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
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
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
Detail of Study
• Depending on Grid• Smallest subbasin area
• Number of channels with no subbasin area
• Shortest Channel – 71.3 feet
Processor Time versusDetail of Study
Minimize processing time and number of channels with no subbasin area
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
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
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
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
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
Longest Flow PathSpatial Analyst – Summarize Zones
Summarize by subbasin to find themaximum distance in the distance grid
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
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