Flooding in Fiji: findings from a 100-year historical series

Abstract

Correspondence from a long-established sugar mill provided the opportunity to construct the longest flood series for a river in Fiji—the Ba River in northwest Viti Levu—from 1892 to 2002. Flood waters reached the mill floor every four years on average. Contrary to common lore, this study could detect no increase in the frequency of major floods over the course of the 20th century, despite intensification of land use and siltation of the river channel over that time. Large, slow-moving tropical cyclones situated northwest of the valley have generated large floods, but so too have tropical rainstorms. Major floods have occurred in months when the Southern Oscillation Index (SOI) was both negative and positive, suggesting that the SOI is a poor indicator of flood potential for the Ba River.     

CYCLES D'ÉVOLUTION DE LA COMPOSITION CHIMIQUE DES EAUX SOUTERRAINES / DANS CERTAINES SITUATIONS HYDROGÉOLOGIQUES TYPIQUES DU BASSIN DE LA MÉDITERRANÉE

Abstract

Results of a comprehensive synoptic-hydrological analysis of major flood events in the Negev (1964–2007) are presented. A low threshold for major flood data was set to be the 10-year recurrence interval of peak discharge and/or flood volume magnitude. Altogether, 75 major flood events, or 133 hydrometrically monitored floods, were extracted. These events were categorized according to synoptic oriented classes by verification of the paired databases of: (a) floods in the study area, and (b) synoptic systems over the Eastern Mediterranean. For the study area, two of the most frequent flood-generating synoptic systems are the autumn Red Sea Trough (RST), 31%, and winter cyclones, 49%. The entire RST series consists of 24 major flood events (55 floods). The synoptic definition was corroborated by analysing the specific form of flood hydrographs and the ratio of flood volume to peak discharge. Regional analysis shows increased contribution of RST events southwards from 30% to 90% with a respective decrease in the number of cyclone events. By comparing two 22-year sub-periods (1964–1985 and 1986–2007), a positive trend in the frequency and magnitudes of RST flood events is discerned. There is also an increased tendency for the occurrence of cyclone floods.

Editor Z.W. Kundzewicz

Citation Shentsis, I., Laronne J.B., and Alpert, P., 2012. Red Sea Trough flood events in the Negev, Israel (1964–2007). Hydrological Sciences Journal, 57 (1), 42–51.     

Suspended sediment transport during tropical‐cyclone floods in Fiji

Flow records, rising‐stage sediment samplers, and a sand suspension model are used to examine suspended sediment concentrations during major floods caused by tropical cyclones TC Joni and TC Kina in the Rewa River, Fiji. The highest concentrations of total suspended solids were measured during the early stages of TC Kina. The suspension model predicts higher sand concentrations for TC Kina compared with TC Joni because of the larger slope and higher shear stresses during Kina. Extremely high wash load concentrations early in TC Kina are at least partly due to remobilization of fine sediment deposited during the earlier TC Joni flood. Samples from the TC Kina had volumetric concentrations larger than 5%, indicating hyperconcentrated streamflows. Mass‐density shear stresses in the hyperconcentrated flows are up 1·6 times larger than clear‐water shear stresses, but they occur early during low stages of the flood and probably do not result in severe bed erosion. Copyright © 2002 John Wiley & Sons, Ltd.     

海南井水位对热带气旋响应特征分析

2001——2010年, 海南省地下流体观测台网记录到多次热带气旋引起的井水位抖动现象．本文以2003年7月21日强热带气旋ldquo;天鹅rdquo;和2005年9月27日台风ldquo;达维rdquo;为例, 系统地研究了这两次热带气旋引起的水位抖动变化的特征．结果表明, 经过高通滤波, 水位抖动变化图像更加明显;通过频谱分析,得知热带气旋引起的水位抖动周期为100——101 min;井水位抖动的起始时间、 幅度最大值的时间与热带气旋通过海南岛陆的时间一致, 且与热带气旋的结构特点、发展和运动过程密切相关,与井孔自身的井-含水层系统对微动态信息响应的能力也有关系．分析认为,气压振荡式升降变化和摩擦是热带气旋引起水位抖动的原因．      

