Probable Maximum Flood Research Articles

PMP and PMF estimations in sparsely-gauged Andean basins and climate change projections

Mixed-regime Andean basins present a complex scenario for flood analysis. In this study, we propose a methodology for incorporating orographic effects influenced by mountainous barriers in the Probable Maximum Precipitation (PMP) estimation method in sparsely-gauged basins. The proposed methodology is applied to the Puclaro Reservoir basin in Chile, which is affected by the Andes. The PMP estimations were calculated by applying statistical and hydrometeorological approaches to the baseline (1960–1999) and climate change scenarios (2045–2065) determined from projections of the ECHAM5 general circulation model. Temperature projections for the 2040–2065 period show that there would be a rise in the catchment contributing area that would lead to an increase in the average liquid precipitation over the basin. Temperature projections would also affect the maximization factors in the calculation of the PMP, as precipitable water content, raising it to 126.6% and 62.5% under scenarios A2 and B1, respectively; the probable maximum flood (PMF) would increase to +175.5% under the A2 scenario. These projections would affect the safety of dam design and would be generalizable to zones with similar mixed hydrology and climate change projections. We propose that the methodology presented could be also applied to basins with similar characteristics.Editor Z.W. Kundzewicz; Associate editor A. Porporato

Hydraulic Modeling of Extreme Hydrologic Events: Case Study in Southern Virginia

AbstractA comprehensive hydraulic modeling effort that was applied to two rural watersheds in southern Virginia is presented in this paper. The 10-, 50-, 100-, and 500-year storms and the probable maximum flood (PMF), are considered in this study with the objectives of assessing the impact of extreme hydrologic events through the generation of inundation maps, characterizing the sediment transport process under severe flooding conditions, and quantifying the uncertainty associated with the floodplains boundary roughness and its influence on the results. The employed methodology includes the construction of a river terrain model that combines geospatial and field-collected data, a calibration procedure based on regression equations and developed stage-discharge predictor curves, and unsteady flow simulations utilizing discharge hydrographs produced by an event-based hydrologic model. The results highlight the severe flooding conditions associated with the PMF and the necessity of considering it as the wors...

A 2000 year natural record of magnitudes and frequencies for the largest Upper Colorado River floods near Moab, Utah

AbstractUsing well‐established procedures for paleoflood hydrology and employing optically stimulated luminescence (OSL) geochronology, we analyzed a very well‐preserved natural record of 44 Upper Colorado River extreme floods with discharges ranging from 1800 to 9200 m3s−1. These are the largest floods occurring during the last 2140 ± 220 years, and this natural record indicates that large floods are much more frequent than can be estimated by extrapolation from the stream gaging record that extends back to 1914. Most of these large floods occurred during the last 500 years, and the two largest floods in the record both exceeded the probable maximum flood (PMF) estimated at 8500 m3s−1(300,000 cfs) for nearby Moab, Utah. Another four floods, with discharges greater than 7000 m3s−1, occurred during the last two millennia. Flood frequency analyses using the FLDFRQ3 model yields the following values, depending on the Manningnroughness coefficients: 100 yr flood—4670–4990 m3s−1; 500 yr flood—6675–7270 m3s−1; 1000 yr flood—7680–8440 m3s−1. The presumed PMF discharge (8500 m3s−1) gets assigned a recurrence interval of about 1000 years, and the largest historical 1884 flood (3540 m3s−1)—a recurrence interval of <100 years. Flood frequency analysis for the Moab Valley based on the gaged record (1914–2012) yield 2730 m3s−1for the 100 yr flood and 3185 m3s−1for the 500 yr flood. This underestimation of the frequency of large floods from the gage data results from effects on that record by modern regulation of upstream river flow and associated water extraction for agriculture.

