CHAPTER 2

TERRAIN AND ENEMY

GENERAL

Commanders maneuver their forces into positions of advantage over the enemy. Engineers analyze the terrain to determine the maneuver potential, ways to reduce natural and enemy obstacles, and how they can deny freedom of maneuver to the enemy by enhancing the inherent obstacle value of the terrain.

ESTIMATE OF THE SITUATION

Commanders and staffs develop estimates of the situation, described in FM 101-5, during the military decision-making process. Terrain and enemy aspects that are applicable to estimates for river-crossing operations are discussed in this chapter. Much of this information directly applies to the intelligence preparation of the battlefield (IPB). Refer to FM 34-130 for more information on the IPB process.

TACTICAL REQUIREMENTS

Although terrain characteristics have a strong influence, tactical requirements ultimately determine the location of the crossing site(s). River conditions must allow the employment of available crossing means and the tactics required for the operation.

The far-shore terrain must support mission accomplishment; otherwise, crossing the river there serves little purpose. Crossing sites must also support the rapid movement of units to the far shore, or the enemy can win the force buildup race. Commanders balance the tactical use of the far-shore terrain against technical crossing requirements at the river to determine suitable crossing locations.

Nearshore terrain must support initial assault sites, rafting and bridging sites, and the assembly and staging areas used by the force. Routes to and from the river must support the quantity of traffic that is necessary for the operation and for the sustainment of the force in subsequent operations.

The enemy's disposition of forces may limit options for the commander. Because the river physically splits his force, he should execute his crossing operation where the enemy is most vulnerable or least able to react. This gives the commander time to mass his force on the far shore before the enemy can concentrate against it.

TERRAIN

The engineer is the terrain expert. He must work closely with the Intelligence Officer (US Army) (S2) during the planning process to determine advantages and disadvantages the terrain gives to both friendly and enemy forces.

CHARACTERISTICS

Rivers form unique obstacles. They are generally linear and extensive and normally cannot be bypassed. Meandering bends in rivers provide far-shore defenders with opportunities for flanking fires and observation of multiple crossing sites. The combined-arms team, as normally configured for combat, needs special preparation and equipment to carry it across river obstacles. After the attacking force crosses the river, it remains an obstacle for all follow-on forces.

A formation cannot breach a river wherever desired, as it can with most field obstacles. Likely crossing sites can be few and equally obvious to both the attacker and defender.

A river provides excellent observation and fields of fire to both the attacker and defender. It exposes the force on the water and makes it vulnerable while entering and leaving the water. It is also an aerial avenue of approach, allowing enemy aircraft low-level access to crossing operations.

Force buildup on the far shore is a race between the defender and the attacker. The river can be an obstacle behind the initial assault force, allowing the enemy to pin and defeat it in detail while preventing rapid reinforcement.

MILITARY ASPECTS

Terrain analysis for a river crossing includes the following military aspects of terrain: observation, cover and concealment, obstacles, key terrain, and avenues of approach (OCOKA). However, many details are peculiar to river crossings. These details include the specific technical characteristics of the river as an obstacle.

CURRENT

The current of a river is a major limiting factor. It imposes limits on all floating equipment, whether rubber assault boats, swimming armored vehicles, rafts, or bridges. The current's velocity determines the amount of personnel/equipment each type of floating equipment can carry or if it can operate at all. Current affects the distance that the floating equipment will drift downstream. Therefore, commanders must either select an offset starting point upstream to reach a desired point on the far shore or take additional time to fight the current. High current velocities make control of a heavy raft difficult; therefore, landings require skilled boat operators and raft commanders and more time.

Current causes water pressure against floating bridges. Bridge companies use boats or an anchorage system to resist this pressure. The higher the current the more extensive the anchorage system must be. Higher currents provide velocity to floating objects, which can damage or swamp floating equipment.

