Antonios,

Your question is remarkably challenging to answer so I shall attempt to answer it in parts.

First, I recommend a key reference which is The CIRIA Beach Management Manual (2010). Pages 782 to 807. It is the prime source for UK practitioners when considering any potential beach control structures or beach management scheme. It was free to download for those registering onto CIRIA's website back in 2010/11 when it was published. From what I can see the free download has been discontinued. Do try to see if you can find a free download from CIRIA, but if not a wider web search for a download of the manual might be productive. This manual is absolutely excellent and I really cannot recommend it strongly enough.

Second, for rock GROYNES the following should affect permeability:

1) The extent to which the groyne projects vertically above the beach surface affects the degree of overtopping. Experience is that projection of less than 0.25m allows significant overtopping transport, but as projection approaches 1m the groyne operates as a more significant barrier. Typically the projection value will vary along the beach profile and also over time due to profile changes. Groynes are thus a more significant barrier on shingle beaches where longshore transport is primarily by bedload. Sand by contrast can overtop even quite high groynes, expecially if the site concerned has a high energy wave climate

2) The extent to which the groyne projects seaward across the beach and breaker zone affects the degree of seaward outflanking. Sand is typically mobile within the nearshore zone and thus a part of drift readily outflanks most groynes, Shingle drift is confined closer to the shore so that groynes that extend to the shingle beach toe can limit outflanking. Outflanking is more likely on higher energy coastlines and/or macro-tidal coasts where the breaker zone can migrate seaward;

3) Due to the uncertainties in the relations above an approximate groyne design is attempted for the permeability desired and the groynes are designed to be adjusted (mainly in height) in the early years following construction according to beach monitoring data in order to achieve the desired permeability;

4) Groyne permeability can be measured by (i) monitoring morphological changes of the beach and determining groyne projection cross ection areas - see Dornbusch (2008) see link below; (ii) tracer studies using tagged sediments and (iii) traps positioned on downdrift sides of a groyne to intercept sediments passing over or around the groyne;

5) Don't forget that a series or "field" of groynes may have a greater affect on littoral drift collectively than a single groyne.

Third, for DETACHED BREAKWATERS permeability is affected by:

(i) Geometry and positioning of the breakwaters. Expecially the distance seaward and the sizes of the gaps between them;

(ii) The extent to which salients form behind each breakwater. If very large salients form then each embayment may become partially self contained and longshore transport could be significantly reduced.

The UK breakwaters that have been studied most are Elmer (West Sussex) and Sea Palling (Norfolk). In their paper concerning Elmer, King et al (2000) reported that their tracing and trapping experiments suggested that breakwaters have

reduced shingle transport by a factor of at least 2 compared to similar open beaches.

KING D M, COOPER N J, MORFETT J C and POPE D J 2000. Application of Offshore Breakwaters to the UK. A Case Study at Elmer Beach, Journal of Coastal Research, 16(1), 172-187.

I hope that this provides some guidance and ideas and I wish you success with your research.

Article Sediment transport through rock groynes on mixed beaches

Obviously, Malcolm has given a very good and detailed answer and kindly mentioned my paper.

Because rock groynes have voids, they are apriory permeable to water and sediment. Groyne efficiency can often be observed by the elevation differential. This differential was found in the paper to be the main predictor for rate of throughput and this was independent on wave environment, probably just due gravity helped reduced being submerged. There must also be a relationship between void size, void connectivity and sediment size.

One could obviously have a finer core, or have a sheet pile spine within the rock groyne to make it less permeable.

