Return Period Storm Research Articles

Regional coastal flood risk assessment for a tidally dominant, natural coastal setting: North Norfolk, southern North Sea

A Coastal Risk Assessment Framework (CRAF) provides two levels of coastal risk and vulnerability assessment, by combining information on the spatially variable hazard and exposure. In Phase 1, areas of greatest risk or ‘hotspots’ are identified. In Phase 2, these hotspots are then analysed in greater detail to identify both direct and indirect extreme event impacts. This approach was applied to the barrier coastline of North Norfolk, eastern England. The CRAF identified high risk coastal hotspots on the basis of both hazard impacts (swash regime (tide + surge + wave runup) and overwash/terrestrial inundation regimes) from a 1 in 115 year return period storm and a range of land use, infrastructure, economic and social vulnerability indicators. Hazard extents and hazard severity, in some locations modified by the presence of intertidal saltmarsh, were calculated for 45, 1–2 km wide sections along the topographically complex coast. When combined with five exposure indicators, eight hotspots were identified along the 45 km long frontage. In a 2nd phase, two of these hotspots, one a chain of small villages (Brancaster/Brancaster Staithe/Burnham Deepdale) and one a small town (Wells-next-the-Sea), were compared in more detail using a suite of coastal inundation and impact assessment models to determine both direct and indirect impacts. Hazards at this higher resolution were calculated using the 1D process-based XBeach model and the 2D LISFLOOD inundation model. Vulnerability to the hazards was calculated using the INDRA (Integrated Disruption Assessment) model with comparison of the two hotspots through the use of a Multi Criteria Analysis (MCA). The selection of hazard hotspots and comparison of hotspots using these techniques allows areas at greatest risk to be identified, of vital importance for coastal management and resource allocation.

Quantifying the impact of community-scale flood mitigation

Federal, State, and local governments, non-governmental organizations, and independent local community organizations spend billions of dollars on watershed management activities. The goals of watershed management projects may span a wide spectrum including, but limited to flood reduction, soil erosion reduction, water quality improvement or protection, and/or increase source of wood fiber for fuel or construction material. Accomplishing these goals is commonly done through revegetation campaigns, construction of small-scale hydraulic works, contour terracing, and urban development planning, among others. An ongoing challenge for these agencies, along with local stakeholders is to understand the various individual and combined impacts of different mitigation measures.The methodology and models developed and applied herein provide important tools for examining the feasibility of construction and the likely impacts of gabion dams and vegetation coverage applications (e.g., hillside and channel revegetation campaigns) in steep mountainous areas upstream of populated areas. The purpose of this investigation is to quantify the impact of changes in the size and location of the gabion dams and of changes in vegetation coverage in the Ravine Millet watershed near Port-au-Price, Haiti. This environment frequently experiences strong convective precipitation events, often the result of hurricane passage over the region.Intensity-Duration-Frequency curves were developed with available data, which were then used to develop design storms associated with different return period frequencies. Hourly satellite-derived precipitation was used to identify the shape and duration of various return period storm events. This investigation then applies some commonly available hydrologic and hydraulic models and various concomitant tools used to construct those models, along with a Kinematic Routing Model to assess the impact of various mitigation strategies. Storm events with multiple peaks of similar magnitude interact with channel geometry and other hydraulic characteristics and may result in peak delay or reduce the time of arrival and may increase or decrease the magnitude of flood peaks. Results indicate that hillslope revegetation impacts to flood peak and the time or arrival of flood peaks at the watershed outlet are limited to relatively low return period storms. Results also indicate that channel vegetation can reduce the peak discharge and significantly delay the arrival of flood peaks. The combined impacts of gabion dams and channel vegetation are highly non-linear, and depend on the shape and duration of storm peaks.