Current can be measured easily (for example, by timing a floating stick) but is normally not constant across the width of the river. Generally, it is faster in the center than along the shore. It is also faster on the outside of a curve than on the inside. A factor of 1.5 times the measured current should be used for planning purposes.

WATER MEASUREMENTS

The depth of the water influences all phases of a river crossing. If the water is shallow enough and the riverbed will support traffic, fording is possible. If the force uses assault boats and the water becomes shallow in the assault area, the force will have to wade and carry their equipment. Shallow water also causes difficulty for swimming vehicles, as the rapidly moving tracks can dig into a shallow bottom and ground the vehicle. The water must be deep enough to float bridge boats and loaded rafts on their crossing centerlines and deep enough in launch areas to launch boats and bridge bays. The depth of the water is not constant across a river. It is generally deeper in the center and in high-velocity areas. Either a bottom reconnaissance with divers or sounding from a reconnaissance boat is necessary to verify the depth.

The width of a river is a critical dimension for bridges (especially, when it determines how much equipment is necessary) and for rafts. The distance a raft must travel determines its round-trip crossing time, which in turn determines the force buildup rate on the far shore.

WATER CHANGES

A swell is the wave motion found in large bodies of water and near the mouths of rivers. It is caused by normal wave action in a larger body, from tidal action, or from wind forces across the water. A swell is a serious consideration for swimming armored vehicles and is less important for assault boats, heavy rafts, and bridges. Hydrographic data and local residents are sources of information on swells. Direct observation has limited use, as a swell changes over time with changing tide and weather conditions.

Tidal variation can cause significant problems. The depth and current of water change with the tide and may allow operations only during certain times. Tidal variation is not the same every day, as it depends on lunar and solar positions and on the current's velocity. Planners need tide tables to determine the actual variation, but they are not always available for rivers. Another tidal phenomenon found in some estuaries is the tidal bore, which is a dangerous wave that surges up the river as the tide enters. It seriously affects water operations. This reverse flow may require that float bridges be anchored on both sides.

Rivers may be subject to sudden floods due to heavy rain or thawing upstream. This will cause bank overflow, higher currents, deeper water, and significant floating debris. If the enemy possesses upstream flood-control structures or dams, it can cause these conditions also.

OBSTRUCTIONS

Most rivers contain sand or mud banks. They are characteristic of low-current areas along the shore and on the inside of the curves of a river, but they can be anywhere. Since they cause problems for swimming vehicles, assault boats, outboard motors, bridge boats, and rafts, troops must find them through underwater reconnaissance or sounding.

Rocks damage propellers, boats, and floating bridges and ground rafts. They cause swimming armored vehicles to swamp if the vehicle body or a track rides up on them high enough to cant the vehicle and allow water into a hatch or engine intake. They can also cause a fording vehicle to throw a track. Rocks are found by underwater reconnaissance or sounding.

Natural underwater obstructions and floating debris can range from sunken ships to wreckage and snags. The current in large waterways can carry significant floating debris, which can seriously damage boats and floating equipment. Usually, debris can be observed after flooding or rapidly rising waters. Underwater reconnaissance or bottom-charting sonar is the only way to locate underwater obstructions.

Man-made underwater obstacles can be steel or concrete tetrahedrons or dragon's teeth, wood piles, or mines. The enemy places them to deny a crossing area and designs them to block or destroy boats and rafts. Underwater reconnaissance or bottom-charting sonar can locate these obstacles.

Vegetation in the water can snag or choke propellers and ducted impellers on outboard motors and bridge boats. Normally, floating vegetation is not a significant problem. Thick vegetation beds that can cause equipment problems are found in shallow water and normally along the shore. As thick vegetation must extend to within 30 to 60 centimeters of the surface to hinder equipment, it can normally be seen from the surface.

THE FRIENDLY SHORE

Concealment is critical to the initial assault across the river. The assault force must have concealed access to the river. It must also have concealed attack positions close to the river from which to prepare assault boats. The overwatching unit prepares concealed positions along the friendly shore, taking full advantage of vegetation and surface contours. Overwatching units must be in position to engage the most likely enemy position(s) on the enemy shore.

