Fording the Imjin River - Korea

During the 2d Infantry Division's preparation for the U.S. Forces Korea's maneuver exercise Foal Eagle '99, the 44th Engineer Battalion (Combat) and the 82d Engineer Company (Combat Support Equipment) received the mission to emplace a tactical ford across the Imjin River. The ford site had to pass an armored task force containing M1 Abrams tanks and M2 Bradley infantry fighting vehicles through an 85-meter-wide water obstacle and support subsequent crossings by smaller groups of mechanized vehicles without significant degradation. For the engineer units involved, this task (from their mission-essential task list) provided an excellent opportunity for the chain of command to exercise engineer reconnaissance skills, integrate earthmoving NCO expertise with combat-engineer judgment, and safely emplace a viable ford on a fast-moving river.

Reconnaissance

As part of the troop-leading procedures, the battalion S3, the company commander, and the commander's key leaders conducted a thorough reconnaissance of the proposed ford site. As shown in the site sketch in Figure 1, the Imjin River at this location runs predominately east-west. At the desired crossing site, a rough access ramp led down to the south bank from the existing unimproved road. The exit ramp on the egress bank was nonexistent. There was a one-lane vehicle bridge (concrete deck and steel girders on concrete piers) just 15 meters west of the proposed ford site, but due to severe weathering of the deck and scouring at the base of the piers, the bridge was rated military load class (MLC) 05 and limited to wheeled traffic.

Using both hasty and deliberate reconnaissance techniques described in FM 5-34, Engineer Field Data, a team of combat engineers and engineer-equipment operators obtained critical information about the proposed ford site. The river reconnaissance was conducted in two parts: First, the engineers determined the key characteristics of the river, to include the water's depth and speed, river-bottom substrate, and composition of the egress and exit banks. The river-bottom composition was determined by wading into pools of still or slow-moving water at different locations, probing, and gathering soil samples. The hasty analysis revealed a 50/50 mix of cobbles and unsorted stones--mostly rounded float from the surrounding hills--and poorly graded sand without any significant deposits of silt or clay. Using available on-site litter (Styrofoam[R] and plastic soda bottles), the reconnaissance team calculated the river velocity at the planned crossing site and various locations upstream. The velocity of the water ranged from 0.5 to 3.0 meters per second across the river's width, with the highest flow in a deep, scoured channel adjacent to the exit bank.

Secondly, the reconnaissance team identified the high-velocity flow pattern across the river channel before placing heavy equipment in the water to construct the ford. The riverbed contained localized regions of above-average depth and increased velocity that shifted after each significant rainfall. Fortunately, the river water was relatively clear, which allowed the reconnaissance team to conduct a hasty field dye test to find those high-velocity flow regions. A map reconnaissance revealed an existing ford approximately 400 meters east of the proposed site (point C on the sketch in Figure 1), which would allow the D7 dozers to enter the water and cross the width of the river. Three dozers entered the water in column and traversed the river from south to north and back in high gear. The height of the water ranged from 1.1 to 1.3 meters (just enough to completely submerge the tracks on the dozers at times).

As the dozers crossed the gap, their tracks stirred up enough silt (our field dye) from the river bottom to reveal the high-velocity flow patterns. Five clearly visible flow channels, depicted in Figure 1, were observed from the existing bridge just west of the proposed work site. As a side note, since the dozers were moving in water that exceeded 1.5 meters per second at certain locations, and the actual depth of the water was unknown across the entire river width, specific safety precautions were prescribed. All equipment operators wore Type III life vests when maneuvering in the water, and a safety dozer with recovery chains and life buoys was positioned downstream to assist if needed.

Course of Action

Since the mission was to emplace the ford site close to the existing bridge, it would be necessary to reduce the flow and depth of the high flow channel along the north bank of the river and distribute the water more evenly across the channel to satisfy the safety limits on water depth and velocity defined in FM 90-13, River-Crossing Operations:

"Combat vehicles can ford shallow rivers that have a limited current velocity and stable beds. Some vehicles have kits to increase fording depth. Fording is possible for current velocities that are less than 1.5 meters per second. Riverbeds at fording sites must be firm and free of large rocks and other obstructions. Vehicle-operator manuals contain specific depth capabilities and required adaptations."

Based on the results of the field dye test and our hasty calculations, the water depth and velocity were too high to place equipment in the river at the proposed ford site. We had to find a way to drop the volume of water entering the northern part of the river channel, reduce the water depth by 1-2 meters and reduce the water's velocity by more than 1 meter per second at the proposed ford site.

In an open-channel flow problem, the quantity or volume of water (Q) flowing through any given cross section is a function of the fluid velocity (V) and the cross-sectional area (A):

Q = VA

Assuming the quantity or volume of water moving through a cross section of the channel at the ford site was constant, the 82d Engineer Company would have to adjust the water depth across the channel and thus reduce the corresponding velocity. If the engineers could get Q to balance between sections A and B as shown in Figure 1, it would be possible to drop the water depth by 1.8 meters on the north side of the river in the vicinity of the ford site. Short of excavating or dredging a new channel, the only option appeared to be a temporary water-diversion technique that required the construction of one or more sizable earthen berms in the middle of the river channel.

