The pitter patter of raindrops Tuesday and today won’t create any issues for keeping city streets devoid of floodwater.

But when it comes to bigger storms — coupled with development of more homes, stores, parking lots, and streets creating more impermeable surfaces and a proportionate increase in runoff — the threat of the current drainage system being overwhelmed and the possibility of more frequent localized flooding increases.

It is why Manteca is spending $1.2 million to devise a game plan to keep storm runoff from increasing localized flooding as the city grow.

The contract the City Council awarded last week to Carollo Engineering combines two critical tasks:

*The update of the storm drainage master plan to make sure the system is improved and expanded in a manner that growth’s impact on runoff is handled as seamlessly as possible.

*Determining what improvements are needed to the French Camp Outlet to carry more runoff to the San Joaquin River as the city keeps growing.

Carrillo previously had been awarded the French Camp Outlet Canal master plan but process was delayed by the seven years it took to update the city’s general plan.

That delay, by the way, was not the root cause of flooding recently in the new Yosemite Greens subdivision along Airport Way next to the municipal golf course.

The issue zeroed in on work that was being done on a project between the neighborhood and the French Camp Outlet that is less than half a mile away.

SSJID crews had cleared the culverts downstream in the outlet to keep the heavy runoff flowing into the San Joaquin River near French Camp.

The storm drainage master plan includes creating an outlet 100 percent under the city’s control to drain much of southwest Manteca into the river at a point west of the Oakwood Lake Shores gated community.

Developers in that area created large temporary retention basins that are able to hold runoff to prevent flooding in southwest Manteca. The future outlet near Oakwood Lake will provide a permanent solution.

Most of the city’s system piggybacks on the SSJID French Camp Outlet along the railroad line that does double duty as the city limits between Manteca and Lathrop.

It originally was out in place to drain the irrigation system long before the city was incorporate in 1918.

The masterplan will address:

* Inventory and assessment of existing storm drain infrastructures

*Updates to the City’s hydrologic and hydraulic models

*Updates to the French Camp Outlet Canal hydrologic and hydraulic models

*Identification of system deficiencies and recommended improvements

*Prioritization of recommended improvements

How the system works

 The city’s elaborate system of storm retention basins — most of which have been put in place since 1989 in conjunction with neighborhood parks in new development — serve as temporary holding ponds.

How the system work is simple.

Telemetry systems sense when water levels are getting too high in the outlet.

It sends messages to divert runoff from rooftops, streets, sidewalks, driveways, parking lot, and other impervious surfaces into the closest storm retention basin.

The water is held in the storm basin until capacity opens up in the outlet canal.

 The SSJID — that owns and maintains the outlet canal that they have allowed much of the city to drain into — seven years ago noted a need to address outlet capacity based on Manteca’s continued growth.

The city agreed and a hydrology study was done to determine what improvements were needed.

The study was completed in 2021. The SSJID accepted the study but the city wanted to relook at the conclusions. That is as far as the outlet study apparently got in terms of devising a game plan and then implementing it.

It is important to remember the French Camp outlet was designed and built to address agricultural needs only when Manteca had less than 2,000 residents.

That means it was designed only to handle any excessive flows from canals during the irrigation as well as runoff in the wet season that makes its way into the canals when there are no irrigation water deliveries.

To contact Dennis Wyatt, email dwyatt@mantecabulletin.com