Defense against amphibious attack at sea remained a Kriegsmarine responsibility and the navy had both deep-water mines and coastal mines for this mission. Deep-water mines are largely outside the scope of this book, but in the event they were not particularly significant on D-Day. Due to the strong currents in the Seine Bay, minefields had to be periodically refreshed to be effective and there were not enough mines available to do so regularly. As a result, Naval Force West's preferred tactic was dubbed "Blitzsperren," or lightning barrage; the idea being that the mine-laying force would wait until an invasion was imminent and then sortie forth and quickly laid minefields. An effort to deploy mines in the Seine Bay off the invasion beaches on the night of May 23, 1944, was frustrated by Allied counteraction after the plans were discovered in advance by an Enigma signals intelligence decrypt, leading to a highly effective Royal Navy and RAF attack on German minelayers.
After the Dieppe raid, the Kriegsmarine showed more interest in deploying controlled minefields in shallow water. These were used primarily in port areas such as Cherbourg and Le Havre, and not off the rest of the Normandy coast due to the time and expense of creating and operating such minefields. The Kriegsmarine developed an inexpensive, mass-produced, shallow-water anti-craft mine called the KMA (kustenmine-A: coastal mine-A), which consisted of a concrete base containing a 75kg explosive charge surmounted by a steel tripod frame with the triggering device. Although cheap and effective, they became available too late. They were first laid in the high-priority areas along the Channel coast from Boulogne south-westward towards Le Havre by early June 1944. The next area to be mined was the Seine Estuary around Le Havre, which was to begin on June 10, but this never took place due to the invasion.
The lack of KMA mines prompted Rommel's headquarters to develop a family of improvised anti-craft devices that could be built in large numbers using locally available materials. In March 1944, Naval Group West developed and tested their own Nussknackermine (Nutcracker mine), which was an improvised copy of the KMA using a concrete base containing an explosive device such as a French high-explosive artillery projectile, with a pivoting steel rod that pressed against the projectile fuse when a landing craft came in contact with it. Other improvised coastal mines used the same concept but different methods of mounting and triggering the explosive charge. Deployment of these began in April 1944, initially at priority locations including the Channel coast and Brest. The performance of these improvised mines was erratic due to the effect of water on submerged munitions not designed for submersion. In addition, German garrisons in some sectors found that the mines tended to be damaged or upset by tidal currents, with the triggering beam being particularly vulnerable.
Simpler anti-craft mines such as the Minenpfahl were created using conventional land mines such as the Teller anti-tank mine, mounted on stakes along the shore. The Schwimmende Balkenmine consisted of several Teller mines strapped to a wooden raft that was held in placed by a rope or chain fastened to a concrete anchor. The Armsperre mine placed a single Teller mine on a float, and then fixed the mine using chain or metal bars to a concrete anchor, creating a cheap coastal equivalent of conventional naval mines. The effectiveness of these mines was mixed due to the effect of seawater on mines not designed for frequent emersion. However, the motto at Rommel's headquarters was "better to do something imperfect than nothing at all"
The Schwimmende Balkenmine was an improvised anti-craft mine consisting of a half-dozen Teller mines strapped to a wooden raft. These were anchored off the invasion beaches to concrete bases or to other obstructions. In the background is one of the ubiquitous Czech hedgehog obstructions.
There were never enough high-explosive devices to create coastal minefields along the entire Normandy coast, so Rommel and his headquarters developed a variety of obstacles to interfere with landing craft. This was Rommel's single most important contribution to the defence of the Normandy coast. During a visit to Hardelot-Plage on February 3, 1944, Rommel was shown a technique developed locally by troops of using a high-pressure water hose instead of a pile driver to emplace wooden stakes. This took only three minutes per stake as compared to 45 minutes using a pile driver. Subsequently, Rommel ordered this technique to be used in Normandy to create extensive obstacle barriers of Hochpfahlen (high stakes) created from telegraph poles, metal beams and other material. In some sectors, such as Sword Beach, the rocky conditions did not permit the use of fire hoses, and the slower pile drivers had to be used. In the haste to create these barriers, little attention was initially paid to their actual effectiveness in stopping landing craft. In mid-February 1944, the Seventh Army tested some of the obstacles using a British landing craft captured at Dieppe. The landing craft plowed through many of the obstacles, especially the stakes. As a result, more substantial Hemmbalk (beam obstructions) were developed based on a tripod design. The less substantial vertical stakes remained in use, but often improved by the addition of mines as mentioned before. Another addition to the stakes was the stahlmesser metal saw teeth to cut into the lower hull of the landing craft.
Hemmbalk were a more substantial obstacle developed in the spring of 1944 when it became evident that the simpler stakes were not effective against an on-rushing landing craft. They were usually topped with a Teller mine to blow a hole in the hull of the landing craft.