AHP를 이용한 농업용저수지 수문학적 안전성평가 방법 개발 및 적용

요 약현재국내일정규모이상댐들에대해서는「안전점검및정밀안전진단세부지침(댐)」기준에따라정밀안전진단을수행하고있다. 하지만여러평가기준중수문학적안전성평가는가능최대홍수량(ProbableMaximum Flood, PMF)에대한기존댐안전성을평가하는것으로대부분의농업용저수지에대해서는적용성이많이떨어진다. 따라서본연구에서는계수화모델과AHP 기법을통해다각적위험요인을고려한농업용저수지의수문학적안전성을재평가하고자한다. 이를위하여농업용저수지의다양한인자를반영하기위한수문학적안전성상·하위평가항목을선정하여계수화모델을개발하였다. 평가항목별지표점수의총합을산출한후전문가집단의응답에대한검증절차를실시하였고, 평가항목의가중치를산정하여최종적으로다각적위험요인을고려한농업용저수지의수문학적안전성평가를위한실용계수를산정하였다. 본연구결과기존평가기준은댐의가능최대홍수량에대한수문학적안전성을평가하는기준으로대부분의농업용저수지에대해서는홍수방어능부족에따른제체의월류로저수지의파괴위험성이큰것으로분류되었다. 그리고가능최대홍수량에대한여유고기준을만족하는저수지에대해서는댐의형식및상태별여유고에대한평가만으로필댐을평가하는것으로나타났다.핵심용어:수문학적안전성평가, 가능최대홍수량, 농업용저수지, 분석적계층화과정AbstractAccording to the「Safety Evaluation Detailed Instructions (Dam)」, precise safety inspection is carried out fordams that exceed a certain scale. However, as the Hydrological Safety Evaluation from various evaluationstandards is designed to evaluate the safety of existing dams considering PMF, the evaluation is much lessapplicable for most agricultural reservoirs. Therefore, the Hydrological Safety Guidelines for agriculturalreservoirs are expected to be re-evaluated considering the diverse risk factors with the coefficient model andAHP in this study. The coefficient model has been developed by selecting the hydrological safetysuperordinate․subordinate evaluation factors to reflect diverse risk factors of agricultural reservoirs. Aftercalculating the sum of indicators score for each evaluation factors, validation procedures were performed forthe questionnaire which a panel answered. The practical coefficient has eventually been estimated for thehydrological safety evaluation considering the diverse risk factors. The conclusions acquired based on thestudy done are that both most agricultural reservoirs were classified as flood defense capability is insufficientand agricultural reservoirs which meet embankment-freeboard standards considering PMF was overestimated.

Lessons From The Lives of Two Dams

Many embankment dams completed during the first six decades of the 20th century have been found deficient relative their ability to resist currently anticipated levels of seismic shaking and probable maximum flood. In this Fourth Victor de Mello Lecture, two recent case histories are described. One is a hydraulic fill structure completed in 1920 that is founded on alluvial material, some zones of which are susceptible to liquefaction. The other is a zoned earthfill dam completed in 1956 that is founded over a channel filled with loose, uncompacted, hydraulically placed tailings from gold mining operations. Each dam has been upgraded in phases over periods of several decades using different strategies and ground improvement technologies to improve stability and reduce failure risks. Several take away lessons from these experiences concerning current risk mitigation strategies, the importance of correct soil and site characterization, and implementation and effectiveness of different ground stabilization and improvement methods are presented.

Development of a Prototype Web GIS-Based Disaster Management System for Safe Operation of the Next Generation Bimodal Tram, South Korea—Focused Flooding and Snowfall

