Downstream Flood Risk Research Articles

Impacts of River Engineering on River Channel Behaviour: Implications for Managing Downstream Flood Risk

Although knowledge of sediment transport has improved over the last 25 years, our understanding of bedload transfer and sediment delivery is still based on a limited set of observations or on models that make assumptions on hydraulic and sediment transport processes. This study utilises repeat lidar survey data of the River Caldew above the City of Carlisle in the UK to investigate the balance of erosion and deposition associated with channel switching from an engineered and managed single thread channel to a naturalising incipient wandering system. Over the 11-year survey period (four bankfull flood events) around 271,000 m3 of sediment were delivered to the river and floodplain and 197,000 m3 eroded suggesting that storage rates of around 7000 m3/annum occurred. The balance of erosion and deposition is influenced by channelisation with very restricted overbank sedimentation and only limited local and transient in-channel bar deposition along the engineered reach (8000 m3 eroded). This contrasts with the activity of the naturalising reach downstream where a developing wandering channel system is acting to store coarse sediment in-stream as large bar complexes and the associated upstream aggrading plane bed reaches and overbank as splay deposits (87,000 m3 stored). Such behavior suggests that naturalisation of channelised systems upstream of flood vulnerable urban areas can have a significant impact on sediment induced flooding downstream. This conclusion must, however, be moderated in the light of the relatively small volumes of material needed to instigate local aggradation in over-capacity urban channels.

Analysis of Flood Storage Area Operations in Huai River Using 1D and 2D River Simulation Models Coupled with Global Optimization Algorithms

This article addresses the issue of flood management using four flood storage areas in the middle section of Huai River in China which protect the important downstream city of Bengbu. The same areas are also used by the local population as residential and agricultural zones. An optimization problem is therefore posed, with two objectives of simultaneously minimizing the downstream flood risk in Bengbu city and the storage areas’ economic damages. The methodology involved development of river flood models using HEC-RAS, with varying complexity, such as 1-dimensional (1D) model with storage areas represented as lumped conceptual reservoirs, and 2-dimensional (2D) models with detailed representation of the terrain, land-use and hydrodynamics in the storage areas. Experiments of coupling these models with global optimization algorithms (NSGA-II, PESA-II and SPEA-II) were performed (using the HEC-RAS Controller), in which the two objective functions were minimized, while using stage differences between the river and the storage areas as decision variables for controlling the opening/closing of the gates at the lateral structures that link the river with the storage areas. The comparative analysis of the results indicate that more refined optimal operational strategies that spread the damages across all storage areas can be obtained only with the detailed flood simulation models, regardless of the optimization algorithm used.

Consequences of dike breaches and dike overflow in a bifurcating river system

Currently, the effect of dike breaches on downstream discharge partitioning and flood risk is not addressed in flood safety assessments. In a bifurcating river system, a dike breach may cause overland flows which can change downstream flood risk and discharge partitioning.This study examines how dike breaches and overflow affect overland flow patterns and discharges of the rivers of the Rhine delta.For extreme discharges, an increase in flood risk along the river branch with the smallest discharge capacity was found, while flood risk along the other river branches was reduced. Therefore, dike breaches and resulting overland flow patterns must be included in flood safety assessments.

Assessing the Impacts of Univariate and Bivariate Flood Frequency Approaches to Flood Risk Accounting for Reservoir Operation

Flood frequency analysis plays a fundamental role in dam planning, reservoir operation, and risk assessment. However, conventional univariate flood frequency analysis carried out by flood peak inflow or volume does not account for the dependence between flood properties. In this paper, we proposed an integrated approach to estimate reservoir risk by combining the copula-based bivariate flood frequency (peak and volume) and reservoir routing. Through investigating the chain reaction of “flood frequency—reservoir operation-flood risk”, this paper demonstrated how to simulate flood hydrographs using different frequency definitions (copula “Or” and “And” scenario), and how these definitions affect flood risks. The approach was applied to the Meishan reservoir in central China. A set of flood hydrographs with 0.01 frequency under copula “Or” and “And” definitions were constructed, respectively. Upstream and downstream flood risks incorporating reservoir operation were calculated for each scenario. Comparisons between flood risks from univariate and bivariate flood frequency analysis showed that bivariate flood frequency analysis produced less diversity in the results, and thus the results are more reliable in risk assessment. More importantly, the peak-volume combinations in a bivariate approach can be adjusted according to certain prediction accuracy, providing a flexible estimation of real-time flood risk under different prediction accuracies and safety requirements.

