Raised Coastal Freshwater Reservoirs at River Mouths:

Seawater intrusion into estuaries is intensifying worldwide as upstream storage, drought and sea-levelrise reduce the freshwater that reaches river mouths. Coastal reservoirs, freshwater lakes built withinthe sea, are an established response, but existing schemes hold their water close to sea level, so watermust usually be pumped to users and the reservoir offers little head for anything else.This study proposes a raised coastal freshwater reservoir: an embanked lake on the shallowshelf next to a river mouth whose water is held above sea level, at about +10 to +30 m. This singlefeature distinguishes the concept from existing coastal reservoirs and gives it four consequences. Water can flow by gravity back into the estuary and through canals to wide areas of the lowland behindthe coast. One-way gates under a positive head keep seawater out at all times. Greater depth lowers the share of storage lost to evaporation. And the head turns the reservoir into a pumped-storagebattery.A seasonal water balance, an evaporation index and a gravity-reach relation are derived, and theidea is illustrated at the Shatt al-Arab mouth by an example named Dana al-Basra Lake: a terracedlake of about 140 km2 off Al-Faw, with a central cell at +30 m, six cells at +20 m, cells at +10 m alongits sea face and a lagoon near sea level. In this layout it stores about 2.4 billion m3, 1.7 billion m3 ofit releasable by gravity, against the roughly 1 billion m3 needed to hold the salt front through a fourmonth summer; an Iranian lake of about 280 km2east of the mouth would add about 3.3 billion m3.Dana al-Basra Lake is fed from the Shatt al-Arab through the Abu al-Khasib–Abu Flus canal, fromthe Shatt al-Basra canal through a settling lake, and from a protected canal of about 1,150 km fromthe Mosul and Haditha dam lakes, which ends in its own settling lake on high ground west of Basraand continues by elevated pipelines straight into the lake’s +30 m central cell. Inside Dana al-BasraLake, water is pumped up at off-peak hours and released down through turbines at peak hours and tothe Shatt al-Arab on the flood tide, so it is kept moving. The study concludes that raising the storedwater above sea level is what turns a coastal reservoir into a source of gravity supply against salinityand for the lowland behind the coast, and that the same reasoning applies to other river mouths. Danaal-Basra Lake is used only to simulate and illustrate the idea: its site, layout and numbers are anexample, and the depressions, routes and shapes are to be chosen by survey and study at the time ofimplementation. Construction methods, a survey line and a hypothetical tide-based gate-control codeare given in appendices.