The Korea Railroad Research Institute (KRRI) has developed a bimodal tram and advanced bus rapid transit (BRT) system which is an optimized public transit system created by mixing the railway’s punctual operation and the bus’ easy and convenient access. The bimodal tram system provides mass-transportation service with an eco-friendly and human-centered approach. Natural disasters have been increasing worldwide in recent years, including floods, snow, and typhoons disasters. Flooding is the most frequent natural disaster in many countries and is increasingly a concern with climate change; it seriously affects people’s lives and productivity, causing considerable economic loss and significant damage. Enhanced conventional disaster management systems are needed to support comprehensive actions to secure safety and convenience. The objective of this study is to develop a prototype version of a Web GIS-based bimodal tram disaster management system (BTDMS) using the Storm Water Management Model (SWMM) 5.0 to enhance on-time operation and safety of the bimodal tram system. The BTDMS was tested at the bimodal tram test railroad by simulating probable maximum flood (PMF) and snow melting for forecasting flooding and snow covered roads. This result could provide the basis for plans to protect against flooding disasters and snow covered roads in operating the bimodal tram system. The BTDMS will be used to assess and predict weather impacts on roadway conditions and operations and thus has the potential to influence economic growth. The methodology presented in this paper makes it possible to manage impacts of flooding and snowfall on urban transportation and enhance operation of the bimodal tram system. Such a methodology based on modeling could be created for most metropolitan areas in Korea and in many other countries.

Comparison of water supply dams' spillway performance in Malaysia

This paper compares the risk of dam overtopping of two rolled compacted concrete (RCC) dams in Malaysia. Uniform methodology was carried out by selecting an appropriate probable maximum precipitation (PMP) series, performing a catchment PMP/PMF (probable maximum flood) rainfall runoff routing and finally, routing the inflow hydrographs through the reservoir water body. The critical PMP/PMF reservoir outflows and their corresponding flood rises were also compared. Both dams are considered safe using the current PMP series. The roles of surcharging volumes above the top water level of the reservoir were also demonstrated via their respective flow attenuation capabilities by their surcharging volume. This paper also demonstrates a close relationship can be forged between the product of spillway width and headroom vis-a-vis the catchment areas draining at the dam sites. The product represents a trade-off between a larger spillway vis-a-vis lower headroom, vice versa. Both Kinta and Bengoh dams’ spillway factors are consistently higher than the regressed curve using a vast database of existing dams and reservoirs in Malaysia.

Physically-based extreme flood frequency with stochastic storm transposition and paleoflood data on large watersheds

Summary Traditionally, deterministic flood procedures such as the Probable Maximum Flood have been used for critical infrastructure design. Some Federal agencies now use hydrologic risk analysis to assess potential impacts of extreme events on existing structures such as large dams. Extreme flood hazard estimates and distributions are needed for these efforts, with very low annual exceedance probabilities ( ⩽ 10 - 4 ) (return periods >10,000 years). An integrated data-modeling hydrologic hazard framework for physically-based extreme flood hazard estimation is presented. Key elements include: (1) a physically-based runoff model (TREX) coupled with a stochastic storm transposition technique; (2) hydrometeorological information from radar and an extreme storm catalog; and (3) streamflow and paleoflood data for independently testing and refining runoff model predictions at internal locations. This new approach requires full integration of collaborative work in hydrometeorology, flood hydrology and paleoflood hydrology. An application on the 12,000 km2 Arkansas River watershed in Colorado demonstrates that the size and location of extreme storms are critical factors in the analysis of basin-average rainfall frequency and flood peak distributions. Runoff model results are substantially improved by the availability and use of paleoflood nonexceedance data spanning the past 1000 years at critical watershed locations.

Flood hazard assessment with multiparameter approach derived from coupled 1D and 2D hydrodynamic flow model

Hydrodynamic flow modeling is carried out using a coupled 1D and 2D hydrodynamic flow model in northern India where an industrial plant is proposed. Two flooding scenarios, one considering the flooding source at regional/catchment level and another considering all flooding sources at local level have been simulated. For simulating flooding scenario due to flooding of the upstream catchment, the probable maximum flood (PMF) in the main river is routed and its flooding impact at the plant site is studied, while at the local level flooding, in addition to PMF in the main river, the probable maximum precipitation at the plant site and breaches in the canals near the plant site have been considered. The flood extent, depth, level, duration and maximum flow velocity have been computed. Three parameters namely the flood depth, cross product of flood depth and velocity and flood duration have been used for assessing the flood hazard, and a flood hazard classification scheme has been proposed. Flood hazard assessment for flooding due to upstream catchment and study on local scale facilitates determination of plinth level for the plant site and helps in identifying the flood protection measures.

