Developers could do more to prevent street flooding. Miami-Dade is cracking down
To combat neighborhood flooding, Miami-Dade County is rolling out new construction rules that require driveways, pool decks and patios to let rainwater pass through to the soil below, instead of running off onto properties next door.
The countywide rules expand drainage standards that are already in place for larger projects, now imposing the same rules on lots smaller than two acres. Instead of traditional asphalt driveways and concrete pool decks, builders of new homes and townhomes would need to use materials designed to let rainwater pass through into the ground below, rather than out into the street or onto neighboring properties.
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The new law Miami-Dade commissioners passed unanimously on Wednesday won’t have much immediate effect, since the rules only kick in for construction of new homes or when an owner seeks a new driveway or pool deck for an existing building.
But the administration of Mayor Daniella Levine Cava sees the new law putting Miami-Dade on track to better manage the downpours that are becoming more intense as the Earth’s climate changes from rising temperatures.
“By increasing the amount of rainfall that can be filtered through and captured in the ground, we’re reducing our community’s risk of flooding and keeping the dirty water out of Biscayne Bay,” Levine Cava said in a statement after the vote.
The change is designed to prevent downhill flooding from new homes that are built to the higher elevations required by modernized flood-plain standards. Those new homes can leave neighbors in older, lower-lying buildings facing runoff rainwater from pricier construction next door.
Flooding by rain — also known as “stormwater” — is one of the main inland concerns as Miami-Dade plans for a future where a changing climate is leading to higher sea levels and more frequent torrential downpours.
With heavy rains able to flood suburban streets miles away from the ocean, higher tides also make it harder for the county’s canal system to collect excess water from the ground and lead it into the sea.
Rainwater that gets swept from a driveway into a street also adds to the flow of fertilizer, animal waste and other suburban pollutants that would otherwise be filtered and retained by backyard soils.
“It really deals with how to better control flooding and how to improve water quality,” said Marina Blanco-Pape, who oversees stormwater regulations within the county’s Department of Regulatory and Economic Resources.
The sweeping law passed without debate at the commission meeting, with no objections voiced by the building industry.
The law applies to Miami-Dade properties not within any city’s limits. The legislation will soon also impose the new requirements on new construction within the county’s 34 cities, with deadlines for municipal governments to adopt their own drainage standards that are at least as strict as Miami-Dade’s new rules.
The rules require lots with new construction to retain all of the water that would fall in the kinds of downpours that are so extreme they’re expected to happen only four times a century — a frequency known as a 25-year storm event. Depending on a lot’s elevation, that could require constructing a retaining wall around the property.
For most home sites, builders will need to swap out traditional materials for ones that prioritize drainage. That could mean replacing an asphalt driveway with pavers that allow grass to grow in between the concrete. Or they could use concrete designed to be more “permeable” — that is, with a surface that allows water to seep through the surface instead of pooling on top.