Photogrammetric and laser altimetric reconstruction of water levels for extreme flood event analysis

This paper assesses the feasibility of estimating water levels using digital photogrammetry. A common problem during an extreme flood event is that automated water level recorders do not record the highest water levels, as a result of instrument malfunctioning. This paper explores two possible solutions to this problem based upon data acquired using synoptic remote sensing methods. The first method requires: (a) high-resolution elevation data (for example, in the form of a digital elevation model for the floodplain); and (b) information on the planimetric position of the maximum flood extent, such as from debris lines (known as wrack lines) visible on aerial imagery flown after the event. The planimetric data can then be used to segment the topographic data in order to identify water level elevations. The second method uses a digitial photogrammetric approach and is suitable where no topographic data are available, but aerial imagery is available, flown after the event. Provided this imagery is of the right scale, digital photogrammetric analysis may be used to identify the elevations of wrack lines visible on the imagery. In this paper, the second of these options is compared with the first. The research shows that desktop photogrammetric methods, using 1:4500 scale imagery, can yield water level estimates that are precise to ±0·147 m, on the basis of check data obtained from lidar data. This is a worst possible estimate of the acquired precision given uncertainties in the lidar data. When compared with the first option, based upon segmenting lidar data using flood outlines, the photogrammetric approach was found to be preferable given both the quality of the lidar and uncertainties over how to segment it.     

Sinuosity evolution along an incising channel: New insights from the Jordan River response to the Dead Sea level fall

The geomorphic evolution of the Jordan River in recent decades indicates that interaction between incision and high-magnitude floods controls sinuosity changes under increasing mouth gradients during base-level fall. The evolution of the river was analyzed based on digital elevation models, remotely sensed imagery, hydrometric data, and a hydraulic model. The response varies along the river. Near the river mouth, where incision rate is high and a deep channel forms, overbank flooding is less likely. There, large floods exert high shear stress within the confined channel, increasing sinuosity. Upstream, near the migrating knickzone channel gradients also increase, incision is more moderate and floods continue to overtop the banks, favoring meander chute cutoffs. The resulting channel has a downstream well-confined meandering segment and an upstream low-sinuosity segment. These new insights regarding spatial differences along an incising channel can improve interpretations of the evolution of ancient planforms and floodplains that responded to base-level decline. © 2018 John Wiley & Sons, Ltd.     

Floods in the IPCC TAR Perspective

Recent floods have become more abundant and more destructive than ever in many regions of the globe. Destructive floods observed in the 1990s all over the world have led to record-high material damage, with total losses exceeding one billion US dollars in each of two dozen events. The immediate question emerges as to the extent to which a sensible rise in flood hazard and vulnerability can be linked to climate variability and change. Links between climate change and floods have found extensive coverage in the Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC). Since the material on floods is scattered over many places of two large volumes of the TAR, the present contribution - a guided tour to floods in the IPCC TAR – may help a reader notice the different angles from which floods were considered in the IPCC report. As the water-holding capacity of the atmosphere grows with temperature, the potential for intensive precipitation also increases. Higher and more intense precipitation has been already observed and this trend is expected to increase in the future, warmer world. This is a sufficient condition for flood hazard to increase. Yet there are also other, non-climatic, factors exacerbating flood hazard. According to the IPCC TAR, the analysis of extreme events in both observations and coupled models is underdeveloped. It is interesting that the perception of floods in different parts of the TAR is largely different. Large uncertainty is emphasized in the parts dealing with the science of climate change, but in the impact chapters, referring to sectors and regions, growth in flood risk is taken for granted. Floods have been identified on short lists of key regional concerns.     

Local flash floods in Central Europe: A case study of Poland

Bryndal, T. 2015. Local flash floods in Central Europe: A case study of Poland. Norsk Geografisk Tidsskrift–Norwegian Journal of Geography. Vol. 69, 288–298. ISSN 0029-1951.

