Increase In Flood Magnitude Research Articles

An increase in the spatial extent of European floods over the last 70 years

Abstract. Floods regularly cause substantial damage worldwide. Changing flood characteristics, e.g., due to climate change, pose challenges to flood risk management. The spatial extent of floods is an important indicator of potential impacts, as consequences of widespread floods are particularly difficult to mitigate. The highly uneven station distribution in space and time, however, limits the ability to quantify flood characteristics and, in particular, changes in flood extents over large regions. Here, we use observation-driven routed runoff simulations over the last 70 years in Europe from a state-of-the-art hydrological model (the mesoscale Hydrologic Model – mHM) to identify large spatiotemporally connected flood events. Our identified spatiotemporal flood events compare well against an independent flood impact database. We find that flood extents increase by 11.3 % on average across Europe. This increase occurs over most of Europe, except for parts of eastern and southwestern Europe. Over northern Europe, the increase in flood extent is mainly driven by the overall increase in flood magnitude caused by increasing precipitation and snowmelt. In contrast, the increasing trend in flood extent over central Europe can be attributed to an increase in the spatial extent of heavy precipitation. Overall, our study illustrates the opportunities to combine long-term consistent regional runoff simulations with a spatiotemporal flood detection algorithm to identify large-scale trends in key flood characteristics and their drivers. The detected change in flood extent should be considered in risk assessments as it may challenge flood control and water resource management.

Flood risk zoning of cascade reservoir dam break based on a 1D-2D coupled hydrodynamic model: A case study on the Jinsha-Yalong River

Large-scale earthquakes and huge landslide surges may cause the failure of cascade reservoirs, resulting in massive downstream flooding. The flood risk zoning of cascade reservoir dam breaks is crucial for local governments to formulate emergency plans and for individuals to garner their awareness of the flood risks. This paper presents a flood risk zoning approach based on a one-dimensional and two-dimensional coupled hydrodynamic model. The flooding area simulated by the coupled hydrodynamic model was divided into a Deadly Zone (D-zone) and an Escape Zone based on whether there is sufficient time for the public to evacuate the danger area under a specific evacuation generation time (EGT). A case study was conducted on the Jinsha-Yalong River (JYR), a confluence river located in Panzhihua City, to map the risk zones. This study considered a dam-break flood in Yalong River encountering two river flood scenarios in Jinsha River (scenario A: the annual average flow; scenario B: a 1000-year return period flood). Two evacuation scenarios (EGT of 30 and 60 min) were also considered; thereafter, four combined evacuation scenarios (A-EGT30, A-EGT60, B-EGT30, and B-EGT60) were simulated. The coupled hydrodynamic model is demonstrated to be effective through the analysis of water balance and hydrographs of special section discharge. The results show that: (1) in comparison to scenario A, scenario B reached its maximum flood extent approximately 5 min earlier, signifying that flood spread speed increases with the increase in flood magnitude; (2) the D-zone area was 127.7 % larger in scenario A-EGT60 compared to A-EGT30 and 120.8 % larger in scenario B-EGT60 compared to B-EGT30. These findings underscore the importance of flood scenarios and EGT on flood risk zones. The proposed risk zoning method can help government agencies in the JYR and major river regions worldwide with flood management and emergency strategy development.

Environmental DNA based biomonitoring for hatchery-raised fish in riverine habitats before and after recordable flood event

It is reported that the magnitudes of flood events in riverine systems have been increasing due to global climate change. Because flood events could displace fish species downstream and/or increase the mortality of fish, it is important to know how the increased floods can affect fish in river networks. In this study, we focused a hatchery-raised amphidromous fish, Ayu Plecoglossus altivelis altivelis in a mainstem-tributary network in Japan and examined the relative fish abundance changes before and after the recordable massive flood using the environmental DNA (eDNA) analysis. We also examined the spatiotemporal patterns of the Ayu eDNA concentrations and the relationships with tributary discharge, width, and depth to examine if the relative abundance changes could be related to tributary size. Our results indicated that Ayu tended to inhabit large, deep tributaries more than small, shallow tributaries even after the flood event. Further, the eDNA concentrations of Ayu have decreased in most study sites after the flood; however, the eDNA concentrations in certain tributaries with lower tributary size have increased during the study period. These results suggest that (1) the habitat conditions could be important for the maintenance of Ayu populations before and after flood events, (2) increase in flood magnitude along climate changes could have impacts on Ayu populations, and (3) not only large tributary, but also small tributaries could be important habitats for the target species to avoid flood disturbances.

