River Flooding: An Introduction

Changing pace a little from actual weather, this provides a very basic overview of river flooding and some information on the Big Black River in Mississippi:

Over the past week or so, a large part of Mississippi and Alabama has received excessive amounts of rain that not only led to flash flooding, but it also has resulted in many area rivers surpassing flood stage and becoming very swelled. The ground can only hold so much water, and when it reaches its capacity/becomes saturated/, excess water just flows as runoff which creates short-term flash flooding and can create longer-term river flooding because of the excess amount of volume of water trying to flow through the river channel.

More particularly, large amounts of rain over a long period of time, such as what has occurred in this case, promotes rivers to slowly rise. With larger river basins, there is generally a lag between the peak in precipitation and the peak in the river's stage, or height. As outlined below, the Big Black River is actually not cresting until this weekend (Bentonia) and early next week (Bovina). Once rivers crest, it slowly will begin to recede its stage; however, a certain river basin could actually be in flood stage for over a week.

For example, the Big Black River's stage, near Bentonia, MS, was 26.9 feet which is almost 5.0 feet above the flood stage of 22.0 feet. Moderate river flooding has been ongoing around this basin, and additional rising of the water is expected through Saturday. The river's stage is expected to crest near 28.0 feet which will most likely result in catastrophic flooding especially to surrounding agricultural lands. (source, NWS Jackson www.srh.noaa.gov/jan )

Another example, the Big Black River 's stage, near Bovina, is 32.4 feet with a crest of 37.5 feet occurring by early next week. This crest is almost 10 feet above its flood stage and at stages near 38 feet, the water will actually flow across the road near the old Highway 80 bridge as well as cause catastrophic agricultural devastation to the lands surrounding the river basin. (source, NWS Jackson www.srh.noaa.gov/jan )

As seen in the graph above, the Big Black River at Bovina has actually been above flood stage for the last 6 days. Our most recent bout of heavy rain actually began on Sunday 1/13, but as discussed above, there is a lag in the river stage's response because of its larger basin relative to other rivers/streams. (source www.usgs.gov )

Given the recent snowfall amounts of 1-4" that have occurred over central portions of Mississippi, as this begins to melt a little later this afternoon, it could also cause some additional rises to area streams/rivers due to excess water.

The data used for this post were collected on the afternoon of 1/17.

IW