A two-parameter design storm for Mediterranean convective rainfall

Abstract. The following research explores the feasibility of building effective design storms for extreme hydrological regimes, such as the one which characterizes the rainfall regime of the east and south-east of the Iberian Peninsula, without employing intensity–duration–frequency (IDF) curves as a starting point. Nowadays, after decades of functioning hydrological automatic networks, there is an abundance of high-resolution rainfall data with a reasonable statistic representation, which enable the direct research of temporal patterns and inner structures of rainfall events at a given geographic location, with the aim of establishing a statistical synthesis directly based on those observed patterns. The authors propose a temporal design storm defined in analytical terms, through a two-parameter gamma-type function. The two parameters are directly estimated from 73 independent storms identified from rainfall records of high temporal resolution in Valencia (Spain). All the relevant analytical properties derived from that function are developed in order to use this storm in real applications. In particular, in order to assign a probability to the design storm (return period), an auxiliary variable combining maximum intensity and total cumulated rainfall is introduced. As a result, for a given return period, a set of three storms with different duration, depth and peak intensity are defined. The consistency of the results is verified by means of comparison with the classic method of alternating blocks based on an IDF curve, for the above mentioned study case.

Geographic information system-based assessment of mitigating flash-flood disaster from green roof systems

Urban flooding is a severe problem and a growing development challenge for many cities around the world. However, urban impervious areas have sharply increased owing to booming construction activities, and this land-use change leads to more frequent flood inundation in urban flood-prone areas. A green roof system is regarded as an effective mechanism to manage rainwater and reduce flooding disaster, as it is capable of retaining rainwater, thus reducing rainfall-runoff. There is still a lack of assessment of this stormwater management tool for flash floods. The issue of flood inundation associated with green roof systems needs to be explored and developed. To evaluate the effects of green roofs on urban flood inundation, this paper aims to construct a framework for modelling urban inundation integrating a hydrological model of green roofs. The approach addresses both urban rainfall-runoff and underground hydrological models for traditional impervious and green roofs. To accurately calculate the spatial variation, we have proposed a hydrological model to evaluate regional runoff on the basis of a catchment mesh. The Deakin University Waurn Ponds campus in Geelong was then chosen as a community-based study case. From geographic information system (GIS) simulation, the results reveal that the green roofs generated varying degrees of mitigation of urban flash floods with storms of different return periods.

The dual effect of vegetation green-up date and strong wind on the return period of spring dust storms

Vegetation phenology changes have been widely applied in the disaster risk assessments of the spring dust storms, and vegetation green-up date shifts have a strong influence on dust storms. However, the effect of earlier vegetation green-up dates due to climate warming on the evaluation of dust storms return periods remains an important, but poorly understood issue. In this study, we evaluate the spring dust storm return period (February to June) in Inner Mongolia, Northern China, using 165 observations of severe spring dust storm events from 16 weather stations, and regional vegetation green-up dates as an integrated factor from NDVI (Normalized Difference Vegetation Index), covering a period from 1982 to 2007, by building the bivariate Copula model. We found that the joint return period showed better fitting results than without considering the integrated factor when the actual dust storm return period is longer than 2years. Also, for extremely severe dust storm events, the gap between simulation result and actual return period can be narrowed up to 0.4888years by using integrated factor. Furthermore, the risk map based on the return period results shows that the Mandula, Zhurihe, Sunitezuoqi, Narenbaolige stations are identified as high risk areas. In this study area, land surface is extensively covered by grasses and shrubs, vegetation green-up date can play a significant role in restraining spring dust storm outbreaks. Therefore, we suggest that Copula method can become a useful tool for joint return period evaluation and risk analysis of severe dust storms.

Modifying the downstream hydrograph with staged labyrinth weirs

Labyrinth and piano key weirs are hydraulically more efficient than linear weirs of common width. As the spillway discharge efficiency increases, the required reservoir detention volume reserved for flood routing reduces and the maximum base-flow operating reservoir pool elevation can subsequently be increased (additional water storage) without a reduction in dam safety. Increased spillway discharge efficiency also causes the reservoir outflow hydrograph to compress temporally and increases the peak outflow discharge, potentially increasing downstream flooding impacts. The influence of linear, labyrinth, and staged labyrinth weir (i.e. cycles with different crest elevations) head-discharge characteristics on the outflow hydrograph behavior were evaluated by numerically routing various flood discharges through a reservoir; peak outflow discharges, the maximum water surface elevation, and the required detention volumes were quantified for each weir alternative. In addition to the benefit of isolating base flows to a subset of the labyrinth weir, the staged labyrinth weir proved to be an effective alternative for modifying (decreasing) the spillway discharge efficiency to limit downstream flooding impact for higher-frequency return-period storm events.

