Guides

What causes lake bank erosion in South Florida

South Florida lake banks erode mainly because the water level rises and falls with the wet and dry seasons, and the band of sandy soil in between is soaked, dried and washed every year until the grass gives up. Runoff from roofs and outfalls, mowing to the edge, wind across open water and fish that dig along the shoreline all speed it up. Most banks have 3 or 4 of these causes working at once, and a repair only lasts if it deals with the ones on your lake.

This guide walks through the causes that are specific to Palm Beach, Broward and Miami Dade, then gives the board a checklist it can take on a walk around its own lakes. For the general signs of a moving bank, see our guide on why lake banks erode.

Why do South Florida lake banks erode differently?

Lakes in most of the country see erosion from steady rain spread across the year. South Florida is different. The South Florida Water Management District puts the wet season at roughly May 15 to October 15, and says the region typically receives about 36 inches, or about 70 percent of its average annual rainfall, in those months. That pattern shapes almost every cause below.

Water level swings between seasons

Community lakes are built to hold a normal water level, called the control elevation, and to rise during storms and drain back through the outfall. Through the dry months, many lakes sit below that level and expose a strip of bank. Through the wet months, the water sits at or above it, and storms push it higher for days at a time.

The strip of bank between low and high water lives a hard life. Turf grass on it is drowned for part of the year and baked for the rest. Once the grass thins, nothing holds the sand, and every rise and fall of the water pulls a little more of it down the slope. This is why the first damage on most South Florida banks is a small cut right at the waterline.

Sandy soil with little to hold it

Much of the fill around South Florida lakes is sand, often the same sand dug out of the lake when it was built. Sand drains well and is easy to grade, but it has very little binding it together once roots are gone. A clay bank can stand at a steep cut for a while. A sand bank slumps into the water as soon as the edge is undercut, which is why sections can look fine one month and drop a foot the next.

Roof downspouts, swales and outfalls

Homes that back onto a lake often send their roof water straight toward it. A downspout that discharges at the top of the bank, or a side yard swale that funnels water between 2 houses, concentrates a summer storm into one path down the slope. That path becomes a gully, and the gully becomes a notch in the waterline.

Outfall pipes do the same thing from the other direction. Water leaving a pipe at speed scours the bank around the headwall, and sand washes out from behind and beneath the concrete. When you see a pipe end that looks like it is sticking out farther than it used to, the bank has moved back around it.

Mowing to the edge

Mowing crews are often told to keep the grass neat all the way to the water. City guidance supports keeping turf to the waterline: the City of Coconut Creek says sod on stabilized banks should be regularly inspected and maintained to the water line. The trouble is how it is done. Heavy mowers on a wet slope leave ruts, scalp the edge and turn on the soft ground where the grass is weakest.

UF/IFAS takes a different view in its Florida Friendly Landscaping guidance, which recommends a minimum 10 foot low maintenance zone at the water with no mowing, fertilizer or pesticides. Each community has to decide what its rules and residents will accept. Either way, the edge should be cut with lighter equipment and a higher blade, and mowers should not turn on the slope.

Wind across open water

On a large, open lake, wind builds small waves across the full length of the water before they hit the bank. The longer that stretch of open water, called fetch, the bigger the waves at the far shore. South Florida lakes are often long and narrow, built to fit between rows of homes, and the bank at the downwind end of a long reach takes the most wear. Look for the section where a cut at the waterline is deeper than anywhere else on the lake. It is often the one facing the longest stretch of open water.

Tilapia nests

Tilapia are common in South Florida stormwater lakes. The Florida Fish and Wildlife Conservation Commission describes male blue tilapia digging large circular nests with their mouths in shallow water over a sandy bottom. UF/IFAS notes that the nests look like craters on the pond bottom, and that nesting stirs up sediment and reduces water clarity, which can harm submerged plants.

The nests are dug in the same shallow band at the foot of the bank that holds up the slope above it. When many nests cluster along one stretch, sand is moved out of that support zone, and the cut at the waterline has less beneath it.

Armored catfish burrows

Sailfin catfish, often called armored catfish, are found in South Florida canals and lakes. The FWC states that they are known to create spawning burrows along shorelines, sometimes undermining canal banks and lake shorelines, and that females lay eggs in shoreline burrows, holes or crevices between April and September.

