DES 606: Watershed Modeling with HEC-HMS

Watershed Subdivision

Theodore G. Cleveland, Ph.D., P.E.

6 OCT 11

Watershed Subdivision

• Example illustrates taking a working model, then subdividing as guided by drainage features.

Base Model

• Use the Woodland’s Ditch C as an example.– Area is 159.3 acres.– Slope is 0.0049

• Build a HMS model and calibrate using a historical event.

Base Model

• For this example I choose to Kerby-Kirpich method to estimate Tc– Small watershed

Base Model

• For this example I choose to Kerby-Kirpich method to estimate Tc– Kerby for Overland Portion– Kirpich for Channel Portion

• Length Estimate– Magenta line is about

3725 feet.– 0-4696 report

suggests overland component be limited to 500 ft.

• Time estimate– Estimate the channel

portion.– Estimate the overland

flow portion.

• Sum of these two times is estimate of Tc for the area.

• We will use this value as the estimate for Lag time in SCS-DUH

• Loss Model– For this example, lets use SCS CN. – Watershed is located in Woodlands, Texas.

• Predominant pine forest• Lissie Sands formation• Hydrologic Soil Group B or C.• CN ~ 55-70 is appropriate• Developed portion is about 20%

• Observed rainfall and runoff– Have a 60 hour event

in a database – stored as elapsed time.

– Invent (or use actual) calendar dates

HEC-HMS Model

HEC-HMS Model

HEC-HMS Model

HEC-HMS Model

HEC-HMS Model

Lumped Model

Subdivide

• Use Grogans Mill road as guide – water would flow in swale along the road

• Flow to drainage ditch from 3 areas

Divide the Watershed

• Use Grogans Mill road as guide– South of Grogans

Mill direct to outlet.– North and West of

the large pond routes to outlet

– East of large pond routes to outlet

Areas, Routing etc.

Route 1064 ft along swale, under road

Route 1596 ft into ditch, under roadto outlet.

23.1 acres17.7 acres

118.6 acres

Parameter Estimates

• Lag Time– Upper Watershed = 58 min.– Lower Watershed = 27 min.– East Watershed = 47 min.

• Routing Lag Times– Upper To Outlet = 18 min.– East To Outlet = 13 min.

HEC-HMS Model

HEC-HMS Model

Summary

• Lumped model– Entire area is a

single sub-basin

Summary

• Subdivided model– Selected portions of

the area are linked– Joined by routing

elements

Summary

• The two different models work about the same in this example (as compared to the observed data)

• Used lag routing, but data suggest substantial storage

• The large pond suggest a storage element too