Hydrology Applications Flashcards
What is catchment routing?
A technique used to predict the changes in the shape of a hydrograph as water moves through a river channel or reservoir. It can be a way of determining whether a pulse of rain reaches a city as a deluge or a trickle.
Describe the Time-Area Runoff routing method
A method of transforming a storm hyetograph to a hydrograph. It accounts for translation only and does not include storage.
In general it looks at the area of the catchment and the intensity of rainfall through it. Assuming that each area drains in a given time period.
What are the assumptions of the Time-Area Runoff method
Assumes that the hydrology computation has time intervals within the time of concentration of the catchment, and based on the time segments the catchment can be divided into corresponding sub-areas.
What is Time of Concentration (Tc)
Tc is the time required for runoff to travel from the hydraulically most distant point in the catchment to the outlet.
Define the ‘hydraulically most distant point’
Not necessarily the point furthest in distance, but the point with the
longest travel time to the outlet
Why is time of concentration important in hydrology?
Tc is a critical factor in hydrological analysis, particularly in methods for
calculating peak discharge.
Helps determine how quickly water reaches the catchment outlet during a
rainfall event.
What is the role of Tc in peak discharge calculations?
Peak discharge occurs when All sections of the catchment area are contributing to runoff simultaneously. Tc helps predict the timing of when this will happen.
Describe the Rational routing method
Widely used for estimating peak runoff in small, urban catchment areas. Gives a general, simplified understanding of the peak flow.
Typically applied to areas with uniform surface cover (e.g., car parks, rooftops).
Helps in the design of storm sewers, channels, and stormwater
structures.
What is the Rational Method equation?
Q=CiA
Where:
Q = Peak runoff (cumecs)
C = Runoff coefficient
i = Average rainfall intensity (m/s)
A = Catchment area (m^2)
What are the limitations of the rational method?
Provides only peak discharge, no full runoff hydrograph (flow variation over time).
May overestimate flow for larger, complex catchments areas.
Assumes uniform rainfall over the catchment.
Results can vary between users due to subjective inputs - eg for the runoff coefficient.
Briefly describe the SCS Triangular Unit Hydrograph
To use a SCS UH you only need to determine the time to peak (Tp) and peak discharge (Qp).
Time concentration, tc (hours)
Duration of effective rainfall, tr (hours)
Lag time, tp (hours) = 0.6tc
Time to peak, Tp (hours) = tr/2 + tp
Peak discharge (cumecs) = 2.08A/Tp
Base time, tb (hours) = 2.67Tp
Base time (hours) can be calculated knowing the LHS of the triangle is 37.5% of the total
What is a frequency analysis?
Evaluates the relationship between:
* Return Period (T) and
* Magnitude of Hydrological Events
What events is a frequency analysis used to estimate?
Used to estimate events like:
* Peak flow discharge
* Rainfall depth or intensity
Designing a bridge for a 1-in-100-year flood, we can find the design discharge
What is the return period (T)?
Average interval between occurrences of a particular event, e.g. a 1-100 year flood
A 1-in-100-year flood does not mean it will happen once every
100 years, floods are not evenly spaced over time. The return period predicts probability, not timing.
What is the annual probability (P%)?
Likelihood of event happening in any
given year.
P% = 100 / Return period (T)
1-in-100-year flood → 1% chance of occurrence each year
