Index: trunk/data/validator/geometry.mapcss
===================================================================
--- trunk/data/validator/geometry.mapcss	(revision 15832)
+++ trunk/data/validator/geometry.mapcss	(revision 15833)
@@ -394,2 +394,17 @@
   suggestAlternative: "building=warehouse";
 }
+
+/* #17533 - Check waterway flow direction */
+*[waterway][waterway!=drain] > node:connection {
+  set notDrain;
+}
+way[waterway=drain] >[index!=1] node:connection {
+  set notDrainFirstNode;
+}
+way[waterway=drain] >[index!=-1] node:connection {
+  set notDrainLastNode;
+}
+way[waterway=drain] > node:connection!.notDrainFirstNode!.notDrain,
+way[waterway=drain] > node:connection!.notDrainLastNode!.notDrain {
+  throwWarning: tr("incorrect drain flow direction");
+}