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:

http://md.water.usgs.gov/publications/sir-2006-5179/index.html
 

Contributed by: BES Co-PI Ed Doheny, USGS