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By Jennifer VerWest

Hydrologic Analysis in Flat Terrain. By Jennifer VerWest. Flat Terrain. Steep/Average Terrain. Differences between Flat and Average/Steep Terrain. Flat Terrain. Steep/Average Terrain. Differences between Flat and Average/Steep Terrain.

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By Jennifer VerWest

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  1. Hydrologic Analysis in Flat Terrain By Jennifer VerWest

  2. Flat Terrain Steep/Average Terrain Differences between Flat and Average/Steep Terrain

  3. Flat Terrain Steep/Average Terrain Differences between Flat and Average/Steep Terrain • Water flows downhill in the direction of steepest slope • Water flows in the direction of least resistance • Overland flow velocities much slower than stream velocities • Overland flow velocities not significantly different from stream velocities

  4. Procedure • Determine Parameters • Hydrologic • Routing • Delineate Watershed

  5. Data • Channel Network • USGS Gage Stations • Soils • Landuse • HEC-2 Model

  6. Data from Harris County (Houston), TX

  7. Watershed Delineation Since the elevation does not necessarily describe where the water goes, it was not used for delineating the watershed. • Spatial Analyst – Assign Proximity • Defined the subbasin as the area that is closest to the channel

  8. Watershed Delineation

  9. Watershed Delineation

  10. Determine Grid Sizefor Analysis • Detail of Study Two Considerations: • Processing Time for Grid

  11. Processing Timefor Grid • Limits of Study • Area of 2835 mi2 (7344 km2) • Computer System Specifications used in Analysis • Pentium III, 800 MHz Processor • 256 MB RAM at 133 MHz

  12. Processing Time for Grid

  13. Detail of Study • Depending on Grid • Smallest subbasin area • Number of channels with no subbasin area • Shortest Channel – 71.3 feet

  14. Detail of Study

  15. Processor Time versusDetail of Study Minimize processing time and number of channels with no subbasin area

  16. Watershed Delineation Convert grid subbasins into vector format

  17. Watershed Delineation Convert grid subbasins into vector format

  18. Hydrologic Parameters • Lag Time, tl • Need longest flow path, LW, and average flow velocity, vW • To get time of concentration, tc (tl=0.6tc) • SCS Curve Number • Need precipitation, P, and curve number, CN • To get excess precipitation, Pe

  19. SCS Curve Number • Curve number, CN • Data • Soils (TNRIS) • Landuse (Harris County GIS) • Create a lookup table • Precipitation, P • Depends on the storm return period and data

  20. Time of Concentration • Average flow velocity, vW • Determined from HEC-2 model tl=0.6tc • Longest flow path, LW • Longest distance in subbasin from outlet

  21. Longest Flow Path • Overland Flow • LW1=overland flow length • vW1=average overland flow velocity • Channel Flow • LW2=channel flow length • vW2=average channel flow velocity

  22. Longest Flow Path Overland velocity is much slower than channel velocity vW1 << vW2 Time of concentration in the channel is negligible to the overland flow time t = L/v Time of concentration for overland flow is much larger than for channel flow tW1 >> tW2

  23. Longest Flow Path Spatial Analyst - Distance

  24. Longest Flow Path Spatial Analyst - Distance

  25. Longest Flow Path Spatial Analyst – Summarize Zones Summarize by subbasin to find the maximum distance in the distance grid

  26. Routing Parameters Reach parameters from HEC-2 model • Muskingham Routing - long channels • Need reach length, Ls, reach velocity, vs, storage parameter, X, and inflow hydrograph, I • To get flow time in reach, K, and outflow hydrograph, Q • Pure Lag – short channels • Need reach length, Ls, reach velocity, vs, and inflow hydrograph, I • To get lag time, tp, and outflow hydrograph, Q

  27. Future Considerations • Network Analyst • Determine flow direction at a junction with more than one downstream reach • Use one way capabilities and costs to set downstream reach • Rating Curves • Flow as a function of depth or elevation • Flow as a function of velocity

  28. Questions ?

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