Sandy soil and a high water table: what it means for drainage in Central Florida

Why does water stand in my sandy yard after rain?

Usually because the sand below is already full of water, not because the sand is slow. In USDA soil survey data, the top layer of Central Florida's common sands passes water at a representative rate of about 13 inches an hour or more, roughly four times the 3.34 inches a 25-year storm drops on Orlando in its worst hour.

The limit is room. Rain soaks down until it meets the water table, below which the soil is saturated. A UF/IFAS rule of thumb is that 1 inch of rain raises the water table about 6 inches in most flatwoods sandy soils. By that rule, a water table 18 inches down has room for about 3 inches of rain before it reaches the surface. A 2-year storm in Orlando is 4.28 inches in 24 hours, and a 25-year storm is 7.53 inches.

Once the soil is saturated it cannot absorb much more, the USGS notes, and further rain runs off or stands. UF/IFAS gives three reasons most Florida flatwoods soils are poorly drained: low elevation, a flat landscape, and a restrictive layer that normally sits below the root zone.

What is a seasonal high water table?

It is the highest level the groundwater reaches in a normal wet season. Florida's wetland rule defines seasonal high water as the elevation ground and surface water can be expected to rise to in a normal wet season, and USDA gives it for each soil as a depth and a number of months.

In Central Florida's flatwoods soils that depth is shallow. USDA gives Myakka and Smyrna a water table within 18 inches of the surface for 1 to 4 months in most years, and says Smyrna's rises above the surface briefly in rainy seasons. On the sand ridges it sits far lower: deeper than 80 inches in Candler.

USDA's hydrology handbook treats a shallow water table as decisive. It puts a soil in group D, its highest-runoff group, solely because the water table is within 24 inches of the surface, even when the soil itself passes water well. If the soil can be drained, it gets a dual group such as A/D: the first letter for drained, the second for undrained.

Which soils are common in Central Florida?

Six sandy soil series cover large parts of Orange, Osceola, Volusia, Brevard and Lake counties. Their USDA descriptions fall into two groups: flatwoods sands with a shallow seasonal water table, and ridge sands where it sits deep.

Swipe the table sideways to see all four columns →

Drainage class and water table: each series' USDA Official Series Description. Acres and runoff group: USDA Soil Data Access, the data behind Web Soil Survey, for the five county surveys, rounded. Group A passes water most freely, D least.

Myakka is Florida's state soil by statute, and its type location, the spot USDA describes it from, is in Lake County. Smyrna's is in Volusia County. NRCS's Florida Drainage Guide lists Myakka, Smyrna, Immokalee and Basinger among the Florida soils that commonly require drainage; Candler and Tavares are not on its list.

When is Central Florida's wet season?

June through September are the wettest months, inside a rainy season that runs from May through October. The St. Johns River Water Management District says that season typically brings up to 70% of the year's rain, with most of its gauges getting 5 to 7 inches in each of the wettest months.

At Orlando International Airport, NOAA's 1991 to 2020 normals give 29.57 inches from June through September, about 57% of the 51.45-inch year. The seasonal high water table is, by definition, the wet-season level, so a big summer storm lands on sand with the least room left. See the 25-year storm for 28 Central Florida cities.

Does a French drain work with a high water table?

Yes, if the water it collects has somewhere lower to go. In wet sand a buried perforated drain takes in groundwater as well as rain, which is how USDA drains these soils: its Florida Drainage Guide gives buried-drain depths and spacings for Myakka, Smyrna, Immokalee and Basinger.

What a high water table takes away is the outlet. A pipe drains by gravity only to a point lower than itself. Where gravity outlets are lacking, the same guide says a dike and pump are normally required, and it gives that note for Basinger. For Myakka and Immokalee it also warns of a high potential for buried drains to plug with iron.

That is why the FAS Drainage Standard measures fall on site before design, at least 1% from inlet to outlet, and why we choose pipe for the fall the ground actually has. How much slope a French drain needs · See yard drainage and French drains.

When is a pump or dewatering needed?

A pump is needed when the water has no workable gravity route. Dewatering is for ground that stays saturated after ordinary surface drainage has not fixed it.

A sump and discharge system collects the water in a basin and pumps it out along a discharge route. We check the incoming water, the basin, the discharge point, power and pump controls. See sump and discharge systems.

Permanent wellpoint dewatering is a permanent pumping approach for water below the surface, used where site conditions support it. The first step is to tell a below-ground water problem from surface runoff, and that takes site information: a flood simulation cannot confirm groundwater conditions. See permanent wellpoint dewatering. Where water collects against the building, start with foundation drainage.

Why does it vary from lot to lot?

Because soil and water table change from place to place. In Lake County alone, the USDA soil survey maps about 100,000 acres of excessively drained Candler and about 61,000 acres of poorly drained Myakka.

NRCS calls drainage highly site-specific and says field conditions and local experience must guide the actual design. So we measure on site. Fall is measured on site before design, the second point of the FAS Drainage Standard, and for water below the surface we gather site information before proposing a system.

General guidance, not a design for your property.