Runoff volume
For US customary units Wrivel uses:
The conversion factor is geometric: one inch over one square foot equals 1/12 cubic foot, then cubic feet are converted to US gallons.
Roof catchment geometry
For roof runoff volume, Wrivel expects the horizontal projected (plan) area draining to the system. Rain falls through a horizontal plane, so entering the larger sloped roof-surface area would overstate event volume. NRCS engineering guidance for rainwater catchment likewise uses the horizontal footprint of the structure for catchment area.
Surface assumptions
The main planner currently uses first-pass runoff coefficients of 0.95 for roof, 0.90 for driveway and 0.85 for other hardscape. These are Wrivel planning assumptions for sensitivity screening—not universal code values, not site-calibrated hydrology and not a substitute for a locally required method. Lawn behavior is likewise intentionally a scenario model, not a published site-design coefficient. The current lawn base values are 0.12 for sandy behavior, 0.24 for loam/mixed behavior and 0.42 for clay/slow-draining behavior, with added increments of 0.05, 0.10 or 0.16 for gentle, moderate or steep slope. The result is capped at 0.68.
These values are useful for sensitivity testing. They are not a substitute for a local hydrologic method, soil survey, infiltration test or jurisdiction-required design coefficient.
Property scale, route length and measured fall
The planner maps the 800 × 540 sketch to the entered property width and depth. Each downspout or wet-spot marker is connected to its nearest marked outlet. Wrivel shows both the individual straight-line source-to-outlet distances and their combined total. Obstacles, bends, pipe fittings and actual trench routing are not included, so these are planning distances rather than material takeoffs.
If the user enters a measured vertical fall for a route, Wrivel calculates an end-to-end grade:
The fall is stored internally in feet and displayed as inches or centimetres depending on the selected units. A positive endpoint grade does not prove continuous fall: intermediate high points, grade breaks, buried conflicts or a downstream restriction can still prevent gravity drainage. A negative result means the marked outlet is higher than the source based on the entered measurement.
Example gravel-trench void storage
The planner's trench value assumes an illustrative 1.5 ft wide × 2 ft deep stone zone with 40% void space. It multiplies that cross-section by the combined sketched route length. This is a comparison aid, not a French-drain design: real aggregate voids, pipe displacement, trench dimensions, inflow rate and outlet capacity all change performance.
French-drain aggregate volume
The material calculator treats the entered stone-filled zone as a rectangular prism, then subtracts the geometric volume occupied by a round pipe over the same length:
An optional ordering allowance is applied only after pipe displacement. The calculator intentionally reports volume rather than tons because aggregate bulk density varies by stone type, grading, moisture and supplier. It also does not infer trench dimensions or installation details; those remain user-entered geometry.
Scenario screening
The shortlist uses only explicit inputs: marked roof sources, wet areas, outlets, selected soil behavior and broad slope category. It intentionally says “screen” rather than “install.” High/low markers are visual notes and do not drive the calculation.
Mitigation what-if
The what-if does not change rainfall or total water generated by the property. It models two explicit management actions: a chosen percentage of roof runoff is treated as sent to a verified safe outlet, then a user-entered empty storage volume is applied to the remaining roof runoff. Residual on-site volume is therefore:
The storage is assumed empty at the start of each storm and no infiltration or drawdown during the event is credited. The same storage capacity is applied across the storm ladder so overflow becomes visible as storm depth increases. This is a comparison workflow, not a hydraulic or legal-discharge determination.
Soil drain-down field worksheet
The Soil Infiltration Test page reports the observed rate at which the water level drops in the user's chosen test hole: water-level drop ÷ elapsed time. It does not convert that observation into saturated hydraulic conductivity or a jurisdictional design infiltration rate. Test-hole geometry, pre-wetting, antecedent moisture, sidewall flow, test depth and local protocol all affect interpretation. EPA rain-garden references on the Sources page are presented as separate screening/design contexts rather than as interchangeable thresholds.
Rainfall data
Users can enter any storm depth. For U.S. precipitation-frequency estimates, NOAA/NWS HDSC is the primary reference linked on the sources page. As reviewed September 22, 2026, NOAA identifies Atlas 14 as the current authoritative precipitation-frequency standard where it applies, while HDSC still publishes current legacy documents for areas not covered by Atlas 14; Atlas 15 is the next-generation update and remains in development. Local codes or agencies may require a specific duration, recurrence interval or different dataset.
Field verification and project brief
The five verification items are user-confirmed checklist states, not facts inferred by Wrivel. The project brief combines planner outputs, entered route-fall measurements and the user's checked/unchecked field items into a handoff for a site visit or quote discussion. Marking every box does not constitute engineering approval, a permit, a utility locate or confirmation that discharge is lawful.
What the model does not do
Wrivel does not model groundwater, pipe hydraulics, inlet capacity, erosive velocity, subsurface geology, floodplains, foundation design, property rights, neighbor impacts, regulatory discharge rights or buried utilities.