Short-duration Rainfall Research Articles

Trends in rainfall intensity for stormwater designs in Ontario

The intensities of short-duration rainfall events are fundamental inputs to the design of stormwater management infrastructure. However, since stormwater infrastructure must function as designed for many decades, if there are long-term trends in rainfall intensities, design storms need to be modified. Evidence demonstrates, using data from 13 rain gauges in Ontario, that storm intensities relevant to urban stormwater (5 year) appear to have changed over the last 30 years. The results show, for example, statistical significance at 80% confidence that the 5-year storm has increased, and 85% that the 2-year storm has increased, for the 1 h storm in Waterloo, using partial duration series (PDS) data. The PDS data indicate intensities are increasing at a rate of 1–3% per year. Results show, for example, that a 5-year recurrence storm for PDS for the period 1970–1984 is now very close in magnitude to a 2-year recurrence storm for the period 1985–2003 for Waterloo, Ontario. The implications for a case study demonstrate that 5 out of 12 storm sewer pipes in a subdivision would need to be increased in diameter to obtain the same level of stormwater performance.

Development of hydrological tools using extreme rainfall events for Dhaka, Bangladesh

Annual daily maximum rainfall data for 57 years (1953 to 2009) in Dhaka, Bangladesh are analysed statistically to prepare two hydrological tools: (1) probable return periods of extreme rainfall events, and (2) intensity-duration-frequency (IDF) curves. Limited availability of short-duration rainfall data leads to difficulties in performing the task through traditional approaches; hence, the Gumbel distribution function and scaling theory have been applied in preparing the probable return periods of extreme rainfall events and IDF curves, respectively. The outcomes of this paper can be used in better designing hydraulic structures in Bangladesh.

Conservative design rainfall distribution for application in arid regions with sparse data

This research aims to analyze short duration rainfall data in arid regions, to propose design storm distributions representatives of the recorded storms. Frequency analyses were carried out on all storm durations. Design storm distributions are compared with published depth duration ratios or entire synthetic storm distributions such as the Bell ratios for less than 2-hour durations and ratio of 24-hour to 1-hour rainfall depths embedded in the Soil Conservation Service (SCS) type II storm distribution, known to be the most conservative among SCS distributions. Rainfall-runoff simulations were carried out to determine the impact of the use of storm distributions on generated peak discharges It was found that Bell ratios are adequate to represent rainfall patterns in arid regions for rainfall durations less than two hours. Furthermore, the SCS type II synthetic storm distribution can be safely used for rainfall stations where extreme storms last more than 3 h on the average. This is true for most of the studied locations. Nevertheless, a modified SCS design storm distribution is proposed to represent regions with more concentrated storms. Peak discharges calculated using the modified SCS storm distributions are equivalent or higher than those calculated using the entire Intensity-Duration-Frequency information.

In situ measurements of post-fire debris flows in southern California: Comparisons of the timing and magnitude of 24 debris-flow events with rainfall and soil moisture conditions

[1] Debris flows often occur in burned steeplands of southern California, sometimes causing property damage and loss of life. In an effort to better understand the hydrologic controls on post-fire debris-flow initiation, timing and magnitude, we measured the flow stage, rainfall, channel bed pore fluid pressure and hillslope soil-moisture accompanying 24 debris flows recorded in five different watersheds burned in the 2009 Station and Jesusita Fires (San Gabriel and Santa Ynez Mountains). The measurements show substantial differences in debris-flow dynamics between sites and between sequential events at the same site. Despite these differences, the timing and magnitude of all events were consistently associated with local peaks in short duration (< = 30 min) rainfall intensity. Overall, debris-flow stage was best cross-correlated with time series of 5-min rainfall intensity, and lagged the rainfall by an average of just 5 min. An index of debris-flow volume was also best correlated with short-duration rainfall intensity, but found to be poorly correlated with storm cumulative rainfall and hillslope soil water content. Post-event observations of erosion and slope stability modeling suggest that the debris flows initiated primarily by processes related to surface water runoff, rather than shallow landslides. By identifying the storm characteristics most closely associated with post-fire debris flows, these measurements provide valuable guidance for warning operations and important constraints for developing and testing models of post-fire debris flows.

