How to investigate flow issues in waterways

When using the Water Overlay, water should usually be able to flow freely through waterways. For some situations, it may be evident that water should not be able to flow further, such as at a properly configured Weir or Pump, or when the bottom of the waterway rises or has an obvious bump.

How to check whether water flows properly

You may be able to see visually on the Overlay where potential issues exist, but by placing a Measurement in the waterway you will be able to see more accurately how much water is where in the waterway.

How to check whether water flows properly through a waterway:

1. Ensure the Water Overlay has a surface last value result type and a surface elevation.
2. Open the Measuring tool.
3. Create a line Measurement, with one endpoint at the start of the waterway, and the other endpoint at the end of the waterway.
4. Set the primary Overlay to surface last value, the base overlay to surface elevation, and activate "sum".
5. Set the Measurement graph display mode to "fit to graph".
6. You now see the water level across the length of the waterway.

See if the resulting summed line is (nearly) flat, or if there is a point where the water level suddenly jumps to a different level.

What can you look for when a flow issue occurs

Because inhibited waterflow can have a number of causes, there isn't a single fix for all situations. Check whether any of the following apply to your situation, and interpret in the context of your own use-case what the desirable solution would be.

Is the surface elevation in that location what you expect?

• In the relevant waterway, use the Measurement made earlier to see whether the height of the bottom of the waterway is as shallow or as deep as you expect.
• Also check whether the height of the terrain is consistent, or inclined, and whether the height of the terrain exceeds the height of the water at any point.

Is there a Building in the waterway which causes a blockage?

• Activate the Water Overlay and see whether there is a Building in that location. See if there is a Weir or a Pump with an angle configured. You can tell by the dam visualized in that location. If so, inspect that Building to see how and when water can flow past it, and how much.
• Also see if there is an Inlet, Pump, or Overflow in that location. It's possible water is added in that location and just flows off in one direction, or that in that location water is removed at such a rate that it doesn't reach the other side of the waterway.

Is the grid cell size so large that the sides of the waterway are recomputed to be more shallow?

• Make sure you have both the general Heightmap Overlay and the surface elevation result type Overlay in your Project. Create a Measurement perpendicular across the waterway, and show both Overlays, one as primary and the other as base. Take note of whether the surface elevation Overlay's results match the depth of the Heightmap Overlay, and whether the incline at the sides matches up. If the incline is shallower, the grid cell size may be too high to allow for the intended steepness.
• If the waterway is narrow, and not aligned to cardinal directions, it's possible a cell partially on one shore of the waterway is adjacent to a cell partially on the other shore of the waterway. In other words there is no cell which is entirely part of the waterway for water to flow through. The result is that the waterway is effectively blocked.

Is there a hydrological feature nearby which blocks the waterway?

• If available, use the base types result type Overlay to inspect the location and see whether cells are marked as Breach, or as outside of calculation area, or otherwise inconsistently with the rest of the waterway.
• If the base types result type is not available, check whether there is a Breach in the indicated location. You can do so by opening the Water Overlay's configuration wizard, open the step concerning "Breaches", and check the locations of the Breaches there (if any).

Is there an edge of the calculation model nearby?

• Check what the current grid cell size is, and check whether the issue occurs near the edge of the project area. Note that the outer two cells of the project area are not part of the calculation.
• Check whether the location of the issue is inside the designated calculation area. You can do so at the weather event step in the configuration wizard, where you can check the configuration for the calculation area. If it is set to Water Areas only, check whether the location of the issue is covered by a Water Area. If it is set to specified areas only, check whether the location of the issue is covered by a Limit Area.
• Check whether the issue takes place near the the edge of the calculation area. The closer to the edge a calculation takes place, the more chance there is of unintended edge effects.