What made the difference in Dorchester

There are more than 2,200 dams that form an important component of Ontario’s aging water management infrastructure. The concept of structured dam safety has been evolving since the early 1980s but has gained significant momentum in recent years as people become more aware of the threats that potential dam failure poses to public and environment safety.

Hatch Energy has always been on the leading edge of this curve and our people are recognized experts in dam safety, regularly contributing to research and development of the Canadian Dam Safety Guidelines.

The original Dorchester Mill Pond Dam was located in the scenic village of Dorchester and was built around 1810, making it one of the oldest dams in the province. It was owned by the Upper Thames River Conservation Authority (UTRCA) and managed by the Municipality of Thames Centre.

Constructed to provide hydraulic power to a sawmill, it consisted of a concrete-faced, rock-filled timber crib. The embankments had deteriorated and remedial works had been implemented on an irregular basis in an attempt to improve safety and reduce seepage. Despite these efforts, deterioration continued to the point where UTRCA commissioned Hatch Energy to undertake a comprehensive dam safety evaluation.

Assessing the original dam
As a first step in the process, we undertook detailed hydro-technical and hydraulic analyses to evaluate the spillway capacity for the site, and to check on the pond and tail water levels during flood events. Advanced engineering techniques included 3D stability analyses, flood routing dam break simulations and on-site condition assessments.

Results verified the hazard potential and design flood for the existing structure, and confirmed that the dam would be overtopped during flood flows. We investigated a number of options ranging from rehabilitation to complete removal. Decommissioning, however, would significantly undermine the established recreational and cultural use of the surrounding area.

Developing a new design
The owner required a modern facility that would require little maintenance at a reasonable cost for a long service life. The large adjacent pond is heavily used by community groups and residents for recreation, so the site’s scenic value and continued access to the area were important to the general public. Safety and environmental stewardship were also critical for all stakeholders.

After an initial design concept was developed, we implemented a public participation process to get the community involved and help ensure that the new dam would satisfy competing usage goals. Interactive presentations and meetings were held with the various stakeholders, as well as a public open house.

As the design progressed, we compared ways of economically implementing a rustic, appropriate stone appearance on the new exposed concrete. We concluded that the use of form liners was an economical and effective solution. A local contractor was selected to implement the design, further enhancing benefits to the local community. Instead of using quarry stone for wingwalls, we collaborated with the contractor and owner to incorporate pre-cast concrete blocks using the stone pattern form liners used for the cast-in-place concrete. Beyond aesthetics and structural stability, this method also reduced project costs.

Construction began on October 25, 2004, and a fast-track schedule was adopted to complete critical concrete placement by the end of that December. We provided full-time, on-site construction management services throughout the process. During construction, mitigation of natural impacts was achieved by implementing best-practices in site management and erosion/sediment control. An ecologically friendly site restoration plan was also employed.

The new dam was designed to re-establish safe public access across the structure, and maintain the existing recreational uses of the Mill Pond. Improved fish habitat areas were also provided downstream.

Key aspects of the new design
A self-regulating concrete overflow dam/spillway was designed to maintain Dorchester Mill Pond water levels. This involved the use of sophisticated computer simulations to design the overflow structures to pass the maximum design flood required for dam safety purposes – while maintaining water levels during the low-flow summer period for recreational users.

As opposed to more conventional designs that use gates or stop logs to pass flood flows, our design resulted in considerable savings in construction, long-term maintenance and operational costs. It also enhanced public safety through automatic flow regulation without the need for a human operator.

Innovative crest level designs were used in order to improve flow conditions and reduce the collection of algae blooms upstream of the dam, a problem identified through our public input process.

In designing the deck and railings for pedestrian access and scenic viewing, it was necessary to meet all current public and workplace safety regulations. However, typical approaches would have required the use of steel.

The public felt that, even when painted, steel railings and decks can be unsightly and have the tendency to peel and rust. As an alternative, we designed a timber deck and hand railing system that satisfied all modern safety requirements but better blended into the natural environment.

The use of patterned concrete form liners gave the new dam the aesthetically pleasing appearance of a 19th century structure while still meeting all modern dam safety requirements. This innovative design concept was a first in Ontario and perhaps Canada. Our engineers determined that this technology will maintain the quality and long-term integrity of the dam, with the added benefit that the cascading/stepped water flow is beneficial for energy dissipation and fish protection.

A healthy future
Accomplishing the multiple goals required an innovative process that went beyond engineering – bringing the public and local community groups into the heart of the design process. Then, using simple yet innovative engineering concepts, we created a dam that blends well with the scenic surroundings, meets modern design flood and safety requirements, and reduces lifecycle costs.

For further information, please contact:

Murray McFarlane
Senior Project Manager
Hatch Energy, Niagara Falls, Ontario
905-374-5200
MMcfarlane@hatchenergy.com