Hydrologic and Hydraulic Models

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Hydrologic and Hydraulic Models

• Hydrologic Modeling
• Demonstration of HEC-HMS
• Uses / Limitations of Hydrologic Models
• Hydraulic Modeling
• Step-Backwater Method
• Demonstration of HEC-RAS
• Uses / Limitations of Hydraulic Models

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Hydrologic Modeling

• Simulate the precipitation-runoff processes of
  dendritic watershed systems
• Q f (cover, rainfall intensity, area)
• Models simplify organization and management of
  complex basins

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Hydrologic Model Uses

• Water availability
• Urban drainage
• Flow forecasting
• Impacts of urbanization
• Reservoir spillway design
• Flood damage reduction
• Floodplain regulation

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Hydrologic Model Uses

• Landuse change hydrologic impacts
• Water resources systems operation
• Restoration studies

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HEC-HMS

• HEC-HMS USACE Hydrologic Engineering Center -
  Hydrologic Modeling System
• Automates precipitation - runoff calculations
• Organizes calculations for complex basins

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HEC-HMS DEMO

• Rancho Seco Powerplant Project
• Objectives
• Determine 100-year discharge with impervious area
  of new plant
• Site Conditions

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HEC-HMS DEMO

• Create basin model
• Run basin model
• Discuss model output

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Hydrologic Model Limitations

• Backwater effects
• Floodplain storage
• Interaction of channel slope and hydrograph
• Configuration of flow networks
• Subcritical and supercritical flow
• Availability of data for calibration

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Hydraulic Modeling

• Simulate characteristics of flow through channels
  (or over floodplains)
• Water Surface Elevation
• Velocity
• Sediment Transport
• Most commonly applied models based on the step
  backwater method

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Step Backwater Method

• Balance energy between cross sections
• EXS1 EXS2
• Requires geometry and other attributes of both
  cross sections

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Step Backwater Method
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Step Backwater Method

• HP1 HV1 HP2 HV2 Losses
• or p rgh 1/2rV2 constant
• HP2 (depth at initial XS) must be known
• Using geometry and flow rate, calculate velocity
  at downstream XS and resulting velocity head
  (HV2) V2/2g
• Guess water surface elevation of upstream XS (HP1)

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Step Backwater Method

• Compute velocity and velocity head (HV1) for
  upstream XS
• Compute losses (friction and eddy)
• Substitute into energy equation
• Iterate until error is acceptable
• Repeat for next upstream cross section

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Step Backwater Method

• Select cross sections to capture
• Changes in flow rate
• Changes in roughness
• Changes in conveyance (geometry, slope)
• Large distances

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HEC-RAS

• HEC-RAS USACE Hydrologic Engineering Center -
  River Analysis System
• Automates step backwater method
• Organizes complex channel calculations

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HEC-RAS DEMO

• Rancho Seco Powerplant Project
• Objectives
• Determine water surface elevations during
  100-year discharge
• Determine velocity in channel and near proposed
  powerplant
• Assess capacity of existing flood control
  infrastructure
• Site Conditions

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HEC-RAS DEMO

• Show network
• Show cross sections
• Run model
• Discuss profile
• Discuss 3D profile

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HEC-RAS Uses

• Flood studies
• Channel alteration design
• Encroachment studies
• Stream restoration design
• Bridge pier design
• Culvert design
• Scour investigations

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HEC-RAS Limitations

• 1-dimension only
• Data required for calibration
• Gauge data vs Normal depth
• The depth at which flow is steady and hydraulic
  characteristics are uniform.
• Tweakability
• Calculations invisible

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Other Hydraulic Models

• 1-D
• Mike-11 (Danish Hydraulic Institute)
• WS-PRO, BRI-STARS (FHWA)
• UNET (USACE)
• 2-D
• AquaDyn (Softkit Technologies)
• Mike-21 (Danish Hydraulic Institute)
• RMA-2 (USACE-WES)

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Other Hydraulic Models

• 3-D
• UnTRIM
• 2-D and 3-D costly, mostly used in research

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Two Dimensional Hydraulic and Sediment Transport
Modeling

• RMA-2 and SED2D
• Calibrated using available information
• Focused on in-channel flows (i.e. 45,000 cfs and
  below)
• Velocities and shear stresses

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Watershed Hydrology

• Watershed includes LMR and East Fork LMR
• Area 1,758 mi2
• Reservoir regulation since 1975
• Caesar Creek Lake and William H. Harsha Lake
  control 33 of watershed area
• Reduced peak flows
• Backwater vs downvalley floods

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Useful urls

• http//www.hec.usace.army.mil/
• http//www.dhi.dk/
• http//water.usgs.gov/software/wspro.html
• http//www.fhwa.dot.gov/bridge/bristars.htm