Analyzing trend and developing an empirical formula for estimating rainfall intensity: a case study Eskisehir city, Turkey

Trend and stationarity analysis conducted for the city center of Eskisehir province, Turkey. The Intensity- Duration – Frequency (IDF) curves and formulas for the estimation of the design rainfall intensity have been developed. Through the application of four different frequency methods, the IDF has been developed for a record of 64 years. The functions employed are; Normal, Log – Normal, Gumbel, and Log –Pearson Type III. A formula for each function developed by determining the parameters with the help of a non-linear regression method. The most appropriate fit of probability distribution determined by goodness of fit test i.e. Chi – Square. The results of the study revealed no evidence of significant trend or nonstationarity in any duration. With an exception to the Normal function which exceeded the critical region in several durations, all the functions found to show high fitting. LPT III identified as the best fit. The coefficient of correlation for the developed formulas found high in all the functions except for LPT III. The study endorsed that the estimation of rainfall intensity for any storm duration and return period can be done for Eskisehir using LPT III and Gumbel formulas as they are evaluated as best fitting.

Flow characteristics of individual lot stormwater detention

In this paper the flow characteristics of stormwater are analyzed as it travels from a roof gutter down-pipe and the turbulent flow generated on entering an individual lot on-site stormwater detention (OSD) unit beneath a residential carport. Comparison was made between a full-scale model and computational fluid dynamic (CFD) simulations to determine the flow characteristics. These modular tanks with multi-unit chambers can capture the roof run-off from a 15-minute, 10-year return period storm. The results from the physical and CFD models matched well, suggesting that turbulent flow occurs when stormwater is directed to an individual lot stormwater detention tank. However, turbulence in the OSD was concentrated around the inlet, after which the pattern changed from turbulent to laminar flow. This work implies that the use of modular underground storage tanks is practical for managing stormwater from a roof.

A Validated Tropical-Extratropical Flood Hazard Assessment for New York Harbor.

Recent studies of flood risk at New York Harbor (NYH) have shown disparate results for the 100-year storm tide, providing an uncertain foundation for the flood mitigation response after Hurricane Sandy. Here, we present a flood hazard assessment that improves confidence in our understanding of the region's present-day potential for flooding, by separately including the contribution of tropical cyclones (TCs) and extratropical cyclones (ETCs), and validating our modeling study at multiple stages against historical observations. The TC assessment is based on a climatology of 606 synthetic storms developed from a statistical-stochastic model of North Atlantic TCs. The ETC assessment is based on simulations of historical storms with many random tide scenarios. Synthetic TC landfall rates and the final TC and ETC flood exceedance curves are all shown to be consistent with curves computed using historical data, within 95% confidence ranges. Combining the ETC and TC results together, the 100-year return period storm tide at NYH is 2.70 m (2.51-2.92 at 95% confidence), and Hurricane Sandy's storm tide of 3.38 m was a 260-year (170-420) storm tide. Deeper analyses of historical flood reports from estimated Category-3 hurricanes in 1788 and 1821 lead to new estimates and reduced uncertainties for their floods, and show that Sandy's storm tide was the largest at NYH back to at least 1700. The flood exceedance curves for ETCs and TCs have sharply different slopes due to their differing meteorology and frequency, warranting separate treatment in hazard assessments.

Measuring eco-roof mitigation on flash floods via GIS simulation

Purpose The rapid and ongoing expansion of urbanised impervious areas could lead to more frequent flood inundation in urban flood-prone regions. Nowadays, urban flood inundation induced by rainstorm is an expensive natural disaster in many countries. In order to reduce the flooding risk, eco-roof systems (or green roof systems) could be considered as an effective mechanism of mitigating flooding disasters through their rainwater retention capability. However, there is still a lack of examining the stormwater management tool. The purpose of this paper is to evaluate the effects on flooding disaster from extensive green roofs. Design/methodology/approach Based on geographical information system (GIS) simulation, this research presents a frame of assessing eco-roof impacts on urban flash floods. The approach addresses both urban rainfall-runoff and underground hydrologic models for traditional impervious and green roofs. Deakin University’s Geelong Waurn Ponds campus is chosen as a study case. GIS technologies are then utilised to visualise and analyse the effects on flood inundation from surface properties of building roofs. Findings The results reveal that the eco-roof systems generate varying degrees of mitigation of urban flood inundation with different return period storms. Originality/value Although the eco-roof technology is considered as an effective stormwater management tool, it is not commonly adopted and examined in urban floods. This study will bring benefits to urban planners for raising awareness of hazard impacts and to construction technicians for considering disaster mitigation via roof technologies. The approach proposed here could be used for the disaster mitigation in future urban planning.

