Auckland Council's flood mapping distinguishes between different overland flow path categories, and whether your site is affected by an OLFP1 (0.469 m³/s at Glendale Rd) or an OLFP2 (4.28 m³/s at the same site) fundamentally changes the engineering response required. Overland flow path protection requirements affect lot layout, building setbacks, fencing, and in some cases the viability of the development altogether. Understanding the difference before you buy the land, or before you finalise your scheme plan, is essential.
What Are Overland Flow Paths?
An overland flow path (OLFP) is the route that stormwater follows across the land surface when the piped network is at capacity. In Auckland, the piped system is typically designed for a 10-year ARI event. When rainfall exceeds that intensity, water must travel overland to reach a stream, drain, or other receiving environment. The path it follows is the overland flow path.
Auckland Council's GeoMaps identify these paths across the region, derived from LiDAR terrain models and hydrological analysis. Each OLFP is categorised based on the peak flow it carries during the 1:100-year ARI event. The two primary categories that affect residential development are OLFP1 and OLFP2.
OLFP1 vs OLFP2: The Engineering Distinction
An OLFP1 is a minor overland flow path carrying less than 1 m³/s in the 100-year event. At Glendale Road, Auckland, the OLFP1 crossing the site carried a calculated peak flow of 0.469 m³/s. This is a manageable flow. It can typically be accommodated within a swale or shallow channel integrated into the lot layout, with building platforms set back from the flow path and floor levels raised above the calculated flood level.
An OLFP2 is a major overland flow path carrying between 1 m³/s and typically up to around 6 m³/s (though this threshold varies by catchment). At the same Glendale Road site, the OLFP2 carried a calculated peak flow of 4.28 m³/s. This is a fundamentally different proposition. A flow of 4.28 m³/s at 100-year ARI requires a dedicated corridor through the site that cannot be built over, fenced across, or obstructed. The corridor width, depth, and roughness must be designed to convey that flow without increasing flood risk to adjacent properties.
The ratio between the two at Glendale Road was roughly 1:9. Same site, two flow paths, with one carrying nine times the flow of the other. The engineering response to each is correspondingly different.
What OLFP Classification Means for Development
Lot layout: An OLFP2 corridor may consume a significant portion of the developable area. At Glendale Road, the OLFP2 corridor required a minimum width to convey 4.28 m³/s at acceptable depth and velocity. This corridor had to be maintained as open space, typically vested as drainage reserve or protected by a consent notice on the title.
Building setbacks: Auckland Unitary Plan rules require buildings to be set back from the edge of an overland flow path. For an OLFP2, this setback is measured from the calculated flood extent, not just the centreline of the path. The practical effect is that building platforms must be located further from the flow path than many developers initially assume.
Fencing: Solid fencing across an overland flow path is generally prohibited because it obstructs flow and raises upstream flood levels. For an OLFP1, pool-style or permeable fencing may be acceptable. For an OLFP2 carrying 4+ m³/s, any obstruction creates a measurable increase in upstream flood level that the consent authority will not accept.
Floor levels: Minimum floor levels are set relative to the calculated 100-year flood level plus freeboard (typically 500 mm in Auckland). For a site affected by an OLFP2, the 100-year flood level is higher and extends further across the site, which pushes finished floor levels up accordingly. This has direct implications for earthworks cost and building design.
The Glendale Road Example
SAE Ltd completed a flood study for Glendale Road, Auckland, where both an OLFP1 and an OLFP2 crossed the site. The study used 2D HEC-RAS modelling to determine flood extents, depths, and velocities for both flow paths under the 100-year ARI event. The results showed that the OLFP2 corridor required a dedicated open channel through the centre of the site, while the OLFP1 could be managed within the lot boundary using a shallow swale.
The critical finding was that the OLFP2 flow path, if not properly accommodated, would have increased flood levels on the neighbouring downstream property by more than 50 mm, which is the threshold Auckland Council applies for adverse effects on third parties. The design had to demonstrate that post-development flood levels at all boundaries were no worse than pre-development conditions.
When to Investigate
The time to check for overland flow paths is before you commit to purchasing a site or before you finalise a scheme plan. Auckland Council's GeoMaps will show you whether an OLFP crosses your site, but the mapped flow path is indicative only. A site-specific flood study, using LiDAR-derived terrain and 2D hydraulic modelling, is required to determine the actual flow, depth, velocity, and extent for design purposes.
If your site is affected by an OLFP2, the engineering cost and the constraints on developable area are materially higher than for an OLFP1. That is not a reason to avoid the site; it is a reason to understand the constraints before committing to a lot layout or a purchase price that assumes the full site is developable.
OLFP1 and OLFP2 are not just mapping labels. They represent fundamentally different flow regimes with different engineering requirements. At Glendale Road, the difference was 0.469 m³/s versus 4.28 m³/s on the same site. Check the classification early, commission a flood study if an OLFP2 is present, and design around the constraint rather than discovering it at resource consent stage.