Storm and Runoff Calculation Standard Review Snowmelt and Climate Change

Transcription

1 Storm and Runoff Calculation Standard Review Snowmelt and Climate Change Presented by Don Moss, M.Eng., P.Eng. and Jim Hartman, P.Eng. Greenland International Consulting Ltd.

2 Map from Google Maps

3 TOBM is a four season resort town situated on Georgian Bay at the gateway to the Bruce Peninsula. TOBM is in the path of the prevailing snow squalls resulting in significant annual snow fall amounts. TOBM has unique drainage challenges, steep slopes and five ski resorts that have snow making capabilities. photo provided by Blue Mountain Resorts

4 In the Town s recent review of it s municipal standards, a review of potential impacts from climate change was to be considered because of it s unique watershed drainage. Although there is not presently a major impact from the ski slopes on the Town s infrastructure could this change in the future? What type of changes could be expected?

5 Interim Measures Typically Being Applied in Municipalities Overview of Findings from Federal/Provincial Investigations Data Sources Available to TOBM

6 In advance of Federal or Provincial directives, other municipalities that have decided to factor in some potential impact due to climate change have included features such as: 15% increase in rainfall intensities; 15% increase in storage volumes; Adjustments to hydrology model parameters; and, Requesting additional i freeboard for lower return period events.

7 A 20 mm 0.5 hr. typical rainfall event will increase 1.05 mm/decade. d Temperature warming 1.8 to 3.4 C by the year Peak rainfall intensities increasing 2.4% with each incremental degree of warming. Trends toward more extreme weather events, with shorter durations (convective T-storms). As weather patterns change there will be a trend for warmer temperatures and extreme rainfall earlier in the year (Spring).

8 Environment Canada Climate Normals for Owen Sound Weather Station Annual Snowfall Accumulation of cm Daily and Hourly Rainfall Records Temperature Records Snow Making accounts for approx. 10 to 15% of SWE annually photo provided by Blue Mountain Resorts

9 Overall, there are five ski resorts drain to the Town s municipal infrastructure photo provided by Blue Mountain Resorts

10 photo provided by Blue Mountain Resorts Largest snowmaking system in Canada

11 As the weather patterns begin to change to show more extremes, rainfall events and warmer temperatures will potentially be more prevalent earlier in the spring. As such, accounting for a potential impact due to climate change can be achieved by determining the impact from a rain-on-snow event. Will this phenomena be complicated by local snow making capabilities? photo provided by Blue Mountain Resorts

12 Snow Melt - Temperature Relationships Snow Pack Development Snow Pack Land Use Relationships Rain-on-Snow Relationships

13 Most Common Method Applied Melt (mm/day) = K (coefficient ) X (T Ta) Ta is the threshold temperature that causes the ice in the base of the snow pack to begin to melt (typically around 2 C). This value has been as high as 5 C for early spring. Ti is the air temperature in C. Melt is measured using the surface area of the contributing watershed. K Coefficient ranges from 3.9 to 5.55 mm (snow water equivalent (SWE)) / C/day with an average rate of 4.1 (Other areas similar latitude)

14 Up to 5% of the snow pack weight in external water can be absorbed b without t any release (referred to as ripe snow); and Snowmelt begins when liquid water equivalent exceeds 15% of total snow pack SWE 80 calories of heat to melt one (1) gram of water. Snow packs in ski areas have been measured to change in density from 400 to 700 kg/m³ (mid season to end of season)

15 The melt equation K coefficient can also be adjusted by the type of land use Open space a= Forest a= Ski slope a= Rango and Martinec

16 Sample Test Area For a 20 ha ski area watershed 40% forest and 60% ski slope warm day in late March or early April (7 C average temperature over full day) with average temperature of 5 C above the threshold temperature (2 C). Melt = (0.358 X 4.1 X X 4.1 X 0.60) X (7 C-2 C) = mm/day or 3,374 m³/day or 39 l/sec

17 The latent heat from rain on a snow pack releases 7.5 times the weight of the water being applied. A 51.2 mm rainfall event can reduce the entire snow pack accumulated through the winter months for TOBM Equivalent summer rainfall event 5 year 6 hour event 50 year 1 hour event Based on Owen Sound records

18 Review of the raw hourly data from Owen Sound station Table 1: Return Period April Rainfall Volumes 6 Hr 1 Hr Rainfall Rainfall (mm) (mm)

19 The magnitude of the April rainfall event volumes are 45.8% of the volumes from the summer rainfall events for the 6 hour 5 year event and 1 hour 50 year events. S l T A Sample Test Area For a 20 ha ski area watershed Melt = 20.0 ha X [(165.8) mm mm of rain] = 22,548 m³/6 hrs = 1.74 m³/sec The 5 year 6 hr event would have a peak flow rate as high as 1.74 m³/sec in early April. A similar analysis with the 1 hour rainfall volume shows the 5 year event peak flow rate would be as high as 4.46 m³/sec. (Summer Q 25 = 4.32 m³/sec)

20 Even though the rainfall event in April is less than ½ the summer rainfall event for the 5 year storm when applied to a ripe snow pack it can generate a larger runoff than the 25 year summer event By 2050 a 3 year April event will generate the same runoff as the present day 25 year summer event

21 The typical 20 ha example would generate a 6 hour volume of: = 37,570 m³ for the 5 year event (rain on snow volume) = 74,120 m³ for the 100 year event (rain on snow volume) Traditional 100 year predevelopment release rate requires 73,440 m³ Facility sizing should consider the volume required from a rain-on-snow event ent as well

22 In areas that receive runoff from ski slopes or heavy snow accumulation: The 25 year summer rainfall event should be considered d to replace the traditional 5 year event design for municipal servicing. Minimum culvert size based on the 25 year summer event should be considered. The spring rain-on-snow event runoff volume should be compared with the post-to to predevelopment volume to be controlled for a summer event. All subject to benefit : cost analysis.

23 Climate change can expect to increase spring rainfall volumes by 4.2 mm by 2050, over the next40years(105mmperdecade); (1.05 The rain-on-snow event should be considered for introduction into the municipal standards for areas that receive drainage from ski slopes or areas of significant snow accumulation; and, The Regulatory (Regional) storm will still govern watercourse floodplain conditions and SWM facility emergency overflow outlet designs.

24 Consider the cost:benefit of incorporating the findings and trends presented herein into municipal criteria.