By Jennifer VerWest

Differences between Flat and Average/Steep Terrain

Flat Terrain Steep/AverageTerrain

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

Procedure

• Determine Parameters• Hydrologic

• Routing

• Delineate Watershed

Data• Channel Network

• USGS Gage Stations

• Soils

• Landuse

• HEC-2 Model

Data from Harris County (Houston), TX

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

Watershed Delineation

Watershed Delineation

Determine Grid Sizefor Analysis

• Detail of Study

Two Considerations:

• Processing Time for Grid

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

Processing Time for Grid

Detail of Study

• Depending on Grid• Smallest subbasin area

• Number of channels with no subbasin area

• Shortest Channel – 71.3 feet

Detail of Study

Processor Time versusDetail of Study

Minimize processing time and number of channels with no subbasin area

Watershed DelineationConvert grid subbasins into vector format

Watershed DelineationConvert grid subbasins into vector format

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 - Distance

Longest Flow PathSpatial Analyst - Distance

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

Questions