The article focuses on the seasonality, geographical distribution, and hydrometeorological aspects of local flash floods in Poland, in Central Europe. Scientific articles describing local flash flood events were studied and a database of such floods in Poland was developed and analysed. The results revealed that local flash flood events were usually generated by rainfall lasting less than two hours, with a mean intensity in the range 20–80 mm·h^-1. The triggering threshold for local flash floods was P = 20 mm·h^-1. Local flash floods may occur from April to October, with higher frequencies in May, June, and July. The floods usually affect mountain and uplands areas differently to lowland and basins regions, and this might be explained by heavy rainfall distribution and topographical conditions. The maxima of the unit discharge (Qs) describe an envelope curve according to the equation Qs = 47A^-0.4 (where A is the catchment area in km²). The maxima appear very consistent with envelope curves proposed for other inland continental European countries. The authors conclude that local flash floods in Poland are similar to those recorded in inland continental areas of Europe, but differ from the floods in the Mediterranean part of the continent.     

River flow forecasting using wavelet and cross‐wavelet transform models

In this study, short‐term river flood forecasting models based on wavelet and cross‐wavelet constituent components were developed and evaluated for forecasting daily stream flows with lead times equal to 1, 3, and 7 days. These wavelet and cross‐wavelet models were compared with artificial neural network models and simple perseverance models. This was done using data from the Skrwa Prawa River watershed in Poland. Numerical analysis was performed on daily maximum stream flow data from the Parzen station and on meteorological data from the Plock weather station in Poland. Data from 1951 to 1979 was used to train the models while data from 1980 to 1983 was used to test the models. The study showed that forecasting models based on wavelet and cross‐wavelet constituent components can be used with great accuracy as a stand‐alone forecasting method for 1 and 3 days lead time river flood forecasting, assuming that there are no significant trends in the amplitude for the same Julian day year‐to‐year, and that there is a relatively stable phase shift between the flow and meteorological time series. It was also shown that forecasting models based on wavelet and cross‐wavelet constituent components for forecasting river floods are not accurate for longer lead time forecasting such as 7 days, with the artificial neural network models providing more accurate results. Copyright © 2008 John Wiley & Sons, Ltd.     

An assessment of engineered log jam structures in response to a flood event in an upland gravel‐bed river

Engineered log jams (ELJs) are employed to address river restoration goals and a range of river management problems including coarse sediment movement. In the Bowmont Water, a dynamic wandering gravel‐bed river in the Scottish Borders, 33 previously untested ELJs primarily designed to capture and store coarse sediment, were installed on a trial basis. Using repeated topographical surveys and field observations, the performance of the ELJs in response to a 5–10 year recurrence interval flood that occurred on the 25 September 2012 was evaluated at two reaches with catchment areas of 28 km² and 57 km². Three of the structures were damaged due to scour of surrounding material that exposed the pile anchors and all the timbers of one structure were completely displaced downstream. Sixteen structures induced geomorphic responses and only four induced significant deposition (>0.3 m) above that which would occur naturally within the adjacent active gravel bar deposition zones. The placement in gravel bars, minor channel blockage ratio created by the structures and their porous nature limited the hydraulic interference and in turn geomorphic responses. Therefore the ELJ placement goal of increasing sediment storage was not fully met. This study contributes to the empirical evidence base for ELJ performance evaluation of different designs in a range of physiographic settings needed to validate performance and refine design. Using these initial findings and knowledge gained from other studies, recommendations for improving the design and placement strategy are proposed. Copyright © 2016 John Wiley & Sons, Ltd.     

Comparison of fish populations in riffles of three braided rivers of Canterbury,New Zealand

Populations of small fish were sampled in 12–20 riffles of the lower reaches of 3 braided rivers in Canterbury, New Zealand, during periods of low, stable flows. In the Ashley, which has been least affected by floods in recent years, the standing stock of fish was severalfold higher than in the Hurunui and Rakaia Rivers, which experienced large floods over much of the time, particularly the Rakaia River. Mean abundance + 1 standard deviation (S.D.) of all species combined, amounted to 5.95 + 2.76 fish m ² for 10 species in the Ashley, 0.59 + 0.60 fish m ² for 7 species in the Hurunui, and 0.23 + 0.11 fish m ²for 6 species in the Rakaia. Mean biomass was 24.85 + 9.59, 2.11 + 1.19, and 2.50 ± 3.60 g m ²in the Ashley, Hurunui, and Rakaia Rivers, respectively. The more common species in the Ashley and Rakaia Rivers were torrentfish, longfinned eel, blue‐gilled bully, and upland bully. The same dominant species complex prevailed in the Hurunui River except that the common river galaxias was present instead of the bluegilled bully. The adverse effects of floods on fish, either directly, indirectly, or both, are considered to be a major limiting factor offish populations in braided rivers characterised by highly unstable flows and river beds, and a lack of suitable cover for fish.     

