Guides

Why lake banks erode, and how to tell if yours is

Lake banks erode because the water works on the waterline every day, and that strip of soil is soaked and dried, washed by runoff, and pushed by small waves until the grass dies and the sand underneath washes into the lake. A bank fails at the waterline first, which is the part of the bank you see least from the clubhouse. If you can find 2 or more of the 7 signs below on a walk around your lake, the bank is already losing soil and will not hold steady on its own.

Why the waterline is where a lake bank fails

A lake bank is a slope of soil held in place by the roots of the grass growing on it. As long as the grass is healthy all the way to the water, the roots knit the top layer of soil together and the bank holds. The trouble starts where the grass and the water meet.

That strip of bank lives a hard life. In the wet season it sits under water or stays soaked for weeks. In the dry season it is exposed to the sun and dries out. Few lawn grasses can survive being drowned for part of the year and baked for the rest of it. Once the grass at the edge dies back, the soil beneath it has nothing holding it, and the next rain or the next windy afternoon starts carrying it away.

What causes lake bank erosion

Most banks are losing soil to several of these causes at once. A good repair addresses the ones that apply to your lake, not just the one that is easiest to see.

• Water level swings. Lakes rise in the wet season and drop in the dry season. The strip of bank in between is soaked and dried every year, and the grass there struggles to survive.
• Runoff. Roof drains, lawn runoff and pipe outfalls send water straight down the slope in a heavy rain. Each storm carries some soil with it.
• Wind and waves. Wind across open water pushes small waves into the bank all day. On a large lake that adds up.
• Sandy soil. Much of the soil around community lakes is sand with very little to hold it together once the grass is gone.
• Mowing too close to the water. Mowers that scalp the edge kill the grass that was holding the bank.
• A slope that is too steep. Banks graded steeper than their permit allows, or that have slumped over the years, lose soil faster.
• Fish nests. In South Florida, tilapia dig nesting holes along the shallow edge, and enough of them undercut the bank.

How these causes work together

The causes feed each other. A mower scalps the edge, so the grass thins. The thin grass cannot survive the wet season, so the edge goes bare. Waves and fish nests cut into the bare sand and leave a small overhang. A heavy rain sends runoff over the top of that overhang, and a section of the slope drops into the lake. Now the slope is steeper than it was, which makes the next round of erosion faster.

Why South Florida lakes have it harder

South Florida's wet season runs from May through October. Heavy afternoon storms arrive day after day, lake levels rise, and the soil stays saturated. Then the dry season pulls the water back down and leaves the edge exposed. Add sandy soil, open water that catches the wind, and fish that nest in the shallows, and most community lakes in Palm Beach, Broward and Miami Dade counties are working against every one of these causes at the same time.

7 signs your lake bank is moving

You do not need an engineer to spot a bank that is failing. Walk the lake with a phone camera and look closely at the edge, not just the lawn.

1. The grass stops 1 or 2 feet short of the water.
2. The ground near the edge gives a little under your shoe.
3. There is a small cliff or overhang where the grass meets the water.
4. Pipes or drain outfalls that used to be buried are now exposed.
5. Fences, patios or pool decks near the water have started to lean or crack.
6. There are dips or soft spots in the lawn a few feet back from the edge.
7. The water near the bank turns cloudy after every rain.

If you see 2 or more of these, the bank is already losing soil. It will not hold steady on its own, and every wet season adds to the job.

What each sign is telling you

The first 3 signs mean the waterline has already given way. The grass has died back, and the water is cutting into bare soil. The 4th sign, exposed pipes, tells you the bank has lost enough soil to uncover something that was built to be buried. The 5th and 6th signs are the most serious. Leaning fences and soft spots back from the edge mean soil is moving out from under the lawn, not just off its face. The 7th sign, cloudy water after rain, means the bank is feeding sand and silt into the lake, which slowly fills it in.

How erosion plays out over several wet seasons

Boards often ask whether they can wait a year. The honest answer is that the bank will not wait with them.

In the first wet season after the grass dies back at the edge, the loss is small. You may see a few inches of bare sand and a slight lip at the water.

By the second or third wet season, the lip has become a small cliff. The slope above it is steeper, and soft spots start to show in the lawn.