Dominant terrain formed by hill masses or river bluffs provides direct-fire overwatch positions. If the dominant terrain is along the shore, it also covers attack positions, AAs, and staging areas. Air-defense (AD) sites should be located on terrain that dominates aerial avenues of approach (one of which is located along the river). When selecting a crossing site, consider the following:

• Dismounted avenues of approach that allow silent and concealed movement of assault battalions to the river.
• Concealed attack positions that are very close to the water along the dismounted avenue.
• Approaches from the attack positions to the water that have gradual slopes and limited vegetation to allow the assault force to carry inflated assault boats.
• Bank conditions that are favorable. Dismounted forces must be able to carry assault boats to the water, and engineer troops must be able to construct and operate rafts with little bank preparation.
• Road networks that feed the crossing sites and support the lateral movement of vehicles between sites. These road networks must be well constructed to carry large amounts of heavy vehicle traffic.
• Potential staging areas that can support large numbers of tracked and wheeled vehicles without continual maintenance.
• Helicopter landing zones (LZs) for embarkation of the assault force.

THE ENEMY SHORE

River meanders form salients and reentrant angles along the shore. A salient on the enemy shore is desirable for the crossing area, as it allows friendly fires from a wide stretch of the near shore to concentrate against a small area on the far shore and limits the length of enemy shore that must be cleared to eliminate direct fire and observation (see Figure 2-1)

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Dominant terrain is undesirable on the enemy shore. Any terrain that permits direct or observed indirect fires onto crossing sites is key terrain. Friendly forces must control it before beginning the rafting or bridging phases.

Natural obstacles must be minimal between the river and the bridgehead objectives. River valleys often have parallel canals, railroad embankments, flood-control structures, swamps, and ridges that can impede more than the river itself. Obstacles perpendicular to the river can help isolate the bridgehead.

Exits from the river must be reasonably good without preparation. Initially, the bank should allow the assault force to land and dismount from the assault boats. This requires shallow banks with limited vegetation. The assault force also requires concealed dismounted avenues up from the river. Bank conditions must allow vehicles to debark from rafts and move up from the river. If banks require earthwork, at least one unimproved crossing site must allow the landing of earthmoving equipment. The most important far-shore requirement is a road network to carry high volumes of heavy vehicletraffic.

INTELLIGENCE

Detailed knowledge of the river and adjacent terrain is critical to both tactical planning and to engineer technical planning. The keys are early identification of intelligence requirements and an effective collection plan. Space-based imaging and weather systems can provide invaluable information to the terrain database. Multispectral imagery (MSI) from satellites can give the engineer terrain detachment a bird's-eye view of the area of operations. Satellite images, the largest 185 by 185 kilometers, can be used to identify key terrain and provide crossing locations. These images can provide information concerning the depth and turbidity of the river and can be used to identify the line of site for weapons and communications systems. With MSI products, prospective construction materials, the locations of existing crossing sites, and nearshore and far-shore road networks can be identified and exploited.

When the MSI is combined with satellite weather receivers, data processors, and the terrain database, it can be used to identify mobility corridors and establish floodplain trafficability. When these space systems are used together, the effects of the weather on terrain can be analyzed and used to develop decision-support products for the commander.

The terrain database is the starting point for obtaining terrain information. Hydrographic studies exist for most rivers in potential theaters of operation around the world. Many of these studies have sufficient detail for identifying feasible crossing sites. Modern information-collection and -storage technology permits frequent revision of existing data.

Engineer terrain detachments at corps and division maintain the terrain database and provide information in the form of topographic products. These products are used with other tools, such as computers and photography, to develop terrain intelligence for staff planners. The planners, in turn, determine initial crossing requirements and estimated crossing rates from their terrain analyses.

