Determination of Leakage Coefficient in Seasonal Rivers Case Study: Shiraz Khoshk River Catchment

Rivers are always one of the important natural resources in meeting the needs and development of human society. Living on the marginal of rivers are associated with risks and problems such as flooding and displacement damages. Floods are an important source of water in arid regions, and alluvial aquifers are recharged by infiltration of seasonal stream flow of rivers. there are many factors affecting floods over the watershed, and it is almost impossible to control and measure all these factors. Therefore, in order to predict floods, the conditions and characteristics must be simulated using hydrological models. River bed usually do not have a permanent flow in dry areas, therefore the flooding water from upstream to lower reaches in the waterway bed is faced with losses due to infiltration and filling of depressions. This situation is more complicated in dry areas, because it is not possible to accurately estimate the amounts of losses in these intervals, and therefore, usually only if there are hydrometric stations on the upstream and downstream, the amounts of losses are calculated in the interval between them. The greatest flood losses, especially decreasing of the peak discharge and its volume, occur when the flood is transferred from the waterways with alluvial beds to the downstream. The Khoshk river that passes through the middle part of Shiraz city has such conditions that flooding every year and remains a lot of damage. Therefore, due to the importance of flood forecasting to reduce downstream risks, it has been selected in this research.This river, which is located in Fars province, is a seasonal river that originate from the west to northwest heights of Shiraz city and after passing through the middle of the city, flows into Maharlu Lake. This river is a combination of two eastern and western branches, the eastern branch was named Nahr-e Aazam and the western one was named Chenarsukhteh. The Khoshk river, as one of the sub basins of the Maharlu Lake, has an area of about 900 square kilometers. Two branches inter shiraz city independently, so that, Nahr-e Aazam branch is 11 km long and 0.00525 percent slope and Chenarsukhteh branch is 13 km long and 0.0153 percent slope in Shiraz city, and then they connect to each other and form the Khoshk river. The Khoshk river passes through the city of Shiraz with a distance of 33.5 km and a slope of 0.00556 percent, and then inter to the Maharlu lake. The course of this river inside the city of Shiraz is in the form of an independent canal and all its banks are separated by a wall. MKE-11, which is a one-dimensional mathematical modeling software, is used to simulate flood routing in such rivers. Flood routing has done hydraulically by solving the Saint-Venant equations in this model. In addition, the solution of Saint-Venant's equations is done by using the finite difference method through the 6-point Abbott grid and solving the continuity and motion equations simultaneously by using the dynamic wave method. This model is adjusted using Manning's coefficient. In the hydrodynamic module, first the Saint-Venant equations were written using the finite difference scheme, and then the equations were analyzed using a grid of points in different places and times. In order to simulate the flow in the Khoshk river, the geometric information of 470 cross sections was used. Manning's coefficient for the branches of Nahr-e Aazam, Chenarsukhteh, and Khoshk river are estimated at 0.024, 0.048, and 0.035, respectively. The accuracy of the values was determined by calibration. Model calibration has been done using Manning's coefficient and observed flood gauge curves with a return period of 100 years, based on the comparison of simulated gauge curves. The measurement curves were compared with the Root Mean Square Error (RMSE). This model has also been recalibrated for leakage coefficient in Nahr-e Aazam, Chenarsukhteh and Iqbal Abad stations. The best value that matched the field characteristics was 10-5 (l/s). The recalibration of the leakage coefficient led to the improvement of the results and the model is accurate enough to simulate the seasonal river. In this research, the flood routing in the Shiraz Khoshk river was carried out and the transfer loss was calculated. Recalibration of the model for the leakage coefficient led to an increase in the agreement between the simulated curves of the gauge and the observed curves. The leakage coefficient is equal to 10-5 (l/s), which was the most consistent. The high slope of the Chenarsukhteh branch compared to the Nahr-e Aazam branch has led to a significant decrease in volume loss in this river branch. Increasing the return period leads to an increase in the volume of transmission loss. It should be noted that the volume of transmission drops in the return period of 25, 50 and 100 years is more than one million cubic meters, which can be a suitable source for feeding the underground water of the region. The percentage of penetration increases with the reduction of the return period; In such a way that in the return period of 5 years 20 percent of the volume of flood penetrates into the Khoshk river.