Remote forcing of water levels by tropical cyclones in southwest Australia

Tropical cyclones (termed hurricanes and typhoons in other regions), are extreme events associated with strong winds, torrential rain and storm surges (in coastal areas) and cause extensive damage as a result of strong winds and flooding (caused by either heavy rainfall or ocean storm surges) in the immediate area of impact. The eastern Indian Ocean, particularly in the northwest region of Australia, is impacted by up to 10 tropical cyclones during the cyclone season, although direct impact of cyclones along the west and southwest coastlines is rare. However, the sub-tidal frequency component of sea level records along the west and south coasts of Western Australia indicates lagged correspondence with the occurrence of tropical cyclones. It is demonstrated that the tropical cyclones generate a continental shelf wave which travels along the west and south coasts of Australia up to 3500 km with speeds of 450–500 km day⁻¹ (5.2–5.8 ms⁻¹) with maximum trough to crest wave height of 0.63 m, comparable with the mean daily tidal range in the region. The shelf wave is identified in the coastal sea level records, initially as a decrease in water level, 1–2 days after the passage of the cyclone and has a period of influence up to 10 days. Amplitude of the shelf wave was strongly affected by the path of the tropical cyclone, with cyclones travelling parallel to the west coast typically producing the most significant signal due to resonance and superposition with local forcing. Analysis of water levels from Port Hedland, Geraldton, Fremantle and Albany together with cyclone paths over a ten year period (1988–1998) indicated that the tropical cyclones paths may be classified into 6 different types based on the amplitude of the wave.     

Yearly tropical cyclone potential impact index in China

A new composite index called the yearly tropical cyclone potential impact(YTCPI)is introduced.The relationship between YTCPI and activities of tropical cyclones(TCs)in China,disaster loss,and main ambient fields are investigated to show the potential of YTCPI as a new tool for short-term climate prediction of TCs.YTCPI can indicate TC activity and potential disaster loss.As correlation coefficients between YTCPI and frequency of landfalling TCs,the frequency of TCs traversing or forming inside a 24 h warning line in China from 1971 to 2010 are 0.58 and 0.56,respectively(both are at a statistically significant level,aboveα=0.001).Furthermore,three simple indexes are used to compare with YTCPI.They all have very close relationships with it,with correlation coefficients 0.75,0.82 and 0.78.For economic loss and YTCPI,the correlation coefficient is 0.57 for 1994–2009.Information on principal ambient fields(sea surface temperature,850 and 500 hPa geopotential heights)during the previous winter is reflected in the relationship with YTCPI.Spatial and temporal variabilities of ambient fields are extracted through empirical orthogonal function(EOF)analysis.Spatial distributions of correlation coefficient between YTCPI and ambient fields match the EOF main mode.Correlation coefficients between YTCPI and the EOF time array for the three ambient fields are 0.46,0.44 and 0.4,respectively,all statistically significant,aboveα=0.01.The YTCPI has the overall potential to be an improved prediction tool.     

Extreme flood‐driven fluvial bank erosion and sediment loads: direct process measurements using integrated Mobile Laser Scanning (MLS) and hydro‐acoustic techniques

This methods paper details the first attempt at monitoring bank erosion, flow and suspended sediment at a site during flooding on the Mekong River induced by the passage of tropical cyclones. We deployed integrated mobile laser scanning (MLS) and multibeam echo sounding (MBES), alongside acoustic Doppler current profiling (aDcp), to directly measure changes in river bank and bed at high (~0.05 m) spatial resolution, in conjunction with measurements of flow and suspended sediment dynamics. We outline the methodological steps used to collect and process this complex point cloud data, and detail the procedures used to process and calibrate the aDcp flow and sediment flux data. A comparison with conventional remote sensing methods of estimating bank erosion, using aerial images and Landsat imagery, reveals that traditional techniques are error prone at the high temporal resolutions required to quantify the patterns and volumes of bank erosion induced by the passage of individual flood events. Our analysis reveals the importance of cyclone‐driven flood events in causing high rates of erosion and suspended sediment transport, with a c. twofold increase in bank erosion volumes and a fourfold increase in suspended sediment volumes in the cyclone‐affected wet season. Copyright © 2016 John Wiley & Sons, Ltd.     

Tropical cyclone activity in the northwestern Pacific associated with decaying Central Pacific El Niños

In this study, a comparative analysis was performed to reveal the relationship between the three types of temporal evolution of the Central Pacific (CP) El Niño (prolonged, abrupt, and symmetric decay) in the tropical Pacific Ocean and tropical cyclone (TC) activity in the Western North Pacific (WNP). TCs in the WNP were found to be sensitive to the different types of temporal evolution of the CP El Niño. During years of prolonged and symmetric decay, TCs were more frequent over East Asia with relatively strong TCs. In contrast, during years of abrupt decay, weak TCs with a relatively short lifespan more likely formed at slightly higher latitudes. The historical record of CP El Niño events is limited, but this study is expected to improve the understanding of complex natural systems and provide fundamental information for the scientific understanding and forecasting of TCs.     

