· · · Repository Hydraulic Engineering Reports

Inventory of the status of the sea dikes in the Red River Delta (northern Vietnam) partly as preparation for improvement works subsidized by the world food program.

Model to determine storm surges due to typhoons along the Vietnmase coast using Delft3d.

The Tarn Giang-Cau Hai lagoon is the most important coastal lagoon of Vietnam located in Thua Thien-Hue province. Basically formed in the late Holocene (more than 2000 years ago), the lagoon is being in the development stage. Its tidal inlets, nowadays are the Thuan An and Tu Hien inlets, are dynamic and ephemeral morphological features. Inlet migration and shoal, breakthrough ofthe sand barrier, erosion ofbeaches and sand dunes affect on socio-economic development and environment of the province to a high degree. Serious consequences of these processes are adverse effects on flooding and inundation, transportation, navigation, fishery, aquaculture, agriculture, lagoon ecosystem and environment. As a primarily step of research on the system, the study is limited on the hydraulic characteristics of the system with the main objectives are to set-up a numerical model to simulate and investigate the hydraulic behaviour of the system; to evaluate the stability situations of the inlets; and to suggest which processes and data are relevant for the successive steps ofthe study on morphology of the system. DUFLOW has been employed to simulate the hydraulic behaviour of the system under different boundary conditions of sea water level, river flow discharge, inlet geometry and configuration. Sensitivities and effect of the uncertainty of sea level rise, storm surges, inlet openings, river flows and tidal parameters on the hydraulic characteristics of the system have been also investigated. Stability ofthe inlets has been evaluated accordingly. Model results indicate that river flows are the most important acting force of the system during floods. Tides, storm surges and inlet openings are also important factors changing the hydrodynamic characteristics of the system in these extreme conditions. In the dry season, the most important factors influencing the hydrodynamic characteristics of the system are tides, sea level rise and inlet openings. Tidal water level, river floods, and sediment transport are the most sensitive acting forces influencing the stability of the inlets. The stability situation of the Thuan An inlet is in a "fair to poor" situation, according to Brunn's P/Mrot criterion. The Tu Hien inlet, which is relatively independent with the openings of other inlets, is always in a "poor" stability condition. Beside of using the model for hydrodynamic simulation of the whole system, it is recommended to employ a morphologic model (preferably 2D) in the successive steps of the study for in detail simulation of the inlets and their vicinity taking into account of effects of tides, waves, river flows, flow circular by wind, density current, sediment transport. The relevant processes and related data are also recommended for future studies.

The coastal areas of Vietnam are repeatedly hit by devastating storms and typhoons. Protective sea dikes are overtopped or breached frequently, with the resulting flooding causing damage to agriculture land, loss of life and crops, and destruction of infrastructure. In the coastal area of the five northern provinces of the Red River Delta, 361 kilometres dikes will be upgraded, assisted by the World Food Program, project number 5325. This project is the follow-up of the WFP project number 4617, which was focused on 7 provinces along the Central Coast. As a result of the strong similarity in physical nature between the Netherlands and Viet Nam on water related issues, the Ministry of Agriculture and Rural Development of the Socialist Republic of Vietnam has requested to the Ministry of Transport, Public Works and Water Management of the Netherlands for technical assistance on flood control issues. As a first step the Dutch Ministry has sent in November 1995 two representatives, mr. Anne van Urk and mr. Ale van der Hoek, to discuss many flood control and water management issues in Viet Nam, on which cooperation can take place. One of the issues with the highest priority was to get a second opinion on the sea dike designs as to be used in the WFP program. So as a second step in the program a mission visited Vietnam, from 26 March to 12 April 1996, in order to evaluate the existing designs and execution options for the five coastal provinces of the Red River Delta.

Among a variety of elements, the nearshore wave climate is considered to be the most important aspect to explain erosion problems along the coast. Due to the fact that available data is limited, numerical models are valuable and necessary tools to apply. This study is initiated with the main objective to study the nearshore wave climate of the project area at Nam Dinh coast in Viet Nam by a 2D wave SWAN model in order to investigate the erosion problem along the coast. SWAN (acronym for Simulating of Waves Nearshore) is a third generation wave model, which has been widely applied to simulate wave conditions in many complex field cases in the Netherlands. However, it has never been used in Viet Nam. Thus the achievement of the study may be considered as documentation for future researches of this model in Viet Nam cases. Within this research, at first a SWAN model was set-up based on available data. This includes a sensitivity analysis and model calibration. Then a series of computations were carried out for the application of the model for a mean statistical year. Subsequently, the computed results from the SWAN model were used to identify sediment transport rates. These were compared to actual values in order to investigate whether the 2D wave model can predict the erosion adequately. One of the main results of this study was that the observed sediment transport was qualitatively in accordance with the sediment transport result of the 2D wave model. This emphasized the importance 2D effects to obtain realistic results. In this study, several drawbacks of SWAN have been identified. They include the inconvenient procedure of pre- and post processing in applying SWAN and the convergence problem when activating all physical processes in the model simulations. The study also showed the importance of sufficient wind data in the computational grid. The assumption of uniform wind field in computational grid may lead to inaccurate results in particular with respect to the mean wave direction. For an improvement of the results, the study recommended amongst others that future research should try to generate a variable wind field in the computational grid.

Evaluation on mitigation of natural disasters, mainly floods, in Central Vietnam, results of the findings of the mission, policy planning, disaster response, sustainability, donor response, prioritized list of mitigation projects, empowerment of communities that live with floods,