Toward a Methodology to Investigate the Downstream Flood Hazards on the American River due to Changes in Probable Maximum Flood due to Effects of Artificial Reservoir Size and Land-Use/Land-Cover Patterns

Abstract Recent research in mesoscale hydrology suggests that the size of the reservoirs and the land-use/land-cover (LULC) patterns near them impact the extreme weather [e.g., probable maximum flood (PMF)]. A key question was addressed by W. Yigzaw et al.: How do reservoir size and/or LULC modify extreme flood patterns, specifically PMF via modification of probable maximum precipitation (PMP)? Using the American River watershed (ARW) as a representative example of an impounded watershed with Folsom Dam as the flood control structure, they applied the distributed Variable Infiltration Capacity (VIC) model to simulate the PMF from the atmospheric feedbacks simulated for various LULC scenarios. The current study presents a methodology to extend the impacts of these modified extreme flood patterns on the downstream Sacramento County, California. The research question addressed is, what are the relative effects of downstream flood hazards to population on the American River system under various PMF scenarios for the Folsom Dam? To address this goal, a two-dimensional flood model, the Flood in Two Dimensions–Graphics Processing Unit (Flood2D-GPU), is calibrated using synthetic aperture radar (SAR) and Landsat satellite observations and observed flood stage data. The calibration process emphasized challenges associated with using National Elevation Dataset (NED) digital elevation model (DEM) and topographic light detection and ranging (lidar)–derived DEMs to achieve realistic flood inundation. Following this calibration exercise, the flood model was used to simulate four land-use scenarios (control, predam, reservoir double, and nonirrigation). The flood hazards are quantified as downstream flood hazard zones by estimating flood depths and velocities and its impacts on risk to population using depth–velocity hazard relationships provided by U.S. Bureau of Reclamation (USBR). From the preliminary application of methodology in this study, it is evident from comparing downstream flood hazard that similar trends in PMF comparisons reported by W. Yigzaw et al. were observed. Between the control and nonirrigation, the downstream flood hazard is pronounced by −3.90% for the judgment zone and −2.40% for high hazard zones. Comparing the control and predam scenarios, these differences are amplified, ranging between 0.17% and −1.34%. While there was no change detected in the peak PMF discharges between the control and reservoir-double scenarios, it still yielded an increase in high hazard areas for the latter. Based on this preliminary bottom-up vulnerability assessment study, it is evident that what was observed in PMF comparisons by W. Yigzaw et al. is confirmed in comparisons between control versus predam and control versus nonirrigation. While there was no change detected in the peak PMF discharges between the control and reservoir-double scenarios, it still yielded a noticeable change in the total areal extents: specifically, an increase in high hazard areas for the latter. Continued studies in bottom-up vulnerability assessment of flood hazards will aid in developing suitable mitigation and adaptation options for a much needed resilient urban infrastructure.

HEC-HMS/RAS와 HEC-GeoRAS 모형을 이용한 홍수범람 시나리오 수립 및 분석

Damages by hurricanes and regional torrential rains due to anomaly climate changes take up the most of recent meteorological disasters in Korea. To reduce these damages, producing flood hazard maps and disaster maps is essential, and accordingly, the interpretation of flood inundation by scenario accounts for an important part. This study established a flood inundation scenario at Sacheon River in Gangneung-si and carried out an analysis using the HEC-GeoRAS model. The rainfall data at the Gangneung General Observation Post from 1961 to 2010(50 years) were used for probable rainfall, and the scenario was analyzed by assuming the case of Typhoon Rusa with 100-year and 200-year frequencies and Probable Maximum Flood (PMF). As a result, flood inundation is predicted if it exceeded the 100-year frequency rainfall while for the 200-year frequency and PMF frequency, flooded areas were simulated at 64.3 ha and 135.9 ha, respectively and maximum flooded depths in 4.1 m and 5.7 m, respectively. It is expected that these results will help predict flood inundation area with rainfall in the duration using the synthetic data of the real-time weather data and the model numerical analysis.

