Shawnee Twin Lakes Dams
The Shawnee Twin Lakes are two municipally owned reservoirs, encompassing Lake #1 (South) and Lake #2 (North). The City of Shawnee constructed the lakes primarily for water supply, flood control, and recreational purposes.
The dams for the Shawnee Twin Lakes are earthen structures connected through an equalization channel. They also share a concrete spillway and both dam crests serve as roadways. The Lake #1 dam was originally built in 1935 and was revised in conjunction with the subsequent construction of the dam at Lake #2 in 1960. The dams are 55 and 50 feet in height and the normal capacity of the lakes are 22,600 and 11,400 acre-feet, respectively. One acre-foot is equivalent to 325,851.4 gallons.
Apart from being the City’s primary source of drinking water, the Shawnee Twin Lakes also offer a range of recreational opportunities such as boating, kayaking, fishing, hunting, camping, and picnicking, all of which attracts visitors to this area. Overall, the Shawnee Twin Lakes have a positive impact on the community by providing a stable water source, recreational outlets, and potential economic benefits from tourism and outdoor activities.
View the Shawnee Twin Lakes Overview Map
Go to the Parks & Recreation information on the Shawnee Twin Lakes
Go to the U.S. Geological Survey Bathymetry and Capacity of Shawnee Reservoir
High Hazard Dam Designation
Dams are vital structures that hold back water to create reservoirs and provide drinking water, and in some cases also generate electricity. However, not all dams are created equal when it comes to the potential risks they pose. The concept of High Hazard Dams revolves around identifying dams that, if they were to fail, could result in significant damage, danger to human life, and property loss downstream.
In the context of dam safety, a system known as "Hazard Potential Classification" is utilized to assess dams based on the potential consequences of their failure. This system assists authorities in prioritizing resources and actions to mitigate risks and protect communities. Visualize a dam as a barrier that holds back water. Dams come in different types:
- Low Hazard Dams: These dams hold back a smaller amount of water. If they were to fail, it might cause a minor mess, but the impact wouldn't be severe. Low Hazard Dams are less risky as their failure wouldn't result in significant harm.
- Significant Hazard Dams: Imagine dams holding back a moderate amount of water. If these dams were to fail, it could lead to more problems. There might be flooding and damage, but the consequences wouldn't be extreme.
- High Hazard Dams: Picture dams holding back a substantial amount of water, like a large lake. If these dams were to fail, it could be very dangerous. High Hazard Dams have the potential to cause major flooding, putting people's lives, homes, and lives at risk.
Go to the OWRB Map of classified dams in Oklahoma
Go to the OWRB Hazard Potential Classification Guidelines for Dams in Oklahoma
Go to the OWRB Interactive "Dam Inventory of Oklahoma” Map
The Oklahoma Water Resources Board (OWRB) oversees the administration of the Oklahoma Dam Safety Program to ensure the safety of more than 4,700 dams within the State of Oklahoma. The program requires regular inspections for all dams based on the presence of downstream development. Dependent on the dam’s risk classification and its potential impact on human life and economic loss, dams must be inspected in intervals ranging from annually to every five years.
Every year, the dams for the Shawnee Twin Lakes are inspected and evaluated by City staff as well as an independent third party. The resulting inspection reports are then submitted to OWRB. Since 2018, the Lake #1 dam has received gradually worsening ratings of “Fair” to “Poor,” whereas the Lake #2 dam has received a consistent rating of “Fair.”
Routine monitoring and a subsequent investigative study have uncovered structural anomalies along a portion of the dam for Shawnee Twin Lake #1. In concert with OWRB, the City of Shawnee is expanding the scope of the field investigation to aid in the preparation for a potential dam repair and rehabilitation project.
View the Shawnee Twin Lake #1 dam inspection reports for 2018, 2019, 2020, 2021, and 2022.
Hazard Mitigation Planning
OWRB requires that all owners of high hazard dams have an associated emergency action plan (EAP) in place. The purpose of an EAP is to reduce the risk of injury or loss of human life and to minimize property damage during an unusual or emergency event at Shawnee Twin Lakes # 1 & 2 dams. The City of Shawnee recently completed its High Hazard Potential Dam Amendment to the Pottawatomie County Hazard Mitigation Plan. It addresses potential issues such as heavy rainfall, flooding, a dam failure, or other events that could affect the safety and integrity of the Shawnee Twin Lakes dams. The purpose of the High Hazard Potential Dam Amendment is to examine the comprehensive risks, define goals, and develop strategic courses of action.
Preceding the High Hazard Potential Dam Amendment was the Dam Breach Analysis and Inundation Mapping report for Shawnee Twin Lake #1, which was completed in November 2013. Under State of Oklahoma law, dam breach inundation maps are required for high hazard dams. Contained within the report is the modeled extent of a probable maximum flood (PMF) as well as the modeled extent of a piping failure referred to as ‘Sunny Day’ event. Both analyses use a theoretical hydrologic model based on light detection and ranging (LIDAR) data and U.S. Geological Service topographic maps (quadrangles). The underlying model was set up with certain assumptions regarding water saturation of drainage area soils, probable maximum storm (PMS) precipitation, inflow conditions, and dam breach location as well as breach width, all of which impacts the results of the model. The report provides an overview of impacted structures and roadways, and it also shows the modeled results in a sequence of maps.
