The difference of SOBEK software is capable to allows simulating system for real-time (Real Time Control - RTC) basing on the observation data on the system which are updated continually to help manage, supervise and operate the exploitation work of water resources in the best way. For example, controlling and operating the work systems for flood control, preventing the tide, flood control; or to form a water supply plan for the work systems of water resources exploitation or monitoring water quality for a system to have a operation plan.

Moreover, with the necessary development of the current commercial software, SOBEK has potential to develop into open source software that allows users to set up, choose the method to solve the problems of Water Resources to suit the conditions and scope of study of Vietnam.

3. OVERVIEW THE APPLICATIONS OF SOBEK SOFTWARE

3.1 The applications of SOBEK software

Thanks to outstanding advantages, SOBEK has been applied in many countries around the world to solve the problems about drought and flood management, morphology simulation, water quality and urban water management. The case studies include a doctoral thesis research of Kuntiyawichai in solving "The interaction between land use and flood management in river basin sub - Thailand" in 2012. Chi river basin in Thailand can not solve the flood problem, especially in low areas, flood occurs frequently. Thus, a program of general flood management has been developed to minimize the negative impacts of floods with different intensities. Hydrological model (SWAT) and hydraulic models (1D / 2D SOBEK) was integrated to simulate flow in the rivers and flooded areas, and analyze the existing systems [7]; The project applied SOBEK 1D WQ module successfully for Punggol-Serangoon basin in Singapore. The project studied and assessed the impact of regulation works to the water quality of city [1]. Zuwen et al, 2003 applied SOBEK 1D module (channel module) successfully to study channel evolution problem at the Yellow estuarine (Yellow River) - China [15]. Verway,2011 studied modeling to support drainage systems planning in Yuen Long, Hong Kong. This is a project having great significance for the managers of Hong Kong. The results of the study are important scientific basises for the urban development and planning in the future.

Besides planning drainage network, SOBEK software is also capable of performing the problem of water supply. To enhance freshwater resources through rainwater storage and collection in the reservoirs to cope with urban water scarcity in many countries. In a study of Bui (2012), with the synthesis model approach, SOBEK was applied to estimate the potential of rainwater in the Punggol-Serangoon urbanized catchments in Singapore. This approach includes: developing hydrological models, 1D flow model, the water drainage and supply network model and a control model according to real time (Real Time Control - RTC) for the operations of water resources exploitation works (water work) as pumping stations, drainage pipes, rolling weirs and channel connectingto the reservoirs. The model establishments is based on a comprehensive collection of survey data about meteorology (such as precipitation, evaporation), land use and topography (eg cross-section of canals, reservoirs depth and height of the ground) in the basin. First, the integrated models is calibrated and validated with the water level observation data of reservoir in the period between 2009 and 2011. After that, the models was calibrated and validated to be used to check the water storage capacity of reservoirs basing on the monthly simulation and measurement water level depth in the reservoirs. The result of study showed that: 1) models which developed can describe the hydrological changes due to the impact of external factors, particularly the operations of hydraulic works; 2) constructing nrolling weir is a useful measure to increase the freshwater productivity of basins. In terms of rainfall during the period from October 2010 to July 2011, the average water storage capacity of Punggol and Serangoon the reservoirs are respectively about 4.28 and 3.46 million m³; 3) rain water which collected from the corresponding basin was more than 100 million m³, this water has the ability to meet 43% of freshwater supplies for all households in Singapore. These findings provide insight for policy makers on the management and planning of water resources in the basin Punggol-Serangoon in Singapore [1].

3.2 The applications of SOBEK software

In Vietnam, at present, MIKE software is being applied commonly for most of the major river basins such as the Red – Thai Binh River Basin, Dong Nai- Saigon river basin , Vu Gia - Thu Bon river basin ... to solve the problems about flood, water quality assessment, determining the minimum flow, or establishing the operating procedures of inter-reservoir ... [6], [9], [12], [17]. Besides, HEC software has also been applied to some problems about forecasting flood, building the operating procedures of inter-reservoir for Huong [14] and Vu Gia - Thu Bon river basin [12]. In addition, ISIS software which is constructed by Halcrow Company and Wallingford University is used in water use programs (WUP) of Mekong River Basin.