Restoration of blanket peat moorland delays stormflow from hillslopes and reduces peak discharge

Over the past 15 years there has been a proliferation of projects aiming to restore the structure and function of UK upland blanket mires, primarily by revegetation of bare peat and the blocking of erosion gullies. These restoration measures have potential to alter stormflow responses and contribute to Natural Flood Management, but their impacts on storm hydrographs are poorly quantified. This paper reports a before-after-control-intervention (BACI) study from three experimental headwater micro-catchments in the South Pennines (UK) representing the first rigorous experimental assessment of the impact of blanket peat restoration on catchment runoff. We evaluate the hydrological impacts of two standard restoration interventions; revegetation of bare peat, and revegetation of bare peat with additional gully blocking. Following revegetation there was a significant decrease in depth to water table and an increase in the prevalence of hillslope overland flow production. There were no significant changes in storm runoff coefficient following either restoration treatment. Storm hydrographs following revegetation had significantly longer lag times (106% increase relative to the control), reduced peak flows (27% decrease relative to the control), and attenuated hydrograph shapes. With the addition of gully blocking the effect is almost doubled. Lag times increased by a further 94% and peak flows reduced by an additional 24% relative to the control. We argue that the primary process controlling the observed changes in storm hydrograph behaviour is retardation of overland stormflow due to increased surface roughness. The significant changes to lag times and peak flow provide evidence that the restoration of degraded headwater peatlands can contribute to Natural Flood Management and the reduction of downstream flood risk, subject to wider catchment scale effects and sub-catchment storm hydrograph synchronicity.

Real-Time Flood Control by Tree-Based Model Predictive Control Including Forecast Uncertainty: A Case Study Reservoir in Turkey

Optimal control of reservoirs is a challenging task due to conflicting objectives, complex system structure, and uncertainties in the system. Real time control decisions suffer from streamflow forecast uncertainty. This study aims to use Probabilistic Streamflow Forecasts (PSFs) having a lead-time up to 48 h as input for the recurrent reservoir operation problem. A related technique for decision making is multi-stage stochastic optimization using scenario trees, referred to as Tree-based Model Predictive Control (TB-MPC). Deterministic Streamflow Forecasts (DSFs) are provided by applying random perturbations on perfect data. PSFs are synthetically generated from DSFs by a new approach which explicitly presents dynamic uncertainty evolution. We assessed different variables in the generation of stochasticity and compared the results using different scenarios. The developed real-time hourly flood control was applied to a test case which had limited reservoir storage and restricted downstream condition. According to hindcasting closed-loop experiment results, TB-MPC outperforms the deterministic counterpart in terms of decreased downstream flood risk according to different independent forecast scenarios. TB-MPC was also tested considering different number of tree branches, forecast horizons, and different inflow conditions. We conclude that using synthetic PSFs in TB-MPC can provide more robust solutions against forecast uncertainty by resolution of uncertainty in trees.

