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Urbanization is known to increase the volume of stormwater runoff and peak flow rates, which leads to changes in the natural flow regime and increases the likelihood of flooding. Low-impact development (LID) practices seek to reduce runoff volume and peak flow and are generally considered to be a more sustainable solution for urban stormwater management. In this study, we present a systematic approach to address nuisance flooding issues in small cities and communities. As an application, the effectiveness of two LID practices, rain barrels and permeable pavements, were explored in mitigating the urban flooding problem of a highly urbanized small coastal watershed in Alabama, USA. The EPA Stormwater Management Model (SWMM) was first calibrated for water depth using data collected at multiple sites within the watershed during the 2014–2015 period. The calibrated model was then used to first identify the areas prone to flooding using design storms with 1, 2, 5-, 10-, 50-, and 100-year return periods. Floodplain maps were generated for those design storms with HEC-RAS. Next, LID options upstream of those flood-prone areas were assessed to potentially minimize the flooding risks. The results indicate that LID controls can have considerable benefits for stormwater management by reducing runoff volume (1–24%), peak flow rates (18–25%), and water depth (5–15%), potentially returning watersheds to their natural flow regimes, thereby minimizing the flooding risk in urbanized areas. However, the effectiveness of LIDs, especially for the runoff volume, quickly diminishes as the return periods of the storms increase. Rain barrels were identified as the most economical and effective LID within the drainage system. [ABSTRACT FROM AUTHOR]

Extreme rainfall in midwestern United States has gotten more common over the last half century, thus increasing flooding events across the region. As a result, traditional flood mitigation measures are commonly overwhelmed by highwater events, illustrating the need for new solutions. Of the 91,000 dams in the US, the vast majority are small and go unused for flood mitigation. Among those that are utilized in flood peak reduction, few are actively managed in which outflows are manipulated through a gated outlet. Instead, small storage locations typically use passive control, allowing impounded water levels to fluctuate without the use of a gated outlet, possibly squandering some of their flow-reduction potential. In this paper, we have evaluated actively managed storage within a distributed network of 130 small dams in a 660-km2 watershed in southeastern Iowa using three operation schemes to increase storage utilization and reduce downstream flows. We developed a module to simulate the dam operation into a distributed hydrologic model that is forced with soil conservation service (SCS) 24-h design storms distributed uniformly across the watershed with 0.2, 0.1, 0.02, and 0.01 exceedance probabilities to evaluate flow reductions. When compared with passive operation, outlet flows were reduced under each proposed iteration of the 24-h design storm. Using the most aggressive operation scheme, outlet flows were reduced by over 70%. These results showcase the need for better understanding of activated flood storage across midwestern watersheds and encourage further work in optimizing this technique for real-time management. [ABSTRACT FROM AUTHOR]

Current design storms used in hydrological modeling, urban planning, and dimensioning of structures are typically point-scale rainfall events with a steady rainfall intensity or a simple temporal intensity pattern. This can lead to oversimplified results because real rainfall events have more complex patterns than simple design series. In addition, the interest of hydrologists is usually in areal estimates rather than point values, most commonly in river-basin-wide areal mean rainfall estimates. By utilizing weather radar data and the short-term ensemble prediction system pySTEPS, which has so far been used for precipitation nowcasting, ensembles of high-resolution stochastic design storms with desired statistical properties and spatial structure evolving in time are generated. pySTEPS is complemented by adding time-series models for areal average rainfall over the simulation domain and field advection vectors. The selected study area is the Kokemäenjoki river basin located in Western Finland, and the model parametrization is carried out utilizing the Finnish Meteorological Institute's weather radar data from the years 2013 to 2016. The results demonstrate how simulated events with similar large-scale mean areal rainfall can produce drastically different total event rainfalls in smaller scales. The sampling method, areal vs. gauge estimate, is also shown to have a prominent effect on total event rainfall across different spatial scales. The outlined method paves the way towards a more thorough and wide-spread assessment of the hydrological impacts of spatiotemporal rainfall characteristics. [ABSTRACT FROM AUTHOR]

