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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]

Short-calendar-time flash flood is one of the serious disasters threatening the safety of transportation of villages and towns in Longnan mountainous area. In order to improve the disaster prevention and mitigation capability of the area, it is of great scientific significance to calculate the degree of flood inundation loss through different rainfall intensities, and to guide the local people in the prevention of flash floods by using the flood risk zoning map. In this paper, based on the HEC-RAS hydrological analysis method, combined with GIS to simulate the flood inundation process of the watershed, and taking Pujiagou in Longnan mountainous area as the research object, the traditional flood risk zoning maps were optimized under the conditions of one-in-5-year, one-in-10-year, one-in-50-year, and one-in-100-year design rainfall, taking into account the multiple factors such as the slope, the land type, the loss rate, the water level, the flow rate, and so on. The results show that compared with the risk zoning map drawn by the traditional method, the optimized risk zoning map by the preferential map method pays more attention to the affected degree of the disaster-bearing body, and solves the shortcomings of the traditional risk zoning map, which is difficult to classify the risk level because of the large span of the risk level of the small areas. In the optimized risk zoning map, the risk level of the upstream and middle reaches of the uninhabited area is reduced, and the risk level of the downstream area of Maquan Village is more clear. Taking Wangjiazui in Maquan Village as an example, under the design rainfall of one-in-50-year, the risk zoning map drawn by the traditional method covers five risk levels, and the area difference of each zone is not significant, which makes it difficult to determine the final risk level. The optimized risk zoning map of the preferred method is more concentrated, and the area of Wangjiazui high-risk area is less than 4% of the medium-risk area, and the risk area of Wangjiazui can be clearly located in the medium-risk area. The optimized risk zoning map of this paper is more advantageous in practicality and adaptability, which can provide help for the early warning and the prediction of flash floods in small watersheds, and are also helpful to disaster prevention and mitigation work. [ABSTRACT FROM AUTHOR]

The design flood for a coastal basin based on model simulations can be impacted by various conditions other than design storm, such as tide level. However, the design storm and tide level are usually investigated separately, and the relation between them has not been sufficiently studied, which may have unexpected impacts on model simulations. This paper presents a bivariate assessment framework that aims to evaluate the coordination between design storm and tide level using a copula-based joint distribution to derive the conditional probability of coincidences between design storm and tide level intervals. We apply this framework to the Tai Lake Basin (TLB) in the eastern coast of China and investigate what tide level along its coastline is appropriate for guiding flood control planning. Our findings reveal that the currently-used tide level along the southern coast of TLB is too low and should be raised by approximately 0.35 m and 0.12 m under maximum 1-day and 3-day storms, respectively. Meanwhile, the tide levels for the north and east coasts are appropriate. This paper provides a comprehensive understanding of the interplay between design storm and tide level in a large coastal basin, offering insights into the design of flood protection systems. [ABSTRACT FROM AUTHOR]

Given the fact that the high frequency of extreme weather events globally, in particular typhoons, has more of an influence on flood forecasting, there is a great need to further understand the impact of typhoon events on design storms. The main objectives of this paper are to examine the magnitude, occurrence, and mechanism of typhoon events in southeast coastal China and their contribution to the design storm study. We take Shanghai, which is a typical metropolitan region in the Yangtze River Delta, China, as an example. The impact of typhoons on the rainfall frequency analysis is quantitatively evaluated using stochastic storm transposition (SST)-based intensity–duration–frequency (IDF) estimates with various temporal and spatial structures under different return periods. The results show that there is significant variability in the storm magnitude within the transposition domain across different durations, highlighting the spatiotemporal heterogeneity over the coastal area. Moreover, the probability of random storm transposition exhibits an uneven distribution. The frequency of typhoon rainfall events within the transposition domain is notably high, and there is considerable variability in the structure of rainfall. Typhoon rainfall amplifies the intensity of design storms, and its contribution increases with return periods. The variability in design storms increases accordingly. Based on the advantages of SST, which retains the spatiotemporal structure of the rainfall in the generated scenarios, the overall framework provides an effective way to examine the impact of diverse characteristics of typhoon rainfall on frequency analysis and facilitate a deeper exploration of the direct impact of various types of extreme storms on the intensity, spatial, and temporal distributions of design storms amidst evolving environmental conditions over this metropolitan region. [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]