Although SOBEK software is applied popularly in countries around the world to solve the problems of water Resources management, the application of this software in Vietnam is still modest. Up to now, only having some research projects using SOBEK software such as projects: "Application of two-dimensional hydrodynamic software for planning, managing floodplains disaster reduction for Quang Ngai province" in 2010 [10]. This project applied SOBEK 2D software to study and calculate two-dimensional hydraulic for Tra Khuc River to identify risk areas affecting by flood. Recently, in 2013, Ho Chi Minh City also applied SOBEK software to research project: "Management of flooding for Ho Chi Minh City" to establish the integrated management plan of flood risk with the participation of the stakeholders to give their views on factors affecting flood and propose mitigation measures [16].

It can be found that, SOBEK software applies popularly in many countries around the world due to the moderate cost and solve many problems about water resource management. However, the application of this software in our country is quite new. Therefore, the application and study of SOBEK software in solving the problems of water resource management in Vietnam is really necessary and meaningful.

4. APPLYING SCS MODEL IN SOBEK-RR MODULE TO CALCULATE FLOOD FLOW FOR BUI RIVER BASIN AT LAM SON HYDROLOGICAL STATION

Rainfall Runoff (RR) module allows us to simulate the composition of basin, calculate the flow from rain in the basin by the losses calculating and flow conversion methods. The method which is chosen to build the model is SCS curve method (Soil Conervation Service) which calculates effective rain.

4.1 SCS method

SCS curve method is proposed by United States soil-protection organization in 1972. The basis of the method as follow: In a rainstorm, the depth of effective rain or flow depth (P_e) never exceed rainfall (P). Similarly, after the process flow starts, the water amount depth which is trapped in the basin (F_a) is always equal or less than the maximum storage amounts (S). Rainfall losses initially which doesn’t create flows as I_a. Therefore, the potential flow is (P - Ia). Based on the collection, monitoring and statistical analysis of data on the basins of American, SCS method hypothesized that the ratio of F_a and P_e real quantities will be equal to the ratio between ( P - Ia) and S potential quantities. From there, the basic equation system of SCS method to calculate the effective rainfall depth or flow directly from a rainfall as follows:

                                                (1)

From the continuity principle, having:

P= P_e + I_a + F_a                                              (2)

Combining (1) and (2) to calculate P_e:

P_e =                                                (3)

S- the depth of maximum storage water layer in the soil; I_a – the depth of initial losses; P_e - the depth of effective rain; F_a – continuous infiltration depth; P- Precipitation.

By studying the experimental results on many small basins, the experience relation has be built in the United States:

I_a = 0.2S

On this basis, having:

P_e =                                               (4)

Establishing the correlation graph between the P and Pe by the data of many basins help find the series of curves, standardize according to CN number. It is a dimensionless number, taking values in the range. With non-porous surfaces or water surfaces, CN is 100; with the natural surfaces, CN is less than 100. The number signs of CN and S curves are interconnected via equation:

The number signs of CN curve is established available spreadsheet which bases on the documents about land classification and land use situation in the United States. SCS method has been using widely in many parts of the world and has the stable and reliable results in the assessment of surface flow. The improvements of SCS about the practice and theory was conducted by Bofu Yu, Tammos, Forest Research Institute Vac-sa-va ... and brang certain efficiencies. Ashish Pandey and colleagues determined the surface flow of Karso basin, combined using GIS and SCS method, received:

Q =                                               (6)

In which: Q - the depth of water layer (mm); P - Precipitation (mm); S - The potential storage amount (with 5 rain days); I_a = 0,3S - the depth of initial losses (mm).

4.2 Applying SCS model to calculate flood flow for Bui river basin, at Lam Son hydrological station

Bui River Basin accounting Lam Son hydrological station has circular shape with the basin area is 33 km². The terrain of river basins has high mountainous, steep roof combining with rock mountains and sparse forests and hills. Inhabitants in the basin are mostly ethnic minorities with low population density.

Bui river basin has only Lam Son hydrological station, which locates in Lam Son commune, Luong Son district, Hoa Binh province. Lam Son hydrological station was established in 1969 and operates until now, measuring the water level, discharge and daily rainfall. Bui river is mountains river with the riverbed slope is big and the up and down floods are very fast, so the timely measurement arrangement and measurement to capture flood peaks exactly is very difficult. For example, in 2008, when the history flood occurred, Lam Son hydrological station didn’t observe flood peaks.

Rain in Bui river basin is by the influence of weather conditions in the region. Generally, heavy rainfall usually occurs in July and August in years. Flood of Bui river basin concentrates from June to September, in which mainly months are July, August and September. With single flood shape having a peak and the up and down intensity of flood is very quick affects to the lives of inhabitants in both sides.