The City of Lenexa in the US state of Kansas has taken a new approach to stormwater management issues in partnership with the local community. Rather than defining stormwater as a problem to solve it now looks upon it as an asset upon which to capitalise. The result has led to an innovatively designed dam and spillway with impressive architectural features.
Lake Lenexa dam was completed in 2007 and its design and associated improvements not only provide for flood control and water quality issues, but have also made a valuable addition to the city’s park system.
Following major storms and flooding in 1998, the City of Lenexa embarked on a new stormwater management programme called ‘Rain to Recreation’. The programme was an opportunity to take a fresh look at the issue from a more environmentally focused perspective. The objectives of the programme include capturing stormwater in the upstream wetlands and removing sediment to improve water quality. In addition, the programme also focuses on the protection and restoration of the natural environment, and providing recreational and educational opportunities for the community. Hence the idea of turning rain into recreation was born.
Standing at 15m high and 244m long, Lake Lenexa dam is located just upstream of a housing development in a part a the city, with a total population of 45,000 residents. The structure is the largest and most visible component of the Rain to Recreation programme which meant that community involvement was a critical component from the outset.
‘The community was very involved in the overall decisions about what the dam was going to look like,’ says Scott R Brand, geotechnical engineer from Black and Veatch who oversaw the design and construction of the dam and spillway. ‘An extensive community outreach programme was held with the community to get a consensus on the design aspects of the project.’
Due to the high visibility of the scheme, aesthetics played an important role in the design of the dam, bridge and spillway sections. Architectural plans were all coordinated to maximise benefit to the community.
The unique layout and principle features of the dam and spillway structure symbolise the cycle of water as it moves from nature into the urban environment and then back to nature (see shaded panel). Elements in the design include a curved dam alignment and an ogee spillway. Unusual architectural spillway features include curvilinear spillway walls, a spout structure, landscaping plans, recreational trails and a stilling basin design.
The spillway bridge is one of the most interesting architectural elements of the project. It consists of a curved concrete section spanning the spillway just downstream of the ogee spillway and upper basin areas. The bridge has a separate viewing canopy on a cantilevered section supported by drilled piers socketed into the underlying bedrock. The idea is for this to be a focal point of the project where visitors can view the cascading pools and fountains from the pedestrian bridge which spans the spillway.
Park and educational facilities surround the dam and reservoir. Stream restoration projects and shoreline improvements have also been made. The effects to downstream habitats were minimised by providing stream flow augmentation.
Design
Preliminary investigations focused on an RCC dam design at Lenexa but onsite materials were not available. However, geotechnical investigations did show potential sources of silts, clays and weathered shale in the reservoir area that could be used for an embankment dam. The final decision was for a zoned earthen dam with a central core embankment, an internal chimney drain and blanket drains to control seepage through the embankment.
The spillway design also included state requirements to pass the 0.4 PMF flows (195m3/sec) through the spillway with a minimum 0.91m of freeboard. The middle portion of the ogee spillway was designed to pass the 100-year storm event (100m3/sec), and the entire spillway was designed to pass flows up to the 0.4 PMF.
The original spillway design included high retaining walls that formed the spillway training walls. These retaining walls were counterfort walls designed to withstand high bearing pressures but were a relatively rigid system. The architect had concerns about these, and adequate bearing capacity and differential settlement of the walls were issues.
Further investigations were carried out to explore cost-effective means to implement the architectural elements of the spillway. In the end the decision was taken to use mechanically stabilised earth (MSE) retaining wall systems to form the high spillway walls. Typically, such walls are used for road and bridge projects but it was determined that they could be used cost-effectively on the spillway. They were also much easier to build and there wasn’t any need for heavy scaffolding or temporary shoring. Other benefits of using MSE walls included:
• A flexible wall system that could undergo some settlement.
• Lower bearing pressures at the toe of the wall.
• Ability to accommodate multiple curves in the spillway design.
A unique application of mixing borrow clay soils with cement-kiln-dust (CKD) provided support for the spillway structural slabs and project features. A mix design testing programme demonstrated that the CKD material, mixed with silts and clays, was capable of reaching design strengths of 300psi.
The value of the CKD mix was demonstrated further. ‘Due to the spillway apron upstream of the ogee, we had to develop a way to prevent underseepage,’ explains Brand. ‘Ultimately, CKD was mixed with the available clay materials to form a very low permeability and strong material that was used as a seepage barrier below the spillway structures and MSE walls.’
All spillway slabs and walls within the spillway area were underlain and backfilled with the CKD materials. Once a section of the slab was poured and forms stripped, the walls were backfilled with CKD up to the next spillway step elevation.
The lead architect was Joel Marquardt of Gastinger, Walker Harden Associates of Kansas City, Missouri.
Impressive project
Construction of the dam started in the summer of 2004 and was completed in 2007. Innovative design and engineering solutions used in the project helped to ensure that budgets were adhered to. ‘Key to bringing this impressive project in within budget was a unique project delivery system,’ said Tom Jacobs, project manager and stormwater engineer. ‘This included contractor selection through interviews rather than the typical design-bid-build system. This allowed the city to team with the designer and contractor for a significant value engineering session to lower the initial project costs.’
David Egger, black-veatch’s vice president for growth and execution of heavy construction projects, such as dams and tunnels within its global water business, says the company strives to consider the ‘triple-bottom-line’ of each project.
‘We carefully consider people, economics, and the environment in developing sustainable solutions,’ he says. ‘Successful completion of this project required not only special expertise in heavy civil engineering and stormwater management but also a true commitment to the city’s goals and budget and an understanding of what people need as aesthetics and quality-of-life amenities.’
The full project is nearing completion with several new roads and work on park facilities all that remain to be finished. A grand project opening and access for the public is anticipated in the near future. ‘The local reaction has been very positive,’ says Brand. ‘I am sure it will be a popular attraction once it opens to the public. The team of designers, architects, city officials and the local community are all very excited about it opening up. We are very pleased with the final result.’
The dam has so far won several awards which include recognition from the American Concrete Institute; Kansas Engineering Excellence Award; American City and County Magazine; and, the American Public Works Association Project of the Year for Kansas.
‘The “Rain to Recreation” programme is about value added,’ says Michael Beezhold, watershed manager with the City of Lenexa. ‘Lake Lenexa dam and spillway has and will continue to add great value to the community for generations to come. The dam and spillway are a marriage of form and function. They are beautifully conceived, designed and constructed. The city is very pleased and proud of the project.’
For further information on Lake Lenexa, please visit www.raintorecreation.org or www.bv.com
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