Understanding Impervious Surfaces in Stormwater Design: How to Calculate Runoff When Site Coverage Matters

Impervious surface area is the single most influential variable in stormwater design for residential and commercial development. It determines how much rainfall becomes runoff, how quickly that runoff reaches the drainage network, and whether the downstream system can handle the load. Every stormwater neutrality calculation, detention sizing exercise, and council engineering plan approval starts with the same question: what is the impervious fraction, before and after development?

This post explains what counts as an impervious surface, how to measure site coverage accurately, how the impervious fraction feeds into runoff calculations, and what NZ councils expect when you report these numbers in an engineering plan.

What Counts as an Impervious Surface?

An impervious surface is any surface that prevents rainfall from infiltrating into the ground. In stormwater engineering, the classification is binary: a surface either allows infiltration or it does not. There is no partial credit.

Surfaces that are always classified as impervious:

• Roofs - all types, including metal, concrete tile, membrane, and green roofs (green roofs may qualify for a reduced runoff coefficient in some council frameworks, but they are still counted as impervious coverage for site area calculations).
• Sealed driveways and parking areas - asphalt, concrete, chip seal.
• Paved courtyards, paths, and patios - concrete, pavers on a sealed base, exposed aggregate.
• Swimming pools - the pool surface and its surrounding paved apron.

Surfaces that are typically classified as pervious:

• Grass and garden beds - unless compacted or on clay subgrade with effectively zero infiltration.
• Gravel driveways - usually classified as pervious, but with a higher runoff coefficient than grass. Some councils treat unsealed gravel on compacted base as semi-impervious.
• Permeable pavers on aggregate base - classified as pervious if installed to manufacturer specifications with an unlined aggregate sub-base. If installed on a sealed or lined base, they are impervious.

The grey areas matter. A gravel car park on compacted clay in the Rangitikei behaves differently from a gravel car park on free-draining Heretaunga Plains gravel. The classification should reflect actual infiltration behaviour, not just the surface material. Where there is doubt, the conservative approach - classifying as impervious - is what councils will accept without further justification.

How to Measure Impervious Coverage

Impervious coverage is expressed as a percentage of the total contributing catchment area. The measurement method depends on the project stage:

Pre-development (existing site): Measure from aerial photography, site survey, or a combination. For greenfield pastoral land, the impervious fraction is typically 0-5% (farm tracks, sheds). For existing urban sites being redeveloped, measure every roofed and paved surface from the survey plan. Do not estimate - measure. A 5% error in pre-development impervious coverage propagates directly into the detention volume calculation.

Post-development (proposed): Measure from the site plan or subdivision scheme plan. Include all proposed buildings at their maximum building platform footprint, all driveways, all paved areas, and any shared accessways. For subdivision where individual house designs are not yet known, use the maximum building coverage permitted by the district plan rule. For Napier City Council residential zones, this is typically 40-50% of the net lot area. For Auckland residential zones under the Unitary Plan, permitted coverage varies by zone but is commonly 35-45%.

The total impervious fraction for the site is then:

Impervious fraction = Total impervious area / Total contributing catchment area

For a typical 6-lot infill subdivision in Napier with 800 m² lots and 45% building coverage, plus shared driveway, the post-development impervious fraction is commonly 65-80%. For a greenfield 20-lot subdivision with road reserve and larger lots, it might be 50-65%.

How Impervious Coverage Drives Runoff

The relationship between impervious coverage and stormwater runoff is not linear - it is worse than linear. Doubling the impervious fraction more than doubles the peak flow rate, because impervious surfaces also reduce the time of concentration (water moves faster across sealed surfaces than across grass).

The two standard methods used in NZ practice handle impervious coverage differently:

Rational Method

The Rational Method uses a composite runoff coefficient (C) that is the area-weighted average of the pervious and impervious runoff coefficients:

C = (C_imp x A_imp + C_perv x A_perv) / A_total

Typical values for NZ conditions:

• C_imp (impervious surfaces): 0.90 - 0.95
• C_perv (grass on free-draining soil): 0.20 - 0.35
• C_perv (grass on clay/pallic soil): 0.40 - 0.55

For a site that moves from 10% impervious (pre-development) to 75% impervious (post-development) on average-draining soil, the composite C value roughly doubles - from approximately 0.30 to 0.75. That means 2.5 times the peak flow from the same rainfall event.

SCS Curve Number Method

The SCS method, used in Auckland (TP108) and for larger catchments, handles impervious coverage through the curve number (CN). Impervious surfaces have CN = 98 (essentially all rainfall becomes runoff after initial abstraction). The composite CN for the site is area-weighted between the impervious CN and the pervious CN, which depends on soil group:

• Soil Group A (free-draining sands/gravels): pervious CN = 39-61
• Soil Group B (moderate infiltration): pervious CN = 61-75
• Soil Group C (slow infiltration): pervious CN = 74-83
• Soil Group D (very slow infiltration, clays): pervious CN = 80-87

The composite CN determines both the total runoff volume and the peak flow rate through the SCS unit hydrograph. Higher impervious coverage pushes the composite CN toward 98, increasing both the volume and peak of the runoff hydrograph.