Comparison of PMP-Driven Probable Maximum Floods with Flood Magnitudes due to Increasingly Urbanized Catchment: The Case of American River Watershed

Abstract Since historical (predam) data are traditionally the sole criterion for dam design, future (postdam) meteorological and hydrological variability due to land-use and land-cover change cannot be considered for assessing design robustness. For example, postdam urbanization within a basin leads to definite and immediate increase in direct runoff and reservoir peak inflow. On the other hand, urbanization can strategically (i.e., gradually) alter the mesoscale circulation patterns leading to more extreme rainfall rates. Thus, there are two key pathways (immediate or strategic) by which the design flood magnitude can be compromised. The main objective of the study is to compare the relative contribution to increase in flood magnitudes through direct effects of land-cover change (urbanization and less infiltration) with gradual climate-based effects of land-cover change (modification in mesoscale storm systems). The comparison is cast in the form of a sensitivity study that looks into the response to the design probable maximum flood (PMF) from probable maximum precipitation (PMP). Using the American River watershed (ARW) and Folsom Dam as a case study, simulated peak floods for the 1997 (New Year's) flood event show that a 100% impervious watershed has the potential of generating a flood that is close to design PMF. On the other hand, the design PMP produces an additional 1500 m3 s−1 peak flood compared to the actual PMF when the watershed is considered 100% impervious. This study points to the radical need for consideration future land-cover changes up front during the dam design and operation formulation phase by considering not only the immediate effects but also the gradual climatic effects on PMF. A dynamic dam design procedure should be implemented that takes into account the change of land–atmospheric and hydrological processes as a result of land-cover modification rather than relying on historical records alone.

Fifteen-stage operation of gated spillways for flood routing management through artificial reservoirs

A procedure to identify sets of operational rules for gated spillways for optimal flood routing management of artificial reservoirs is proposed. The flood retention storage of a dam having a gated flood spillway is divided into 15 sub-storages whose surface elevations are identified as critical levels. The most suitable operation set for the downstream conditions and for the dam can be chosen from many derived operation sets. The spillway gates are operated in an optimum way for any floods from very small magnitudes to the probable maximum flood (PMF), without having to forecast the actual magnitude of the incoming flood hydrograph. Decision floods are formed by dividing the PMF into 15 sub-hydrographs by 5 and 10% increments in the ranges 5–50% and 50–100% of the PMF, respectively. Many potential spillway gate openings from closed to fully open are chosen initially. As a result of a series of routing simulations of 15 decision floods, a set of 15 gate openings is determined such that all floods from very small magnitudes to the PMF may be routed without overtopping the dam crest. Next, a few more 15-stage operation rules are determined such that the gate openings of the initial stages are decreased as their critical levels are increased stepwise, with the objective of attenuating smaller floods more effectively and releasing higher outflows for larger floods close to and including the PMF. The developed model is applied to the Catalan and Aslantas dams in Turkey, both of which serve for flood mitigation as well as hydropower generation. Citation Haktanir, T., Citakoglu, H., and Acanal, N., 2013. Fifteen-stage operation of gated spillways for flood routing management through artificial reservoirs. Hydrological Sciences Journal, 58 (5), 1013–1031. Editor Z.W. Kundzewicz; Associate editor A. Montanari

