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Recurrence Interval (recurrence + interval)
Selected AbstractsRainfall thresholds for shallow landsliding derived from pressure-head monitoring: cases with permeable and impermeable bedrocks in Boso Peninsula, JapanEARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2007Yuki Matsushi Abstract Rainfall thresholds for shallow landslide initiation were determined for hillslopes with two types of bedrock, permeable sandstone and impermeable mudstone, in the Boso Peninsula, Japan. The pressure-head response to rainfall was monitored above a slip scarp due to earlier landslides. Multiple regression analysis estimated the rainfall thresholds for landsliding from the relation between the magnitude of the rainfall event and slope instability caused by the increased pressure heads. The thresholds were expressed as critical combinations of rainfall intensity and duration, incorporating the geotechnical properties of the hillslope materials and also the slope hydrological processes. The permeable sandstone hillslope has a greater critical rainfall and hence a longer recurrence interval than the impermeable mudstone hillslope. This implies a lower potential for landsliding in sandstone hillslopes, corresponding to lower landslide activity. Copyright © 2007 John Wiley & Sons, Ltd. [source] Flume experiments on the horizontal stream offset by strike-slip faultsEARTH SURFACE PROCESSES AND LANDFORMS, Issue 2 2004Shunji OuchiArticle first published online: 4 FEB 200 Abstract Flume experiments, in which the middle section of an erosion channel is displaced horizontally, have been conducted to assess the response of streams to horizontal displacement by a strike-slip fault. The experimental erosion channel was developed in a mixture of sand and clay, which provided relatively stable banks with its cohesiveness. Horizontal displacement of a strike-slip fault perpendicular to the channel is expected to add a ,at section to its longitudinal pro,le along the fault line. The experimental stream eliminated this ,at section with downstream degradation, upstream aggradation, and lateral channel shift. As a result, a roughly continuous longitudinal pro,le was maintained. This maintenance of a continuous longitudinal pro,le along channel is considered to be the principle of stream response to horizontal displacement by a strike-slip fault. Downstream degradation was the dominant process of this stream response in the overall tendency of erosion without sand supply. When the rate of fault displacement was low (long recurrence interval), the experimental stream eroded the fault surface, jutting laterally into the channel like a scarp, and de,ected the channel within the recurrence interval. This lateral channel shift gave some gradient to the reach created by fault displacement (offset reach), and the downstream degradation occurred as much as completing the remaining longitudinal pro,le adjustment. When the rate of fault displacement was high (short recurrence interval), the lateral erosion on the ,rst fault surface was interrupted by the next fault displacement. The displacement was then added incrementally to the existing channel offset making channel shift by lateral erosion increasingly dif,cult. The channel offset with sharp bends persisted without much modi,cation, and downstream degradation and upstream aggradation became evident with the effect of the offset channel course, which worked like a dam. In this case, a slight local convexity, which was incidentally formed by downstream degradation and upstream aggradation, tended to remain in the roughly continuous longitudinal pro,le, as long as the horizontal channel offset persisted. In either case, once the experimental stream obtained a roughly continuous gradient, further channel adjustment seemed to halt. Horizontal channel offset remained to a greater or lesser extent at the end of each run long after the last fault displacement. Copyright © 2004 John Wiley & Sons, Ltd. [source] Initial hydrologic and geomorphic response following a wildfire in the Colorado Front RangeEARTH SURFACE PROCESSES AND LANDFORMS, Issue 10 2001John A. Moody Abstract A wildfire in May 1996 burned 4690 hectares in two watersheds forested by ponderosa pine and Douglas fir in a steep, mountainous landscape with a summer, convective thunderstorm precipitation regime. The wildfire lowered the erosion threshold in the watersheds, and consequently amplified the subsequent erosional response to shorter time interval episodic rainfall and created both erosional and depositional features in a complex pattern throughout the watersheds. The initial response during the first four years was an increase in runoff and erosion rates followed by decreases toward pre-fire rates. The maximum unit-area peak discharge was 24 m3 s,1 km,2 for a rainstorm in 1996 with a rain intensity of 90 mm h,1. Recovery to pre-fire conditions seems to have occurred by 2000 because for a maximum 30-min rainfall intensity of 50 mm h,1, the unit-area peak discharge in 1997 was 6.6 m3 s,1 km,2, while in 2000 a similar intensity produced only 0.11 m3 s,1 km,2. Rill erosion accounted for 6 per cent, interrill erosion for 14 per cent, and drainage erosion for 80 per cent of the initial erosion in 1996. This