Burrows show up as rows of holes just below the waterline, easiest to see when the water drops in the dry season. A bank riddled with burrows can collapse in sections once the soil above has nothing solid under it.

Hurricanes and king tides

Hurricanes and tropical storms hit inland lakes through heavy rain and wind. The lake can rise well above its control elevation for days, and wind driven waves work on a part of the bank that normally sits above the water. Our guide on lake bank damage after a hurricane covers what to do after a storm.

King tides are a coastal issue. Most community stormwater lakes are not open to the tide, so king tides do not act on their banks directly. If your lake drains to a tidal canal near the coast, ask your engineer whether high tides slow its drainage.

Banks that were too steep to begin with

The SFWMD ERP Applicant's Handbook, Volume II, section 5.3.2(e)(1), calls for wet detention side slopes no steeper than 4:1 from the top of bank to at least 2 feet below the control elevation. A bank that was built steeper, or that has drifted steeper, loses soil faster to every cause above. Our guide on the lake bank slope rule explains how to check.

How the causes show up on the bank

Lake bank walk checklist

Walk the banks twice a year if you can: in late spring near the end of the dry season, when the water is low and the foot of the bank is exposed, and in November after the wet season, when the summer damage is fresh. Bring a phone, a tape measure and a copy of the site plan.

1. Start at a fixed point and walk each lake in the same direction every time.
2. At each section, note the lots it sits behind.
3. Photograph the bank from the same spots every visit.
4. Look for a cut or cliff at the waterline and measure its height.
5. Note where the grass stops and how far that is from the water.
6. Check every downspout and swale that points toward the lake.
7. Check every outfall pipe and headwall for washouts behind or beneath it.
8. Look for ruts or scalped grass from mowing along the edge.
9. Note which shore faces the longest stretch of open water.
10. In low water, look for round nest craters and rows of burrow holes along the foot of the bank.
11. Measure the distance from the top of the bank to any fence, patio, pool deck or irrigation line.
12. Write it up the same day and file it with the board's records.

What to do with what you find

Sort each section into stable, moving or urgent. Sections within a few feet of a fence, patio, pool deck or pipe come first. Fixing the cause matters as much as fixing the bank: redirect downspouts, change mowing practices and deal with outfall scour, or the new bank will face the same forces as the old one. Our guide on lake bank repair options compared explains the choices.

If you would rather have the walk done for you, we offer a free lake bank walk. We measure and photograph every section and give the board a written report. When the board is ready, we rebuild the bank from the waterline up with leading edge technology. You can also take the Lake Bank Risk Score or call (561) 309 9603.

Questions boards ask

What is the most common cause of lake bank erosion in South Florida?

The seasonal rise and fall of the water is behind most of it. The band of bank between dry season and wet season levels is soaked and dried every year, the grass there dies back, and the sand beneath it washes into the lake.

Do armored catfish really damage lake banks?

Yes. The FWC states that sailfin catfish dig spawning burrows along shorelines that sometimes undermine canal banks and lake shorelines. Look for rows of holes just below the waterline when the water is low.

Should we stop mowing at the water's edge?

UF/IFAS recommends a 10 foot low maintenance zone at the water, while some cities ask for sod maintained to the water line. Whichever your community follows, keep heavy mowers off the soft edge, raise the blade and do not let crews turn on the slope.

Do king tides affect our lake?

Most community stormwater lakes are not open to the tide, so king tides do not act on their banks directly. If your lake drains to a tidal canal near the coast, ask your engineer whether high tides slow its drainage.

When is the best time to walk our lake banks?

Late spring, when the water is low and the foot of the bank is visible, and November, right after the wet season. Walking at both times shows the full band of bank that the water works on.

Sources: SFWMD wet season news release · City of Coconut Creek Canal and Lake Bank Stabilization · UF/IFAS Florida Friendly Landscaping Principle 9 · FWC Blue Tilapia profile · UF/IFAS Tilapia in Stormwater Ponds · FWC Sailfin Catfish profile · SFWMD ERP Applicant's Handbook Vol II

More guides

• Why lake banks erode, and how to tell if yours is
• Retention pond bank repair: what boards and managers need to know
• Why regrading and sodding a lake bank does not last