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

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

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

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

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:

TR-55 methodology: CN zCN range 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

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

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

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

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)

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.

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”

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

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 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)

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