Superstandard overland flow generalized computing method comes up to solve the channel flood calculation, which is influenced by urban terrain and consequently receipt more rainfall flood in ungauged region. This method used design storm as an exchange, analyzed the design storm in water conservancy industry traditional atlas and the design storm in municipal industry approved formula, compared the differences, explored the recurrence interval cohesion way of the two industries and finally conversed to the superstandard overland flood value. This research took the Xiaokongtong channel in Qingyang City, Gansu Province as an example, calculated three sections, including the tableland flood, superstandard overland flood and pipe network catchment flow, provided some theoretical reference values for the channel cases that took the superstandard overland flow into consideration but entirely rely on the urban storm flood model and achieved less effect from it. [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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It is time to shift our paradigm of small watershed design from a graphic (or tabular) to a theoretical method, because (1) the recent general unit hydrograph (UH) model can convert a design hyetograph to a design hydrograph simply, accurately, and theoretically; (2) the Natural Resources Conservation Service (NRCS) has recommended that the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 rainfall data of depths and distributions at a specific site, which is often called the NRCS-NOAA rainfall distributions, should be used for small watershed design if runoff data are unavailable; and (3) in this paper, we have presented a design procedure that formulates design hydrographs from the NRCS-NOAA Atlas 14 rainfall distributions and the general UH model automatically, using the MATLAB convolution function. A literature review indicated that the current practice for design hydrographs in small watersheds from hyetographs is laborious because both hyetographs and UHs are discrete. By contrast, the theoretical general UH model can significantly simplify this process. In this research, we first found analytic design hydrographs from rectangular and triangular hyetographs, which were next used to validate the MATLAB convolution method. We then proposed a double exponential rainfall distribution for both asymmetric and symmetric hyetographs. After that, we modified the symmetric exponential distribution model to describe NRCS-NOAA Atlas 14 data for site-specific hyetographs, which are finally convolved with the general UH model for site-specific design hydrographs, using the MATLAB convolution function. It is noteworthy that the proposed method extends the classic rational method from the peak discharge to the whole hydrograph; and it is valid for both continuous and discrete hyetographs. Hence, it provides a powerful tool in urban development, agriculture land use, roadway planning, and airport construction; it can also be used to evaluate an existing drainage system under various meteorologic–hydrologic conditions. Finally, we expect that this research will shift our current design practice and university UH teaching from an empirical to a theoretical paradigm in the near future. [ABSTRACT FROM AUTHOR]

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]

As climate change advances, the stationarity assumption that governs traditional precipitation analysis is becoming untenable. Studies that incorporate nonstationarity typically use global circulation model (GCM) projections to determine the magnitude and direction of expected precipitation changes. However, the high computational costs and the coarse spatial resolution of GCMs make this method unsuitable for local precipitation analysis. In this study, nonstationarity is represented by a precipitation probability distribution with time-varying parameters. Three generalized extreme value (GEV) distributions are fitted: (1) the shift model, where the GEV location parameter varies linearly with time, (2) the stretch model, where the GEV location and scale parameters both vary linearly with time, and (3) the stationary model, a time-invariant distribution provided for the purpose of comparison. This procedure is applied to 24-h annual maximum precipitation records for ninety years (1900-1989) at five long-term measuring sites in Pennsylvania. Results varied among the five sites, suggesting that localized climate effects can cause precipitation differences at a small spatial scale. No significant nonstationarity was detected in two of the five locations. In three locations, however, increases in GEV location and scale combined to create a substantial, though not always significant, rise in the frequency of extreme precipitation. These trends were extrapolated forward over 30 years (1990-2019) and compared with an observed distribution for that year. The nonstationary models appeared to perform better at sites with stronger precipitation trends, which suggests a simple procedure for selecting sites where nonstationary analysis is most needed. [ABSTRACT FROM AUTHOR]

