(WSB photo, Sunday in Sunrise Heights)

After two more water-main breaks in West Seattle on Sunday and Monday, following the two on Thursday, we had a few followup questions for Seattle Public Utilities. Here’s the reply we received today from SPU spokesperson Sabrina Register, incorporating answers to our questions:

While we don’t know the cause of the water main breaks on 100th SW (Monday) and 29th SW (Sunday), they are likely not related to the two water main breaks that occurred last Thursday, March 2. The above-mentioned locations are in different pressure zones than last week’s breaks.

With regard to the two leak locations Sunday and Monday:

29th SW – installed 1945 – Cast Iron – 585 pressure zone*
100th SW- installed 1938 – Cast Iron – 585 pressure zone

We tend to experience more water main breaks whenever we have cold weather, dipping below freezing or near freezing. Pipes are carrying very cold water, and the ground freezing around the pipes can play a role. The contraction and expansion of pipes during cold weather can cause pipes to break.

To give context to the number and location of water main breaks around Seattle, in the past month Seattle Public Utilities has investigated and/or repaired nine leaks in Seattle south of Denny and 12 leaks investigated and/or repaired north of Denny.

SPU’s rate of breaks is currently about 7-9 breaks per 100 miles of pipe per year. While there isn’t an “industry standard” threshold for water main breaks, utilities generally consider below 15-20 main breaks per 100 miles of pipe per year as an acceptable rate.

(*Pressure zone: “A water system subsection operating from sources at a common hydraulic elevation.”)

We also asked about some big-picture info, such as plans for updating the city’s aging pipes – there’s some overview info here and a project list here, though nothing currently planned in our area; for the really big picture, the water-system plan is linked here, and Register recommends Volume 1, Chapter 5 (we’ll be reading that eventually for another followup).