Torrential rains cause sewer overflows

Monday’s storm resulted in four overflows in four locations in the Sewer Authority Midcoastside (SAM) system. SAM provides sewer service from Half Moon Bay to Montara. Locations of the overflows, with links to maps:

• Montara at the Date/Harte pump
• Mirada and Magellan in Miramar
• Surfer’s Beach
• Controlled overflow at the Montara Storage Facility

"Last year’s rain was a 10 to 20 year event," said SAM board member Scott Boyd, speaking of the deluge that resulted in a well-publicized overflow last year.  "This year was bigger."  And the overflow was worse as well.

According to Jack Foley, manager of SAM, because rain flushed the sytem before the overflow and because of the amount of water involved, the sewage overflow was diluted ten to twenty times. And, of course, the continuing rains flushed it further.

The rain on Monday was tremendous.  With 4.7 inches falling in Montara in less than 24 hours, more then 5 in Pacifica and more than 3 in Half Moon Bay. One inch fell in Montara between 5 and 7am, and 0.6 inches in just half an hour.

The result of all this water was to "max out the system" according to Scott Boyd. This, despite the half-million gallon storage facility that was added recently.  The challenge for the system is that there are multiple points where water is collected and moved on in the system and each of these is a potential bottleneck.

Optimizing the system requires complex modeling, which has been ongoing at SAM. In addition to SAM’s continuing review of their system in light of recent events and their plans for additional storage to accomodate flooding, the authority is under review by the Environmental Protection Agency for last year’s overflow. This review could well result in additional federal requirements on the system.

Events of this nature were considered extreme, but it’s beginning to look like extreme events are the norm.

The letup in the rain over the last 24 hours has allowed SAM to empty its system of all the extra water. This is good news. Another big storm is expected to dump more rain on the Coastside starting Wednesday