Site and drainage design for a mobile batch concrete yard in Palmetto, Manatee County

What the project was

A 1.47-acre industrial infill site in Palmetto, in Manatee County, was being developed as a mobile batch concrete yard with a service shop. The proposal is an industrial concrete mixer, a cement silo, aggregate stockpiles, a loading pit, and a 6,000 square foot building holding a warehouse and a small office, with the paving, drainage and utilities to support them.

The site is three platted lots in an existing industrial park rather than one parcel, and the county made their consolidation into a single lot a condition of the building permit. RSP was the civil engineer, and carried the site and drainage design through the county's final site plan review and a separate state permit for the wastewater system.

The constraint

A concrete batching yard is a demanding stormwater use on a small site. Washdown, aggregate stockpiles and a loading pit all generate runoff that carries material, and the site is only 64,051 square feet in total, so there is little room for a conventional pond and no margin for a drainage layout that gets in the way of the operation. The yard has to stay a working yard, with room for mixers to turn and for stockpiles to be loaded.

The second constraint was below ground. The design water table sits at elevation 8.00 feet, and the site's drainage therefore had to work in a narrow band: deep enough to store and infiltrate, shallow enough to stay above the water table for most of the year. Hydraulic conductivity was taken from the project's geotechnical report rather than assumed, at 8.27 x 10-4 cubic feet per second per square foot per foot of head.

The third was the sanitary connection. There was no gravity sewer the building could fall to, so wastewater had to be pumped, and a pumped connection to a public system is a state matter in Florida rather than only a county one. That put a Department of Environmental Protection permit on the critical path for a system of modest size.

What the work involved

Stormwater is handled by an exfiltration trench system rather than a pond: 1,418.88 feet of trench, 6.00 feet wide and 6.10 feet deep, giving a storage volume of 12,769.93 cubic feet, laid out in runs along the site's edges and beneath the yard. The design uses a safety factor of two on 150 per cent of the water quality volume, which is the conservative end of the method, and the storage sits under the pavement where it does not take operating area.

The trench network is fed by 18-inch corrugated HDPE pipe between ten drainage structures, at slopes from 0.5 to 0.9 per cent. Every structure carries a pollution-retardant baffle. On a site that disposes of its stormwater into the ground, that detail is what keeps floating material out of the aquifer, and on a concrete yard it is doing real work rather than satisfying a note.

The building sits at a finished floor of 13.50 feet NAVD 88, with the yard graded to fall away from it to the inlets. Cross sections were drawn along four lines so the proposed surface can be read directly against existing grade, which on a site being regraded for truck movement is where the design is actually checked.

For wastewater, the design is a private lift station with approximately 74 linear feet of two-inch force main, connecting to an existing force main in the adjacent street and from there to the Manatee County North Regional Water Reclamation Facility. Water service is a one-inch meter with backflow prevention, and fire coverage was confirmed against an existing hydrant flow tested at 1,125 gallons per minute, within the required distance of the proposed buildings.

A stormwater pollution prevention plan was prepared for the construction phase and for the use itself, covering concrete and masonry washout, stockpile containment and sediment control - the specific risks a batching operation brings rather than a generic erosion control sheet.

Agencies and approvals

Manatee County approved the final site plan on 22 February 2024 under PLN2308-0059, project FSP-23-126. The approval runs across the county's own review sections - planning and zoning, public works development review, floodplain, impact fees and the fire district - and it carries conditions, including the consolidation of the three lots into one before a building permit issues and a certificate of level of service valid to February 2027. The floodplain section recorded no objection, the site lying in flood zone X on panel 12081C0168F.

The Florida Department of Environmental Protection's Southwest District issued permit CS41-0182186-501-DWC/CL on 17 July 2025, authorising construction of the domestic wastewater collection and transmission system under Chapter 403 of the Florida Statutes. The permit runs to 2030 and, as these permits do, requires the Department to clear the system before it is placed into service for anything other than testing.

The outcome

The project holds county site plan approval and a state wastewater construction permit, obtained in that order over about eighteen months.

The site keeps 21,649 square feet of open space, 33.8 per cent of its area, on a use that could easily have paved everything. The stormwater storage that made that possible is underground, in trench rather than in a pond, which is what let a yard this small carry a batching operation and still meet its drainage requirement.

For an owner weighing a similar site, the useful observation is about sequence rather than about drainage. A pumped sanitary connection brings a state agency into a project that otherwise sits entirely with the county, and it is worth establishing early whether a site can fall to gravity sewer, because the answer changes who has to approve the work and how long it takes.