Operations & Maintenance
Dykes prevent the spread of water outside the normal capacity of the channel. They are constructed of material that compacts into a solid structure while maintaining a degree of impermeability. There is usually a durable running surface 3.66 m to 4 m wide, enabling access of heavy equipment in the event of an emergency. Dyke side slopes are designed to be smooth at a 2:1 or greater slope. Trimmed grass is encouraged, as it aids in the reduction of surface erosion.
The Pemberton Valley Dyking Network is composed of approximately 40 km of dykes. The individual dykes that make up this network are: 1) Smuks Dyke, 2) Forestry Dyke, 3) Ryan Dyke, 4) Strobl Dyke, 5) Hungerford Dyke, 6) Miller/Lillooet Dyke, 7) Ayers Dyke, 8) Adventure Ranch Dyke, and 9) Pemberton Creek Dyke.
- Dyke Maintenance and Monitoring
- Rip Rap Bank Protection
- Ditches and Culverts
- Gravel Management
Dyke Maintenance and Monitoring:
A sizable maintenance and monitoring program is required in order to attend to all 40 km of dykes. The PVDD carries out an annual inspection and corresponding report to the Ministry of Environment, Inspector of Dikes office. Additional inspections are conducted throughout the year in order to locate deficiencies. Examples of deficiencies include sink holes, slumping, burrowing animals, and trails cutting into dyke slopes from herds or pedestrians.
Vegetation management is also critical. Trimmed grass is good for stabilizing dykes, however, trees and shrubs are not. Trees can root deep into the structure and possibly right through it. This can result in piping, where pressurized water from the river seeps along the roots removing material from the dyke body, potentially destabilizing the dike. Larger trees rooted in dykes have the potential to cause damage if they blow over or are struck by floating debris, tearing the root mass out of the dyke. For these reasons the dyke network is mowed each year. Additionally, a brushing program has recently been adopted to manage shrubs and trees along the toe of the dyke. For a more detailed description of Dyke Maintenance and Monitoring carried out by the PVDD please refer to Ministry of Forests, Lands and Natural Resource Operations, Guidelines for Management of Flood Protection Works in BC
Today’s dykes are designed to provide protection from a 1:200 year flood event – the flood that statically occurs once every 200 years. Dykes are assessed and designed using detailed river and dyke survey information, rainfall and river flow data, river characteristics and watershed mapping. Once gathered, this information is used to generate a computer model identifying the 200 year water level in relation to the crest of the dyke or river bank. Since 2000, the PVDD has commissioned several engineering reports to assess flood protection in the valley. Recommendations from these reports are considered and prioritized by the PVDD according to feasibility, funding availability and social impacts. The following Reports are available:
Rip Rap Bank Protection:
Historically, creeks and rivers in the Pemberton Valley meandered from one side of the valley to the other. This is a natural process that can occur suddenly or over a long period of time. Movement or realignment of river flow patterns can occur as result of log jams, gravel deposits, erosion or large flood events.
Rip Rap is rock that has been placed along a dyke or river bank to stabilize the course of a river by reducing bank erosion. Although rip rap is very durable it is susceptible to large floating debris, ice, erosion and sometimes vegetation. Life expectancy of rip rap is subject to the quality of rock, quality of the original construction and maintenance.
Maintenance and Monitoring of Rip Rap:
The PVDD inspects the condition of rip rap throughout the year. Inspections can be carried out from the river bank or dyke and at least once a year from a boat. Any loss or movement of rock is closely inspected. Sometimes trees rooted in rip rap can cause damage when they fall dislodging their root ball and the rip rap. Once water has breached rip rap significant damage and loss of land can occur over a short period of time.
Deficiencies in rip rap are typically repaired during low flows unless considered an emergency. The PVDD periodically removes vegetation from rip rap in critical areas if it is considered threatening. Wide spread removal of vegetation on rip rap is not feasible nor environmentally responsible/permissible. Most of the repairs to rip rap are relatively small and are intended to keep rip rap continuous and free of voids. It is largely a proactive approach that is considered to have the least amount of environmental impacts as well as being cost effective.
