Lecture 6a Flashcards
what are the streamflow generation mechanisms
saturation overland flow (direct runoff)
return flow (subsurface flow intersects with land surface)
baseflow (groundwater)
what are some drainage basin characteristics that influence runoff response
slope
roughness
storage
drainage density
slope length
what effect does slope have on runoff response
gentler slope, damped peaks
steeper slope, exaggerated peaks
what effect does storage have on runoff response
little storage, larger peak
more storage, damped
what effect does roughness have on runoff response
less rough, larger peak
more rough, damped
what effect does drainage density have on runoff response
high density, larger peak
low density, damped
what effect does slope length have on runoff response
shorter length, higher peak,
longer length, damped
what is the inflection point of a stormflow hydrograph
where the slope of logQ vs t changes
what is the area under the stormflow hydrograph
direct runoff and baseflow
what are the bounds of direct runoff on the stormflow hydrograph
the beginning of the storm and the inflection point
what is the baseflow in the stormflow hydrograph
the area under the curve that is not direct runoff
What is runoff
effective precipitation or Precipitation minus Abstraction
what are three kinds of abstractions in runoff
interception, infiltration, depression storage
what are three types of runoff predictions
peak runoff rates
total runoff volume
complete hydrograph
what is the time of concentration
the time for a wave to travel from the most hydraulically distant point to the catchment outlet
what is the lag time
the time between the centroid of rainfall and peak flow of the response hydrograph
when should you use the rational equation
when analyzing small urban watersheds
what are the assumptions of the rational method
the entire catchment is contributing flow at the watershed outlet (the duration of the storm must be greater than or equal to time of concentration)
rainfall is distributed uniformly over the entire watershed
all catchment losses can be incorporated into one empirical coefficient
in urban hydrology the time of concentration is sometimes called
inlet times
once time of concentration is reached what condition can be assumed
an equilibrium condition in which the runoff rate is equal to rate of effective rainfall
what kinds of flow regimes might exist along the hydraulic length
sheet flow
shallow concentrated flow
channel flow
how is sheet flow characterized
by runoff that occurs as a continuous sheet of water flowing over the land surface
what does shallow concentrated flow represent
the channelization of overland flows into small rills and channels
when does channel flow exist
in larger watersheds where overland flows eventualy are routed throgh larger defined stream channels
which flow regime has the fastest velocities? which has the lowest?
sheet flow has the lowest velocity
shallow concentrated is in the middle
channel flow has the highest velocity
what parameters are commonly used in equations estimating time of concentration
slope
length of overland flow
rainfall intensity
parameter that descibres the catchment surface
the kinematic wave equation is generally regarded as the most
physically based tc formula
what is the kirpich formula used for
estimating tc of small uniform catchments
how does SCS go about estimating time of concentration
it sums time of concentration for each of the flow regimes
when is shallow concentrated flow assumed to begin dominating
after 100m
how is shallow concentrated flow time of concentration estimated
using the mannings equation
how long can sheet flow last
a maximum of 100m
to find channel time of concentration..
use mannings equation to find velocity then use T=L/V
what basin charactersitics do the runoff coefficient depend on
groundcover
soil type
land use
slope
rainfall intensity
