Preheat oven to 180 degrees C. Grease and flour a 20x30cm pan. In a small bowl, mix chopped rhubarb with 1/2 cup sugar; set aside. Sift together the flour, bicarb soda, salt, cinnamon and nutmeg. Set aside.
In a large bowl, cream together 125g butter and 1 1/2 cup sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the buttermilk. Fold in the rhubarb mixture. Stir in the coconut and chopped almonds. Pour batter into prepared pan.
To make streusel topping: In a small bowl, combine 4 tablespoons flour and 4 tablespoons brown sugar. Cut in butter until mixture resembles coarse crumbs. Sprinkle on top of cake.
Bake in the preheated oven for 30 to 35 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.