diff --git a/swash/processing/mat_npz.py b/swash/processing/mat_npz.py
index 5a603cd..0d4b6bc 100644
--- a/swash/processing/mat_npz.py
+++ b/swash/processing/mat_npz.py
@@ -27,8 +27,10 @@ inp.mkdir(parents=True, exist_ok=True)
 
 log.info(f"Reading swash output from '{sws_out}'")
 raw_tsec = sio.loadmat(sws_out.joinpath("tsec.mat"))
-i = np.fromiter((k[5:] for k in raw_tsec.keys() if re.compile(r"^Tsec_").match(k)), dtype="U10")
-t = np.fromiter((raw_tsec[f"Tsec_{k}"][0,0]*10**3 for k in i), dtype=np.uintc)
+i = np.fromiter(
+    (k[5:] for k in raw_tsec.keys() if re.compile(r"^Tsec_").match(k)), dtype="U10"
+)
+t = np.fromiter((raw_tsec[f"Tsec_{k}"][0, 0] * 10**3 for k in i), dtype=np.uintc)
 np.save(inp.joinpath("t"), t)
 del raw_tsec
 raw_xp = sio.loadmat(sws_out.joinpath("xp.mat"), variable_names="Xp")
@@ -39,14 +41,33 @@ del raw_xp
 raw_botl = sio.loadmat(sws_out.joinpath("botl.mat"), variable_names="Botlev")
 botl = raw_botl["Botlev"][0]
 np.save(inp.joinpath("botl"), botl)
-del raw_botl
+del raw_botl, botl
 
 raw_watl = sio.loadmat(sws_out.joinpath("watl.mat"))
-watl = np.asarray([raw_watl[i0][0] for i0 in np.char.add("Watlev_", i)], dtype=np.single)
+watl = np.asarray(
+    [raw_watl[i0][0] for i0 in np.char.add("Watlev_", i)], dtype=np.single
+)
 np.save(inp.joinpath("watl"), watl)
-del raw_watl
+del raw_watl, watl
 
 raw_vel = sio.loadmat(sws_out.joinpath("vel.mat"))
 vel_x = np.asarray([raw_vel[i0][0] for i0 in np.char.add("vel_x_", i)], dtype=np.single)
 np.save(inp.joinpath("vel_x"), vel_x)
-del raw_vel
+del raw_vel, vel_x
+
+raw_zk = sio.loadmat(sws_out.joinpath("zk.mat"))
+n_zk = (len(raw_zk.keys()) - 3) // t.size
+zk = np.asarray(
+    [
+        raw_zk[i0][0]
+        for i0 in np.char.add(
+            np.char.replace("zki_", "i", np.arange(n_zk).astype("U1"), count=1)[
+                None, :
+            ],
+            i[:, None],
+        ).reshape(-1)
+    ],
+    dtype=np.single,
+).reshape((t.size, n_zk, x.size))
+np.save(inp.joinpath("zk"), zk)
+del raw_zk, zk