Parsons Brinckerhoff "has made huge miscalculations on Severn Estuary tidal lagoons’ costs"

December 14, 2008 

The infamous (,_Massachusetts))) American engineering firm Parsons Brinckerhoff (“PB”) has submitted cost numbers on power from tidal lagoons (25p) that are roughly 800% higher than all the previous studies of Tidal Electric Limited’s tidal lagoon power conducted by UK engineering giant WS Atkins (3.1p) and corroborated by AEA Technology, OFGEM, Rothschild Bank, Montgomery Watson Harza, and several private energy companies. PB has arrived at their extraordinarily high numbers by ignoring the technology developer’s design parameters and introducing their own design and therein making four costly design errors:

1.      Depth. The PB tidal lagoon is partially in 10 meters depth whereas the Tidal Electric Limited design calls for a maximum depth of 5 meters. The added depth in PB’s design increases the cost of those sections by about 250%.

2.      Output Calculations. PB calculated the output from a tidal lagoon in a 10 metre tidal range by using the output from a tidal lagoon in a 6.5 metre tidal range. The output from a tidal lagoon is a function of the square of the tidal range and thus the power output is underestimated by about 100:42 ratio. This is the most basic fact regarding tidal power and it is difficult to understand how PB is unaware of the relevance of the size of the tidal range, given that they have been tasked with discerning which tidal technologies are best suited for use in the Severn.

3.      Two-way generation. The tidal lagoon is designed to generate on both the ebb and the flood tides, but PB simply says it is more productive to generate only on the ebb tide. This error reduces the output estimate by about half. Tidal Electric Limited has demonstrated in meticulous detail how two-way generation is more productive than one-way generation, despite the intuitive obviousness of the fact. Note: it is the practice of barrage advocates to pronounce that two-way generation is unproductive, perhaps because it is unproductive for the barrage.

4.      Crest width. PB insists that the top of the wall must be 5 metres in width while the Atkins design calls for a 3 metre crest width. When questioned, PB responded that 5 metres is required for “health and safety” reasons. Further inquiry reveals that PB proposes that the wall be constructed to accommodate a road for inspection vehicles and that the inspection vehicles require 5 metres width for “health and safety.” Tidal Electric Limited has always assumed that the offshore location is more suitable for boats to serve as inspection vehicles and had not planned for a road and presumably a car park somewhere on the structure. The increased crest width adds about 20% to the cost and a road and car park cause the cost to jump significantly. Tidal Electric Limited regards the road as impractical and unnecessary and a likely danger to the public as an “attractive nuisance.”

Please see below for a more detailed view of the discussion.


Peter Ullman (

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