To Manage Water, We Want to Know How Often a Flow Is Equaled or Exceeded
DURATION CURVE - % of Time a Flow Is Equaled or Exceeded
Plot of Discharge vs Probability of Occurrence
To create: Rank Flows, as m, From Highest to Lowest 1 to N
If 2 Values Are Equal, Assign Separate Rank
P = 100 * (m/(n+1))
Normalize flow by basin area to compare basins of different size
Q___
BasinArea
log scale
More severe floods e
e.g.
g crystalline rock
rock,
very little soil, poor infiltration, rapid
runoff, high peak flows, low base flows
Moderate conditions in glacial outwash
with good infiltration but rapid through
flow, moderate peak and base flows
Mild floods e.g. alluvial basin with
good infiltration and moderate
ability to transmit groundwater, low
peak flows, high base flows
0.01
50
99.99
Probability that Flow is Equaled or exceeded
Steeper
Whatcurve
does indicates
a steeper more
curvesevere
indicate?
floods
Estimate Velocity With Manning Equation
V = 1.49 R 2/3 S 1/2
n
where: V = average velocity in fps
R=h
hydraulic
d
li radius
di (flow
(fl
area [ft2]/wetted
]/ tt d perimeter[ft])
i t [ft])
S = slope of energy gradient
n = Manning friction factor
R = (40ft * 0.75ft) / (40ft + 0.75ft + 0.75ft) = 0.72
S = (5816ft – 5775ft) = 0.02276
550m 1 ft 39.3in
12i 1
12in
1m
n ~ 0.05
V ~ 3.6 ft/sec
Q ~ 109 cfs
1
Drainage area = 400 mi2
very rough average annual baseflow? ~ 40 cfs?
average annual ground water discharge ~ 1.26x109 ft3
Drainage area = 400 mi2
What is the baseflow discharge in 1987?
2
Drainage area = 400 mi2
1987
Qo = 150 cfs
tlog = time for Q to drop 1 log cycle = 0.6yr
t = time for recession = 0.7yr
1 yr
1 log
cycle Q
3
TOTAL GW THAT COULD DISCHARGE AT START OF RECESSION, Vtp:
Vtp is evaluated ∫
∞
0
Vtp =
Qotlog
2.3
TOTAL GW THAT COULD DISCHARGE AT END OF RECESSION, VR:
VR is evaluated
∫
∞
t @ end
VR =
1987, 1988, 1989, 1993
Qo ~ 150 cfs
tlog = time for Q to drop 1 log cycle ~ 0.6 yr
t = time for recssion
recession~~0.7
0.7yryr
Vtp ~ 1.2x109 ft3
VR ~ 8.4x107 ft3
Vdischarged ~ 1.2x109 ft3 ~26,000 AF
Qotlog
⎛ tt
2.3⎜10 log
⎜
⎝
⎞
⎟
⎟
⎠
4