Heavy rainfall over the Christmas holiday weekend in Missouri has led to Record flooding on the Meramec River on the southern side of St Louis. The stage was partially set with above normal rainfall in November. Then with much above normal rainfall in December including widespread 5 to 10+ inches just with the Christmas weekend storm, the soils could no longer absorb the rain. Creeks and streams quickly swelled and lead to flash and areal flooding.
The water quickly moved in to the Meramec River and it started rising as early as Saturday morning. Even with the local hilly terrain, it has taken a few days for the river to crest. As noted in the graphic below, the National Weather Service has four River Forecast points on the Meramec River before it empties into the Mississippi River, south of St Louis.
Meramec River at Pacific
The first forecast location (farthest west) is the Meramec River at Pacific. It crested Tuesday Afternoon/Tuesday Night with a provisional height of 33.42 ft. which is just under the 33.6 ft. flood of record level. This location did however rise above the highest point on the Rating Curve.
The rating curve, also referred to as the Stage Discharge relationship, provided the correlation between the amount of water in the river or flow and the river height or stage level. This correlation curve is very important as it allows hydrologists to translate the model flow forecasts into something that is more meaningful to emergency responders and the public, a stage or elevation forecast. So when the river is forecast to go above the rating curve, there is a HIGH degree of uncertainty of how the river will rise as it climbs out of its banks, spread through the flood plain and encounters obstacles including structures, trees, grasses, bridges, etc. For further information about the stage discharge relationship, the USGS has a more detailed and technical explanation.
Meramec River at Eureka
The next forecast location downstream, is the Meramec River near Eureka. This location crested Wednesday Afternoon/Night at a provisional level of 46.06 ft. This is a new record stage and surpassed the previous flood of record of 42.9 feet by 3.16 ft! It also topped the Rating Curve by several feet; so that created uncertainty in the forecasts.
Meramec River at Valley Park
The third location is the Meramec River at Valley Park. Valley Park also exceeded the historical flood of record by 4.41 ft. This location also topped the rating curve. By the pictures from this area, the river impacted several nearby structures including over-topping Interstate 44. The picture below shows the river over-topping I 44 in several locations. The Drury and Hampton Inns which are located just south of the Interstate are not flooded. More pictures from this area are available in a St Louis Post Dispatch picture gallery.
The graphic below shows the flood plain analysis for the area around the Valley Park River Gauge. The area in the magenta box highlights the area shown in the picture. The areas shaded in light blue show the 1% risk area, aka the 100 year flood plain, while the area in the orange show the 0.2% risk area, aka the 500 year flood plain. Note that the blue shading over-tops I 44 to the west of the hotels, while there are several areas that are over-topped by the 0.2% risk.
The USGS will make the final determination of the flood flow frequency for the Valley Park river gauge. But based off subjective analysis with the pictures, one can conclude that this flood is likely a 100 year flood and potentially a 500 year flood. There are several things the USGS must consider when making the finalized analysis of a flood flow frequency determination for an event including if there were any debris constrictions and other issues that may have contributed to a higher stage level and a lower flow. The USGS has more information about the 100 year flood.
Meramec River near Arnold
The last forecast point before the Meramec River empties into the Mississippi River is the gauge located near Arnold. This location had a broader crest Wednesday Night/Thursday of 47.26 ft. Arnold also exceeded the flood of record of 45.3 ft, provisionally by 1.98 ft.
Recession and Clean Up
With dry weather forecast for the next couple of days, the Meramec River will continue to fall. Clean up however will take some time after the waters recede. If you were impacted by the flood or know someone who was impacted, the Red Cross has some flood clean up tips. Also contact your local Flood Plain management and/or emergency manager for local guidance and to have someone evaluate your property for damages.