Design storms guarantee uniformity regarding quality and operation standards of engineering projects and have been employed widely in urban drainage system design. Commonly used urban design storms, such as the Chicago (K-C) storm and Pilgrim and Cordery (P-C) storm, are calculated using prescribed or historical hyetographs. A prescribed hyetograph is unsuitable for a particular urban region in most cases, and a historical hyetograph takes no account of the spatiotemporal variations between the rainfall pattern at the rain station and that within the calculated region. Additionally, neither method can make adaptive adjustments for climate change. To obtain a more practical design storm with consideration of spatiotemporal variations and climatic changes, this study introduced proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) to urban design storm study. To demonstrate the feasibility and advantages of the proposed methodology, four cities (London, New York, Sydney, Wuhan) in different continents with different climatic characteristics were selected as case studies. The principal results are as follows. (1) Breaking the assumption of a uniform precipitation distribution, the proposed DMD-POD method is effective in capturing design storms under current climatic conditions and is sufficiently flexible to adapt to climate change. (2) The low-order representation of the rainfall field indicates substantial change in the storm patterns in urban areas. The peak discharge in New York and Wuhan is almost 10% higher than before urbanization, while that in Sydney and London is more than 10% lower on average. The peak time is largely unchanged in New York and Sydney, while it is 4 and 5 h later in London and Wuhan, respectively. (3) Compared with the K-C storm and P-C storm, the flood peak of POD storm increases and appears slightly earlier. The flood peak time in Wuhan simulated with the POD storm is 1 h (2 h) earlier than that simulated with the K-C storm (P-C storm). The peak flow obtained by the POD storm is 9.55% (25.05%) greater than that obtained by the K-C storm (P-C storm), which means that the POD design storm demands a higher level of safety for an engineering project under the same return period. [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.

The replacement of natural pervious surfaces with impervious surfaces due to urbanization, construction, and development causes excess stormwater runoff and results in cities experiencing localized flooding events. The installation of green stormwater infrastructure (GSI) is one way of reducing flooding events and preventing downstream erosion and damage. In this study, computer rainfall-runoff simulations were performed to analyze GSI's effectiveness in mitigating stormwater runoff when applied to sites with different soil types and for which different design storms were established by regulation. A mixed-use development site was used as a hypothetical site on which to perform the analysis. The study applied the same design to six small- to medium-sized cities in the southeastern United States with different design storm magnitudes. The cities’ ordinances were reviewed, and none required GSI. Therefore, this study revised some of the stormwater management requirements to stress GSI implementation, and then stormwater modeling was conducted to see how regulatory changes would affect runoff. The HydroCAD stormwater modeling tool was used to perform hydrologic simulations for the hypothetical building site in each of the six cities using the design storms and small storms of the cities. Even though GSI has been commonly implemented in large cities, small and medium-sized cities can also prevent excess stormwater by incorporating GSI in their ordinances for new developments and site retrofits. Based on the hydrologic simulation results, municipalities with lower magnitude design storms and low infiltration soils have the most to benefit from GSI and could benefit from ordinances requiring GSI. For smaller, more frequent storms, GSI alone can meet the pre-development peak flow requirements.

It is now well established that our warming planet is experiencing changes in extreme storms and floods, resulting in a need to better specify hydrologic design guidelines that can be projected into the future. This paper attempts to summarize the nature of changes occurring and the impact they are having on the design flood magnitude, with a focus on the urban catchments that we will increasingly reside in as time goes on. Two lines of reasoning are used to assess and model changes in design hydrology. The first of these involves using observed storms and soil moisture conditions and projecting how these may change into the future. The second involves using climate model simulations of the future and using them as inputs into hydrologic models to assess the changed design estimates. We discuss here the limitations in both and suggest that the two are, in fact, linked, as climate model projections for the future are needed in the first approach to form meaningful projections for the future. Based on the author's experience with both lines of reasoning, this invited commentary presents a theoretical narrative linking these two and identifying factors and assumptions that need to be validated before implementation in practice. This article is part of a discussion meeting issue 'Intensification of short-duration rainfall extremes and implications for flash flood risks'. [ABSTRACT FROM AUTHOR]

Study region: The present study focuses on Emilia-Romagna (northern Italy), a densely populated region affected in the last few years by several destructive flood events. Study focus: A rich and detailed regional dataset of annual maximum series (AMS) of sub-daily rainfall is considered with a three-fold aim: to detect possible changes in (1) seasonality and (2) magnitude of sub-daily rainfall extremes, (3) to assess the current reliability and accuracy of a previously developed regional model for rainfall frequency estimation based on the local value of mean annual precipitation (MAP). New hydrological insights for the region: The main findings of the study include: (1) delay (towards Autumn) in the mean timing of sub-daily rainfall extremes, more marked for higher elevations and longer durations; (2) significant increase in the magnitude of sub-daily extreme events for some areas, especially in the Apennine; (3) high sensitivity of the regional predictions of the frequency regime of sub-daily rainfall extremes to local changes in AMS and MAP.