represents about a 200-fold increase in erosion rates on hillslopes which had a recovery or relaxation time of about three years. About 67 per cent of the initially eroded sediment is still stored in the watersheds after four years with an estimated residence time greater than 300 years. This residence time is much greater than the fire recurrence interval so erosional and depositional features may become legacies from the wildfire and may affect landscape evolution by acting as a new set of initial conditions for subsequent wildfire and flood sequences. Published in 2001 by John Wiley & Sons, Ltd. [source] Net spinning caddisflies as stream ecosystem engineers: the influence of Hydropsyche on benthic substrate stabilityFUNCTIONAL ECOLOGY, Issue 3 2004B. J. CARDINALE Summary 1Organisms that physically modify or create habitat (ecosystem engineers) can have a profound influence on community and ecosystem dynamics. 2Here evidence is presented that one of the most abundant and widely distributed lotic insects could act as an ecosystem engineer in streams by increasing the stability of benthic substrates during flooding. 3Natural densities of larval net spinning caddisflies (Hydropsychidae) were established in stream channels that had standardized physical properties. The mobility of three particle sizes were measured during simulated flooding and the fraction of particles eroded compared with that of control streams. 4Larvae increased the initial velocity required to erode sediments by 10,30%. At velocities sufficient to scour 87% of particles from control channels, 57,100% remained stable in channels colonized by larvae. 5Assuming larvae have similar effects in natural streams, caddisflies could be expected to increase the recurrence interval of a substrate scouring flood from 1·67 year to 2·41 year, corresponding to a 17% decrease in the probability of bed scour per year. 6Our study suggests these insects could play an important role in generating the spatial ,refuges' that moderate the resistance of lotic communities to flooding. It is argued that, as has occurred in marine systems, a better understanding of how freshwater organisms engineer their physical environment has much potential to complement our historical focus on the abiotic forces that constrain populations and communities. [source] Enigma variations: the stratigraphy, provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group, south Pembrokeshire, WalesGEOLOGICAL JOURNAL, Issue 5 2006Richard G. Thomas Abstract The Lower Devonian (Lochkovian-Emsian) Cosheston Group of south Pembrokeshire is one of the most enigmatic units of the Old Red Sandstone of Wales. It consists of a predominantly green, exceptionally thick succession (up to 1.8,km) within the red c. 3,km-thick fill of the Anglo-Welsh Basin, but occupies a very small area (27,km2). Four formations,Llanstadwell (LLF), Mill Bay (MBF), Lawrenny Cliff (LCF) and New Shipping (NSF),group into lower (LLF,+,MBF) and upper (LCF,+,NSF) units on stratigraphical and sedimentological criteria. Two palynostratigraphic associations (Hobbs Point and Burton Cliff) are recognised in the LLF. Overall, the Cosheston succession comprises a fluvial, coarsening-upward megasequence, mostly arranged in fining-upward rhythms. It is interpreted as the fill of an east-west graben bounded by faults to the north and south of the Benton and Ritec faults, respectively. Both ,lower Cosheston' formations were deposited by east-flowing, axial river systems draining a southern Irish Sea landmass. Drainage reversal, early in the deposition of the LCF, resulted in ,upper Cosheston' lateral, SW-flowing rivers which carried predominantly second- and multi-cycle detritus. The ,lower Cosheston' is characterized by an abundance of soft-sediment deformation structures, probably seismically triggered by movements along the graben's northern bounding fault. A minimum average (, mesoseismic) earthquake recurrence interval of c. 4000,yr is estimated for the MBF. This and the correlative Senni Formation of south-central Wales form a regionally extensive green-bed development that represents a pluvial climatic interval. Copyright © 2006 John Wiley & Sons, Ltd. [source] Low and high flow analyses and wavelet application for characterization of the Blue Nile River systemHYDROLOGICAL PROCESSES, Issue 3 2010Assefa Melesse Abstract The low and high flow characteristic of the Blue Nile River (BNR) basin is presented. The study discusses low and high flow, flow duration curve (FDC) and trend analysis of the BNR and its major tributaries. Different probability density functions were fitted to better describe the low and high flows of the BNR and major tributaries in the basin. Wavelet analysis was used in understanding the variance and frequency-time localization and detection of dominant oscillations in rainfall and flow. FDCs were developed, and low flow (below 50% exceedance) and high flow (over 75% exceedance) of the curves were analysed and compared. The Gravity Recovery and Climate Experiment (GRACE) satellite-based maps of monthly changes in gravity converted to water equivalents from 2003 to 2006 for February, May and September showed an increase in the moisture influx in the BNR basin for the month of September, and loss of moisture in February and