fix: NaN in CourbeRemous results due to negative Y values in Calc_Y_RK4()

refs #339

src/open-channel/remous.ts

@@ -757,7 +757,7 @@ export class CourbeRemous extends SectionNub {
         }
 
         // let k2 = this.Calc_dYdX(Y + Dx / 2 * k1);
-        const rDXDY2: Result = this.Calc_dYdX(Y + rDx / 2 * k1);
+        const rDXDY2: Result = this.Calc_dYdX(Math.max(Y + rDx / 2 * k1, 0));
         if (!rDXDY2.ok) {
             return rDXDY2;
         }
@@ -768,7 +768,7 @@ export class CourbeRemous extends SectionNub {
         }
 
         // let k3 = this.Calc_dYdX(Y + Dx / 2 * k2);
-        const rDXDY3: Result = this.Calc_dYdX(Y + rDx / 2 * k2);
+        const rDXDY3: Result = this.Calc_dYdX(Math.max(Y + rDx / 2 * k2, 0));
         if (!rDXDY3.ok) {
             return rDXDY3;
         }
@@ -779,14 +779,14 @@ export class CourbeRemous extends SectionNub {
         }
 
         // let k4 = this.Calc_dYdX(Y + Dx * k3);
-        const rDXDY4: Result = this.Calc_dYdX(Y + rDx * k3);
+        const rDXDY4: Result = this.Calc_dYdX(Math.max(Y + rDx * k3, 0));
         if (!rDXDY4.ok) {
             return rDXDY4;
         }
         const k4 = rDXDY4.vCalc;
 
         // tslint:disable-next-line:variable-name
-        const Yout = Y + rDx / 6 * (k1 + 2 * (k2 + k3) + k4);
+        const Yout = Math.max(Y + rDx / 6 * (k1 + 2 * (k2 + k3) + k4), 0);
 
         // if ($this ->rDx > 0 xor !($Yout < $this ->oSect ->rHautCritique)) { return false; }
         if (XOR(rDx > 0, !(Yout < hc))) {