Getting to Work

Using the four D7 dozers available on-site, a test berm was constructed at a 45-degree angle across one of the five water-flow patterns to determine if this technique would produce a noticeable change in the water distribution across the channel. After 1 hour of earthmoving, the water depth began to drop on the north side and rise on the south side of the channel. Based on the test berm, we calculated that the primary diversion berm would run at approximately the same angle across the river's width for a total of 250 meters. We estimated that the berm, in its final form, would have to be 2.5 meters high and 6 meters wide to withstand small fluctuations in the river's flow for the duration of the exercise.

The dozers worked from southeast to northwest, building several smaller berms diagonally across the width of the river. To meet the desired flow speed in the deepest channel, three of the five major flow channels had to be successfully diverted. These smaller earthen berms were constructed of mostly river-bottom material, consisting of large angular gravel, cobbles, and small boulders that were concentrated along several sandbarlike deposits in the river channel. Eventually, these smaller berms were completed and connected to form the primary diversion berm.

Once the primary berm was in place, several hundred cubic meters of small boulders were dumped into the deepest flow channel at the point where the berm and channel intersected on the north side of the river, approximately 275 meters east of the ford site and 25 meters from the northern bank. This significantly slowed the flow of the water in that particular channel, resulting in a reduction of more than 1.3 meters in the water level and a reduced flow velocity of approximately 2.5 meters per second.

The dozers leveled the river bottom from south to north and removed large rocks or small boulders that could hamper tracked-vehicle movement. Working with two 20-ton dump trucks and a 5-yard loader, 400 cubic meters of material of unsorted rocky bank material--generally large gravel and small cobbles--were stockpiled at a borrow site on the south bank of the river. Material was pushed from the shore, then hauled by the 5-yard loader through the water and dumped into the deeper channel on the north side of the ford. As the water level continued to drop, two dozers--pushing perpendicular to the ford--piled additional river-bottom material into the ford. This effort began to lift the ford (in essence, creating a weir) so that water would pool upstream and slow below 1.5 meters per second as it moved across the ford.

After two days, the work was completed. Actual work time in the water was 16 hours. The water on the southern half of the ford was 0.5 meters deep. On the northern portion, originally the deepest part of the river channel, the water was less than 1 meter deep. There was decreased flow velocity across the entire width of the river, with a maximum river velocity across the ford of under 1.5 meters per second. The ford site also required significant entrance- and exit-bank preparation in addition to the work in the river itself. This bank work was done in conjunction with the berm and weir preparation. The site sketch in Figure 2 shows the final results.

Lessons Learned

The units learned several important lessons, to include the following:

* Troop-leading procedures work! Whenever possible, conduct a personal recon along with key leaders. In this case, we included our highly skilled engineer-equipment operators and their NCO supervisors. These soldiers knew the equipment capabilities and offered valuable insights on how to safely tackle the problem. Fortunately, the existing bridge served as an excellent vantage point for discussing plans and observing movements on the ford.

* Regardless of the mission, always test the water before placing soldiers or equipment in the water.

* Comply with all local environmental laws and coordinate with all affected landowners or residents.

* Ensure that after conducting a comprehensive risk assessment, all unnecessary risks are eliminated and all required operations are properly resourced, planned, rehearsed, supervised, and executed.

This river-fording mission offered the soldiers and leaders of the 82d Engineer Company and the 44th Engineer Battalion an excellent training opportunity structured around a mission that they could be called on to execute during a hostile situation.

Major Landry is completing a doctorate in civil engineering at Rensselaer Polytechnic Institute in Troy, New York. He was previously the operations officer for the 44th Engineer Battalion, 2d Infantry Division, in the Republic of Korea and commander of C/12th Engineer Battalion, 1st Armored Division in Germany and taught in the Civil and Mechanical Engineering Department at West Point. MAJ Landry is a graduate of the United States Military Academy and the Command and General Staff College; Air Assault, Airborne, and Ranger qualified; and a professional engineer in the state of Virginia.

Captain Dorf is assigned with the Corps of Engineers in Bahrain. He previously served as the Assistant Brigade Engineer, 2d Brigade, 2d Infantry Division, Republic of Korea; and commander, 82d Engineer Company (Combat Support Equipment). CPT Doff is a graduate of the University of Maine, Orono.

Engineer Reconnaissance Data

Facts:

* Imjin River is 85 m wide

Part A: 30 m wide, 3 m deep, [greater than]2 mps

Part B: 55 m wide, 0.3 m deep, [less than]1 mps

* MLC 05 bridge

* 4 D7 dazers

* 2 20-ton dump trucks

* 1 5-yard loader

* 1 grader

* Extensive supply of large river rock available on south bank of river.

* Field dye test revealed water flows as shown by arrows on site sketch.

COPYRIGHT 2001 U.S. Army Maneuver Support Center
COPYRIGHT 2004 Gale Group