I gave this talk at a stormwater conference to help people think through some of the reasons for modelling, and how to get the most from their modelling efforts.
19. How certain do you need to be?
Figure: Cooperative Research
Center for Catchment Hydrology
75%?
99%?
Within 0.5 feet elevation?
Within 100 cubic feet per second?
Which parameter will be used for
calibration and error analysis?
Flow?
Water elevation?
Perform sensitivity analysis and
calibration to increase confidence
Data is hard to find for small
watersheds
Can another similar watershed be
used for calibration?
33. Model Structure
Figure: Cooperative Research
Center for Catchment Hydrology
Empirical‐based on statistical analysis of other watersheds
Conceptual‐based on a conceptual understanding of watershed
processes
Physical‐based on physical processes that can be tied directly to
measured characteristics
36. Empirical Hydrologic Models:
Rational Method
Table: NOAA Atlas 14 for University of Tennessee
Knoxville Monitoring Station
Q=CiA
Q=flow (ac‐in/hr≈cfs)
i = rainfall intensity for time
of concentration (in/hr)
A = area (acres)
Peak flows only
Best for small urban watersheds
Can lead to paradoxical results
60. Streambank Erosion
Photo: Mary Halley
Use model to check that
natural streams will be
kept in equilibrium (e.g. no
net erosion or deposition)
Requires knowledge of soils
Shear stress method
Table method
Geomorphologic method
64. Design Information (Input) MINOR MAJOR
Type of Inlet Type =
Local Depression (additional to continuous gutter depression 'a' from 'Q-Allow') aLOCAL = 1.0 1.0 inches
Total Number of Units in the Inlet (Grate or Curb Opening) No = 1 1
Length of a Single Unit Inlet (Grate or Curb Opening) Lo = 6.00 6.00 ft
Width of a Unit Grate (cannot be greater than W from Q-Allow) Wo = N/A N/A ft
Clogging Factor for a Single Unit Grate (typical min. value = 0.5) Cf-G = N/A N/A
Clogging Factor for a Single Unit Curb Opening (typical min. value = 0.1) Cf-C = 0.10 0.10
Denver No. 14 Curb Opening
H-Vert
H-Curb
W
Lo (C)
Lo (G)
Wo
WP
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Q for 1/2 Street (cfs)
QIntercepted&Bypassed(cfs),FlowSpreadT&T-Crown(ft),FlowDepth(inches)
QIntercepted(cfs) QBypassed(cfs) SpreadT (ft),Limited
byT-CROWN
SpreadT (ft),Not Limitedby
T-CROWN
FlowDepthd(inches)
Gutter Geometry (Enter data in the blue cells)
Maximum Allowable Width for Spread Behind Curb TBACK = 5.0 ft
Side Slope Behind Curb (leave blank for no conveyance credit behind curb) SBACK = 0.1000 ft. vert. / ft. horiz
Manning's Roughness Behind Curb nBACK = 0.1000
Height of Curb at Gutter Flow Line HCURB = 6.00 inches
Distance from Curb Face to Street Crown TCROWN = 13.0 ft
Gutter Depression a = 1.64 inches
Gutter Width W = 1.50 ft
Street Transverse Slope SX = 0.0200 ft. vert. / ft. horiz
Street Longitudinal Slope - Enter 0 for sump condition SO = 0.0300 ft. vert. / ft. horiz
Manning's Roughness for Street Section nSTREET = 0.0150
Minor Storm Major Storm
Max. Allowable Water Spread for Minor & Major Storm TMAX = 5.0 10.0 ft
Max. Allowable Depth at Gutter Flow Line for Minor & Major Storm dMAX = inches
Allow Flow Depth at Street Crown (leave blank for no) X = yes
H
y
d xS
S wa
S treet
C row n
W
T , T .
Tx
Q xwQ
T .C R O W N
C U R B
SBA C K
T .B AC K
M AX
Minor Storm Major Storm
Max. Allowable Gutter Capacity Based on Minimum of QT or Qd Qallow = 1.5 5.5 cfs
Inlet Capacity