A new equivalent exponential storm model for long-term statistics of ocean waves

The paper proposes a new solution, in the context of equivalent storm models, for long-term statistics of ocean storms. These models are essentially based on the concept of equivalent sea which consists of substituting the actual storms at a given location with a sequence of equivalent storms, achieved from a statistical perspective.Firstly the Equivalent Triangular Storm (ETS) was proposed in which triangular storms were considered. The model was then generalized assuming a power law for a more realistic description of the storm history in time domain. The two models were developed following the same logic and assuming the equivalent storm symmetric with respect to its peak. In both cases the solution for the calculation of return period of a sea storm whose maximum significant wave height is greater than a given threshold is achieved, but only for ETSs it has a closed form and for all the others shapes it requires a numerical calculation.The main focus of the present work is to combine in a unique model the main advantages of the two previous ones, that is to obtain an optimized shape (as for EPS) and closed from solutions for the long-term statistics (as for ETS). An exponential law is introduced, taking into account asymmetry in storm profile. The long-term statistics of ocean storms is then investigated via this new Equivalent Exponential Storm (EES) model, by analyzing the effects of asymmetry between the growth and decay phases of each storm. Data from National Data buoy Center (NDBC) are processed for the new EES model validation and comparison with previous ETS and EPS. The results show that the assumption of asymmetric storm profile does not lead to significant changes of the long-term statistics, but could be very useful in other kinds of application as it enables a more realistic representation of storm history. Finally, in comparison with ETS and EPS, it has been seen that EES provides a better representation of actual storm duration.

Post-Evaluation of Flood Hazards Induced by Former Artificial Interventions along a Coastal Mediterranean Settlement

Inappropriate artificial activities can escalate the frequency, the intensity, and the magnitude of floods and aggravate their aftermath. Additionally, urban sprawl is responsible for increasing the manifestation of flood events in coastal areas. In this study, the effects of diachronically formulated artificial stream-channel modifications and registered urban expansion on flood hazards were quantified. Initially, land-use changes were monitored over time and a detailed field topographic survey was performed. The peak discharge for a 50-year return period storm event was found to be equal to 42.3 m3/s, as computed by the rational method. A steady flow analysis with a mixed flow regime was then performed so as to compute the water velocity and surface runoff elevation at 49 discrete cross sections for the years of 1945, 1981, and 2009, whereas the inundation extent was found equal to 12,208, 10,519, and 9,530 m2 for the same years by implementing another model. Although the water level and inundation extent were respectively reduced by 5.5 and 9.2% in 2009 compared with that in 1981, more houses had been constructed on the riverbed, and thus the flood hazard was greater in 2009 than in 1981. Despite the fact that the flood area was larger by 16.1% in 1945 compared with 1981, the flood hazards were diminished in 1945 because the households were settled more than 78 m away from the stream centerline. Thus, in 1945, the stream would have successfully sustained flood events even with a 50-year recurrence period.

Storm Surge Hindcast and Return Period of a Haiyan-Like Super Typhoon

The extremely intense Typhoon Haiyan (local name Yolanda), struck the Philippines in November 2013, catastrophically impacting the region. It is important to understand the storm surge characteristics and occurrence probability of such super typhoons for reconstruction of the devastated area. This study mainly examines two topics. First, the characteristics of Typhoon Haiyan and its related storm surge were analyzed with a series of numerical experiments using an empirical typhoon model and storm surge model. The maximum surge height and inundation area were estimated and validated against measurements of tides and wind speeds obtained from local agencies NAMRIA and PAGASA, as well as against post-event surveys of maximum inundation heights. Based on the analysis of best hindcast results, the maximum storm surge level was about 5 m at Tacloban on Leyte Island. Second, the return period of a Haiyan-like typhoon and associated storm surge was estimated with NOAA's long-term observed IBTrACS typhoon dataset and a stochastic tropical cyclone model. Statistical analysis of typhoons around Tacloban was performed, and the estimated return period of storm surge levels seen during Typhoon Haiyan is 240–360 years.

