Selecting an Appropriate Routing Technique within HECHMS Hydrologic

Selecting an Appropriate Routing Technique within HEC-HMS Hydrologic Engineering Center 1

Objectives • Assist modelers in the identification of an appropriate hydrologic or hydraulic routing method for a given problem • Discuss important study objectives and physical considerations • Provide rules of thumb Hydrologic Engineering Center 2

Keep In Mind… • All routing models are simplifications of the real world • An appropriate method captures the significant aspects of the system in question • An appropriate method provides the necessary information to meet study objectives Hydrologic Engineering Center 3

Questions to Ask Yourself • For each routing method: – How does it represent open channel flow processes? – What are it’s advantages and disadvantages? – What input data is required? – Can it supply the required output information? Hydrologic Engineering Center 4

Additional Factors to Consider • What type of study / what will be considered? – Reservoir design – Channel modifications – System effects (changes to the watershed) • Level of detail – Feasibility, planning, or design • Characteristics of the physical system in question • Familiarity and experience of the modeler • Time and Costs Hydrologic Engineering Center 5

Backwater Effects • Caused by tides, confluences, hydraulic structures, bridges, culverts, channel constrictions, and other obstructions • Hydrologic methods generally do not simulate backwater effects – Modified Puls can • Hydraulic methods generally do account for backwater – Kinematic Wave cannot Hydrologic Engineering Center 6

Floodplains • Can have significant impact on routed hydrograph • Important factors include: – Width – Slope – Roughness • Separate overbanks from main channel Hydrologic Engineering Center 7

Floodplains • Methods that can simulate flood plain effects: – Muskingum-Cunge w/ an eight point cross section – Modified Puls • Very wide and flat floodplains may require 2 D modeling – 2 D Diffusion Wave method Hydrologic Engineering Center 8

Confluences and Flow Splits • If no significant backwater: – All hydrologic methods can apply • If significant backwater, use: – Diffusion Wave – Modified Puls • For full networks with flow splits and reversals, full dynamic wave equations should be applied – e. g. HEC-RAS Hydrologic Engineering Center 9

Supercritical Flow • Hydrologic methods do not know about flow regime • If supercritical for a long reach, or stage is important, supercritical reach should be isolated • Intermittent supercritical can be ignored for hydrologic routing purposes Hydrologic Engineering Center 10

Observed Data • If observed data is not available: – Physically based methods should be used • Easier to estimate input data • More reliable results • When observed data is available: – Must calibrate and validate – Use statistical metrics Hydrologic Engineering Center 11

Channel Slope • As slope gets flatter, hydrologic methods begin to fail – Simplifications become more and more important – e. g. acceleration terms w/in Momentum equation Hydrologic Engineering Center 12

Slope and Rate-of-Rise Rules of Thumb • When bed slope > 10 ft/mi: – Most hydrologic routing methods are okay to use • When bed slope < 2 ft/mi: – Most hydrologic routing methods will produce erroneous results • When bed slope is between 2 and 10 ft/mi: – Depends upon the channel properties, chosen method, and rate-of-rise of the hydrograph Hydrologic Engineering Center 13

Ponce (1978) Criterion • Kinematic Wave routed peak within 5% provided the following is satisfied: where: Hydrologic Engineering Center T = Hydrograph duration in seconds So = Friction or Bed Slope (ft/ft) Uo = Reference mean velocity (ft/sec) do = Reference flow depth (ft) 14

Ponce (1978) Criterion • Diffusion Wave routed peak within 5% provided the following is satisfied: where: Hydrologic Engineering Center T = Hydrograph duration in seconds So = Friction or Bed Slope (ft/ft) g = Gravitational acceleration (ft/sec 2) do = Reference flow depth (ft) 15

Factors to Consider Appropriate Methods NOT Appropriate No observed hydrograph data available for calibration -Diffusion Wave -Kinematic Wave -Muskingum-Cunge -Modified Puls* -Lag -Muskingum Significant backwater that will influence hydrograph -Diffusion Wave -Modified Puls* -Lag -Kinematic Wave -Muskingum-Cunge Flood will go out of bank into the floodplain -Diffusion Wave -Modified Puls* -Muskingum-Cunge -Muskingum -Lag -Kinematic Wave *with storage-discharge curves produced by a hydraulic model Hydrologic Engineering Center 16

Factors to Consider Appropriate Methods NOT Appropriate Slope > 10 ft/mi and: -Most, if not all, methods -None 2 < Slope < 10 ft/mi and: -Diffusion Wave -Muskingum-Cunge -Modified Puls* -Muskingum -Kinematic Wave -Lag Slope < 2 ft/mi and: -Diffusion Wave -Muskingum-Cunge -Modified Puls* -Kinematic Wave -Muskingum -Lag Slope < 2 ft/mi and: -Full Dynamic Wave -All other methods Hydrologic Engineering Center T = Hydrograph Duration [sec] S 0 = Friction Slope [ft/ft] u 0 = Average Velocity [ft/s] d 0 = Flow Depth [ft] *with storage-discharge curves produced by a hydraulic model 17

Conclusion • The most appropriate method depends upon many considerations • Important physical considerations include backwater effects, floodplains, and channel slope • Slope rule of thumb and Ponce criterion can be used to narrow the list of appropriate methods Hydrologic Engineering Center 18