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The increasing threat of post-wildfire hazards creates an imperative for improved post-wildfire flooding and debris flow prediction capabilities. Because rainfall is a primary driver of predictive hydrology and debris flow initiation and inundation models, recent efforts have emphasized the need for interdisciplinary collaboration between meteorology and post-wildfire hazard science that develops more accurate rainfall estimates with longer lead times. In this work, we identified critical knowledge gaps for developing rainfall estimates and filled those gaps by reviewing recent literature and synthesizing pre-existing datasets. Gap areas were organized into the following general topics: a) rainfall intensity-duration-frequency relations, b) time-varying rainfall, c) spatially varying rainfall, and d) rainfall regimes. Recent key research advances include the increasing availability of gridded quantitative rainfall estimates, the expanded use of distributed hydrologic and erosion models that incorporate spatial and temporal variability in rainfall, and the linking of concepts and modeling from the atmospheric and climate sciences with post-wildfire hazard science. We prototype a rainfall regime regionalization schema that captures self-similar properties of rainfall intensity (k, the maximum rainfall intensity) and temporal scaling (n, the decay rate). Our k - n relations schema could serve as a framework for organizing, interpreting, and predicting post-wildfire hydrologic and erosional responses. Finally, we summarize salient gaps for implementing spatiotemporally varying rainfall as the driver of post-wildfire hydrologic models designed to improve the prediction of flooding and debris flow hazards to the built environment for emergency managers. [ABSTRACT FROM AUTHOR]

Rainfall events largely control hydrological processes occurring on and in the ground, but the performance of climate models in reproducing rainfall events has not been investigated enough to guide selection among the models when making hydrological projections. We proposed to compare the durations, intensities, and pause periods, as well as depths of rainfall events when assessing the accuracy of general circulation models (GCMs) in reproducing the hydrological characteristics of observed rainfall. We also compared the sizes of design storm events and the frequency and severity of drought to demonstrate the consequences of GCM selection. The results show that rainfall and extreme hydrological event projections could significantly vary depending on climate model selection and weather stations, suggesting the need for a careful and comprehensive evaluation of GCM in the hydrological analysis of climate change. The proposed methods are expected to help to improve the accuracy of future hydrological projections for water resources planning. [ABSTRACT FROM AUTHOR]

Urban flooding is becoming more severe due to increases in extreme rainfall amounts and impervious land cover. Design storms are critical for developing infrastructure and land use solutions to these flood challenges. In this study, we examine urban design storms using Stochastic Storm Transposition (SST). SST is a technique that uses probabilistic resampling from a" storm catalog" of observed rainfall events selected from a pre-defined regional "transposition domain". Resampled storms are then spatially transposed to estimate distributions of extreme rainfall intensity, duration, and frequency (IDF). This study presents an SST-based rainfall analysis for Shanghai, China. Results show that there is spatial heterogeneity in rainfall across study's transposition domain which must be accounted for during the transposition step of SST. SST can reproduce reasonable IDF estimates and provide design storms with realistic spatiotemporal rainfall structures. Large variations in spatio-temporal structure of these SST-based design storms under different return periods call into question the conventional hydrologic engineering assumption of design storm rainfall that is spatially-uniform and temporally idealized. [ABSTRACT FROM AUTHOR]

