Transcript NRCS TR55 - Small Watershed Hydrology

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.
Topology of WinTR-55
Empirical vs. Conceptual
Stochastic vs. Deterministic
Lumped vs. Distributed
Continuous vs. Event-based vs. Peak discharge
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
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:
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
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:
 1000  
L0.8 
 CN 10 1
Tc 
1900 S 0.5
0 .7
 
 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
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
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
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)
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.
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”
Schematic Example
Sub-area 2
(Channel Routing)
Outlet
Sub-area 3
Legend
Sub-area 1
Storage Area
Sub-Area Inflow
Points
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 < 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 userdefined
24-hour
II (average)
Application of TR-55
Raingarden design for Votey parking lot runoff