The land morphology approach to flood risk mapping: An application to Portugal

In the last decades, the increasing vulnerability of floodplains is linked to societal changes such as population density growth, land use changes, water use patterns, among other factors. Land morphology directly influences surface water flow, transport of sediments, soil genesis, local climate and vegetation distribution. Therefore, the land morphology, the land used and management directly influences flood risks genesis. However, attention is not always given to the underlying geomorphological and ecological processes that influence the dynamic of rivers and their floodplains.Floodplains are considered a part of a larger system called Wet System (WS). The WS includes permanent and temporary streams, water bodies, wetlands and valley bottoms. Valley bottom is a broad concept which comprehends not only floodplains but also flat and concave areas, contiguous to streams, in which slope is less than 5%. This will be addressed through a consistent method based on a land morphology approach that classifies landforms according to their hydrological position in the watershed. This method is based on flat areas (slopes less than 5%), surface curvature and hydrological features.The comparison between WS and flood risk data from the Portuguese Environmental Agency for the main rivers of mainland Portugal showed that in downstream areas of watersheds, valley bottoms are coincident with floodplains modelled by hydrological methods. Mapping WS has a particular interest in analysing river ecosystems position and function in the landscape, from upstream to downstream areas in the watershed. This morphological approach is less demanding data and time-consuming than hydrological methods and can be used as the preliminary delimitation of floodplains and potential flood risk areas in situations where there is no hydrological data available.The results were also compared with the land use/cover map at a national level and detailed in Trancão river basin, located in Lisbon metropolitan area, an urbanized basin that suffered heavy flooding in the last decades. This study also contributes to a better understanding of the basin morphology at a local-scale and the effects of soil sealing in downstream flood risks.This work will contribute to the understanding of the morphology, ecology and land use of watersheds that could be used to reduce runoff and downstream flood risk. This can be accomplished by using natural water retention and infiltration methods or higher-level based planning instead of a reaction to local decisions on flood hazards. This morphological approach to map landforms, including wet system, is a valuable tool to assist policy makers and planners in flood risk and land use management, floodplain restoration, agricultural land management practices, and location of human activities according to ecological suitability.

Realising the potential of natural water retention measures in catchment flood management: trade‐offs and matching interests

Natural water retention measures (NWRM) are a multifunctional form of green infrastructure that can play an important role in catchment‐scale flood risk management. While green infrastructure based on natural processes is increasingly recognised as being complementary to traditional flood control strategies based on grey infrastructure in urban areas, there are a number of outstanding challenges with their widespread uptake. At a catchment scale, it is widely accepted that NWRM in upstream areas based on the concept of ‘keeping the rain where it falls’ can help reduce the risk of downstream flooding by enhancing or restoring natural hydrological processes including interception, evapotranspiration, infiltration, and ponding. However, both the magnitude of flood risk reduction and the institutional structures needed for widespread uptake of NWRM are inadequately understood. Implementing NWRM can involve trade‐offs, especially in agricultural areas. Measures based on drainage management and short rotation forestry may help ‘keep the rain where it falls’ but can result in foregone farm income. To identify situations where the implementation of NWRM may be warranted, an improved understanding of the likely reductions in downstream urban flood risk, the required institutional structures for risk management and transfer, and mutually acceptable farm compensation schemes are all needed.

Urbanização e Impactos no Ciclo Hidrológico na Bacia do Mineirinho

Este trabalho teve como objetivo comparar cenários de ocupação urbana e seus efeitos no ciclo hidrológico da Bacia do Mineirinho. Buscou-se avaliar os riscos de enchentes causados pelo aumento das áreas de impermeabilização que tem ocorrido com a implantação do novo Campus da USP, São Carlos, SP. Foram estabelecidos quatro cenários de ocupação urbana: cenário pré-urbanização (1972); cenário 2000, cenário 2025 com Plano Diretor (CPD); e cenário 2025 sem Plano Diretor (SPD). Para comparação dos diferentes cenários realizaram-se simulações hidrológicas com modelo hidrológico IPH II. Pode-se observar que no cenário 2025 SPD a vazão máxima aumentou 388,0% quando comparada ao cenário 1972 e 319,4% quando comparada ao cenário 2000. Entre os cenários 2025 CPD e 2025 SPD há diminuição de 22,3% na vazão máxima e aumento no tempo de pico é de 50 minutos. Mesmo com aplicação de diretrizes do PD os riscos de inundações continuam altos.

An advanced method for flood risk analysis in river deltas, applied to societal flood fatality risk in the Netherlands

Abstract. This paper discusses a new method for flood risk assessment in river deltas. Flood risk analysis of river deltas is complex, because both storm surges and river discharges may cause flooding and the effect of upstream breaches on downstream water levels and flood risk must be taken into account. This paper presents a Monte Carlo-based flood risk analysis framework for policy making, which considers both storm surges and river flood waves and includes effects from hydrodynamic interaction on flood risk. It was applied to analyse societal flood fatality risk in the Rhine–Meuse delta.

