GIS and SMS in Numerical Modeling of Open Channel Flow

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GIS and SMS in Numerical Modeling of Open Channel
Flow

• Ryan Murdock
• November 28, 2000

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SMS Surface Water Modeling Modeling System

• Graphical pre and post processor for numerical
  surface water models

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Data Collection
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Detailed Stream Elevation Data

• DEMs not suitable
• Small scale TIN needed
• TIN representing Pecan Bayou SE of Brownwood, TX
• Based on 2 elevation contour data from aerial
  photogrammetry

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TIN of Pecan Bayou SE of Brownwood, TX
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Image Data

• Displayed in the background
• For on-screen digitizing
• To enhance display of SMS model
• For model placement (with registered images)

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DRG of Brownwood, TX Quad Map
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Flow Data for Pecan Bayou

• USGS gaging station
• 27 mi downstream
• Downloaded last 3
• years of data
• Apply max, mean,
• and min flows to
• SMS model

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Data Processing

• More happened here than meets the eye

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TIN Data

• Clip down data
• Very large (gt200 MB), hard to work with
• Surface Create Contours using Spatial Analyst
• Clip contours using Geoprocessing Wizard
• Recreate TIN using 3D Analyst
• Now what?
• Need to convert to scatter point for SMS
• SMS could not read my TIN directly

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TIN Data
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Convert TIN to Grid

• Theme Convert to Grid using Spatial Analyst
• Grid extents based on a smaller theme
• Specify cell size

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Convert Grid to ASCII Points

• Arc workstation new name gridascii(tingrid)
• SMS import ascii points as an arc view file
  (.arc)

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Conceptual Model andMesh Building in SMS
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Conceptual Model

• Created using GIS objects (map module)
• Constructed independently of the mesh
• Description of the site
• Geometric features (main channel and banks)
• Boundary of domain to be modeled
• Flow rates and wsels of boundary conditions
• Material zones (Mannings n values, etc)

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Conceptual Model

• River boundary feature objects

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Projection Challenges

• Built conceptual model and mesh based on
  registered quad image
• Elevation data (scatter points) based on TIN
• Data in different projections
• TIN- Albers Equal Area
• Quad Image- UTM

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Tracing channel in GIS using TIN
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Import Shapefiles into SMS
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Finish Conceptual Model

• Redistribute vertices
• For good element creation
• Define polygons
• Assign meshing parameters
• For creating finite elements
• Patch desirable when direction of flow known
• Apply boundary conditions
• Flowrate and head
• Assign materials

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Construction of Mesh Network

• Convert feature objects to a mesh
• Interpolating to the mesh
• (scatter point module)
• Renumber mesh (mesh module)
• Organizes node and element ordering from
  specified boundary condition nodestring

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Finite Element Mesh
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Finite Element Modeling

• Represent something physical with a mathematical
  equation
• Want as few elements as possible, but still
  represent geometries accurately
• Numerical model takes the conceptual model and
  gives results
• SMS has capabilities for 1, 2, 3 D models

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2D Work

• Mesh was set up to run FESWMS, a 2D hydrodynamic
  model
• Equations based on continuity of mass and
  momentum
• Depth-averaged flow (downstream cross-stream)
• Input model parameters (eddy viscosity, n, etc.)
• Model crashed
• Not a push button exercise

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1D Modeling

• Given position 1, find head and velocity at
  position 2
• Balance energy at each cross section
• Conservation of continuity momentum
• Cross-section averaged flow

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Near Future Work

• Perform an advanced WSPRO analysis on the mesh
• WSPRO is a one dimensional water surface profile
  computation model
• Assumes flow normal to cross sections, steady
  state conditions, flat wsel across cross section
• Examine results (depths and velocities) based on
  a few different characteristic flowrates at
  various cross sections
• Refine roughness/land use estimates