Folly Beach 1-yr Post-Construction Monitoring Report

Nicole Elko, Ph.D.

September 8, 2015

Purpose

• Monitor sediment placed during 2014 federal renourishment– Planning, budgeting, and design

• Life-cycle analysis– How long will the project last?

• Meet USACE requirements

• USACE CONTRACT• Great Lakes Dredge & Dock

• 1.4 M cy sand• $30.7 M (15% City cost)

• January – June 2014

• MONITORING• August 2014• June 2015

2014 Federal Project recap

OCRM Monuments

USACE O&M Manual

Beach Nourishment Performance Evaluation

“periodic nourishment will be initiated upon a mutual agreement between the Government and the Local Sponsor based on the results of annual inspections and surveys. Periodic nourishment is deemed necessary when 25% of the length of the project storm berm has reached a 15 foot width at elevation 9.0 NGVD. At this time an approximate high tide beach width of 60-75 feet will exist.”

8.0 ft NAVD88

1. 8-ft berm & MHW width2. Volume remaining

PEL(Perpetual Easement Line)

200 ft wi-20 ftaverage+

65 ft wi-1.5 fttapers19%

5 of 26 profiles = 19%

Life Cycle Analysis8-ft berm & MHW

  Shoreline Length

# of Monuments

Shoreline length/# of Monuments required to meet trigger

5798 ft 7 monuments

# of years until trigger is met 4 yrs 3 yrs     

Year trigger will be met: 2019 2018

Rudimentary – based on only 2 points

SW-15 to -30 cy/ft

Volumetric Analysis

Total volume placed by 2014 project (cy) 1,400,000

Total volume in place: Aug 2014 (cy) 1,093,513

Volume change Aug 2014 - June 2015 (cy) -334,862

Volume remaining in June 2015 (cy) 758,652

Just over half of the placed volume remains

Life Cycle AnalysisVolume trends

4yrs

Life Cycle AnalysisVolume trends

3yrs

Life Cycle AnalysisVolume trends

0yrs

Summary of Volume Trends

• If trends continue, performance may be better than 3-4 yrs predicted

Sum

mary

of

Monit

ori

ng • Between Aug 2014 & Jun 2015

– 20 ft shoreline (MHW) erosion– “8-ft berm” eroded 1.5 ft– Lost 15-30 cy/ft volume

• As of June 2015, on average,– beach is 200 ft wide– “8-ft berm” is 65 ft wide– ~half nourishment volume remains

• USACE trigger– “8-ft berm” < 15ft at 5 of 26 profiles (19%)– Trigger will be met in 3 to 4 years (2018-19)

Folly Beach 1-yr Post-Construction Monitoring Report

Nicole Elko, Ph.D.

September 8, 2015