Amplified Extreme Floods and Shifting Flood Mechanisms in the Delaware River Basin in Future Climates

AbstractHistorical records in the Delaware River Basin reveal complex and spatially diverse flood generating mechanisms influenced by the region's mountains‐to‐plains gradients. This study focuses on predicting future flood hazards and understanding the underlying drivers of changes across the region. Using a process‐based hydrological model, we analyzed the hydrometeorological condition of each historical and future flood event. For each event, at the subbasin scale, we identified the dominant flood generating mechanism, including snowmelt, rain‐on‐snow, short‐duration rain, and long‐duration rain. The rain‐induced floods are further categorized based on the soil's Antecedent Moisture Condition (AMC) before the event, whether dry, normal, or wet. Our historical analysis suggests that rain‐on‐snow is the primary flood mechanism of the Upper Basin. Although most frequent, the magnitude of rain‐on‐snow floods is often less severe than short rain floods. In contrast, historical floods in the Lower Basin are primarily caused by short rain under normal AMC. Given the uncertainties in climate projections, we used an ensemble of future climate scenarios for flood projections. Despite variations in regional climate projections, coherent perspectives emerge: the region will shift toward a warmer, wetter climate, with a projected intensification of extreme floods. The Upper Basin is projected to experience a marked decrease in rain‐on‐snow floods, but a substantial increase in short rain floods with wet AMC. The largest increase in flood magnitude will be driven by short rains with wet AMC in the Upper Basin and by short rains with normal AMC in the Lower Basin.

Spatial and Temporal Characteristics of Channel Disturbance Zones in Big River, Southeast Missouri (1937–2018)

abstract: Ozarks watersheds have responded to land clearing and settlement disturbances by transporting large amounts of gravel and sediment to main valleys of medium-large river systems. Historical gravel and sediment accumulate in disturbance zones, which are areas of excessive channel activity that can be detected over time using historical aerial photographs. This project uses a series of aerial photos from 1937–2018 to identify disturbance zones in the Big River of southeast Missouri, which has a history of lead mining and ore processing that has caused widespread contamination of the channel and floodplain deposits. Variations in active areas of the historical meander belt show that disturbance zones account for 24 percent of the channel length at an interval of one per 2.5 km of channel. Megabars and extensions are the most prominent types of disturbance zones, but translations and cutoffs are larger with higher potential for sediment storage. From 1937 to 1976, areas of channel activity have cycled between expansion and contraction. However, after a period of recovery prior to 1976, disturbance zone areas have been expanding over the last 40 years likely in response to an increase in flood magnitude and frequency from more high intensity rainfall events since 1973.

Spatial and Temporal Characteristics of Channel Disturbance Zones in Big River, Southeast Missouri (1937-2018)

Ozarks watersheds have responded to land clearing and settlement disturbances by transporting large amounts of gravel and sediment to main valleys of medium-large river systems. Historical gravel and sediment accumulate in disturbance zones, which are areas of excessive channel activity that can be detected over time using historical aerial photographs. This project uses a series of aerial photos from 1937-2018 to identify disturbance zones in the Big River of southeast Missouri, which has a history of lead mining and ore processing that has caused widespread contamination of the channel and floodplain deposits. Variations in active areas of the historical meander belt show that disturbance zones account for 24 percent of the channel length at an interval of one per 2.5 km of channel. Megabars and extensions are the most prominent types of disturbance zones, but translations and cutoffs are larger with higher potential for sediment storage. From 1937 to 1976, areas of channel activity have cycled between expansion and contraction. However, after a period of recovery prior to 1976, disturbance zone areas have been expanding over the last 40 years likely in response to an increase in flood magnitude and frequency from more high intensity rainfall events since 1973.

