The Feni-river closure dam reviewed

Abstract

The subject of this study is the Feni River Closure Dam in Bangladesh, which was designed in 1983 and subsequently built in 1985. In this final report the influence is analyzed of advanced design methods, which have become available since 1983, on the geometric dam profile and sea side slope protection. For this purpose the Feni dam was redesigned using these advanced design techniques. First the methods used for the original design were analyzed, with regard to the geometric profile and sea side slope protection. Subsequently the Feni dam was redesigned, using advanced design methods and considering the afore mentioned items. Contrary to the original design, the distinction between monsoon and cyclone conditions was made, because of the different character of both hydraulic situations. As a result of the closure of the Feni river by means of the Feni dam, a huge area downstream of the dam has accreted. The geometric redesign and structural redesign of the slope protection was carried out twice: With and without accretion influence. In this way, the influence of a hypothetical method to predict the morphological process, which was not available at the time the original design was originally made, is demonstrated. In order to derive the hydraulic loading conditions first the water levels were analyzed. The influence of the accretion process on the water levels near the dam was assumed to be zero. By analyzing gauge readings near the dam site, a probability distribution for water levels during monsoon conditions was derived. The probability distribution of water levels for cyclone conditions was obtained by means of a numerical storm surge model. After calibrating this model, 40 synthetic situations were derived, of which the probability of occurrence could be derived from the meteorological cyclone record since 1900. By computing the maximum water levels for these 40 situations and by means of relations between gauge stations the probability distribution for cyclone conditions near the Feni dam could be obtained. For the wave climate the influence of the accretion process is not negligible. As no wave observations have been carried out in the neighbourhood of the Feni dam, again a numerical model was used. By means of a wave hindcast model, the wave climate could be estimated during cyclones and monsoon winds and for the two cases: with or without accretion influence. It was found out that the wave height 3 years after closure was about 10 to 20% lower than at the time of closure for the same probability of exceedance. This effect is caused by the accretion of the fore shore.