Evaluation of watershed management measures on flooding using HEC-HMS model in Ramian watershed

Before the formation of significant runoff production, floods could be controlled with less cost and problems. For this reason, watershed measures are implemented to control the flood in a place where precipitation occurred. Watershed management measures are taken to protect water and soil by creating vegetation or constructing check-dams. Given the importance of watershed management projects in managing natural resources and due to the high cost involved in implementing these projects, ensuring of their efficiency is an important part of water and soil conservation measures. In spite of the increasing development of watershed management measures, inadequate study has been done to evaluate their efficiency. The evaluation is generally quantitative or qualitative or combination of them. In this research, a quantitative evaluation is done and the status of the watershed is compared in two periods of after and before constructing watershed management measures. Ramian watershed is a sub-catchment of Voshmgir dam basin in Golestan province, Iran. In this watershed, because of flood dangers, local Watershed Management Directorate has carried out by biological (including forestry, planting pile, forage extension, seedling) and mechanical measures (including gabion), since 1993. Considering the necessity of achieving proper results from implementation of these projects, in this research the performance of watershed management measures has been evaluated. The efficiency of watershed management measures can be quantifying by selecting proper indicator which is appropriate to the type of project, type of assessment, required accuracy and available data. Watershed management measures have been implemented in different parts of the Ramian watershed; therefore they should be evaluated with a determined method to achieve acceptable results. In this research, the effect of watershed management measures is evaluated using flood indices (peak flow and flow volume). The research data were collected by: field survey, literature review, locating and checking the specifications of the check-dams in the main stream of each sub-basin, and mapping the cross-section of the routing reaches. Then, flow simulation was performed using HEC-HMS. To evaluate the effect of biological and mechanical measures, curve number and time delay parameters were used. In order to estimate the curve number, land use map was prepared in pre and post period of biological measures. By comparing land use maps in these two periods using kappa index, it is shown that the amount of land use changes in the Ramian watershed is high. This is due to the fact that in addition to biological measures, other watershed measures (including protection and exclusion and the displacement of villagers by the Department of Natural Resources and the abandonment of agricultural lands, and as a result of changing their vegetation), also affected the land use status of the catchment area. Therefore, the impact of biological measures on flood reduction has been significant. Changes in the time delay is studied by considering the changes in the length and gradient of each main stream, with and without check dams. As most check dams were not constructed in the main stream, the number of structures in the main streams was not high. The changes in the length and gradient of the river in different sub-basins of the Ramian watershed were 1.2% and 3.4% respectively, which was not so tangible. According to small changes in the slope and the length of the main stream, the amount of increase in the delay time was also small and about one percent. Before simulating rainfall-runoff, the daily discharge was investigated to gain favorable changes. A daily statistical analysis of discharge showed that the maximum of daily discharge and the flow coefficient decreased by 45% and 9%, respectively. Similar to the results of the statistical survey, the results of flow simulation through the HEC-HMS hydrologic model confirmed the effectiveness of watershed measures. So, according to the results, the flood has decreased in different return periods after the watershed management measures. The highest decrease of peak and volume of discharge occurred for 10 and 20 year return period. However, for greater return periods the impact of project on the peak discharge decreased. In addition, the major reduction in flood was influenced by biological measures, and mechanical measures played a minor role in the watershed management. Generally, the impact of the watershed management measures in Ramian watershed on flood discharge is evaluated as positive.