Index: trunk/data/validator/unnecessary.mapcss
===================================================================
--- trunk/data/validator/unnecessary.mapcss	(revision 13377)
+++ trunk/data/validator/unnecessary.mapcss	(revision 13378)
@@ -54,2 +54,13 @@
   fixRemove: "payment:cash";
 }
+
+/* see #10346, #15667 */
+way[waterway][oneway?] {
+  throwWarning: tr("{0} is unnecessary for {1}", "{1.key}", "{0.key}");
+  group: tr("unnecessary tag");
+  fixRemove: "{1.key}";
+}
+way[waterway][oneway=-1] {
+  throwWarning: tr("{0} is unnecessary for {1}. The flow direction is defined by the way direction.", "{1.key}", "{0.key}");
+  group: tr("unnecessary tag");
+}