MORPHOLOGICALLY RELATED INTEGRATIVE MANAGEMENT CONCEPT FOR RECONNECTING ABANDONED CHANNELS BASED ON AIRBORNE LiDAR DATA AND HABITAT MODELLING

Most of the large rivers are heavily degraded and lack near‐natural conditions due to high human pressure (agricultural use and settlements) especially on former inundation areas. Hence, it is rarely possible to ‘restore’ predisturbance conditions of rivers and their floodplains. Further, river or floodplain restoration programs are often based on type‐specific reference conditions. Those reference conditions are mainly determined on the basis of historical maps not giving any information of, for example, sediment supply, flood frequency and vegetation cover (density). Especially for improving the ecological status of rivers with abandoned channel features, key habitats for target fish species have to be restored by reconnecting floodplains and their secondary channel system. In addition, because of the necessity of improving the ecological status, there is growing interest in interdisciplinary river restoration techniques. Within the presented article, an integrative concept is derived based on Light Detection and Ranging measurements and numerical modelling with respect to river dynamics (hydrologic and morphological). Further habitat modelling, based on unsteady depth‐averaged two‐dimensional hydrodynamics, is applied with a focus on the mesounit scale. For testing the conceptual model, various river reaches at the Morava River were selected, featuring different morphological characteristics. It was found that the applied management concept allows considering the important issues of river dynamics (morphological/hydrologic) using a flow‐ and flood‐pulse approach for identifying bottlenecks of target species at the Morava River. The reconnection of abandoned channels will result in an increase of hydromorphological heterogeneity and/or woody debris within the study reach. This might be of high relevance for habitat features (e.g. backwater habitats) especially for flow pulses between low flow and mean flow and/or in reaches without abandoned channels between low‐flow and the bankfull stage. Copyright © 2012 John Wiley & Sons, Ltd.     

Discrepancies in Flood Modelling Approaches in Transboundary River Systems: Legacy of the Past or Well-grounded Choices?

Flood modelling in transnational rivers requires efficient cross-border collaboration among the riparian countries. Currently, each country/region usually uses a different hydraulic modelling approach, which may hinder the modelling of the entire river. For the sake of accurate and consistent river modelling there is a necessity for the establishment of a framework that fosters international collaborations. This study investigates the current hydraulic modelling approach across the whole length of the River Meuse, the main course of which crosses three north-western European countries. The numerical models used by French, Belgian, and Dutch agencies and authorities were interconnected by exchanging boundary conditions at the borders. At the central part of the river, the Belgian hydraulic model assumed steady flow conditions, while the rest of the river was modelled in unsteady mode. Results for various flood scenarios revealed a distinctive pattern of water depths at the Belgian-Dutch border. To clarify whether this is a bias induced by the change in modelling approach at the border (steady vs. unsteady), we remodelled a stretch of the river across the Belgian-Dutch border using a consistent unsteady modelling approach. The steady and unsteady approaches led to similar patterns across the border, hence discarding the hypothesis of a bias resulting from a change in the employed model. Instead, the pattern in water depths was attributed to a change in the topography of the Meuse Valley, where there is a transition from a narrow steep corridor with limited water storing capacity in Ardennes massif to wide floodplains in the Dutch lowlands. The associated flood damping for the 100-year discharge is less than 1% in the Ardennes and exceeds 15% in the Dutch lowlands. It can be inferred that the current differences in regional hydraulic modelling approaches for the River Meuse are generally well-grounded and not just a legacy of the past.

     

Ecohydraulic modelling of anabranching rivers

In this paper we provide the first quantitative evidence of the spatial complexity of habitat diversity across the flow regime for locally anabranching channels and their potential increased biodiversity value in comparison to managed single‐thread rivers. Ecohydraulic modelling is used to provide evidence for the potential ecological value of anabranching channels. Hydraulic habitat (biotopes) of an anabranched reach of the River Wear at Wolsingham, UK, is compared with an adjacent artificially straightened single‐thread reach downstream. Two‐dimensional hydraulic modelling was undertaken across the flow regime. Simulated depth and velocity data were used to calculate Froude number index, known to be closely associated with biotope type, allowing biotope maps to be produced for each flow simulation using published Froude number limits. The gross morphology of the anabranched reach appears to be controlling flow hydraulics, creating a complex and diverse biotope distribution at low and intermediate flows. This contrasts markedly with the near uniform biotope pattern modelled for the heavily modified single‐thread reach. As discharge increases the pattern of biotopes altered to reflect a generally higher energy system, interestingly however, a number of low energy biotopes were activated through the anabranched reach as new subchannels became inundated and this process creates valuable refugia for macroinvertebrates and fish, during times of flood. In contrast, these low energy areas were not seen in the straightened single‐thread reach. Model results suggest that anabranched channels have a vital role to play in regulating flood energy on river systems and in creating and maintaining hydraulic habitat diversity.     

