Saturday, June 13, 2009

Locks, Dams and Bridges

The low-head dam is the most dangerous type of dam. They are not be easily spotted because the top of a low-head dam can be several feet below the water's surface. Because of their small size and drop, low-head dams do not appear to be dangerous. However, water going over a low-head dam creates a strong recirculating current or backroller, sometimes called the "boil" at the base of the dam. Even on small rivers, the force of the backroller can trap your vessel against the face of the dam and pull you under the water, even while wearing your personal life jacket. Be aware that on large rivers or during high water the backroller or boil may be located more than 100 feet downstream of the dam. Avoid low-head dams.

Large-structure dams are more easily spotted because of their powerhouses and spillways. They can be dangerous to boaters and swimmers both below and above the dam. These areas are usually off-limits.

Low-head dams pose a serious danger to vessel operators. Surface currents below low-head dams can suck vessels toward the face of the dam. Currents above low-head dams can sweep vessels over the dam. The recirculating currents and turbulent waters below these dams can swamp vessels and drown boaters.

Lock attendants are present at most locks to help you through safely.
A series of dams on a river help maintain enough water depth to allow river traffic to operate year-round. As a result of a dam, there will be two levels of water at the dam site, one level above the dam and a different one below. Locks safely transport boats from one water level to another.

Approaching the locks
Be aware that commercial traffic always has priority over recreational boats. Wait at least 400 feet away from the lock for the signal to enter the lock. Alert the lock attendant that you wish to go through the lock. You can sound one prolonged blast followed by one short blast of your boat's sound-producing device. You also may contact the lock attendant using your VHF marine radio on Channel 13, but never interrupt commercial communication. Enter the lock only after you've been signaled to enter by the lock's traffic lights or by the lock attendant. Otherwise, stay clear of the lock.

Signal Lights at Locks
Flashing red light means stay well clear of the lock and do not enter. Allow plenty of room for boats to exit the lock.
Flashing amber light means approach the lock at a safe speed and under full control.
Flashing green light means enter the lock.

When using locks, boaters should:
Have fenders and at least 100 feet of rope to use in securing your boat inside the lock.
Follow the lock attendant's instructions and proceed slowly.
Avoid passing another boat when inside the lock, unless directed to do so by the lock attendant.
Wait for the lock attendant's signal to exit the lock.

Most states have laws requiring that you pass under bridges at a slow speed. You should always reduce your speed and proceed with caution near any bridge or man-made structure that decreases visibility and passage. Many bridges are high enough to allow normal boat passage. However, some bridges provide only low clearance during normal conditions or periods of high water. Many drawbridges open and close when a boat arrives. To request passage, contact the bridge operator using sound signals or a VHF marine radio. Be aware that debris can collect around pilings of bridges and create dangerous obstructions. Sailboat operators should always check clearance of the boat's mast before passing under bridges. This can be very difficult to determine from the operator's position on the boat. On charted waters, the chart will indicate bridge clearance at a particular water level. Current water level and tide must be factored in to determine present clearance.

Changing Water Levels
Fluctuating water levels can cause special hazards for boaters. Water levels can change rapidly due to tides, flooding rivers, or water released through dams. Any of these conditions can cause boats to run aground in areas where navigation may have been safe earlier. Any change in water level also can affect docking to a fixed pier.

Tides on Coastal Waters
Tides are created by the sun and moon exerting a pull on the earth. High tides and low tides are predictable, and each one normally occurs twice daily at approximately six-hour intervals.

Boat operators in coastal waters need to be aware of the effect of tides. The rise and fall of tides can cause water levels to fluctuate by several feet and also can generate strong currents. Some tidal currents are strong enough that some boats cannot make headway against the current.
As a boat operator, you need knowledge of the tides in your boating area. It is a good idea to learn how to read the tide tables found in newspapers in coastal areas. Tide schedules also can be found on weather radio channels.