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Guidelines for Unit Hydrograph Analyses
Published in Saeid Eslamian, Faezeh Eslamian, Flood Handbook, 2022
Richard H. McCuen, Tianming Zhao
A runoff hydrograph represents the direct response of the watershed to a storm event. Maidment (1993) showed the importance of runoff velocities to the shape and scale of unit hydrographs. With actual measured rainfall and runoff data, the rainfall characteristics (e.g., the complexity and peakedness) will be reflected in the runoff hydrograph. In many cases, a rainfall hyetograph and a runoff hydrograph may show little similarity because the watershed processes smooth the variation of the rainfall hyetograph. For example, a multi-peaked rainfall hyetograph often produces a single-peaked runoff hydrograph, or a very peaked rainfall hyetograph produces a fairly flat runoff hydrograph. These cases are evidence that the watershed acts as a smoothing function. This fact needs to be fully understood when selecting a unit hydrograph model and when calibrating unit hydrograph parameters from measured rainfall-runoff data.
Surface water and the atmosphere
Published in Ian Acworth, Investigating Groundwater, 2019
Generally, the analysis of rainfall data requires the consideration of information obtained from rain gauges. These data can be presented in two forms: Mass Curve – A plot of cumulative rainfall depth as a function of time. The mass curve is the integral of the hyetograph, or the hyetograph is the differential of the mass curve. The instantaneous rainfall intensity is given by the slope of the mass curve. Figure 2.29 shows hyetographs for a number of rainfall stations during a storm event at Fowlers Gap, western NSW.Hyetograph – The hyetograph is a plot of incremental rainfall depth as a function of time. It is generally shown as a histogram and gives the depth of rainfall over a given time period.
Urban Flood Modelling
Published in Ahmad Fikri Bin Abdullah, A Methodology for Processing Raw Lidar Data to Support Urban Flood Modelling Framework, 2020
An alternative approach that might be termed a surface water budget approach incorporates the loss mechanism into the catchment model. In this way, the incident rainfall hyetograph is used as an input, and the estimation of infiltration and other losses is made as an integral part of the calculation of the runoff. This approach implies that infiltration will continue to occur as long as the average depth of excess water on the surface is finite. Clearly, this may continue after the cessation of the rainfall.
Assessing the mitigation effect of deep tunnels on urban flooding: a case study in Guangzhou, China
Published in Urban Water Journal, 2019
Huabing Huang, Lin Zhang, Lin Liu, Xianwei Wang, Xina Wang, Cuilin Pan, Dashan Wang
The mitigation effect of drainage facilities on urban flooding depends not only on the designed capacity, but also on features of the rainfall, such as average intensity and temporal distribution (Guan, Sillanpää, and Koivusalo 2016; Isidoro, de Lima, and Leandro 2012; Ten Veldhuis et al. 2018; Zhu and Chen 2017). The intensity of rainfall in Guangzhou has changed significantly over the past two decades (GBWA 2011), and a new code for average storm intensity has thus been formulated. The storm hyetograph describes the temporal distribution of rainfall depth during a storm event, thus determining the peak runoff in a catchment, especially in an urban catchment where over half of rainfalls turn into runoffs (Ewea et al. 2016; Gong et al. 2016). The Chicago curve (Keifer and Chu 1957) is widely applied to design storms in China. A previous investigation (Pan et al. 2017) found that it tends to underestimate the peak flooding volume in the southern urban catchments of Downtown Panyu, Guangzhou and the Improved Huff curve (Pan et al. 2017) could better represent the temporal distribution of storm rainfalls in Guangzhou. Further investigation is needed into how the difference between these curves influences the response of the drainage system to storm events, and the conclusion can support a more reasonable design of urban drainage facilities.
Temporal patterns for design hyetographs in New Zealand
Published in Australasian Journal of Water Resources, 2018
Shailesh Kumar Singh, George A. Griffiths, Alistair I McKerchar
In contemporary flood estimation where interest lies with predicting not only the peak but also the volume of a catchment outflow hydrograph, it is necessary to prescribe a hyetograph(s) of storm rainfall of given duration and return period. The variation of rainfall with time or hyetograph is characterised by a maximum rainfall depth and a temporal pattern or shape. It has long been known that cumulative hyetographs of rainfall in convective or frontal type storms tend to have their peak rainfall rates early in a storm while cyclonic events tend to have their peak rates near the middle of the storm duration (Eagleson 1970). Moreover, results from rainfall-runoff modelling show that hyetograph shape has a strong influence on the shape of a catchment outflow hydrograph (Marani et al. 1997; De Lima and Singh 2002).
A multi-criteria risk-based approach for optimal planning of SuDS solutions in urban flood management
Published in Urban Water Journal, 2022
Mozhgan Karami, Kourosh Behzadian, Abdollah Ardeshir, Azadeh Hosseinzadeh, Zoran Kapelan
This study used synthetic design storms for the rainfall simulation in the UDS. Note that continuous simulation by using actual historic data of long-term rainfall record can provide more accurate and robust comparison of the long-term water balance and hydrologic performance of alternative stormwater management options. However, synthetic design storms were selected here as they are typically used for designing the UDS and use of actual historic rainfall requires a long-term rainfall record (e.g. 30–50 years) with high time resolution (e.g. 5–10 minutes) that access to this level of data was not possible for the case study. Hence, the Intensity-Duration-Frequency (IDF) curves of the rainfall of the closest weather station (i.e. the Mehrabad station) to the project site were selected. Each IDF curve represents the relationship between rainfall intensity and duration for a specific frequency (i.e. inverse of return period) of the rainfall. The analysis of rainfalls with various intensities and durations in the IDF curves shows rainfalls with a 6-hour duration are the most critical condition corresponding to the maximum surface runoff in the UDS (Karami, Ardeshir, and Behzadian 2016). Therefore, rainfalls with return periods of 2, 10 and 100 years (that are typical return periods for the UDS design in the local standards) and a duration of 6 hours are considered here to evaluate risk assessment of flooding and surface runoff pollution. The corresponding average intensity of rainfall obtained from the IDF curves of the case study are 1.94 mm/hr for 2 years, 3.04 mm/hr for 10 years and 5.94 mm/hr for 100 years. Moreover, the basic hyetograph suggested by Yen and Chow (1980) was used here to generate the temporal distribution of rainfall due to its simplicity. This hyetograph is represented by a triangular shape with the time to peak intensity approximately 0.375 times rainfall duration and the peak intensity estimated as a function of total rainfall depth, duration and peak intensity.