Uncertainty Estimation in Flood Inundation Mapping: An Application of Non‐parametric Bootstrapping

Disaster prevention planning is affected in a significant way by a lack of in‐depth understanding of the numerous uncertainties involved with flood delineation and related estimations. Currently, flood inundation extent is represented as a deterministic map without in‐depth consideration of the inherent uncertainties associated with variables such as precipitation, streamflow, topographic representation, modelling parameters and techniques, and geospatial operations. The motivation of this study is to estimate uncertainties in flood inundation mapping based on a non‐parametric bootstrapping method. The uncertainty is addressed through the application of non‐parametric bootstrap sampling to the hydrodynamic modelling software, HEC‐RAS, integrated with Geographic Information System (GIS). This approach was used to simulate different water levels and flow rates corresponding to different return periods from the available database. The study area was the Langat River Basin in Malaysia. The results revealed that the inundated land and infrastructure are subject to a flooding hazard of high‐frequency events and that the flood damage potential is increasing significantly for residential areas and valuable land‐use classes with higher return periods. The proposed methodology, as well as the study outcomes, of this paper could be beneficial to policymakers, water resources managers, insurance companies and other flood‐related stakeholders. Copyright © 2017 John Wiley & Sons, Ltd.     

Estimating Sediment Budget at a River Basin Scale Using a Process-Based Distributed Modelling Approach

The paper presents a process-based distributed modelling approach for estimating sediment budget at a river basin scale with partitions of suspended and bed loads by simulating sediment loads and their interactions. In this approach, a river basin is represented by hillslopes and a network of channels. Hillslopes are divided into an array of homogeneous grid cells for modelling surface runoff and suspended sediments. Channels are defined by incorporating flow hydraulic properties into the respective hillslope grids as sub-grid attributes for modelling both suspended and bed loads. Suspended sediment transport is modelled using one dimensional kinematic wave approximation of Saint-Venant’s principles of conservation of mass and momentum. Transport capacity of runoff or streamflow is used to set the limit of suspended sediment transport rate. Bed load in channels is estimated based on the instantaneous water and hydraulic parameters. Fractional interchange between suspended load and bed load is then back calculated. The performance of the model was evaluated through a case study application in a large river basin in Japan. The model satisfactorily calculated the sediment transport and total sediment budget in the basin. The simulated bed load was found to be reasonable and consistent with the water flow and suspended sediment flux. The results showed the bed load can be expressed as a linear function of the suspended load. The fractions of different sediment loads also exhibit linear relationships with water discharge for the rising and recession limbs of the flood hydrographs. The case study has demonstrated that the process-based distributed modelling approach can efficiently describe the basin-scale sediment budgets with due consideration of the suspended and bed loads and their interactions in the hillslopes and channels.     

Neural networks for fast fluvial flood predictions: Too good to be true?

Accurate models with low computational times are required to predict the consequences of fluvial floods in real-time. Even though detailed hydraulic models can predict flood water levels and corresponding inundation extents with high accuracy, their computational times limit their applicability to being used as a flood early-warning system. Therefore, conceptual models were developed in the literature for many years. These types of models do not attempt to represent the complex dynamic flood generation processes but are based on simplified hydraulic concepts, generally only using a digital elevation model as input. However, a shift in this research field is currently present from conceptual models to data-driven models, and more specifically to neural networks. This paper discusses the benefits and drawbacks of both modelling approaches and speculates which method is most promising to be used as a flood warning system to predict the consequences of fluvial floods in real-time.     