Changes in seasonal compound floods in Vietnam revealed by a time-varying dependence structure of extreme rainfall and high surge

Compound floods due to intense rainfall and storm surges in coastal areas have shown an increasing trend in some parts of the world, and many studies suggested a strong link with climate change. Yet, such link has not been fully explored and quantitively assessed. In this paper, we demonstrate the development and application of a nonstationary framework to determining different compound scenarios, where individual drivers and their interactions have altered under climate change. The framework has been applied to one of the most flood-prone areas: the Ho Chi Minh City of Vietnam, to help analyze the present and future compound flood risks in both the dry and wet seasons driven by the joint effect from heavy inland rainfall and high skew surge. Over the period of 1980–2017, the two drivers are found to be significantly correlated in March and April, corresponding to the transition from dry-to-wet seasons. We also find that the commonly-used traditional multivariate statistical models underestimate the flood magnitudes for both the current (represented by 2020) and future (represented by 2050) scenarios, when compared with the results produced by the nonstationary methods. In addition, the results reveal that the dry season is expected to receive more floods triggered by the increased intensity and frequency of rainfall extremes, with the magnitude reaching a similar level to that of the wet season. This is in line with the climate projections under RCP4.5 and 8.5 scenarios although the duration of dry spells is expected to increase and the total annual rainfall to decrease in Vietnam. The simulated flood inundations indicate remarkable increases in flood magnitude and extension, especially at the locations identified as low risk by the stationary models.

Diverging projections for flood and rainfall frequency curves

Flood damages are projected to increase with climate change due to intensification of extreme rainfalls, with water scarcity also projected to increase due to longer periods of drought leading to less runoff from flood events. In order to adapt to climate change, precise and robust projections of flood events are required of a magnitude (or rarity) relevant to engineering design and water resources planning. However, due to the complexity of catchment-specific processes which influence flood response, many studies disagree on the direction and magnitude of the projected change: some studies project increases in flood magnitude, even above projected increases in rainfall maxima, while other studies project decreases. Using a combination of continentally downscaled rainfall projections for Australia and rainfall-runoff models locally calibrated to rare floods using a novel objective function, we show that projected changes in flooding do not follow projected changes in extreme rainfall, with changes in flooding depending on region, runoff mechanism, and event severity. Decreases in frequent flooding up to the 1 in 5 annual exceedance probability are projected across the continent, with increases only projected for frequent flooding in some tropical regions, diverging from the projected increases in rainfall maxima across a majority of the continent. For rare events of annual exceedance probability 1 in 50, the flood magnitude is projected to increase across the northern and eastern coasts by the end of the century – commensurate with the median projected increase of 20% in rainfall maxima. However, decreases in both rainfall maxima and flooding are still projected for south-western Australia, even for the rarest events. The regional coherence of our results suggests the patterns in the projected changes may be applicable to other tropical, temperate, and arid regions around the world.

Climate impact emergence and flood peak synchronization projections in the Ganges, Brahmaputra and Meghna basins under CMIP5 and CMIP6 scenarios

The densely populated delta of the three river systems of the Ganges, Brahmaputra and Meghna is highly prone to floods. Potential climate change-related increases in flood intensity are therefore of major societal concern as more than 40 million people live in flood-prone areas in downstream Bangladesh. Here we report on new flood projections using a hydrological model forced by bias-adjusted ensembles of the latest-generation global climate models of CMIP6 (SSP5-8.5/SSP1-2.6) in comparison to CMIP5 (RCP8.5/RCP2.6). Results suggest increases in peak flow magnitude of 36% (16%) on average under SSP5-8.5 (SSP1-2.6), compared to 60% (17%) under RCP8.5 (RCP2.6) by 2070–2099 relative to 1971–2000. Under RCP8.5/SSP5-8.5 (2070–2099), the largest increase in flood risk is projected for the Ganges watershed, where higher flood peaks become the ‘new norm’ as early as mid-2030 implying a relatively short time window for adaptation. In the Brahmaputra and Meghna rivers, the climate impact signal on peak flow emerges after 2070 (CMIP5 and CMIP6 projections). Flood peak synchronization, when annual peak flow occurs simultaneously at (at least) two rivers leading to large flooding events within Bangladesh, show a consistent increase under both projections. While the variability across the ensemble remains high, the increases in flood magnitude are robust in the study basins. Our findings emphasize the need of stringent climate mitigation policies to reduce the climate change impact on peak flows (as presented using SSP1-2.6/RCP2.6) and to subsequently minimize adverse socioeconomic impacts and adaptation costs. Considering Bangladesh’s high overall vulnerability to climate change and its downstream location, synergies between climate change adaptation and mitigation and transboundary cooperation will need to be strengthened to improve overall climate resilience and achieve sustainable development.

