The Halcrow Report

Report by Halcrow HPA for Sea Palling, Norfolk, England >>>

Project Happisburgh to Winterton Stage 3b Date    
Note Undercurrent Stabilizers (Holmberg Technologies) Ref   

1 Introduction

As part of the on-going works on the Happisburgh to Winterton frontage the Environment Agency (the Agency) have asked Halcrow to review the potential to use undercurrent stabilisers. Note—this refers to Holmberg Technologies’ patented Undercurrent Stabilizer Systems.

This technical note will present details of a initial review of information available in the public domain, and will explain the general form of under current stabilisation, the theoretical manner in which the method works and its potential suitability for the Happisburgh to Winterton frontage.

2 Principles

Undercurrent Stabilizers are an erosion control technology patented by Holmberg Technologies Inc, which uses geotextile tubes filled with concrete to form low profile structures extending at right angles to the shore, these are designed to artificially create a shallower bottom profile near the shore.

The geotextile tubes are transported to the construction site, put in place, and filled with concrete pumped from land-based trucks. They are placed in groups along a shoreline with the intention that it dissipates and reduces the energy of sediment carrying waves and currents, directing energy away from the bottom and protecting the near-shore from downcutting and scour.

Unlike traditional groynes, undercurrent stabilizers taper as they get farther from shore. It is claimed that the stabilizers reduce wave reflection and turbulence creating a "low-energy beach" allowing more sand to be deposited.

Holmberg has a fundamental disagreement with many coastal engineers about the source of beach sand. While the traditional model has sand moving along the beach - meaning that structures capturing and retaining sand in one area, are depriving it from another - Holmberg maintains that ‘the primary source of beach sand is the offshore shelf, and therefore undercurrent stabilizers do not deprive adjacent beaches.’

(http://www.forester.net/ec_0007_beachfront.html). It is unclear as to the basis for this statement, if this was the case then groynes would equally not deprive adjacent beaches, and many situations on the coast in the UK clearly show this not to be the case.

3 Advantages

The following advantages of this technique are suggested by Holmberg Technologies;
In-situ casting process may reduce construction time,
Reduced problems with weather delays common to the traditional marine construction field,
Eliminates the use of large equipment working from floating platforms.
Minimal site disturbance
Aesthetic impact much less than more traditional forms of sea defence.
Maintains Longshore transport.

4 Disadvantages

Although it is claimed that the technology has been used successfully to protect around 100 different sites in the USA, there still is little research done into factors such as the longevity of this system and the downdrift effects on the surrounding coast.

5 Suitability for the Frontage

The current Strategy for this frontage, to achieve the “hold the line” recommendation of the Shoreline Management Plan is one of maintaining the current beach management regime whilst periodically recharging the beach (This means adding dredged fill) to counteract the effect of the inbalance between the material entering the frontage from the north and the material leaving the frontage to the south. 

Undercurrent Stabilisers are intended to maintain beach levels to ensure stability of the existing defences in a similar way to other beach management systems (e.g. groynes), and therefore may be suitable as an alternative. The current Strategy shows that the most economical way of maintaining the current beach management regime is to carry out the periodical replacement of groynes as they reach the end of their design life, to change the form of beach management to the use of undercurrent stabilisers would require the complete replacement of all groynes.
 
The Happisburgh to Winterton frontage is particularly sensitive to longshore transportation, and specifically because of the environmentally sensitive Winterton Ness are to the south of the site which relies upon the sedimentary feed from this frontage. The current defence strategy relies heavily upon beach recharge to maintain beach levels along the frontage. The claim (of Undercurrent Stabilizers) to maintain longshore transport may make this system suitable for the Happisburgh to Winterton frontage, but significant modelling would be required to determine what short, and long term effects a change of defence form would have upon the sedimentary regime of this area of coastline and adjacent frontages.

The possibility of adopting an alternative form of beach management structure would need to be considered at Strategy level, and therefore cannot be considered as part of the current PAR for works under the existing Strategy. It is recommended that these structures are considered at the next Strategy Review. At this time it is recommended that an outline review of the potential cost of implementing a scheme is carried out to consider the economic viability of such a scheme when compared to the present value cost of implementing works as recommended by the current strategy. Dependant upon this initial review a full assessment of the effect on coastal processes may be required.

6 References
http://www.erosion.com/index.asp, Holmberg Technologies Inc, USA
http://www.forester.net/ec_0007_beachfront.html , Erosion Control, Beach Reinforcement, Feature Article, July/Aug 2000,
 http://www.marinet.org.uk/coastaldefences/canute.html, Why Canute Failed, Our Defences against the Sea, Article by Pat Gowen. Nov 2005

Have you read Commissioner Feind's letter?
Be sure to read the rebuttal!

(click above for larger image)

Reefs at Sea Palling:

Offshore breakwaters designed by Halcrow, and the sand that was brought in to create the beach in front of them.

The bays the breakwaters created had to be later reinforced with rock revetments.

(click above for larger image)

Happisburg erosion:
This historic English town was threatened when the nearby Sea Palling breakwaters interrupted sand flow and caused erosion down-current.