Geotubes vs. riprap vs. concrete mats: four methods compared

Why not just riprap?

Riprap is stone laid on a shaped bank over a filter fabric.

What is true about it. It is the simplest method to picture: stone on a bank. People ask for it because it looks natural. The South Broward Drainage District lists riprap, next to sheet piling and geotubes, among the methods used successfully in the district.

What it asks of the owner.

• Upkeep. Riprap is not maintenance-free. Stone moves downslope over time, so the rule is: inspect yearly, top up as needed. The U.S. EPA’s fact sheet on riprap reads the same way: “Inspect riprap areas annually and after major storms.” Weeds grow through the stone, and someone has to keep them down.
• Lawn. Riprap needs a slope between 2:1 and 4:1: two to four feet across for every foot down. That slope comes out of the lawn. And riprap gives you a stone edge at the water, not a lawn.

On a stormwater lake the permit sets the slope before the stone does. In the South Florida Water Management District these lakes are designed with side slopes no steeper than 4:1 (SFWMD Applicant’s Handbook Vol. II, section 5.3.2(e)), so a 2:1 stone slope is not available on every lake.

If you want lawn down to the water, we would not recommend riprap. A geotube is covered with sod down to the water.

The four methods at a glance

Method	What it is	What to weigh	We would not recommend it when
Geotube	A sleeve of woven geotextile, filled with sand, that rebuilds the edge of the bank. Sod covers it down to the water.	The drop-off, a shelf under the water to seat the tube on, and a structure that holds the water level steady.	The level swings about two feet or more between the rainy and the dry season. Or there is no shelf and the drop-off runs past five or six feet.
Riprap	Stone laid on a shaped slope over filter fabric. The stone is typically six to twelve inches across.	Whether you want a stone edge at the water, room for the slope, and someone who will keep it up.	You want lawn down to the water. An excavator and a skid steer cannot get to the bank. Or the water is about six feet deep at the toe of the bank.
Flexamat concrete block mat	A mat of small concrete blocks, unrolled over the prepared bank. Grass grows up through the gaps, and the concrete is still there.	Where water runs: channels and swales that carry stormwater, pipe outfalls and inlets, canal banks. A shaped slope, machine access, and whether concrete that shows at low water is acceptable.	The bank is a steep drop-off: the mat needs a shaped slope, so a collapsed bank has to be rebuilt first. A machine cannot reach the bank. Or you expect a green edge all year: on Flexamat the concrete shows when the water drops.
Vinyl sheet piling	A vertical wall of vinyl sheet piles driven side by side, with a concrete cap.	Whether there is a shelf, and how deep the drop-off runs: a wall fits where there is no shelf under the water and the drop-off runs past five or six feet. Or on a canal bank that drops about four to five feet, where a slope would narrow a channel that has to stay open.	The problem is about six inches of erosion on a lake bank. A wall is the wrong answer there.

We check these on the free site visit, the shelf with a grade rod, not a guess. We do not recommend a method from a photo or from a calculator.

We also do gabion work, by individual inquiry: see gabion baskets.

How each one fails

• Geotube. Bare fabric left in the sun, the wrong fill, or a cut at the waterline. Each is explained in Why geotube installations fail.
• Riprap. The stone slides: rounded stone, undersized stone and a slope that is too steep go first. Or the toe is not keyed into a trench. Or the fabric is cut or exposed, and soil starts leaving.
• Flexamat. A base that was not compacted lets the mat settle. An edge that was not trenched in gets undermined; the manufacturer asks for a toe trench at least 18 inches deep at every edge that takes concentrated flow. For block revetments in general, the Federal Highway Administration’s circular HEC-23 describes water getting under the mat and lifting it, or washing the soil out. That guideline is written for heavier articulating concrete block systems, not for Flexamat.
• Vinyl sheet piling. Water builds up in the soil behind the wall and pushes it toward the canal; missing or corroded anchors let it go. Soil that disappears behind a wall is leaving somewhere; two common paths are under the wall and through a crack.

How long each one lasts, and what it needs afterwards

Which of the four lasts longer has no verified answer, so we do not rank them. We promise no number of years for any of them. This is what is on record.

• Geotube. The City of Miramar, which installs geotubes through its own program, states that they “normally last upwards of twenty (20) years if properly maintained”, and it requires residents to keep the tube covered with grass. Afterwards: mowing on top is fine, and edging at the waterline is the one thing to stop.
• Riprap. The question is upkeep. Someone has to inspect the bank every year and look for two things: stone that has slid down the slope, and fabric that shows. We give no interval for top-ups: that depends on what the yearly inspection finds.
• Flexamat. The manufacturer’s specification and installation guide, as we read them, state no design life in years and no warranty term, so we state none. Its maintenance guide gives four points: mowing is routine once the area is stabilized; do not spray a non-selective weed or grass killer on it; repair rills and gullies at the edges; keep bare soil beside the mat seeded.
• Vinyl sheet piling. CMI, which makes the vinyl sheet piling we use (ShoreGuard), states a 50-year warranty on its vinyl. That is the manufacturer’s warranty on the material, on the manufacturer’s terms. A warranty period is not a service life, and we do not turn one into the other.

What is the same for all four

• Warranty. Five years on workmanship, on what we build.
• Price. We do not rank the methods by cost. Most jobs fall between $50,000 and $500,000, and the realistic minimum is about $10,000. What moves the price is in our guide to cost.
• Permits. For geotube, Flexamat and riprap we prepare and file the paperwork, and the proposal says which permit your job needs. For a wall, which approvals apply varies by city and county; we confirm it for your address.

Next step

The free site visit covers one lake, and we come within 7 days (usually sooner). A written proposal with a price range follows within 2 days of the visit. All four methods are on the technology page.