function setVisibility(element, isShown){
 if (isShown) {
   element.style.visibility = "visible";
 }else element.style.visibility = "hidden";
}

function SlideDown(){
	var residentialSpotDredging = document.getElementById("dredgingmenu");
	setVisibility(residentialSpotDredging, true);
}

function SlideUp(){
	var residentialSpotDredging = document.getElementById("dredgingmenu");
	setVisibility(residentialSpotDredging, false);
}