古学水电站厂房导流设计与度汛分析

介绍了四川古学水电站引水式围堰的类型和结构布置,重点论述了在汛期被水流冲毁的围堰局部位置及冲毁的原因,对围堰转角处的流速进行了计算与分析,提出了防冲措施,可供类似工程参考。     

流域洪水模拟

通过对流域水流运动规律的分析，将流域的水流概化为调蓄单元的零维模拟、河道水流的一维模拟、行洪区水流的二维模拟、联系的处理及无资料径流的处理，对各部分的特点，采用相应的数值方法，统一进行求解，即构成了流域洪水模型，最后以淮河中下游段为试验区，进行了洪水还原计算，结果令人满意。     

闽江流域洪水预报模型比较研究

应用新安江模型与垂向混合产流模型对闽江流域干流以上的15个子流域进行了模拟验证,进一步论证了这两种概念性流域水文模型结构的合理性,并得出了如下结论:大闽江流域,新安江模型与垂向混合产流模型都适用,且均能取得良好的效果。由于不同的洪水预报模型考虑产流机理的不同,采用的结构与参数不同,对不同洪水特征的计算结果也有差别,在实际使用中,设置不同结构的洪水预报模型是必要而且是有效的。同时,还对历史洪水根据其涨洪前段的实测流量过程与模型计算流量过程的确定性系数最大作为选择模型的标准,进行模型选择模拟预报。由此还得出另一个结论:在实时洪水预报中,根据不同的预知信息实时选择最佳的模型,能提高洪水预报的精度。     

基于决策树技术的场次洪水拦洪比推理模型研究

为缓解水资源缺乏以及发展区域经济,人们在流域内修建了大量的水利工程,改变了流域的下垫面条件,从而影响了蒸发、入渗、产流、汇流等特性,导致传统的洪水预报模型难以反映流域产汇流规律。文章文通过决策树技术,挖掘上游水库群拦蓄或泄放洪水的规律,建立场次洪水拦洪比推理模型,对洪水预报予以修正,有效提高洪水预报精度。     

2D Finite Element Inundation Modelling in Anabranching Channels with Sparse Data: Examination of Uncertainties

Flood inundation modelling in developing countries is severely limited by the lack of high resolution terrain data and suitable imagery to map flood extents. This study assessed the predictive uncertainty of modelled flood extents generated from TELEMAC2D model using low-cost, sparse input data commonly available in developing countries. We studied a river reach characterised by anabranching channels and river islands in eastern India. In this complex fluvial setting, we analysed computational uncertainty as a function of error in both satellite-derived flood-extent maps using a Generalised Likelihood Uncertainty Estimation (GLUE)-based approach. The model performance was quite sensitive to the uncertainty in the inflow hydrograph, particularly close to the flood peak. Evaluation of the flood inundation probability map, conditioned upon deterministic and probabilistic observed flood extents, reveals that the effect of using probabilistic observed data is only evident for portions of the model domain where the model output is free from consistent bias (over or under prediction) likely created by the imperfect terrain data.     

三门峡水库汛期浑水发电优化调度方式研究

通过对１９８９￣１９９４年三门峡水电站汛期浑水发电试验期间水库水沙资料进行分析，得出水库汛期的优化调度方式为：在汛期发电开始前，应先利用洪水或通过降低库水的办法使水库近坝段冲出０．５￣０．６亿ｍ＾３的调沙库容；在发电运行期再根据机组运行情况来水来沙情况，昼多用底孔泄流，增加底孔出流比例，从而达到过机含沙量和细化过机泥沙粒径的目的。汛期发电水库运用水位可按如下方法控制：汛期发电开始后，库水位按３０５     

Modelling Uncertainty in Flood Studies

Estimates of average annual damage (AAD) are a key central component of the hydrologic, hydraulic, and economic information developed in the evaluation of flood damage reduction plans. AAD or the expected value of annual damage, in dollars or other monetary units, is a probability-weighted average of the economic losses associated with the full range of possible flood magnitudes. Economic benefits are assessed as the reduction in AAD which would result from implementation of a particular plan. Traditional methodologies for performing hydrologic, hydraulic, and economic studies and estimating AAD have recently been expanded to explicitly incorporate analysis of the uncertainties inherent in the data collection and modelling effort. This paper presents a strategy for modelling uncertainties in the estimation of AAD for ungauged urban floodplains.     

A perspective on dryland river ecosystems

The ecosystem concept should be reappraised as a basic model for rivers, with regard for flow as an organizing variable. This would facilitate comparisons between the large rivers of humid climates, where flow regimes are comparatively regular, and those of arid and semi-arid areas, where river regimes are highly variable. Ecosystem processes might be modelled by combining the river continuum and flood pulse concepts, with refinements to accommodate a complex flood pulse (e.g. variations in stage amplitude, timing, duration, rates of rise and fall). Patch boundaries (ecotones) such as the riverine littoral zone warrant close study because they strongly influence the structure and dynamics of the ecosystem. The general model needs a quantitative basis, perhaps focused on the balance of processes involved in the physical transport and biological transformation of carbon. The ultimate test of such a model will be in its capacity to predict the effects of flow regulation. Further development, however, is limited by data. In both research and management monitoring programmes need to be established to provide information and to develop a sustained, comprehensive approach to dryland rivers as ecosystems.     

