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Flood Hazard Zonation Using Hydraulic Model of HEC-RAS in GIS
This study which combines GIS with hydrologic model is based on some of the previous research works. Smith (1995) created a hydrological information development system, by using GIS and hydrological watershed parameters such as design storm, soil hydrology, time of concentration, runoff coefficient, etc (6). OLivera and Maidment (1992) used GIS technique, obtained primary stages data including the elevations, separated the reach network and sub-basins, recognized the hydrological elements, created continuity among them and finally input them in the hydrological model (7 and 8).
The results obtained from using HEC-Ras showed that determination of spatial parameters are easy by hydrological model systems and the results can be extendable.
MATERIALS AND METHODS
To recognize the Neka watershed, the vegetation, geology, soil and other information are prepared in the form of maps. Then through GIS and evaluation of the effect of the application of pre-processor of GIS for hydrological system, the channel vegetation of the basin is created to obtain a regional model of channel and watershed characteristics. Information from early model of elevations is prepared from GIS processor and is transferred into hydrological model. To create a rainfall model in GIS medium, it has been tried to practically evaluate it in the hydrological model. To compare the results of hydrological model with observed flow data for calibration of the model, the evaluation of the correctness of the input to the model and the modeling system have been performed. For the regional modeling, simulation and flooding levels GIG techniques has used such as; ArcView software and HEC-GeoRAS extension v.3.1, Spatial Analysis and 3D Analysis (9).
To determine the water velocity in the selected reaches, a cross section was surveyed between the highest and lowest main channels in each hydrological unit. These sections should be regular and constant in shape. After estimation of roughness coefficient by Chow (1988) method, the water velocity was calculated for the main river and each tributary using Manning equation.
The lag time of the sub-basins was calculated from US-SCS formula described by Chow et al. (10):
Tc =L 0.8 W[(1000/CN)]-9]0.7 31.68S 0.5................(1)
Finally for the flood zonation of the 5.5 km section of the Neka watershed 7 return period of 2, 5, 10, 25, 50, 100 and 200 years has selected which are determined with different height, level, depth and velocity along the channel.
RESULTS AND DISCUSSION
Discharge Estimation for Different Return Period
After preparing the probability maximum discharges in Ablo station, between different statistical distributions of discharges, using Hyfa software the best one with different return period for each station has determined. In this study Pearson type III has shown as the best statistical distribution. The flow rates used in simulation of flood discharge in Neka watershed are shown in Table 1.
| Distr./Ret.(yr) | 2 | 5 | 10 | 20 | 25 | 50 | 100 | 200 |
| Log. Normal (2 parameter) | 73 | 189 | 310 | 468 | 527 | 742 | 1010 | 1338 |
| Log. Normal (3 parameter) | 73 | 231 | 371 | 532 | 589 | 783 | 1005 | 1258 |
| Pearson III | 62 | 113 | 229 | 406 | 475 | 719 | 1008 | 1337 |
| Log. Pearson III | 74 | 180 | 303 | 480 | 553 | 837 | 1239 | 1801 |
| Gamble | 108 | 312 | 447 | 577 | 619 | 745 | 871 | 997 |
| Selected Pearson III | 62 | 113 | 229 | 406 | 475 | 719 | 1008 | 1337 | Table 1. The best statistical distribution data used for simulation of flood discharge in Neka watershed.
Manning roughness coefficient
For determination of Manning coefficient firstly all characteristics of the selected sections in right, left and the main bed of the river channel separately have measured in the field (Table 2).
| No. Section | Right Bank | Left Bank | Main Channel | Number for each section |
| 1 | 0.050 | 0.050 | 0.040 | 1 to 46 |
| 2 | 0.035 | 0.035 | 0.030 | 46 to 93 | Table 2. Determination of coefficient for each section
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