The ABCs of protecting your dam for safe overtopping using articulated concrete blocks (ACBs)

Communities throughout most of the United States have seen steadily worsening precipitation events over the last decade, including a phenomenon known as "rain bombs,” when concentrated intense rainfall occurs in specific areas within a relatively short period of time.

This weather trend poses challenges for dam owners. During these “rain bombs,” water levels in lakes and reservoirs rise quickly due to the large inflow from runoff, a formerly extreme and rare situation that has become increasingly common. Many dams have inadequate spillway capacity to handle such large inflows, which may cause the dam to be overtopped. During an overtopping event, the dam is susceptible to erosive forces which may lead to weakening of the dam structure, dam failure, catastrophic downstream flooding, legal actions, and enormous repair costs.

To protect the dam from overtopping events and to increase spillway capacity, dam overtopping protection systems are becoming more popular. One of the more effective dam overtopping protection systems utilizes articulated concrete blocks (ACBs). These standardized, interlocking blocks are typically laid down in mats, with the help of heavy equipment, to form a hard surface over which rushing water can flow without eroding the underlying earthen embankment.

ACBs vs. other technologies 

Three of the most common alternatives to ACBs for dam overtopping protection systems include reinforced concrete slabs, grouted riprap, and gabions (large, wire baskets filled with rocks). ACBs can be a more appealing approach than these competing technologies for several reasons:

• ACB installation is generally less labor intensive than constructing reinforced concrete slabs, which makes the installation more cost effective.
• In areas where a dam is subjected to high foot traffic, like parks and recreational areas, ACBs can provide a safer walking surface than grouted riprap and gabions.
• ACBs can accommodate vehicle traffic, unlike grouted riprap and gabions.
• ACBs are generally found to be more aesthetically pleasing than other overtopping protection systems. The blocks can be overlaid with topsoil and seeded to form a grass or wildflower cover that allows the installation to blend into the natural environment.
• ACBs generally have a longer service life than gabions as the blocks are more resistant to damage from erosion and weathering.

In designing overtopping protection systems, a key first step for dam owners to ensure an ACB system succeeds is to have hydrological and hydraulic (H&H) analyses performed to determine expected hydraulic conditions over the ACBs during the required spillway design storm. Owners are also well advised to collaborate with their preferred ACB manufacturer to ensure, based on the H&H analyses conducted, the ACB system being specified will meet all regulatory requirements and will stand up to projected maximum flows during the spillway design storm.

Two additional features well worth including in the design are:

1. Well-engineered filtered drains to control and collect seepage through the embankment below the ACBs to protect against internal erosion, and
2. Seepage control measures upstream of the ACB system to reduce the risk of under-seepage forces eroding the subgrade of the ACBs and negatively impacting the integrity of the overtopping protection.

ACB case study

GZA GeoEnvironmental recently worked on a project at Lake Tamarack in Stockholm, New Jersey, in the northwestern corner of the state, that exemplifies the importance of these measures. An early generation of ACBs had been designed and installed, by others, along the embankment of the South Dam in the early 2000s to increase the spillway capacity. The dam historically had issues with seepage through the upstream bulkhead wall that were not successfully addressed when the original ACBs were installed. Following that ACB installation, the seepage issues worsened, resulting in loss of subgrade material below the ABCs and subsequent settling and depressions.

Then, in 2018, an extreme storm event caused the lake level to rise quickly and overtop the dam embankment under a few inches of water. Concentrated flow through the joints of the ACBs was observed, which further eroded the underlying embankment soils. The damage was clearly visible from additional settlement of the ACBs from this relatively minor overtopping event. It was clear that the original ACBs were not capable of safely passing overtopping flows during the larger spillway design storm, during which the dam is expected to overtop by nearly 3 feet.

The team completed updated H&H analyses and worked with an ACB manufacturer that conducts hydraulic performance testing on their products to design a new ACB system for the dam. Making the necessary repairs and upgrades provided an opportunity to implement current best practices and standards that have been proven out since the first generations of ACBs were implemented. At Lake Tamarack, the new proposed ACB system included mats of cable-tied, tapered, open-cell ACB units installed over a stone drainage layer. The new system also included staggered joints between the blocks, rather than the straight joints that were used in the original system which allowed for concentrated flow paths during overtopping events. The project also included replacing the existing upstream bulkhead wall and constructing an upgraded embankment drainage system to address the dam’s historical seepage problems. Construction began in November 2020 and was completed within six months, by April 2021, allowing the lake to refill before the summer recreation season.

When designed, installed and maintained properly, ACBs can be an effective and affordable alternative for protecting your dam against overtopping and the more frequent severe precipitation events.