2 Estuary storage
Estuaries may be used as reservoirs to store water. The water in estuaries is a mixture of fresh river water and seawater. To store fresh water, a barrage can be built across the mouth of the estuary to keep seawater out of the whole estuary, or embankments can be constructed, enclosing smaller river-fed freshwater reservoirs within the estuary.
Estuary storage has many advantages. It avoids flooding large areas of land for reservoirs, and the large lake created in an estuary could also be used for recreation. Cities are often located around estuaries, so water would be available where there is a demand for it. It is sometimes possible to combine an estuary storage scheme with a road or rail link across the estuary, to improve communications in the area. On the other hand, estuary storage also has many disadvantages. The water is stored at the lowest point of the river, at sea level, and it would have to be pumped to all users. River water often contains a high proportion of effluent at river mouths, so the water is usually of poor quality and would require expensive treatment. A barrier would restrict navigation. There may also be ecological problems as estuaries with tidal mudflats are the feeding grounds of many coastal birds and other animals.
There are two large estuary storage schemes in the Netherlands where water storage is combined with land reclamation, flood protection and communication links across barrages. The Zuyder Zee in the Netherlands used to be a large tidal lagoon, and has been converted into a number of polders (land reclaimed from the sea, below sea level, e.g. Figure 3) and a large freshwater lake, the Ijsselmeer. The Rhine delta scheme involves barrages across the channels in the delta, a combination of flood protection and water storage.
In Britain, proposals were made in the 1960s for barrages to be built across Morecambe Bay to provide more water for the Manchester area, across the Dee estuary for Liverpool, and across the Wash for East Anglia. However, all the proposals for estuary storage in Britain were dropped in the late 1970s because of their expense and inflexibility, and concern about environmental changes.