Flood modeling in bandsar using cell model technique

Introduction
Farmers in arid lands construct long levees along contour lines called bandsars adjacent to an ephemeral stream in floodplain areas to harvest floodwater and cultivate crops. The flood water stored in reservoirs behind the bandsars ensures soil moisture through flood harvesting. The use of bandsars contributes to the development of domestic agriculture. Thereby, seasonal and flush floods are prevented from being wasted into the desert. This method is common in the provinces of South Khorasan, Razavi Khorasan, Kerman and some other regions of Iran and as well as neighboring countries of Afghanistan and Pakistan. Bandsars can be directly constructed across the ephemeral streams or at their flood plains in a short distance. The purpose of this study is to model the flow in dry rivers and bandsars based on MODCEL algorithm to route the incoming flood and assess the amount of stored water in this traditional structure to evaluate its role in flood alleviation. Flow modeling is performed in two modes of controlled and uncontrolled inflow to bandsar and the importance of controlling the inflow is investigated. The program code is written in MATLAB software.
Methodology
The structure of the cell model (MODCEL) is based on the concept of flow cells, inwhich the study area is divided into a set of different cells that the surface or channel currents between them are expressed via hydraulic laws. This arrangement forms a quasi-two-dimensional hydrologic-hydraulic model, although all relationships are written in one dimension. The cellular model is based on the laws of conservation of mass, energy and momentum. Model cell types include river/ canal, normal surface, and reservoir cells. River / canal cells are used to model flow in open channels. Superficial cells are used for natural surfaces where runoff occurs. These cells are usually larger than the river cells. The reservoir cell is used to simulate a temporary pond or reservoir and is represented by an area-volume-height curve. The hydraulic relationship for surface flows is based on the St. Venant dynamic equations. The output flow from the reservoirs is calculated from the classical formula of wide-crested weirs. Cells are storage elements where the principle of mass conservation is applied to them. Therefore, at any time increment, the amount of water stored in a cell is related to the inflow-outflow rates from the adjacent cells plus the shares of rainfall and penetration. The volume of water inside each cell is a function of the water level in the cell center and the discharge between two adjacent cells at any given time.
Results and Discussion
The performance of bandsars in Akbarieh village (33°41′N, 59°06′E), in Qaen County of South Khorasan are studied in this research. An ephemeral stream is bifurcated into two channels at the apex of an alluvial fan in the region. The bandsars are constructed over the floodplains of the ephemeral stream, in alignment with the contours of the alluvial fan. The maximum height of bandsars are 0.95 m above the bed with a 3. m wide spillway at 0.5m height. A five-year flood hydrograph was developed from the available climatological data using HEC-HMS and GIS tools and was given to the model as an upstream boundary condition. The inflow hydrograph was routed for the system of stream-bandsar using MODCEL model and the time history of water elevation was obtained for each bandsar, separately. The effects of infiltration and evaporation were added to the model as well. Computational results indicated that the volume of floods at stations R1 and R2 located at the downstream of each bifurcation decreased by 91 and 94 percents, respectively, when the bandsars were active. The peak discharge of hydrographs at the same stations were reduced by 81 and 89 percents as well. The bandsars were able to control a total of 92% of the volume of the incoming flood. This amount of flood penetrated into the soil surface layer, making the land suitable for cultivation. The high efficiency of bandsars in the region reduced the volume of floods in the village downstream of the bandsar complex. On the other hand, if the inflow to the earthen bandsars are not controlled, water level in some of the upstream dikes overflow the levees and breaking them, successively. Moreover, the downstream bandsars will not be able to drew water from the stream.

Conclusions
Inflow, outflow and the amount of water storage in traditional levees or bandsars were modeled using the MODCEL algorithm. These structures were able to control 92% of the total volume of an incoming 5-year flood. The bandsars had an effective role in reducing the peak and volume of flood flow and delay its lag time.