Water Research Laboratory | School of Civil & Environmental Engineering

Considerations for the Design of Rock Armoured Coastal Protection on Fringing Reef CoastlinesMatt Blacka, Kristen Splinter, Rebecca Killalea

Outline

Problem

Thoughts

Modelling

More thoughts

Conclusions (kind of)

Problem

cotK

HM

D3

3

Mass of

rocks

Density of

rocks

submerged

density

slope

wave height

fudge

factor

Hudson equation established & verified for non-fringing reef environments -

How do we work out H -

Problem

cotK

HM

D3

3

ThoughtsX-Section

Plan View

H

Hs = d varies with bed slope

d

Thoughts

Thoughts

Hs = d = 0.55dFor flat bathymetric profile

Goda (2007); Barthelemy (2013)

H

d

X-Section

Plan View

Modelling

• 2D scale physical model testing program in wave flume

• Undertaken to investigate:

1. Are coastal protection rock sizes predicted using depth limed design wave

height and Hudson equation acceptable for reef fringed locations?

2. Even if we use more accurately predicted wave heights with Hudson equation,

is the equation able to accurately predict the required armour rock sizes?

Modelling

• 2D scale physical model testing program in wave flume

• 44 m long, 1.2 m wide, 1.8 m deep wave channel

Wave Flume Physical Model Profiles

Modelling

• Rock armoured seawalls located inshore of 2 locations (75 m & 150 m lagoon width)

• 2 different (non-overtopped) revetment slopes

• 2 different rock armour sizes, 2T & 6T rocks

• 2 different wave/water level conditions

Modelling - Results

• Initial desktop predictions using Hudson equation and assuming simplistic depth limited

waves suggested:

• For smaller wave condition, rocks as small as 0.5T should have been adequate

• For larger wave condition, rocks of the order of 2T should have been adequate

• Scale physical model tests indicated that for the smaller wave condition:

• 2T armouring suffered minor damage, 6T armouring insignificant damage

• Scale physical model tests indicated that for the larger wave condition:

• with narrow reef width, 2T armouring failed, 6T armouring adequate but minor

damage

• With wide reef width both 2T and 6T adequate but minor damage

• General conclusion is that required rock armour size is under-predicted by Hudson

equation and simplistic depth-limited wave height calculation, especially for narrow lagoon

widths

More thoughts……

• Previous physical modelling experimental results on the same bathymetric profiles (Blacka

et al., 2015) provided an improved equation for predicting depth limited wave heights that

also considered lagoon width

Reef Top Wave Height r=𝐻𝑟𝑚𝑠

𝑑𝛾𝑟 = 0.3576 + 0.439𝑒−0.0126𝑥𝑟

More thoughts……

• Water depths on the reef and lagoon are very complicated and can be dominated by

infragravity surges

Offshore

Water Surface

Reef Top

Water Surface

More thoughts……

• How well does the Hudson equation work if we plug the wave

and water level characteristics in that were measured right at

our test revetment in the physical model study?

cotK

HM

D3

3

• ………..Rubbish

• Equations predicted that ridiculously large rock armour sizes would be

required

Conclusions (kind of)

• Existing equations for design of rock armour on coastal protection structures were

not developed or intended for use in locations with fringing coral reefs

• If these equations are going to be used for locations with a fringing reef, additional

effort needs to go in to considering:

• The width of reef/lagoon and its impact on wave heights at the seawall

• The impact of long period infragravity surges on water levels

• Even if you predict the environmental design conditions well, the available armour

sizing equations are not particularly accurate

• Ongoing physical modelling is required to refine:

• Exactly which wave parameters we should be feeding into the equation

• Better values for the Kd fudge factor that improve the equation for reef fringed

locations

Questions?