For years, Lake Austin homeowners have built stout vertical walls along the shore to keep their land from slowly slipping into the water.

The so-called bulkheads — usually made of metal, stone blocks or concrete — repel waves stirred up by big boats.

But city scientists say the redirected waves wipe out plants along the shore that serve as habitat for fish and animals and that filter pollutants from the lake, a source of Austin's drinking water.

The city is proposing new rules, which the City Council is scheduled to consider Oct. 28 , that would require any new such walls to be sloped and their exteriors made of natural materials, such as rocks and plants.

Some contractors and property owners say the rules go too far, but supporters say they will protect the shoreline from erosion and more gently absorb waves.

"Lake Austin is critical to our drinking water supply and is a huge recreational resource, so it's important to keep it in good shape," said Carol Lee , a city parks board member who supports the changes.

The rules would also clarify that lakeside property owners must get city permits to build trams that shuttle homeowners from the lake's cliffs down to the shore and that city scientists say also threaten the lake's ecology.

There are about two dozen trams along Lake Austin, and vertical walls line about half the shore. Landowners would not have to tear down those features, only get the city's permission if they want to make major changes to them or build new ones.

The city already requires permits for bulkheads but does not have clear enough design rules for them, said Andrew Clamann, an environmental scientist in the city's Watershed Protection Department.

Future walls would have to be stepped or sloped — less than a 45-degree angle — and be made of materials the city recommends to soften waves. The walls also couldn't add to a homeowner's property by extending their shoreline or filling in land that is currently under water.

Some who build and design bulkheads say the new types of walls and materials will cost more, be less effective and limit homeowners' access to the water.

Many lakeside homeowners, especially those on coves or inlets, don't have enough waterfront to fit slopes of rocks and plants, said Rusty Signor , a marine contractor who for 35 years has built bulkheads and docks along Lake Austin and other Texas lakes.

"This would disenfranchise them from having usable lakefront property," he said.

A vertical bulkhead creates a defined edge with sand and shore beside it, making it easy for people and boats to enter the water, said John McIntyre , a structural engineer who has designed bulkheads along Lake Austin for 20 years. Rocks stacked into a slope can be dangerous for people or boats, he said. They can also generate slippery algae, collect trash and serve as a haven for rodents and snakes, he said.

There are variations on vertical walls that would work just as well without harming the shore's ecosystem, he said.

"There is not just one configuration of shoreline along Lake Austin. It varies from very steep to very shallow soils," McIntyre said. "We don't want to be confined into only doing a vegetated shoreline, or be hamstrung by artificial rules."

Mary Neely , a supporter of the changes and a member of the city's environmental board, said residents could still have docks or find other safe ways to enter the water.

This effort "is about the future of the lake. The shore is an area of important biological activity that we can't disregard," she said.

The vertical walls cost about $200 per linear foot; the new designs and materials could cost at least $350 per linear foot, Signor said. He also said there is also no proof that slopes made of rocks and plants are strong enough to repel the waves generated during busy boating days on the lake.

Sloped bulkheads "just don't work. They aren't going to mitigate waves to the degree that the city thinks they will, especially not when some boats throw three or four wakes," said Brian Bailey , who has owned a home on the lake for three years and built a new vertical bulkhead there.

Clamann said engineers would still have leeway to come up with innovative designs. The materials the city would require have a wide range of cost and have proved effective in other places, he said.

The aim of the changes is to "protect both the landowner and the environment," he said.