Through-flow

Many urban water systems contain blind angles, for example, ditches without culverts, running perpendicular to a dam or dyke, watercourses with weirs or far corners of a pond. These are locations featuring stagnant water, which may cause issues with, e.g., excessive growth of duckweed, floating algae beds or (blue-green) algae. A watercourse is highly vulnerable if the water cannot continue to flow, for example, in a blind watercourse. A watercourse that is temporarily closed is moderately vulnerable. Watercourses connected to other watercourses, in which water can flow freely, are not vulnerable.

Periods of drought often require letting water into a water system from a main water system in order to keep the water level up to par. Water that is let in will flow through the system. An inlet system can also be used to flush a water system suffering from poor water quality, for example, because the water has been stagnant for a prolonged period. A water system without any options for letting in additional water is highly vulnerable. A system is moderately vulnerable if water cannot be let in during drought. A system that can always let in sufficient inlet water to ensure its through-flow is not vulnerable.

In minor water systems, such as wells and brooks, seepage ensures a constant flow. In such systems, seepage supplies sufficient water to create a flow. This is not the case in larger water systems, such as ponds and canals. Here, stagnant water adds to the probability of issues arising from, for example, duckweed, floating algae beds or (blue-green) algae. A system is not vulnerable if its flow rate exceeds 0.5mm of water per day.

Aquatic plants obstruct the through-flow of water. The extent to which aquatic plants obstruct such through-flow depends on the flexibility of the aquatic plants, their volume and their spread across the water system. For example, an open flow course prevents stagnation of water, whereas in an overgrown watercourse, water may remain stagnant for quite a long period of time. Is the through-flow of a water system visibly limited because of aquatic plants? Then the system will be highly vulnerable to issues arising from, for example, excessive duckweed, floating algae beds or (blue-green) algae. Are aquatic plants visibly limiting through-flow at a few locations? Then the system will be moderately vulnerable to such issues. If the through-flow is not affected by aquatic plants, the water system will not be vulnerable.

Culverts are continuous connecting pipes between watercourses. If a culvert is situated too low, duckweed, branches and other debris will pile up and block the culvert, thus obstructing through-flow of the water. In order to guarantee through-flow, the top of a culvert must always rise some distance above the waterline. Is a culvert situated entirely under water, and do you see duckweed piling up in front of it? Then the water system will be highly vulnerable to issues arising from, for example, duckweed, floating algae beds and (blue-green) algae. A culvert that is situated sufficiently high in order to allow duckweed to pass but is partially blocked will render the water system moderately vulnerable to such issues. A system is not vulnerable if it has no culverts or if duckweed can easily pass the culverts and the culverts are not blocked.

Culverts are continuous connecting pipes between waterways. Culverts that were installed in the past are now increasingly too narrow to transport all the water during heavy rainfall. The water then comes to a temporary standstill. Is this the case? If so, it makes the water system very vulnerable to problems caused by, for example, duckweed, flab, and (blue-)green algae. Is the culvert large enough to transport all the water during heavy rainfall? Then it does not make the system vulnerable.