Climatology of River Flooding during the
in the Blacksburg County Warning Area
by Jan Jackson and Robert Stonefield
There are 26 river forecast points in the WFO Blacksburg Hydrologic Service Area (image below).
Late winter to early spring is climatologically the peak for river flooding for our area. There have been 277 documented river forecast point flood events from 1994 to 2007 in the WFO Blacksburg Hydrologic Service Area. The floods occurred in every month of the year (image below). However, there were high numbers in each month of the late cool season from January through March. January had the highest number of flood events for all months, with 61, which is 22% of the total. February had 47 events, or 17% of the total, and March had 37 events, or 13% of the total. The three month period of January through March had over half the river flood events, with a total of 145, or 52%. The 4 month period of January through April accounted for 61% (170) of the river flood events. The high percentage of flood events in the late cool season is likely attributable to efficient runoff due to saturated or frozen soils, dormant vegetation and melting snow cover. Average discharge on the rivers is also climatologically higher during this period.
The severity of river flooding is based on three categories: minor, moderate and severe. These categories correspond to pre-defined ranges of levels above flood stage, which varies at each forecast point. Because impacts to areas around the river gage are much greater beginning at the moderate flood stage, moderate and major river floods are considered significant floods. Of the 277 events, 95 were classified as moderate, and 21 were classified as major (see graph below).
Significant flooding accounted for 42% (116 of 277) of the river flood events in the Blacksburg HSA. Significant river flooding occurred in every month of the year (see graph below). However, January (32), had the highest number of significant river flood events, and the 3 month period of January through March accounted for over 50% (63 of 116) of all significant river flood events. The 4 month period of January through April accounted for over 60% (72 of 116) of all significant river flood events. September (21), also had a secondary peak of significant river flooding, corresponding to the peak for widespread rainfall associated with the remnants of tropical systems.
The number of floods at each forecast point (see images below) during the 14 years of the study ranged from zero, (Hinton, WV), to 44, (South Boston VA). There are 5 river forecast points with dams immediately upstream from the gauge, which reduces the flood potential in various amounts. Hinton, WV has never flooded since the Bluestone dam immediately upstream was built in 1945. Wilkesboro, NC has only flooded 5 times since the dam was built there in 1962, and only once during the period of study. The other 3 locations with dams upstream: Covington, VA had 2 floods during the period of study, Radford, VA had 4 floods, and Altavista, VA had 8 floods. Other than the influence of dams, the other main factor in the number of floods at a particular forecast point was related to location along the river. On each of the 6 major rivers with more than one forecast point, (James, Roanoke, Dan, Yadkin, New and Greenbrier), headwater points generally had the fewest floods with the number of floods increasing with each forecast point downstream. For example, along the Roanoke River, the city of Roanoke at the headwaters had 11 floods, Altavista downstream had 8 floods with some protection from a dam, Brookneal had 14 floods, and Randolph had 27 floods. Three forecast points at the ends of major rivers, (Bremo Bluff on the James, Randolph on the Roanoke, and South Boston on the Dan), accounted for 35 % (95 of 270) of all river floods.
Number of flood events by scale at each forecast point during the period 1994 to 2007: