Runoff analysis and Unit Hydrograph Flashcards
Rainfall
loss = surface (overland) runoff
Infiltration excess runoff
when rainfall intensity > infiltration capacity
Saturation excess runoff
when the soil is saturated
for good vegetation cover
Drainage area - Catchment
follows the surface topography
Underground component
in-soil flow and ground water relies on local geology (may not agree with the surface topography)
Drainage area influencing factors
slope of the catchment
catchment orientation
stream frequency
soil moisture deficit
lake and reservoir area
shape of the catchment
Rational Method
used to estimate flood peak for small urban and rural catchment
no hydrograph
Rational Method Equation
Qp = CPA
where
Qp - Peak flow
P - rainfall intensity
A - catchment area
C - runoff coefficient
What is concentration time
the time needed for water travelling from the most remote point to the outlet of the catchment
River Flow - form
when surface runoff reaches a channel, stream goes to river
river channel may cut deep enough to interact with the aquifer and ground water
Effluent streams
fed by ground water
Influent streams
feed surrounding ground water and aquifer
Intermittent streams
sometimes both influent and effluent
Perennial streams
flow all year round
Ephemeral streams
sometimes flowing sometimes dry
velocity-area method
measuring the velocity distribution over a cross section and then multiply the averaged velocity by the area
What does ADCP mean?
Acoustic Doppler Current Profiler
ADCP definition
obtaining detailed flow velocity distribution over cross section using ADCP
What is the directive cause of surface runoff
Effective rainfall
Effective rainfall is also called…
Net rain
Runoff above the base flow is also called
Surface runoff
direct runoff
due to allowing to distinguish those in soil flow and base flow
tp - time to peak
also loosely referred to the catchment concentration time
The amount of the effective rainfall is equal to…
surface/ direct runoff
Effective rainfall definition
the part of the rainfall that produces runoff
Direct rainfall definition
the consequences of the effective rainfall
Concept of balance
for a given event, the amount of effective and direct should be identical
To calculate loss/ effective rainfall - Proportional loss method
treating a fixed fraction (proportion) of rainfall as loss
To calculate loss/ effective rainfall - Infiltration curve method
the loss is accounted for by a time-varying function of infiltration
To calculate loss/ effective rainfall - Phi Index
the loss is assumed to be fixed at a constant intensity. only those with intensity higher than it can generate net rain
To calculate loss/ effective rainfall - CWI method
the ratio of loss is calculated based on the ‘wetness’ of the catchment (the wetter it the smaller ratio of loss).
What does the wetness represent in the CWI method
how much rainfall falls over a catchment in a prescribed time period immediately before the event
What does CWI mean
Catchment Wetness Index
UH theory assumes
1) storm of equal duration produce runoff hydrographs of equal time base regardless of magnitude of rainfall
2) time distribution of runoff for a given storm is independent of precipitation resulting from antecedent of subsequent storms
3) hydrograph ordinates are direcly proportional to the rainfall intensity provided that the rainfall is uniformly distributed in time
Assumptions for UH aplications
1) losses can be determined and separated
2) rainfall is uniformly distributed
3) rainfall intensity constant
4) principle of superposition is applicable
5) base of direct runoff hydrograph is constant for fixed duration of effective rainfall
6) the UH does not change with time
7) base flow separation is possible
Change duration of UH - short to long
Superposition model
Change duration of UH - long to short
S - curve model