Changes in Duration-Related Characteristics of Late-Summer Precipitation over Eastern China in the Past 40 Years

Abstract Duration is a key feature of rainfall events that is closely related to rainfall mechanisms and influences. This study analyzes the decadal change in the duration-related characteristics of late-summer (July–August) precipitation over eastern China during 1966–2005. Accompanying the southern-flooding and northern-drought (SFND) pattern of rainfall amount over the eastern China in recent decades, the duration-related rainfall structure also experienced significant changes. In North China, the frequency of short duration rainfall events decreased and their intensity increased. The decadal decreases of rainfall amount over North China are largely contributed by long duration rainfall events, especially those occurring between midnight and morning. In the mid-to-lower reaches of the Yangtze River valley, both the frequency and amount of long duration precipitation have significantly increased. The mean and maximum duration time of late-summer precipitation has increased 0.85 and 7.61 h, respectively. Considerable increases of rainfall amount of two kinds of precipitation, the short and medium duration rainfall events in the late afternoon and the long duration rainfall events in the early morning, contribute to the “southern-flooding.” Despite the differences between the northern and southern region, there is a common feature of their decadal precipitation changes that the intensity of short duration rainfall in the late afternoon has shown an increasing trend.

Stationarity of AMAX series of short-duration rainfall for long-term Canadian stations: detection of jumps and trends

Short-duration rainfall data are key inputs for design of stormwater infrastructure. To address concerns raised regarding the validity of relations based on these data, 13 Canadian climate stations were selected: one station for each province and territory, having a “long” record length (27 to 63 years), minimal missing years and potential significance for urban infrastructure design. For each station, a state-of-practice time series analysis was completed for the nine intensity–duration–frequency (IDF) durations (5 min – 24 h) to identify non-stationarities in the form of trends or jumps. Approximately, 30% of the time series showed statistically significant trends or jumps. Data sets not satisfying the stationarity criterion should be critically reviewed before computed quantiles are applied for design. Arbitrary split sample analysis was investigated as an alternative method for identifying non-stationarities and was found to be an unreliable indicator of significant changes.

Mechanism and modelling of shallow soil slope stability during high intensity and short duration rainfall

Shallow landslides in nearly saturated uncohesive to slightly cohesive soils are triggered by high intensity, short duration rainfall which infiltrates into soil and changes intergranular friction and effective stresses. For this, the especially developed Soil–Water Interaction Modelling System (SWIMS) was used with CL-ML type soils. For simplicity, rainfall intensity and duration were kept constant. Results showed that (1) All 35° slopes were failed by translational failure. For the other (15°,25°) slopes, no failures were observed; (2) For all slopes, FOS increased with increasing compaction degree and decreased with increasing slope angle; (3) Other parameters, such as soil density, porosity, saturation degree, water contents, and water permeability may also affect shear strength/slope stability, especially for low degrees of saturation (S<95%), compared to high degrees of saturation (S=,>95%). (4) A correlation of SWIMS tests observed that average wetting band depths (hobser), with the calculated wetting band depths from the Lump Equation (hLE), were poor, as hobser values were much higher than hLE values. Differences increased for very low degrees of saturation (S), compared to S>95%. This meant that the Lump equation underestimated wetting band depths. Further, if the Lump equation is still considered valid, this would imply either water-permeability increases, porosity decreases or both occur towards full saturation; a process where the last possibility is the most probable occurrence.

Temporal and spatial characteristics of extreme hourly precipitation over eastern China in the warm season

Based on hourly precipitation data in eastern China in the warm season during 1961–2000, spatial distributions of frequency for 20 mm h−1 and 50 mm h−1 precipitation were analyzed, and the criteria of short-duration rainfall events and severe rainfall events are discussed. Furthermore, the percentile method was used to define local hourly extreme precipitation; based on this, diurnal variations and trends in extreme precipitation were further studied.

Long-term precipitation change by hourly data in Haihe River Basin during 1961–2004

Hourly summer precipitation data recorded at 21 stations during 1961–2004 in the Haihe River Basin in North China were analyzed. The results show that the precipitation frequency and amount and the morning peak mainly relating to long-duration rainfall events decreased during this period, whereas the normalized afternoon peak mainly relating to short-duration events increased, which may suggest that the proportion of short-duration rainfall has increased as the total summer rainfall has decreased. For short-duration events, the mean intensity and peak intensity increased at most stations and the time to peak intensity decreased, which may be attributable to the higher thermal contrast between the warmer lower surface and cooler upper level. In the case of long-duration events, the total amount was significantly correlated with the East Asian summer monsoon index for the period 1961–2001 (correlation coefficient of 0.63). Although the total amount of rainfall in long-duration events decreased in the basin, the mean intensity and peak intensity, as well as the extreme hourly precipitation, increased in the western basin and decreased in the eastern basin.

