Transcript Water Balance in MIKE-SWMM Melissa Figurski
Water Balance in MIKE-SWMM Melissa Figurski
Blocks in SWMM
Runoff
Transport
Extended Transport (Extran)
Runoff Block
Simulates quantity and quality of runoff
Rainfall
Snowmelt
Converts to runoff (Inlet hydrographs) and pollutant loads (pollutographs)
Simulated Events
Can be run for minutes to years
Less than 2 weeks:
Single-event simulation
Greater than 2 weeks:
Continuous simulation (w/ no time limit)
Calculations are exactly same for both
Except for snow
Subcatchment Representation
Up to 200 idealized subcatchments
Impervious with depressions
Impervious without depressions
Pervious with depressions
Each subcatchment drains to a channel/pipe or inlet
Single slope
Data Input to Runoff Block
Precipitation
Up to 10 hyetographs for 10 gages
Surface Quantity
Surface Quality
Evaporation Rate
Land Use
For snowmelt – air temp and wind speed
Time Step
WET
Globally wet
WETDRY
No precip, water still in surface storage
DRY
No precip or surface storage
Runoff Loses
Evaporation
Evapotranspiration
Infiltration
Groundwater flow
Percolation
Surface Water Outflow
Evaporation
Average Monthly Rate
Continuous simulation – value for all months
Evaporation Time Series
Daily, weekly, or monthly
‘Loss off the Top’
Evapotranspiration
Upper Zone Evapotranspiration
Fraction of total maximum evaporation rate
Lower Zone Evapotranspiration
Constant over time step
Infiltration (1)
Horton’s Equation
f p = f c + (f o – f c )e -kt f p f c
= infiltration capacity, ft/sec = min or ultimate value of fp, ft/sec
f
o
t
= maximum or initial value of fp, ft/sec = time from beginning or storm, sec
k
= decay coefficient, sec -1
Infiltration (2)
Green-Ampt Equation
For F < F s :
f = i and F = S u IMD/(i/K s – 1) for i >K s
For F > F s :
f = f p and f p = K s (1 + (S u IMD/F) F i
= infiltration rate, ft/sec = rainfall intensity, ft/sec
F
= cumulative infiltration volume, ft
F s
= cumulative infiltration volume for ponding, ft
S u
= average capillary suction at wetting front, ft water
IMD
= initial moisture deficit for this event, ft/ft
K s
= saturated hydraulic conductivity of soil,. ft/sec
Groundwater Flow
Can be routed to a channel/pipe
Can be ‘lost’ from system
Both methods are accounted in continuity check
Percolation
Uses Darcy’s Law for Unsaturated Flow
Water Balance
Continuity Balance
Reference
Storm Water Management Model: Version 4, User’s Manual.
Section 4: Runoff Block