Importance sampling for efficient modelling of hydraulic loads in the Rhine–Meuse delta

During flood events, breaching of flood defences along a river system can have a significant reducing effect on downstream water levels and flood risks. This paper presents a Monte Carlo based flood risk framework for policy decision making, which takes this retention effect into account. The framework is developed to estimate societal flood risk in terms of potential numbers of fatalities and associated probabilities. It is tested on the Rhine–Meuse delta system in the Netherlands, where floods can be caused by high flows in the Rhine and Meuse rivers and/or high sea water levels in the North Sea. Importance sampling is applied in the Monte Carlo procedure to increase computational efficiency of the flood risk computations. This paper focuses on the development and testing of efficient importance sampling strategies for the framework. The development of an efficient importance sampling strategy for river deltas is more challenging than for non-tidal rivers where only discharges are relevant, because the relative influence of river discharge and sea water level on flood levels differs from location to location. As a consequence, sampling methods that are efficient and accurate for one location may be inefficient for other locations or, worse, may introduce errors in computed design water levels. Nevertheless, in the case study described in this paper the required simulation time was reduced by a factor 100 after the introduction of an efficient importance sampling method in the Monte Carlo framework, while at the same time the accuracy of the Monte Carlo estimates were improved.

Flood exposure and settlement expansion since pre-industrial times in 1850 until 2011 in north Bavaria, Germany

During the past century, land use change, such as settlement development and intensification of agriculture, has led to a decreased capacity of natural floodplains to provide flood regulation across large areas of Europe. The expansion of built-up areas in flood zones has reduced retention areas, potentially leading to an increase of downstream flood risk and an increased demand for flood regulation measures. The goal of this paper was to analyse the historic development of settlements from the 1850s until 2011 in relation to flood exposure in the upper reaches of the river Main, Germany. The settlement area of pre-industrial times was derived from historical land cover maps with the aid of object-based classification. Current settlement areas were extracted from the real estate map of Bavaria. Topographic and flood exposure variables were utilized for the statistical analysis of settlement change. Results showed a strong increase in settlements in all administrative districts of the case study area since the 1850s. The total built-up area within the flooding zone of the investigated section of the Main river increased almost fivefold. Such expansion of settlement into the natural floodplains indicates a high growth in demand for flood regulation. Furthermore, our results suggest that flood exposure affected site selection of settlement in the past much stronger than it does today. Technical flood control (e.g. channels, dams and retention areas) reduced the area of settlement at risk for many towns in the case study region, but for some, it remains remarkably high.

Measurement of flood peak effects as a result of soil and land management, with focus on experimental issues and scale

As a result of several serious flood events which have occurred since 2000, flooding across Europe is now receiving considerable public and media attention. The impact of land use on hydrology and flood response is significantly under-researched, and the links between land use change and flooding are still unclear. This study considers runoff data available from studies of arable in-field land use management options, applied with the aim of reducing diffuse pollution from arable land, in order to investigate whether these treatments also have potential to reduce downstream flooding. Intensive monitoring of 17 hillslope treatment areas produced a record of flood peak data covering different mitigation treatments for runoff which occurred in the winter of 2007–2008. We investigated event total runoff responses to rainfall, peak runoff, and timing of the runoff peaks from replicates of different treatments, in order to assess whether there is a significant difference in flood peak response between different mitigation options which could be used to mitigate downstream flood risk. A mixed-modelling approach was adopted in order to determine whether differences observed in runoff response were significant. The results of this study suggest that changes in land use management using arable in-field mitigation treatments can affect local-scale runoff generation, with differences observed in the size, duration and timing of flood peaks as a result of different management practices, but the study was unable to allow significant treatment effects to be determined. We suggest that further field studies of the effects of changes in land use and land use management need to upscale towards farm and catchment scale experiments which consider high quality before-and-after data over longer temporal timescales. This type of data collection is essential in order to allow appropriate land use management decisions to be made.

The potential for agricultural land use change to reduce flood risk in a large watershed

Effects of agricultural land management practices on surface runoff are evident at local scales, but evidence for watershed-scale impacts is limited. In this study, we used the Soil and Water Assessment Tool model to assess changes in downstream flood risks under different land uses for the large, intensely agricultural, Raccoon River watershed in Iowa. We first developed a baseline model for flood risk based on current land use and typical weather patterns and then simulated the effects of varying levels of increased perennials on the landscape under the same weather patterns. Results suggest that land use changes in the Raccoon River could reduce the likelihood of flood events, decreasing both the number of flood events and the frequency of severe floods. The duration of flood events were not substantially affected by land use change in our assessment. The greatest flood risk reduction was associated with converting all cropland to perennial vegetation, but we found that converting half of the land to perennial vegetation or extended rotations (and leaving the remaining area in cropland) could also have major effects on reducing downstream flooding potential. We discuss the potential costs of adopting the land use change in the watershed to illustrate the scale of subsidies required to induce large-scale conversion to perennially based systems needed for flood risk reduction. Copyright © 2013 John Wiley & Sons, Ltd.

