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API · /runoff-api

Stormwater Runoff API

healthy 4,516 Subscribers

Stormwater-runoff civil-engineering maths as an API, computed locally and deterministically. The rational endpoint computes the peak runoff from a catchment with the Rational Method, Q = C·i·A — in metric form Q(m³/s) = C·i·A/360 with rainfall intensity i in mm/h and area A in hectares, or in US form Q(cfs) = C·i·A with intensity in in/h and area in acres — where the runoff coefficient C is the fraction of rain that runs off (about 0.9 for paving and 0.2 for lawns). The time-of-concentration endpoint computes how long water takes to flow from the most remote point of the catchment to the outlet with the Kirpich formula, tc = 0.0195·L^0.77·S^(−0.385) minutes, from the flow-path length and slope; this sets the design-storm duration. The detention endpoint gives a first-order estimate of the detention-pond storage needed to throttle a peak inflow down to an allowable outflow over a storm duration, (Q_in − Q_out)·duration. Coefficients are dimensionless, intensities in mm/h or in/h, areas in ha or acres, lengths in m and flows in m³/s. Everything is computed locally and deterministically, so it is instant and private. Ideal for civil-engineering, drainage, urban-planning, landscape and flood-risk app developers, sewer-sizing and detention tools, and hydrology education. Pure local computation — no key, no third-party service, instant. Live, nothing stored. 3 endpoints. This is stormwater runoff; for open-channel flow use a Manning API and for pipe friction a Darcy API.

#stormwater #runoff #rational-method #hydrology #civil #developer-tools
api.oanor.com/runoff-api
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Machine-readable spec so AI agents can integrate this API.

/api/runoff-api/openapi.json
/api/runoff-api/llms.txt

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API health

healthy
Uptime
100.00%
Server probes · 24h
Avg latency
97 ms
Server probes · 24h
Subscribers
4,516
active
Total calls
24
last 7 days
status Full status page → · 28 probes/24h

Pricing

Pick a tier — billed monthly, cancel anytime.

Free

Free

  • 2,480 calls / month
  • 2 requests / second
  • Hard cap (429 above quota, no overage)
  • 2,480 calls/month
  • 2 req/sec
  • Rational + time of concentration + detention
  • No credit card
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Starter

€9.00 /month

  • 38,000 calls / month
  • 6 requests / second
  • Hard cap (429 above quota, no overage)
  • 38,000 calls/month
  • 6 req/sec
  • Metric & US units, Kirpich tc
  • Email support
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Pro

€24.00 /month

  • 244,000 calls / month
  • 15 requests / second
  • Hard cap (429 above quota, no overage)
  • 244,000 calls/month
  • 15 req/sec
  • Drainage & detention-pond pipelines
  • Priority support
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Mega

€73.00 /month

  • 1,650,000 calls / month
  • 40 requests / second
  • Hard cap (429 above quota, no overage)
  • 1,650,000 calls/month
  • 40 req/sec
  • Platform scale
  • Dedicated SLA
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Weir Flow API

Weir flow maths for open-channel discharge measurement as an API, computed locally and deterministically. The rectangular endpoint computes the flow over a rectangular sharp-crested weir, Q = (2/3)·Cd·b·√(2g)·H^1.5, from the crest width and the head of water above the crest — and solves the head back from a known discharge. The vnotch endpoint computes the flow over a triangular V-notch weir, Q = (8/15)·Cd·√(2g)·tan(θ/2)·H^2.5, from the notch angle and head, the most accurate weir for small flows because the discharge varies with the head to the power 2.5. The broadcrested endpoint computes the flow over a broad-crested weir, Q = Cd·(2/3)^1.5·√g·b·H^1.5 ≈ Cd·1.705·b·H^1.5, the rugged field structure used for river gauging. Each device carries its standard discharge coefficient (rectangular 0.62, V-notch 0.58, broad-crested 0.85) which you can override, and each solves either the discharge from a measured head or the head required for a target discharge. Everything is computed locally and deterministically, so it is instant and private. Ideal for hydrology, irrigation and civil-engineering tools, flow gauging in channels and treatment plants, stormwater and water-resource apps, and fluid-mechanics education. Pure local computation — no key, no third-party service, instant. Live, nothing stored. 3 endpoints. This is weir overflow discharge; for uniform open-channel flow use a Manning API and for differential-pressure pipe metering use an orifice API.