Characteristic features of hourly rainfall in India

AbstractHourly rainfall data recorded at 72 Self Recording Raingauge Stations (SRRG) for the period 1969–2006 are utilized to study the characteristic features of hourly rainfall and diurnal variations of rainfall in India. Temporal changes in the short duration (less than 12 h) rainfall extremes are examined. Time distribution of a heavy rain spell of 24 h duration, which is an important component in water resources management and flood control studies, is analysed. Different aspects of hourly rainfall such as, average number of rain hours in a year, empirical probability distribution functions have also been examined. Analysis indicates, many stations in India have recorded more than 10 cm of rainfall in an hour's duration. Such stations are located along the west coast, foothills of Himalayas and along the tracks of monsoon disturbances. Extreme rainfall events of various duration (3–12 h) show increasing trends, significant at 5% level, at many stations located in central India and peninsular India except for southern part of west coast. Time distribution of hourly rainfall shows that, on average, 25% of the daily rainfall can be received in just 3 h, while 80% can occur in 12 h during the heavy rain spells of 24‐h duration. Copyright © 2011 Royal Meteorological Society

Limitations of instantaneous water quality sampling in surface-water catchments: Comparison with near-continuous phosphorus time-series data

Summary The validity of load estimates from intermittent, instantaneous grab sampling is dependent on adequate spatial coverage by monitoring networks and a sampling frequency that reflects the variability in the system under study. Catchments with a flashy hydrology due to surface runoff pose a particular challenge as intense short duration rainfall events may account for a significant portion of the total diffuse transfer of pollution from soil to water in any hydrological year. This can also be exacerbated by the presence of strong background pollution signals from point sources during low flows. In this paper, a range of sampling methodologies and load estimation techniques are applied to phosphorus data from such a surface water dominated river system, instrumented at three sub-catchments (ranging from 3 to 5 km2 in area) with near-continuous monitoring stations. Systematic and Monte Carlo approaches were applied to simulate grab sampling using multiple strategies and to calculate an estimated load, Le based on established load estimation methods. Comparison with the actual load, Lt, revealed significant average underestimation, of up to 60%, and high variability for all feasible sampling approaches. Further analysis of the time series provides an insight into these observations; revealing peak frequencies and power-law scaling in the distributions of P concentration, discharge and load associated with surface runoff and background transfers. Results indicate that only near-continuous monitoring that reflects the rapid temporal changes in these river systems is adequate for comparative monitoring and evaluation purposes. While the implications of this analysis may be more tenable to small scale flashy systems, this represents an appropriate scale in terms of evaluating catchment mitigation strategies such as agri-environmental policies for managing diffuse P transfers in complex landscapes.

Trends and Variability in Extreme Rainfall Events in British Columbia

This paper analyzes hourly rainfall data from a collection of tipping bucket rain gauges in British Columbia. The hourly rainfall data are used to define peaks over threshold (POT) rainfall events for durations of 1, 2, 6, 12 and 24 hours. This database is then used to define, on an annual basis, the number of over threshold events, the average magnitude of the over threshold events, and the largest over threshold event. Trend analysis is conducted for these three variables for each duration and for several common analysis periods drawn from the period 1966 to 2005. The identification and estimation of trends is conducted using the Mann-Kendall nonparametric test for trend. The global, or field, significance of the trend results is established using a bootstrap resampling approach. The research reveals generally increasing trends in extreme rainfall, especially for the summer season and for the short duration rainfall events.

The challenges of developing rainfall intensity-duration-frequency curves and national flood hazard maps for the Caribbean

In many Caribbean countries a lack of established good practice methods means that engineers and planners are often unable to plan for and mitigate floods effectively. In most Caribbean states rainfall intensity – duration – frequency (IDF) curves are not readily available. This is a result of the limited quantity of short duration rainfall data available and also because the few IDF curves that have been developed in the region are generally not in the public domain. The lack of readily available IDF curves in the region often results in engineers responsible for the design of key infrastructure inappropriately “transferring” IDF curves developed for islands, where rainfall is less intense, for use in their designs. There are no countries in the Caribbean with nationally consistent flood hazard maps. This often leaves spatial and emergency planners with insufficient information to make important strategic decisions. This paper details the challenges that were faced in producing rainfall IDF curves for return periods up to 1 in 50 years and nationally consistent extreme fluvial flood extent maps with limited data for selected countries within the Caribbean. Recommendations are made for the future development of rainfall IDF curves and national flood maps in the region both in terms of data and organisational requirements.

