Flood simulation on compaction forms by geomorphological indices and hec-ras hydraulic model (Case study of Pardisan town, Qom)

. Alluvial fan flooding occurs as a flood occurring on the surface of an alluvial fan or similar terrestrial shape that originates from the source at the peak and is characterized by high velocity flows, active erosion processes, sediment transport and unpredictable flow paths, any point in the Active alluvial fan may be at risk of flooding, and flooding changes over time.

Lack of attention to the geographical features of an area in relation to urban development causes instability of landscapes and geographical phenomena. Floods are one of the Risks that have challenged Pardisan town. Floods cause a lot of damage to the area every year. The location of this area of Qom city in the area of floods and its acceptance, as well as its rapid development, requires comprehensive studies in this place. There is access from the slope, air conditioning, access to transportation roads as well as smooth space in urban development, so that this area can be studied logically and in principle. The purpose of this study is to answer the question whether the development of Qom city in the study of geographical features has been done or not? To answer this question, it is necessary to conduct field evaluation and review patterns and models.

Methodology

The study area is located in the area of Qamroud river and in longitude 50°41'14" west and50°54'17" east and latitude34°18'50"south and 34°34'15" north. The desired area is 222 hectares. The general slope of the region is south to north, where the southern part is mainly composed of old alluvial fan deposits and the northern part is composed of new deposits. Field and satellite images, reports, land use maps and topography of the regional water organization and natural resources of Qom province are among the important sources used in this research. Due to the size of the area, we divided the area into 15 basins. In this research, two qualitative and quantitative methods have been used. In the qualitative method, geomorphological indices of flood risk zones were determined and in the quantitative method, using numerical data in hec-Ras software environment, a 100-year flood risk map was drawn and finally the results of both methods overlapped and a final map was presented.

Results and Discussion

The term active refers to areas of the alluvial fan where deposition, erosion, and instability of flow paths are possible. If floods and sedimentation have occurred areas of the alluvial fan over the last 100 years, such areas can clearly be considered active.

Such conclusions can be drawn from historical data, photographs, aerial photographs taken at different times, and engineering and morphological data. To study the flood zones, we first need to analyze the current situation in the region. To do this, using hazardous images, vegetation maps, waterway network maps and field visits, hazard zone analysis was performed on alluvial fans. Taking into account the desired indicators, the initial flood risk area in the area of Pardisan town was drawn. The indicators presented can give us an overview of the current situation, which is basically accompanied by flaws and shortcomings. We need mathematical modeling for better and more accurate representation.

Before entering the flood modeling, it is necessary to prepare hydrometric data from the study area. For this purpose, we will first extract this data. In this study, due to the lack of a hydrometric station in the study area, a Rational method has been used to calculate the maximum flood flow

Equation(1) Q=1/360 CIA

Q= peak flood discharge in cubic meters per second C = surface runoff coefficient, I = maximum rainfall intensity in millimeters per A = basin area per hectare

To analyze the intensity, amount and duration of floods for flood warning programs, we need a flow diagram. Which is obtained from the following relations;

Equation(2) Qp=0.208A/Tp

Equation(3) tp=D/2+ti

E D is the required continuity for rainfall (hours), t is the latency of the basin (hours), A is the area of the basin (square kilometers), Qp is the peak flow rate of the unit hydrograph in cubic meters per second for one millimeter of runoff, and tp is the peak time.

Equation(4) D=0.133Tc

Equation(5) ti=Tc/1.66

Tc is the precipitation concentration time which is used in SCS method with delay method using the following formula.

Equation(6) Tc=0.000142L^0.8 〖(25400/CN-228.6)〗^0.7 S^(-0.5)

Where L is the length of the canal in meters, S is the slope of the main canal in percent. The curve number (CN) is determined by the soil characteristics, the type of land use and the previous soil moisture conditions. According to the diagram, 15p, 14p, 9p, 7p, 9p, 2p basins introduce the largest volume of flood into the alluvial fans.

After obtaining hydrological data from the existing relationships, we proceed to simulate floods from the study area. After creating the geometric model, a one-dimensional model was created for all the main channels. For this purpose, riverbeds, shorelines, flow path determination, cross sections and surfaces resulting from the intersection of cross sections were extracted. After constructing the geometric model of the main channel, it should be combined with the geometric model of the entire study area (floodplain) and an integrated model suitable for hydraulic modeling should be obtained. In creating a computational network, it is necessary to enter the desired cell dimensions.