Identifying areas sensitive to land use/land cover change for downstream flooding in a coastal Alabama watershed

This paper explored the linkage between historic, current and future land use/land cover (LULC) conditions and peak flow and runoff volumes in a coastal community in Alabama in an effort to identify critical areas for downstream flooding. The study demonstrated that critical areas cannot be determined intuitively without conducting modeling studies. The study watershed, Eightmile Creek, experienced approximately 48 % forest loss between 1966 and 2011 largely due to urbanization. Residential development is expected to continue mostly in the central part of the watershed in the near future. Historic, current and future LULC maps were developed by processing aerial imagery, which were used in the HEC-HMS hydrologic model to study flood risk. An index method was applied to estimate the contribution of different parts of the watershed to downstream peak flows. The model showed a significant increase in peak flow and runoff volume from 1966 to 2011 and from 2011 to 2022 due to urbanization. The sensitivity of peak flows to LULC change decreased with increasing storm return periods, but the order of importance of different parts of the watershed, in terms of flooding, did not change significantly. Results of this study demonstrate the need for sustainable development by targeting areas that can have the least impacts on downstream flooding. The methodology presented in this paper can help decision makers propose land use alternatives to minimize adverse environmental impacts.

Regionalization of Rainfall Intensity-Duration-Frequency (IDF) Curves in Botswana

A regional analysis of design storms, defined as the expected rainfall intensity for given storm duration and return period, is conducted to determine storm Rainfall Intensity-Duration-Frequency (IDF) relationships. The ultimate purpose was to determine IDF curves for homogeneous regions identified in Botswana. Three homogeneous regions were identified based on topographic and rainfall characteristics which were constructed with the K-Means Clustering algorithm. Using the mean annual rainfall and the 24 hr annual maximum rainfall as an indicator of rainfall intensity for each homogeneous region, IDF curves and maps of rainfall intensities of 1 to 24 hr and above durations were produced. The Gamma and Lognormal probability distribution functions were able to provide estimates of rainfall depths for low and medium return periods (up to 100 years) in any location in each homogeneous region of Botswana.

SWMM-based evaluation of the effect of rain gardens on urbanized areas

Taking an area of Xi’an city as an example, the Storm Water Management Model was established according to the measured data after calibration and validation. Three different return period storms (once in 2 years, once in a decade, and once in 20 years) were designed to simulate the water quantity and quality of the study area with or without rain gardens. Rain gardens with different proportions (1, 2, and 4 %) of the study area were set for simulation and comparison. Finally, the control effects of rain gardens with and without discharge facility were compared. Rain gardens can reduce the total amount and peak flow of urban runoff. Such gardens can also reduce pollutant peak concentration and pollution load. The peak time of flow and concentration can be delayed. However, the control effect or reduction rate of rain gardens will decrease with increasing rainfall intensity. A larger area proportion of rain gardens results in a better control effect on urban runoff. The control effect of rain gardens without discharge facility is better than that of rain gardens with discharge facility under the same conditions of rainfall and area ratio. The control effect difference between rain gardens with and without discharge facility decreases with the increase in the area ratio of rain gardens.

Design and model confirmation of the intermediate scale VolturnUS floating wind turbine subjected to its extreme design conditions offshore Maine

AbstractFloating offshore wind turbines are gaining considerable interest in the renewable energy sector. Design standards for floating offshore wind turbines such as the American Bureau of Shipping (ABS) Guide for Building and Classing Floating Offshore Wind Turbine Installations are relatively new and few if any floating wind turbines have yet experienced the prescribed design extreme environmental conditions. Only a few pilot floating turbines have been deployed in Europe and Japan. These turbines have been designed for long return period storm events and are not likely to see their extreme design conditions during early deployment periods because of the low probability of occurrence. This paper presents data collected for an intermediate scale floating semi‐submersible turbine intentionally placed offshore Maine in a carefully selected site that subjects the prototype to scale extreme conditions on a frequent basis. This prototype, called VolturnUS 1:8, was the first grid‐connected offshore wind turbine in the Americas, and is a 1:8 scale model of a 6 MW prototype. The test site produces with a high probability 1:8 scale wave environments, and a commercial turbine has been selected so that the wind environment/rotor combination produces 1:8‐scale aerodynamic loads appropriate for the site wave environment. In the winter of 2013–2014, this prototype has seen the equivalent of 50 year to 500 year return period storms exercising it to the limits prescribed by design standards, offering a unique look at the behavior of a floating turbine subjected to extreme design conditions. Performance data are provided and compared to full‐scale predicted values from numerical models. There are two objectives in presenting this data and associated analysis: (i) validate numerical aeroelastic hydrodynamic coupled models and (ii) investigate the performance of a near full‐scale floating wind turbine in a real offshore environment that closely matches the prescribed design conditions from the ABS Guide. Copyright © 2015 John Wiley & Sons, Ltd.

