avoid valuerror with empty coastline list (#717)

* avoid valuerror with empty coastline list * updated whatsnew
Deltares · Dec 13, 2023 · f921fcc · f921fcc
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diff --git a/dfm_tools/coastlines.py b/dfm_tools/coastlines.py
@@ -67,8 +67,11 @@ def get_coastlines_gdb(res:str='h', bbox:tuple = (-180, -90, 180, 90), min_area:
         coastlines_gdb_L3 = gpd.read_file(file_shp_L3, include_fields=include_fields, where=f"area>{min_area}", bbox=bbox)
         coastlines_gdb_list.append(coastlines_gdb_L3)
 
-    # remove empty geodataframes from list to avoid FutureWarning and concatenate
+    # remove empty geodataframes from list to avoid FutureWarning
+    # escape for empty resulting list and concatenate otherwise
     coastlines_gdb_list = [x for x in coastlines_gdb_list if not x.empty]
+    if not coastlines_gdb_list:
+        return gpd.GeoDataFrame()
     coastlines_gdb = pd.concat(coastlines_gdb_list)
     print(f'{(dt.datetime.now()-dtstart).total_seconds():.2f} sec')
 

diff --git a/docs/whats-new.md b/docs/whats-new.md
@@ -5,6 +5,9 @@
 - automatically parse epsg from FM mapfile to crs in `dfmt.open_partitioned_dataset()` by [@veenstrajelmer](https://github.com/veenstrajelmer) in [#685](https://github.com/Deltares/dfm_tools/pull/685)
 - added spatial/temporal subsetting and retrieving of insitu observation data with `dfmt.ssh_catalog_subset()` and `dfmt.ssh_retrieve_data()` by [@veenstrajelmer](https://github.com/veenstrajelmer) in [#711](https://github.com/Deltares/dfm_tools/pull/711)
 
+### Fix
+- prevent ValueError upon concatenation of only emtpy coastlines geodataframes by [@veenstrajelmer](https://github.com/veenstrajelmer) in [#717](https://github.com/Deltares/dfm_tools/pull/717)
+
 
 ## 0.18.0 (2023-12-08)