The Influence Of Water-related Engineering On Flood-control In Mountainous Watershed On Mike21FM

In recent years, with the economic development of mountain area and extreme climate (such as local heavy rain) changes, extreme hydrological events of river system and frequency of disasters constantly increase. The torrential rain of Beijing "7·21" happened in 2012, triggered a major natural disaster. The severe-hit area such as Fangshan outbreaks flood disasters in many rivers and debris flow in local area, resulting in flood control facilities severely damaged, threatening downstream flood safety; roads, bridges and infrastructure seriously damaged, traffic block; destroyed farmland, houses, cars, causing loss of life and property. The current flood-control impact study mainly focused on the informative and important water-related engineering of large rivers, although the flood-control problems of mountainous watershed are increasingly more and more prominent, water-related engineering research is less for lack of data information. The paper is selected one of the worst-hit watershed in the Beijing "7·21" rainstorm-the Hongluogu watershed in Fangshan District for the study, Using a two-dimensional hydrodynamic and mathematical model based on unstructured grids--Mike21FM, though collecting hydrological climate, planning data and field investigation, to study on flood control effect of water-related engineering in mountainous watershed. The main contents and conclusions are as follows:1. Based on the damage survey of the Hongluogu watershed in Beijing after the "721" rainstorm, it is concluded that the heavy precipitation intensity, low river flood control standard, narrow channel beam, insufficient space on the flood discharge caused by water-related engineering and too much loose debris need to transport are the important reasons resulting in seriously damaged by flood in the Hongluogu watershed.2. It is selected the main channel of the Hongluogu watershed as the model calculation area, and 10a design flood standard and the "721 "measured peak flow as the control boundary condition, establishing Mike21FM plane 2D water mathematical model of the channel, then generalized the river water-related engineering in a reasonable manner. It is carried out the model of calibration and verification, and simulated flood process in 10 years and 20 years. Due to the lack of measured data of research reach, model validation used with 10 years flood conditions based on the HEC-RAS software. Though comparing water level, the results meet the requirements of the water line similar, therefore, eddy viscosity of plane 2D water mathematical model is 0.28, manning number of main and side beach of the model are respectively taken 33.3 m1/3/s and 22.2 m1/3/s.3. Using Mike21FM model simulates different water level, velocity changes before and after construction of river-crossing engineering, cascade and near-river buildings. Though the analysis of water-blocking effect about water-related engineering and present flow capacity in the channel, the results showed that:(1) The Hongluogu watershed status quo cannot meet the standards of mountain river in ten years, and under the condition of the"7-21 "flood river, water-related buildings, bridges, fields and so on are badly damaged. River-crossing engineering in the mountainous area has a larger effect on flood-control. The water-blocking effect of the large crossing bridge is the most obvious, because the backwater of large crossing bridge is too big, which indicates that the location of the bridges is severely unreasonable, in the conditions of the original water section cannot meet the requirement of the flood, the construction of the large crossing bridge can cause flood depth and flood area further increase. The water-blocking effect of medium and small bridges are influenced by piers’size and structure, Piers forms, distance between upstream and downstream of the bridges and other factors; at the same time, backwater height and backwater range are increased with increasing skew angle, which not only affects the flood safety, but also be a direct cause of the bridge damage. The water-blocking effect of submersible bridges is relatively obvious compared with medium and small bridges, and increased with increasing its design heights.(2)design three different kinds of cascades’sizes and forms:①current condition of six drop-sills, height of 1.0 m (scheme 1);②six the design height of 0.5 m drop-sills (scheme 2); ③three the design height of 1.0 m drop-sills (scheme 3).Though simulating and analyzing the change of water level, velocity and range of flood when the cascades built in the early and after silting flat. The results show that in the upstream of cascades, the water level significantly rises even overflows and flow velocity decreases, and backwater values and backwater range are increased with the increasing of the height of drop-sills; while in the downstream of cascades, water level drops obvious and velocity increased significantly.No matter it is built in the early or after silting flat, present cascades’forms may cause the greatest impact on hydraulic parameters, followed by scheme 3, and scheme 2 is relatively small; Under the same scheme, backwater of cascades built in the early can affect only a certain area, but water level and overflow range of cascades after silting flat rise significantly. So the status quo of cascades’forms don’t meet the requirements of flood control.(3) near-river buildings on flood control analysis show that when the flood standard is 10 years, huangshan buildings group and huangshan permanent safety resettlement buildings crowd flood discharge in river space severely, resulting in insufficient cross-sectional flow capacity and increasing the main river channel flood discharge pressure, in this condition, overflow on both sides is serious. Shuanma village buildings has a little impact on river flood, but there is no vent channel when river overflow floods into the village, so it is should be constructed blocked measures such as blocking dam in overflow points to protect the village.4. Though applying mathematical model to simulate the process of design flood in 10 years and in 20 years, we obtain the hydraulic elements of different location in the channel, including water level, flow velocity, flow direction, flow, etc. By analyzing water line, flow velocity and flow capacity along the channel in different design flood conditions, it may provide the train of thought to re-optimize the layout of the main water-related engineering; by analyzing the different regional flood situation, it may provide the basis for bank protection and embankment design; by controlling the river water cross section optimization analysis, it may provide the basis for broaden and deepen design of river channel.