Automatic Discretization and Parameterization of Watersheds using a Digital Elevation Model

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1 Automatic Discretization and Parameterization of Watersheds using a Digital Elevation Model Ellen Hachborn, Karen Finney, Rob James, Nandana Perera, Tiehong Xiao WaterTech 2017 Computational Hydraulics International, Guelph, Ontario, Canada

2 Hydrologic watershed models state of practice 3

EPA SWMM5 engine Tools discussed in this

3 Modelling approach EPA SWMM5 hydrologic and hydraulic model Physically based, deterministic model PCSWMM Professional 2D Spatial decision support system for the EPA SWMM5 engine Tools discussed in this presentation were implemented in PCSWMM US EPA SWMM 5 4

4 Overland flow schematization

5 Effect of subcatchment width 6

6 Current state of practice Watershed is divided into a coarse number of lumped subcatchments Parameters such as slope, imperviousness and infiltration variables are averaged across each subcatchment Parameters are calibrated to match flows observed at downstream locations 8

7 Problems with the current state of practice In reality, there are many complex processes occurring on watershed surfaces Although SWMM5 uses a physically based approach to modeling surface hydrology, there is still only one flow equation representing runoff for each subcatchment, ostensibly representing sheet flow Sheet flow is normally limited to 100m or less Modeling known channelized flow as sheet flow potentially results in calibrating parameters beyond their reasonable bounds and can produce unrealistic head water hydrographs 9

8 Hypothesis By further discretizing the watershed, the processes involved with channelized flow and sheet flow can be better represented 10

9 Hypothesis Channelization represented as sheet flow Watersheds Channelization represented using irregular transects 11

10 Watershed Delineation Tool (WDT) in PCSWMM 12

11 WDT subcatchment delineation Subcatchments are automatically delineated by computing the flow direction, flow accumulation and stream delineation based on userdefined thresholds The following slides go through the process 13

12 Digital Elevation Model (DEM) 14

13 Burn in streams layer Optional layer PCSWMM will cut or burn the streams into the DEM 15

14 Fill pits layer Removes low elevation areas in DEM that is completely surrounded by higher terrain Removed by raising elevations 16

15 Flow direction layer Assigns each cell a number indicating the direction of flow Grids will find the lowest neighbouring cell for every 3x3 cell neighbourhood 17

16 Slope layer Calculated based on flow direction Calculated using the elevation difference from the two points used to calculate flow direction 18

17 Contributing area layer 19

18 Stream threshold layer 20

19 Flow path layer 21

20 Watersheds layer 22

21 Original hydraulic model 23

22 WDT hydraulic model 24

23 WDT additional laterals 25

Currently PCSWMM does not automatically generate the missing transect data for the

24 Transect creator tool WDT created conduits are assigned with a circular conduit shape. Currently PCSWMM does not automatically generate the missing transect data for the WDT generated hydraulics. Transects can be automatically generated using the transect creator tool in PCSWMM thus avoiding having to collect additional field data. 26

25 Transect creator tool 27

26 Transect creator tool 28

27 Subcatchment parameterization by the WDT Area computed from polygon GIS area Slope computed as average DEM slope Width computed as per Guo and Urbonas (2009) Imperviousness area weighted from land use Depression storage area weighted from land use Roughness area weighted from land use Green-Ampt infiltration area weighted from soils map 29

28 Subcatchment width -Guo and Urbonas (2009) Uses kinematic wave approach to convert irregular shaped subcatchments to equivalent rectangular planes as used in SWMM. Subcatchment width and slope are estimated considering the location and longitudinal elevation drop of the receiving concentrated flow channel. Provides a reproducible methodology that was automated in PCSWMM. Guo, J. and Urbonas, B. (2009). Conversion of Natural Watershed to Kinematic Wave Cascading Plane. J. Hydrol. Eng., 14(8),

1.2 million residents Half of the watershed is devoted to housing CHI

29 Case study: Toronto and Region Conservation (TRCA) Don River Watershed Don River watershed situated in Southern Ontario Total area ~ ha Home to 1.2 million residents Half of the watershed is devoted to housing CHI completed the original Don River model in 2010 and updated the model in

30 30 subcatchments Coarse Model Average subcatchment area = 538 ha Total number of subcatchments = 67 32

31 30 subcatchments Detailed Model Average subcatchment area = 74 ha Total number of subcatchments =

32 Peak flow comparison Coarse Model Detailed Model 34

33 Events comparison plots Coarse Model Detailed Model Observed Data 35

34 Conclusions For the modeler, the benefits of a calibrated / validated model include: Better understanding of the modeled system Improved confidence and reliability in model results For the elected officials (who ultimately make the infrastructure decisions) and general public (who have to live with these decisions), benefits include: Model credibility Wider acceptance of recommendations C O M P U T A T I O N A L H Y D R A U L I C S I N T E R N A T I O N A L 36

35 Conclusions Significant infrastructure expenditures are based on the application of stormwater models it s important to get them right! Limited modeling budgets and short time lines likely are here to stay However, we have the technology and the data resources to improve model calibration and validation 37

36 38

Efficiency and Accuracy of Importing HEC RAS Datafiles into PCSWMM and SWMM5

5 Efficiency and Accuracy of Importing HEC RAS Datafiles into PCSWMM and SWMM5 Karen Finney, Rob James, William James and Tiehong Xiao An advantage of USEPA s SWMM5 is its capability to dynamically model