Spatial identification and optimization of upland wetlands in agricultural watersheds

Wetland ecosystems are considered as potential ecological solutions for increasing the capacity of watersheds to store runoff waters upstream, and thereby, decrease risk of downstream flooding. Especially in tile-drained agricultural landscapes, wetlands constructed to intercept these tiles can serve as storage basins for agricultural runoff, leading to both reduction in peak runoff flows and diminished transport of agricultural nutrients. The objective of this study was to develop a watershed-scale methodology for identifying potential sites for wetlands in a tile-drained landscape in the Midwestern USA, and for optimizing the spatial distribution of these wetlands for reductions in peak runoff flows. The benefits of this methodology is demonstrated by using it for selecting appropriate wetland restoration and/or creation sites in Eagle Creek Watershed (ECW), located 10 miles northwest of Indianapolis, IN, USA. Results show that a large number of potential sites could be identified (e.g., 2953 sites in ECW), and with a choice of effective wetland design parameters and with spatial optimization of their areas, locations, and drainage areas, it is possible to achieve significant peak flow reductions with fewer sites and smaller wetlands.

Impact of Artificial Reservoir Size and Land Use/Land Cover Patterns on Probable Maximum Precipitation and Flood: Case of Folsom Dam on the American River

The design of the dams usually considers available historical data for analysis of the flood frequency. The limitation of this approach is the potential shift in flood frequency due to physically plausible factors that cannot be foreseen during design. For example, future flood extremes may change, among other factors, due to strong local atmospheric feedbacks from the reservoir and surrounding land use and land cover (LULC). Probable maximum flood (PMF), which is the key design parameter for hydraulic features of a dam, is estimated from probable maximum precipitation (PMP) and the hydrology of the watershed. Given the nonlinearity of the rainfall-runoff process, a key question that needs to be answered is How do reservoir size and/or LULC modify extreme flood patterns, specifically probable maximum flood via climatic modification of PMP? Using the American River Watershed (ARW) as a representative example of an impounded watershed with a large artificial reservoir (i.e., Folsom Dam), this study applied the distributed variable infiltration capacity (VIC) model to simulate the PMF from the atmospheric feedbacks simulated for various LULC scenarios (predam, current scenario, nonirrigation, and reservoir-double). The atmospheric feedbacks were simulated numerically as PMP using the regional atmospheric modeling system (RAMS). The RAMS-generated PMP scenarios were propagated through the VIC model to simulate the PMFs. Comparison of PMF results for predam and current scenario conditions showed that PMF peak flow can decrease by about 105 m3/s, while comparison of current scenario with nonirrigation PMF results showed that irrigation development has increased the PMF by 125 m3/s. On the other hand, the reservoir size had virtually no detectable impact on PMP and consequently on PMF results. Where downstream levee capacity is already underdesigned to handle a dam’s spillway capacity, such as for the case study, such increases indicate a likely impact on downstream flood risk to which any flood management protocol must adapt. The premise that modern dam design and operations should consider an integrated atmospheric-hydrologic modeling approach for estimating proactively potential extreme precipitation variation due to dam-driven LULC change is well-supported by this case study.

The potential of sand dam road crossings

In many African dryland regions, culverts are built under low-volume rural road river crossings. The capacity of culverts is often inadequate to accommodate the peak floods and, over time, sedimentation reduces this capacity further. As a result, culverts are vulnerable to being washed away. Sand dams that incorporate a ford are an alternative to culverts and offer significant benefits. Correctly designed sand dams manage flood flows and are a robust, cost-effective alternative. Among additional benefits, sand dams recharge the aquifer, reduce downstream flood risks and provide a reliable, year-round water supply in water-scarce environments. This paper documents a case study from Kenya where a sand dam acts as both a road crossing and the source for a water pipeline. The paper describes the principles for siting, design and construction of sand dam road crossings, where this approach is appropriate and the barriers and enablers to wider adoption.

