BC Hydro Update: Puntledge River Operations for Flood Risk Management From Subtropical Storms
February 6, 2015
From BC Hydro, 2:00 pm:
"That was a close one. The brunt of the subtropical storms are nearing the end though another significant wave is expected this afternoon and tonight.
BC Hydro would like to provide a few points of interest in the nuances of the storm event to date and downstream flood risk management. The rain fall amount in the watershed above the Comox Lake Reservoir over the 24 hour period on Thursday was 100 mm. That's a large rain event that had two significant rain bands early Thursday morning and then in the evening. Over the last 36 hours, 136 mm of rain has fallen.
The Cruikshank River peaked at about 160 m3/s. The peak water inflow into the Comox Lake Reservoir was around 375 m3/s.
The concern was the high tide at around 7:30 am today and it was essentially a near miss. The tide was supposed to be around 4.8 m yet the storm surge from winds increased the tide height by 0.6 metres.
Flooding in isolated areas near the 5th Street bridge can begin above a river gauge level of 4.0 m, though the general threshold is thought to be 4.2 m. More broader flooding may begin at 4.4 m. The gauge hit 4.03 m during the morning high tide.
The Browns River had fortunately come off its peak flow of about 140 m3/s to about 90 m3/s during the high tide. That's 50 m3/s of flow or the equivalent of about 0.2 of a metre in gauge height at the 5th Street bridge.
The Tsolum River was still increasing and flowing at about 180 m3/s (and is still flowing at that rate) at the high tide, and BC Hydro had reduced, beginning at 1:00 am, its discharge from the dam down the Puntledge River from about 125 m3/s to about 50 m3/s. The current rate of discharge from the dam is 145 m3/s.
The general rule is a high ocean tide and a total river flow of 400 m3/s may cause some isolated flooding. At this morning’s high tide the river flow was at 350 m3/s. The ocean storm surge made it a potentially challenging situation.
The next heavy band of rain is expected to drop anywhere from 50-100 mm this afternoon through tonight. Given the level of watershed saturation and the potential for storm surge, for the high tide on Saturday, BC Hydro plans to reduce the discharge from the dam down to about 35 m3/s. There is a possible risk for some isolated flooding tomorrow morning. It will depend on the peaking of the Browns and Tsolum rivers.
The weather on Saturday and Sunday is wet though generally benign with daily rain totals of around 20 mm.
The Comox Lake reservoir got to a low of 133 m on Wednesday afternoon. It is currently at 133.7 m and rising at a fairly good rate. BC Hydro still has available water storage in the reservoir. It free spills over the dam at 135.3 m."
February 5, 2015 update from BC Hydro
"An update on the weather forecast, the water inflows and BC Hydro operations.
The rain event over these four days is changed, particularly for Saturday and Sunday where this storm system will be shifting further south. This means a reduction in total rain fall. There is a bit of a break in the rainfall this afternoon, and then the most intense rain from this storm may be tonight and into Friday. Precipitation levels may amount to 150 mm or so in the mountain areas above the Comox Lake Reservoir though to Sunday. Rain amounts down by the ocean will be less.
Since the storm systems began to hit around Wednesday at midnight, about 46 mm of rain has fallen in the upper watershed.
The temperatures are warmer than normal and may cause some snow melt.
The water inflows and adjacent river systems in the Comox Valley began to react to the rain earlier this morning. The flow rates at the Cruikshank, Browns and Tsolum rivers have all come up but are currently well below any thresholds of flood risk management concerns. They were all in the 3 to 10 m3/s range before the storm started and are currently flowing at:
- Cruikshank River – 43 m3/s;
- Browns River – 50 m3/s; and
- Tsolum River – 76 m3/s.
The Cruikshank flows into the Comox Lake Reservoir, and the Browns and Tsolum rivers all feed into the Puntledge River near Courtenay. If the total Puntledge River flow rates at the 5th Street bridge hit about 400 m3/s during a high tide, there is the potential for some isolated flooding such as Lewis Park. With the current weather forecast, and BC Hydro’s ability to hold back water in the reservoir, any potential risk for isolated flooding may be the Friday morning high tide. Other than that there appears to be no flood risk.
BC Hydro has kept the two spillway gates at the Comox dam wide open the past few days in advance of the storms and the reservoir has dropped about 0.6 of a metre and is currently at 133 metres. The level is now beginning to level off as the rain run-off flows into the reservoir. BC Hydro has been releasing about 120 m3/s and will begin to decrease all the way down to about 40 m3/s during high ocean tides until the storm is concluded. The reservoir level is in good shape for these storms to absorb most of the forecasted inflows. There is 2.3 metres of available reservoir water storage.
On the weekend, with the reduced forecasted rain fall, BC Hydro may back off its high discharges from the dam as we consider reservoir levels. BC Hydro continues to advise the public to stay away from the Puntledge River through to early next week. The river flows may be very high and dangerous, and BC Hydro will be fluctuating its discharges from the dam in consideration of ocean tides on Friday and Saturday.
This may be BC Hydro’s final update on our operations for this storm event. It there is a significant shift in the weather for the worse an update may be provided."
February 3, 2015 update from Stephen Watson, BC Hydro:
"BC Hydro’s forecasters are following a series of subtropical storms that will be hitting the West Coast of North America beginning Wednesday night. Unlike a relatively narrow band of precipitation directed at the coast from the Southwest with high precipitation levels, known as an atmospheric river, these systems look to have a very wide band of heavy rain that will hit the United States and Canada from the South. For Central Vancouver Island, our forecasts show a potential for between 120 and 300 mm of rain over four days starting Thursday. Temperatures will be warm and freezing levels high, and what limited snow pack we do have will have further melting and add to the water inflows into the Campbell River system and other river systems. There is the potential that these storms could create very high river flows over those four days and beyond.
In my 15 years of involvement on BC Hydro operations on the Puntledge River system it’s hard to recall large subtropical storm systems hitting the area in February. The last such storm was in 2003. Typically they are in the November to January timeframe.
The winds look to be moderate and out of the south to southeast. This may cause a bit of storm surge up the Courtenay estuary.
The ocean tides are fairly high though certainly not King Tides, which will be helpful to limit downstream flooding impacts. The high tides are at 4.8 metres over these four days.
The Comox Lake Reservoir is in the middle elevation range and BC Hydro will be in good shape to absorb some of these forecasted water inflows. The reservoir is currently at 133.6 metres. Beginning Tuesday evening, BC Hydro will increase the discharge out of the Comox dam to the maximum capacity, and given the level of the reservoir, that’s about 140 m3/s. Flow rates will go from about 35 m3/s to about 140 m3/s on Tuesday night with BC Hydro backing off each day for the high tides. This will lower the reservoir level further in advance of the storms.
BC Hydro is providing a public safety advisory to stay away from the Puntledge River beginning Tuesday night through next Wednesday. The river flows may be very high and dangerous, and will be fluctuating as BC Hydro considers ocean tides.
BC Hydro will provide an update on Wednesday afternoon as the weather forecast becomes clearer."