.