May. It was also shown that 2004 and 2005 were drier with less moisture influx compared to 2003 and 2006. On the basis of the Kolmogorov-Smirnov, Anderson-Darling and Chi-square tests, Gen. Pareto, Frechet 3P, Log-normal, Log-logistics, Fatigue Life and Phased Bi-Weibull distributions best describe the low and high flows within the BNR basin. This will be beneficial in developing flow hydrographs for similar ungauged watersheds within the BNR basin. The below 50% and above 75% exceedance on the FDC for five major rivers in addition to the BNR showed different characteristics depending on size, land cover, topography and other factors. The low flow frequency analysis of the BNR at Bahir Dar showed 0·55 m3/s as the monthly low flow with recurrence interval of 10 years. The wavelet analysis of the rainfall (at Bahir Dar and basin-wide) and flows at three selected stations shows inter- and intra-annual variability of rainfall and flows at various scales. Copyright © 2009 John Wiley & Sons, Ltd. [source] Hydrological response to timber harvest in northern Idaho: implications for channel scour and persistence of salmonidsHYDROLOGICAL PROCESSES, Issue 17 2008Daniele Tonina Abstract The potential for forest harvest to increase snowmelt rates in maritime snow climates is well recognized. However, questions still exist about the magnitude of peak flow increases in basins larger than 10 km2 and the geomorphic and biological consequences of these changes. In this study, we used observations from two nearly adjacent small basins (13 and 30 km2) in the Coeur d'Alene River basin, one with recent, relatively extensive, timber harvest, and the other with little disturbance in the last 50 years to explore changes in peak flows due to timber harvest and their potential effects on fish. Peak discharge was computed for a specific rain-on-snow event using a series of physical models that linked predicted values of snowmelt input to a runoff-routing model. Predictions indicate that timber harvest caused a 25% increase in the peak flow of the modelled event and increased the frequency of events of this magnitude from a 9-year recurrence interval to a 3·6-year event. These changes in hydrologic regime, with larger discharges at shorter recurrence intervals, are predicted to increase the depth and frequency of streambed scour, causing up to 15% added mortality of bull trout (Salvelinus confluentus) embryos. Mortality from increased scour, although not catastrophic, may have contributed to the extirpation of this species from the Coeur d'Alene basin, given the widespread timber harvest that occurred in this region. Copyright © 2008 John Wiley & Sons, Ltd. [source] A regional frequency analysis of United Kingdom extreme rainfall from 1961 to 2000INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 11 2003H. J. Fowler Abstract Multi-day rainfall events are an important cause of recent severe flooding in the UK, and any change in the magnitude of such events may have severe impacts upon urban structures such as dams, urban drainage systems and flood defences and cause failures to occur. Regional pooling of 1-, 2-, 5- and 10-day annual maxima for 1961 to 2000 from 204 sites across the UK is used in a standard regional frequency analysis to produce generalized extreme value growth curves for long return-period rainfall events for each of nine defined climatological regions. Temporal changes in 1-, 2-, 5- and 10-day annual maxima are examined with L-moments using both a 10 year moving window and the fixed decades of 1961,70, 1971,80, 1981,90 and 1991,2000. A bootstrap technique is then used to assess uncertainty in the fitted decadal growth curves and to identify significant trends in both distribution parameters and quantile estimates. There has been a two-part change in extreme rainfall event occurrence across the UK from 1961 to 2000. Little change is observed at 1 and 2 days duration, but significant decadal-level changes are seen in 5- and 10-day events in many regions. In the south of the UK, growth curves have flattened and 5- and 10-day annual maxima have decreased during the 1990s. However, in the north, the 10-day growth curve has steepened and annual maxima have risen during the 1990s. This is particularly evident in Scotland. The 50 year event in Scotland during 1961,90 has become an 8-year, 11-year and 25-year event in the East, South and North Scotland pooling regions respectively during the 1990s. In northern England the average recurrence interval has also halved. This may have severe implications for design and planning practices in flood control. Copyright © 2003 Royal Meteorological Society [source] Sedimentary record of a tsunami during Roman times, Bay of Cadiz, SpainJOURNAL OF QUATERNARY SCIENCE, Issue 5-6 2002L. Luque Abstract Historical data show that the Gulf of Cadiz has been exposed to destructive tsunamis during at least the past 2000 yr. The last tsunami was generated by the AD 1755 Lisbon earthquake, which affected the Atlantic coasts of Spain, Portugal and Morocco. Today, these littoral areas are intensely populated and the expected damage could be much greater. Tsunami studies are of great importance in helping to determine the recurrence interval of these events. The presence of washover fan deposits on the inland margin of