Description

1) Managing North Carolina’s Flood Risk by Monitoring and Managing the State’s Aging Dams
Tasnuva Mahjabin, PhD, Atkins, Tasnuva.Mahjabin@atkinsglobal.com
Co-presenters: Kenneth Hunu, Kenneth.Hunu@atkinsglobal.com; Tom Langan, Tom.Langan@ncdps.gov

Abstract: North Carolina has over 6,000 dams, many of which are classified as high-hazard dams. The failure of these dams may cause loss of life or serious damage to homes, industrial and commercial buildings, schools, important public utilities, primary highways, or major railroads. To improve public safety for those living downstream of a dam, North Carolina Department of Public Safety (NCDPS) in conjunction with the Natural Resources Conservation Service (NRCS) has started the development of the DamWatch tool. DamWatch is an interactive web-based application intended to provide real-time monitoring of rainfall, snowmelt, and streamflow that could pose potential threats to dam safety and provide alerts to essential personnel when dams are experiencing a critical event. Geospatial products that document flooding risk in the event of dam failure is an important component of the DamWatch Tool. ATKINS was tasked to execute simplified, cost-effective approaches for modeling dam breaches and create inundation maps and other dam risk products for all the high- and medium-hazard dams in North Carolina. Since the traditional approach to create inundation maps and other dam risk products could take years to complete and could be cost-prohibitive, ATKINS developed innovative technical approaches to complete these tasks at a fraction of the cost and time. This presentation will provide an overview of the innovative approaches developed to complete the tasks. Dam breach modeling was done using HEC-RAS and dam risk products including shapefiles and rasters were generated from HEC-RAS after the completion of the dam breach simulation. These mapping layers allow the end user to visualize the extents of flooding and the magnitudes of maximum depths, water surface elevations, and velocities in the inundated areas. In addition to the above, shapefiles of impacted roads and buildings with information about flooding depths, water surface elevations and the earliest arrival time of the peak flood wave were created. A series of arrival time cross-sections were also generated to document the arrival times of the peak breach flood wave as it travelled downstream. These dam risk products can provide essential information for identifying communities and infrastructure at risk in the event of a dam failure. Shapefiles of the impacted buildings were also populated with information about the depth of flooding and potential damage costs and provide a useful resource for calculating property insurance. The array of dam risk products described above used in conjunction with the DamWatch interface enhances community preparedness and equips emergency managers and community leaders with vital information for emergency management and allocating resources during the aftermath of a dam failure.

2) Recognizing Risk – The Prioritization of High Hazard Potential Dams
Barrett Slate, EI, Freese and Nichols, barrett.slate@freese.com
Co-presenters: None

Abstract: The FEMA HHPD grant funding bases eligibility on whether a dam is high hazard, has an EAP, fails to meet minimum state dam safety standards, and poses unacceptable risk to the public. In FY21 these criteria still apply while FEMA has begun to further define what “unacceptable risk” really means. Freese and Nichols, Inc. conducted a screening level risk assessment using FEMA’s new risk assessment tool, with changes to best fit North Carolina’s specific needs, in order to prioritize NC’s high hazard dams in a consistent manner. Freese and Nichols, Inc. assessed 73 dams by analyzing the total annual probability of failure and comparing this to downstream consequences to find the state’s dams which required the most urgent action and funding. The process was completed as a “desktop” review using only existing data retained by NC Dam Safety to expedite the process. This presentation will explore this pilot study in depth, while also discussing its various benefits and challenges.

3) Following the Advice of Albert Einstein for a Dam Break Study
Christine Suhonen, GZA, christine.suhonen@gza.com
Co-presenters: None

Abstract: Two dimensional dam break modeling, which is used to create inundation maps for emergency preparedness, can be performed with a wide variety of tools and software. Some of these software packages include DSS-WISE Lite and HEC-RAS. DSS-WISE Lite was funded by the Federal Emergency Management Agency and developed by the National Center for Computational Hydroscience and Engineering at the University of Mississippi’s School of Engineering. The software is free and hosted online. HEC-RAS is a free software provided by the US Army Corps of Engineers. Both software solve the 2D shallow water equations. GZA has used these software to perform dam break modeling for multiple projects. After several projects’ completion, we have reviewed the projects in the context of this advice from Albert Einstein: “everything should be made as simple as possible, but not simpler.” We reviewed whether we had selected the best software for the assigned GZA staff and for the intended purpose of the analysis. We have additionally looked at whether the development of the inundation maps could have been further simplified by removing the use of software altogether. Using empirical equations to compute dam break peak flow and a given cross section’s water surface elevation, GZA compared the results from each software with hand calculations. This presentation will focus on an in-depth analysis for one dam, to illustrate these differences. Our evaluation showed that each approach has advantages and disadvantages that vary based on the size of the dam, the complexity of the dam’s downstream area, and the modeling experience of the staff.