Summer Storms Overwhelm Drainage

Town relies on retention basins instead of storm drains, leaving community prey to flooding

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Dennis Fendler/Roundup

Last week’s flooding in Payson revealed limitations in the town’s drainage system.

The intense, localized flooding that shocked some business owners in Payson last weekend unfortunately didn’t surprise Town Engineer LaRon Garrett.

Alas, the hilly town’s lack of a major drainage system based on storm drains and big washes means that monsoon flooding will always be with us.

So businesses in low pockets should probably keep sand bags on hand for those intense summer storms, noted Garrett.

New development will probably make things better — but not by much and not in the older areas of town.

“We have no plans to go to a storm drain system, which would be very expensive,” said Garrett.

The town’s street maintenance crews spent all week sweeping dirt from the streets and digging out clogged culverts and drains.

Many business owners in town were surprised to see streets fill up, parking lots turn to lakes and water come sloshing in the front door last Friday and Saturday, as a series of intense storms set a July record. Some of the unofficial rain gauges in town registered two inches of rain in an hour or two.

Many residents who have moved to Payson from places like Phoenix are used to centralized storm drain systems, where water collects in the streets, drops into big buried drains and empties into the Salt River or other major collectors.

Payson perhaps could have laid the groundwork for a storm drain system back in the 1920s and 1930s when the town started developing, but opted instead for a casual, almost unplanned system.

But growth in the town’s core graded and covered natural drainage paths, which means the cost to go back and install a drainage system could prove prohibitive, Garrett said.

The town incorporated in 1973 and didn’t even adopt a comprehensive drainage ordinance until about 1982.

About 40 percent of the neighborhoods in town were built before the town had a drainage ordinance and much of the flooding remains concentrated in those neighborhoods.

The town council overhauled the drainage ordinance in 1999 by requiring any new construction to include retention basis that could hold 100 percent of the rainfall that fell on the lot in 2-, 10- and 100-year storms. That means the retention basins have to be designed to stop and hold both small flows and the heavy rains generated by those once-in-a-century regional storms.

The town raised the standard again in 2006, with a requirement that new development have a grading plan and retention basis that would hold on the site 125 percent of the rain that falls in 2-, 10- and 100-year storms.

That retention-basis based plan has made some some sloping lots all but unbuildable and the town council has already approved exceptions for some developments.

However, even the larger retention basins with the new, tougher requirement still don’t connect to a centralized storm drain system to carry excess water safely through town.

And that means Payson will always suffer flooding from these intense, summer monsoon storms.

That’s because the standards for 2-, 10- and 100-year storms anticipate big storms that hit a large area. They don’t account for the intensity of small, monsoon storms that can dump 1 to 3 inches in an hour in a relatively small area — overwhelming even the best of retention basins. Moreover, the new standards apply only to undeveloped areas.

Payson currently has a population of about 17,000, but the General Plan anticipates a build-out population closer to about 38,000. The plan anticipates relying on a retention basin system even at build-out — which means even the new areas of town will remain subject to localized flooding.

The town probably can’t afford to put in storm drains now and also can’t afford to put in big enough retention basins to handle such intense, brief storms, said Garret.

Retrofitting the town with storm drains would cost millions.

Several years ago the Arizona Department of Transportation did a study on improving the drainage along the highway, but dropped the idea when the price tag for the key stretch came in at $10 million.

Moreover, building big enough retention basins to avoid spot flooding during intense storms would gobble up a huge amount of real estate. “If we designed the system for monsoon storms, half the town would be a retention basis,” said Garrett.

Residents have also long complained that new development often makes the problem worse instead of better. For instance, people working at Famous Sam’s, which was flooded last Friday, blamed water running off a half completed development behind the shopping center.

Garrett said that development got stalled when it lost its financing and so hasn’t completed its drainage improvements, but that new developments that adhere to the current code should actually improve the situation — at least a little — thanks to the requirement that builders retain 125 percent of the rain on site.

But that still won’t prevent flows exceeding the capacity of the required retention basins from gushing on down the hill and causing problems for downstream property owners, he said.

“There’s just no way in this town we can design for the worst possible flood,” said Garrett.

“The new development will help some, but in the older areas — you’re not going to see much change. Until we have a place to dump the water, we’re kind of stuck.

In the meantime, keep those sand bags on hand.

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