From Houston to Toronto, a common theme has been flooding caused by the increase in impermeable surfaces. In June, 2017, part of Southwestern Ontario was hit with a 1 in 100 year storm event, and some areas received 6.7 inches (170 mm) of rain in 7 hours. In August, 2018, Toronto received 2.95 inches (75 mm) of rain, and Black Creek rose 11.5 ft (3.5 metres) in 90 minutes.
So, what does this have to do with rural drainage? Lots!
First, let’s consider changes in the area of farmsteads. There are factors that lead to an increase in the size of farm buildings. Farm safety in the poultry industry is moving buildings from multi-story facilities to single story buildings, and more of them. Animal welfare standards are increasing the number of square footage per animal. Technology and efficiency in barn design are resulting in larger areas for traffic and transportation. Bunk silos are replacing upright silos.
As farm buildings get larger, the volume of water that runs off the roof and paved areas during storm events also increases. I spent a decade asking farmers to divert clean upslope water away from livestock yards and manure storages, and this usually involved downspouts and tile. The speed at which it is discharged to the receiving drainage ditch or channel after a storm event also increases.
Once fields are tiled, what is the impact when new structures are built, and downspouts tiled into the system? When designing municipal drainage systems, the engineer will account for the area requiring drainage and the land use. Is the land use primarily row crops, permanent pasture, or forested? What they will rarely account for is impermeable surfaces.
Let’s use an example. There are 3 poultry barns, approximately 9800 ft2 per structure. If we got 2 inches of rain in one storm event, that would equal 1633 ft3 of water per structure, or 4899 ft3 from the roofs alone. Without the buildings, this water would simply infiltrate the soil, or run off the farmstead into adjacent fields for infiltration. Some erosion may occur in the field areas sloping away from the farmstead.
If the barn downspouts are directed into the tile drainage system, this is a large volume of water for the system to absorb in a relatively short period of time. The tile systems may discharge into a municipal drain, and both systems must be able to handle it. The age of the tile system and municipal drain, the frequency of maintenance, and original design capacity will all affect the severity of the implications of storm event. Even if the drains can hold the volume of water in open channel systems, they might not be able to handle the speed at which the water enters the system. The direct discharge of the water from the farmstead can influence the degree of fluctuation in water levels, similar to the Black Creek experience. In addition, some of the storm events are happening during the winter period, when infiltration will be non-existent. The stability of the banks and the capacity of the drain to slow the water flow is vital to reducing the erosive forces.
One farmer pointed out to me that there were always farmsteads with impermeable surfaces. Wouldn’t the volume of water be the same? Instead of a farmstead on every farm lot, now there are fewer farmsteads but they are larger. True! Comparing aerial photography from 1950 to today, we could calculate the difference in impermeable surface. However, we also have an increase in tile drainage, larger field sizes, straighter drainage channels and more roads. Although I have no concrete (no pun intended) proof, even if the square footage of impermeable surface was the same, the environmental and landscape conditions are much different.
If those structures were in an urban area or even used for something other than agriculture, there would be a requirement for a stormwater management plan. One of the reasons for this is that urban areas tend to be more restricted in area available to manage water, so it needs to be considered during site planning. I am not suggesting that we should move in a regulatory approval site-specific direction, but rather consider the many unique opportunities for water conservation or reuse in rural areas.
In Ontario, we don’t usually have a need for irrigation, but in future, maybe we will. The economics of this would play out better in higher value crops.
What challenges or opportunities would there be in stormwater ponds or even natural channel designs on drains that would incorporate water retention? The poultry operators I spoke to were concerned with open ponds that would encourage wildlife. This may pose a biosecurity risk for the industry. However, ponds could be situated far enough away from the barn to reduce the risk.
I have spoken to farmers whose well water is not of sufficient quantity or quality for their farm operations. Some use cisterns, with appropriate treatment, for water sources for their livestock or for pressure washing. Could we incorporate a rural cistern sort of system where stored water could be used for fire-fighting?
For farmers and tile contractors, the time to think about this is in the pre-construction period. There is opportunity in agriculture to find solutions that work.
Source : Field Crop News