عنوان مقاله [English]
Flood plain is a key ecological component in arid and semi-arid areas, with soil and water flowing as the most important pillar of agriculture as well as the habitat of humans.
However, what always threatens these ecologically sensitive areas is the occasional riot flood and irreparable financial and psychological damage. (Enzel et al,1993: 2289).
Undoubtedly, the occurrence of floods with rainfall intensity and frequency of storms and hydrological characteristics of the basin on the one hand, and anthropogenic activities and climate change caused by it on the other hand, has a direct relationship. It is also anticipated that suddenly occurred floods will increase dramatically in the near future due to global warming.(Kleinen and Petschel-Held, 2007:286. deMoel et al, 2009:291).
Therefore, in order to understand this natural phenomenon, it is necessary to have reliable hydrodynamic models that can correctly estimate the flood occurrence to reduce its risk and hazard.
Recently, advances in computational resources, data collection and the development of multiple numerical codes have increased the use of hydrodynamic modeling techniques to simulate flood plains.
Numerous studies have examined and modelled the flood flows in Ligvan River with different viewpoints so far. However, in these studies, one dimensional modeling in which flood plain has not been considered has been used. These models do not provide details of flood spreading and flood changes between the river channel and the plains floods. This paper presents a quasi-two-dimensional modeling in Mike 11 software used for simulating the flood flow in the Lighvan River. In quasi-two dimensional simulation, main river and flood plain modeling are conducted separately, and their connection are connecting channels. In this approach, the flood plain around the main river are introduced to the model as two virtual rivers in both sides of the main river. In addition, the main and the virtual river (which represents the flood plain) are connected to each other by connecting channels.
As long as the level of the water level in the main river has not reached the upstream level of the channel, discharge in virtual river is zero. As soon as the water level in the main river reaches the upstream level of the main channel, this part of flood enters the virtual river.
In order to simulate a quasi-two-dimensional model for Lighvan basin, the flow rate of 200 m3/s (maximum flood occurred in this river) is considered as the boundary condition of the upstream, and Q-H is employed for the downstream boundary, which is calculated by the software by using Manning’s relationship and cross-sectional information of desired location. In order to get better results, the roughness coefficient in the main part and flood plain were provided by using Chow table and field visits which was eventually introduced to the software. This research approved the potential of a quasi-two-dimensional hydrodynamic approach in comparison to one-dimensional and two-dimensional methods. Results indicated that flood dynamics in the flood plain plays an important role in flood simulation. Based on similar results, the amount of water flow in main channel of the river and at the upstream point is 112 m3/s, and the amount of water flow transmitted by the connections is equivalent to 87 m3/s showed a well-established continuity in these connective structures. Studies also show that a quasi-two-dimensional model is working similar to two-dimensional models in flood dispersion and also in inter and discharge flow from river channel, and offers acceptable accuracy in the calculation, while one-dimensional modeling with same cross sections does not show flood peak changes and does not take into account the decrease and increase in the volume of flood, occurred due to the dissemination and return flow to the river, and only shows the rerouting along the river course.
Other results of quasi-two-dimensional simulation showed that, since the Ligvan River is in high mountains and has a relatively high slope, its floods have less concentration time (one and half hours after the simulation) in the one hand, and relatively have a high velocity on the other hand which increases the ability of flood destruction. Also, the results depicted that the highest risk of flood is in 6th and 7th sections and the lowest one is in first and second sections which are consistent with the realities.
The zoning maps of the velocity, depth and risk of floods show that vegetation in different areas of the flood plain affects the characteristics of flood propagation. The results show that with decreasing vegetation, the depth of flood waves decreases while the discharge current increases. Increasing the high risk zone around the main canal is associated with reduced vegetation cover.
In addition, many studies have shown that compared to two-dimensional models, the MIKE 11 quasi-2D is able to provide accurate information on streaming; In general, it works appropriately to estimate flood spreading area specially for flooding up to 325 cubic meters per second. But in particular, some hydrological processes, such as loss of evapotranspiration and interaction between river water and groundwater are ignored.