Flood Hazard Zonation Using Hydraulic Model of HEC-RAS in GIS

M. Mohseni
Former Master of Sciences Student in Watershed Management,
University of Mazandaran, Sari, Iran.

K. Solaimani
Associated Professor,
Faculty of Natural Resources,
University of Mazandaran, POBox 737 Sari, Iran
solaimani2001@yahoo.co.uk

Abstract
Flood plain and area near to rivers, because of their special condition are suitable condition for social and economical activities and usually, they are affected by different flood hazards. However in these zones determination of flood zone and its height and also detecting properties of floods in different return periods is most important. So flood zonation is necessary for suitable development and main parameter for investigating of ecological and environmental effect. Interaction between some river hydrologic models and geographical information systems (GIS) cause some gains and methods, which are sensible to the planners. This study was carried out with the purpose of using hydraulic model of HEC-RAS with Arc View software to estimate the flood zone of 5-km distance on Neka River in northern part of Iran. In order to flood zonation for different periods of 2, 5, 10, 25, 50, 100 and 200 years at 1:1000 scale topography maps were used. The results can be concluded that combination of GIS and HEC-RAS model in this study is useful and efficient. Finally the flood zone of 25 years is predicted to be more dangerous than the other periods for the selected area.

INTRODUCTION
With increase in agricultural activities along rivers and concentration of population around submergible areas, the flood-induced damages are in increasing trend. The complete flood protection with installation of great flood control structures like flood dams are not justified due to its high cost. It is not environmentally, socially and economically an optimum idea either. For this reason, the flood zonztion can have a considerable role in flood management through logical utilization of weir gates and dam reservoirs. In this direction, different systems have been innovated in different countries of the world, but lack of equipment and tools and also high cost of installation are the limiting factors in Iran (3). Recently the flood return period has decreased in Neka Basin of northern Iran, so a suitable method of decreasing the flood-damages is required, by flood zonation. The overall aim of this study is to find out the efficiency of GIS to create the main inputs to simulate a comprehensive hydrological model. So that the main requirement of a hydrological model is the description of flow channel characteristics and land surface as input to watershed model. The flood zonation is in fact the development and perfection of the applied engineering hydrology and its aim is to obtain real time data of rainfall and river flow by short wave, radio and satellite network and using them in rainfall-runoff models to forecast and also zonation of floods in consecutive time and space intervals (4). The quality of flood forecasting systems depends mainly on the quality and the amount of basic collected data about hydrology and the hydrological yield of the corresponding watershed (5).

The study area with 5.5 km long of Neka watershed is located between Chaman and Bezminabad villages. According to UTM the coordinates of the study area is as flow;

XMin=699000
XMax =704000
YMin=4058000
YMax=4062000

Which is located in north latitude, in northern Iran?
The mean annual rainfall of the region is estimated to 800 mm by Isohyets method and 816.2 mm by Theissen method. The main part of the basin has very humid climate and the northern part of the basin with lower elevations has humid climate by De marten method. Based on Emberge method, the middle and northern part has cold humid and southern part of the basin with higher elevations has mountain climates.

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

Figures 1 and 2 show the hydrological elements of water level profiles during different return period.

Figure 1. Long profile of water level for the return period of 2, 5 and 10 year.

Figure 2. Long profile of water level for the return period of 25, 50 and 100 year.

The amounts obtained in (I) condition for events 2, 3 and 4 are shown in Figures 3, 4 and 5 as simulated and observed hydrographs in hydrometry station.

Figure 3. Flood Zonation for return period of 5 year

Figure 4. Flood Zonation for return period of 50 year

Figure 5. Flood Zonation for return period of 200 year

The gained results of this study can be concluded that as most Iranian watersheds are submergible and the water resources plans are in progress, use of flood zonation models and techniques like GIS became more important than before. That is because the method is a combination of management and structures issues which makes the system more efficient and decreases the flood–induced damages. This research which is to investigate the efficiency of using GIS as a preprocessor for a comprehensive flood zonation models and to improve the flood forecasting systems in Neka watershed, is a sensible gate in comparison to other methods on this subject and in northern watersheds of Iran. In this study evaluation of the efficiency and use of GIS in hydrological model system was successful. GIS as a suitable technique prepares the great and accurate files of the watershed and is favorable for locations with ground data available. For example, the rainfall distribution and conversion to readable format by hydrological model system which combines these components in an applied hydrological models.

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