Your Inundation layer should be added to the map as an overlay
and not as a basemap
(tiled layers can be overlays as well as basemaps).
First you'll want to distinguish between your basemap layers and your overlays.
Your basemap layers (streets, satellite, etc) are those layers that are mutually exclusive and should always be on the bottom.
Your overlay layers are not mutually exclusive and will always sit on top of the basemap layers.
The following example is from the Leaflet LayersControl docs page
var cloudmadeUrl = 'http://{s}.tile.cloudmade.com/API-key/{styleId}/256/{z}/{x}/{y}.png',
cloudmadeAttribution = 'Map data © 2011 OpenStreetMap contributors, Imagery © 2011 CloudMade';
var minimal = L.tileLayer(cloudmadeUrl, {styleId: 22677, attribution: cloudmadeAttribution}),
midnight = L.tileLayer(cloudmadeUrl, {styleId: 999, attribution: cloudmadeAttribution}),
motorways = L.tileLayer(cloudmadeUrl, {styleId: 46561, attribution: cloudmadeAttribution});
var map = L.map('map', {
center: new L.LatLng(39.73, -104.99),
zoom: 10,
layers: [minimal, motorways, cities]
});
var baseMaps = {
"Minimal": minimal,
"Night View": midnight
};
var overlayMaps = {
"Motorways": motorways,
"Cities": cities
};
L.control.layers(baseMaps, overlayMaps).addTo(map);
At this point, the basemaps (Minimal and Night View) will always be underneath the overlays (Motorways and Cities).
Make sure your Inundation layer is added to the map as an overlay
and not a basemap
.