Site inputs
Pre-filled values are illustrative examples only — replace every one with site-specific values. Sources for the examples: pre-dev CN 74 = TP108 pasture, hydrologic soil group C; post-dev CN 90 ≈ composite 60% impervious MPD lot (0.6×98 + 0.4×74 = 88.4); Tc from the site's longest flow path per TP108 (example 30 min pre / 15 min post reflects the faster post-development response).
Real NIWA HIRDS v4 depths (2 km grid, historical climate). Verify against the site's HIRDS point report for design.
Council presets apply the sourced duration/ARI augmentation (e.g. Auckland GD15, ΔT 3.8 °C RCP8.5 at 2110). Recorded in the results provenance.
Optional extras — pre-fill from an address or a design plan
Fills the areas with the legal parcel area and selects the nearest rainfall zone — confirm the contributing catchment.
Recommended design
Per-event compliance — check the workings
Rainfall provenance
Tank screening
Weighted CN, initial abstraction, S, runoff index,
runoff depth/volume, and time of concentration (eq 4.3) are verified directly
against the ARC TP108 (1999) source document and its own worked examples,
including the split pervious/impervious method below. Peak flow, if a rainfall
location is chosen, comes from this tool's SCS unit-hydrograph engine rather
than TP108's own graphical chart (Figure 5.1 — not digitised here, see
docs/reference/).
Read the
methodology and verification record (PDF).
Step 1 — locate the site
Step 2 — draw the catchment
Draw the contributing catchment (polygon) and the longest flow path (line) using the drawing tools at top-left. Each shape is measured as soon as you finish it, and the measurements drop straight into step 3. The red boundary is the LINZ parcel, shown for reference: the contributing catchment is often not the parcel.
Area, flow path length and rainfall zone below are filled from what you drew. Review every value before running. Start and end RL are not filled automatically: read them off the contours on the map, which are 0.5 m LiDAR intervals at site scale.
Split models the pervious area (plus any unconnected impervious) and the piped-connected impervious area as separate sub-catchments with their own CN/Ia/tc, then sums their hydrographs at the outlet — the more rigorous method for mixed post-development sites.
Catchment
Fields mirror the TP108 desktop assessment worksheet. Pre-filled values are illustrative examples only.
Direct input, used as entered. TP108 minimum of 10 min applies regardless. Fill in the flow path below for a suggested value from TP108 eq 4.3 — shown after running, not applied automatically.
Flow path (for the suggested tc)Pervious sub-catchment
Include any unconnected impervious area here (impervious area that drains onto pervious ground, per TP108 §3) — enter its area and it will be weighted in with CN 98. Pre-filled with the ARC TP108 (1999) Pakuranga worked example.
Impervious sub-catchment (connected)
Impervious area connected directly to the piped network. CN 98, Ia 0 mm — fixed, per TP108 §3.
Rainfall
Shared by both catchment models above — the same storm applies to the whole site.
Optional — adds a computed peak flow via the SCS unit-hydrograph engine. Leave unset for the worksheet-style Q/V cross-check only.