Numerical modelling dam break analysis for water supply project

Dam provides many benefits to the society, but it can also cause extensive damage to downstream area when it fails. Dam failure can cause extensive damage to properties and loss of human life due to short warning time available. In general, dam spillway was designed to drain the maximum discharge from the dam during the Probable Maximum Flood (PMF). The spillway is functioned to prevent the dam from failure due to overtopping, which can lead to the dam failure. Dam failure will result in large volume of water travelling at very high velocity to the downstream area of the dam. It can cause extensive property damage, destruction of important facilities, and significant loss of human life along the way. Due to the potential of high hazard it poses to the downstream area, a dam break analysis is considered very essential. This paper focuses into the dam failure analysis for Kahang Dam by prediction of breach flow hydrographs and generation of inundation map at downstream area. From the PMF scenario simulation, the maximum inflow is 525.12 m3/s and peak discharge from the dam during dam failure is 6188m3/s. The results are able to provide information for preparation of Emergency Response Plan (PMF), in which appropriate steps can be taken by relevant authorities to avoid significant loss of human lives.

Probable Maximum Precipitation (PMP) over mountainous region of Cameron Highlands- Batang Padang Catchment of Malaysia

The Cameron Highland Batang Padang (CHBP) catchment situated on the main mountain range of Peninsular Malaysia is of large economical importance where currently a series of three dams (Sultan Abu Bakar, Jor and Mahang) exist in the development of water resources and hydropower. The prediction of the design storm rainfall values for different return periods including PMP values can be useful to review the adequacy of the current spillway capacities of these dams. In this paper estimates of the design storm rainfalls for various return periods and also the PMP values for rainfall stations in the CHBP catchment have been computed for the three different durations of 1, 3 & 5 days. The maximum values for 1 day, 3 days and 5 days PMP values are found to be 730.08mm, 966.17mm and 969.0mm respectively at Station number 4513033 Gunung Brinchang. The PMP values obtained were compared with previous study results undertaken by NAHRIM. However, the highest ratio of 1 day, 3 day and 5 day PMP to highest observed rainfall are found to be 2.30, 1.94 and 1.82 respectively. This shows that the ratio tend to decrease as the duration increase. Finally, the temporal pattern for 1 day, 3day and 5 days have been developed based on observed extreme rainfall at station 4513033 Gunung Brinchang for the generation of Probable Maximum Flood (PMF) in dam break analysis.

Simulation of Breach Outflow for Earthfill Dam

Dams have been built for many reasons such as irrigation, hydropower, flood mitigation, and water supply to support development for the benefit of human. However, the huge amount of water stored behind the dam can seriously pose adverse impacts to the downstream community should it be released due to unwanted dam break event. To minimise the potential loss of lives and property damages, a workable Emergency Response Plan is required to be developed. As part of a responsible dam owner and operator, TNB initiated a study on dam breach modelling for Cameron Highlands Hydroelectric Scheme to simulate the potential dam breach for Jor Dam. Prediction of dam breach parameters using the empirical equations of Froehlich and Macdonal-Langridge-Monopolis formed the basis of the modelling, coupled with MIKE 11 software to obtain the breach outflow due to Probable Maximum Flood (PMF). This paper will therefore discuss the model setup, simulation procedure and comparison of the prediction with existing equations.

An assessment of Bengoh dam overtopping scenario

Dams are supposedly designed and built with stringent criteria for no overtopping under the most adverse meteorological event. This may not be the case for dams that were built earlier before current methodological guidelines and practices are followed. The objective of assessing the safety criteria, i.e., overtopping of the Bengoh dam structure was achieved by carrying out the independent tasks of derivation of probable maximum precipitation (PMP) by reviewing other studies and works in Malaysia: a PMP-to-PMF (probable maximum flood) routing using a modified version of the synthetic unit hydrograph approach and a conventional reservoir routing procedure, i.e., modified Puls procedure to estimate the flood rise during a fateful PMP/PMF event. In essence, the task carried out is to evaluate and assess the adequacy of the step spillway capacity of the Bengoh dam. Bengoh dam, which is made of rolled compacted concrete (RCC) with a step spillway, is considered safe in the event of a PMP/PMF event, with a flood rise generally lower than the embankment crest level, i.e., +86.2 m MSL (mean sea level).

