Measure/What is a contributing drainage area
Definition · terrain
What is a contributing drainage area?
A contributing drainage area is the land whose surface runoff arrives at one chosen point. Every square meter of it drains, by some path across the ground, to that point. Move the point and you get a different area.
It is the same idea as a watershed, only the outlet is not a river mouth or a stream gauge. It is a few meters of a wall, a driveway apron, or the low corner of a garden.
Why it is the number that decides the argument
Grade tells you which way water leaves the last three meters. It does not tell you how much water shows up.
Two walls can be graded identically and be nothing like each other. One receives runoff from a strip of lawn and a garden path. The other receives a hillside, two roofs and half a road. Same slope, two entirely different quantities of water.
That is why a per-property figure is almost useless and a per-wall figure is not. A house is a collection of walls with different exposures, and the contributing area at each one is its own question.
The intuition
Stand at a wall and imagine tipping the whole neighborhood slightly. Every drop that would eventually run past your boots came from somewhere. The set of "somewheres" is the contributing area. It is usually far larger than the parcel, and it does not respect property lines.
How it is actually computed
From a bare-earth elevation surface — the ground with the trees and the buildings stripped out — in four steps, each of which has a way of going wrong.
1. Fix the surface before routing anything
A real measured surface is full of tiny closed hollows, most of them artefacts a few millimeters deep. Route water naively and it falls into them and stops.
How much does that matter? Measured on real ground: 282,572 m² of contributing area in Chicago and 201,312 m² in Pittsburgh drained into interior pits two to four millimeters deep and simply terminated — against largest surviving channels of 1,034 m² and 33,931 m². Nearly the entire drainage network was being eaten by pits you could not see with a magnifying glass.
So every closed depression is raised until it overflows, so no water is left with nowhere to go. Water spills over a 2 mm lip. The depth each hollow was raised is kept rather than thrown away, because a real hollow beside a house is exactly the thing worth telling you about.
2. Route the water the way water moves
The lazy method hands all of a cell's water to its single steepest neighbor. That is fast, and it manufactures hard channels across ground where flow really spreads.
Instead the modeled water spreads the way overland flow actually behaves — near-single-channel in a steep gully, dispersive across a lawn.
3. Put the buildings back
A bare-earth surface has had the buildings removed, which means water will happily route straight through a house. Buildings are put back in as walls before anything is routed, so flow goes around a structure rather than through it — and the area arriving at the upstream wall is the area that actually arrives.
4. Sample at the wall, and admit the uncertainty
Accumulation values change sharply across a flow boundary. A single sample taken 30 cm to one side can differ from its neighbor by an order of magnitude, and quoting either as "the" answer is fiction.
So the value is sampled at nine points beside the wall and published with a range. A range is not a hedge. It is the honest shape of the measurement, and a single decimal figure would be a worse answer wearing better clothes.
An independent check that the routing is sane
Where a city digs its storm inlets is a yardstick nobody here tuned. Pittsburgh Water publishes the location of every inlet it maintains, so modeled channels can be scored against ground somebody else surveyed for their own reasons.
The limits are not small. Published inlet positions carry one to two meters of georeferencing error, and an inlet sits where a crew could dig, which is not always where the water goes. Agreement here is evidence the routing is sane, not proof that it is right.
What a contributing area is not
- Not a design figure. It carries no rainfall intensity, no soil infiltration, no runoff coefficient and no engineer's seal. It says where the water comes from, not what pipe to buy.
- Not a legal boundary. Water crossing a parcel line is a fact about topography. What that means for anybody's rights is a question for a lawyer.
- Not a prediction of water. A large upstream area is not a finding that a structure has ever taken water. That was not measured.
- Not permanent. A new curb, a re-graded driveway or a neighbor's retaining wall can move a divide. The surface describes the ground as measured.
What this is not
This is not a finding that any property has a drainage problem, a wet basement, or any defect. It is a measurement of ground slope and surface flow. Whether water has ever reached a particular structure was not measured and is not claimed.
See the contributing area at your own walls
The free look finds your building, puts it on the survey and shows you the shape of what was measured — the model, the flow paths and the hollows. The numbers are the product, so those come with the $25 report.
Common questions
What is a contributing drainage area?
The area of land whose surface runoff arrives at one chosen point. Every square meter of it drains, by some path over the ground, to that point. Change the point and the area changes.
Is it the same as a watershed or a catchment?
The same idea at a different scale. A watershed is usually named for a stream gauge or a river mouth; a contributing area at a house wall is the same computation with the outlet moved to a few meters of masonry.
Why does it matter more than slope?
Slope tells you which way water leaves the last three meters. Contributing area tells you how much water arrives. Two walls can carry identical grade while one receives a garden path and the other a hillside.
Why do you publish a range instead of a number?
Because the value is sampled at nine points beside the wall, and near a flow boundary a single sample is a coin toss. A decimal point would be false precision.
Can I use this figure to size a drain?
Not as a design input. It comes from a public bare-earth surface with a stated uncertainty, not from a site survey, and it carries no rainfall intensity, no infiltration and no engineer's seal.