The link between land-use management and fluvial flood risk

There is much policy interest in the possible linkages that might exist between land use and downstream fluvial flood risk. On the one hand, this position is sustained by observations from plot- and field-scale studies that suggest land management does affect runoff. On the other, upscaling these effects to show that land-management activities impact upon flood risk at larger catchment scales has proved to be elusive. This review considers the reasons for why this upscaling is problematic. We argue that, rather than it reflecting methodological challenges associated with the difficulties of modelling hydrological processes over very large areas and during extreme runoff events, it reflects the fact that any linkage between land management and flood risk cannot be generalized and taken out of its specific spatial (catchment) and temporal (flood event) context. We use Sayer’s (1992) notion of a ‘chaotic conception’ to describe the belief that there is a simple and general association between land management and downstream flood risk rather than the impacts of land management being spatially and temporally contingent in relation to the particular geographical location, time period and scale being considered. Our argument has important practical consequences because it implies that land-management activities to reduce downstream flood risk will be different to traditional flood-reduction interventions such as levees. The purpose of demonstration projects then needs careful consideration such that conclusions made for one project are not transferred uncritically to other scales of analysis or geographical locations.

Large-scale assessment of flood risk and the effects of mitigation measures along the Elbe River

The downstream effects of flood risk mitigation measures and the necessity to develop flood risk management strategies that are effective on a basin scale call for a flood risk assessment methodology that can be applied at the scale of a large river. We present an example of a rapid flood risk assessment methodology for the Elbe River. A 1D hydraulic routing model is extended by including the effect of planned (regulated and unregulated) and unintended retention (dike breaches) on the peak water levels. We further add an inundation model for dike breaches due to dike overtopping and a macroscale economic approach to assess the flood damage. The flexible approach to model the effects of measures by means of volume storage functions allows for rapid assessment of combinations of retention measures of various proposed dimensions and at multiple locations. The method allows for the comparison of the flood risk at the scale of the main river trajectory, which has not been possible for the Elbe River to date. The model is applied to a series of exemplary flood risk mitigation measures to show the downstream effects and the additive effects of combinations of measures on the flood risk along the river. We further demonstrate the increase in the downstream flood risk resulting from unilateral decisions to increase the dike height at upstream locations. As expected, the results underline the potential effectiveness of increased retention along the river. The effects of controlled retention at the most upstream possible location and largest possible extent generate the most pronounced reduction of average annual damage. As expected, the effect of uncontrolled retention with dike relocations is significantly lower.

Environmental risk of dissolved oxygen depletion of diverted flood waters in river polder systems – A quasi-2D flood modelling approach

River polders are retention basins contained by levees alongside rivers into which water from the main river channel is diverted during extreme floods in order to cap the peak discharge of the flood hydrograph and to alleviate downstream flood risk by reducing the water levels. The retained water, however, is stagnant and the organic material in the water and the bottom sediments imposes a strong oxygen demand on the water. This paper presents a quasi two-dimensional computer-based methodology to assess the environmental risk exhibited by the operation of polders with which the concentration of dissolved oxygen in river and polder water can be simulated. A Monte-Carlo analysis allows the probability distribution of all the outcomes of the minimum dissolved oxygen levels in the water to be derived. From this analysis, the environmental risk of the dissolved oxygen concentrations in the polder water falling below 2 mg O 2/L (the level considered critical for aquatic ecosystems) can be determined. The August 2002 extreme flood event on the Elbe River, Germany, with a proposed polder system variant was used to calibrate the model. A daily time step was used to for the simulations for a time frame 12–21 August 2008. The results show plausible spatial and temporal variations in the dissolved oxygen concentrations within the polders. The quasi-2D approach was successful in simulating the spatial distribution of water quality constituents in the polder system. There is up to approximately 20% risk that dissolved oxygen levels fall below 2 mg/L in the polders. This risk can potentially increase if sediment oxygen demand increases due to crop residue and water temperatures in polders increase. High nutrient transport in the river during flooding can cause a spurt of phytoplankton growth in the polders.