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

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

3. Topology of WinTR-55 zEmpirical vs. Conceptual zStochastic vs. Deterministic zLumped vs. Distributed zContinuous vs. Event-based vs. Peak discharge

4. History of TR-55 zDeveloped by SCS (now NRCS) for agricultural watersheds in 1954 zWidely accepted, yet no peer review zAdapted for urban watersheds zPoor performance in forested watersheds

5. TR-55 methodology: Runoff zRainfall (P) separated into: yRainfall excess (Q) yInitial abstraction (I a ) – interception, infiltration, and depression storage yRetention (F) – proportion retained, infiltrated zBasic assumption: zCN is a function of S, the potential maximum retention z→ Runoff equation:

6. TR-55 methodology: CN zCN range 0-100 y0 = no runoff y100 = complete runoff zFunction of: yHydrologic soil group yCover type yTreatment practice yHydrologic condition yImpervious area yARC – antecedent runoff condition zCan be adjusted for ARCI and ARCIII

7. TR-55 methodology: T c zTime it takes water to travel from most hydrologically distant portion of watershed to the outlet. zMany ways to calculate: yWith limited data: yNRCS method: xSheet flow f(length, slope, Manning’s n) xShallow concentrated flow f(length, slope, Manning’s n) xChannel flow f(length, slope, Manning’s n, channel dimensions) zOther methods incorporate rainfall intensity

8. TR-55 methodology: Hydrographs zUnit hydrograph approach: yQ = P * unit hydrograph zDefined 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) zDefault DUH is average shape of a large number of ag watersheds nationwide zUser-specified DUH can be input

9. TR-55 methodology: Routing z Muskingum-Cunge method z Most widely used method of stream-channel routing z O j+1 = C 1 I j+1 + C 2 I j + C 3 O j z Constants are based on travel time through reach

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

11. TR-55 Methodology: Sub-area/Reach Concepts z WinTR-55 represents the watershed as a system of sub-areas and reaches. z “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 z“Reaches” represent the configuration of flow paths within the watershed. zStorage routing (Lakes, Structures, Wetlands, etc.) and Channel Routing take place within a Reach. zAll WinTR-55 modeled watersheds end with the final stream reach terminating at an “Outlet”

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

14. Other TR-55 Criteria zMaximum Area zNumber of Sub-areas zTc for any sub-area zNumber of reaches zTypes of reaches zRainfall Depth zRainfall Distributions zRainfall Duration zAntecedent 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)

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