• An original spatio-temporal statistical downscaling procedure was proposed. • An original procedure for modeling extreme rainfall processes over a wide range of spatial and temporal scales. • It is feasible to establish the linkages between GCM climate projections and rainfall extremes at a local site. • A robust procedure for accurate assessment of climate change impacts on local rainfall extremes. • An illustrative application using 21 GCMs and extreme rainfall data for Ontario region was presented. The main challenge in the assessment of climate change impacts on the estimation of extreme rainfalls (ERs) for urban drainage systems design is how to establish the linkages between the climate projections given by Global Climate Models (GCMs) at global scales and the observed extreme rainfalls at a given local site. Downscaling approaches have been proposed in many previous studies to downscale global-scale GCM daily information to regional-scale daily climate projections. However, these daily downscaled data are still considered too coarse in both spatial and temporal resolutions and hence not suitable for climate change impact studies at a local site or for small urban watersheds. The present study proposes therefore an innovative statistical downscaling (SD) approach for establishing the linkage between daily extreme rainfalls at regional scales and daily and sub-daily extreme rainfalls at a local (point) scale. The feasibility and accuracy of the proposed method were assessed for a case study in Ontario (Canada) using observed ER data from seven raingauges and climate simulation outputs from 21 GCMs that have been downscaled by NASA to a regional 25-km scale for the RCP 4.5 scenario. Results based on various graphical and numerical comparison criteria have indicated the feasibility and accuracy of the proposed SD approach. In addition, a robust assessment of the climate change impacts on the ERs for urban drainage system design was performed using a series of statistical tests in sequence to evaluate the significant changes of rainfalls among different time periods. It was found that significant increases by 8% to 18% in extreme design rainfalls of return periods up to T = 25 years, and insignificant increases by 3% to 8% in the 50-year and 100-year design rainfalls for many locations, except for one location with a significant increase of 18%. The confidence intervals were also computed for these estimated design rainfalls with widths varying from 5% to 22%. [ABSTRACT FROM AUTHOR]

Rainfall is a key hydro meteorological variable. Climate change is disrupting the hydrological cycle and altering the usual cycle of rainfall, which frequently results in long-lasting storms with significant rainfall. A first step in hydrologic design of project is to determine the design storm or rainfall events to be used. For deriving design storm, researchers concluded that instead of using generalized readily available curves or maps, it is better to estimate design storm based on site specific historical rainfall data. The objective of the study is to analyze the rainfall data in the koyna watershed area in order to evaluate the design storm, which will be further used as an input data for HEC-HMS event based hydrological modelling of flood peak attenuation of design storm flow at koyna dam during extreme rainfall event. In this study, 40 years (1982–2021) of rainfall data from 8 rain gauge stations in Koyna Dam Catchment area is used initially for performing trend analysis through statistical and graphical techniques and then for Isopluvial analysis. The Sen's slope test and the Mann-Kendall test are the statistical techniques employed, and Innovative Trend Analysis is the graphical technique used. IDF approach is used for deriving design storm, and using Gumbel's frequency distribution method Isohyetal maps, IDF tables and curves are prepared for 2,10,25,50,75 and 100 year return periods and 6,12,24,48 and 96 h durations. Results obtained from statistical and graphical trend analysis of annual rainfall series are consistent. No statistically significant trend in annual rainfall series is observed, however there is rising and falling trend was observed in annual as well as monthly rainfall series. From the results of design storm study, the design storm hyetograph of 10 years return period and 96 h duration is selected, which gives the rainfall intensity of 10.88 mm/h for the koyna catchment. There are various dams nearby koyna catchment, The Isohyet maps, IDF curves and table output available from this study can be more reliably used during planning and design of hydraulic structure for other areas near by koyna catchment. • Through rainfall analysis design storm,evaluated and further used as an input data for HEC-HMS hydrological modelling. • Annual and monthly Trend analysis of 40 years rainfall from 8 rain gauge stations is studied using statistical tests, and graphical technique. • Using IDF method with Gumbel's frequency distribution, Isohyetal maps, IDF curves and tables were prepared. • No statistically significant trend was found and results from graphical and Statistical Trend Analysis methods are consistent. • The results obtained are site specific, so more reliable, hence can be used with greater confidence for deriving design storm. [ABSTRACT FROM AUTHOR]

