What happens when thousands of litres of rainwater start gushing down a mountainside?
Follow along as we investigate a couple considering buying land and building a home in El Salvador. Before they buy, we're putting the property through a series of real-world tests, from what's beneath the ground to whether the land can actually support the home they imagine. Will they ultimately buy it? The file's still open.
The house would sit on a ridge. Water running away from it sounded like good news.
But, can this property really handle a house, a large terrace and a pool?
The proposed building area occupies high ground along a ridge. There is no significant mountainside above it collecting rainfall and sending runoff toward the proposed house.
The land already wants to shed water away from the building area, not toward it.
After investigations into the soil, the groundwater, and the rainwater the roof could capture, drainage looks almost embarrassingly straightforward. Rain falls on high ground. Gravity takes it downhill. Done.
But that's the property as it currently sits, untouched. The buyer isn't planning to leave it that way. Before construction, the land decides where the water goes. After construction, the design does.
A roof gathers rainfall that used to land across open ground and concentrates it into gutters and downspouts. But building changes the flow. A terrace adds hard surface. A driveway can collect and redirect runoff. Grading changes the contours that once decided where water travelled naturally.
And Investigation 03 adds one more thing: a rainwater cistern, built specifically to capture a substantial share of the roof's runoff.
None of this creates more rain. It changes where the rain collects, how quickly it moves, and where it eventually has to go.
Investigation 03—The Rainwater—asked whether the roof could become another water source for the house. The answer was promising. But now the rainwater comes back into the story.
Storage has a ceiling. During prolonged rain, or when storage is already high from an earlier storm, the tank eventually reaches its limit.
At that moment the water changes roles. The next litre isn't a resource anymore. It's runoff.
The Rainwater investigation asked how much of this property's rain could be kept.
The Drainage begins with what it can't.
The first instinct was to look for an obvious natural destination: a river, or a quebrada. So where could all that water actually go? Answering that meant first learning what a quebrada was: a natural ravine or drainage channel that may carry water seasonally or mainly during heavy rainfall rather than year-round.
To find out, the buyer turned to El Salvador's Ministry of Environment and Natural Resources—MARN—and its official hydrographic maps. Those maps classify the country's waterways, including quebradas. No mapped quebrada was identified crossing the proposed building area. MARN official hydrography
But the absence of a mapped quebrada turned out not to be the obstacle it first appeared to be.
The buyer's next stop was OPAMSS, the planning authority responsible for reviewing development and drainage requirements in the area. And there, the investigation took an unexpected turn.
Because this wasn't simply a question of how to drain the property. Before that could happen, the proposed drainage had to be shown to be feasible.
And OPAMSS has a process for exactly that.
It's called Factibilidad de Aguas Lluvias—stormwater drainage feasibility—or, in the regulations, simply FALL.
FALL has two pathways. FALL-1 applies when a property does not border or have a river or quebrada crossing it. FALL-2 applies when it does.
Based on what the mapping had shown, this property appeared to fall into FALL-1.
In other words, not having a mapped quebrada wasn't the dead end the buyer had feared.
The buyer had been looking for somewhere the water already went.
OPAMSS was asking a different question:
Where could it safely be sent?
The terrain answers one part of the problem immediately. The property has substantial natural fall away from the proposed house. Moving water downhill isn't difficult here.
But downhill is only a direction.
Once rainfall has been concentrated by roofs, gutters and overflow, a steep descent adds speed. And uncontrolled water moving quickly down a mountainside can create erosion rather than solve a drainage problem.
So the challenge isn't getting the water to move. It's controlling it. The drainage plan has to manage both volume and velocity before the water reaches an accepted outlet.
Then the regulations produced a number the buyer had already seen in the previous investigation. Suddenly, 150 square metres mattered.
OPAMSS scales stormwater requirements according to a project's total impermeable surface — not just the roof, but the impermeable area already present plus the hard surfaces the project proposes to add. OPAMSS drainage regulation
And that's where another unfamiliar term entered the investigation: SUDS — Sustainable Urban Drainage Systems, or Sistemas Urbanos de Drenaje Sostenible.
The name sounds more complicated than the idea. Instead of allowing rainwater from hard surfaces to rush away all at once, SUDS uses measures that capture, store, slow, infiltrate, filter or otherwise control runoff before it reaches its eventual outlet.
OPAMSS increases the drainage requirements as the project's impermeable footprint grows. Once that total passes 150 square metres, a SUDS proposal has to be backed by calculations. Above 500 square metres, it steps up again to a full hydrological and hydraulic study.
The buyer already had one number to put into that calculation: the estimated roof area from Investigation 03.
Now add the terrace. The driveway. And any other surface that stops rain soaking naturally into the ground.
Suddenly, drainage had a price tag.
Which produced another useful surprise: the cistern from Investigation 03 can be part of that strategy.
OPAMSS's own SUDS guidance recognizes rainwater-collection tanks as a form of source control. The tank wasn't only storing water for the house. While it still has capacity, it was also holding roof runoff back from the slope. OPAMSS SUDS guidance
Of course, a full tank has little storage left for the next storm. The cistern doesn't eliminate drainage. It simply becomes one piece of the drainage strategy.
The final impermeable footprint will determine how far the drainage requirements step up. If the project remains within the lower bands, the buyer is dealing with a SUDS proposal and, once the total impermeable surface exceeds 150 square metres, supporting calculations.
But if the project exceeds 500 square metres of total impermeable surface, OPAMSS requires a hydrological and hydraulic study, together with the mitigation system.
And that study has a price tag.
It has to be prepared as part of the professional drainage work. A reliable project-specific cost cannot be determined until the final footprint and scope are known, but it is another professional service and another pre-construction cost the buyer needs to add to the project budget.
By this point, the investigation could finally answer the question it started with: can the water actually go somewhere?
Yes.
The site has the fall needed to move stormwater away from the proposed house. The regulations provided a way to manage it.
What remains is to calculate the project's final impermeable footprint, choose the appropriate drainage measures, and establish the accepted outlet through the formal design and approval process.
That is professional design work. It is not evidence that the site has failed the drainage investigation.
But after following the water, checking the regulations and accounting for the steep terrain, the investigation found no drainage condition that provides a reason to walk away from the property.
THE DRAINAGE — BUILDABILITY: PASS
But rainwater isn't the only water this house will eventually have to send somewhere.
Every shower.
Every sink.
Every toilet.
And there is no sewer waiting at the road.
Which leaves another question...
Where does that water go?
INVESTIGATION 04 · THE DRAINAGE — COMPLETE
4 QUESTIONS REMAIN
This is Investigation 04 of CAN I BUILD HERE?, a Build El Salvador series following the real-world due diligence behind a potential land purchase in El Salvador. Interested in sponsoring this investigation or a future installment? Become a Build El Salvador sponsor
