Arn Canal Upgrades 2010
Over the past several years the PVDD has undertaken several projects on Arn Canal designed to increase conveyance and capacity. The most notable project involved pulling back the banks of the Canal in key areas from the Hwy 99 Bridge to the Poplar St. Bridge. During heavy rainfall in November 2009, the work was put to the test. Water in the canal overtopped the lowered banks inundating the constructed storage areas as designed and illustrated in the picture below.
In July 2009, the survey and hydraulic assessment of the Arn Canal was finalized. The intent of the assessment was to determine the current carrying capacity of canal, identify flow impediments, develop a working survey for future projects and to begin to develop a long term strategy for improvement projects on the canal.
Miller Creek Sediment Trap Weir
In 2004, a Sediment Trap Weir was constructed in the upper fan area on Miller Creek. The objective of the sediment trap is to manage gravel accumulations in an area where there will be less impact to fish habitat. Historically, this activity was carried out in downstream locations that are now considered to be valuable habitat.
The annual bedload aggradation rates are estimated to be approximately 5,000m3 annually. This is a rough estimate and can vary from year. Improvements to road infrastructure, drainage and hillside stability within the watershed have been carried out by Capital Power and are expected to reduce gravel input to Miller Creek.
The PVDD has developed a monitoring program to assess bedload movement, quantify gravel removal requirements and assess the performance of the sediment trap weir. This is done with survey of established cross-sections within the sediment basin and downstream locations and is considered part of a long term management plan.
A small working window has been established in late April in conjunction with Fisheries and Oceans. This timeframe allows for minimal disturbance to fish and capitalizes on low water conditions for carrying out the work. The PVDD is anticipating working within this timeframe to re-establish the storage capacity of the sediment basin.
Arn Canal and Grandmother Slough Vegetation Management Program
In 2008, the PVDD sent a crew into Arn Canal. The goal was to begin to manage, in an environmentally friendly manner, the uncontrolled growth of trees and brush which were considered flow impediments in the canal. The crew not only removed branches and trees but also pallets, shopping carts, bicycles and one front end loader tire.
The project was considered a success generating two dump truck loads of material over a short 400m section of canal. There still remains much work to do on the canal in order to find the balance between good drainage and a healthy aquatic ecosystem. The PVDD will continue with this project on the Arn Canal and will be doing the same on the Grandmother Slough.
Pemberton Creek Gravel Management
In December 2007 and January 2010 wet avalanches (Slushies) occurred on Pemberton Creek where by snow and ice formed blockages in the channel. In the 2007 event Highway 99 was closed for 12 hours due to overtopping. Both events required excavators to remove the snow and ice blockage from approximately ¾ km of Pemberton Creek. Although no gravel was removed at this time the work could potentially have negative impacts on fish habitat. A variety of conditions are suspected to trigger these debris jams. These include a low highway bridge with insufficient conveyance, gravel bedload in low gradient reaches and changing climatic conditions.
The PVDD is working with Fisheries and Oceans Canada to develop improvements to the channel in order to enhance conveyance during wet avalanche conditions. This work will include resurveying cross-section from the mid 1980’s to assess the level of aggradation. Once this data is collected a determination can be made as to possible enhancements
Lillooet River Gravel Removal
The PVDD has been working with Ministry of Environment and the Department of Fisheries and Oceans to develop a long term gravel management plan for the Lillooet River. In 2009, previous engineering and survey work was presented to DFO and an environmental assessment was initiated. As a result, further information was requested by DFO quantifying direct hydraulic benefits of gravel removals. Consequently, additional survey of the channel will be carried out in 2010 in order to run a hydraulic model that calculates water levels in relation to the dyke crest. If it is determined that the water level profile is negatively impacting the surrounding flood protection works then this justification can be incorporated into the environmental assessment.
The PVDD has been working to develop a Ditch Classification and Management Plan. Some of the ditches in the valley were once natural watercourses and valuable fish habitat. Arn Canal is an example of one. The Management Plan will streamline the approval process for ditch maintenance in the Valley. In 2009, work was carried out in association with property owners to improve conveyance in key areas on several drainage ditches across the valley. This program is ongoing.
Vegetation Management/the Dykes
The PVDD will be continuing to focus on removing larger vegetation in close proximity to the dykes. Roots from larger trees can grow through a dyke creating seepage pathways and the potential dyke failure in extreme cases and therefore it is essential to maintain this vegetation.
The PVDD will be conducting a provincial dyke and rip rap inspection in 2010. Deficiencies identified during the inspection will be noted and repaired under a general maintenance program or if considered a capital project a shelf ready or emergency plan will be developed to carryout the work.