Drainage redesign for an industrial outdoor storage redevelopment in Medley, Miami-Dade County

What the project was

An industrial site of just over seven acres in Medley, in Miami-Dade County, was being redeveloped as an outdoor industrial storage facility. The site was already industrial, and that is the part that mattered: it was entirely impervious. Every square foot of the 7.04 acres the drainage study computes on was building or pavement, drained by a 689-foot underground detention run installed for its previous use.

RSP was the civil engineer for the stormwater design and carried it through three separate approvals: the Town of Medley for the site plan, Miami-Dade County's Department of Regulatory and Economic Resources for the construction documents, and Miami-Dade DERM for the surface water management system itself.

The constraint

A redevelopment on a fully impervious site starts from an unusual position. There is no pre-development greenfield to compare against and no margin to give back: the existing condition is already the worst case, and the question a reviewer asks is not whether the project makes drainage worse but whether it can be made meaningfully better while the site stays operationally useful.

Outdoor industrial storage is demanding in that respect. The site's value is its yard, so area given to drainage is area taken from the use. The design therefore had to find its storage underground and at the edges rather than in the middle of the yard, and had to do it in south Florida conditions, where the water table is high and storage is won by exfiltration into the surrounding ground rather than by deep basins.

Two further constraints were fixed before design began. The site sits in FEMA flood zone X, and the building was to be flood-proofed to elevation 8.00 feet. And the Town of Medley's own approval made the sequence explicit: DERM approval had to be confirmed before the Town would issue a building permit, so the drainage design could not be left to be resolved later.

What the work involved

The starting point was a drainage study of the existing and proposed conditions, computed on the site's own land use rather than on assumptions. The pre-development breakdown establishes the baseline that everything else is judged against: 306,662 square feet, of which 19,128 square feet was building and the remaining 287,534 square feet other impervious surface, at an average grade of about 6.7 feet.

Runoff was modelled by the SCS method against South Florida Water Management District rainfall for the 5-year 24-hour, 10-year, 25-year and 100-year 72-hour storms. On the pre-development site, with no infiltration available anywhere, rainfall excess equals rainfall: the 100-year 72-hour storm produces 17.4 inches of rainfall and 10.21 acre-feet of runoff, with no part of it retained on site.

The water quality calculation ran the same way, pre against post. Miami-Dade requires treatment of the greater of the first inch of runoff over the site or a depth set by the percentage of imperviousness. On a wholly impervious site the second governs at the full 2.50 inches, giving 1.467 acre-feet to be treated.

The proposed system was then sized against those numbers. It replaces the 689-foot detention run with an underground network of 18-inch perforated pipe on exfiltration trench, laid out across the yard in runs of roughly 150 to 260 feet at slopes between 0.4 and 0.8 per cent, with a retention area added at the site's edge. Trench length was computed with the standard exfiltration equation, using a hydraulic conductivity of 2.53 x 10-4 cubic feet per second per square foot per foot of head, a trench width of 6.5 feet, and a safety factor of two on 150 per cent of the water quality volume. That calculation requires 2,625 feet of trench; the design provides 2,943 feet.

Alongside the drainage, the site was regraded to drive water to the new inlets, repaved in heavy-duty asphalt for the loads outdoor storage puts on a yard, and planted to the Town's landscape requirements. Every exfiltration trench carries a pollution-retardant baffle set at least 1.5 feet below control elevation, which is what keeps floating pollutants out of the ground in a system that discharges by infiltration rather than to a surface outfall.

Agencies and approvals

Three separate authorities reviewed this project, each looking at something different, and the design had to satisfy all three.

Miami-Dade DERM's Water Control Section approved the drainage system under Surface Water Management General Permit 13-80171-P, DERM file 230509-80. DERM's review is of the surface water management system as a system: whether the storage and treatment proposed are adequate, and whether a design that disposes of its water into the ground does so without carrying pollutants with it.

The Town of Medley issued administrative site plan approval PSPR2023-0008 on 9 November 2023. The approval covers the land use side: a 2,700 square foot structure with bathroom facility, lot coverage of 3.82 per cent, 12.32 per cent open space, eleven parking spaces, and landscaping of 106 trees and 1,074 shrubs, all of them native, with ten street trees at no more than 35 feet apart. That approval carries the condition that DERM approval be confirmed before a building permit issues.

Miami-Dade County's Department of Regulatory and Economic Resources stamped the construction document set, including the drainage report, as a Job Copy on 12 March 2024.

The outcome

The redevelopment leaves the site draining better than it found it, which is not the usual direction of travel for an industrial project.

Impervious cover falls from 100 per cent to 87.7 per cent. The 37,772 square feet of pervious area created - 12.3 per cent of the site - includes a retention area of 0.447 acres, where before there was none. Underground detention goes from 689 feet to 2,943 feet of pipe on exfiltration trench, more than four times the storage length, now with infiltration capacity the previous system did not have.

The modelled runoff falls at every storm tested. The 100-year 72-hour storm drops from 10.21 to 9.53 acre-feet, the 25-year from 7.57 to 6.90, the 10-year from 6.04 to 5.39, and the 5-year 24-hour from 4.05 to 3.42. The water quality volume required falls too, from 1.467 to 1.279 acre-feet, because the treatment requirement is driven by imperviousness and the imperviousness went down.

For an owner, the useful summary is that the yard stayed a yard. The storage that made those numbers move sits under the pavement and at the site's edge, and the operational area the redevelopment was for was not spent on drainage.