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The Gorai River, a distributary of the Ganges, is an important artery for Bangladesh, as it is the source of fresh water for the south-western part of Bangladesh. The river is used for navigation, fisheries, agriculture and domestic purposes. Besides this, the fresh water flow of the river is also important to the ecology and economy (logging) of the mangrove forests situated along the coast. During the eighties and nineties the flow in the river gradually slowed down, especially during the dry season. The river discharge was decreasing and the annual sedimentation rate was significantly increasing. This led to a vicious circle causing difficulties for the people living along its banks and detrimental effects to the mangrove forests. To get the river flowing again, a number of solutions were considered.
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The eco-dynamic design of the Gorai river includes the use of the natural flow conditions. By restoring a natural flow in the river, the river will be more self-sustaining with limited maintenance dredging. Important in the execution of such a project is a certain flexibility in the design and construction phase.
Traditionally a river like the Gorai would be embanked by a permanent structure of dams and dikes. The flow of the river would be man-made and a continuous struggle against natural forces in terms of dredging and river training would be necessary.
General Project Description
Title: The Gorai Re-excavation Project
Location: Gorai River, Bangladesh
Date: 1998 - 2001
Involved parties: Boskalis, Dredging International, HAM, VOACZ
Abstract: To get the Gorai River flowing again and minimize the period of complete drought, a channel was dredged.
Topics: River, channel, mangrove restoration, morphology, reduction salinity intrusion
In the 1980’s and 1990's the water flow in the Gorai River had gradually decreased and sedimentation had increased, causing the river to fall dry during increasingly longer periods. This, in combination with the increased salinity intrusion in the downstream part, had a negative impact on river-dependent activities such as agriculture, trade, transport, fisheries, logging, environment and domestic use. Therefore, a solution was searched to oppose the drying up of the Gorai River. Especially the mangrove forest, the Sundarbans in the delta of the Gorai, is of high environmental value. The aim of the project was not only to restore the river’s historic flow conditions, but also to explore the possibilities for long-term restoration of the river. And together with restoration of the flow, the project should improve the water quality and promote ecological restoration in the Subarbans and the Gorai River corridor.
To get the Gorai River flowing again, a channel was dredged through the shoal which blocked the bifurcation from the Ganges and the upper part of the river. Keeping the inherent uncertainty of the river morphology in mind, the design, planning and construction were executed with a high degree of flexibility.
EDD-dimension: Conserving biodiversity in the Subarbans was an important driver of the project in all phases. Moreover, the social aspects of the river contributed to the integral approach of the project. The solution chosen can be seen as building with nature, as it builds on the natural river processes and strives for long-term sustainability. Ideally, the river would have become self-sustaining with limited maintenance dredging.
Planning and Design
The project's design was to create a low-water channel from the bifurcation to some 30 km further downstream. This channel would cover only part of the cross-section and restore the river’s natural cross-sectional shape. Due to the widely varying discharges, however, the morphological response to this intervention was rather uncertain. Therefore detailed planning made little sense and adjustments to the actual morphological development has to be possible during construction.
During the wet Monsoon season, some parts of the dredged channel would fill in. Therefore, dredging continued over three successive years in the wet season, expecting the river to be able to maintain itself afterwards. In the end the river should have become a self-sustaining ecosystem, combined with limited maintenance dredging to keep the bifurcation open.
EDD-dimension: The principal driver of the project was to conserve existing environmental values and creating new ones by restoring the river discharge and thus reducing salinity intrusion into the Sundarbans area. Moreover, the increasing flow would positively influence the water quality of the river and the connected surface waters, thus indirectly the level of well-being (nutrition, poverty alleviation, employment) of the Bangladesh people. Moreover, the flexibility and adaptability needed in the design and its execution is characteristic of the EDD-approach.
Part of the sand dredged from the low-water channel was placed on the high-water banks, low enough not to hamper the flow during high water and high enough to concentrate the flow during low water. Another part of the sand was removed from the river bed and used as construction material by the inhabitants.
During construction the river's morphology was closely monitored and the design of the works was adjusted accordingly. Dredging was done during the wet season and took three successive years.
EDD-dimension: Utilising and subtly steering the morphological processes in the river can be considered as building with nature. The natural forces help to shape the design, instead of the design shaping the natural forces like in conventional designs. The set-up of the works enabled adaptation to the hydrodynamic and morphological processes in the river.
Operation and Maintenance
Each year during the wet Monsoon the channel tends to widen and shoal, but during the three-year project the dredging effort to keep the river flowing became less each year. On that basis, a blend of maintenance dredging and natural forces was expected to be optimal to keep the Gorai flowing. Maintenance dredging would concentrate at critical places, indicated by close monitoring of the hydrodynamics and the morphology of the river.
It has to be noted, however, that upon evaluation of the project consultants concluded that continued maintenance dredging and storage of the material on the high river banks may not be sustainable in the long run. They suggested a different design, including groins and revetments. This design has not been implemented so far, by lack of money and/or political will. For the same reason the long-term maintenance scheme was never realised and the river has largely returned to its degraded state.
After three years of dredging the river discharge had increased and the river was flowing all year round, thus increasing its navigability. The year-round supply of water to the delta led to a reduction of the salinity intrusion into river and the surrounding areas. Consequently, the fish population increased. However, maintenance dredging turned out to be crucial, as the river starts silting up as soon as dredging is stopped.
Because of uncertainty it made little sense to set dredging volumes and locations on beforehand. In an uncertain environment like this more flexible measures are needed. Also, the time between design-survey and approval of dredging proposals needs shortening, as the river morphology changes fast.
Close monitoring and associated modelling has led to new insights into the behaviour of the river and how to best go about it.
This examples shows that the approach promoted by Building with Nature and applied here can give a technically sustainable solution. Due to governance complications, however, this particular project could not be shown to be sustainable in the long run.
- Groot, J.K. de, P. van Groen, 2001. The Gorai Re-Excavation Project. Terra et Aqua 85: pp. 21-25.
- Hydronamic, 2000. Gorai River - re-excavation pilot priority works. Port and Waterway Engineering, Project Development
- PPW, 2000. PPW Final Report. Gorai River Contractors