The rainfall data is available for India majorly as 24-hour records, taken between 08:30 hours on consecutive days. Indian Meteorological Department (IMD) is establishing continuous recording rain gauge network, but sufficient sub-daily data would become available only after a few decades. Consequently, practicing engineers resort to generic time distribution curves to obtain sub-daily rainfall. Time distribution curves for rainfall available for India includes Central Water Commission-Indian Meteorological Department-Ministry of Surface Transport (CWC-IMD-MoST) curves for various zones and IMD-CWC curves for few river basins, respectively. Time distribution curves of rainfall are also available in international literature, namely, Standard Project Storm (SPS) and Technical Report No. 55 (TR55), both developed for USA. In this article, all these curves are evaluated for accuracy at a coastal site near Mumbai, India, which predominantly receives south-west monsoon rainfall. From the study, it is concluded that whereas SPS (index rainfall: 8 inches) and TR55 curves would, in general, provide safe (though uneconomic) design, adoption of SPS (index rainfall: 20 inches) curve, CWC-IMD-MoST curve (subzone 5a & 5b) or IMD-CWC curve for Catchment No. 102 for west flowing rivers would result in unsafe hydraulic design for this site. The deviations could be attributed to different rainfall characteristics at foreign locations or due to temporal, areal and ensemble averaging for the river basins in India. Rather than adoption of generic curves from literature, site specific time distribution curve for rainfall would be preferable for safe and economic design of important hydraulic structures. [ABSTRACT FROM AUTHOR]

This paper evaluates the design peak flow for several return periods using two approaches: 1) a flood frequency analysis; 2) rainfall-runoff model applied to a design storm. The design storm originates from the design rainfall depth, which is distributed throughout the rainfall duration via synthetic hyetographs. In the flood frequency analysis, the design peak flow was obtained from a 30-year series of flows estimated from rainfall observations at a five-minute time step. The design storm was routed with the same rainfall-runoff model and parameters to yield the design peak flow. The hyetographs tested were categorized as having advanced, delayed, alternating, uniform and empirical blocks that were built from rainfall observations. All design storms analysed undersized the hydraulic structures. The results, which were valid for the study region, indicated the need to rethink the concept of design storms, which associate the design storm depth with synthetic hyetographs, for a conservative standard. [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]

Estimation of urban runoff peak and volume is a fundamental step in determining the transferring capacity of urban drainage systems. The main aim of this study was to present an application of the Storm Water Management Model (SWMM) in order to estimate urban flooding of a semi‐arid area (Zanjan city in the northwest of Iran). The performance of an urban drainage system in the study area was also investigated. According to the results, SWMM is an effective tool for urban flood estimation in a semi‐arid area. In this study, urban peak flow was simulated via a calibrated model with acceptable accuracy. Based on the results of the model simulation, the capacity of the main canals in the study area is sufficient for peak runoff transferring for a design storm with 50 year return periods, without retrofitting. Whereas, based on local observation and model results, localized and surface flooding can be observed in some urban areas. [ABSTRACT FROM AUTHOR]

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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: Distributions of rainfall with the time and space resolutions of minutes and kilometers, respectively, are often needed to drive the hydrological models used in a range of engineering, environmental, and urban design applications. The work described here is the first step in constructing a model capable of downscaling rainfall to scales of minutes and kilometers from time and space resolutions of several hours and a hundred kilometers. A multiplicative random cascade model known as the Short‐Term Ensemble Prediction System is run with parameters from the radar observations at Melbourne (Australia). The orographic effects are added through multiplicative correction factor after the model is run. In the first set of model calculations, 112 significant rain events over Melbourne are simulated 100 times. Because of the stochastic nature of the cascade model, the simulations represent 100 possible realizations of the same rain event. The cascade model produces realistic spatial and temporal patterns of rainfall at 6 min and 1 km resolution (the resolution of the radar data), the statistical properties of which are in close agreement with observation. In the second set of calculations, the cascade model is run continuously for all days from January 2008 to August 2015 and the rainfall accumulations are compared at 12 locations in the greater Melbourne area. The statistical properties of the observations lie with envelope of the 100 ensemble members. The model successfully reproduces the frequency distribution of the 6 min rainfall intensities, storm durations, interarrival times, and autocorrelation function. [ABSTRACT FROM AUTHOR]

