DES 606 : Watershed Modeling with HEC-HMS

1. DES 606 : Watershed Modeling with HEC-HMS Module 11 Theodore G. Cleveland, Ph.D., P.E 29 July 2011

2. Channel Routing • Example 5 illustrated lag-routing for simplistic channel routing. • Lag routing does not attenuate nor change shape of the hydrograph • Conflicting arguments on where applicable • probably adequate for hydrographs that stay in a channel (no floodway involvement) and travel distance is short (couple miles). • Other methods are required where attenuation and shape change is important

3. Channel Routing • Methods that attenuate and change shape of hydrographs in HMS include • Storage • a type of level pool routing • Muskingum • a type of storage routing that accounts for wedge (non-level pool) storage • Kinematic • A type of lag routing where the lag is related to channel slopes and accumulated reach storage – considered a hydraulic technique

4. Channel Routing • Consider each individually using an example • Develop required input tables • Enter into HMS • Examine results

5. Channel Routing • Channel storage routing is essentially an adaptation of level pool reservoir routing • Principal difference is how the storage and discharge tabulations are formed. • In its simplest form, the channel is treated as a level pool reservoir.

6. Channel Routing • The storage in a reach can be estimated as the product of the average cross sectional area for a given discharge rate and the reach length.

7. Channel Routing • A rating equation is used at each cross section to determine the cross section areas.

8. Channel Routing • A known inflow hydrograph and initial storage condition can be propagated forward in time to estimate the outflow hydrograph. • The choice of Dt value should be made so that it is smaller than the travel time in the reach at the largest likely flow and smaller than about 1/5 the time to peak of the inflow hydrograph • HMS is supposed to manage this issue internally

9. Channel Routing Example • Consider a channel that is 2500 feet long, with slope of 0.09%, clean sides with straight banks and no rifts or deep pools. Manning’s n is 0.030.

10. Channel Routing Example • The inflow hydrograph is triangular with a time base of 3 hours, and time-to-peak of 1 hour. The peak inflow rate is 360 cfs.

11. Channel Routing Example • Configuration: Input hydrograph Routing Model Output hydrograph Q(t) t

12. Data preparation • Construct a depth-storage-discharge table • Construct an input hydrograph table • HMS • Import the hydrograph and the routing table information • Simulate response

13. Depth-Storage • Compute using cross sectional geometry • Save as depth-area table (need later for hydraulics computations) • Multiply by reach length for depth-storage Depth Length A4 A3 A2 A1 Area A1 A1+A2 A1+A2+A3

14. Depth-Perimeter • Compute using cross sectional geometry • Save as depth-perimeter table (need later for hydraulics computations) Depth Length W4 W3 W2 W1 Wetted Perimeter W1 W2 W3

15. Depth-Discharge • Compute using Manning’s equation and the topographic slope

16. Inflow Hydrograph • Create from the triangular input sketch

17. HEC-HMS • Create a generic model, use as many null elements as practical (to isolate the routing component)

18. HEC-HMS • Storage-Discharge Table (from the spreadsheet) Note the units of storage

19. HEC-HMS • Meterological Model (HMS needs, but won’t use this module) Null meterological model

20. HEC-HMS • Set control specifications, time windows, run manager – simulate response Observe the lag from input to output and the attenuated peak from in-channel storage

21. HEC-HMS • Set control specifications, time windows, run manager – simulate response Lag about 20 minutes Attenuation (of the peak) is about 45 cfs Average speed of flow about 2 ft/sec Observe the lag from input to output and the attenuated peak from in-channel storage

22. Muskingum Routing • A variation of storage routing that accounts for wedge storage Muskingum Muskingum-Cunge Level-pool Q Inflow Depth-Up Wedge storage Kinematic Wave Outflow Depth-Down

23. Muskingum routing is a storage-routing technique that is used to translate and attenuate hydrographs in natural and engineered channels, but avoids the added complexity of hydraulic routing. • The method is appropriate for a stream reach that has approximately constant geometric properties.

24. At the upstream end, the inflow and storage are assumed to be related to depth by power-law models

25. At the downstream end, the outflow and storage are also assumed to be related to depth by power-law models

26. Next the depths at each end are rewritten in terms of the power law constants and the inflows

27. Then one conjectures that the storage within the reach is some weighted combination of the section storage at each end (weighted average) • The weight, w, ranges between 0 and 0.5. • When w = 0, the storage in the reach is entirely explained at the outlet end (like a level pool) • When w = 0.5, the storage is an arithmetic mean of the section storage at each end.

28. Generally the variables from the power law models are substituted • And the routing model is expressed as • z is usually assumed to be unity resulting in the usual from

29. Generally the variables from the power law models are substituted • And the routing model is expressed as • z is usually assumed to be unity resulting in the usual from

30. For most natural channels w ranges between 0.1 and 0.3 and are usually determined by calibration studies • Muskingum-Cunge further refines the model to account for changes in the weights during computation (better reflect wedge storage changes)

31. In HEC-HMS • Use same example conditions • From hydrologic literature (Haan, Barfield, Hayes) a rule of thumb for estimating w and K is • Estimate celerity from bankful discharge (or deepest discharge value) • Estimate K as ratio of reach length to celerity (units of a time, essentially a reach travel time) • Estimate weight (w) as

32. In HEC-HMS • Use same example conditions

33. In HEC-HMS • Use same example conditions Muskingum Parameters Results a bit different but close Difference is anticipated

34. In HEC-HMS • Change w to 0.0, K=20 minutes, NReach=2 • Level Pool Muskingum Parameters Results almost same as level-pool model

35. In HEC-HMS • Change w to 0.5, K=20 minutes, NReach=2 • Lag Routing Muskingum Parameters 20 minute lag routing

36. Muskingum-Cunge • More complicated and in HMS almost a hydraulic model • Data needs are • Cross section geometry (as paired-data) • Manning’s n in channel, left and right overbank • Slope • Reach length • Will illustrate data entry using the same example

37. Muskingum-Cunge • Cross section geometry • “Glass walls”

38. Muskingum-Cunge • Associate the section with the routing element • Other data included

39. Muskingum-Cunge • Run the simulation • Result comparable to level-pool.

40. Summary • Examined channel routing using three methods • Level pool routing (Puls) • Muskingum • Muskingum-Cunge • Selection of Muskingum weights allows analyst to adjust between level-pool and lag routing by changing the weighting parameter • All require external (to HMS) data preparation • Muskingum-Cunge hydraulically “familiar)

41. Summary • Parameter estimation for Muskingum method requires examination of literature external to HMS user manuals • Instructor preferences for routing • Lag routing (if can logically justify) • Level-pool • Muskingum-Cunge (as implemented in HMS) – very “hydraulics” • Muskingum (I would only choose if had calibration data) • Kinematic-wave (outside scope– limited geometries)