Hydrologic Evaluation of a Mediterranean Watershed Using the SWAT Model with Multiple PET Estimation Methods

The Penman-Monteith (P-M) method suggested by the Food Agricultural Organization in irrigation and drainage paper 56 (FAO-56 P-M) was used in the Soil and Water Assessment Tool's (SWAT) water balance simulation at the outlet of an experimental watershed in Sicily, Italy. A sensitivity analysis determined that the model was more sensitive to this potential evapotranspiration (PET) parameter than to the other six parameters impacting surface runoff in this small Mediterranean watershed. The FAO-56 P-M method was compared to the three existing SWAT PET methods from 1997 to 2003. The watershed's water balance was more realistically simulated by the FAO-56 P-M method than by the other PET methods. The traditional P-M method incorporated into SWAT overestimated total (surface and base flow) runoff volumes observed during the entire period by approximately 50%; however, total runoff volumes were underestimated by only 17% when the FAO-56 P-M method was used. The surface runoff simulation results using the FAO-56 P-M PET equation for calculating daily values was sufficient at the monthly time interval (Nash-Sutcliffe efficiency >0.75) during the calibration and validation periods. The incorporation of the FAO-56 P-M method has broadened the SWAT model's applicability to watersheds that are in semi-arid environments with high-intensity, short-duration rainfall events.

Subseasonal Characteristics of Diurnal Variation in Summer Monsoon Rainfall over Central Eastern China

Abstract Subseasonal characteristics of the diurnal variation of the summer monsoon rainfall over central eastern China (25°–40°N, 110°–120°E) are analyzed using hourly station rain gauge data. Results show that the rainfall in the monsoon rain belt is dominated by the long-duration rainfall events (≥7 h) with early-morning peaks. The long-duration rainfall events and early-morning diurnal peaks experience subseasonal movement that is similar to that of the monsoon rain belt. When the monsoon rainfall is separated into the active and break periods, the long-duration early-morning precipitation dominates the active period, which is in sharp contrast to the short-duration (≤6 h) rainfall with leading late-afternoon diurnal peaks during the break period. The combination of different diurnal features of monsoon rainfall in the active and break monsoon periods also explains the less coherent diurnal phases of summer mean rainfall over central eastern China. The cause of the early-morning peak of rainfall during the active monsoon period is discussed.

Short Duration Rainfall Extremes in Ireland: Influence of Climatic Variability

A widely-noted change in the North Atlantic circulation in the 1970s affected the spatial distribution and seasonal pattern of rainfall over Ireland. To examine if this was accompanied by a change on short duration precipitation extremes, multi-decadal time series from the second half of the twentieth century of thirteen hourly precipitation stations in Ireland have been analysed for the occurrence of extreme values over several durations of up to 24 h. Strong evidence was found for a change since the late 1970s in short duration rainfall depths, particularly in the west of the country. Precipitation depth-duration-frequency analyses over two sub-periods showed that at several locations, storm event magnitudes which corresponded to a 30 year return period before 1975 had a return period close to 10 years in the post-1975 period. The widespread increase in spring and autumn rainfall and the local increases in the frequencies and magnitudes of severe rainfalls have implications for engineering hydrology, flood risk analysis and water resources management. The necessity of using up-to-date data to derive design storm magnitudes is stressed, due to the possible influence of underlying climatic shifts. Furthermore, as non-stationarity has been demonstrated, the use of long timeseries extending beyond thirty years into the past will result in underestimation of storm intensities in many areas.

Evidence of debris flow occurrence after wildfire in upland catchments of south-east Australia

Numerous reports of “flash floods”, “mud torrents” and “landslides” in burnt landscapes of south-east Australia were only recently linked to debris flows and recognised as a significant process that warrant more detailed investigation. This paper provides a systematic documentation of high-magnitude erosion events after wildfire in south-east Australia, focusing on small (< 5 km 2), upland catchments in eastern Victoria that were burnt by wildfire between 2003 and 2009. The aims of the study were to i) collect and show evidence of debris flow occurrence after wildfire; ii) quantify erosion rates from debris flows and; iii) identify rainfall thresholds and key hydrological properties. The result showed that 13 out of the 16 recorded high-magnitude erosion events were runoff generated debris flows. These occurred in dry eucalypt forests burnt at high or very high severity in steep headwater catchments throughout the eastern uplands of Victoria. The debris flows were triggered by intense, short duration rainfall events ( I 30 35–59 mm h −1) with annual exceedance probability in the order of 20%. This is the first paper to document the occurrence of post-fire runoff generated debris flows in Australia, so the discussion draws on literature from the western USA, where a large body of research has been dedicated to evaluating the risk posed by post-fire debris flows and their role in landscape processes. Typical features common to both systems include low infiltration capacity of burnt catchments; widespread sheet erosion and levee lined rills on steep upper hillslopes; and severe channel erosion initiated in response to convergent flow in previously un-scoured drainage lines. The depth of sheet erosion on surveyed slopes in the upper catchments (4.6 ± 0.96 mm to 18.4 ± 2.7 mm) indicates that hillslope material provides an important source of sediment. The average channel entrainment rate of three debris flows ranged from 0.6 to 1.4 m 3 m −1. Runoff generated debris flows were not recorded in wet or damp forest types suggesting that this process is unlikely to operate in these forest environments. One isolated case of mass failure generated debris flow was recorded in wet forest. The outcome of the study indicates that runoff generated debris flows in dry eucalypt forest are an important process to be considered during post-fire risk assessment of hydrological hazards.