Timescale-based flood typing to estimate temporal changes in flood frequencies

ABSTRACT

The flood peak is the dominating characteristic in nearly all flood-statistical analyses. Contrary to the general assumptions of design flood estimation, the peak is not closely related to other flood characteristics. Differentiation of floods into types provides a more realistic view. Often different parts of the probability distribution function of annual flood peaks are dominated by different flood types, which raises the question how shifts in flood regimes would modify the statistics of annual maxima. To answer this, a distinction into five flood types is proposed; then, temporal changes in flood-type frequencies are investigated. We show that the frequency of floods caused by heavy rain has increased significantly in recent years. A statistical model is developed that simulates peaks for each event type by type-specific peak–volume relationships. In a simulation study, we show how changes in frequency of flood event type lead to changes in the quantiles of annual maximum series.     

A new seasonal design flood method based on bivariate joint distribution of flood magnitude and date of occurrence

Abstract

Seasonal design floods which consider information on seasonal variation are very important for reservoir operation and management. The seasonal design flood method currently used in China is based on seasonal maximum (SM) samples and assumes that the seasonal design frequency is equal to the annual design frequency. Since the return period associated with annual maximum floods is taken as the standard in China, the current seasonal design flood cannot satisfy flood prevention standards. A new seasonal design flood method, which considers dates of flood occurrence and magnitudes of the peaks (runoff), was proposed and established based on copula function. The mixed von Mises distribution was selected as marginal distribution of flood occurrence dates. The Pearson Type III and exponential distributions were selected as the marginal distribution of flood magnitude for annual maximum flood series and peak-over-threshold samples, respectively. The proposed method was applied at the Geheyan Reservoir, China, and then compared with the currently used seasonal design flood methods. The case study results show that the proposed method can satisfy the flood prevention standard, and provide more information about the flood occurrence probabilities in each sub-season. The results of economic analysis show that the proposed design flood method can enhance the floodwater utilization rate and give economic benefits without lowering the annual flood protection standard.

Citation Chen, L., Guo, S. L., Yan, B. W., Liu, P. & Fang, B. (2010) A new seasonal design flood method based on bivariate joint distribution of flood magnitude and date of occurrence. Hydrol. Sci. J. 55(8), 1264–1280.     

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

Abstract

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     

海岛地震台网自然因素对地脉动干扰分析

利用海南地震台网宽频带地震仪连续观测资料,分析不同自然环境下地脉动扰动信号,结果显示:在不同自然环境下,地震仪记录的地脉动扰动信号的形态、振幅、频率、扰动强度不同,其中登陆海岛的热带气旋引起的震颤信号频带窄、能量大、扰动强度较大,对地脉动干扰影响大,其次是天文潮期,远离海岛的热带气旋对地脉动干扰影响较小;不同路径、距离的热带气旋引起的震颤信号频率带一致,但扰动强度区别明显;不同自然环境对不同观测台站记录的地脉动信号均存在影响,只是程度稍有不同。     

西北太平洋热带气旋活动对大尺度环境场影响的线性回归分析

本文使用西北太平洋累积气旋能量(ACE)月尺度异常序列,对850 hPa相对涡度、地表纬向风、柱水汽含量、OLR、降水和SST等大尺度环境场变量进行时滞回归分析,讨论了月尺度TC活动对大尺度环境场的影响及其与周尺度的区别和联系.结果表明:(1)月尺度ACE回归出的纬向风无论是强度还是范围都要明显大于周尺度ACE回归结果,TC的频繁发生,尤其是强度大且持续时间长的近赤道TC,对于激发或加强TC活动区域南侧的低纬地区西风异常有一定的积极作用,持续并且强度较大的西风异常可能导致西风的爆发,而西风爆发会在很大程度上影响ENSO事件的发生和演变.(2)在月尺度上,OLR、柱水汽含量、降水和SST等物理量均呈现出较为明显的E1Nino型分布,而在周尺度上,仅SST呈现出明显的El Nino型分布,这在一定程度上反映了月尺度TC活动和太平洋ENSO信号之间存在更密切的关联.(3)在TC发生后1-2月,TC的主要生成区域柱水汽含量减少、OLR增大,这会在一定程度上降低该区域生成TC的潜能.虽然TC所引起SST降低的空间尺度很小,但其通过大气和海洋的传导会扩大到更大尺度上,由于这种反馈具有一定的滞后性,所以月尺度TC对大尺度环境场影响的信号更为显著.     

