What happens when it rains in urban landscapes?
When it rains on urban and suburban landscapes, impervious surfaces like parking lots, streets, and other concrete exterior prevent the water from soaking into the ground. When water can’t properly seep through the surface, it goes downhill. Along the way, it picks up pollutants. These are often harmful chemicals, metals, trash, and other contaminants. These contaminants ultimately end up in rivers, lakes, or oceans, polluting the water and potentially harming any aquatic life in these systems. So, despite lots of rain, much of the water is just wasted as runoff and polluted in the same process.
How do we make sure runoff water doesn't cause trouble in the absence of open spaces?
The issue of runoff is serious, but the good news is that there are solutions too. In this article, we’ll understand the alternatives we’ve got to address this issue.
The traditional way of managing surface runoff
The traditional way of cleaning and storing run-off in open ponds and sewers (above-surface storage) has been in practice for ages. In this system, the runoff is stored above the surface of the ground in ponds, sewers, or man-made tanks. The process is simple – collect the runoff gushing down from higher points and store it in open areas. The often huge ponds are designed specifically to handle stormwater are capable of storing runoff water. Common pollutants like pieces of debris and waste either get filtered out at the inlet of the pond or settle down at the bottom of the pond. The process is common and for good reason. It’s extremely effective if we have a lot of undeveloped land at our disposal. With a severe lack of undeveloped land, however, this system is at its limits.
Water quality issues
Other than space saving, there’s another grave concern which the above-ground stormwater system must address – The issue of maintaining the quality of water. Stormwater ponds are designed to contain and/or filter out pollutants that enter the system together with the runoff. Usually, runoff water remains in the open ponds or tanks for a significant amount of time. This allows the sediment to settle down at the bottom of the pond. But there’s an issue with that. Water that stands still in open ponds for long periods of time becomes a breeding ground for mosquitoes and pathogens. Geese and other wild animals may also become unwanted residents. All of these put together degrade water quality significantly.
Runoff water is collected in these open ponds so that it could be used at a later stage. However, a lot of water simply evaporates. The larger the area of a pond, the higher the evaporation rate. A pond can lose up to 1% of the water pumped into it in an hour. When summers are dry, you want to be able to store as much as you can, without losing it.
Public safety concern
Standing water in open ponds often causes public safety concerns for children playing in and around the ponds. Steep slopes and unfenced boundaries are unsafe which can easily give way to fatal accidents, if people are not cautious enough.
Maintenance of the detention or retention ponds cna be another concern for this system. It’s a time-consuming and costly affair to remove the sediment once the pollutants have entered the pond. While routine activities like removing debris and mowing are a year-round low-skilled process, removing accumulated sediment requires skilled labor and specialized equipment. In fact, activities like repairing critical structural features of the embankment and pipe might even require the services of a civil engineer.
Clearing clogged water inlets or outlets of ponds is time-consuming to say the least. Clogging happens when debris or sediment accumulates, blocking the flow of water.
Luckily, there are more solutions available to work with. The next one is yet another above-ground system. Green Infrastructure has couple of aces up its sleeve which explains why it got quite popular among smaller communities.
This system imitates a natural ecosystem in terms of catching run-off. Green infrastructure strategies reduce the flow of stormwater and locally manages stormwater through infiltration (water soaking into the ground), capture and reuse of the water. The collected water can also be used to develop rain gardens, vegetated swales, and wetlands.
Because of this, many of the open pond drawbacks simply don’t apply for this system, making it flexible and just an all-round solution to the problem of managing stormwater runoff.
Though developing green infrastructure seems like a solution without any issues, it has some major limitations which cannot be overlooked.
The biggest drawback of the system becomes apparent when the flow rate of stormwater reaches higher levels. The erosion, development cost, and maintenance cost make it a less desirable solution in the long run and can only be remedied with highly specialized materials.
On top of that, the facility for developing green infrastructure is also completely dependent on the availability of land space which, especially in urban areas, is a scarce resource. Under these circumstances, utilizing valuable commercial space for stormwater management doesn’t seem like the wisest option.