An extremeness threshold determines the regional response of floods to changes in rainfall extremes

Precipitation extremes will increase in a warming climate, but the response of flood magnitudes to heavier precipitation events is less clear. Historically, there is little evidence for systematic increases in flood magnitude despite observed increases in precipitation extremes. Here we investigate how flood magnitudes change in response to warming, using a large initial-condition ensemble of simulations with a single climate model, coupled to a hydrological model. The model chain was applied to historical (1961–2000) and warmer future (2060–2099) climate conditions for 78 watersheds in hydrological Bavaria, a region comprising the headwater catchments of the Inn, Danube and Main River, thus representing an area of expressed hydrological heterogeneity. For the majority of the catchments, we identify a ‘return interval threshold’ in the relationship between precipitation and flood increases: at return intervals above this threshold, further increases in extreme precipitation frequency and magnitude clearly yield increased flood magnitudes; below the threshold, flood magnitude is modulated by land surface processes. We suggest that this threshold behaviour can reconcile climatological and hydrological perspectives on changing flood risk in a warming climate.

Ubiquitous increases in flood magnitude in the Columbia River basin under climate change

Abstract. The USA and Canada have entered negotiations to modernize the Columbia River Treaty, signed in 1961. Key priorities are balancing flood risk and hydropower production, and improving aquatic ecosystem function while incorporating projected effects of climate change. In support of the US effort, Chegwidden et al. (2017) developed a large-ensemble dataset of past and future daily streamflows at 396 sites throughout the Columbia River basin (CRB) and selected other watersheds in western Washington and Oregon, using state-of-the art climate and hydrologic models. In this study, we use that dataset to present new analyses of the effects of future climate change on flooding using water year maximum daily streamflows. For each simulation, flood statistics are estimated from generalized extreme value distributions fit to simulated water year maximum daily streamflows for 50-year windows of the past (1950–1999) and future (2050–2099) periods. Our results contrast with previous findings: we find that the vast majority of locations in the CRB are estimated to experience an increase in future streamflow magnitudes. The near ubiquity of increases is all the more remarkable in that our approach explores a larger set of methodological variation than previous studies; however, like previous studies, our modeling system was not calibrated to minimize error in maximum daily streamflow and may be affected by unquantifiable errors. We show that on the Columbia and Willamette rivers increases in streamflow magnitudes are smallest downstream and grow larger moving upstream. For the Snake River, however, the pattern is reversed, with increases in streamflow magnitudes growing larger moving downstream to the confluence with the Salmon River tributary and then abruptly dropping. We decompose the variation in results attributable to variability in climate and hydrologic factors across the ensemble, finding that climate contributes more variation in larger basins, while hydrology contributes more in smaller basins. Equally important for practical applications like flood control rule curves, the seasonal timing of flooding shifts dramatically on some rivers (e.g., on the Snake, 20th-century floods occur exclusively in late spring, but by the end of the 21st century some floods occur as early as December) and not at all on others (e.g., the Willamette River).

