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Flooding kept at minimum

Manteca’s park/retention basins doing their job

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Flooding kept at minimum

Manteca city worker Joe Chavez works on unclogging storm drains along Pebble Way on Sunday.

HIME ROMERO/The Bulletin


POSTED December 3, 2012 12:02 a.m.

The three storms that dropped over 1.6 inches of rain on Manteca over five days - including .73 inches Sunday  - caused little flooding except for water logged neighborhood street corners were storm drain inlets were clogged by leaves.

Had the same succession of storms hit 30 years ago sand bags would have been the order of the day in many parts of Manteca.

Flooded streets - and even businesses and ground floor apartments - were once the norm after downpours and steady rains. That’s because the city’s storm drain system once was easily overwhelmed  causing water to back up onto surface streets thanks in part to Manteca’s flat terrain.

How Manteca’s system works

How the city’s storm system works is part high-tech and part low-tech.

It starts as a light rain.

And then before you know it, the skies open up.

Rainwater runs off your roof, your driveway, your patio and other impervious surfaces.

It trickles into the gutter in a steady stream joining the run-off from nearby homes as well as the streets and sidewalks.

A running stream of water punctuated with a steady downpour flows into the storm drain and disappears beneath the streets of Manteca.

It’s below the surface in storm drains ranging from 12 inches to 48 inches or more that your rain run-off joins that from hundreds of other homes and commercial ventures.

But the trip ultimately to the San Joaquin River and into the San Francisco Bay is interrupted.

The city’s storm system is becoming overloaded. Pumps kick on and the water is taken out of the system into a nearby park that has been designed to provide double duty as a storm retention basin. It is here the water will pool with other run-off waiting for the system to clear up room so it can continue its journey.

The goal is to take pressure off trouble spots to avoid street flooding and backing water up on to private property by controlling flow with a telemetry system that has been programmed to respond automatically when workers detect problems in the city as the torrential cloud burst continues.

The water level in the storm retention basin rises providing an effective safety valve against flooding. Then capacity is freed up in the system and a second pump starts removing the water from the basin and putting it back into underground drains that make their way to the west near the Union Pacific Railroad tracks. It is here that the water comes gushing out through large screens covering culverts and into a San Joaquin Irrigation District canal.

The water will then make its way north to the French Camp Slough about eight miles away before turning west and heading past the northern levees on the northern edge of Weston Ranch in Stockton and into the San Joaquin River to continue its journey to the Bay Area and ultimately into the San Francisco Bay and beyond.

There are more than 2,800 storm drains citywide. All of the storm drains are cleaned before and after storms to prepare for the next rain. Even so, it is a big help when citizens keep leaves and other debris away and off grates to the storm system.

System wouldn’t work without SSJID canals

The system wouldn’t work without access to SSJID canals and drains that are between Manteca and the river. The city works closely with the district and shares in the cost to improvements along the drain that heads to French Camp Slough where all of the city’s storm drain water that enters pipes currently goes.

The city is dealing with a fact it can’t escape -any improvement to land increases the water run-off.

An acre of farmland that used to grow crops typical has a one-tenth of a rating for run-off.

 It increases when you put grass on it, build homes, or put in streets.

It is the same reason why California’s rivers are flooding more often. There is more run-off because more land is being made impervious to percolation during heavy storms.

The city doesn’t have a separate storm system crew but instead relies primarily on sewer system workers. Whenever they work on storm system matters, their time is billed internally to that account since state law requires enterprise fund accounts that serve specific users such as those who access sanitary services can’t legally pay for service other than what the fee is collected to pay for.

Additional improvements and deferred maintenance

The City Council in February set aside $218,000 in federal Community Development Block Grant funds go to helping improve the storm system in older sections of Manteca. The council earmarked $150,000 of the block grant funds to add to existing money set aside to purchase land and construct a storm surge basin along Moffat Boulevard. Another $68,000 will go to making additional improvements to prevent flooding at Center and Main streets.

Engineers have noted that it would be cost prohibitive to design and construct a system that would eliminated absolutely all prospects of flooding in the older parts of Manteca. But many of the ways to improve operation of the existing system that has been upgraded several times since the early 1990s in the central district is to make it easier for storm run-off to flow unobstructed into underground pipes.

Improvements in the past decade have substantially reduced flooding potential at historical trouble spots such as Moffat and Main, Center and Main and along other streets between Main Street and Powers Avenue. There can still be flooding simply because the water can’t get into the system.

The surge basin would allow for water to be taken out of the system and stored creating more capacity for run-off. Stored water would then be released back into the system once capacity is freed up.

Manteca has unfunded storm system needs

City leadership in 2001 dropped a $2.35 a month utility tax that was raising $690,000 a year to go toward storm system improvements and maintenance. The tax was repealed after the legality of the utility users’ tax was questioned. It was adopted on Nov. 20, 1989 as a way to fund storm drainage system improvements and maintenance to alleviate street flooding, particularly in the downtown district.

The issue of funding storm drain improvements has not been addressed in the last 11 years meaning that the storm maintenance and upgrade of the existing system is a drain on the general fund budget.

Manteca still has three years of payments on the original loan from 1989 to make that totals more than $1 million. That debt repayment - $223,000 a year – plus ongoing maintenance – takes a $500,000 to $700,000 annual bite out of the general fund. The utility tax – which would have brought in $900,000 this year if it were still in place – would have covered that tab plus have money to go toward over $1 million in upgrades and replacement of aging infrastructure that staff has recommended but hasn’t been funded.

To address some of the issues the city back in 2009 changed its policy of having water discharged from various park storm retention basins within 48 hours. That requires much larger pipes that add to the cost. The newer strategy calls for a water discharge rate of 96 hours from storm retention basins instead of 48 hours. That represents the longest accepted maximum time that water would sit in a retention basin before it is released into a series of underground pipes and eventually into the San Joaquin River.

Currently, it is rare for water to stand in any retention basin more than 24 hours.

In order to meet the 100-year flood protection standard that will remain unchanged, the city may have to alter future retention basins to either make them deeper or larger.

The problem, however, is how to fund improvements that can’t legally be charged off to growth.

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