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Land Development Code: Density and impervious cover

Tuesday, November 26, 2019 by Jessi Devenyns

With the first reading of the new Land Development Code draft expected by Council in two short weeks, the Watershed Protection Department is working to finalize its recommendations.

Despite the tight timeline, Environmental Officer Chris Herrington told the Environmental Commission at its Nov. 20 meeting that his staffers have achieved a task he had previously deemed impossible. Herrington said his department was able to straddle Council’s direction to increase housing units while improving water quality, decreasing flood risk and maintaining the current level of impervious cover. “We came much closer than I thought would be physically possible,” he said.

In the draft code, which proposes an additional 397,000 units, the overall increase in the city’s impervious cover is one-fifth of 1 percent. “More than half (of lots) had a decrease in impervious cover or no change in the maximum allowable (from the) zoning entitlement,” said Herrington.

Achieving this minimal effective increase in impervious cover stems in part from proposed additional detention and mitigation requirements for redevelopment. Currently, developers must only provide water detention and quality solutions relative to the amount of additional impervious cover added. In the proposed code, developers will have to provide detention and water quality enhancement for all of the impervious cover on a site, even previously existing cover that was untreated.

This approach, Herrington told the commissioners, will treat 10,000 existing acres of impervious cover that today have no mitigation.

Herrington told the Austin Monitor that smaller developments are now eligible to pay a fee in lieu of providing water detention and quality controls. Small developments are defined as 2 acres or less for water quality treatment. For water detention, these properties are defined as 1 acre or less for residential and less than half an acre for commercial and multifamily development. The city will use the payments from these sites to upgrade drainage infrastructure or construct a new water quality facility that will have a greater impact on the overall water management citywide.

Council Member Leslie Pool told the Monitor that collecting a fee-in-lieu is a conventional approach that Austin has employed for decades. “It’s almost always cheaper than doing (mitigation) on-site,” she said. However, she noted that the fees that are collected rarely add up to the amount of money necessary to construct large water quality treatment or detention facilities.

Nevertheless, she said, when this approach does generate the necessary funds, it has the advantage of allowing for infrastructure where the city has oversight to ensure continued quality.

Lots in the transition zones were carefully considered by Watershed staff. Of those properties slated for an increase in impervious cover, calculations by the Watershed Protection Department demonstrate that almost no additional flooding will occur even if zoning entitlements for impervious cover are maxed out. “It is counterintuitive, but it is based on explicit science,” explained Herrington. The science showed that existing drainage infrastructure is already overwhelmed, so an additional increase in runoff becomes negligible.

Commissioner Pam Thompson noted that the issue is not only the quantity of runoff but the velocity at which water flows. “Although it is a small percentage, where it goes is certainly important,” she said.

Council Member Leslie Pool told the Austin Monitor that the calculation for the effects of additional impervious cover incorporates land from the entire city and does not provide an accurate picture. “If they are giving us a number for the entire city, that would tend to diminish concerns,” she said. To get a clearer understanding of how increasing impervious cover will affect the runoff and drainage in specific segments of the city, she said she’s asking to see the effects of impervious cover on a watershed by watershed basis.

“When we’re talking about things like flooding … I get really concerned that we are not doing enough and we are not being specific enough,” she said.

Transition zone properties are tracts that will be entitled to an increase in impervious cover allowances. However, city staff members are proposing taking extreme caution when allowing for development in transition zones that intersect with floodplains. In the draft, Herrington highlighted that staffers recommend not allowing additional density to be placed in the floodplain unless it is consistent with the regulations recently adopted by Council.

In the draft code, development regulations associated with construction on slopes will also be extended into the urban watersheds where it does not currently apply.

The pervious cover that is left after development is just as critical as the ground that has been covered. To maintain the porous nature of the natural ground, draft code specifies that developers must decompact any soil that was compacted during construction. Herrington told the Monitor that protecting pervious cover is as important as limiting impervious cover. With this change, “we actually think we’ll be adding in functionality,” he said.

Another point of concern was the Save Our Springs Ordinance. “I do firmly believe it is time we revisit the redevelopment exception,” said Herrington. “I do believe that the time is critical for us to address that pollution is occurring every time it rains on the Barton Springs Zone.”

However, he told commissioners that after discussions with stakeholders, the revision of this particular ordinance will take place separately from the Land Development Code rewrite. He said that those conversations will arrive before commissioners next year.

Photo by pato garza made available through a Creative Commons license.

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