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Water Balance in MIKE-SWMM

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.

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Water Balance in MIKE-SWMM

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  1. Water Balance in MIKE-SWMM Melissa Figurski

  2. Blocks in SWMM • Runoff • Transport • Extended Transport (Extran)

  3. Runoff Block • Simulates quantity and quality of runoff • Rainfall • Snowmelt • Converts to runoff (Inlet hydrographs) and pollutant loads (pollutographs)

  4. 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

  5. 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

  6. 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

  7. Time Step • WET • Globally wet • WETDRY • No precip, water still in surface storage • DRY • No precip or surface storage

  8. Runoff Loses • Evaporation • Evapotranspiration • Infiltration • Groundwater flow • Percolation • Surface Water Outflow

  9. Evaporation • Average Monthly Rate • Continuous simulation – value for all months • Evaporation Time Series • Daily, weekly, or monthly • ‘Loss off the Top’

  10. Evapotranspiration • Upper Zone Evapotranspiration • Fraction of total maximum evaporation rate • Lower Zone Evapotranspiration • Constant over time step

  11. Infiltration (1) • Horton’s Equation fp = fc + (fo – fc)e-kt • fp = infiltration capacity, ft/sec • fc = min or ultimate value of fp, ft/sec • fo = maximum or initial value of fp, ft/sec • t = time from beginning or storm, sec • k = decay coefficient, sec-1

  12. Infiltration (2) • Green-Ampt Equation • For F < Fs: f = i and F = SuIMD/(i/Ks – 1) for i >Ks • For F > Fs: f = fp and fp = Ks(1 + (SuIMD/F) • F = infiltration rate, ft/sec • i = rainfall intensity, ft/sec • F = cumulative infiltration volume, ft • Fs = cumulative infiltration volume for ponding, ft • Su = average capillary suction at wetting front, ft water • IMD = initial moisture deficit for this event, ft/ft • Ks = saturated hydraulic conductivity of soil,. ft/sec

  13. Groundwater Flow • Can be routed to a channel/pipe • Can be ‘lost’ from system • Both methods are accounted in continuity check

  14. Percolation • Uses Darcy’s Law for Unsaturated Flow

  15. Water Balance • Continuity Balance

  16. Reference • Storm Water Management Model: Version 4, User’s Manual. Section 4: Runoff Block

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