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.
#image_titl
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
Mud Lake Dam Rehabilitation
Olivenhain Dam Power Line Access
References
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)
Sustainable vegetation in channels can be achieved with the GEOWEB TRM system. Applications that benefit include roadside ditches, stormwater channels, shoreline embankments, dams, spillways, and pond overflow systems. The GEOWEB/TRM system is a more sustainable—and environmentally friendly solution than rip rap for these common applications.
We have a great team here at Presto Geosystems, so we thought it would be fun to share a bit more about each member through a new blog series. For our inaugural Meet the Presto Geosystems Team blog post, we are thrilled to introduce José Pablo George—Presto Geosystem’s International Business Manager (BDM).
In the United Kingdom, Network Rail encountered extremely soft soils with low shear strengths during track modifications to the North West Electrification Programme. Due to soft subgrade conditions, conventional track design methods resulted in cross-sections as thick as 1 meter. Poor soil conditions along the track route required a soil stabilization solution to improve undertrack stiffness and provide a more cost-effective solution. The Network Rail Track Bed Investigation (TBI) team elected to evaluate an alternative solution using geocells to reduce required cross-section thicknesses. The GEOWEB Soil Stabilization System has been used under track in the United Kingdom since the 1980s; however, very limited information was collected at that time to document the resulting improvement in performance. Therefore, the TBI team used in-house numerical modelling to validate the design approach, and results indicated that a geocell-stabilized track performed as well as the conventional full-thickness cross-section. Based on this information, combined with the demonstrated long term stabilization of the above-referenced early installations, the TBI team elected to use the alternate track bed design incorporating GEOWEB Geocells. The North West Electrification Programme subsequently approved the use of geocells on a number of sections with the goal of reducing construction depth, in turn, reducing costs associated with track enhancement and long term maintenance.
After installing the GEOWEB Soil Stabilization System on the North West Electrification Programme in 2017, track quality improved significantly. The reduction in the required track bed construction (40% reduction in granular fill material), reduced the cost of track enhancement by approximately 22% (Wehbi, et al., 2018). Network Rail also realized the benefit in the ability to use granular fill or course sand as ideal infill materials. Network Rail’s experience using geocells has shown substantial construction cost savings and benefits to the structural integrity of the track bed (Wehbi, et al., 2018). Network Rail has also monitored Willesden North on the London North East and Newham Bog on the London North West in addition to the Northern West Electrification Programme, which show similar results and benefits.
The use of the GEOWEB 3D slope cover system best addresses critical details when designing or remediating geomembrane covers. With the aging infrastructure of dams, impoundments, and landfills, design engineers are looking for innovative and cost-effective solutions to build and repair new and existing facilities. The adaptability of the 3D system provides geomembrane protection while contributing to an easier and faster installation process.
Measure the performance of the GEOWEB (GW) material combined with a turf reinforcement mat (TRM) (integrated system) with topsoil infill and vegetation under varying shear stresses and flow rates to quantify both hydraulic forces and
The Research Facility
Exposed to the extreme flows, and despite ordinary topsoil infill and typical TRM staple patterns, the system showed no measurable soil loss. It was observed that the vegetation had decreased stem and blade count during the total testing timeframe, however at a decreasing rate of loss for each incremental test.
The results of this integrated system testing can be applied to highway drainage ditches, spillways, dam, and pond overflow systems and other vegetated channels exposed to high shear forces and intermittent, longer-duration velocities. This system replaces rip rap with a less expensive, low maintenance, aesthetically pleasing green solution.
With geocells, it is not uncommon to see an overall reduction in the required thickness of the base layer or leveling course in a load support application by 50% or more, along with an overall improvement in allowable bearing capacity. This applies to foundations as well as permanent or temporary site features, such as access roads or construction platforms.
Presto Geosystems and the USACE tested various resin blends and concluded that virgin high-density polyethylene (HDPE) provided superior weld consistency and structural strength. Presto Geosystems introduced the 
800.548.3424
