6.1: Catchment Response Flashcards
What is the Rational Method used for?
It is appropriate for estimating peak discharges for small drainage areas with no significant flood storage
What is the effect on the catchment hydrograph of a narrow vs. wide area near the outlet?
Narrow: delayed peak
Wide: advanced peak
What is the effect of steep vs. mild slope on a catchment hydrograph?
Steep slope: large & advanced peak. short duraction
Mild slope: small peak, long duration
What is the effect of low vs. high permeability on a catchment hydrograph?
Low permeability: large peak, ‘flashy’
High permeability: small peak, ‘sluggish’
What is the effect of an advancing vs. receding storm (moving towards vs. away from outlet) on a catchment hydrograph?
Advancing: more pronounced peak
Receding: less pronounced peak, ‘stretched out’
What is the effect of a developed vs. undeveloped catchment on a hydrograph?
Developed: more pronounced peak
Undeveloped: less pronounced peak
What causes a double peak on a catchment hydrograph?
Two large catchment areas
What is the time of concentration
The travel time from the most remote point in the catchment to the outlet
What are isochrones?
Contours of equal travel time in a catchment, used to partition large catchments for runoff modelling
What are the 4 methods of estimating rainfall runoff?
- Rational method (simplest)
- Unit hydrograph
- Runoff routing
- Physics based
What is the Rational Method equation and what is it used for?
Qy = CyIyA/360
Qy [m3/s], Cy [-], Iy [mm/h], A [ha]
Used to estimate the peak flow of design ARI from an average rainfall intensity of the same ARI. Should only be used for small catchments.
What is an IDF curve?
IDF: intensity-duration-frequency
Intensity [mm/h] vs. duration [h]
- Graphed for different ARI values
- Intensity decreases as duration increases
- Intensity increases with increasing ARI
- The time of concentration is used for the design duration
What is the Bransby Williams formula? When is it used?
Hint: used to calculate tc
tc = 58L / (A^0.1 x Se^0.2)
tc [min], L [km], A [km^2], Se [m/km]
Used for ungauged catchments
What is Se and how is it calculated?
Slope calculated using the equal area slope method. This involves drawing a line on a graph of height vs. distance from outlet that has an area above the line equal to the deficit area below the curve.
