Image credit: A new community redevelopment project in Seattle includes a natural drainage system.
From Stormwater (The Journal for Surface Water Professionals).
The only time we realize that our streets are actually part of a larger-scale ecosystem is during a heavy downpour when storm water decides to use our streets as
temporary rivers.
Engineers deal with storm water the same way they deal with any input-output problem. The equation is simple: to cope with the incoming volume, increase the throughput so that you get maximum output. That is the dogma -and that's how we've been dealing with floodwaters on our streets ever since: Move it out as fast as possible to the nearest body of water. Big storms require big drains and a bigger the flood control system. (Some of our widest roads have five meter diameter pipes as stormwater drains.)
(The biggest example of this thinking in Metro Manila is the
Manggahan Floodway which channelized portions of a river and then built a long canal to drain the low lying parts of the eastern metropolis into the Laguna Lake.)
The
drain-the-waters-as-fast-as-you-can approach, works, to a certain point. It fails if: 1) the input volume is bigger than what you designed the system for; or 2) if the throughput gets clogged.
The throughput does get clogged. Often. Which requires a lot of maintenance, a lot of keeping the channels clean -of building even bigger pipes to drain the water. And every year, we get the same complaint:
trash clogs the drainways. At the same time, we've been paving over more and more of our land area causing more water to run off into our streets and overwhelming our flood control systems.
Year after year, the floods get worse (especially the flash floods on EDSA and the Buendia-SLEX intersection) and year after year, the proposed solution is the same: more maintenance, or proposals for bigger flood control projects.
Time for a rethink.Natural systems approach water differently. Nature prefers to
slow down the water, allowing it to pool around tree roots, slowing the flow with grass, collecting it in seasonal ponds and generally letting it percolate into the soil. This has two effects: 1) it reduces surface water runoff so the creeks and waterways don't get overwhelmed and 2) it preserves precious topsoil, keeping it from getting washed away.
Engineers have realized that the environmentalists do have something to say and have made the connection that speeding up water is ineffectual and damaging to the environment. (You can draw a straight line cause and effect from the
flood control projects in the Mississippi river basin to the tragedy of New Orleans.)
They have also begun to rethink how to handle water runoff on streets and roadways. Yes, you want to drain the roadway, but it doesn't mean channeling the water out to the nearest body of water. It is better to hold and percolate -not shunt it out but allowing it to recharge the groundwater aquifers. (And yes, our aquifers do need recharging. See this
pdf report -2.6mb).
So, landscaping -which was used primarily as ornamentation (and
apparently still is) is now seen as an approach to handling water-runoff. Instead of trying to drain the rainwater off into pipes, landscape systems (like shared tree wells,
permeable pavements and
bioswales) hold the water locally and drain it into the soil rather than draining it into a flood system. The landscape becomes a natural flood control system. (Guess what, it also reduces water pollution -by eliminating nonpoint source pollution and by using the soil to filter the water before it gets to the aquifers.)
So the
clean-and-green proponents and the
save-the-pasig movement need to sit down with our traffic and the flood control engineers to re-think the way our streets deal with water.
Well designed streets, which reconceptualize the network of roads as green infrastructure -so that it not only carries vehicular traffic but also protects the environment, will do more than just provide us with grassy medians, sidewalk trees or flower beds. Well designed streets will probably save our cities and revive our dying rivers and lakes.
NEW! UPDATED! NOW WITH NUMBERS!
A conversation with a friend led me to running some number scenarios of what the stormwater runoff volume for Metro Manila might look like, and though the math is rudimentary, the numbers are startling:
If you assume that 30% our megacity's land area (636 sq.km.) is devoted to pavement (roadways) and 5% devoted to waterways (very generous assumption), then you would get:
- 191 sq.km of roads
- 32 sq. km. of waterways
If a storm dumps water at the volume of
10cm per hour (that's about 4 inches of rain -not so spectacular for the metro), the 191 sq.km of roadways would produce a runoff volume in the order of
190.8M cubic meters. (And we're only looking at the roadway runoff -and naively assuming that nothing else (parking lots, driveways, etc.) drains into the roadways.
Assume, again rather generously, that your 32 sq.km. of waterways has an average depth of 2 meters -then the total carrying capacity of the waterways would be
63.6M cubic meters -or roughly just 1/3 of the runoff our roadway will produce. (And we're assuming that the throughput is at 100% -no trash or blockages.)
That means our waterways will have to
drain completely every 20 minutes if it is to absorb and remove just 1 hour of (10cm) of stormwater runoff from our streets!
If only 50% of our streets (paved areas) had some natural drainage/bio retention system -of say 1M depth holding capacity for every square meter of street, we could absorb half of that runoff (95.4M) locally. No need to wait for the waterways to drain.