the Valdelagrana Spit bar (Cadiz, Spain) indicates the occurrence of a high energy marine event ca. 2300 cal. yr BP. Historical, geomorphological, sedimentological, palaeontological and geochronological data suggest that a tsunami could have affected the area during Roman times. Copyright © 2002 John Wiley & Sons, Ltd. [source] CHANNEL STABILITY DOWNSTREAM FROM A DAM ASSESSED USING AERIAL PHOTOGRAPHS AND STREAM-GAGE INFORMATION,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 3 2000Kyle E. Juracek ABSTRACT: The stability of the Neosho River channel downstream from John Redmond Dam, in southeast Kansas, was investigated using multiple-date aerial photographs and stream-gage information. Bankfull channel width was used as the primary indicator variable to assess pre- and post-dam channel change. Five sin-mile river reaches and four stream gages were used in the analysis. Results indicated that, aside from some localized channel widening, the overall channel change has been minor with little post-dam change in bankfull channel width. The lack of a pronounced post-dam channel change may be attributed to a substantial reduction in the magnitude of the post-dam annual peak discharges in combination with the resistance to erosion of the bed and bank materials. Also, the channel may have been overwidened by a series of large floods that predated construction of the dam, including one with an estimated 500-year recurrence interval. [source] On the use and evaluation of prospective scan methods for health-related surveillanceJOURNAL OF THE ROYAL STATISTICAL SOCIETY: SERIES A (STATISTICS IN SOCIETY), Issue 1 2008William H. Woodall Summary., We review some prospective scan-based methods that are used in health-related applications to detect increased rates of mortality or morbidity and to detect bioterrorism or active clusters of disease. We relate these methods to the use of the moving average chart in industrial applications. Issues that are related to the performance evaluation of spatiotemporal scan-based methods are discussed. In particular we clarify the definition of a recurrence interval and demonstrate that this measure does not reflect some important aspects of the statistical performance of scan-based, and other, surveillance methods. Some research needs in this area are given. [source] Determination of bankfull discharge magnitude and frequency: comparison of methods on 16 gravel-bed river reachesEARTH SURFACE PROCESSES AND LANDFORMS, Issue 11 2006O. Navratil Abstract Bankfull discharge is identified as an important parameter for studying river morphology, sediment motion, flood dynamics and their ecological impacts. In practice, the determination of this discharge and its hydrological characteristics is not easy, and a choice has to be made between several existing methods. To evaluate the impact of the choice of methods, five bankfull elevation definitions and four hydrological characterizations (determination of duration and frequency of exceedance applied to instantaneous or mean daily data) were compared on 16 gravel-bed river reaches located in France (the catchment sizes vary from 10 km2 to 1700 km2). The consistency of bankfull discharge estimated at reach scale and the hydraulic significance of the five elevation definitions were examined. The morphological definitions (Bank Inflection, Top of Bank) were found more relevant than the definitions based on a geometric criterion. The duration of exceedance was preferred to recurrence intervals (partial duration series approach) because it is not limited by the independency of flood events, especially for low discharges like those associated with the Bank Inflection definition. On average, the impacts of the choice of methods were very important for the bankfull discharge magnitude (factor of 1·6 between Bank Inflection and Top of Bank) and duration of exceedance or frequency (respectively a factor 1·8 and 1·9 between mean daily and instantaneous discharge data). The choice of one combination of methods rather than another can significantly modify the conclusions of a comparative analysis in terms of bankfull discharge magnitude and its hydrological characteristics, so that one must be cautious when comparing results from different studies that use different methods. Copyright © 2006 John Wiley & Sons, Ltd. [source] Hydrological response to timber harvest in northern Idaho: implications for channel scour and persistence of salmonidsHYDROLOGICAL PROCESSES, Issue 17 2008Daniele Tonina Abstract The potential for forest harvest to increase snowmelt rates in maritime snow climates is well recognized. However, questions still exist about the magnitude of peak flow increases in basins larger than 10 km2 and the geomorphic and biological consequences of these changes. In this study, we used observations from two nearly adjacent small basins (13 and 30 km2) in the Coeur d'Alene River basin, one with recent, relatively extensive, timber harvest, and the other with little disturbance in the last 50 years to explore changes in peak flows due to timber harvest and their potential effects on fish. Peak discharge was computed for a specific rain-on-snow event using a series of physical models that linked predicted values of snowmelt input to a runoff-routing model. Predictions indicate