What NZ Councils Expect

Every council that requires stormwater neutrality or stormwater management needs the impervious coverage reported clearly in the engineering plan. The specific requirements vary:

Napier City Council (NCC SW-S1): The stormwater neutrality standard requires pre-development and post-development impervious areas to be stated in square metres and as a percentage of the contributing catchment. The engineering report must include a site plan showing the catchment boundary and all impervious surfaces colour-coded or hatched. NCC engineers will check these figures against the scheme plan and the district plan coverage rules.

Auckland Council (TP108 / SWCoP v4): Auckland's Stormwater Code of Practice requires impervious coverage to be reported for each sub-catchment. For subdivision where individual lot designs are unknown, Auckland accepts the use of maximum permitted building coverage from the Unitary Plan zone rules, plus an allowance for driveways and accessways. The TP108 methodology requires the impervious fraction as a direct input to the SCS curve number calculation.

Rangitikei District Council / Horizons RC: No single codified standard, but the engineering report for any subdivision requiring stormwater discharge consent must include a clear impervious area schedule. Given the pallic soils in the Rangitikei, even small increases in impervious coverage produce significant additional runoff because the pervious surfaces already have high runoff coefficients.

Hamilton City Council (PC12): Hamilton requires impervious coverage reporting for both the retention (reuse) and detention (neutrality) calculations. The retention system is sized based on the volume of runoff from impervious surfaces for a defined small-storm event. The detention system is sized based on the peak flow differential, which is driven by impervious coverage.

Common Mistakes

These errors appear repeatedly in stormwater reports that SAE reviews or inherits from other consultants:

1. Underestimating post-development coverage. Using the building footprint from a concept plan rather than the maximum permitted coverage. If titles are being created and future building is uncontrolled, the design must use the maximum coverage the district plan allows. Anything less will be queried by council.
2. Ignoring driveways and accessways. A shared accessway serving six lots at 3.5 m width and 40 m length is 140 m² of impervious surface. On a compact site, this alone can shift the impervious fraction by 5-10%.
3. Using the wrong pre-development baseline. The pre-development condition for stormwater neutrality is the natural or original land use, not the current condition. If the site is currently a sealed car park but was historically pasture, the pre-development impervious fraction is approximately zero, not 95%.
4. Not accounting for connected vs disconnected impervious areas. A roof that discharges to a garden (disconnected) behaves differently from a roof piped directly to the stormwater network (connected). Some methods, including TP108, distinguish between connected and disconnected impervious area. Treating all impervious area as connected is conservative but may oversize detention.
5. Rounding or estimating rather than measuring. A "roughly 60% impervious" assumption applied to a 2,000 m² site means an uncertainty of plus or minus 200 m². That uncertainty translates directly to detention volume uncertainty - potentially 3,000-5,000 litres of storage that may or may not be needed.

Worked Example: Typical 4-Lot Infill, Napier

Consider a 2,400 m² site in the Napier residential zone, currently containing one dwelling with a gravel driveway. The proposed development is a 4-lot subdivision with new shared accessway.

Pre-development:

• Existing dwelling roof: 180 m²
• Existing gravel driveway (classified pervious): 60 m²
• Remaining grass/garden: 2,160 m²
• Pre-development impervious fraction: 180 / 2,400 = 7.5%

Post-development (using maximum permitted coverage):

• 4 lots at 500 m² net, 45% building coverage = 4 x 225 = 900 m² roof
• 4 driveways at 35 m² each = 140 m²
• Shared sealed accessway: 150 m²
• Paved patios (estimated 20 m² per lot): 80 m²
• Total impervious: 1,270 m²
• Post-development impervious fraction: 1,270 / 2,400 = 52.9%

Using the Rational Method with Napier HIRDS v4 rainfall data for the 10-year ARI storm (with RCP 8.5 climate change uplift), the pre-development peak flow is approximately 4.2 L/s and the post-development peak flow is approximately 11.8 L/s. The detention system must attenuate the post-development peak back to 4.2 L/s. The required storage volume, determined by routing the inflow hydrograph through the proposed tank and orifice system, is approximately 9,500 litres for the critical storm duration.

This is a straightforward result. The impervious fraction increased from 7.5% to 52.9%, the peak flow nearly tripled, and the detention system is four 2,500-litre tanks with a calibrated orifice plate. The entire calculation chain starts with accurate measurement of impervious area.