api.oanor.com/weir-api

Flood & River Discharge API

Global river-discharge and flood forecasting as an API, powered by the GloFAS (Global Flood Awareness System) model via Open-Meteo. For any coordinate on Earth, get a daily river-discharge forecast of up to 30 days — with the ensemble spread (mean, max and min across forecast members) so you can gauge uncertainty — plus up to 90 days of recent discharge history, and a quick current-situation summary with today's discharge and a 7-day outlook (peak day, max/min and rising/falling/stable trend). Discharge is reported in cubic metres per second. Ideal for flood early-warning and monitoring, insurance and reinsurance risk, agriculture and irrigation planning, hydropower, and environmental research. Data covers modelled rivers worldwide (none over open ocean). Open data via Open-Meteo / GloFAS.

api.oanor.com/flood-api

USGS Streamflow API

Live river and stream conditions from the USGS Water Services network. The catalogue of 16,900+ active US streamflow gauges is searchable by name, state or coordinate; the live endpoint returns the latest observation for any gauge: streamflow / discharge (in cubic feet per second and m³/s) and gauge height (in feet and metres), with the station name, observation time and coordinates. Find the gauges nearest any lat/lon. Ideal for flood monitoring, hydrology, whitewater/fishing & recreation apps, drought tracking and environmental dashboards.

api.oanor.com/streamflow-api

Balloon Decor API

Party-balloon maths as an API, computed locally and deterministically — the helium-lift and balloon-count numbers a party planner or balloon artist decorates by. The helium endpoint gives a balloon’s lift from its inflated diameter: net lift is the inflated volume times the difference between air and helium density, about 1.046 grams per litre, so a fully inflated 11-inch latex balloon (around 11.4 litres) lifts roughly 12 grams gross and about 9 after its own weight, while a 36-inch giant lifts hundreds of grams. The float endpoint flips it around — how many balloons to float a payload = the weight divided by the net lift per balloon, rounded up, so a 50-gram card floats on six 11-inch balloons. The garland endpoint sizes an organic balloon garland or arch from its length: about 12 balloons per foot in a mix of sizes — roughly 40 % 5-inch, 45 % 11-inch and 15 % 16-inch for that full, textured look — so a 10-foot garland takes about 120 balloons, denser if you want it lush. Everything is computed locally and deterministically, so it is instant and private. Ideal for party-planning, event-decor, balloon-artist and celebration app developers, decor-estimator and shopping-list tools, and event software. Pure local computation — no key, no third-party service, instant. Inches and grams. Live, nothing stored. 3 compute endpoints.

api.oanor.com/balloon-api

Frequently asked questions

Quick answers about pricing, quotas, and integration.

How do I get an API key for Stormwater Runoff API?
Sign up for free at oanor.com, generate an API key from the developer dashboard, and call Stormwater Runoff API with the x-oanor-key header. No credit card needed for the free tier.
What's the rate limit for Stormwater Runoff API?
Free tier allows 1 request per second. Paid plans scale up to 50 requests per second on the Mega tier. Hard limits return HTTP 429 above the quota — no surprise overage charges.
How much does Stormwater Runoff API cost?
Stormwater Runoff API has a free tier with 100 calls / month. Paid plans start at €9.00 / month with higher quotas and faster rate limits.
Can I cancel my subscription anytime?
Yes. Plans are billed monthly and you can cancel anytime from your billing dashboard. No long-term contracts and no cancellation fee.
Is Stormwater Runoff API GDPR-compliant?
All requests to Stormwater Runoff API go through our EU-based gateway. Your upstream API key never leaves our server and no personal data is shared with the upstream provider beyond the request you send.

Pick an endpoint from the list on the left to see its details and try it.

Code snippets

Sign up to get an API key, then call any path under your slug.

curl https://api.oanor.com/runoff-api/SOME_PATH \
  -H "x-oanor-key: oanor_test_..."
const res = await fetch("https://api.oanor.com/runoff-api/SOME_PATH", {
  headers: { "x-oanor-key": "oanor_test_..." }
});
const data = await res.json();
$ch = curl_init("https://api.oanor.com/runoff-api/SOME_PATH");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, ["x-oanor-key: oanor_test_..."]);
$response = curl_exec($ch);
import requests
r = requests.get(
    "https://api.oanor.com/runoff-api/SOME_PATH",
    headers={"x-oanor-key": "oanor_test_..."},
)
print(r.json())

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