The contribution of tropical cyclones to rainfall in Mexico

Investigating the contribution of tropical cyclones to the terrestrial water cycle can help quantify the benefits and hazards caused by the rainfall generated from this type of hydro-meteorological event. Rainfall induced by tropical cyclones can enhance both flood risk and groundwater recharge, and it is therefore important to characterise its minimum, mean and maximum contributions to a region or country’s water balance. This work evaluates the rainfall contribution of tropical depressions, storms and hurricanes across Mexico from 1998 to 2013 using the satellite-derived precipitation dataset TMPA 3B42. Additionally, the sensitivity of rainfall to other datasets was assessed: the national rain gauge observation network, real-time satellite rainfall and a merged product that combines rain gauges with non-calibrated space-borne rainfall measurements. The lower Baja California peninsula had the highest contribution from cyclonic rainfall in relative terms (∼40% of its total annual rainfall), whereas the contributions in the rest of the country showed a low-to-medium dependence on tropical cyclones, with mean values ranging from 0% to 20%. In quantitative terms, southern regions of Mexico can receive more than 2400 mm of cyclonic rainfall during years with significant TC activity. Moreover, (a) the number of tropical cyclones impacting Mexico has been significantly increasing since 1998, but cyclonic contributions in relative and quantitative terms have not been increasing, and (b) wind speed and rainfall intensity during cyclones are not highly correlated. Future work should evaluate the impacts of such contributions on surface and groundwater hydrological processes and connect the knowledge gaps between the magnitude of tropical cyclones, flood hazards, and economic losses.     

柘林水库修河流域降水时空分布特征与旱涝研究

根据1953—1983年降水资料,分析了柘林水库修河流域降水时空分布特征和旱涝规律。降水主要集中在6月中、下旬,有时可推迟到7月中旬,此时暴雨频数高,强度大,历时长,是柘林水库洪水运行管理的关键时期。为此,需要了解梅雨晚期发生的大暴雨的形成条件和时空分布,以及梅雨期旱涝情况和采用预报的风险程度。本文在这方面进行了一些综合性分析,特别是对5,6月旱涝年份的统计规律、环流特征作了较详细的气候学对比研究。     

On the Prediction of Tropical Cyclones over the Indian Region Using a Synthetic Vortex Scheme in a Mesoscale Model

The tropical cyclones form over the oceanic regions where conventional meteorological observations are not available. This contributes to a poor initial analysis of the cyclonic vortex and hence inadequate forecast. One way of overcoming the above problem is to modify the initial analysis by replacing the weak and ill-defined vortex in the initial analysis with a synthetic vortex having the correct size and intensity at the correct location. In this study we are investigating the effect of inclusion of a synthetic vortex based on Rankine as well as on Holland wind profiles, using NCAR-AFWA bogussing scheme for the prediction of four tropical cyclones, which formed over the Bay of Bengal during November 2002 and 2005, December 2005 and over the Arabian Sea during May 2004, using the MM5 model. Two numerical experiments are designed in this study for each of the above four cyclones. In the first experiment the model is integrated with a synthetic vortex based on Rankine wind profile while in the second experiment we utilize the Holland wind profile. For the November 2002 cyclone, in both the experiments the model is integrated from 10 November 2002 18 UTC to 12 November, 2002 12 UTC with the synthetic vortex inserted at the initial time. The results of the study for the November 2002 cyclone show that the model simulation with the Holland vortex has produced a stronger cyclone in terms of minimum sea-level pressure and maximum wind speed. Also, the results for the November 2002 cyclone with the Holland vortex showed a better longitudinal height section of the horizontal wind speed across the center of the cyclone. The track error of the cyclone for the November 2002 cyclone is less in the model simulation with the Holland vortex at the initial time and at 24 hours of forecast. The results for the November 2002 cyclone with the Rankine vortex showed greater vertical wind speed as compared to the Holland vortex. However, for the November 2002 cyclone there were no significant differences in the spatial distribution of precipitation for both the experiments. In order to provide an adequate number of case studies for a good statistical sample, the present study is extended for three additional cyclones over the Indian region. All four cyclones studied here show that the Holland vortex has produced a stronger cyclone in terms of the minimum sea-level pressure and maximum wind speed. The Holland vortex showed a better vertical structure of wind speed in the longitudinal height section at 24 hours of forecast for the November 2005 cyclone while the structure was better for the Rankine vortex for the remaining two cyclones. There were no significant differences in the spatial distribution of precipitation for the two experiments corresponding to all four cyclones. Some statistical results pertaining to all four cyclones are provided such as the average track error as well as the average difference between the observed and the model minimum sea-level pressure and the maximum wind speed. The statistical results corresponding to the average of all the four cyclones are at only a slight variance with the results corresponding to the November 2002 cyclone.     

