diff --git a/B06_Section/B06_Section_UI_Cross_Body_Guard.ts b/B06_Section/B06_Section_UI_Cross_Body_Guard.ts
index 640af0277..b3fdefb2b 100644
--- a/B06_Section/B06_Section_UI_Cross_Body_Guard.ts
+++ b/B06_Section/B06_Section_UI_Cross_Body_Guard.ts
@@ -271,3 +271,20 @@ export function withoutGuardedFill<T extends BoxLayout | FordLayout>(
     label: { ...ford.label, text: `${ford.label.text} L${Math.ceil(lengthM - 1e-6)}m` },
   } as T;
 }
+
+/** 세월교 · 박스 유입 흙막이 수평공간 시작 = 본체 바닥(세월교 바닥판 · 박스 저판 상면) 높이의
+ *  흙막이 전면 자리(89-3). 본체가 이 측점에 없으면 null(배관 규칙). */
+export function guardFloorStart(
+  section: CrossSection,
+  base: BodyGuardBase | null,
+  wall: WallLayout,
+): OffsetPoint | null {
+  const end = base?.ends.inlet;
+  if (!end) return null;
+  const slab = section.culvert?.body === "box" ? (section.box?.slab_thickness_m ?? 0) : 0;
+  const floor = end.invert + slab;
+  return {
+    offset: wall.points[2].offset + wall.outward * wall.lean * (wall.topJoint.elevation - floor),
+    elevation: floor,
+  };
+}
diff --git a/B06_Section/B06_Section_UI_Cross_Culvert_Geom.ts b/B06_Section/B06_Section_UI_Cross_Culvert_Geom.ts
index 7c3b5bbd9..14c2cecde 100644
--- a/B06_Section/B06_Section_UI_Cross_Culvert_Geom.ts
+++ b/B06_Section/B06_Section_UI_Cross_Culvert_Geom.ts
@@ -25,6 +25,7 @@ import { buildRevetWallGeometry } from "./B06_Section_UI_Cross_Wall";
 import {
   bodyGuardAxes,
   bodyGuardBase,
+  guardFloorStart,
   bodyGuardHeight,
   fordPipeFit,
   mergeGuardTrim,
@@ -596,7 +597,7 @@ export function computeCulvertLayout(
     outletBottom: outletWall?.bottomBack.elevation ?? 0,
     basinStart: culvert.hidden_pipe
       ? inletWall
-        ? pipeCorners.inlet.bottom
+        ? (guardFloorStart(section, body, inletWall) ?? pipeCorners.inlet.bottom)
         : null
       : basinFillStart,
     basinBottom: culvert.hidden_pipe ? (inletWall?.bottomBack.elevation ?? 0) : basinFillBottom,
diff --git a/resources/tester/helper_79_guard_shape.cjs b/resources/tester/helper_79_guard_shape.cjs
index 69a30ad20..aacab3ac3 100644
--- a/resources/tester/helper_79_guard_shape.cjs
+++ b/resources/tester/helper_79_guard_shape.cjs
@@ -154,6 +154,19 @@ out.cutBerm2 = cutTails(
   sections.map((s) => (s === at(352.14) ? berm2 : s)),
 );
 
+// ⑤b 유입 수평공간 시작 높이 = 본체 바닥(세월교 바닥판 상면 · 박스 저판 상면 · 89-3)
+out.inletFloor = [352.14, 173.087].map((chainage) => {
+  const section = at(chainage);
+  const layouts = computeStoredLayouts(section, sections);
+  const start = layouts.culvert.basinFill.segments[0]?.points[0];
+  const inletLeft = (section.uphill_side ?? "left") === "left";
+  const floor = layouts.ford
+    ? layouts.ford.sides.find((s) => s.role === "inlet").slabTopElevation
+    : layouts.box.sides.find((s) => (s.role === "left") === inletLeft).bottomElevation +
+      layouts.box.box.slab_thickness_m;
+  return { start: start ? r3(start.elevation) : null, floor: r3(floor) };
+});
+
 // ⑥ 관 보호공 기본 자리 = 제일 위(성토선 0 지점 · d 0) · 관 길이 m 단위
 const pipe = clone(at(180));
 pipe.culvert = {
diff --git a/resources/tester/test_79_guard_shape.py b/resources/tester/test_79_guard_shape.py
index f33cbcafe..f12a573dc 100644
--- a/resources/tester/test_79_guard_shape.py
+++ b/resources/tester/test_79_guard_shape.py
@@ -87,6 +87,12 @@ def test_흙막이_끝_절토는_수평공간_뒤(shape: dict) -> None:
         assert tail["berm"] == pytest.approx(2.0, abs=1e-3)
 
 
+def test_유입_수평공간_시작은_본체_바닥(shape: dict) -> None:
+    """89-3 — 박스 저판 상면 · 세월교 바닥판 상면 높이에서 수평 → 절토."""
+    for case in shape["inletFloor"]:
+        assert case["start"] == pytest.approx(case["floor"], abs=1e-3)
+
+
 def test_관_보호공_기본_자리는_제일_위(shape: dict) -> None:
     pipe = shape["pipe"]
     assert pipe is not None