Constructed Waterway Designer

I increased the width of my Low Flow Channel (LFC) by 5m but the total width of the cross-section increased by less than 5m. What's going on?

This screen shot shows an example of a cross-section prior to the LFC being widened.

This cross-section has been widened as shown by the yellow dashed lines.

Because the width of the Low Flow Channel has increased, the water depth required to carry the 3 month or 1 year flow is less:

Figure 3 - New water levels

The increased LFC width means that the main waterway has a lot larger capacity than the original cross-section at any water surface level. Consequently the depth to carry the 1% flow doesn't need to be as much as it was for the original cross-section, and the total cross-section width doesn't increase by 5m.