What Causes Shoreline Erosion on Petenwell and Castle Rock

Wave Action

Both Petenwell (23,000 acres) and Castle Rock (16,000 acres) have significant open water fetches — the distance wind can travel across open water before reaching your shoreline. Larger fetches produce larger waves. Summer afternoon winds on these lakes can produce sustained wave action that erodes unprotected sandy or loamy banks consistently over the course of a season. Properties on south or west-facing shorelines exposed to prevailing wind directions experience more wave erosion than sheltered coves or north-facing shores.

Wake from Boat Traffic

Wake from powerboats, pontoons, and personal watercraft contributes to shoreline wave energy even when direct wind waves are minimal. High-traffic areas of both lakes experience cumulative wake erosion that compounds over the course of a summer season.

The Drawdown Cycle

The annual fall drawdown exposes the upper shoreline — the zone between summer full pool and winter low pool — to freeze-thaw cycling, precipitation erosion, and the physical disturbance of exposed sediment. Unprotected shorelines in this zone accumulate erosion damage during each drawdown period that compounds year over year.

Approved Erosion Control Methods

Riprap

Rock armoring (riprap) is the most durable and widely used erosion control method on these lakes. Properly installed riprap — native stone of appropriate size and gradation placed on a geotextile filter fabric base — effectively dissipates wave energy and prevents bank erosion. Riprap installation on Petenwell and Castle Rock requires a Wisconsin DNR permit under Chapter 30. The DNR has a streamlined general permit for riprap replacement or repair under 200 linear feet. Larger or more complex riprap projects require individual permits. Castle Rock and Petenwell are specifically called out in the DNR's shoreline erosion permitting guidance.

Bio-Stabilization

Bio-stabilization uses native shoreline plants, live stakes, coir fiber rolls, and similar organic materials to stabilize the bank and reduce erosion. It is more ecologically aligned with Wisconsin's shoreland buffer zone requirements than hardened armoring. Some bio-stabilization methods may be eligible for abbreviated or general permit processes. The drawback for high wave-energy shorelines is that bio-stabilization alone may be insufficient — it is often most effective as a complement to riprap in the upper bank zone above the wave energy line.

Seawalls

Concrete or steel seawalls are permitted for severe erosion situations but are subject to more restrictive DNR review than riprap. Seawalls that improperly deflect wave energy can actually increase erosion on adjacent properties. The DNR generally favors riprap over seawalls for lake shoreline erosion control. Seawall installation on these lakes requires an individual Chapter 30 permit.

What Requires a DNR Permit

The General Rule

Most shoreline alteration work within the ordinary high water mark of navigable waters in Wisconsin requires a Wisconsin DNR permit under Chapter 30 or Chapter 281 unless it qualifies for a specific exemption. This includes: placing riprap, installing or replacing seawalls, placing fill in the water or on the shoreline, and dredging. For Petenwell and Castle Rock specifically, individual permits are required for breakwater installations regardless of size.

The General Permit Option

Wisconsin DNR offers general permits (streamlined review) for certain shoreline erosion control work that meets specific criteria — including riprap replacement or repair under 200 linear feet on inland lakes and rivers. If your project qualifies for a general permit, the process is faster and less burdensome than an individual permit. Apply through the DNR's ePermitting system. Always check current DNR requirements before starting any shoreline work.

When to Do the Work

The annual drawdown period — fall and winter when the shoreline is exposed — is the optimal window for shoreline erosion assessment and repair. Exposed erosion damage is visible and accessible for inspection. Riprap installation and repair work in the exposed upper bank zone can be completed before the shoreline refills. Plan shoreline maintenance work in fall following dock removal, when the shoreline is both accessible and visible at its worst condition.

Frequently Asked Questions