Roadside swales have multiple functions. A major function is to maintain a dry roadbed. Keeping the centerline of the swales at least 1.1 feet below the centerline of the roadway helps to keep the roadbed above the water-table. This helps to make the road last longer. Another function is to store run-off from the roadway to mitigate the effect of the impervious surface of the roadway. The immediate function of the swale in a rainfall event is to clear the roadway of stormwater.
To understand the relationship between the multiple functions it is useful to identify the four states of the swale.
- Dry
- Puddling
- Low flow
- High flow.
In the first state, the swale is dormant. This is the condition during a dry period, with a low water table. The second state is where there is some residual water left in the swale. This is normal because the storage water is being absorbed into the ground. This is typical for periods of three days to a week after a rainfall event. The water stays in the swale until the water table subsides enough to absorb the water. This is also the way small amounts of water used for irrigation are meant to be handled. The third state is typical for the first three days after a rainfall event. There will be a low velocity flow along the swale in the general direction of the outfall. Usually only the center of the water will show any movement. It takes a long time for the surface vegetation to release all the rainwater gathered during a rain, so it seems to just keep coming even though there is no readily identifiable source. In some cases, the flow will continue indefinitely if the water table is high enough to supply the flow. The fourth state is the one that demonstrates the system's ability to remove Stormwater at the full capacity for which it was designed. The swale capacity has been calculated to handle certain statistical rainfall events efficiently and quickly. Most of this flow will be finished within an hour after the storm has stopped. The conditions will then return to the third state.