4.2.1 Model setup
Collecting and processing input data is the initial stage using mathematic model tools. Basing on the requirements of the model, collecting and processing the concerning documents and data to serve calculation. The document types include meteo-hydrological document and data in the basin; the plan of the study area; digital elevation map (DEM) ... In which, discharge and rainfall data is the most important. The data are collected according to hours and synch about time.
The study basin has only Xuan Mai rainfall stations with 12 hours and day measurement rainfall data. However, the problem of flood flow calculation must consider the change of rainfall in a short period (1h or 2h) to be able to improve the accuracy of the study results, especially with the small and medium basin as the basins of Day River. Therefore, the article uses the hour rainfall data of Ha Dong station to zoom the hour rainfall data for Xuan Mai stations basing on total rainfall with 12 hour period of Xuan Mai stations.
To simulate flood on Bui river basin, at Lam Son station, the model parameters are determined through calibration for floods in December, 1981 and validation for flood in September, 1985. Then, applying the parameters which are calibrated and tested to perform the problems of flood flow simulation of Bui river basin with flood in October, 2008.
To assess the model parameters can proceed by comparing the two processlines of calculation and observation on a chart and combing the evaluation of criteria such as NASH, the difference of calculation and observation flood peak (DQmax) and the difference of calculation and observation time peak (Dtmax).
4.2.2 The validation and calibration results of SCS model
The validation and calibrationresults of SCS model parameters in SOBEK RR module for Bui River Basin at Lam Son station achieve good results. The parameters which are selected are fairly consistent, the comparison results between the calculated and measured values with the criteria are good as NASH is 0.75, DQmax is less than 6 percent and Dtmax is identical. It demonstrates that SCS model in SOBEK- RR module can use to calculate flood flows for Bui River Basin. However, to assert the characteristic parameters for Bui river basin need to calibrate and validate SCS model with many different floods. The validation and calibrationresults and the parameters of SCS model is shown in below
Figure 3: The validation and calibration result of model
Table 2: The criterias to assess the model parameters
|
Year |
Q calculated max (m3/s) |
Q observed max (m3/s) |
DQmax (%) |
NASH |
Dt (hour) |
|
1981 |
415 |
392 |
5.87 |
0.75 |
0 |
|
1985 |
344 |
356 |
3.37 |
0.77 |
0 |
Table 3: The parameters of SCS model for Bui river basin at Lam Son station
|
No |
Parameter |
ID |
Unit |
Value |
|
1 |
Average Basin Slope |
i |
m/m (%) |
0.0013 |
|
2 |
Flow path length to outlet |
L |
m |
9,000 |
|
3 |
Curve number |
CN |
- |
70 |
|
4 |
Unit Hydrograph |
UH |
- |
SCS UH |
|
5 |
Initial AMC |
IAMC |
- |
III (Wet) |
|
6 |
Time lag |
TL |
hour |
1 |
By analyzing and evaluating the validation and calibration results of SCS model for Bui River Basin, can make an initial assessment that model parameters are consistent with the meteo - hydrological conditions and different conditions of basin, although the floods to calibrate and validate the model are limit. So, these parameters can use to simulate flood of Bui river basin
4.2.3 The result of flood calculation for Bui river basin
To simulate flood flow for Bui river basin, the paper uses rainfall data of Xuan Mai station which are zoomed basing on hour rainfall data of Ha Dong station. Specifically, the paper will use hour rainfall data of flood from October 30th to November 2th, 2008 of Ha Dong station. The calculated result of the flood process line is shown in the following figure:
Figure 4: The calculated result of flood flow at Lam Son hydrological station (flood in 2008)
Through the simulation results for flood from October 30th to November 2th , 2008 by SCS model at Lam Son station on Bui river basin, it is shown that the flood in October, 2008 appears from 6 a.m on October 31th to 7 a.m on November 1th and the appeared time of flood peak is at 6 p.m on October 31th with Qmax is 620 m3 / s. Comparing to the day average value of observed flood discharge which is Corresponding the time occurring flood at Lam Son station, found that values the average flood flow in October 31th is 54.2 m3 / s down 11 times the flow value instantaneous peak flows. Basing on the statistical discharge data from 1970 to 2010 of Lam Son station, the ratio between the immediate flood peak discharge and the day average discharge is from 3 to 7 times. Thereby, can see that with the small catchment as Bui river basin (F = 33 km2), the study and consideration of rain with short period is very important in forecasting flood to prevent and reduce damages that are caused by floods. Therefore, need to invest to build more short period rain gauges to implement data serving the flood management and study for Bui river basin.
5. CONCLUSION
SOBEK software is a powerful tool to calculate and forecast flood, optimize drainage system, control water system, design sewer, simulate the morphology of river, simulate salinization and surface water quality. Besides, SOBEK is the ideal tool to study the problems of dam and dike break and the flood in urban areas ... 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. The paper has also applied successfully to simulate flood flow by SCS model in SOBEK-RR module for Bui river basin. Initial result have built model to simulate the flood flow of Bui river basin basing on collection documents. This result has important implication for forecasting and warning flood in the basin, as well as supporting the measurement and monitoring of heavy flood period. SOBEK software has been applying widely around the world, so it has moderate cost and solves many problems of water resources management. So the SOBEK software use in the problems of water resources management must be studied and applied. At the same time, it needs to be developed and introduced to solve different problems for different areas in Vietnam.
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