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 acres. –Slope is 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. – 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 ~ 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

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 road to outlet acres 17.7 acres 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

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