Early in the mission analysis, planners identify further terrain-intelligence needs for the crossing. They provide this to the Assistant Chief of Staff, G2 (Intelligence) (G2) for inclusion in the intelligence-collection plan. The plan specifies that intelligence systems are used to gather essential terrain information for a more detailed analysis. Information on specific river segments and the surrounding terrain is obtained and verified by aerial and ground reconnaissance.

PRIORITY INTELLIGENCE REQUIREMENTS (PIR)

The following tactical and technical information is often PIR for executing a successful crossing:

• Enemy positions that can place direct or observed indirect fires on crossing sites and approaches.
• The location and type of enemy obstacles, particularly mines, in the water and on exit banks.
• The location of enemy reserves that can counterattack assault units.
• The location of enemy artillery that can range crossing sites, staging areas, and approaches.
• The location and condition of existing crossing sites.
• The width, depth, and velocity of the river.
• The condition and profile of the river's bottom.
• The height, slope, and stability of the bank.
• The condition of nearshore and far-shore road networks.
• Previous enemy tactics for defending water obstacles.
• Floodplain trafficability.

INFORMATION COLLECTION

Engineer units have the primary responsibility to collect the terrain information needed for river crossings. If the river is under friendly control, engineer units collect river, bank, and route information. If it is not, space- (satellite) or computer-based intelligence should be accessed, or maneuver units with attached engineer reconnaissance teams should conduct reconnaissance operations to obtain needed information. Engineer light diving teams obtain far-shore, nearshore, river-bottom, and underwater-obstacle information. Local inhabitants provide additional information about bridges, the flow of a river and the stability of its banks, road networks, ford sites, and other river conditions. Aviation assets can provide aerial and video reconnaissance to greatly enhance the IPB for river-crossing operations. Normal intelligence-collection assets develop the picture of the enemy's defense that is necessary for templating.

THREAT

Leaders who understand enemy tactics can defeat the enemy at the river for a successful crossing. Many potential enemies use doctrine from the former Soviet Union, making their tactics the most likely ones US forces must overcome during a crossing. Therefore, the discussion in the following paragraphs describes an opposing-force (OPFOR)-style defense and an attack at a river as the most likely threat. See US Army Training and Doctrine Command (TRADOC) Pamphlet 350-14 for details on an OPFOR defense and TRADOC Pamphlet 350-16 for OPFOR water crossings.

RIVER DEFENSE

The threat considers a water obstacle to be a natural barrier, enabling a strong defense on a wide front with small forces. Units must be prepared to conduct operations in a high level of mission-oriented protective posture (MOPP). The threat prefers to defend on a riverbank that is under its complete control. It can, however, defend forward or to the rear of a river. Its choice depends on the terrain, the forces available to it, and their strengths. The threat considers the defensive characteristics of the terrain. It weighs the severity of the obstacle, the effect of lost crossing sites, and the possibility of severed supply lines.

The threat may defend forward when the terrain is favorable, when it has sufficient reserve combat power, or when it plans to resume the offense immediately. When defending forward, it intends to defeat the crossing force before it reaches the river. The threat will place its defensive forces as far forward of the river as possible.

First-echelon regiments of a division in the main defensive belt forward of a river establish initial defensive positions 10 to 15 kilometers from the river. Second-echelon regiments occupy positions within a few kilometers of the river. These positions are astride major avenues of approach to block attacking forces so that a counterattack can destroy them.

When defending along a river, the threat places most of its forces as close to the exit bank as defensible terrain permits. Their mission is to protect the crossing sites and defeat the crossing force while it is divided by the river. The arrangement of defensive belts is similar to the defense forward of the river, except that the distance between first- and second-echelon regiments may be less. This concentrates more force to defeat assault forces on the exit bank.

Threat engineers destroy existing bridges and mine known crossing sites. They keep only a few sites open for the withdrawal of the predominantly amphibious security force. Threat engineers also emplace obstacles along approach and exit routes, including the riverbanks. As time and assets permit, they add obstacles such as floating mines and underwater obstructions to further disrupt crossing efforts.