Event-based design tool for construction site erosion and sediment controls

This paper provides additional discussion surrounding the novel event-based soil loss models developed by Trenouth and Gharabaghi (2015) for the design of erosion and sediment controls (ESCs) for various phases of construction – from pre-development to post-development conditions. The datasets for the study were obtained from three Ontario sites – Greensborough, Cookstown, and Alcona – in addition to datasets mined from the literature for three additional sites – Treynor, Iowa, Coshocton, Ohio and Cordoba, Spain. Model performances were evaluated for each of the study sites, and quantified using commonly-reported statistics. This work is nested within a broader conceptual framework, which includes the estimation of ambient receiving water quality, the prediction of event mean runoff quality for a given design storm, and the calculation of the required level of protection using adequate ESCs to meet receiving water quality guidelines. These models allow design engineers and regulatory agencies to assess the potential risk of ecological damage to receiving waters due to inadequate soil erosion and sediment control practices using dynamic scenario forecasting when considering rapidly changing land use conditions during various phases of construction, typically for a 2- or 5-year design storm return period.

Analysis of Climate Change and 24-Hour Design Storm Depths for a Range of Return Periods Across South Carolina

A warming climate leads to a moister atmosphere and more rapid hydrologic cycle. As such, many parts of the country are predicted to experience more total rainfall per year and more frequent extreme rainfall events. Most regions of the country have stormwater systems designed to a standard that matches outflow rates to pre-development values for specified return period storms. Increases in these return period storm depths, as predicted by many global climate models, will stress existing stormwater infrastructure. This paper examines how rainfall patterns will change over the remainder of the century across the state of South Carolina. Rainfall simulations from 134 realizations of 21 global climate models were analyzed across the state of South Carolina through 2099. Results show that there will be increases in both annual total rainfall (ATR) and 24-hour design storm depth for a range of return period storms. Across South Carolina, ATR is predicted to increase by approximately 2.3-4.0 inches over the forecast period while the 100 year design storm depth is predicted to increase by 0.5-1.2 inches depending on location. However there are significant regional variations with the Savannah River Basin experiencing smaller increases in ATR compared to the rest of the state.

Effects of wildfires on peak discharges in watersheds

Wildfires can alter the hydrological processes in watersheds resulting in increases in peak discharge - one of the most important hydrological variables used in water resources applications. It thus follows that the standard methods used to calculate rainfall runoff should be modified in order to model the potential changes in watershed response under post-fire conditions. However, no reliable methodology for quantitatively assessing the effects of wildfires on hydrological parameters, such as curve numbers or runoff coefficients, has been identified to date. The approaches currently used are usually site-specific, mainly based on personal experience or very simple empirical strategies and then affected by a degree of uncertainty. This paper addresses issues regarding the estimation of the Soil Conservation Service Curve Number (SCS-CN or CN) and considers the case study of San Giuliano, L’Aquila (Italy), a small urban basin recently affected by a wildfire that resulted in a significant reduction in forest cover. The effects of the fire on runoff are modelled by adjusting CNs according to existing approaches from the literature in order to perform a sensitivity analysis for post-fire conditions; this allows us to examine the effects of the variability in model input parameters (estimates of post-fire CNs) upon expected peak discharges related to different return period storms. The fire effect ratio, which can be seen as a global parameter for describing alterations in the watershed response due to fire, is calculated by dividing post-fire peak discharge by pre-fire peak discharge. For the present case study, this ratio ranged between 1.1 and 2.3, indicating the urgent need for quantitative research on the effects of wildfires on the hydrological variables affecting runoff calculations.