Written By: Samantha Justice, P.E.
Dams and Spillways Are a Critical Part of U.S. Infrastructure
With estimates of 84,000 structures nationwide, dams and spillways are essential for controlling flooding, water distribution and management, and providing hydroelectric power. Unfortunately, these structures cannot last forever. The average age of dams and spillways in the US is 57 years, seven years over the typical 50-year lifespan of these structures. Aging infrastructure can lead to serious consequences if safety precautions are not taken, or measures are not implemented to address identified problems in a timely manner. The most important measure that any dam manager can apply is continual inspection and upkeep.
The US Army Corps of Engineers Infrastructure Report Card rating for 2021 was a C-, and the health of dam structures was a significant part of that low rating. State and federal regulations provide a framework for the assessment and maintenance of dam and spillway structures-at a minimum yearly audit inspections that identify areas that need repair or replacement are required. Performing these repairs can help extend the lifetime of dams to help keep essential services in place without excessive cost or increased failure potential.
Understanding Areas of Concern for Existing Structures
The vast majority of America’s rivers and lakes have existing dams and spillways, and as such, very few new structures are being built. With new construction, safety measures can be incorporated during the design phase to extend the lifetime of the project and help prevent failures. The true threat is with existing structures that have gone past their intended lifetimes or have seen areas of potential failure.
A recent example of the potential for catastrophic damage due to a dam failure is the 2017 Oroville Dam crisis in Northern California. Extremely heavy rainfall over a number of days raised the level of Lake Oroville, increasing the flow over the main spillway to above-average levels. Almost immediately, damage was observed in the lower half of the spillway, with a large section of the concrete path collapsing. The emergency spillway was utilized to help prevent further damage to the main spillway, however, excessive erosion occurred to the emergency spillway path, and emergency repairs were subsequently required to address damage in both spillway areas. Further damage occurred when more rainfall increased the lake level yet again, including blocking the downstream river and requiring the immediate shutdown of the Oroville hydroelectric power plant. Luckily, total collapse of the dam did not occur, but more than 180,000 residents of the Feather River Basin were required to evacuate for multiple days, and over the next year, more than 1,000 people worked more than 2 million hours to rebuild the spillways to ensure the safety of downstream communities.
With the passage of the 2021 Infrastructure Investment and Jobs Act, states will have access to funds to complete repairs and upgrades of aging dams and spillways before failure can occur. The failure at the Oroville Dam was preceded by rejection of a 2005 upgrade proposal to build a concrete emergency spillway that could have handled the high water flows seen in the 2017 event. Re-evaluating existing structures to ensure that they are still able to withstand 100-year and 500-year flood events is crucial to the longevity of the dam network within the US. Maintaining both upstream and downstream dam faces and spillways is an ongoing process, fighting against wave action and erosion, as well as any potential impact damage caused during storm events. Even simple maintenance of roads and work pads over dams can have a lasting effect on the health of these structures by allowing workers access to inspect and repair the structures quickly and easily.
GEOWEB® Geocells Are a Repair Solution for Dams and Spillway Sites
GEOWEB geocell technology is a versatile geosynthetic system that can be used to create long-term solutions for many of the common dam and spillway problem areas. Geocells function as the support structure for unpaved roadways, capable of supporting maintenance and repair vehicles. They also function as surface erosion control solutions, preventing the formation of rills or the collapse of unstable soils due to water flow, wave action, and storm events.
GEOWEB geocells can be placed on the upstream face of a dam structure to mitigate the effects of wave action on the dam, supporting existing riprap areas, or replacing them entirely with vegetation, gravel or concrete. The flexibility of the GEOWEB system allows for the use of mixed infill materials, such as topsoil above normal water levels for grass growth and small aggregate below the water level for erosion prevention. Comprised of high-density polyethylene (HDPE), GEOWEB geocells are formulated for long term durability to resist weathering, chemical attack, and ultraviolet radiation, and are therefore suitable for use in applications where the material will be subjected to cyclic wetting and drying, permanently submerged, or full sun exposure. The material is not prone to degradation or corrosion due to environmental factors, and can be placed on the downstream face of, or within, a spillway structure. The system is also compatible with concrete infill to accommodate extremely high flow velocities. For comparison, Table 1 summarizes allowable velocities and shear stresses for various channel lining alternatives.
In emergency spillway areas, topsoil infill with vegetation can be used to allow for a natural camouflaged look, while still preventing erosion and uncontrolled water flow, and outperforming traditional unreinforced channel lining alternatives.
Staging areas and maintenance roads are also integral parts of a dam site, and when necessary, these features provide vital access and adequate ground support for vehicles and heavy equipment to perform inspections, routine maintenance, and repairs. The GEOWEB system can be used in a variety of load support applications, including unpaved access roads, laydown areas, and parking lots. Reinforcing these roads means significantly reduced maintenance requirements, reduced rutting, and access to areas that might otherwise be unable to support heavy loads due to soft soil conditions. Minimizing stresses on top of dam structures is critically important to preventing the formation of cracks or slumps within the structure that could lead to failure. The GEOWEB road system can be integrated with the slope protection system on the upstream and downstream faces of the dam for a continuously protected berm from water, vehicle, and impact loads.
Design Support & Resources for the GEOWEB System Applications
The engineering team at Presto Geosystems works closely with engineers and project planners, offering free project evaluation services and on-site support. Our recommendations will deliver a technically sound, cost-effective solution based on four decades of accredited research and testing data. Please contact our knowledgeable staff and network of qualified distributors and representatives to discuss your project needs today.
Related Articles and Case Studies
United States Department of Agriculture, Natural Resources Conservation Service, (2007) Part 654 Stream Restoration Design, National Engineering Handbook, Chapter 8, Threshold Channel Design, (viewed 23 March 2022 and available https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=17784.wba as a link directly to Chapter 8). Table 8-11 “Allowable velocity and shear stress for selected lining materials” referenced from 8-37.
Colorado State University, Engineering Research Center (2009) Hydraulic Testing and Data Report for GEOWEB 30v with Concrete, research summary courtesy Presto Geosystems, (viewed 23 March 2022 and available Colorado State University Testing PDF)