A quantitative model for danger degree evaluation of staged operation of earth dam reservoir in flood season and its application

Based on the natural disaster risk evaluation mode, a quantitative danger degree evaluation model was developed to evaluate the danger degree of earth dam reservoir staged operation in the flood season. A formula for the overtopping risk rate of the earth dam reservoir staged operation was established, with consideration of the joint effect of flood and wind waves in the flood sub-seasons with the Monte Carlo method, and the integrated overtopping risk rate for the whole flood season was obtained via the total probability approach. A composite normalized function was used to transform the dam overtopping risk rate into the danger degree, on a scale of 0-1. Danger degree gradating criteria were divided by four significant characteristic values of the dam overtopping rate, and corresponding guidelines for danger evaluation are explained in detail in this paper. Examples indicated that the dam overtopping danger degree of the Chengbihe Reservoir in China was 0.33-0.57, within the range of moderate danger level, and the flood-limiting water level （FLWL） can be adjusted to 185.00 m for the early and main flood seasons, and 185.00-187.50 m for the late flood season. The proposed quantitative model offers a theoretical basis for determination of the value of the danger degree of an earth dam reservoir under normal operation as well as the optimal scheduling scheme for the reservoir in each stage of the flood season.

Determination of minimum flood flow for regeneration of floodplain forest from inundated forest width-stage curve

Floods are essential for the regeneration and growth of floodplain forests in arid and semiarid regions. However, river flows, and especially flood flows, have decreased greatly with the increase of water diversion from rivers and/or reservoir regulation, resulting in severe deterioration of floodplain ecosystems. Estimation of the flood stage that will inundate the floodplain forest is necessary for the forest＇s restoration or protection. To balance water use for economic purposes and floodplain forest protection, the inundated forest width method is proposed for estimating the minimum flood stage for floodplain forests from the inundated forest width-stage curve. The minimum flood stage is defined as the breakpoint of the inundated forest width-stage curve, and is determined directly or analytically from the curve. For the analytical approach, the problem under consideration is described by a multi-objective optimization model, which can be solved by the ideal point method. Then, the flood flow at the minimum flood stage （minimum flood flow）, which is useful for flow regulation, can be calculated from the stage-discharge curve. In order to protect the forest in a river floodplain in a semiarid area in Xinjiang subject to reservoir regulation upstream, the proposed method was used to determine the minimum flood stage and flow for the forest. Field survey of hydrology, topography, and forest distribution was carried out at typical cross sections in the floodplain. Based on the survey results, minimum flood flows for six typical cross sections were estimated to be between 306 m3/s and 393 m3/s. Their maximum, 393 m3/s, was considered the minimum flood flow for the study river reach. This provides an appropriate flood flow for the protection of floodplain forest and can be used in the regulation of the upstream reservoir.

The Coupling Effect of Flood Discharge and Storm Surge on Extreme Flood Stages: A Case Study in the Pearl River Delta, South China

The low-lying Pearl River Delta in South China is subject to severe flood threats due to watershed floods,sea level rise,and storm surges.It is still unknown to what extent and how far inland storm surges and sea level rise impact the extreme flood stages.This study investigated the coupling effect of flood discharge and storm surge on the extreme flood stages in the Pearl River Delta by using on site observations and simulations generated by the Hydrologic Engineering Center-River Analysis System model.The results show that flood discharges dominated the flood stages in the middle and upper Pearl River Delta,while the storm surges had maximum impact near the river mouth.The storm surges and flood stages showed a significant increase after 2002 in the Hengmen waterway.The design flood stages for the post-2002 period were 0.23–0.89 m higher than the pre-2002 ones at Hengmen at the six return periods from 5 to 200 years examined in this study.Their difference declined toward the upper waterway and reduced to zero about 23 km away from the Hengmen outlet.The coincidence of extreme flood discharges and storm surges further escalates the extreme flood stages in the lower 30 km of estuarine waterways.Our results quantify the severe threats due to sea level rise and intensified storm surges in the lower Pearl River Delta,and are significant for urban planning and designing and managing flood control facilities in the Pearl River Delta and in other coastal fluvial deltas.

Changes of Water stage in the middle Yangtze River influenced by human activities in the past 70 years

Water stages play a critical role on flood control,water supply,navigation,and ecology in rivers.Investigation of water stages provides better understanding of riverbed evolution processes and river management.Based on the hydrological observation in past 70 years,the changes of low-flow and flood stages were investigated by a combination of Mann-Kendall test,moving t-test,and wavelet analysis.1)In accordance with the location,the middle Yangtze River was divided into upper reach,middle reach,and lower reach.Water stages in the upper reach show a decreasing trend,while that in the middle reach present an increasing trend,and the lower reach are mainly dominated by natural evolution.2)The mutation year of water stages in the upper reach was around 1985,indicating that the Gezhouba Dam facilitated the reduction of water stages.The trend mutation in the middle reach was in 1969,which was consistent with the implementation of Jingjiang Cutoff.3)Human activities aggravated the change of water stages,leading the primary period of water stage time series to exceed 20 years.4)In the upper reach,the reductions of water stages were attributed to the riverbed erosion induced by human activities.While in the middle reach,the recent falling effects of riverbed erosion can hardly offset the rising effects of the channel resistance on water stages.5)In the future,the increasing trend in the middle reach may be arrested due to the riverbed erosion induced by the Three Gorges Dam.Long-term observation of the flood stage must be conducted in the middle Yangtze River.