View the High Hazard Potential Dam Amendment
View the Pottawatomie County Hazard Mitigation Plan, Chapter 1 & 2 - Introduction & Planning Overview, Chapter 3 - Hazard Identification (large file, will load slowly), Chapter 4 - Mitigation Strategy, and Chapter 5 - Update, Prioritization and Review.
View the Dam Breach Analysis and Inundation Mapping report for Shawnee Lake 1
Recent Discovery of Anomalies
First, a small longitudinal crack was discovered in the pavement of the roadway of the Lake #1 dam. Since this discovery, a dip in the roadway has surfaced and subsequently grew in size. As this dip has grown, a geo-technical investigation was recommended in the annual inspection report. Initiated based on that report recommendation, the recently completed field investigation focused on the area in which the roadway dip and an associated slippage crack occurred. The geo-technical investigation included shear wave testing and soil borings that measured resistance and friction.
- Shear Wave Testing – Shear wave testing involves sending seismic waves through materials like soil and rock to assess their properties. Shear waves travel differently through different materials, helping to identify changes in subsurface conditions.
- To create a repeatable and consistent seismic wave, a heavy weight, often in the form of a sledgehammer, is lifted and then allowed to strike the ground surface. The generated seismic waves are detected using specialized equipment, such as geophones or accelerometers, which record the wave's arrival times and characteristics.
- Conducted directly on the dam, the testing provides a profile of the subsurface layers, indicating the depth at which different materials or soil types transition, all of which helps identify the dam's composition and potential weak zones. Changes in wave speed or reflections in shear wave testing can indicate the presence of groundwater as well as identify anomalies such as voids, discontinuities, or variations in soil density, which may affect the dam's stability.
- Cone Penetration Testing – Cone penetration testing (CPT) is a geotechnical method used to assess soil properties and conditions below the ground's surface. It involves pushing a cone-shaped penetrometer into the soil and recording various measurements.
- During CPT, a cone-shaped probe is pushed into the ground at a constant rate. The resistance and friction encountered by the probe are continuously measured, providing valuable data about soil behavior. CPT provides information about soil properties such as cone resistance, sleeve friction, pore water pressure, and local soil behavior.
- CPT is efficient and provides real-time data without the need for sample retrieval. It offers rapid insights into subsurface conditions, making it useful for assessing soil strength, compressibility, and stratification.
View the August 21, 2023, Presentation to the City Commission
View the introductory PowerPoint Presentation
Immediate Next Steps
The immediate next steps towards the dam’s rehabilitation will include:
- Shear wave testing along the entire length of the Lake #1 dam will be conducted. (For information on this testing, refer to the previous Section on “Recent Discoveries of Anomalies”)
- Cone penetration testing of select locations along the entire length of the Lake #1 dam will be conducted. (For information on this testing, refer to the previous Section on “Recent Discoveries of Anomalies”)
- An investigation for the potential presence of water on the downstream slope through non-intrusive near-infrared and thermal imaging will be undertaken.
- Infrared and thermal imaging involve capturing the heat emitted by objects in the form of infrared radiation. Different objects, including soils, emit varying amounts of heat depending on their temperature, composition, and water content. Moist soil retains water near its surface, resulting in cooler temperatures compared to drier areas. Thermal imaging can detect these temperature variations, revealing areas of higher and lower water content.
- Infrared and thermal imaging allows for non-intrusive, rapid assessment of soil moisture over large areas. It can identify trends, anomalies, and areas of concern, aiding in resource management and decision-making. Various factors, including soil type, vegetation cover, and surface properties, can influence thermal patterns. These must be considered when interpreting thermal images.
- At OWRB’s direction and pending further State guidance, the City is has taken steps to begin a gradual, precautionary drawdown the water contained in Lake #1.
- At this time, the City of Shawnee has switched to pulling the majority of its drinking water needs from Lake #1, drawing down approximately 4 million gallons per day. This rate is not quite double the normal draw rate, but will not result in a rapid reduction in the level of the Lake.
- If the Oklahoma Water Resources Board advises the City to increase the draw-down rate, the City will immediately share this information to allow owners of boat docks and watercraft to plan accordingly.
Involvement of Applicable State Agency
OWRB regulates more than 4,700 dams in Oklahoma and is responsible for enforcing dam safety regulations. The City of Shawnee will continue to closely coordinate with the State agency related to emergency preparedness as well as the potential design and construction of a dam repair and rehabilitation project for the Shawnee Twin Lake #1.
Go to the OWRB Frequently Asked Questions
Please check this page regularly. As new recommendations, action steps, and mitigation measures are formulated, City staff will send out public notices and upload the new information here. For questions, please contact Andrea Weckmueller-Behringer, City Manager at 405-878-1601 or firstname.lastname@example.org.