Note on a modified return period scale for upper-truncated unbounded flood distributions

Probability distributions unbounded to the right often give good fits to annual discharge maxima. However, all hydrological processes are in reality constrained by physical upper limits, though not necessarily well defined. A result of this contradiction is that for sufficiently small exceedance probabilities the unbounded distributions anticipate flood magnitudes which are impossibly large. This raises the question of whether displayed return period scales should, as is current practice, have some given number of years, such as 500years, as the terminating rightmost tick-point. This carries the implication that the scale might be extended indefinitely to the right with a corresponding indefinite increase in flood magnitude. An alternative, suggested here, is to introduce a sufficiently high upper truncation point to the flood distribution and modify the return period scale accordingly. The rightmost tick-mark then becomes infinity, corresponding to the upper truncation point discharge. The truncation point is likely to be set as being above any physical upper bound and the return period scale will change only slightly over all practical return periods of operational interest. The rightmost infinity tick point is therefore proposed, not as an operational measure, but rather to signal in flood plots that the return period scale does not extend indefinitely to the right.

Changes of extreme drought and flood events in Iran

Located in an arid and semi-arid region of the world, Iran has experienced many extreme flood and drought events in the last and current century. The present study aims to assess the changes in Iran's flood magnitude and drought severity for 1950–2010, with some time span variation in some stations. The Mann-Kendall test for monotonic trend was first applied to assess changes in flood and drought severity data. In addition, to consider the effect of serial correlation, two Pre-Whitening Trend (PWT) tests were also applied. It was observed that the number of stations with statistically significant trends has increased after applying PWT tests. Both increasing and decreasing trends were observed for drought severity and flood magnitude in different climate regions and major basins of Iran using these tests. The increase in flood magnitude and drought severity can be attributed partly to land use changes, an annual rainfall negative trend, a maximum rainfall increasing trend, and inappropriate water resources management policies. The paper indicates a critical situation related to extreme climate change in Iran and the increasing risk of environmental changes in the 21st century.

Geomorphic effects, flood power, and channel competence of a catastrophic flood in confined and unconfined reaches of the upper Lockyer valley, southeast Queensland, Australia

Flooding is a persistent natural hazard, and even modest changes in future climate are believed to lead to large increases in flood magnitude. Previous studies of extreme floods have reported a range of geomorphic responses from negligible change to catastrophic channel change. This paper provides an assessment of the geomorphic effects of a rare, high magnitude event that occurred in the Lockyer valley, southeast Queensland in January 2011. The average return interval of the resulting flood was ~2000years in the upper catchment and decreased to ~30years downstream. A multitemporal LiDAR-derived DEM of Difference (DoD) is used to quantify morphological change in two study reaches with contrasting valley settings (confined and unconfined). Differences in geomorphic response between reaches are examined in the context of changes in flood power, channel competence and degree of valley confinement using a combination of one-dimensional (1-D) and two-dimensional (2-D) hydraulic modelling. Flood power peaked at 9800Wm−2 along the confined reach and was 2–3 times lower along the unconfined reach. Results from the DoD confirm that the confined reach was net erosional, exporting ~287,000m3 of sediment whilst the unconfined reach was net depositional gaining ~209,000m3 of sediment, 70% of the amount exported from the upstream, confined reach. The major sources of eroded sediment in the confined reach were within channel benches and macrochannel banks resulting in a significant increase of channel width. In the unconfined reach, the benches and floodplains were the major loci for deposition, whilst the inner channel exhibited minor width increases. The presence of high stream power values, and resultant high erosion rates, within the confined reach is a function of the higher energy gradient of the steeper channel that is associated with knickpoint development. Dramatic differences in geomorphic responses were observed between the two adjacent reaches of contrasting valley configuration. The confined reach experienced large-scale erosion and reorganisation of the channel morphology that resulted in significantly different areal representations of the five geomorphic features classified in this study.