Non-linear growth of cumulative flood losses with time

Statistical data over the past 30 years show that the cumulative sum of losses caused by floods S(t) has been increasing with time approximately as t^1·3, i.e. faster than the linear growth expected for a stationary process. (Losses are evaluated by the number of homeless caused by floods, since these data are the most systematically reported.) At the same time, the factors determining flood losses (the rate of floods and single loss distribution) appear to be stationary over the period of observation. An explanation of this paradox is suggested based on a heavy-tail distribution function of losses, i.e. a distribution function with infinite expectation value. The proposed stochastic model predicts a faster than linear growth of the cumulative losses until some limiting time, which corresponds to the recurrence period of the maximal possible single loss. Similar pseudo-non-stationary effects can be observed for other types of catastrophes and hydrological characteristics with heavy-tail distributions © 1998 John Wiley & Sons, Ltd.     

Dependence between flood peaks and volumes: a case study on climate and hydrological controls

Abstract

The aim of this paper is to understand the causal factors controlling the relationship between flood peaks and volumes in a regional context. A case study is performed based on 330 catchments in Austria ranging from 6 to 500 km² in size. Maximum annual flood discharges are compared with the associated flood volumes, and the consistency of the peak–volume relationship is quantified by the Spearman rank correlation coefficient. The results indicate that climate-related factors are more important than catchment-related factors in controlling the consistency. Spearman rank correlation coefficients typically range from about 0.2 in the high alpine catchments to about 0.8 in the lowlands. The weak dependence in the high alpine catchments is due to the mix of flood types, including long-duration snowmelt, synoptic floods and flash floods. In the lowlands, the flood durations vary less in a given catchment which is related to the filtering of the distribution of all storms by the catchment response time to produce the distribution of flood producing storms.

Editor Z.W. Kundzewicz     

Flood-type trend analysis for alpine catchments

ABSTRACT

In many places, magnitudes and frequencies of floods are expected to increase due to climate change. To understand these changes better, trend analyses of historical data are helpful. However, traditional trend analyses do not address issues related to shifts in the relative contributions of rainfall versus snowmelt floods, or in the frequency of a particular flood type. We present a novel approach for quantifying such trends in time series of floods using a fuzzy decision tree for event classification and applied it to maximal annual and seasonal floods in 27 alpine catchments for the period 1980–2014. Trends in flood types were studied with Sen’s slope and double mass curves. Our results reveal a decreasing number of rain-on-snow and an increasing number of short rainfall events in all catchments, with flash floods increasing in smaller catchments. Overall, the results demonstrate the value of incorporating a fuzzy flood-type classification into flood trend analyses.     

Reconstructing extreme post‐wildfire floods: a comparison of convective and mesoscale events

In much of western United States destructive floods after wildfire are frequently caused by localized, short‐duration convective thunderstorms; however, little is known about post‐fire flooding from longer‐duration, low‐intensity mesoscale storms. In this study we estimate and compare peak flows from convective and mesoscale floods following the 2012 High Park Fire in the ungaged 15.5 km² Skin Gulch basin in the northcentral Colorado Front Range. The convective storm on 6 July 2012 came just days after the wildfire was contained. Radar data indicated that the total rainfall was 20–47 mm, and the maximum rainfall intensities (upwards of 50 mm h^?1) were concentrated over portions of the watershed that burned at high severity. The mesoscale storm on 9–15 September 2013 produced 220–240 mm of rain but had maximum 15‐min intensities of only 25–32 mm h^?1. Peak flows for each flood were estimated using three independent techniques. Our best estimate using a 2D hydraulic model was 28 m³ s^?1 km^?2 for the flood following the convective storm, placing it among the largest rainfall‐runoff floods per unit area in the United States. In contrast, the flood associated with the mesoscale flood was only 6 m³ s^?1 km^?2, but the long‐duration flood caused extensive channel incision and widening, indicating that this storm was much more geomorphically effective. The peak flow estimates for the 2013 flood had a higher relative uncertainty and this stemmed from whether we used pre‐ or post‐flood channel topography. The results document the extent to which a high and moderate severity forest fire can greatly increase peak flows and alter channel morphology, illustrate how indirect peak flow estimates have larger errors than is generally assumed, and indicate that the magnitude of post‐fire floods and geomorphic change can be affected by the timing, magnitude, duration, and sequence of rainstorms. Copyright © 2017 John Wiley & Sons, Ltd.