overtopping of seawalls

Overtopping of Seawalls  Tony Mignone

National Weather Service Caribou Maine

anthony.mignone@noaa.gov  

“Avenues” Basin  

“Scituate Ave” Basin  

Staff Gauge

Staff Gauge

Seawall at Scituate Massachusetts

Test site for this project

Avenues Basin Looking Southeast

Avenues Basin Looking Northeast

Seawall

Time Stamp = 27th/0754 EST Water Level = 15.476 feet

Seawall Seawall

Stop Sign 7th & Ocean Side Dr.

Avenues Basin Flooded from Overtopping

January 27, 2015

Overwash Water Ponding in Basin

landward of Seawall

Astronomical

Tide

7th Avenue Staff Gauge

Flooding is only result of overtopping

Max level on Staff Gauge Seawall Crest 21.5 MLLW

Max Storm Tide Level 13.1 ft MLLW

Flood Water is above maximum storm tide level

Overtopping Parameterization J.P. de Waal and J.W. van der Meer

Off shore wave steepness

Dimensionless Freeboard for slopes

Adjusted Dimensionless Freeboard for slopes (Bruce)

Plunging wave dimensionless discharge

Mean overtopping discharge m³/s  per  meter  of  structure  length

Data Logger

Avenues Basin Staff Gauge

& Data Logger

Data logger records water depth every 6 minutes

and is deployed prior to the storm event.

Reduction Factors

= Reduction factor for Berm/Sandbar

= Reduction factor for shallow water

= Reduction factor for foreshore roughness

= Reduction factor for angle of wave attack

Seawall Crest

Still Water Level (SWL)

Seawall toe

Algorithm Parameters Freeboard & Slope

Freeboard

Slope measured from the +2.0 MLLW level

to the seawall toe

Shallow water reduction factor J.P. de Waal & J.W. van der Meer

Larger waves break in deeper water

Smaller waves break closer to the seawall

Waves will be reduced as they shoal over a sandbar

Reduction factor for a Berm or Sandbar

J.P. de Waal & J.W. van der Meer  

Reduction factor for roughness J.P. de Waal & J.W. van der Meer

Surface Type: !  Smooth Slope = 1.00 !  Grass = 0.90 to 1.00 !  Rock (1 layer) = 0.55 to 0.60 !  Rock (two or more layers) = 0.50 to 0.55

Wave Direction

Seawall

Short crested waves

Long crested waves

Angle of wave attack reduction factor

J.P. de Waal & J.W. van der Meer

046⁰

Wave Direction 075⁰

075⁰ 095⁰

GIS ArcMap 10 used to determine basin Volume

1.  Create a Tin surface 2.  Activate 3D Analyst 3.  Cut out basin using Tin

Editing Tool 4.  Use Surface Volume Tool to

compute the basin volume 5.  As an alternative the polygon

volume tool can be used

2  

4  

3  

1  

5  

8 ft

9 ft

8 ft

11 ft

Manhole Catch basin

18 inch R.C.P. Pipe

Drainage System

Outlet 36 inch A.C.C.M.P. Pipe

Manning Equation

"  Q = quantity of flow ft³ per second "  n = manning coefficient for roughness "  A = cross sectional area of pipe "  R = Hydraulic radius "  S = Hydraulic slope

Catch Basins

Grate is partially blocked by ice, snow and debris

Grate Flow

Tidal response

No tidal response to drainage rate with constant 43.6 m³/min

rate for 30+ hours

Slope = 0.134

Slope = 0.142

Slope = 0.132

Slope = 0.131

Slope = 0.090

Seawall Divided into Sectors based on survey of slope

Surface Chart January 27, 2015 2100Z

Test Event January 27, 2015

Surface Chart November 2, 2014 1500Z

Test Event November 1, 2014

Surface Chart December 9, 2014 1800Z

Test Event December 9, 2014

Questions ??