Characterization of subsurface fluxes at the plot scale during flash floods in the Valescure catchment,France

This study focuses on a 10-m² plot within a granitic hillslope in Cevennes mountainous area in France, in order to study infiltration and subsurface hydrological processes during heavy rainfalls and flash floods. The monitoring device included water content at several depths (0–70 cm for the shallow soil water; 0–10 m for the deep water) during both intense artificial and natural rainfall events, chemical and physical tracers, time-lapse electrical resistivity tomography. During the most intense events, the infiltrated water was estimated to be some hundreds of millimetres, which largely exceeds the topsoil capacity (≤40 cm deep in most of the cases). The weathered/fractured rock area below the soil clearly has an active role in the water storage and sub-surface flow dynamics. Vertical flow was dominant in the first 0–10 m, and lateral flow was effective at 8–10 m depth, at the top of the saturated area. The speed of the vertical flow was estimated between 1 and 10 m/hr, whereas it was estimated between 0.1 and 1 m/hr for the lateral flow. The interpretation of the experiments might lead to a local pattern of the 2D-hydrological processes and profile properties, which could be generic for most of the mountainous catchments under Mediterranean climate. It suggests that fast triggering of floods at the catchment scale cannot be explained by a mass transfer within the hillslope, but should be due to a pressure wave propagation through the bedrock fractures, which allows exfiltration of the water downstream the hillslope.     

唐宋暖期我国东部重大气象、水文灾害的时空分布特征

唐宋时期是我国气候中的重要暖期,开展该时期区域灾害时空格局研究,可为气候变暖的影响研究提供实证案例,为潜在极端气候风险防范提供依据。基于《中国古代重大自然灾害和异常年表总集》,以10年为时间分辨率,将气象、水文类灾种整合归类为冷害、热害、旱灾、水灾、其他灾害,并以省为基本单元统计唐宋时期我国东部地区(南北方)重大气象、水文灾害的发生频次,分析其时空分布特征。结果显示:唐宋暖期灾害可分为3个高发期和4个低发期,存在"较少发-高发-少发-较高发"循环的特点;南、北方自然灾害高发期和低发期并不同步;灾害发生具有空间集中性(多发于浙江、河南、陕西、山东、河北和江苏),南方多水灾、旱灾,北方多水灾、冷害。唐宋时期灾种构成与现今相似,重大水灾发生频次是旱灾的3～4倍。受经济发展地域范围和经济水平、灾害记录详略及灾害等级评判标准差异的影响,现代灾害发生频次增大,具有区域分布集中性,主要发生于华北、华中、华南,区域分布范围明显南扩。在气候变暖情况下,应重点防范水灾、旱灾和冷害。     

SOIL GEOMORPHOLOGY AS AN INTEGRATIVE APPROACH IN PHYSICAL GEOGRAPHY: A REVIEW OF FOUR RECENT BOOKS

Relict barrier beaches occur around the margins of many former pluvial lakes in the California desert. In common with modern barrier beaches along ocean coasts, these relict barriers possess geometric and compositional properties indicative of beach-forming processes, notably waves and currents. Because lake waves are a direct response to wind action, past wave climates may in turn be used to infer paleowind regimes. Where beaches of known age occur, wave climates and wind regimes may be invoked for specific time intervals. Using barrier beaches from former Lake Mojave and Lake Manly, this paper explores the extent to which the above theory may be applied to invoke past processes and wind regimes. The data indicate beach emplacement mainly by wave-related accretion and overwash generated by winds similar in direction to those of today, doubtless influenced then as now by topographic corridors. However, textural properties, predicted wave heights and entrainment velocities indicate that sustained southerly winds were probably twice as strong (by a factor ranging from 1.2 to 2.2 or more) and probably more persistent than today, at least towards the close of pluvial conditions. [Key words: barrier beaches, wave climate, paleoclimate, paleowinds, pluvial lakes, Quaternary, California desert.]