Ditches and Culverts:
Drainage ditches are constructed to collect and convey water from low laying areas in the valley to creeks and rivers. Some drainage ditches in the valley are quite long, up to 14km and pass through many properties on route. A drainage ditch relies primarily on slope to effectively convey water. Other factors that may impact the effectiveness of a ditch include how obstructed the flow of water is, improperly sized culverts or bridge crossing, sediment buildup and intensity of a rainstorm or snowmelt.
Ditch Maintenance and Water Stewardship:
Many of the ditches in the valley, including Arn Canal, were originally small streams. Historically, streams meandered across the valley floor connecting wetlands to creeks and rivers and supported a wide variety of habitat. In many cases, these watercourses were straightened and turned into a network of ditches to dry out the land for agricultural or development purpose. In doing so, habitat was significantly impacted. Today managing and maintaining these ditches is a delicate issue.
Maintenance typically consists of removing vegetation and sediment by hand or by machine. In some cases, these activities may have negative impacts on habitat and subsequently are regulated by the Ministry of the Environment and Department of Fisheries and Oceans. In these situations a strong case must be built for in channel work. A detailed hydraulic assessment, habitat assessment, and mitigation plan must be submitted for approvals before work commences.
In 2008 the PVDD began to classify fish values in key drainages in the valley in order to streamline maintenance activities wherever possible. The PVDD has also commissioned a survey and hydraulic assessment of the Arn Canal. The results of these assessments will facilitate informative decision making for future projects on the Arn Canal.
Culverts with Flap Gates:
How does water on the inside of the dykes escape? Drainage ditches in the valley pass through dykes eventually connecting to a creek or river. Culverts equipped with flap gates, sometimes referred to as a “Flood Box”, facilitate this connection. Culverts installed through the dyke allow water to escape the ditch. The trick is to keep water from the creek or river from flowing back up the culvert and flooding adjacent properties. This is achieved by using a flap gate, which is attached to the riverside of the culvert. The flap gate is hinged to the culvert and when river levels rise, water pressure closes the gate preventing water back flow into the culvert. In doing so, it also prevents drainage ditch water from escaping resulting in localized pooling on the inside of the dyke. Once river levels moderate the flap gates open and drain land on the inside of the dykes. Without flap gates both storm water and river water would collect inside of the dykes causing wide spread flooding.
Culvert and Flap Gate Maintenance:
Culverts play a critical role in flood protection and must be consistently monitored and maintained. Maintenance activities range from clearing debris from culvert inlets and outlets to replacing culverts that are considered no longer structurally sound.
The movement of gravel through a watercourse occurs naturally over time as a result of erosion. Gravel has a tendency to accumulate at different locations while being moved downstream. These locations are typically called gravel bars and are elevated areas within the river channel. As they grow within the channel they force the flow of the river elsewhere. All of the creeks and rivers within the Pemberton Valley have flowed in a different course to where they are now as a result of this natural process.
Some of the dyking and rip rap work in the valley limited the ability of rivers to change their course. Consequently, rivers and their gravel bed load are confined to their individual channels. In some cases, if left unchecked, gravel can accumulate in a channel forcing water levels to exceed the levels of the dykes.
Historically, gravel has been removed from Valley rivers to reduce the water level profile at a specific locations, based on visual inspection of gravel bed load. Other times, gravel has been removed to facilitate construction projects for preload or structural fill. Over the past century, a greater understanding of the complexity of river ecosystems has been developed. As a result, gravel management is now based on science and engineering. Regulating agencies require information indicating hydraulic impacts and subsequent threats to public safety caused by gravel bed load. Detailed survey work and computer generated models of the river flow characteristics are used to provide justification. In addition, environmental assessments must be conducted to identify habitat impacts. Once this information has been compiled, a plan is developed to carry out the work and mitigate impacts to habitat.
The PVDD monitors gravel bed load on all creeks and rivers in the Valley both visually and through survey. Several management plans have been adopted to deal with bed load when it is considered to hydraulically impact the level of protection offered by the dyke and were minimal impact to the river ecosystem will occur. For more information on gravel management plans please see: