08 December 2011

Urban Flash Index

Definition: The ratio between the instantaneous peak discharge and the mean discharge associated with a precipitation-runoff event.  This ratio can be useful when applied to, and in comparison of continuous discharge data from small, urban watersheds in describing the stream's flashiness.  A flashy stream or watershed is one that tends to produce narrow, steeply peaked storm hydrographs that rise and fall very quickly.

Example: A stream whose discharge peaked at 1,000 cubic feet per second as a result of a particular rainfall event, but whose mean discharge-response to the event was 50 cubic feet per second would have an urban flash index of 20, and would be described as a very flashy stream.  A stream whose discharge peaked at 100 cubic feet per second but, whose mean discharge-response to the event was 50 cubic feet per second would have an urban flash index of 2, and would not be considered to be very flashy at all.

The urban flash index was discussed in detail and applied at the U.S. Geological Survey stream gage on Minebank Run in Baltimore County, MD as part of a hydrologic investigation on the before and after effects of stream restoration.


Table 1. Storm duration, rainfall, and storm-intensity data for 18 selected storm events in the Minebank Run watershed, water years 2002 through 2004.

[EST, Eastern Standard Time; hrs, hours; in., inches; in/h, inches per hour]
 
Date(s) of storm
Time
(EST)
Storm duration
(hrs)
Storm rainfall total
(in.)
Average storm intensity
(in/h)
Maximum storm intensity
(in/h)

November 25, 2001
1610–1935
3.42
1.80
0.53
2.88
April 18, 2002
1350–1655
3.08
0.93
0.30
2.76
April 19, 2002
1515–1550
0.58
0.80
1.38
4.68
April 27–28, 2002
2155–1055
13.00
1.57
0.12
0.48
May 2, 2002
0200–0725
5.42
0.98
0.18
1.92
June 6, 2002
1655–1720
1735–1800
0.83
1.06
1.28
3.12
August 3, 2002
0050–0150
1.00
1.18
1.18
3.24
October 10–11, 2002
0535–1255
31.33
3.06
0.10
1.32
February 22, 2003
0610–1440
8.50
2.13
0.25
1.68
June 12, 2003
1620–1820
1910–2025
3.25
2.27
0.70
3.48
June 13, 2003
1835–1945
1.17
0.81
0.69
3.12
August 4, 2003
0155–0945
7.83
0.64
0.08
0.60
September 18–19, 2003
1310–0115
12.08
0.64
0.05
0.48
September 22–23, 2003
1905–0715
12.17
3.15
0.26
2.04
October 14–15, 2003
1840–0020
5.67
2.07
0.37
3.00
November 19, 2003
1315–2130
8.25
2.09
0.25
3.60
June 25, 2004
1520–1645
1.42
0.75
0.53
2.28
July 7, 2004
1405–1655
2.83
1.92
0.68
2.52













Table 2. Comparison of instantaneous peak discharge and daily mean discharge for the largest storms recorded at U.S. Geological Survey station 0158397967, Minebank Run near Glen Arm, Maryland, water years 2002 through 2004.

[ft, feet; ft3/s, cubic feet per second]


Date
Peak gage height
(ft)
Instantaneous peak discharge
(ft3/s)
Daily mean discharge
(ft3/s)
Ratio of instantaneous to daily mean discharge

June 12, 2003
8.61
1,390
42
33.1
July 7, 2004
7.97
945
27
35.0
July 27, 2004
7.93
919
31
29.6
September 23, 2003
7.78
834
44
19.0
June 13, 2003
7.58
730
29
25.2
August 3, 2002
7.58
725
15
48.3
May 17, 2004
7.55
720
14
51.4
November 19, 2003
7.50
700
35
20.0
June 6, 2002
6.88
466
13
35.8
October 14, 2003
6.84
411
23
17.9
April 19, 2002
6.79
401
8.0
50.1
November 25, 2001
6.73
367
18
20.4
June 25, 2004
6.29
295
14
21.1
August 16, 2003
6.13
272
8.3
32.8
May 25, 2004
6.09
266
10
26.6
September 19, 2003
6.02
256
11
23.3
February 22, 2003
6.00
253
50
5.1
August 4, 2003
5.97
248
8.4
29.5
May 2, 2002
5.96
247
13
19.0










Why important: The flashiness of a stream provides some general insight into that stream's impact in various aspects, such as flooding and potential for flash flooding, public safety hazards, sediment transport, stream channel stability, and potential for riparian habitat degradation. 

For more information:
Contributed by: BES Co-PI Ed Doheny, USGS

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