Identifying the rainfall characteristics and understanding the rainfall-related processes is one of the key factors in the scientific management of water resources. Selection of the design storm is the first step in the estimation of the design flood. Determining temporal rainfall patterns is very important as one of the design rainfall properties in flood estimation and the design of drainage systems. This study was concerned with the pattern of rainfall depth during its occurrence at the Dez dam station. In order to plot Huff curves in the Dez dam station, the recorded data of the rain gauge was used. For this purpose, all 280 storms from 1972 to 2016 in different seasons were classified into the five distinct classes including i) 0-2 hours, ii) 2-6 hours, iii) 6-12 hours, iv) 12-24 hours, and v) more than 24 hours. Furthermore, for each class and in each of the seasons, the Huff curves were plotted using all storms information single class. Moreover, for the considered station, all recorded events were considered in a single class and the general Huff curve was plotted using the probability of 50%. In general, the highest number of storms in the Dez dam (about 32.9%) was grouped in the second quartile. However, it was about 55, 48.5 and 50.4 percent in the case of short storms (0-2 hours) for spring, autumn and winter, respectively. In contrast, low percentages were obtained for the rainfalls having long durations. The Logistic equation was extracted for all Huff curves. These curves would be useful in efficient water resources management. The value of the correlation coefficient between the amounts obtained from the Logistic model and the corresponding values extracted from the curves was more than 0.99, which was significant in 1 percent. [ABSTRACT FROM AUTHOR]

This study aims at plotting the Huff curves and designing storm hyetographs in the stations of Izeh, Idonak, Abdolkhan and Ahvaz. A total of 1811 recorded storms in the period 1966-2016 in different seasons were classified into five distinct time groups according to their rainfall duration. The Huff curves were plotted for each class in each of the seasons using the whole set of storms. Results showed that classification of storms led to a better distinction of rainfall pattern in different seasons and durations. In order to compare the time distribution of rainfalls, three indices of S, I, and Q were defined that consider the ratios of non-dimensional cumulative rainfall curves from the 10% probability Huff curve obtained in the 25%, 50%, and 75% of time durations to their corresponding values from the 50% probability Huff curve. The results showed that in the four mentioned stations, the values of S index were greater than I, and both of them were greater than Q. The range of S index varied from 1.47 (in Idonak station for spring storms having the duration between 12-24 hours) to 9.63 (in Abdolkhan station for spring storms with the duration of less than 2 hours). Whereas the range of Q index varied from 1.03 (in Ahvaz station for spring and winter rainfalls having the duration of less than two hours, and in Idonak station for spring storms with the duration of less than two hours) to 1.44 (in Abdolkhan station for spring storms with the duration of 6-12 hours). The range of I index varied from 1.07 (in Ahvaz for spring storms with the duration of 2-6 hours) to 2.12 (in Abdolkhan for winter storms with the duration of less than two hours). For each of the Huff curves, design storm hyetographs were derived and presented using the 50% probability Huff curve. [ABSTRACT FROM AUTHOR]

Low-impact development (LID) is generally regarded as an effective solution for urban stormwater management. However, the effects of LID on storm flooding on an urban drainage systems scale have not been fully understood under different rainfall characteristics. In this paper, using a high-density residential catchment in China as a case study, the PCSWMM model is applied to simulate the control effects of seven LID land layout scenarios on water quantity and quality of rainfall runoff under different rainfall return periods, durations, and locations of peak rainfall intensity. The simulations indicated the following results: (1) All seven LID scenarios are more effective in flood control during smaller rainfall return periods and longer rainfall duration storms, and the influence of locations of peak rainfall intensity on the rainfall runoff is not obvious. (2) Permeable pavement is the best choice in single LID measures, and the combination of permeable pavement, vegetative swale, and a green roof is the best choice among combined LID layouts. This paper gives insight into the performance of LID designs under different rainfall characteristics, which is essential for effective sponge city construction. [ABSTRACT FROM AUTHOR]

Extreme flood estimates for dam safety are routinely obtained from hydrologic simulations driven by selected design storms. The temporal structure of such design storms can be obtained from Rainfall Mass Curves (RMCs), which are adimensionalized curves of the cumulative precipitation depth as a function of event duration. This paper assesses for the first time the spatialand temporal variability of observed RMCs for Switzerland, an Alpine region with complex topography. The relevance of the detected RMC variability for extreme flood estimation is illustrated based on an application to a high elevation catchment, the Mattmark dam catchment in the Swiss Alps. The obtained results underline that quantile RCMs represent a simple yet powerful tool to construct design storms for dam safety verification and that regional, seasonal and event-duration effects on RMCs are small enough to justify the use of a unique set of Swiss-wide quantile RMCs. The presented analysis could be refined in the future by explicitly accounting for orographic, convective or frontal precipitation events. [ABSTRACT FROM AUTHOR]