Floodplain Management in Urban Developing Areas. Part I. Urban Growth Scenarios and Land-Use Controls

Some of the most critical flood problems occur in urban areas where values at risk are higher and damages tend to be heavier. Fast urban developing regions raise very specific problems because of the unsteady situation of these regions in terms of catchment land-use and urban encroachment. A realistic approach to flood management in these situations requires the consideration of urban growth scenarios and the simulation of the corresponding flood conditions. Coastal towns are frequently located in floodplains and are subject to flood hazard. This is the case for many coastal areas in Southern Europe that are still developing fast due to migration of the population to these areas. In such cases, flood management cannot be dissociated from land-use management, and non-structural measures for flood control can play a crucial role. It is important to make an ex-post evaluation of these types of measures in areas where they have been adopted. These issues are addressed in two Portuguese catchments, representative of urban growth and related to flood problems in Portugal and other Southern European regions. The Laje catchment is used for the ex-post evaluation of nonstructural measures, and the Livramento catchment is used for the modelling of urban growth scenarios. Quantitative results and policy recommendations are presented based on these two case-studies. Floodplain management is better done with GIS, especially if it is linked to hydrologic and hydraulic modelling capabilities. The use of GIS for conducting these studies is presented in Part II of this article.     

机械锁键锚杆在三门峡水利枢纽坝前清污栈桥中的应用

三门峡水电站是三门峡水利枢纽工程的重要组成部分，水电站承担着防汛、发电、灌溉等综合利用任务。由于黄河汛期来污量较多，严重影响汛期防洪和发电效能。为此，黄河明珠集团在坝前污物综合治理方案中对污物运输方式进行了探索与研究，坝前清污栈桥就是对污物运输方式的一种探索。坝前清污栈桥施工的技术重点和难点是清污栈桥的承重支撑结构与坝体连接的固定装置：十字机械锁键锚杆。本文着重介绍十字锁键的特性、加工制作和施工流程工艺，以期为类似工程提供一种新方法、新技术。     

防洪调度图在二滩水库调度中的应用

二滩水库是以发电为主的大型水库,因设计时不承担下游的防洪任务,正常高水位以下未留防洪库容,水库在汛期要长期弃水运行,防洪调度比较被动。本文以二滩水库为例,根据不同入库流量编制出二滩水库防洪调度图,依据入库流量大小预留出相应的防洪库容,不但解决了防洪与发电之间的矛盾,使二滩水库获得可观的防洪库容和较大的防洪效益,而且为金沙江下游及长江干流的防洪削峰、错峰创造了条件,还可为汛末蓄水关闸时间的选择提供了可靠的依据。该防洪调度图也是汛期水库防洪、发电和水位控制的实时调度图,应用灵活主动,并可在其他以发电为主(或引水较有规律)的大型水库应用和推广。     

Floodplain Management in Urban Developing Areas. Part II. GIS-Based Flood Analysis and Urban Growth Modelling

In Part I of this article the very dynamic nature of floodplain management was discussed and the need for modelling the urban growth processes and formulating scenarios of urban development was emphasised. In this second part, the use of Geographic Information Systems (GIS) for addressing those problems is presented. GIS have been recognised as a powerful means to integrate and analyse data from various sources in the context of comprehensive floodplain management. Adequate information and prediction capability is vital to evaluate alternative scenarios for flood mitigation policies and to improve decision making processes associated with flood management. A framework for the comprehensive evaluation of flood hazard management policies is also addressed in this article. This comprehensive approach to flood problems is more than an attitude or a philosophical starting point. It makes use of specific technological tools conceived to be used by different actors, some of them being nonexperts in flood analysis. These tools, based on GIS, are very appropriate for a participatory approach to flood policy formulation and floodplain management because they help communicating with the public in a scientifically correct and yet rather simple manner.     

小浪底水利枢纽的初期运行管理实践

小浪底水利枢纽是一座以防洪、防凌、减淤为主，兼顾供水、灌溉和发电的综合性特大型枢纽控制工程，具有规模宏大、技术结构复杂、泥沙问题突出、社会效益显著的特点。为此对初期运行进行了总结、分析。水库调度、调水调沙、库区异重流方面的实践，对小浪底水利枢纽下阶段的运行管理和类似枢纽的运行管理具有一定参考意义。