## 7 Discussion  
Earth embankments are a tried and tested method for forming a barrage but not necessarily for the lengths and depths required for tidal range schemes. They require more materials than caissons, but most comprise dredged sand and gravel, plus an impermeable core. They can be prone to storm damage during construction until the final rock armour is positioned.  
* The armour weight will vary with design wave height; as the seabed increases in depth, the weight and hence cost will increase.
* The loss of fill due to erosion by tides and waves during construction could be significant.
* Settlement of an embankment could be significant on granular soils, possibly in the order of a metre, or more especially if there is clay and/or silt content in the strata below.  
\begin{table}
\begin{tabular}{l c c} \hline \hline \multirow{2}{*}{**All-Up Rates, Based on Gross Volume**} & \multicolumn{2}{c}{**\(\mathcal{E}\)/m\({}^{3}\)**} \\ \cline{2-3}  & **April 2008** & **April 2023** \\ \hline average rate for plain precast concrete caissons & 130.0 & 197.6 \\ from DECC report & 7.0 & 10.6 \\ dredged sand core & 43.2 & 65.7 \\ Earth \& rock embankment construction & 13.0 & 10.6 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Indexed all-up rates for embankment and caisson construction from the 2008 Severn Estuary proposals.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Seabed Level (m OD)**} & \multicolumn{2}{c}{**Vert Caissons \(\alpha\) = 0 \({}^{\circ}\)**} & **Horiz Caissons \(\alpha\) = 35 \({}^{\circ}\)** & **Horiz Caissons \(\alpha\) = 45 \({}^{\circ}\)** & **Embankment \(\alpha\) = 68.2 \({}^{\circ}\)** \\ \cline{2-7}  & **Area (m\({}^{2}\))** & **Cost \(\mathcal{E}\)k/m** & **Area (m\({}^{2}\))** & **Cost \(\mathcal{E}\)k/m** & **Area (m\({}^{2}\))** & **Cost \(\mathcal{E}\)k/m** \\ \hline \(-10.0\) & 444 & 88 & 462 & 91 & 564 & 111 & 1078 & 71 \\ \hline \(-14.0\) & 765 & 151 & 625 & 124 & 776 & 153 & 1536 & 101 \\ \hline \(-18.0\) & 1060 & 209 & 810 & 160 & 1020 & 202 & 2074 & 136 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Cost estimates based on sectional area.  
Vertical caissons are becoming more popular for docks and breakwaters, especially in deep water. They require precasting cells in a deep-water dock or on a jack-up large. They are towed into position and held by winches. The ballast of dredged sand and gravel is gradually added so that the caisson sinks slowly onto its prepared foundation. The crest is wider than required solely for stability but would suit a barrage that includes a road or rail link across an estuary. Horizontal construction also offers the potential to provide routes to carry pipes and cables between the coastal docking points of the barrage. Recent interest in a barrage across Morecambe Bay was spurred by the need to provide a route for a high-voltage cable linking Heysham Nuclear Power Plant to the planned nuclear facility at Moorside in West Cumbria.  
Horizontal caissons are like those used for immersed tube tunnels except for the open tops. A removable roof covering will be required when floating out to sea. The positioning and ballasting are the same as for vertical caissons. These units can be precast in coastal sites and floated out in shallower water than the vertical units. The relative quantity of reinforced concrete may be less than that for vertical caissons as they form an outer inclined face.  
Good-quality marine concrete should last for 120 years without major problems, especially if new materials can be used to avoid traditional steel reinforcement. These are gravity structures, so there is little stress within the concrete members. The surface of the precast concrete slopes can be overlaid with new slabs at a future time when it is necessary to raise the height and width to accommodate greater SLR.