Abstract: In combination with land use change, climate change is increasingly leading to extreme weather conditions and consequently novel hydrologic conditions. Rainfall Area intensity‐duration‐frequency (IDF) curves, commonly used tools for modeling hydrology and managing flood risk can be used to assess hydrologic response under extreme rainfall conditions. We explore the influence of land use change on hydrologic response under designed extreme rainfall over the period 1976 to 2006 in the Kamo River basin. Run‐off for all six designed rainfall shapes under 2006 land use is higher than that under 1976 land use, but the timing of peak discharge under 2006 land use occurs at roughly the same time as that under 1976 land use. Results indicate that run‐off under 2006 land use yielded higher discharge than under 1976 land use, and rainfall shape six leads to the most extreme hydrologic response and most dangerous conditions from the perspective of urban planning and flood risk management. Future hydrologic response will differ from present due both to changes in land cover and changes in extreme rainfall patterns requiring modification to Area IDF curves for catchments. [ABSTRACT FROM AUTHOR]

Design storm is one of the most important tools to design hydraulic structures, hydrologic system and watershed management, mostly extracted by intensity- duration - frequency (IDF) curves for a given specific duration and return period. As for conventional methods to calculate IDF curves, the precipitation should be recorded for different durations so that foregoing curves can be extracted. Such data can be collected from rain gauge stations. In many areas, just daily precipitation data are available by which IDF curves cannot be extracted as per conventional methods. The aim of this research is to make IDF curves for short-term durations according to time scaling model as well as daily rainfalls. The relationships of this method are characterized with three variables including mean ( μ ) and standard deviation ( σ ) of daily rainfall intensity, and scaling exponent (H) by which all IDF curves might be drawn. The method used in present paper entails for less computational steps than conventional methods and by far has low parameters considerably than others in turn increases reliability. Scaling method is used to extract the IDF curves in rain-gauge stations in Khuzestan province located in southwest Iran and results proved the efficiency and robustness of the scaling method. Also ability of scaling concept method was examined in constructing of regional IDF. [ABSTRACT FROM AUTHOR]

The storm hyetograph is critical in drainage design since it determines the peak flooding volume in a catchment and the corresponding drainage capacity demand for a return period. This study firstly compares the common design storms such as the Chicago, Huff, and Triangular curves employed to represent the storm hyetographs in the metropolitan area of Guangzhou using minute-interval rainfall data during 2008-2012. These common design storms cannot satisfactorily represent the storm hyetographs in sub-tropic areas of Guangzhou. The normalized time of peak rainfall is at 33 ± 5% for all storms in the Tianhe and Panyu districts, and most storms (84%) are in the 1st and 2nd quartiles. The Huff curves are further improved by separately describing the rising and falling limbs instead of classifying all storms into four quartiles. The optimal time intervals are 1-5 min for deriving a practical urban design storm, especially for short-duration and intense storms in Guangzhou. Compared to the 71 observed storm hyetographs, the Improved Huff curves have smaller RMSE and higher NSE values (6.43, 0.66) than those of the original Huff (6.62, 0.63), Triangular (7.38, 0.55), and Chicago (7.57, 0.54) curves. The mean relative difference of peak flooding volume simulated with SWMM using the Improved Huff curve as the input is only 2%, -6%, and 8% of those simulated by observed rainfall at the three catchments, respectively. In contrast, those simulated by the original Huff (-12%, -43%, -16%), Triangular (-22%, -62%, -38%), and Chicago curves (-17%, -19%, -21%) are much smaller and greatly underestimate the peak flooding volume. The Improved Huff curve has great potential in storm water management such as flooding risk mapping and drainage facility design, after further validation. [ABSTRACT FROM AUTHOR]