First-echelon defensive forces maneuver to bring maximum defensive fire on the crossing force. These defensive forces engage the crossing force with all possible organic and support weapons at crossing sites while it is crossing. Their mission is to defeat the crossing force before it can establish a bridgehead.

Second-echelon battalions, astride major egress routes from the river, block assault elements so counterattacking forces can engage and destroy battalion or smaller assault elements. Second-echelon regiments occupy positions 4 to 5 kilometers behind the first echelon. They provide depth to the defense. It is from this area that the threat launches local counterattacks.

The threat undertakes a defense to the rear of a river when time or terrain precludes a defense forward of the river or on the exit bank. In this situation, security elements deploy on the exit bank to harass and disrupt the attacker's assault and support forces. These security elements delay the attacker to provide time to establish the main defense.

A significant threat capability against a river crossing is artillery. Therefore, if the S2 indicates that the threat has formed artillery groups (regimental artillery groups [RAGs], division artillery groups [DAGs], or Army artillery groups [AAGs]), then it has the capability to saturate crossing sites. In this case, it is not sufficient to eliminate the threat's observation of the river before building bridges, as the concentration of artillery fires can deny an entire bridging site without the necessity for observed fires. The threat can also place rafting operations at risk, as it can place artillery fires on the entrance bank, the exit bank, and the raft centerline simultaneously. Therefore, this requires counterbattery fire to be planned and coordinated to counter threat artillery attacks on the crossing sites.

OFFENSIVE RIVER CROSSING

The threat's offensive river-crossing capability has a significant effect on retrograde crossings by US forces. Threat doctrine espouses direct and parallel pursuit. The threat's ability to force a crossing on a flank and cut off friendly elements before they can complete the retrograde crossing is a major concern.

The threat is well prepared to cross water obstacles. On the average, it anticipates that a formation on the offense will cross one water obstacle of average width (100 to 250 meters) and several narrower ones each day. It considers the crossing of water obstacles to be a complex combat mission but regards this as a normal part of a day's advance.

The threat has two assault-crossing methods. The first one is an assault crossing from the line of march. This it does on the move, having prepared its subunits for the crossing before they approach the water obstacle. The other method is the prepared assault crossing-the main forces deploy at the water obstacle and cross after making additional preparations. The success of the threat's crossings is determined by the following:

• Careful preparation.
• Reconnaissance of opposing forces and the water obstacle.
• Surprise.
• Air superiority.
• Destruction of opposing forces by fire.
• Timely advance of crossing resources.
• Personnel and equipment control at the crossings.
• Strict compliance with safety measures.

Threat doctrine calls for relentless pursuit to prevent the opponent from disengaging, to seize available crossing sites quickly, and to cross the obstacle on the heels of withdrawing forces. Forward detachments and advanced guards have a large role in this. A forward detachment reaches the water obstacle as quickly as possible, bypassing strongpoints, and captures existing bridges or river sections suitable for an assault crossing. It crosses the water, seizes key terrain on the opposite bank, and holds it until the main force arrives.

The threat achieves protection from its opponent along routes to the river by using concealing terrain and creating vertical screens out of vegetation and metallic camouflage nets. Once the crossing begins, the threat uses smoke and thermal decoys to defeat precision-guided munitions.

Threat tactical doctrine recognizes that time is a decisive factor in the success of an assault crossing from the line of march. The threat anticipates that it should take a forward detachment (battalion) 1 to 1 1/2 hours, a first-echelon regiment 2 to 3 hours, and a division 5 to 6 hours to cross a river of moderate width (100 to 250 meters).

When an assault crossing from the line of march is not feasible, the threat uses the prepared assault crossing. Here, the main force deploys at the water obstacle with subunits in direct contact with the opponent. The threat then makes more thorough preparation for the crossing. Success depends on covertness, so the crossing usually takes place at night.