2008 Illinois Lake Management Association Shoreline Erosion Control Failures and How To Avoid Them by: Hank Sutton, President, Lake Rip Rap, Inc. www.lakeriprap.com - www.macoupinboats.com Setting the stage for a natural shoreline healing
We enjoy our Successes
but we learn from our Failures.
Circa 1970 2007 37 years of Shoreline Erosion
To avoid failures, we need to have a basic understanding of the shoreline erosion process.
Terms of the Typical Eroding Shoreline Limits of wave energy (Zone B ).... Wave energy is confined to Zone B. The soil in Zone B is taken into suspension by wave energy and is transported to deeper water, where it settles to the bottom and remains indefinitely as sediment. Wave energy undermines Zone A and soil falls into Zone B where it too is removed by wave energy. The soil in Zone C is not effected by wave energy. As soil is removed from the bank to deeper water, the bank retreats inland.
Shoreline erosion after 2 years Shoreline erosion of an unprotected shoreline
Shoreline erosion after 2 years Shoreline erosion after 30 years For Illustration Only
original slope original shoreline full pool
Under Designed No Filter Fabric Not enough riprap to dissipate the wave energy.
Full Pool Low water reveals a failed revetment No Filter Fabric - No Apron
No filter fabric & no apron
Riprap subsidence without filter fabric The riprap subsides as the underlying soil is removed. Filter Fabric - Isolates the substrate from. wave energy & - Spreads the weight of. the riprap
Placing Filter Fabric
Pinning Filter Fabric in Place Pin Driver
Pontoon boat helps keep fabric spread
Building the apron Free fall of the riprap should be held to a minimum to prevent damaging the fabric
Another method of protecting the fabric is to allow the riprap to drop onto the existing rock and then cascade onto the fabric.
Filter fabric not fully spread (bunched-up).
Filter fabric too narrow. proposed toe Minimum width required 15 wide rolls work well in most cases.
Upper limit of Zone B full pool Riprap subsides as substrate material is lost.
Failed due to substrate loss No Filter Fabric
effective lean revetment unprotected shoreline
The result of substrate loss & riprap subsidence Full pool 40 feet This shoreline has retreated 40 feet since the failure. No Filter Fabric (Water level: 60 below full pool )
Failed in less than 10 years Original shoreline 42 below full pool No Filter Fabric Inadequate Toe Protection & Low Quality Riprap
apron Low water reveals an effective apron on filter fabric
Looks Good.
Except
The structure is being threatened by underwater substrate loss. No filter fabric and no apron. Filter fabric and a well built apron could have prevented a possible failure at this location
~ 1:1 surface slope under size RR-3 One mile fetch Fabric Exposed Full Pool Surface slope too Steep for the Riprap Size
3:1 surface slope under size RR-3 One mile fetch Smaller riprap can be used if the surface slope is reduced.
It would not be practical to reduce the surface slope at this location, therefore, larger riprap must be used to provide stability.
Under-Designed Surface slope too steep for the riprap size No filter fabric
Specify DOT Approved, High Quality Riprap Low quality riprap is broken down by the stresses of freezing & thawing. This 400 pound riprap was reduced to rubble in only 15 years
1997
1999
2002
2005 original riprap location original slope Low water reveals a failed riprap revetment Low Quality Riprap & No Filter Fabric
Breakwater built by: Macoupin Boats Shoreline naturally colonized. Before
Bank sloughing & Natural Healing
An example of natural healing
An example of natural healing
After 5 years After 3 years An example of natural healing
1999 Early Spring 2004 7 years of Post Construction Geomorphology Summer 2004 2006 Nearing its angle of repose No vegetation yet
Accurate Quality Control insures effectiveness without wasting material.
Stakes indicate the limit of the apron
Monroe City Reservoir Bio-engineered Shoreline Stabilization If the rock toe subsides, the bio-engineered elements will be undermined and are likely to fail. rock toe apron filter fabric To avoid subsidence, the rock toe should be designed and built with: High Quality Stone Filter Fabric & Protective Apron
Boat Damage Run-away duck blind Physical damage caused geo-tubes to fail. Snow mobile damage Failed geo-tube
Repaired Geo-tube Structure with natural plant colonization.
No filter fabric & no toe protection With filter fabric and toe protection, sand bags filled with dry concrete mix can be effective.
Coconut fiber log with no additional toe protection. Failed in the existing high wave energy at these locations
Coconut fiber logs can be successful in lower wave energy.
No filter media behind the wall - No toe protection
16 lost Full pool Original Bottom No Toe Protection Wave energy is reflected off a bulkhead in all directions - energy reflected horizontally creates intersecting waves - energy reflected up creates a splash - energy reflected down erodes the unprotected substrate
The foundation of this sea-wall has been undermined by substrate erosion. Note: Photographed at Low water
Failed steel sea-wall. No toe protection Inadequate embedment
Soil lost behind the failed sea-wall.
Riprap toe protection corrects two serious problems: 1) Toe Erosion (Scour) & 2) Wave Reflection Some Lakes require riprap toe protection for all sea-walls. This area was starting to failing when the riprap was installed.
The Failure of Over Design - Unnatural appearance - Wasted material - Impedes vegetation. and wildlife access
The Failure of Over Design This bank could have healed naturally with a lean revetment.
With a high, steep bank, Lake Rip Rap usually recommends: 1) In remote areas: Depending on the littoral slope, either an off-shore breakwater or lean riprap revetment followed by natural healing. Lean Riprap Revetment Setting the stage for a natural healing If quicker results are needed: 2) In residential or park areas: Bio-engineered upper bank with a Riprap Toe on Filter Fabric.
Shoreline erosion control using conventional equipment can be very destructive. Macoupin Boat working in the Shawnee National Forest with no environmental damage to the wooded slopes..
Shoreline Improvement? Or is it For Illustration Only
Conclusions: Most Common Causes of Shoreline Failures 1. No Filter Fabric or fabric not properly installed 2. Inadequate toe protection (no apron or toe trench) 3. Under-designed or Over-designed 4. Surface slope too steep for the size of riprap used 5. Low Quality riprap If these 5 problems are avoided, and if the structure is monitored and maintained, it should be affordable & effective for a life time.
Why does the west bank erode as fast as the east bank?
The east bank of our local reservoirs receive more wave energy than the west bank because of the prevailing wind direction. N Prevailing Wind Lake Shelbyville West Bank East Bank
Coffeen Lake Montgomery County, Illinois West Bank East Bank
Clinton Lake DeWitt County, Illinois North East Bank South West Bank
Late Fall Undisturbed, compact, cohesive soil is fairly resistant to wave energy. Frost heave greatly weakens cohesive soil, increasing its erodibility in the Spring. After the Spring thaw, the weakened soil is quickly removed by very little wave energy. Early Spring Photo Credit: Jack Nawrot
Kinkaid Lake, Murphysboro, Illinois Early Spring Photo credit: Jack Nawrot The bank retreat in March alone may be 10 times that of all the other months combined. An annual event. A few weeks later
This soil weakened by frost heave was removed within 3 weeks after the Spring Thaw. Gillespie New Lake
Other factors to consider Stronger winds in the spring Different Soil Types & Strength (varying mixtures of sand, silt & clay) Depth of frost penetration Soil Moisture Content
Is it possible that the existing soil is so extremely weakened by frost heave that: Shoreline erosion is as severe on the west bank as on the east bank, even though the west bank receives less wave energy. Most shoreline erosion and retreat occurs within a month after Spring thaw and Frost Heave may actually effect shoreline erosion as much as prevailing wind direction?