river discharge Flashcards
Runoff equation
Runoff (L/T) = discharge (L^3/T) / catchment area (L^2)
Streamflow definition
volume of water flowing through stream channel
Stream discharge definition
volume of water that flows past a given point in a stream in a unit of time
Stream discharge equation
Discharge = width x depth x velocity
-reported in cubic feet per second (cfs) or m^3/sec or L/sec
single-instance flow estimation methods
-floating object and measurement of travel time (apply correction factor)
-manual measurment of width & depth AND dilution gauging for velocity
-velocimeters
continuous flow estimation
-continuous water depth & velocity measurements (costly & logistically difficult)
-continuous water depth AND rating curves
Dilution gauging
instantaneous water velocity for a given location obtained by dumping dye in upstream water and calculating the time before dye in the stream reaches someone downstream
-useful for small and rough stream
velocity area equation
discharge = average flow velocity x transversal section area
velocimeters method
give instantaneous water velocity for given river location
-should be measured at 60% of full river depth (where average velocity occurs)
Acoustic Doppler Current Profilers (ADCP)
measure instantaneous/continuous water velocity AND depth
-using soundwave to measure 3D velocity of whole water column
Pressure transducers & velocimeters
measures continuous water depth & velocity
-can be directly in river OR in gauge house connected to river by pipe
Pressure transducers definition
measures water pressure exerted on top of instrument and use calculations to transform in into water depth
weirs & flumes
artificial controls to force flow through critic depth at which Q is known
-flume: concentrate flow at one particular location
-weirs: using V-shape to concentrate flow at bottom of V
rating curves usage
used to convert water depth into discharge based on statistical relationships
rating curves considerations
errors occur when measuring flow velocity
diff rating curves for diff seasons
changes shape when river floods
Steps to build rating curves
- measure water velocity at diff water lvl
- use stream flow equation to calculate discharge
- build relationship between water lvl and discharge value
- use rating curve to predict discharge from any water lvl value
hydrograph definition
graphical representation of stream discharge VS time
-how watershed as a whole responds to precipitation event
-can be for single event, season, year, etc.
2 types of hydrograph assessment
-hydrograph separation
-hydrograph analysis
hydrograph separation components
breaking hydrograph to distinguish generic flow components
-stormflow: portion fed by rapidly moving runoff in response to rainfall or snowmelt (subsurface & overland flow)
-baseflow: portion fed by slowly moving runoff even in absence of rainfall or snowmelt (GW)
hydrograph separation methods
-Linsley’s fixed interval method
-recession limb method
-isotope-based hydrograph separation
-chem hydrograph separation
-straight line method
Linsley’s fixed interval characterstics and formula
formula: A^0.2
-only need watershed area
-create multi-segment line
straight line characteristics and formula
formula: 0.05 pi^3 x h^-1 x A
-only for watershed smaller than 20 square miles
-slope proportional to watershed area
-most frequently used
rising limb definition
period when stream discharge increase in response to rainfall or snowmelt event
falling limb definition
period when stream discharge decreases after rainfall or snowmelt event
rising limb shape and runoff types
steep: quickly generates & fast-moving runoff (overland flow)
less steep: slower moving runoff (subsurface flow)
Lag time 2 definitions:
1: time interval between center of mass of precipitation and center of mass of event stream hydrograph
2. Time interval between peak precipitation and peak of event stream hydrograph
-short = overland flow/ slow = subsurface flow
time to peak
time from beginning of rising limb to occurrence of peak discharge
-good indication of dominant runoff (short = more surface runoff)
Time to concentration
time from end of precipitation to inflection point of recession limb of event hydrograph
-how much time until last raindrop feed stream via runoff
time base
duration of the event stream hydrograph during which quick flow occurred
elongated VS circular watershed timing
circular: shorter distance in watershed = short time of concentration / higher peakflow/ shorter lagtime/ shorter peak duration
elongated: longer time of concentration/lower peak value/ longer lag time/ longer peak duration
Runoff ratio definition and formula
Amount of precipitation that was made into runoff and made it to the outlet -> computed by dividing the volume of stormflow by the volume of water input
2 formulas option:
- Rp = total stormflow/total precipitation
-Ry = total stormflow/ effective precipitation
9 Most IMP factors affecting runoff and storm hydrographs
-watershed area
-stream order
-shape of watershed
-regional climate/ type of precipitation/ antecedent moisture conditions
-topography
-geology and soil
-land use and land cover
-drainage condition -> (natural drainage density, tile drains, etc.)
-depression storage feature -> (wetlands, man-made reservoirs, etc.)
arid VS humid climate hydrograph
arid:
-soil characteristics & hydrophobicity -> HOF = shorter lagtime
-larger volume of water with overland flow = higher peak
-quick flows because overland = small peak duration
Humid:
-mostly subsurface flow = longer lag time
-lower & flatter peak -> water doesn’t reach stream all at same time
climate as natural watershed factor
-precipitation
-antecedent conditions
-time of year/season
-other conditions affecting evapotranspiration
characteristics of watershed as natural factor
-watershed area
-shape of watershed
-elevation
-slope
-lenght & slope of valley
river network as natural factor
-routing and detention
-drainage density
-river conditions
-size of river (stream order)
surface depression as natural factor
-storage, hydrologic balance
reservoir as natural factor
-store IMP volume
-prevent/delay runoff
-water extracted reduced stream discharge
-dry reservoir = temporary storage during flood
geological & soil characteristics as natural factors
-bedrock permeability
-soil permeability
-thickness of soil
-infiltration capacity
-initial conditions
Land use as anthropogenic watershed factors
-irrigation & drainage ditches
-tillage on wetlands and subsoil compression
-urbanization
-direct influence on retention capacity & many water budget components
measures to mitigate high streamflow
-increase afforestation
-increase meadow area at expense of arable land
-limit construction of impervious surface
-promote implementation of infiltration areas & permeable pavements
flood frequency use
long term analysis of daily/ weekly flows allowing determination of important seasonal differences
perennial stream definition
water flowing year-round with water table above the streambed for most of the year -> GW is primary source of water and direct precipitation and runoff are supplemental ones
intermittent stream
water is flowing during certain time of the year only when GW provides water to the stream (when the water table is high enough)
-may not have flowing water during dry period
ephemeral stream
water flowing only for short period of time after rain events as event runoff triggers streamflow (streambed above water table = no GW as source)
flow duration curve definition
cumulative frequency curve showing the percentage of time that a specified discharges value are equaled or exceeded
-highest flow -> lowest % of exceedance
flood definition
occurs anytime a waterbody overtops natural/ artificial banks in any reach of a stream -> water above bankful level
high water definition
highest water level reached during a flood
flood implication for envr and society
-high risk & damages to human with extreme floods
-regular natural events that trigger ecological processes and shape river channel
-floodplain are important ecological system that help attenuate flood waves and are used for agriculture
what is a 100-year flood
it is a flood of given size that has a probability of 1 over 100 chance of occurring every year based on historical data
structural flood control measures
measures linked to engineering efforts
-embankments
-channelization
-diversion
-storage structures (dams & reservoirs)
non-structural flood control measures
measures linked to sound floodplain management
-identification of high-risks areas
-zoning regulations for development
