Hawaiian Dam Safety Seismic Hazard Assessment

PROBABILISTIC SEISMIC HAZARD ANALYSES FOR HAWAII’S HIGH-HAZARD DAMS INFORM RISK MANAGEMENT AND EMERGENCY PLANNING TO SAFEGUARD ISLAND COMMUNITIES

Our Client’s Challenge

Deep beneath the islands that form the state of Hawaii, volcanic and tectonic activities shift and shake the land, resulting in thousands of earthquakes each year. Seismic hazards can vary significantly from one island to the next, with the highest hazard on the island of Hawaii and the lowest on Kauai. While most seismic events are only detectable by sensitive instruments called seismometers, some can cause significant risks to the state’s dams, potentially leading to failure and threatening public safety and the water supply.

Hawaii has 126 state-regulated dams, most of which were constructed before 1940 to support sugarcane plantations. The once-thriving plantations are gone, but the aging dams remain. According to the Association of State Dam Safety Officials’ dam safety performance report, nearly 70% of Hawaii’s dams are privately owned. The American Society of Civil Engineers gave the dams a “D” on its 2019 infrastructure report card.

The Hawaii Department of Land and Natural Resources (DLNR) plays a critical role in regulating and promoting the safety of these dams by encouraging high safety standards, maintaining a dam inventory, promoting continuous guideline updates, and engaging in emergency preparedness. As part of the state dam safety program, the DLNR provides technical training for dam owners and operators and partners with them to develop or update emergency action plans.

The DLNR retained Gannett Fleming to perform a probabilistic seismic hazard analysis (PSHA) for 118 high-hazard potential dams on the islands of Hawaii, Maui, Molokai, Oahu, and Kauai.

Our Solution

A PSHA provides a comprehensive framework for estimating the likelihood of seismic events of varying magnitudes occurring within a specific timeframe, which is vital for assessing the seismic vulnerability of dams. By incorporating potential seismic scenarios, a PSHA helps engineers and policymakers develop more resilient dam designs and retrofitting strategies to withstand seismic forces, safeguarding areas downstream from significant property damage.

The PSHA results inform the DLNR’s risk management and emergency preparedness planning. It provides a consistent methodology to evaluate the seismic hazard at each dam site and helps provide essential input parameters needed to assess the dynamic stability of high-hazard dams.

As part of the PSHA, we characterized seismic sources, including foreshocks and aftershocks, to provide hazard calculations that considered all sources of shaking. We developed a scaled backbone ground motion model characterization from seismic recordings in Hawaii to provide seismic hazard results specifically for the islands.

The full report includes:

Precise documentation of input source and ground motion models in text and logic tree format.
Detailed geologic maps with dam sites plotted.
A discussion of the differences between the DLNR model and the United States Geological Survey’s National Seismic Hazard Model for the State of Hawaii.

The report contains detailed PSHA results, including uniform hazard spectra, hazard curves, deaggregations, and fractiles for all dams evaluated.

Key Features

Probabilistic seismic hazard analysis for 118 dams on the islands of Hawaii, Maui, Molokai, Oahu, and Kauai.
Report containing consistent ground motion modeling criteria and dam site maps.

Outcomes

Improved understanding of seismic risk.
Enhanced dam safety-related decision-making.
Increased ability to assess the dynamic stability of high-hazard dams.
Increased ability for dam owners to analyze and retrofit their dams for seismic hazards.