Rainfall accumulation depths over a given area are strongly dependent on the shape of the storm together with its direction of advection. A method to produce design storms exhibiting anisotropic spatial scaling is presented by combining a state-of-the-art stochastic rainfall generator STEPS with the linear generalized scale invariance (GSI) notation. The enhanced model is used to create ensembles of design storms based on an extreme storm with a distinct rainband shape observed in Melbourne, Australia. Design storms are generated both with and without accounting for anisotropy. Effect of anisotropy on precipitation characteristics is studied using the entire region covered by the radar ( radar scale) and at a significantly smaller catchment scale. A rainfall-runoff model is applied to route the rainfall through the catchment into streamflow. Accounting for anisotropy allows for a more realistic description of precipitation features at the radar scale. At the catchment scale, anisotropy increases the probability of high rainfall accumulations, which translates into greater flood volumes. No discernible difference was observed in streamflow characteristics after controlling for the accumulation over the catchment. This could be explained by a lower importance of anisotropy relative to other factors affecting streamflow generation, and by the difficulties in creating representative rainfall temporal properties at the catchment scale when the radar scale is used for model calibration. The proposed method provides a tool to create ensembles of design storms when the anisotropic shape of the fields is of importance. [ABSTRACT FROM AUTHOR]

Abstract: Low impact development (LID) is generally regarded as a more sustainable solution for urban stormwater management than conventional urban drainage systems. However, its effects on urban flooding at a scale of urban drainage systems have not been fully understood particularly when different rainfall characteristics are considered. In this paper, using an urbanizing catchment in China as a case study, the effects of three LID techniques (swale, permeable pavement and green roof) on urban flooding are analyzed and compared with the conventional drainage system design. A range of storm events with different rainfall amounts, durations and locations of peak intensity are considered for holistic assessment of the LID techniques. The effects are measured by the total flood volume reduction during a storm event compared to the conventional drainage system design. The results obtained indicate that all three LID scenarios are more effective in flood reduction during heavier and shorter storm events. Their performance, however, varies significantly according to the location of peak intensity. That is, swales perform best during a storm event with an early peak, permeable pavements perform best with a middle peak, and green roofs perform best with a late peak, respectively. The trends of flood reduction can be explained using a newly proposed water balance method, i.e., by comparing the effective storage depth of the LID designs with the accumulative rainfall amounts at the beginning and end of flooding in the conventional drainage system. This paper provides an insight into the performance of LID designs under different rainfall characteristics, which is essential for effective urban flood management. [Copyright &y& Elsevier]

Abstract: The purpose of this paper is to develop an empirical formula to estimate design rainfall intensity based on intensity–duration–frequency (IDF) curves. These curves have been generated from a 32-year recorded rainfall data for Riyadh region. Rainfall intensity–duration–frequency curves describe rainfall intensity as a function of duration for a given return period which are important for the design of storm water drainage systems and hydraulic structures. The formula is derived using the analysis of results of three different frequency methods, namely: Gumbel, Log Pearson III, and Log normal. These methods are used to obtain the IDF curves for six different durations (10, 20, 30min, and 1, 2, 24h) and six frequency periods (2, 5, 10, 25, 50, 100years) where the best method of IDF estimation is recommended for future analysis. The equation can predict rainfall intensity in Riyadh region for any return period with a given storm duration and calibrated parameters obtained from IDF curves. Good match was achieved between its results and other analytical methods results such as Gumbel method. Moreover, it allows incorporating data from non-recording stations, thus remedying the problem of establishing IDF curves in places with a sparse network of rain-recording stations. [Copyright &y& Elsevier]

This article presents the elements involved in the design of a bioretention planter/trench infiltration-detention system as part of a very large-scale urban retrofit project. The prototype system was designed to intercept all of the runoff from a synthetic 5.08-mm 24-h rainfall event. Diverted flows were conveyed into bioretention planter for treatment. The bioretention systems were fingerprinted into areas comprising 0.8% of the contributory drainage areas, with an associated stone trench comprising another 3.4%. As layered systems, an approach that is capable of modeling vertical flows in addition to dynamic routing of outflows is used. The system was first modeled using HydroCAD, a design storm event modeling software. A four-compartment node system is used to model the dynamics of flow through the layers. The system was then modeled using SWMM 5.0.014 continuous simulation software. The resulting response to a design storm was computed by both of these models to compare the results of each method. The resulting SWMM model was then run on the 2005 design year rainfall distribution. Under existing conditions, over 60% of annual runoff volume exceeded the 3.50 L·s-1·ha-1(0.05 cfs-ac-1) threshold for initiation of combined sewer overflows (CSOs). Nearly all runoff was intercepted by the planter/trench infiltration system and even with a soil infiltration rate of only 2.54 mm·h-1, 47% was infiltrated, and less than 6% was discharged at rates that could initiate CSOs. The number of CSO exceedance pulses was reduced from 233 to 6, a reduction of 97%. The volume of flows exceeding the CSO threshold decreased by 90% in the planter/trench system. [ABSTRACT FROM AUTHOR]