Assessing of IDF curves for hydrological design by simple scaling of 1-day precipitation totals

Assessing of IDF curves for hydrological design by simple scaling of 1-day precipitation totalsIn this paper the scaling properties of short term extreme rainfall in Slovakia were investigated. The simple scaling theory was applied to the intensity-duration-frequency (IDF) characteristics of a short duration rainfall. This method allows for the estimation of the design values of rainfall of selected recurrence intervals and durations shorter than a day by using only the daily data. The scaling behavior of rainfall intensities was examined, and the possibility of using simple scaling in Slovakia was verified. The methodology for the simple scaling of rainfall is demonstrated using an example of the meteorological station in Ilava.

Recent changes in rainfall characteristics and their influence on thresholds for debris flow triggering in the Dolomitic area of Cortina d'Ampezzo, north-eastern Italian Alps

Abstract. In this paper, we examine variations in climate characteristics near the area of Cortina d'Ampezzo (Dolomites, Eastern Italian Alps), with particular reference to the possible implications for debris-flow occurrence. The study area is prone to debris-flow release in response to summer high-intensity short-duration rainfalls and, therefore, it is of the utmost importance to investigate the potential increase in debris-flow triggering rainfall events. The critical rainfall threshold is agreed to be a crucial triggering factor for debris-flows. Data from a monitoring system, placed in a catchment near Cortina (Acquabona), show that debris-flows were triggered by rainfalls with peak rainfall intensities ranging from 4.9 to 17.4 mm/10 min. The analyses of meteorological data, collected from 1921 to 1994 at several stations in the study area, show a negative trend of annual rainfall, a considerable variation in the monthly rainfall distribution, and an increase in the temperature range, possibly related to global climate changes. Moreover, high-intensity and short-duration rainfall events, derived from data collected from 1990 and 2008, show an increase in exceptional rainfall events. The results obtained in a peak-over-threshold framework, applied to the rainfall data measured at the Faloria rain gauge station from 1990 to 2008, clearly show that the interarrival time of over-threshold events computed for different threshold values decreased in the last decade. This suggests that local climatic changes might produce an increase in the frequency of rainfall events, potentially triggering debris flows in the study area.

A generalized regional design storm rainfall model for Botswana

Design of drainage and dam structures involves a full understanding of the duration, magnitude and volume of peak flood flows anticipated. For gauged catchments a number of established flood frequency models and rainfall-runoff models are used widely. However, most planned developments for bridge or dam or any other water development structures in a catchment are located at a site in a stream where there are no discharge records. In a catchment, records of short duration rainfall data might not also be available in order to transform them to design flood. In this study, a generalized parametric model for rainfall intensity-duration-frequency (IDF) relations is developed for the estimation of rainfall extremes in Botswana. The model is calibrated on available records on rainfall IDF relationships that are in use by the Botswana Roads Department. The proposed formulation allows one to estimate the expected rainfall depth for a duration ranging from 5 minutes to 2 hours and beyond and for low and medium return periods (up to 50 years) in any location within Botswana. A simplified experiment is developed in order to assess the reliability of the proposed formulation, whose performances are tested also in comparison with other regionalization approaches recently proposed by the scientific literature. The proposed model reproduced the observed IDF both qualitatively and quantitatively with high values of R2 ranging between 92.9 to 99.9 % during model calibration and 92.3 to 99.8% during model verification. Given the lack of recorded continuous and short duration rainfall information, the model can be used to assist hydrologists in the design of drainage and other water control structures for ungauged sites in Botswana.Keywords: Design storms, IDF model, Calibration, verification, parameter sensitivity analysis, Botswana