WinTR-55: Introduction and Background

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

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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

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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, Mannings n)
• Shallow concentrated flow f(length, slope,
  Mannings n)
• Channel flow f(length, slope, Mannings 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
• Oj1 C1Ij1 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.

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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

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Schematic Example
Reach 2
Sub-area 2
Outlet
(Channel Routing)
Sub-area 3
Reach 1
(Storage Routing)
Legend
Sub-area 1
Storage Area
Sub-Area Inflow Points
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Other TR-55 Criteria

• 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 lt Tc lt 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)

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Application of TR-55

• Raingarden design for Votey parking lot runoff