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More rooftops & paving means more flooding

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POSTED February 23, 2017 1:11 a.m.

No sooner had Coyote Creek overflowed in San Jose on Monday forcing 225 residents to flee knee deep water that the buzz started about how climate change is making flooding worse.
Let’s get real for a second. The nine-county San Francisco Bay Area had 4.6 million residents in 1970, 6 million in 1990 and 7.7 million today.
The population has increased by 60 percent. That means more roof tops, more shopping centers, more streets, more office complexes, more sidewalks, and more patios.
All of those urban improvements are impervious. Water runs off instead of being soaked into the ground. The more homes and the more development, the more run-off.
It means a storm of the same magnitude that pelted the Bay Area 30 years ago would have generated significantly less storm runoff.
California in 1965 when the State Water Project was completed had 16.8 million residents. Today we have 39.5 million. Again that means there are a lot more rooftops, roads and other paved over surfaces.
Not only do more people create a demand for more water but they also create a bigger demand for flood control whether it involves rivers or creeks.
This isn’t climate change, folks. It’s basic hydrology.
If the world was run by engineers — the ones that build things that don’t involve silicon chips and the Internet — things might be a bit different.
Take storm run-off as an example. There is now a federal mandate requiring developments of a certain size to contain or significantly reduce storm run-off onsite as a way of easing the impact on streams for reasons primarily concerned with keeping pollutants out of the water.
To a degree, it makes a lot of sense. Rain water off a parking lot carries oil residue and such as well as dirt and even litter. The more water than can be contained onsite, the less runoff into streams.
Some areas such as Los Angeles have gone as far as incorporating cisterns in residential backyards to harvest rain for irrigation purposes. It essentially kills two birds — water demand and runoff control  — with one stone.
The biggest urban development trend in response to the federal mandate is the employing of French drains. This involves trenches filled with gravel or rock. French drain systems are already in place in Manteca especially along Moffat Boulevard at the Manteca Veterans Center and Manteca Veterinary Clinic.
While you will see more and more French drains as Manteca and other communities keep developing the real question is why teeth aren’t being added to systems engineers have designed.
Manteca’s storm drain system that includes retention basins and pipes that typically flow into irrigation canals and into the French Camp Slough to reach the San Joaquin River also has a passive component factored in — residential front yards. It is why every residential lot created in Manteca by municipal ordinance can only have a set percentage of front yards covered with impervious surfaces such as concrete. That is to allow some of the rain water to stay in yards and soak into the ground instead of running off into the gutter, then down a storm drain and out into the river.
While many of us seem to be against the idea of more government regulation it is arguable more of us are against flooding.
It is why it is curious that very few jurisdictions have permit requirements for driveway expansion that is done essentially to pour concrete to park more cars or recreational vehicles.
Pouring more concrete in front yards sends more rainwater down the storm drain and into rivers. Runoff from 200 square feet of cement might not sound like a lot but multiple it by a million homes and you’re talking some serious water.
It would make sense for Manteca to require driveways expanded beyond what is in place when homes are first built to be “grass driveways” employing a pattern of pavers and grass to allow for drainage.
Such a concept may sound alien to homeowners in 2017 as it is reminiscent of 1930s era “Hollywood driveways” consisting of two ribbons of hard material such as concrete with grass or other or low lying vegetation in between. It also serves the same purpose as paver-grass driveways. Both have the added benefit of slightly reducing ground heat generation in the summer months.
The city might also want to consider homes with bigger impervious footprints of 2,500 plus square feet of roof top to have 1,000-gallon underground cisterns installed at the time the home is built.
All of this might strike you as ridiculous but consider this: Demographers project California will have 60 million residents by 2050 or 50 percent more than out current population. That means 50 percent more roof tops, parking lots, sidewalks, and such.
Not only do we need to plan for securing water supplies through more storage but we also need to plan to reduce the inevitable increase in runoff that means we will have more and more Coyote Creeks with greater frequency.

Disclaimer.


 



This column is the opinion of executive editor, Dennis Wyatt, and does not necessarily represent the opinion of The Bulletin or Morris Newspaper Corp. of CA.  He can be contacted at dwyatt@mantecabulletin.com or 209.249.3519.

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