After only a few years of operation, an extensive rain gauge network provides fruitful information on temporal patterns of local storms, helping urban water, managers with in the difficult choice of appropriate design storms. A total of 1470 1-h storms were identified for the period 1999-2005 in Québec City based on rainfall depth and interevent time criteria. Taking advantage of a clustering technique, the Kohonen neural network, 1-h storms were divided into 16 clusters depending on similarities in their temporal patterns, and then lumped into four groups. The database revealed that about one-third of all storms have a uniform intensity, one-third are early-peaking, and one-third are either symmetrical or late-peaking. Early-peaking patterns include the highest maximal 5-min intensity: 0.22-0.30 of the rainfall depth range, therefore in the same range as common Canadian 10-year design storms. [ABSTRACT FROM AUTHOR]

Summary: This study focuses mainly on increases in peak discharges with reductions in recurrence intervals for design hydrographs due to growing imperviousness. Rainfall–runoff simulation is a major basis for analyzing the hydrological effects of urbanization. Available recordings of 50 rainfall–runoff events during 1966–2002 were used as the study sample. Forty events were calibrated to determine the relationships between impervious areas and hydrological parameters in the Nash and SCS models. Block Kriging and non-linear programming methods were used to estimate the mean rainfall and its hourly excess value, respectively. The remaining 10 cases were used to test the established relationships. Calibration and verification results confirm that the methods used in this study effectively illustrate the hydrological and geomorphic conditions in complex urbanization processes. The rainfall–runoff routings, by using the design storm approach and the established relationships, demonstrated the following: (1) peak flows of the design flood hydrographs increased by about 127, 266, 375, 440, 515, 564, 593, and 629m3/s for eight return periods, respectively; (2) peak times were individually shortened from 9 to 6h due to urbanization; (3) the recurrence intervals of 200, 100, 50, and 25 years before urbanization were reduced to about 88, 33, 16, and 8 years also due to urbanization if they would have occurred at the present time within the Wu–Tu watershed. Finally, these analytical results can be obtained easily from the designed diagram that shows the relationships among peak discharges, impervious areas and return periods. This research can be used to prevent disasters, loss of life and property damage. [Copyright &y& Elsevier]

• Maximum rainfall intensity versus time window follows single or broken power laws. • Most convective storms follow broken power law showing transition at about 17 min. • Scaling transition implies the limitation of single power function for design storm. During a storm event, rainfall intensity rarely remains uniform but rather shows radical temporal variation. Given the time window w within a storm, the maximum rainfall intensity over w , namely i w decreases as w increases. Our analysis of individual storm events using ground measurements of 1-minute temporal resolution reveals that the i w – w relationship follows either a single or broken power law. Such scale-invariance is likely associated with known fractal or multi-fractal characteristics of rainfall structure. In broken power-law events, two power-law exponents of β m (at minute time scale) and β h (at hourly time scale) are fitted for segments separated by the characteristic time τ. Mostly β m < β h , implying the persistence of high rainfall concentration prior to τ. This reflects the characteristics of convective storms, and the range of τ agrees with the known time span of convective storm duration. The more concentrated storm event exhibits the stronger persistence (a smaller β m) prior to τ and the faster moisture depletion (a greater β h) thereafter. The knowledge gained from this study provides important implications to the existing design storm formula, often called the Mononobe formula: observed scale-free intra-storm variability is in accordance with this formula while the scaling transition at sub-hourly time scale explains the known limitations of its practical applications. [ABSTRACT FROM AUTHOR]