(continued)

Saugeen Conservation predicts spring flooding but maybe
not severe

Weather

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Ron McManus of the Saugeen Valley Conservation Authority (SVCA) gets ready to take some snow course readings
photos courtesy of Saugeen Conservation

This winter has been one of the coldest in the past 40 years, leaving the area with an accumulated snow pack that has never seen before, according to the Saugeen Valley Conservation Authority (SVCA).

Based on snow course readings taken by SVCA staff at 14 different stations across the watershed, the average snow depth is 17.75 inches (47 centimetres).

There are a number of other factors involved with forecasting spring watershed conditions. Once snow course readings are taken, staff then determine the actual ‘water content’ or equivalent amount of water in the snow. This figure, based on March 1 readings, is 7.3 inches (185 millimetres), a little over double the historical average amount for early March.

Snow density is also an important factor. Snow density values help determine when the snowpack is saturated. 

A snow density value of 45 per cent typically indicates that the snowpack is “ripe” or that run-off can be expected at any time favourable temperatures occur. Currently, the average snow density across the watershed is 39 per cent.

Staff also looks at ice cover. This year, the ice cover is the most extensive this area has seen in a number of years. While some areas along the Saugeen River have historically been prone to ice-jamming, jams can occur anywhere along waterways at any time during the initial stages of break-up.

It’s probably no surprise that given the amount of snow and ice in the area this winter, there is a possibility of significant spring run-off. The extent, however, will depend entirely on weather conditions.

Should an abrupt and prolonged period of mild and wet weather prevail, there may be a rapid break-up and melt, that, combined, could generate much higher flows than seen in the last number of years.

Ideally, temperatures above freezing during the day and below freezing overnight (also perfect maple syrup weather), serve to gradually deplete the snowpack over a period of two to three weeks, and allow the rivers to flush out the ice and excess water in a naturally controlled manner.

Does the potential for flood exist? The Saugeen River is 192 kilometres in length, the third largest river system in southern Ontario. The potential for flooding always exists, but usually is most prominent at this time of year as the winter’s accumulation of snow melts. 

SVCA staff will be continuously monitoring conditions as spring approaches to inform and prepare municipalities and watershed residents of potential flood events. Should flood-related messages be issued, there are three types of condition statements that watershed residents should be aware of, depending on the severity of the situation. 

It is important to know the difference between the three. These are also standard across the province for all 36 conservation authorities.

  1. Watershed Conditions Statement – issued when conditions suggest high run-off potential that could lead to localized flooding in low-lying areas. These have been typically issued during March Break to keep parents and children aware of dangerous river and stream conditions.

  1. Flood Watch Statement – issued when the potential for generalized flooding exists throughout the watershed or for specific municipalities.

  2. Flood Warning – issued to specific flood damage centres where significant flooding is definitely possible.

Interesting Facts on the Saugeen River watershed area:

  • On average, the Saugeen Watershed receives about 40 inches (1,000 millimetres) of annual precipitation (60% liquid/40% snow), of which 45 per cent runs off to Lake Huron and the remaining 55 per cent is retained on the land to help recharge groundwater reserves.

  • Soil characteristics influence run-off. The predominantly clay soils that exist downstream of Walkerton and along the lakeshore have less infiltration capacity, hence, a higher percentage of run-off and a faster response to rain and melting snow. Conversely, the coarser soils in the upper watersheds (i.e. above Walkerton), serve to store more water and reduce/delay run-off. It is also this area that provides virtually all the baseflow in the summer for the Saugeen River.

 

Ron McManus of the Saugeen Valley Conservation Authority (SVCA) takes snow course readings in the Saugeen watershed

  • Slope or topography also plays a role with respect to melt and run-off. South-facing slopes, for example, warm up more quickly and promote earlier run-off.

  • Latitude and altitude also influence snow melt. In the south part of the watershed (i.e. the South Saugeen), melting will usually occur slightly earlier than in the North Saugeen. Similarly, river systems near Lake Huron tend to melt earlier than the higher, upper reaches of the watershed. The elevation difference, between the Osprey wetlands (where the Saugeen begins its journey), to Lake Huron is 1,150 feet (351 metres).

  • Forest cover plays a significant role in determining snow melt. Snow accumulated in these areas is always the last to melt. During the great land clearings of pioneer settlement, much of the natural storage and run-off controls (i.e. forest cover, wetlands, etc.) were lost. Settlement pressures to maximize productive arable land on the clay plains and lakeshore areas have intensified this even more. Run-off in these areas is concentrated, swift, and often beyond the capacity of what the natural drainage network can handle. On the other hand, the gradual change in land use from active agricultural usage to a more forested and recreational usage in the upper watershed has helped to gradually re-establish some of these natural controls.


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Saturday, March 08, 2014