Timberborn's Dams and Droughts Versus a Real Detention Pond

A developer once asked me, in all seriousness, why his site plan needed a pond when the lot already "drained fine." He was right that it drained. Before he paved it, two acres of grass soaked up most of a summer storm and released the rest slowly. After paving, the same storm would leave the site in minutes and land on the neighbor downhill all at once. The pond was not for his lot. It was for everyone below it.

I spent eleven years stamping site plans, and that conversation repeated more often than I would like. Lately I have found a better way to explain it, thanks to a game about beavers. Timberborn is, underneath the cute art, one of the best water storage simulators I have played. It also gets the purpose of storage exactly backwards from a detention pond, and that contrast turns out to be the clearest explanation I have.

Water in Timberborn

Timberborn, from the Polish studio Mechanistry, puts you in charge of a beaver colony in a river valley. Water flows from sources at the map's edge and through the valley in a real simulation: it spreads, pools, finds low ground and spills over. Every so often a drought arrives and the sources stop. Later versions add badtides, when the sources pour out contaminated water instead.

To survive, you build dams, levees and floodgates. Dams hold water back to a set height. Floodgates let you raise or lower the crest so you control how much passes. Players build huge reservoirs during wet seasons so the colony has water through a drought that may last a dozen days or more. Getting the storage volume right is the core puzzle of the game.

Why the game's goal is the opposite of a detention pond

In Timberborn, stored water is a resource. You hold it as long as possible and release it only when you need it. Engineering would call that retention or supply storage.

A stormwater detention pond does not want to keep water at all. Its job is to catch a burst of runoff from a storm and release it slowly over the next day or so, so that the peak flow leaving the site after development is no higher than the peak before development. Once the storm passes, a dry detention pond should drain empty and wait for the next one.

That difference changes everything about the design. A Timberborn reservoir is judged by how much it holds. A detention pond is judged by how slowly it lets go during a storm and how reliably it empties afterward. One wants a tight seal. The other wants a carefully sized hole.

A reservoir stores water for later. A detention pond stores time.

The arithmetic I walk owners through

Here is a stripped-down example for a two acre commercial lot using the rational method, which is still common for small sites. It estimates peak flow as Q = C x i x A, where C is a runoff coefficient, i is rainfall intensity in inches per hour, and A is area in acres.

Paving more than doubles the peak, from 2.46 to 6.15 cubic feet per second. The rainfall intensity here is a 10-year, short-duration value I have seen in parts of the Midwest. Yours comes from local rainfall data and the reviewing agency's design storms, usually several of them, often from the 2-year to the 100-year event.

The quickest triangle approximation, the extra 3.69 cfs held for a 15 minute peak, says roughly 3,300 cubic feet of storage. When I ran the full critical duration check for a real site of about this size, the required volume came out closer to 5,000. Short approximations tend to undersize, which is one reason reviewers ask for the full calculation.

The outlet is the floodgate

This is where Timberborn's floodgates become a genuinely useful picture. A detention pond's outlet structure is usually a small orifice near the bottom for frequent storms, a larger opening or weir higher up for big storms, and an emergency spillway at the top for anything beyond design.

A 4 inch orifice under 3 feet of water passes roughly 0.7 cfs, using the standard orifice equation with a typical coefficient. That small number is the point. It throttles the outflow so the pond fills during the storm and drains slowly afterward. Players who have tuned a floodgate to keep a reservoir level while letting a trickle through to farms downstream already understand the idea. In the game you adjust the gate by hand. In a pond, the opening sizes are fixed at design, so they have to be right on paper.

Where I see the most field failures is not the math. It is the orifice getting clogged with leaves and trash because nobody installed a trash rack or nobody maintains it. A clogged orifice turns a detention pond into a retention pond by accident, and then the emergency spillway starts doing work it was never meant to do often.

The "bigger is safer" instinct

Timberborn rewards building the biggest reservoir you can. Owners and some contractors bring the same instinct to stormwater: make the pond bigger and the problem goes away. A bigger basin with the same outlet does hold more, but an oversized outlet, or a basin built in the wrong spot on the site, can still pass too much flow at the moment it matters. Size is only one of three variables. Location and outlet matter just as much, and they all start with reading how water naturally moves across the lot.

The other instinct I push back on is treating the pond as optional landscaping that can be value-engineered out late in the project. I have watched that happen. The approvals described in what a grading permit actually requires usually make the pond a condition, and removing it means redesigning drainage across the whole site.

What the game leaves out

Timberborn has no neighbors downstream to sue you, no reviewing engineer and no maintenance budget. Water in the game is also clean unless a badtide arrives, while real runoff from a parking lot carries oil, sediment and trash, which is why many jurisdictions now pair detention with water quality treatment. Erosion is missing too. A fresh outlet that discharges onto bare soil will cut a gully in one season, a problem covered in why a freshly graded slope fails after the first storm.

More of these crossovers are in the games section.

A check to do on your own site this week

If your property has a detention pond or basin, walk to the outlet structure after the next rain. Look for the low orifice and check whether it is open or blocked by debris. If the basin still holds water two days after a storm and it was designed to be dry, the outlet is likely clogged. Clearing it is usually a half hour job with gloves and a rake, and it restores the protection the pond was built to provide.