In June 2008, Eastern Iowa experienced the largest flood ever recorded, including the flooding of Cedar Rapids in the Cedar River basin and Iowa City in the Iowa River basin, respectively. Although the 2008 rains, in themselves, were not the worst in history, the resulting flood was the worst in some areas.
For example, in the Cedar River basin, runoff resulting from rain that fell in the upper watershed on June 8 moved downstream to coincide with rain falling into the lower part of the watershed on June 12. These consecutive storms combined to produce an otherwise unexpected, rapid rise of the river and a single well-defined and extremely large flood peak in Cedar Rapids on June 13. The travel time of runoff from the upper watershed to the downstream confluence took four days, which coincided with the time difference of two rainstorms occurring on June 8 and June 12, respectively. It might not be an exact example of a single storm movement, but, this is a clear and compelling example that shows how rainstorm movement in the same direction as runoff can exacerbate the magnitude of flood peaks (Seo et al., 2013).
It has been known for a long time that storm movement is the norm rather than the exception (Singh, 1997). Most storms tend to be moving storms (Shearman, 1977; Marshall, 1980) with typically preferred directions in different seasons (Huff, 1979; Shearman, 1977; Upton, 2001). For example, Huff (1979) found that in Illinois 84% of severe heavy rainstorms exhibited motion with a westerly component. The direct influence of rainfall movement on the peak and shape of the runoff hydrograph has been long recognized (Maksimov, 1964; Yen and Chow, 1968; Wilson et al., 1979; Jensen, 1984; Niemczynowicz, 1991; Singh, 1998). In general, a storm moving in the downstream direction shows a late peak, greater peak discharge, steeper rising limb and shorter base time compared with a storm moving upstream.
What can we do even in the most severe condition?
“In general, it has been found that the very heavy storms tend to have nearly W-E orientations. Heavy, but less severe storms, are usually oriented WSW-ENE or WNW-ESE. Moderately heavy storms, especially those of short duration (1 to 3 hours) are frequently oriented WSW-ENE or SW-NE. It is quite fortunate that the orientation of most of the major drainage basins in northeast Illinois are closer to a north-south than a west-east orientation.”
(Huff, 1976)
Is it a fortune that we can rely on even in the most severe condition? There should be something that we can change, which would mitigate even the most severe flood caused by the spatio-temporal varation of rainfall.
Depending on network configuration, the response from a drainage basin changes. A sinuous network produces less peaks and vice versa (Seo and Schmidt, 2012; 2013).
It is not a fortune that dictates us. I think network configuration is one among various alternatives that deals with the impact from storm movements.
References
[1] Bradley, J., A. Allen (2010). "What causes floods in Iowa?" A watershed year : anatomy of the Iowa floods of 2008, C. F. Mutel, ed., University of Iowa Press, Iowa City, xvii, 250 p., 210 p. of plates.
[2] Huff, F. A. (1979). Hydrometeorological characteristics of severe rainstorms in Illinois, Illinois State Water Survey, Urbana.
[3] Huff, F. A., and Vogel, J. L. (1976). Hydrometeorology of heavy rainstorms in Chicago and northeastern Illinois : Phase 1, historical studies, Illinois Water Survey, Urbana.
[4] Jensen, M. (1984). "Runoff pattern and peak flows from moving block rains based on a linear time area curve." Nord Hydrol, 15(3), 155-168.
[5] Krajewski, W. F., and Mantilla, R. (2010). "Why were the 2009 floods so large?" A watershed year : anatomy of the Iowa floods of 2008, C. F. Mutel, ed., University of Iowa Press, Iowa City, xvii, 250 p., 210 p. of plates.
[6] Linhart, S. M., and Eash, D. A. (2010). "Floods of May 30 to June 15, 2008, in the Iowa and Cedar River basins, eastern Iowa: U.S. Geological Survey open-file report 2010–1190." 99.
[7] Maksimov, V. A. (1964). "Computing runoff produced by a heavy rainstorm with a moving center." Soviet Hydrology, 5, 510-513.
[8] Marshall, R. J. (1980). "The estimation and distribution of storm movement and storm structure, using a correlation-analysis technique and rain-gauge data." J Hydrol, 48(1-2), 19-39.
[9] Niemczynowicz, J. (1991). "On storm movement and its applications." Atmos Res, 27(1-3), 109-127.
[10] Seo Y, and Schmidt, A. R. (2012). "The effect of rainstorm movement on urban drainage network runoff hydrographs." Hydrol Process, 26(25), 3830-3841.
[11] Seo Y, and Schmidt, A. R. (2013). "Network configuration and hydrograph sensitivity to storm kinematics." Water Resour Res, 49(4), 1812-1827.
[12] Seo Y, Schmidt, A. R., and Sivapalan, M. (2012). "Effect of storm movement on flood peaks: Analysis framework based on characteristic timescales." Water Resour Res, 48.
[13] Shearman, R. J. (1977). "The speed and direction of movement of storm rainfall patterns with reference to urban storm sewer design." Hydrological Sciences Bulletin, XXII(3), 421-431.
[14] Singh, V. P. (1998). "Effect of the direction of storm movement on planar flow." Hydrol Process, 12(1), 147-170.
[15] Upton, G. J. G. (2002). "A correlation-regression method for tracking rainstorms using rain-gauge data." J Hydrol, 261(1-4), 60-73.
[16] Wilson, C. B., Valdes, J. B., and Rodriguez-Iturbe, I. (1979). "Influence of the spatial-distribution of rainfall on storm runoff." Water Resour Res, 15(2), 321-328.
[17] Yen, B. C., and Chow, V. T. (1968). A study of surface runoff due to moving rainstorms, Dept. of Civil Engineering, University of Illinois, Urbana,.
[18] Yen, B. C., and Chow, V. T. (1969). "A laboratory study of surface runoff due to moving rainstorms." Water Resour Res, 5(5), 989-1006.
