Lower Mississippi River Probabilities: A Collaborative Investigation

In scoping an investigation of the Lower Mississippi River flow probabilities, the US Army Corps of Engineers (USACE) looked for existing, watershed-wide continuous simulation hydrologic models with established confident results covering several decades of simulation or forecasting. USACE and the National Weather Service (NWS) established a partnership where the NWS generated two simulations using the Community Hydrologic Prediction System and Flood Early Warning System (CHPS_FEWS). The basin-wide hydrologic analysis was accomplished through the partnership across five River Forecast Centers (RFC): NCRFC (North Central), MBRFC (Missouri Basin), OHRFC (Ohio River), ABRFC (Arkansas/White/Red Basin) and LMRFC (Lower Mississippi River).

The Regulated Simulation represents today’s hydrology and an Unregulated Simulation “turned-off” the regulation at most flood control reservoirs. The period simulated for the scenarios leveraged the NWS historic precipitation and temperature time series spanning from about 1952 to 2017. USACE prepared temperature and precipitation grids using the Livneh 1/16 resolution gridded daily datasets, that extended the simulation period back to 1915. The 2018 Mississippi River &Tributaries (MR&T) Flowline Assessment HEC-RAS model was leveraged to route and combine major tributaries to simulate the Lower Mississippi River from Cairo, Illinois to the downstream boundary condition at West Pointe a La Hache, Louisiana (River Mile 48.76). The HEC-RAS geometry represents MR&T geometry (levee heights) but, for the unregulated simulation, removed the three major M&RT diversions at Old River, Morganza and Bonnet Carre to an approximate pre-project geometry.

USACE developed an R-script to compare simulated flows for the Regulated Simulation to the observed flow at a set of primary USGS gage locations for the last 20-30 years of modern record. A series of timeseries plots and statistical comparisons demonstrated the fit of the simulated data to observed. The R script exported a standardized report for each of the NWS RFCs, and coordination meetings were held between USACE and the RFCs to verify the results.

The two simulated time series are used to initially prepare frequency curves at key gage locations in the Lower Mississippi River and major tributaries. The Regulated Simulation timeseries is a stationary data set for the modern-day condition and displays a picture of how the flood control reservoirs would have impacted the watershed’s response to floods that occurred before the construction of major reservoirs in the system. The Unregulated Simulation is a unique dataset that can be used to help define the upper end of the frequency curves and is helpful in quantifying the impact of reservoirs throughout the watershed. This collaboration between the USACE and NWS Agencies provides unique datasets that will have numerous uses for watershed planning scenarios in the future.