SAVANNAH, Ga. – Every year, more than six-million visitors travel to walk the cobblestone steps of River Street in historic, downtown Savannah, Ga. An unobstructed view of the Savannah River offers visitors the unique opportunity of viewing some of the largest cargo ships in the world as they pass by under the Talmadge Memorial Bridge on their way to the Georgia Ports Authority’s Garden City Terminal. What they don’t see through the dark water under the passing ships, however, is the 47-feet deep river channel bottom, recently deepened and annually maintained by the U.S. Army Corps of Engineers, Savannah District.
To keep marine transportation functioning in a way that is safe and efficient requires information and regular assessment of the water’s depth. Leading the effort to peer through the murky water, find the true depths, and report the data to federal and state authorities is the responsibility of a small team of hydro-survey geographers and engineers stationed at the Savannah District Engineer’s Depot on Hutchinson Island.
“Marine transportation plays a critical role in providing our region with goods and freight and the opportunity to export our own resources to places all over the world,” said Chris Wheeler, Savannah District Hydro-Survey Section Chief. “In order to keep the shipping channel safe for transiting ships, we use single-beam and multi-beam sonar to map the seafloor and river channel bottom in order to maintain an effective depth and ensure there are no obstructions.”
Single and multibeam sonars are shallow water instruments that provide full bottom coverage and are ideal for the inland waters and river operations covered by USACE.
Recent advances in sounding, coupled with increasing computing capabilities, have allowed the Savannah District to achieve higher efficiencies in performing its hydrographic surveying mission.
“Multibeam sonar in particular has given us a distinct advantage to create clear and accurate representations of what is going on at the bottom of the river,” said Miles Saunders, Savannah District hydro-survey geographer. “Basically, the sonar head emits a spread of 512 beams from the bottom of our survey vessel. As the beams hit the bottom of the river, they are bounced back to sonar where a computer program measures the readings from each beam, compiles the information, calculates distance and depth, and creates a picture of what it looks like down there.”
The high resolution and accuracy of multibeam survey systems has enabled to be used for dredge payment surveys in channel deepening operations where highly accurate digital terrain models DTMs are needed.
“The technology we are using today to map the river bottom is a tremendous asset for our district and for our stakeholders,” Wheeler said. “We’re essentially able to paint the entire river bottom, with each beam acting like a brush that strokes back and forth to create a very detailed pictured.”
Environments for hydrographic surveying range from muddy, low slope rivers with high concentrations of suspended particulate matter and very soft bottoms, to dynamic coastal areas and estuaries with rapidly changing water characteristics.
“We also use single-beam sonar because the Savannah River is very muddy and it is great for penetrating all the mud and silt flowing downstream,” said Danny Durden, Savannah District hydro-survey geographer. “Our single beam sonar can create a 25-degree cone of low frequency sound that can get through all the particulate and provide us with an accurate measure of depth.”
Since Gen. James E. Oglethorpe and his 114 colonists landed on what was known as Yamacraw Bluff on the Savannah River, hydrographers have used many techniques to understand and measure the topography of the Savannah River.
“This is actually a very old branch of science,” Wheeler said. “For centuries Sailors and geographers measured depths with either a sounding pole or a hand lead line with positions determined by three-point sextant fixes to mapped reference points. Those basics have not really changed a whole lot in our modern surveying, but the tools with which we use to gather those data have changed significantly.”
The main intent for hydrographic sonar survey is the safety of navigation, to improve the nautical charts so that commerce can flow from port to port. In a harborage as historically contested as Savannah, it can even find unexpected objects and treasures long-lost beneath the dark waters.
“After one of our harbor dredges brought up what looked like old pieces of cannons and anchors, a hydro-survey team from Savannah used multi-beam sonar technology to discover the wreckage of the CSS Georgia, and more recently, the 19 cannon found at 5-Fathom Hole that are housed here at the depot,” Wheeler said. “As the technology continues to advance and provide us with better and better images, there’s no telling what other objects and relics we’ll end up finding on the bottom of the Savannah River.”