In the case of ungauged catchments, different procedures can be used to derive the design hydrograph and design peak discharge, which are crucial input data for the design of different hydrotechnical engineering structures, or the production of flood hazard maps. One of the possible approaches involves using a hydrological model where one can calculate the design hydrograph through the design of a rainfall event. This study investigates the impact of the design rainfall on the combined one-dimensional/two-dimensional (1D/2D) hydraulic modelling results. The Glinšˇcica Stream catchment located in Slovenia (central Europe) is used as a case study. Ten different design rainfall events were compared for 10 and 100-year return periods, where we used Huff curves for the design rainfall event definition. The results indicate that the selection of the design rainfall event should be regarded as an important step, since the hydraulic modelling results for different scenarios differ significantly. In the presented experimental case study, the maximum flooded area extent was twice as large as the minimum one, and the maximum water velocity over flooded areas was more than 10 times larger than the minimum one. This can lead to the production of very different flood hazard maps, and consequently planning very different flood protection schemes. [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]

Copyright of Tecnología y Ciencias del Agua is the property of Instituto Mexicano de Tecnologia del Agua (IMTA) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This study integrates and develops methods, namely low impact development (LID) selection method and an LID spatial planning model, to enable decision-making to minimize pluvial flooding for a community. The objective is to minimize the flood risk under the worst case of the design storm within the budget constraints. Design storms in current and future climate scenarios are analyzed as input to the Storm Water Management Model (SWMM). Then, LID practices are selected based on the proposed procedure and a spatial planning model is built to identify the optimal LID layouts using the simulated annealing (SA) algorithm. The lower and upper bounds of the generated rainfall intensities of a five-year 1-h duration design storms for the Hadley Centre Global Environment Model version 2 for the atmosphere and oceans (HadGEM2-AO), the Norwegian Earth System Model (NorESM1-ME), and the CSIRO-Mk3.6.0 Atmosphere-Ocean GCM (CSIRO-Mk3.6.0) during 2021-2040 are derived. The LID selection helps efficiently identify appropriate LID. Results show that nearly no flood occurs under the optimal LID layouts found by the LID spatial planning model. Moreover, it is more optimal to invest in LID in the lower sub-catchments in LID planning when the budget is limited. These methods are generally applicable for a community using LIDs as adaptation measures against pluvial flooding. [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]

While the size of single low impact development (LID) unit determines the performance in attenuating water quantity and quality from storm runoff, the performance of multiple LID units is sensitive to both their size and arrangement order. This study describes a methodology to obtain the best performance for multiple LID units under varying by time-to-peak of storm with the same intensity and duration using storm water management model (SWMM), a popular model for rainfall-runoff and water quality simulation. The hypothetical temporal distributions were designated by Huff curves, which provided characterizing storm mass curves, along with the relationship of intensity-duration-frequency to determine storm intensity for 1-h in 2-year return period. Three types of LID units (rain barrel, infiltration trench, and vegetative swale) were selected to develop aggregate LID scenarios using the SWMM. The results indicated that, when compared with other field experiments, the SWMM successfully estimated change in flow discharge and suspended solid (SS) loss reflecting different storm patterns at the final outlet of and urbanized sub-catchment as well as the effects of LID practices. The performances of aggregate LID scenarios including lag time of peak runoff, peak runoff reduction, volume reduction, and SS loss reduction were sensitive to arrangement order and time-of-storm peak-to-storm duration. Scenario 5, which had the order of vegetative swale, rain barrel, and infiltration trench, showed the most effective serial arrangement, as it exhibited the more consistent results across the storm patters. This study thus provides insights into the effective design of aggregate LID scenarios considering different storm characteristics. [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]