Estimation of the summer-fall PMP and PMF of a northern watershed under a changed climate

[1] This study is focused on assessing a summer-fall probable maximum precipitation (PMP) under recent climate conditions and then applying it under a future projected climate using output information from a regional climate model. The estimated PMPs are forced into a hydrological model to investigate potential changes in probable maximum flood (PMF) values. The PMP method is based on the moisture maximization method developed by the World Meteorological Organization. Precipitable water amounts are evaluated using data from the Canadian Regional Climate Model. The approach was tested on the Manic-5 River basin in Canada. Results show the PMP intensity could increase by 0.5–6% for 48 h and 72 h PMP values by the 2071–2100 horizon. These PMP values were used to assess projected PMF values. A lumped conceptual hydrological model was calibrated under recent climate values and model parameters were kept unchanged for modeling future hydrological regimes. Hydrological modeling results indicate modest changes of the PMF values for future climate projections, with no clearly identified upward or downward trends. The study also highlighted the need for a coherent approach to estimate PMF values under future climate conditions using projected PMP estimates, since the current practice of simulating the PMF value by inserting half of a PMP six days prior to the PMP event in a meteorological time series did not produce consistent results. An approach based on a random insertion of the PMP into a meteorological time series is a promising avenue to explore.

Assessing the impact of climate and land use changes on extreme floods in a large tropical catchment

Summary In the wake of the recent catastrophic floods in Thailand, there is considerable concern about the safety of large dams designed and built some 50 years ago. In this paper a distributed rainfall–runoff model appropriate for extreme flood conditions is used to generate revised estimates of the Probable Maximum Flood (PMF) for the Upper Ping River catchment (area 26,386 km2) in northern Thailand, upstream of location of the large Bhumipol Dam. The model has two components: a continuous water balance model based on a configuration of parameters estimated from climate, soil and vegetation data and a distributed flood routing model based on non-linear storage–discharge relationships of the river network under extreme flood conditions. The model is implemented under several alternative scenarios regarding the Probable Maximum Precipitation (PMP) estimates and is also used to estimate the potential effects of both climate change and land use and land cover changes on the extreme floods. These new estimates are compared against estimates using other hydrological models, including the application of the original prediction methods under current conditions. Model simulations and sensitivity analyses indicate that a reasonable Probable Maximum Flood (PMF) at the dam site is 6311 m3/s, which is only slightly higher than the original design flood of 6000 m3/s. As part of an uncertainty assessment, the estimated PMF is sensitive to the design method, input PMP, land use changes and the floodplain inundation effect. The increase of PMP depth by 5% can cause a 7.5% increase in PMF. Deforestation by 10%, 20%, 30% can result in PMF increases of 3.1%, 6.2%, 9.2%, respectively. The modest increase of the estimated PMF (to just 6311 m3/s) in spite of these changes is due to the factoring of the hydraulic effects of trees and buildings on the floodplain as the flood situation changes from normal floods to extreme floods, when over-bank flows may be the dominant flooding process, leading to a substantial reduction in the PMF estimates.

Maximum observed floods in China

Maximum observed floods (MOF) and their envelope curves are useful to hydrological engineers when estimating probable maximum floods or design floods. The World MOF and its envelope curve were developed originally in 1967 and modified in 2009 by our team. Based on MOF concepts and observed hydrological data in China, the China MOF and its envelope curve are presented, and their characteristics analysed. The results will be useful for flood design, for example for dam spillways, in China and in similar regions, in particular where no data are available, but cannot be used without modification and comparative analyses. Editor Z.W. Kundzewicz Citation Li, C., Wang, G., and Li, R., 2013. Maximum observed floods in China. Hydrological Sciences Journal, 58 (3), 728–735.