HYDROLOGIC CONNECTIVITY OF FLOODPLAINS, NORTHERN MISSOURI-IMPLICATIONS FOR MANAGEMENT AND RESTORATION OF FLOODPLAIN FOREST COMMUNITIES IN DISTURBED LANDSCAPES

Hydrologic connectivity between the channel and floodplain is thought to be a dominant factor determining floodplain processes and characteristics of floodplain forests. We explored the role of hydrologic connectivity in explaining floodplain forest community composition along streams in northern Missouri, USA. Hydrologic analyses at 20 streamgages (207–5827 km2 area) document that magnitudes of 2-year return floods increase systematically with increasing drainage area whereas the average annual number and durations of floodplain-connecting events decrease. Flow durations above the active-channel shelf vary little with increasing drainage area, indicating that the active-channel shelf is in quasi-equilibrium with prevailing conditions. The downstream decrease in connectivity is associated with downstream increase in channel incision. These relations at streamflow gaging stations are consistent with regional channel disturbance patterns: channel incision increases downstream, whereas upstream reaches have either not incised or adjusted to incision by forming new equilibrium floodplains. These results provide a framework to explain landscape-scale variations in composition of floodplain forest communities in northern Missouri. Faust (2006) had tentatively explained increases of flood-dependent tree species, and decreases of species diversity, with a downstream increase in flood magnitude and duration. Because frequency and duration of floodplain-connecting events do not increase downstream, we hypothesize instead that increases in relative abundance of flood-dependent trees at larger drainage area result from increasing size of disturbance patches. Bank-overtopping floods at larger drainage area create large, open, depositional landforms that promoted the regeneration of shade-intolerant species. Higher tree species diversity in floodplains with small drainage areas is associated with non-incised floodplains that are frequently connected to their channels and therefore subject to greater effective hydrologic variability compared with downstream floodplains. Understanding the landscape-scale geomorphic and hydrologic controls on floodplain connectivity provides a basis for more effective management and restoration of floodplain forest communities. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.

Potential increase in floods in California’s Sierra Nevada under future climate projections

California’s mountainous topography, exposure to occasional heavily moisture-laden storm systems, and varied communities and infrastructures in low lying areas make it highly vulnerable to floods. An important question facing the state—in terms of protecting the public and formulating water management responses to climate change—is “how might future climate changes affect flood characteristics in California?” To help address this, we simulate floods on the western slopes of the Sierra Nevada Mountains, the state’s primary catchment, based on downscaled daily precipitation and temperature projections from three General Circulation Models (GCMs). These climate projections are fed into the Variable Infiltration Capacity (VIC) hydrologic model, and the VIC-simulated streamflows and hydrologic conditions, from historical and from projected climate change runs, allow us to evaluate possible changes in annual maximum 3-day flood magnitudes and frequencies of floods. By the end of the 21st Century, all projections yield larger-than-historical floods, for both the Northern Sierra Nevada (NSN) and for the Southern Sierra Nevada (SSN). The increases in flood magnitude are statistically significant (at p <= 0.01) for all the three GCMs in the period 2051–2099. The frequency of flood events above selected historical thresholds also increases under projections from CNRM CM3 and NCAR PCM1 climate models, while under the third scenario, GFDL CM2.1, frequencies remain constant or decline slightly, owing to an overall drying trend. These increases appear to derive jointly from increases in heavy precipitation amount, storm frequencies, and days with more precipitation falling as rain and less as snow. Increases in antecedent winter soil moisture also play a role in some areas. Thus, a complex, as-yet unpredictable interplay of several different climatic influences threatens to cause increased flood hazards in California’s complex western Sierra landscapes.

Assessment of climate change impact on floods using weather generator and continuous rainfall‐runoff model

AbstractOne of the potential impacts of climate change (CC) is the change on flood frequency and magnitude. Assessment of impact on floods involves many uncertainties. Difficulty in preparing reliable continuous time series of climate variables with fine time step and capturing extreme rainfalls for future climate scenarios has led to limited studies aimed at investigation of the change of flood regime due to CC. Weather Generators (WGs) have the potential for downscaling of GCMs (General Climate Models) outputs. However, WGs inabilities in relation to producing extreme rainfalls and low‐frequency variabilities have caused WGs to receive limited attention in flood studies. In this study, a new WG model, that removes the aforementioned WGs' inabilities, is coupled with a continuous daily rainfall‐runoff model to assess the CC impacts on floods of a basin in Iran. Changes in different characteristics of climate variables are applied in the downscaling procedure. Results indicated that, aside from the large uncertainty of emission scenarios, the climate change will lead to considerable increase in flood magnitude in the study basin. Copyright © 2011 Royal Meteorological Society

