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WinTR-55: Introduction and Background

WinTR-55: Introduction and Background. Objectives: The basics History of WinTR-55 Nuts & bolts of algorithms Demo of model interface. Why model?. To estimate conditions where measurements are not available or possible. To test system understanding. To facilitate design.

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WinTR-55: Introduction and Background

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  1. WinTR-55: Introduction and Background • Objectives: • The basics • History of WinTR-55 • Nuts & bolts of algorithms • Demo of model interface

  2. Why model? • To estimate conditions where measurements are not available or possible. • To test system understanding. • To facilitate design.

  3. Topology of WinTR-55 • Empirical vs. Conceptual • Stochastic vs. Deterministic • Lumped vs. Distributed • Continuous vs. Event-based vs. Peak discharge

  4. History of TR-55 • Developed by SCS (now NRCS) for agricultural watersheds in 1954 • Widely accepted, yet no peer review • Adapted for urban watersheds • Poor performance in forested watersheds

  5. TR-55 methodology: Runoff • Rainfall (P) separated into: • Rainfall excess (Q) • Initial abstraction (Ia) – interception, infiltration, and depression storage • Retention (F) – proportion retained, infiltrated • Basic assumption: • CN is a function of S, the potential maximum retention • → Runoff equation:

  6. TR-55 methodology: CN • CN range 0-100 • 0 = no runoff • 100 = complete runoff • Function of: • Hydrologic soil group • Cover type • Treatment practice • Hydrologic condition • Impervious area • ARC – antecedent runoff condition • Can be adjusted for ARCI and ARCIII

  7. TR-55 methodology: Tc • Time it takes water to travel from most hydrologically distant portion of watershed to the outlet. • Many ways to calculate: • With limited data: • NRCS method: • Sheet flow f(length, slope, Manning’s n) • Shallow concentrated flow f(length, slope, Manning’s n) • Channel flow f(length, slope, Manning’s n, channel dimensions) • Other methods incorporate rainfall intensity

  8. TR-55 methodology: Hydrographs • Unit hydrograph approach: • Q = P * unit hydrograph • Defined as temporal distribution of runoff resulting from a unit depth (i.e., 1 cm) of rainfall excess occurring over a given duration (i.e., 24 hrs) • Default DUH is average shape of a large number of ag watersheds nationwide • User-specified DUH can be input

  9. TR-55 methodology: Routing • Muskingum-Cunge method • Most widely used method of stream-channel routing • Oj+1 = C1Ij+1 + C2Ij + C3Oj • Constants are based on travel time through reach

  10. TR-55 methodology: Detention • Outlet flow from detention pond: • Pipe orifice flow assumed • V-notch or rectangular weir • All flow routed through structure (no overflow option) • Assumes no losses from pond (i.e., infiltration)

  11. TR-55 Methodology: Sub-area/Reach Concepts • WinTR-55 represents the watershed as a system of sub-areas and reaches. • “Sub-areas” are the watersheds that generate hydrographs that feed into the upstream end of a reach.

  12. TR-55 Methodology: Sub-area/Reach Concepts • “Reaches” represent the configuration of flow paths within the watershed. • Storage routing (Lakes, Structures, Wetlands, etc.) and Channel Routing take place within a Reach. • All WinTR-55 modeled watersheds end with the final stream reach terminating at an “Outlet”

  13. Schematic Example Sub-area 2 Reach 2 Outlet (ChannelRouting) Sub-area 3 Reach 1 (StorageRouting) Legend Sub-area 1 Storage Area Sub-Area Inflow Points

  14. Maximum Area Number of Sub-areas Tc for any sub-area Number of reaches Types of reaches Rainfall Depth Rainfall Distributions Rainfall Duration Antecedent Runoff Condition 25 square miles 1-10 0.1 hour < Tc < 10 hours 0-10 Channel or Structure 0-50 inches (0-1,270mm) NRCS Type I, IA, II, III, NM60, NM65, NM70, NM75, or user-defined 24-hour II (average) Other TR-55 Criteria

  15. Application of TR-55 • Raingarden design for Votey parking lot runoff

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