by jennifer verwest
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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By Jennifer VerWest
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Differences between Flat and Average/Steep Terrain
Flat Terrain Steep/AverageTerrain
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Differences between Flat and Average/Steep Terrain
Flat TerrainSteep/Average
Terrain• 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
• Water flows downhill in the direction of steepest slope
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Procedure
• Determine Parameters• Hydrologic
• Routing
• Delineate Watershed
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Data• Channel Network
• USGS Gage Stations
• Soils
• Landuse
• HEC-2 Model
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Data from Harris County (Houston), TX
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Watershed Delineation
• Spatial Analyst – Assign Proximity
Since the elevation does not necessarilydescribe where the water goes, it was not used
for delineating the watershed.
• Defined the subbasin as the area that is closest to the channel
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Watershed Delineation
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Watershed Delineation
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Determine Grid Sizefor Analysis
• Detail of Study
Two Considerations:
• Processing Time for Grid
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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
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Processing Time for Grid
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Detail of Study
• Depending on Grid• Smallest subbasin area
• Number of channels with no subbasin area
• Shortest Channel – 71.3 feet
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Detail of Study
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Processor Time versusDetail of Study
Minimize processing time and number of channels with no subbasin area
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Watershed DelineationConvert grid subbasins into vector format
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Watershed DelineationConvert grid subbasins into vector format
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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
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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
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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
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Longest Flow Path
• Channel Flow• LW2=channel flow
length
• vW2=average channel flow velocity
• Overland Flow• LW1=overland flow
length
• vW1=average overland flow velocity
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Longest Flow Path
Time of concentration in the channel isnegligible to the overland flow time
Time of concentration for overland flowis much larger than for channel flow
tW1 >> tW2
t = L/v
Overland velocity is much slower than channel velocityvW1 << vW2
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Longest Flow PathSpatial Analyst - Distance
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Longest Flow PathSpatial Analyst - Distance
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Longest Flow PathSpatial Analyst – Summarize Zones
Summarize by subbasin to find themaximum distance in the distance grid
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Routing Parameters
• 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
Reach parameters from HEC-2 model
• Pure Lag – short channels• Need reach length, Ls, reach velocity, vs, and
inflow hydrograph, I
• To get lag time, tp, and outflow hydrograph, Q
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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
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Questions