The impact of mangrove tree models has been studied under controlled model conditions, using a wide flume in a hydraulics laboratory.

The hydroinformatics software model DIVAST (Depth Integrated Velocities And Solute Transport) was modified to account for drag force and blockage due to mangrove vegetation. The hydraulic model was verified against experimental data. A visual BASIC graphical user interface and animated high quality graphics output were developed to make the model accessible to engineers and planners with limited knowledge of hydrodynamic and water quality models.

The model was then set up for two mangrove estuaries in: India (1) and Malaysia (2). For case study (1) the model was extended to predict selected water quality parameters and the impact of mangrove vegetation on pollutant transport in Malad Creek, near Bombay, India. In case study (2) the model has been set up to quantify the impact of logging and forest management on the coastal hydrodynamics of the Merbok Estuary in Malaysia

Guidelines for the calibration and application of the DIVAST model for water quality management have been developed. A manual has been produced and two workshops have been organised at the Department of Irrigation and Drainage, in Kuala Lumpur, Malaysia, to train local engineers and planners. Furthermore, a student from the University of Science, Malaysia, has been trained in the use of the model. A socio-economic survey has also been carried out for the Merbok Estuary, to establish which stakeholders would be affected most by water pollution problems.

The model has been used by NEERI (National Environmental Engineering Research Institute) in India to study the design of the Bombay Sewage Outfall project. This study has been undertaken in association with Binnie and Partners (now Binnie, Black and Veatch), UK.

The impact of mangrove vegetation on the hydrodynamic processes has been studied in a hydraulic flume. The effects of tree diameter, density, spatial distribution and the presence or absence of stilt roots have been tested for different flow conditions. It was established that mangrove vegetation increased the velocity and water depth in nearby creeks and caused an appreciable headloss. Tracer simulations showed that mangrove vegetation enhanced the advective transport of solutes in nearby creeks. The transport of solutes was lower in the adjacent vegetated areas due to low flow velocities, but local turbulent mixing may be enhanced.

A widely used numerical model (DIVAST) has been modified to include drag force and blockage due to mangrove vegetation. A graphical user interface and high quality graphical output device have also been developed, making the model easier to use as a management tool. The model has been verified against experimental data. Values for the drag coefficients of mangrove vegetation were obtained through a systematic calibration process. The predicted impact of mangrove vegetation was found to be mainly dependent on the vegetation surface and on the drag coefficient. The latter was independent of the flow conditions tested but not on the vegetation surface.

Application of the model to two field sites showed that inclusion of mangrove vegetation significantly improved the agreement between the modelled and measured tidal velocities. The results obtained in Malad Creek, India, suggested that mangrove vegetation increased the flushing of the main creek and, hence, improved the predictions of the overall water quality characteristics. However, this effect may be small compared with other parameters that influence the flushing rate of a tidal creek. Transport in the vegetated areas was reduced and this was thought to be a more important factor in mangrove areas, potentially leading to a stronger impact on the fate of pollutants released within mangrove swamps.

The results from the socio-economic survey suggested that aquaculturists and fishermen were most likely to be affected by water pollution problems in mangrove areas.

Mangrove vegetation has a considerable effect on the hydrodynamics of nearby water bodies, which have been predicted much more accurately using the modified DIVAST model.

The graphical used interface and the graphical output devices have made the hydroinformatics model much more user-friendly to engineers and planners with little or no knowledge regarding the use of sophisticated hydrodynamic and water quality models.

Application of the model to two field sites indicated that mangrove vegetation influenced the transport and flushing processes in the estuaries and the pollutant transport fluxes within the swamp areas.

The carrying capacity of a mangrove estuary may be defined on the basis of its hydrodynamic flushing characteristics and its ability to support the biological transformation and degradation of pollutants

The modified model can help to assess the water management options in mangrove estuaries and assist in the design of water quality surveys.

Struve J. and Falconer R.A., "A Review of Water Quality and Hydraulic Processes in Mangrove Areas". Submitted to the ASCE Committee on the Restoration of Wetlands. To be published in Journal of Coastal Research, 1999.

Wu, Y., Falconer, R. A. and Struve, J., "Modelling of Flows in Mangrove Forests with Particular Application to the Merbok Estuary". Proceedings of International Conference on Hydrology and Water Resources of Humid Tropics, Ipoh, Universiti Sains Malaysia, Paper 1, November 1998, pp.1-11.

Franks, T. and Falconer, R.A., "Developing Procedures for the Sustainable Use of Mangrove Systems", Agricultural Water Management, Elsevier Science B.V., 1999 (in press).

Acknowledgements

The Principal Investigator would like to acknowledge the considerable contribution made to this study by his co-researchers, namely Ms Juliana Struve (now at the Environment Agency, Thames Region) and Dr Yan Wu (Cardiff University), Professor Ong and Dr Gong at the University of Science, Malaysia, and Dr Khanna and his staff at NEERI, India.

Journal publications are in preparation

DIVAST was distributed free of charge to the Department of Irrigation and Drainage, Kuala Lumpur (Malaysia), to the National Environmental Engineering Research Institute (NEERI, India) and to the Universiti Seins Malaysia, Penang. A training course will be provided to staff at NEERI, shortly.

Contacts are being established in Vietnam and Thailand with a view to providing the model to research organisations in these countries.

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