Thank you for that lead, Kutscha. Much is now clearer.
It appears that 617 Squadron and 9 Squadron (from October 1944) were the only ones trained to drop Tallboys, of which about 700 were used during hostilities.
The Roer Dams, like the Ruhr Dams, consisted of two types; the walled masonry Urft dam (like the Möhne and Eder), and the earth-covered concrete-cored Schwammanauel dam (like the Sorpe).
In December 1944 BC handled each differently, but both unsuccessfully.
The Urft is a walled masonry gravity dam, 58m high and 226m long.
It was allocated to 8 Group, which presumably included 617 and 9 Squadrons.
The following excerpt argues that Upkeep (the bouncing bomb) was more appropriate than Tallboy for destroying such dams, but Upkeep was no longer operational in December 1944.
“Most interestingly, the (12,000-lb, 6-ton Tallboy) bomb was used against dams (one of the principal targets for the big bombs from their inception, of course). The Kembs Barrage (that is a weir) in Alsace and part of the Rhine canal system, was successfully breached on 7th October 1944, thus preventing the Germans using it to flood U.S. troops approaching from Belfort. Uniquely on this raid, the Tallboys were dropped at low level for accurate positioning; use of Upkeep again would have been more appropriate, but for the Lancasters but there was not time to train the new crews for delivery of the weapon. The Sorpe Dam was also attacked with Tallboys (by 9 Squadron conventionally from 14,000ft) on 15th October 1944 and hit by two Tallboys (plus several near misses) but not breached (one Tallboy was found in the mud when the dam was partially drained in 1958, and successfully defused). Three raids were also mounted against the Urft Dam, 30 miles SW of Cologne, during December, again to prevent it being used to flood troops, and the lip of the dam was damaged, but the Germans prevented further damage by lowering the water level.”
http://www.sirbarneswallis.com/Bombs.htm
Upkeep was not appropriate for an earth dam with a concrete core, as was discovered when the Sorpe Dam survived two direct hits. Again this excerpt from the same website; “Sorpe Dam: Unlike the Möhne and Eder Dams which were walled dams, the Sorpe was constructed of a concrete core flanked by earth banking on both sides. Different tactics were thus employed against the dam, which was to be attacked along its crest, and the mine dropped without spin. McCarthy made the first attack, making nine dummy runs as the target was covered in mist; on the tenth run, the mine was dropped and exploded on the dam crest. Brown also made several dummy runs, dropping his mine on the sixth run. It also exploded on target, but although the crest was damaged, the dam was not breached and no seepage through the core (as hoped for) resulted. The damage to the crown of the dam, however, required the Germans to half empty the reservoir to effect repairs, so there was some water loss as a result of the attack.”
As seen above, the Sorpe Dam was also proved resistant to Tallboys.
The Schwammanauel Dam like the Sorpe is a massive earth dam with concrete core, and the second largest barrage in Germany.
It was given to 3 Group to destroy, and presumably they used 1,000-lb medium bombs without effect.
In conclusion, then, it appears that BC lacked the means in December 1944 of destroying dams like the Sorpe and Schwammanauel until the introduction of the 22,000-lb (10-ton) Grand Slam. “Its first use was against the Bielefeld Viaduct; 3,000 tons of bombs (including Tallboys) had already been dropped on it with little result, but Grand Slam brought it down on 14th March 1945 (the first Grand Slam had been test dropped in the New Forest the day before). Grand Slam was used against similar targets to Tallboy (often a raid would include both types of bomb), and again caused remarkable destruction wherever it was used - the Arnsberg, Arbergen, Neinburg and other bridges were also to be felled by the bomb. In total, 41 Grand Slams were dropped during the war.
After BC had withdrawn Upkeep (the bouncing bomb) they then lacked the means of destroying walled masonry dams. They could find no means of placing Tallboy alongside the dam wall. Masonry walled dams are presumably resistant to earthquakes.
The bottom line is that BC lacked the means of destroying either the Urft or Schwammanauel Dams. But they did try valiantly - that is clear.
Does anyone know whether Grand Slam was ever used against a walled masonry dam, and if so with what result.
Any comment or observation on the above conclusion, or on anything else, would be most welcome.
And again thank you, Kutsch and Bruce Dennis, for your help.
Tony