After several wet seasons, the bank has cut back toward the homes. Fences lean, patios crack, and pipes stick out of the bank. At that point the repair is no longer about putting grass back. It is about rebuilding soil that is gone, and there is more of it to rebuild every year.

The size of the repair grows with each season of waiting. A board that acts when the signs first show up is rebuilding a few feet of edge. A board that waits is rebuilding the slope.

How a board should document lake bank erosion

Good records help the board in 3 ways. They show residents the problem is real, they let contractors price the same job, and they give the association a history if questions come up later about who is responsible. That last question should go to the association's attorney, and our guide on who is responsible for lake bank erosion explains the common arrangements.

A simple record looks like this:

• Photograph each section of bank from the same spot every few months, ideally once in the dry season and once near the end of the wet season.
• Note which of the 7 signs you see in each section, and how far the grass stops from the water.
• Mark exposed pipes, leaning fences and soft spots on a site map.
• Keep any resident complaints with the date and the address.
• Write down when mowing contracts change, since a new crew can change how close mowers run to the edge.

If you want a starting point, our Lake Bank Risk Score asks a short set of questions about your lake and tells you where it stands. For a full record, we offer a free lake bank walk where we measure and photograph every section and give the board a written report it can use whether it hires us or not.

Common mistakes boards make with eroding banks

The most common mistake is waiting for the bank to get bad enough to justify the cost. The repair only gets bigger, and the soil that washes into the lake does not come back on its own.

The second is approving a repair that puts the bank back the way it looked without fixing what holds it. Pushing loose soil back onto the slope and laying sod on top looks finished on the day the crew leaves, but nothing holds the bottom of the bank. We explain the pattern in why regrade and sod fails.

The third is treating each section as a separate emergency. When boards patch whichever bank a resident complains about, they pay for a crew to come back over and over, and they never get a plan. A lake wide survey and a repair phased by lake give the board a schedule it can budget for.

The fourth is ignoring mowing. If the mowing crew scalps the edge, even a well built repair will be under pressure. Ask your grounds contractor to keep mowers back from the water.

What to do once you see the signs

Start with a record of where the bank stands today. Then ask any contractor you talk to the same questions, so the answers can be compared:

1. Which of the causes on our lake does your repair address?
2. What holds the bottom of the bank after you leave?
3. What will residents see while the work is happening, and after?
4. What does your warranty cover in writing, and for how long?
5. Will you inspect the work after it is finished, and how often?

Our approach is to rebuild the slope from the waterline up with leading edge technology that lets the grass root into the bank so it holds. We keep the work at the bank and out of residents' yards. Our work carries a 5 year material and 5 year labor warranty in writing, with inspections in years 3, 5, 7 and 9, each with a written report for the minutes. We do every foot of the work ourselves with no subcontracted crews. Call us at (561) 309 9603.

Questions boards ask

Can a lake bank stop eroding on its own?

Not once the grass at the waterline has died back. The bare soil has nothing to hold it, and the water keeps working on it every day. Without a repair, the bank keeps losing soil and the slope keeps getting steeper.

How fast does a lake bank erode?

It depends on the soil, the slope, the size of the lake and how much runoff reaches the bank. What is consistent is that each wet season makes the next one worse, because every loss leaves a steeper, more exposed slope. Photographs taken at the same spot each season are the best way to measure your own lake.

Is cloudy water near the bank a sign of erosion?

It often is. If the water along the edge turns cloudy after every rain, soil is washing off the bank and into the lake. Over the years that soil fills the lake in and reduces how much stormwater it can hold.

Do tilapia really cause erosion?

Yes. Tilapia dig nesting holes along the shallow edge, and enough of them can undercut the bank. On their own they rarely cause a failure, but on a bank already stressed by water swings and runoff they speed it up.

Should we fix the worst section first or the whole lake?

Start with a survey of the whole lake so you know what you are dealing with. Then fix the worst sections first and phase the rest by lake. Patching only the section a resident complains about usually means paying for the same visit many times.

More guides

• Retention pond bank repair: what boards and managers need to know
• Why regrading and sodding a lake bank does not last
• 7 ways to repair an eroding lake bank, compared