A model-based assessment of the effects of projected climate change on the water resources of Jordan

This paper is concerned with the quantification of the likely effect of anthropogenic climate change on the water resources of Jordan by the end of the twenty-first century. Specifically, a suite of hydrological models are used in conjunction with modelled outcomes from a regional climate model, HadRM3, and a weather generator to determine how future flows in the upper River Jordan and in the Wadi Faynan may change. The results indicate that groundwater will play an important role in the water security of the country as irrigation demands increase. Given future projections of reduced winter rainfall and increased near-surface air temperatures, the already low groundwater recharge will decrease further. Interestingly, the modelled discharge at the Wadi Faynan indicates that extreme flood flows will increase in magnitude, despite a decrease in the mean annual rainfall. Simulations projected no increase in flood magnitude in the upper River Jordan. Discussion focuses on the utility of the modelling framework, the problems of making quantitative forecasts and the implications of reduced water availability in Jordan.

Depositional character and preservation potential of coarse-grained sediments deposited by flood events in hyper-arid braided channels in the Rift Valley, Arava, Israel

Four coarse-grained braided channels undergoing rapid deposition in the hyper-arid Arava Rift Valley have been studied to determine the dynamics of strata deposition by monitored flood events and to ascertain the sedimentologic character of these event deposits. The systems span granule-sand to sandy-gravel bed material having longitudinal bars with simultaneously increasing grains size, slope and bed relief. Scour and fill were monitored along cross-sections using scour chains and repeat surveys, while event strata (=fill) were described with reference to sediment calibre, type of support, grading, thickness and location. Unlike many previous investigations of gravel sedimentology, the source of information for this study involves not only stratigraphic description but also observation of the concomitant dynamics of the channel bed as expressed by the extent of event deposition. The texture of the deposited sediment generally coarsens in sympathy with the surface bed texture of the studied channels. The thickness of event-deposited sediments increases as the magnitude of flow events increases. The stratigraphic record is characterized by: (1) sub-parallel stratification, (2) no grading, (3) clast-supported gravel infilled with sand-granule matrices and (4) rapid sedimentation rates. This character is hypothesized to mirror the elevated bedload fluxes in this hyper-arid fluvial setting. Lacking densely-vegetated banks, the proximal braidplain of this arid environment often contains no extensive fine-textured floodplain successions. The depth of fill as well as the extent of strata obliteration increase with flood magnitude, explaining the increased preservation potential of event strata with increase in flood magnitude. It also explains why most event strata, including large ones, are only partially preserved.

Scaling and regionalization of flood flows in British Columbia, Canada

AbstractA regionalization of flood data in British Columbia reveals a common scaling with drainage area over the range 0·5×102&lt;Ad&lt;104 km2. This scaling is not a function of flood return period, which implies that simple scaling—consistent with a snowmelt‐dominated flow regime—applies to the province. The observed scale relation takes the form $Q \propto A_{d}^{0{\cdot}75}$, similar to values reported in previous studies. The scaling relation identified was used to define the regional pattern of hydroclimatic variability for flood flows in British Columbia after discounting the effect of drainage area. The pattern was determined by kriging a scale‐independent runoff factor k for the mean annual flood, 5 year flood and 20 year flood. The analysis permits quantification of uncertainty of the estimates, which can be used in conjunction with the mapped k‐fields to calculate a mean and range for floods with the identified return period for ungauged basins. Owing to the sparsity of data, the precision is relatively poor. The standard error is generally less than 75% of the estimate in the southern half of the province, whereas in the northern half it is often between 75 and 100%. Examination of the relative increase in flood magnitude with increasing return period reveals spatially consistent but statistically insignificant differences. Flood magnitude tends to increase more rapidly in the western regions, where rain events may contribute to flood generation. The relative increase in flood magnitude with return period is consistently lower in the eastern mountain ranges, where snowmelt dominates the flood flow regime. Copyright © 2002 John Wiley &amp; Sons, Ltd.