So, when land comes at a premium, what can you do? What is economically viable?
Well, much like a lack of land led to a never ending race of developing the highest skyscraper in today’s cities, the solution seems to be to use layers.
Only this time, instead of going up, we go down.
Sub-surface stormwater management. A serious contender?
A sub-surface stormtank system claims to address most of the objections raised against an above-ground system for managing stormwater. While not universally applicable, let’s find out if this system can really provide an economical and sustainable solution to the problem of stormwater in urban areas.
A good sub-surface stormwater management system is one that significantly reduces the flow of runoff, removes debris and oil from water effectively and has enough storage to temporarily store even a huge quantity of water to be used as and when required.
That’s right. Instead of diverting water to a pond, you’ll be storing in on-site. All of it.
Since one of our concerns is saving space, you need to be able to build on top of it. So that means that a good sub-surface system should also have load-bearing capacity to support infrastructure development over its surface.
LOAD BEARING SUB-SURFACE STORMWATER MANAGMENT?
No more ponds or marshes. Instead, the entire thing is contained in a tank below the ground. Not a normal tank, but we’ll get into that later.
The stormtank system is a sub-surface system and is installed below the surface of the earth while the land above could be used for any load bearing purposes like the development of parking lots, the creation of athletic fields, and pavements. This way, there is zero waste of land and what’s more, the land can still be used for commercial purposes. On the contrary, all above-ground stormwater management systems rely on undeveloped land.
Maintaining the quality of runoff
In a sub-surface system, the utmost emphasis is placed on maintaining the quality of the runoff. The debris and oils that come along with runoff flow are separated at the entrance of the system with the help of a stormtank shield. The collected runoff can either be left in the chamber to get seep into the native soil or can be discharged off to the nearby streams. It can also be used for irrigation. After all, the water is devoid of pollutants.
While above-ground wet ponds also segregate the waste at the inlet, a common problem is clogging. This happens more often when the height difference between the source and the receiving body is insufficient. In the sub-surface stormwater management system, the storm chamber is always built below the surface of the land. Because of that, there will always be a substantial height difference. The water falls into the chamber at high speeds while the stormtank shield acts as a deterrent to debris and oil.
Huge storage capacity
A subsurface system should be built to take care of an enormous quantity of runoff. The problem with water swales and detention tanks is that they are just not capable of handling huge flow of stormwater which then leads to flooding and soil erosion. A subsurface system can easily slow the flow of runoff, collect the entire runoff, filter out the pollutants and finally keep the clean water for further usage.
Replenishes water table
Naturally, stormwater soaks into the soil and maintains the level of water under the ground. Re-building the water table below the surface is very important for sustainability. Though the above-ground systems like detention ponds ensure the water table is maintained, water is held for a longer time which raises public health concerns. Sub-surface stormwater systems recharge groundwater while also making sure the water is clean and compliant to environmental regulations.
Let’s look at a real-world application.
Stormtank Case Study
What does this look like in a real-world scenario?
Let’s take a look at the Seattle Hill Road project. Here, the road needed to be widened but since it was common for the road to flood, handling stormwater was a critical issue.
With no pond or marsh available nearby, the options for managing stormwater were limited.
This was the goal: widen 1.6 miles of roadway which on a normal day supports 15,000 passenger vehicles and 67 buses, introduce dedicated sidewalks and bike lanes, planter strips, tree box filters, grass-lined retention areas, and porous concrete sidewalks.
This was no back road and, as a consequence, load baring capacity was important. So if the stormwater system was going to be place below the road, it needed to be able to handle this amount of traffic.
This is where it gets interesting. The current setup was a typical storm drain and dry-well system. That meant that a lot of space was needed to handle the runoff, leaving travelers with a narrow road. By shifting the entire stormwater structure to a space below the road, all this space became available for a wider road, sidewalks and bike lanes.
Now, using a subsurface stormtank system, rated to take high loads, they could replace the existing storm drain and save space.