that timber harvest caused a 25% increase in the peak flow of the modelled event and increased the frequency of events of this magnitude from a 9-year recurrence interval to a 3·6-year event. These changes in hydrologic regime, with larger discharges at shorter recurrence intervals, are predicted to increase the depth and frequency of streambed scour, causing up to 15% added mortality of bull trout (Salvelinus confluentus) embryos. Mortality from increased scour, although not catastrophic, may have contributed to the extirpation of this species from the Coeur d'Alene basin, given the widespread timber harvest that occurred in this region. Copyright © 2008 John Wiley & Sons, Ltd. [source] The efficacy of topical intralesional BCG-PSN injection in the treatment of erosive oral lichen planus: a randomized controlled trialJOURNAL OF ORAL PATHOLOGY & MEDICINE, Issue 7 2009C. Xiong Background:, Nowadays, it has been widely accepted that the local cell-mediated immunologic disorders may play an important role in the pathogenesis of oral lichen planus (OLP). Therefore, we sieved out polysaccharide nucleic acid fraction of bacillus Calmette-Guerin (BCG-PSN) from various immunomodulators to evaluate the short-term therapeutic efficacy and clinical safety of intralesional BCG-PSN injection for erosive OLP. Methods:, A total of 56 OLP patients were randomly assigned to receive either intralesional injection of 0.5 ml BCG-PSN every other day (31 of 56) or 10 mg triamcinolone acetonide (TA, a positive-controlled group, 25 of 56) every week for 2 weeks. After the cessation of treatment, those cured from erosion were followed up for 3 months. Another two researchers measured erosive areas and recorded visual analog scale (VAS) scores both at the start and the end of the treatment. We also registered adverse reactions and the recurrence intervals. Results:, After 2-week treatment, 27 of 31 BCG-PSN-treated patients (87.1%) and 22 of 25 TA-treated patients (88.0%) healed. There were no statistical differences between the two groups in erosive areas (27.86 ± 27.97 vs. 25.68 ± 34.65, P = 0.801) and VAS scores (2.45 ± 1.64 vs. 2.40 ± 1.38, P = 0.946). Three of 31 BCG-PSN-treated patients (9.7%) vs. 2 of 25 TA-treated patients (8.0%) experienced the swelling or burning sensation (P = 0.827). A total of 49 of 56 patients were followed up. There were no statistical differences in the recurrence rates (33.3% vs. 45.5%, P = 0.386) and intervals (80.89 ± 26.83 vs. 73.48 ± 28.11, P = 0.419). Conclusions:, Topical intralesional BCG-PSN injection is as effective as TA for erosive OLP, which suggests that topical intralesional BCG-PSN injection can be a promising therapeutic alternative for erosive OLP, especially for those insensitive, or even resistant, to glucocorticoids. [source] EFFECTS OF DAM IMPOUNDMENT ON THE FLOOD REGIME OF NATURAL FLOODPLAIN COMMUNITIES IN THE UPPER CONNECTICUT RIVER,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 6 2002Keith H. Nislow ABSTRACT: Understanding the effects of dams on the inundation regime of natural floodplain communities is critical for effective decision making on dam management or dam removal. To test the implications of hydrologic alteration by dams for floodplain natural communities, we conducted a combined field and modeling study along two reaches in the Connecticut River Rapids Macrosite (CRRM), one of the last remaining flowing water sections of the Upper Connecticut River. We surveyed multiple channel cross sections at both locations and concurrently identified and surveyed the elevations of important natural communities, native species of concern, and nonnative invasive species. Using a hydrologic model, HEC-RAS, we routed estimated pre-and post-impoundment discharges of different design recurrence intervals (two year through 100 year floods) through each reach to establish corresponding reductions in elevation and effective wetted perimeter following post-dam discharge reductions. By comparing (1) the frequency and duration of flooding of these surfaces before and after impoundment and (2) the total area flooded at different recurrence intervals, our goal was to derive a spatially explicit assessment of hydrologic alteration, directly relevant to natural floodplain communities. Post-impoundment hydrologic alteration profoundly affected the subsequent inundation regime, and this impact was particularly true of higher floodplain terraces. These riparian communities, which were flooded, on average, every 20 to 100 years pre-impoundment, were predicted to flood at 100 , 100 year intervals, essentially isolating them completely from riverine influence. At the pre-dam five to ten year floodplain elevations, we observed smaller differences in predicted flood frequency but substantial differences in the total area flooded and in the average flood duration. For floodplain forests in the Upper Connecticut River, this alteration by impoundment suggests that even if other stresses facing these communities (human development, invasive exotics) were alleviated, this may not be sufficient to restore intact natural communities. More generally, our approach provides a way to combine site specific variables with long term gage records in assessing the restorative potential of dam removal. [source] | |||||||||||||