While many Mayan cities and compounds were built on hills located near a river, they never carried water from the rivers to their hilltop cities. Over the past decade, archeologists exploring Bolivia discovered a secret of hydraulic engineering hidden in a very recently discovered Mayan hilltop enclave that spouted water. They initially believed that the enclave had been built over a spring and estimated that it had been abandoned around the year 1700, the same year of the birth of Daniel Bernoulli whose research forms the basis of modern hydraulic engineering.
Further investigation of the Mayan enclave revealed a hidden water reservoir that received a steady stream of water that literally flowed uphill through a man-made tunnel of decreasing cross-sectional area. Mayan engineers may have built cofferdams over 1000 years ago to access water from below the surface of a river. They also knew how to install hydraulic structures of rock to accelerate water to higher velocity prior to diverting a portion of it into a tunnel of decreasing cross-sectional area that supplied water to a reservoir located at higher elevation.
The precedent of the Mayan engineers may be applicable to modern, small-site hydroelectric power generation along river and oceanic channels in place of free-flow kinetic turbines. Water would be re-directed to a coastal reservoir with water surface at 0.7m to 4m above the water surface of the source channel. While open-stream turbines may convert energy at 30% to 45% efficiency, a low-head turbine operating over 0.7M head could convert energy at about 70% efficiency and at 84% efficiency over a head of 3m or 11 feet.
A turbine built into a low-head dam could generate greater output at greater efficiency than an open-stream turbine of equivalent size. It could offer greater structural strength and be easier to access, maintain, remove and install/re-install that an open stream turbine placed in powerful streams. There are channels where the water flow velocity and the strength of the current would exceed the capability of kinetic turbines and impede the installation, repair and removal of kinetic turbines in severe water currents. There are channels where kinetic turbines may conflict with marine navigation or impede the movement of fish to upstream locations.
There are numerous locations around the world where nature has already provided the rudiments of a coastal reservoir. The channels formed by the Button Islands on the north side of Gray Strait in Northeastern Canada provide the walls of what could become the holding reservoir of a low-head oceanic dam. A powerful current races from Hudson Strait through Gray Strait and into the Atlantic Ocean for 2 cycles of over 5 hours each every 24 hour period. There is no kinetic turbine built anywhere in the world other than a recycled ship propeller that could provide sufficient structural strength capable of withstanding the force of the powerful current.
The kinetic energy of the current that flows through Gray Strait has been calculated to be in excess of 6000MW. There are 3 channels at that location that could be converted into reservoirs with ramp-over at the western entrances and low-head dams at the eastern exits into the Atlantic Ocean. There is an inlet from the Atlantic side that extends across to a point near the western entrance of the channel to the immediate north of it. A total of 4 oceanic reservoirs with low-head power dams in that region could generate up to 1000MW for 2 cycles of 5 hours each every day.
If predictions come true that most of the Arctic ice could melt within the next few decades, oceanic coastal power generation could prevail at several locations across Northern Canada. Powerful oceanic tidal currents capable of destroying a free-flow turbine also flow through offshore channels along Canada's Pacific coast, where coastal reservoirs and low-head oceanic dams may be a possible option. There are numerous other oceanic coastal channels around the world where the strength of the water current combined with other factors could provide an opportunity to consider coastal reservoirs with low-head dams to generate electric power.
There are also river channels with currents that are too powerful for kinetic turbines or where the installation of free-flow turbines conflicts with some other activity such as marine navigation or fish migration. There has opposition to the proposed mass installation of kinetic turbines along a section of the Mississippi River by the marine industry, along a section of the St Lawrence River by a federal regulator and along sections of several other significant rivers across North America by parties concerned with fish migration. The hydraulic engineering precedent pioneered by the engineers of the Mayan empire may form the basis of an aspect of modern small-site hydroelectric power generation.