Flood duration vs. flood magnitude: Repeat sedimentation measurements for large floods along the lower Mississippi River over hydrologic years 2020, 2018-2019, 2011

Because most flood sedimentation studies are focused on discreet events it remains challenging to distinguish the importance of different processes on overbank sedimentation, particularly flood duration relative to flood magnitude.  We report new field data pertaining to sedimentation thickness (mm) and particle size (mm) for the 2020 hydrologic year along the lower Mississippi River, which extends studies reported from large flood events in 2018-2019 and 2011. This study benefits from having repeat measurements at the same location for different types of flood events to consider flood magnitude relative to flood duration.

The study area is a ~25 km long segment of the lower Mississippi alluvial valley between Natchez, Mississippi and Red River Landing, Louisiana, a reach that includes the entire North American drainage of the Mississippi basin which annually undergoes extensive flooding.

Flooding in hydrologic year 2020 (at Natchez, MS) occurred from January 15 to June 21, an event of 159 days that is nearly two months longer than the average flood duration. The 2018-2019 compound flood event was overbank a record 286 days. These two sequential events are of much greater duration than the notorious high magnitude events of 2011 and 1973 with a flood duration of 53 days and 90 days, respectively.

Particle size of flood deposits from the 2020 event varied most within the first ~200 m of the channel, and then somewhat by depositional setting, including fine-sand (d50 = 0.17 mm) at natural levee crest to fine silt (d50 = 0.011) atop natural levee backslopes, meander scroll -ridge, meander scroll-swale, abandoned channel, and backswamp environments. Despite differences between the 2018-2019 and 2020 events, the average particle size in 2020 is 0.040 mm, somewhat coarser than 0.029 mm of 2018-2019.

Recent long duration flood events have finer grained deposits than sedimentary deposits from the record 2011 flood, which averaged 0.049 mm. Additionally, recent long duration flood events (2018-2019, 2020) produced less sand than the 2011 flood, averaging 32% (2011), 14% (2018-2019), and 22% (2020). This points to the importance of flood magnitude in distributing sand across a wider swath of the floodplain, while the high energy event likely flushed fine-sediment downstream and inhibited slackwater sedimentation, which characterized the 2018-2019 and 2020 events.

Flood deposit thickness at 41 sites in 2020 averaged 33 mm, notably less than 2018-2019 event (85 mm avg.). Flood deposits from the 2011 event averaged 39 mm in thickness. Sediment thickness should be contextualized against the period over which flood sedimentation occurs. The influence of flood duration results in unit (daily) sedimentation rates for the 2020, 2018-2019, and 2011 flood events being 0.21 mm/day, 0.30 mm/day, and 0.74 mm/day, respectively. Across large lowland floodplains flood duration is more important than flood magnitude to the total amount of sedimentation. Regardless of flood magnitude or duration, a comparison of recent flood sedimentation amounts with the infamous 1973 flood event reveals the persistent decline in Mississippi sediment loads since dam construction of the mid-1900s. Study results are further contextualized by considering (upper basin) sediment province and event-based discharge – suspended sediment dynamics.