LAND SURVEY PROJECT PROFILE   

Crescent Dam Laser Scan Survey

2010 - American Council of Engineering Companies (ACEC) New York, Platinum Award for Engineering Excellence for the New York Power Authority (NYPA) Cresent Dam Laser Scan in Crescent, New York.  

In the summer of 2008, C.T. Male was retained by the New York Power Authority (NYPA) to survey and map areas of spalling on part of the Crescent Dam structure on the Mohawk River in the Town of Colonie, Albany County, New York.  The dam, constructed in 1912, is just upstream of the confluence of the Mohawk and Hudson Rivers and is comprised of three dams, two crescent shaped dams and another smaller dam which impounds additional water slightly downstream from the upper dams.  All three structures are concrete gravity dams built on a rock foundation.  Dam A is 53 feet high with a length of 902 feet, Dam B is 33 feet high with a length of 537 feet and Dam C (the dam at the focus of our laser scan) is 16 feet high with a length of 526 feet.  The dams impound 10 miles of the upstream Mohawk River for hydroelectric power, drinking water and navigation purposes.  Lock E-4 Embankment Dam, also known as Waterford Flight (Erie Canal) is adjacent to the dam structure is 34 feet high and has a length of 2,860 feet.  

After nearly 100 years of heavy ice and debris flowing over Dam C, areas of spalling, cracks and leaks became noticeable to NYPA.  More information was needed to evaluate the extent of the repair necessary to the dam. The best way for NYPA to obtain the high level of detail needed to develop a method for repair was to perform a High Definition Laser Scan Survey.  C.T. Male used its Lecia HDS 3600 High Definition Laser Scanner to perform the job.  The laser scanner has the ability to make thousands of survey grade measurements per second and create a Point Cloud (a group of millions of highly accurate points used to create a 3-D model).  

Stable survey level control points were set prior to the scan on the powerhouse side of the dam, the island side of the dam and downstream of the dam on rocks in the middle of the river that protruded above the water elevation.  These control points were set using conventional surveying methods during the late summer hoping that the Laser scan could be done within a few weeks time when the water flows subsided.  A 14’ rowboat was used to access the control points on the island and in the river.   

The Laser scan of the 526 foot long dam structure was scheduled to start in August during times of typical low water. However, in the summer of 2008, upstate New York was battered by fierce storms and torrential rains, raising the water levels on the river and delaying the scheduled start date to December.  

The maximum discharge of the structure is 548,300 cubic feet per second. Its capacity is 70,000 acre feet with normal storage being 50,000 acre feet.  The impounded area drains an area of approximately 3,453 square miles.   

On December 8^th, 2008 the water level of the 3.1 square miles of upstream Mohawk River surface area was lowered to a level below the spillway to allow access to the dam face without water flowing over it.  Water flow levels needed to be low enough to enable NYPA to divert the entire flow of the river through the power house to keep the water from spilling over the crest of the dam. 

On the day of the scan, one of the hydroelectric turbines was down for routine maintenance so NYPA was not able to divert as much water as if all turbines were operational.  In addition, NYPA could only lower the water level for a short time.  The schedule for turbine maintenance called for two additional turbines to be taken offline within two days of the start of the scan.  With impending winter weather on the way and rising water levels, timing was critical to complete the scan of the structure. On the day of the scan the temperature was seven degrees fahrenheit.  An inch and a half of ice had formed on some of the still backwaters of the river.  This complicated the field work as two of the control points previously set were accessible only by row boat. Instead of seeing water flow over the dam, crews saw as much as one inch of ice in places on the face of the dam. Water that was seeping through the cracks in the dam produced ice sheets on the remainder of the face of the dam.     

To further complicate the work, the laser scanner had never been used in such extreme weather and it was uncertain if the scanner would even operate in single degree temperatures.  The specifications from the manufacturer called for the lowest operating temperature to be 32 degrees.   

The extremely delicate scanner was transported to the control points by rowboat.  Due to the frozen backwaters of the Mohawk River, it was necessary for the crew to kneel in the bow of the 14’ rowboat while chopping through an inch and a half of ice with a sledge hammer.  Further problems were encountered from the swirling eddies that were created by the water current downstream of the tail race.  The crew got stuck in one of the powerful eddies and had a difficult time getting the boat out of the swirling water.  After the scan, the crew had to row back against strong late day winds pushing them farther and farther away from shore.  The field crew was unquestionably pushed to their limits but persevered through the second day to complete the job. 

Despite the extreme weather conditions, the Leica HDS 3600 worked flawlessly.  The data collected was of excellent quality and was used to create a Point Cloud of the structure.  From the Point Cloud, a 3-D model was produced for NYPA engineers to show a realistic and highly accurate model of the spalling and cracks in need of repair.  The raw point cloud consisted of over 16 million individual points each with highly accurate x, y and z values.  Seamless panoramic digital photos were taken of the dam with the scanner and registered as a background to the point cloud.  The digital 3-D model with registered digital photo background enabled engineers to visualize and quantify the cracks and spalling in the Dam.  Data was coordinated to the site control, processed and imported into Leica’s Cloudworx.  In Cloudworx, mapping began to create the 3-D model of the dam.   Isometric views were mapped as well as 142 cross sections, a Digital Terrain Model (DTM) and Topographic Contour Map across the face of the dam.  The DTM surface of the main area of horizontal spalling was tipped up from a vertical face (elevation view) to a horizontal (plan view) in order to show contours of the crack, as if you were looking down into a hole.  From the DTM other leaks and cracks in the dam were identified, mapped and their volumes computed to determine the amount of fill needed to repair the dam.  Some of the vertical seams in the concrete were also showing signs of spalling, so, detailed cross sections and volume computations were prepared for these as well.  Some of the areas of spalling and cracks were big enough to fit a person inside of them.  

Based on C.T. Male’s drawings and computations the reconstruction project went out to bid to contractors and repairs to the cracks and spalling on the Crescent Dam began in the summer of 2009 to bring the face of the dam back to a smooth symmetrical surface. NYPA was extremely pleased with the quality of the data captured during the Laser Scan Survey. 

As a result of C.T. Male’s dedication to the project, the final product was detailed and comprehensive and exceeded the original expectations of the client.  NYPA commended C.T. Male on its ability to meet the needs of its engineers.