Water Management – classroom presentations

Presentations are:

Analysis of Hurricane Helene Record Rainfall-Does This Change PMP and Are These Storm Changing?, presented by Bill Kappel, Applied Weather Associates.

Hydrologic Insights from 14,000 Square Miles: Understanding the Unique Dynamics of Extreme Rainfall and Flood Analysis, presented by Patrick Miles, Freese and Nichols, Inc.

MIDH2O – A Real-Time Water Supply Forecasting System with Hydrologic State Tracking and Reservoir Operations Modeling, presented by Tom Evans, RMA

Details about each presentation and the speakers are below:

Analysis of Hurricane Helene Record Rainfall-Does This Change PMP and Are These Storm Changing?
Presented by Bill Kappel, Applied Weather Associates.

Widespread heavy rain caused by a combination of the remnants of Hurricane Helene and the highly variable topography produced historic rainfall and catastrophic flooding over and near the southern Appalachian Mountains. Portions of western North Carolina bore the brunt of this storm’s fury, causing extensive damage to infrastructure and many fatalities. Many areas are still recovering a year later.

Applied Weather Associates (AWA) used its Storm Precipitation Analysis System (SPAS) to quantify the rainfall accumulation in across region. This storm's precipitation rivaled and, in some cases, exceeded the largest observed events in the region, including the July 1916 Alta Pass storm. SPAS outputs show average precipitation depths over a 10 square mile area greater than 30 inches, and over a 100 square mile area greater than 25 inches for both 48-hour and 72-hour durations.

This information has been evaluated against the numerous Probable Maximum Precipitation (PMP) studies AWA has completed in the region, including the Tennessee Valley Authority and several statewide and FERC studies. This information was compared to PMP depths which were developed without the storm included to see if the storm exceeded PMP development for dam design and evaluations. Many interesting findings resulted. It was often the largest tropical storm rainfall event in the region at small area sizes and for longer durations (48- and 72-hours), but other storm types exceeded the Helene rainfall for these area sizes and durations. This demonstrated the rarity of the storm but also showed that it did not exceed the PMP envelopment for the study location analyzed. This is important because it demonstrates the utility of the storm-based PMP process and the need to evaluate PMP for all relevant storm types.

This presentation will detail the importance of capturing the impacts of this type of rare event to help inform detailed reconstruction of extreme rainfall for dam safety and design. This adds another significant storm to the AWA’s extensive storm database of nearly 1000 extreme storm events that are used in PMP development. And most importantly, shows again that these storms have happened before and will happen again, and the water power/dam safety community therefore needs to be properly prepared for these types of rare events.

Hydrologic Insights from 14,000 Square Miles: Understanding the Unique Dynamics of Extreme Rainfall and Flood Analysis
Presented by Patrick Miles, Freese and Nichols, Inc.

Whether constructed based on engineering judgment and observations in the early era of dam building, nationwide hydrometeorological reports in the latter half of last century, or site-specific studies in the modern era, our dams exist in their current size and function for specific reasons. While they may have conformed to original design standards, those standards may have evolved, along with watershed conditions and engineering advancements, and may warrant reevaluation.

Probable maximum precipitation (PMP) and the associated flood (PMF) have been topics of academic and industry interest for decades. Many variable factors contribute to our understanding of PMP, but meteorological science has grown dramatically in the past few decades. Likewise, hydrologic and hydraulic modeling capabilities have opened new methods to understand the nature of runoff during extreme flood events.

This presentation will discuss the many factors, advancements, and critical decision points associated with extreme rainfall and flood analyses and the associated dam safety design implications.

As a relevant case study, the presentation will also discuss the recent site-specific PMP/PMF study for the Coosa and Tallapoosa Basins. The project modernized all aspects of the hydrometeorological and hydrologic assessments for the 11 Alabama Power Company hydropower dams in these basins. This state-of-the-practice analysis uses 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 2D modeling approach in HEC-RAS, which more accurately captures the influence of river bends, floodplain storage, and backwater tributaries.

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.

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. In total, more than 400 model scenarios were analyzed to identify the controlling PMP/PMF scenario.

Overall, the study identified both increases and decreases in PMF reservoir elevations relative to previous studies. Notable differences in flood response were observed between the wide, dendritic reservoirs and the more linear, flashier watersheds. This meant that some dams were more sensitive to steeper temporal distributions and more intense spatial patterns while other dams saw decreases.

MIDH2O – A Real-Time Water Supply Forecasting System with Hydrologic State Tracking and Reservoir Operations Modeling
Presented by Tom Evans, RMA

Accurate and timely water supply forecasting is critical for effective reservoir operations, especially in regions where snowpack and precipitation patterns significantly impact water availability. This presentation describes an operational hydrologic forecast and reservoir modeling system developed for the Merced Irrigation District (MID). Designed to improve operational decision-making, the MIDH2O forecasting system is built from cutting-edge forecasting tools used by the US Army Corps of Engineers to support operations at reservoir systems across the US and integrates modeling tools with real-time hydrologic and meteorologic data to enhance forecasting accuracy.

MIDH2O has adapted publicly available versions of the Corps tools to work with MID’s in-house hydro-meteorological database and has leveraged features of the HEC-HMS hydrology model to produce a system that continuously tracks hydrologic states – include the evolving snowpack – within the Merced basin and progresses those states forward in time every 6 hours. This maintains current, spatially distributed estimates of SWE and Soil Moisture throughout the basin.

A key recent innovation in MIDH2O allows the modeled state of the snowpack to be updated with SWE observations from Airborne Snow Observatories in near-real time. The SWE estimates are now incorporated into the model state as soon as the field data becomes available, improving the accuracy and timeliness SWE and reservoir inflow estimates, and enhancing MID’s ability to manage water resources efficiently.

Flow estimates from the system’s hydrologic forecasts become inputs to the ResSim and RAS models, allowing MID operators to see estimated flows and stages resulting from planned operations going forward.

From these continuously created and updated hydrologic states, two forecast products are generated: 1) The 7-day hydrograph at all gage locations within the Merced River watershed, and 2) the seasonal (April-July) volumetric inflow forecasts to Lake McClure. Since the hydrologic states are kept current, the forecast products are generated every 6-hours, based on the latest 7-weather forecasts and current hydrologic states.

By combining state-of-the-art modeling with real-time data integration, MIDH2O represents a significant advancement in operational water management, providing a robust framework that can be adapted for other hydropower and water resource systems across the U.S.

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