PLT - Hydrologic Insights from 14,000 Square Miles: Unique Dynamics of the Coosa and Tallapoosa Site-Specific PMP & PMF Study

The last comprehensive hydrometeorological studies of the Coosa and Tallapoosa River Basins were performed by Alabama Power Company (APC) over 30 years ago. In partnership with Applied Weather Associates, the FNI team developed a site-specific PMP/PMF update to modernize all aspects of the hydrometeorological and hydrologic assessments for the eleven hydropower dams in these basins. This state-of-the-practice analysis makes use of the latest hydrologic and hydraulic modeling techniques, advanced meteorologic analyses, and improved stream and reservoir monitoring data.

Comprehensive calibrated hydrologic models covered the 14,000 square mile study area, and hydraulic routing was performed using a fully two-dimensional modeling approach in HEC-RAS, which more accurately captures the influence of river bends, floodplain storage, and backwater tributaries. The overall 2D model geometry was sub-divided at each dam to optimize model runtimes and consisted of over one million cells representing 2,000 square miles of riverine, reservoir, and floodplain routing environments. Gate operations were modeled within HEC-RAS utilizing detailed operational scripts, which were then validated with HEC-ResSim.

One unique finding related to full-2D reservoir routing was reservoir stage sensitivity to bathymetry. Testing indicated that lack of bathymetry in the terrain led to lower water depths and higher velocities, increasing the effect of roughness and causing increased lag and attenuation. These influences were mitigated by incorporating available bathymetric surveys and estimating a bathymetric surface for reservoir areas without available data.

Rigorous storm maximization helped determine the critical scenario for each dam by evaluating the entire river basin as a system, applying alternate spatial and temporal patterns, and optimizing storm area to produce the greatest runoff to each reservoir. In total, more than 400 model scenarios were analyzed to identify the controlling PMP/PMF scenario.

Managing the suite of hydrologic and hydraulic models and associated data outputs for all these scenarios relied on manipulating HEC-RAS with a robust HECRASController script, which allowed the project team to batch compute large, interdependent models and evaluate multiple scenarios seamlessly and efficiently.

Overall, the study identified both increases and decreases in PMF reservoir elevations relative to the previous studies. For example, notable differences in flood response were observed between the wide, dendritic reservoirs in the Upper Coosa and the more linear, flashier watersheds of the Lower Coosa. This meant that Lower Coosa dams were more sensitive to steeper temporal distributions and more intense spatial patterns and thus saw relative increases where Upper Coosa dams saw decreases.