In response to Uwe's key insights I can add the following details from my own experiences:

During fieldwork on newly constructed rock groynes on Lancing beach (freshly replenished shingle) back in 1997 we found that voids often trapped our tracers which then remained immobile. We concluded that only when voids fill and sediment levels increase around the rocks towards the groyne crest can connectivity between partially filled voids develop. I observed shingle "channels" to develop through the rock groyne as beach levels increased around the rocks. We constructed a "fence" trap on the assumed downdrift side of one such channel and found that under moderate wave conditions it did collect significant quantities of shingle. However those quantities were only a small fraction of the drift that could have been expected on the beach without groynes. Although the fence trap worked well it was labour intensive to construct and keep clear of shingle between transport events (as it filled the surface gradient within deterred further entry of shingle). Whenever drift altered direction it ceased to function!. 

Our impression using tracers was that moderate to strong wave energy was required for any transport through the groyne. We deployed tracers near the groyne toe but were unable to detect significant outflanking to seaward by shingle even during storms when the shingle beach toe extended to the groyne toe.. Of course every rogk groyne has unique features and with hindsight our selected groyne being newly constructed did not have significant elevation differentials.

In consideration of Uwe's experiences against our own I would propose that the following are required before elevation differential can become a dominant factor::(i) Sediment build up to reduce the crest projection of the groyne and (ii) infilling of voids to create interconnected transport paths between the projecting rocks. Conditions favourable to transport across groynes clearly occur during longer periods of unidirectional drift when beach sediments will preferentially accumulate on one side of the groyne reducing the relative crest projection and increasing the elevation differential between the updrift and downdrift sides.

Those interested in further details of the fieldwork that I have referred to should obtain the full report via the link included below.

Technical Report Shingle Beach Transport Project:

Dear Dr Malcolm Bray and Dr Uwe Dornbusch,

I would like to thank you for thoroughly sharing with me your theoretical and practical knowledge  of the permeability issue with respect to rock groynes and breakwaters. About the detached rock breakwaters I was wondering whether I could firstly calculate the altered wave height of a wave passing through the rock breakwater (e.g.via a relative transmission coefficient), and subsequently, calculate the new value of the long-shore sediment transport rate behind the breakwater (via a relative formula). Could the corresponding value of the long-shore transport rate for the case that a rock breakwater exists divided by the value of the long-shore transport rate for the case of open sea conditions be equal to the permeability coefficient of the detached rock breakwater?       

Antonios,

Your follow-up proposal for estimating longshore permeability coefficient should also consider that the angle of approach of a wave passing through a rock breakwater will almost certainly be affected by its passage. Thus, not only would the total wave energy alter, but the proportion of wave energy that can power littoral drift on the shore behind the breakwater could vary too. It could be that on sand beaches the magnitudes and durations of the longshore current (tide or wave driven) flowing between the breakwaters and the shore are the critical factors. This has certainly been found to be significant at Sea Palling detached breakwaters in Norfolk where a significant quantity of field measurements have been undertaken. I attach links to a relevant paper, but many others are available frequently authored by Professor Chris Vincent (see link below). I would recommend literature searches using "Sea Palling" and "breakwaters" to pick up all of the publications by different research teams.

I would recommend use of field data to calibrate initially simple transport formulations because the environment is so complex. In fact, practising engineers typically use physical models (wave tank) rather than computational models to refine breakwater scheme configurations during the design phase because of complexity. One way to reduce complexity would be to focus on microtidal or non-tidal environments so that attention could be focussed on wave driven processes alone. Indeed, breakwater schemes are potentially most efficiently designed in environments with low tidal range as less rock is typically needed.

When considering the effects of rock groynes on sandy beaches is is possible that the degree to which the groyne structure retards longshore currents could offer opportunities for estimating rates of overtopping in situations where the suspended load (NOT bedload) transport dominates. That said there is still likely to be very significant longshore sand transport by seaward outflanking of the groyne toes.

Apologies for my very delayed response due to the start of our new teaching term.

Oops,

I've forgotten to add the links that I referred to in my answer immediately above so I attach them below.

I wish you success with your research.

https://www.researchgate.net/profile/Christopher_Vincent

Article Beach response to shore-parallel breakwaters at Sea Palling,...