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Rational method
Published in James C.Y. Guo, Wenliang Wang, Junqi Li, Urban Drainage and Storage Practices, 2023
James C.Y. Guo, Wenliang Wang, Junqi Li
By definition, the time of concentration of the watershed is the travel time required for stormwater to travel from the most upstream point along the waterway to the outlet. To estimate the time of concentration, it is recommended that the longest waterway be selected to represent the watershed. A waterway often begins with overland flows for a short distance and then becomes a gully or swale flow due to the concentration of flows. Farther downstream, a waterway is formed by well-defined cross-sections through reaches. The time of concentration along a waterway is the cumulative flow times through the reaches as: Tc=To+∑j=1j=N(Tf)j
Flood Hydrograph and Baseflow Separation Using the Web- Based Hydrograph Analysis Tool
Published in Saeid Eslamian, Faezeh Eslamian, Flood Handbook, 2022
The shape of the basin influences the time required for water to travel from the distal parts of a basin to its outlet. The shape of the basin affects the shape of the hydrograph by affecting the time of concentration. The effect of shape can best be demonstrated by considering the hydrographs of discharges from at least two differently shaped basins with the same surface area, subject to rainfall of the same intensity. In most elongated basins it takes a longer time for the hydrograph to reach the peak. A broad-shaped basin results in peak flow occurring soon because of less time of concentration, a narrow hydrograph with a high peak. Time of concentration is the time taken by a water parcel to travel from the hydraulically most distant point in the watershed to the outlet. Fan-shaped or nearly semi-circular-shaped river basins generally yield the high-peaked and narrow hydrographs occurring at a longer time interval because of the longer time of concentration, whereas elongated river basins give the low-peaked and broad hydrographs (Subramanya, 2008).
Characteristics of Watershed
Published in Sandeep Samantaray, Abinash Sahoo, Dillip K. Ghose, Watershed Management and Applications of AI, 2021
Sandeep Samantaray, Abinash Sahoo, Dillip K. Ghose
It is a basic parameter of a watershed. The time required for a water particle to flow from most distant point in the watershed to its outlet is called time of concentration. It is computed as the time for runoff to flow from most hydraulically distant point of drainage area to point under study. It is applied for computing peak discharge of a watershed. Peak discharge is based on Tc as it is a function of intensity of rainfall. There can be numerous potential paths taken into consideration to determine longest travel time. Mark and Marek (2011) suggested that designers must detect the flow route alongside which the longest travel time is expected to follow. Sometimes Tc is also computed as the time difference between end of rainfall excess and inflection point.
Evaluation of an urban drainage system using functional and structural resilience approach
Published in Urban Water Journal, 2022
Mitthan Lal Kansal, Deepak Singh Bisht
As discussed in section 3.4, the surface flow () was estimated using eq. (10) whereas the flow velocity is computed at different levels of depths (d), i.e. 0.1d to 0.9d (in m/s), as shown in Table 2 to estimate the time of flow () using eq. (11). The time of surface flow () and time of flow () are determined as 0.1 h and 3.12 h, respectively. The concentration time is, therefore, found to be 3.22 h. Since the temporal resolution of GPM-IMERG rainfall data used in the study is half-hourly, a duration of 3.5 h is taken as critical storm duration for further analysis in computing the design storm of the T-year return period. The duration of the design storm must be at least equal to the time of concentration in order to generate the maximum flow from the catchment. However, it should not be longer, because storms with longer durations have statistically lower intensities (Butler et al. 2018).
Semi-distributed parameter optimization and uncertainty assessment for an ungauged catchment of Deduru Oya Basin in Sri Lanka
Published in International Journal of River Basin Management, 2020
The concept of time of concentration decides the time needed for water to flow from the most remote point in the catchment to its outlet. In SWAT model, the time of concentration is controlled by five parameters: CH_L(1) (Longest tributary channel length in the sub-basin), CH_S(1) (Average slope of tributary channels), CH_N(1) (Manning’s ‘n’ value for the tributary channels), SLSUBBSN (Average slope length) and OV_N (Manning’s ‘n’ value for overland flow). During the OAT analysis, only the CH_N(1) parameter was received as the parameter that influences time of concentration in the Maguru Oya catchment. Since the selected catchment is relatively flat in nature, the area does not have any significant slope differences. Hence the parameters related to slope such as CH_S(1) and SLSUBBSN do not make any significant impact on simulated flow. The time of concentration process considers two types of Manning’s roughness coefficients – one coefficient for tributary channel flow (CH_N(1)) and other coefficient for overland flow (OV_N). The coefficient related to channel flow varies with the characteristics of the channel and the coefficient related to overland flow varies with the land use type. Since there is no significant land use variation in the Maguru Oya catchment, the coefficient related to channel flow influences for simulated flow rather than the coefficient related to overland flow.
An approach for flood assessment by numerical modeling of extreme hydrological events in the Zat watershed (High Atlas, Morocco)
Published in Urban Water Journal, 2020
Myriam Benkirane, Nour-Eddine Laftouhi, Bouabid El Mansouri, Ismail Salik, Meriem Snineh, Fatima Ezzahra El Ghazali, Safia Kamal, Zineb Zamrane
The transform prediction model in HEC-HMS simulated the process of the direct runoff of excess precipitation on the watershed, and transformed the precipitation excess into point runoff. The model transform the rainfall excess into direct surface runoff through a unit hydrograph, the input parameters are follow: Time of Concentration (Tc): defined as the time needed for water to flow from the most remote point in a watershed to the watershed outlet, it was calculated using the Giandotti Equation (4)