North by Northwestern

Icy buildup that blocked Evanston water plant pipes was cleared early Friday afternoon, according to a press release from the City of Evanston.

“Between the hours of 4:30 a.m. and 12:30 p.m. on Friday, January 30, the City of Evanston treatment plant was unable to bring water into the treatment process due to an icing condition on the intakes,” Interim City Manager Rolanda Russell said in the press release.

Frazil and anchor ice formed in water pipes overnight. Northwestern University, along with other big facilities in Evanston, was notified at 8 a.m. on Friday and asked to reduce its water usage.

“The cooperation from these facilities was excellent and allowed the Water Utility more time to try to rectify the problem,” Russell said. “However, because the situation did not improve, at 11:45 a.m. the City issued a press release asking all customers to conserve water.”

Despite the pipes being cleared, Russell advised to continue curtailing water usage until Saturday.

“Customers are asked to continue voluntary conservation measures until at least 1:00 p.m. on Saturday, January 31,” she said.

Similar conditions could eventually reoccur in the near future.

“Customers should be aware that weather forecasts indicate that conditions will remain optimal for ice formation to reoccur for the next several days.” Russell said. “Therefore, it is recommended that customers draw and store drinking water before going to bed tonight in the unlikely event that the situation reoccurs over the weekend.”

The press release asked to cut down on “non-essential water usage, such as laundry, running the dishwasher, etc.”

Read the statement below.

EVANSTON WATER PLANT RESUMES NORMAL OPERATION;
CONTINUED VOLUNTARY WATER CONSERVATION REQUESTED

The City of Evanston’s water plant has resumed normal operations after water intake was restricted due to ice build-up, but customers are asked to continue voluntary conservation measures until at least 1:00 p.m. on Saturday, January 31. This involves curtailing non-essential water usage, such as laundry, running the dishwasher, etc.

Customers should be aware that weather forecasts indicate that conditions will remain optimal for ice formation to reoccur for the next several days. Therefore, it is recommended that customers draw and store drinking water before going to bed tonight in the unlikely event that the situation reoccurs over the weekend.

Residents are requested to check for status updates on the City Web site (www.cityofevanston.org), Evanston Community Media Center’s cable channel 16, and radio station (1650-AM). A recorded message will be posted at (847) 448-8058 after 5:00 p.m. on January 30.

We appreciate the citizens’ efforts during this period.

Read on for further information about today’s events:

Between the hours of 4:30 a.m. and 12:30 p.m. on Friday, January 30, the City of Evanston treatment plant was unable to bring water into the treatment process due to an icing condition on the intakes. During this time, the Evanston Water Plant continued to supply high-quality water to its customers at a reduced rate from onsite storage. At no point in time was the quality of water in the system compromised. Communities which receive water from Evanston were asked to conserve water and their water supply was not curtailed.

Background:
This situation can occur during winter months, and the Evanston Water Plant modifies its operation anticipating that this icing condition may occur. Pumping is decreased at night and increased during the day to fill all of the potable water storage facilities. There is normally sufficient storage of potable water at the water plant and in the distribution system to wait out this type of situation.
Customers which use large amounts of water, such as Northwestern University, St. Francis Hospital and the North Shore Medical Center, were contacted at 8:00 a.m. and asked to voluntarily reduce usage deemed non-essential (i.e. laundry, dishwashing, etc.). The cooperation from these facilities was excellent and allowed the Water Utility more time to try to rectify the problem. However, because the situation did not improve, at 11:45 a.m. the City issued a press release asking all customers to conserve water.

Read on for more information on frazil/ anchor ice formation:
The formation of frazil and anchor ice depends on meteorological conditions. In 1982, Gee Tsang outlined the conditions for frazil and anchor ice formation in simple terms:
In common language, one says that frazil and anchor ice are likely to form on a cold, clear night when the wind is strong, the humidity of the air is low, especially if such a night follows a cold, windy, cloudy day.
As Tsang says, in mid-latitudes anchor ice tends to form at night. On mornings following anchor ice formation events, the sun warms the water, and anchor ice is released.
The drawing of lake water through the intakes causes the water to super-cool (i.e. the water temperature is cooled to below the freezing point). When this happens, ice can grow in the water column. The first ice to appear is frazil (small disks or spicules of ice suspended in the water column). The drawing of the water into the intakes causes the frazil ice to stick to the intake pipe itself, forming anchor ice (ice attached or anchored to something).
Super-cooling is the process of chilling a liquid below its freezing point, without the phase change from liquid to solid. Super-cooling of water is ubiquitous in nature, and is a necessary precondition for the formation of frazil and anchor ice. Water in rivers, lakes, and seas is only super-cooled to about 0.01 oC.