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What is an Overland Flow Path?

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An Overland Flow Path (OFP) is an important concept in the flood engineering real, as an OFP is the path that water takes over the surface of the land when it exceeds the capacity of the underground drainage system and follows the natural topography. Alternatively, if there is no drainage system, the concentration of stormwater runoff is what creates an OFP when rainfall intensity surpasses the capacity of the soil, or hits an impervious surface, hence creating runoff. These localised paths are vital in assessing flood risks for any given area, as they have the potential for flood risk and damage to persons and property.

Overland Flow Paths can take many shapes and forms, from slight depressions in the landscape (ponding) to established channel beds, and even man-made structures in urban environments. In an urban environment, an overland flow path could occur within a street (which typically is designed to cater for a level of overland flows), a park, or even a parking lot, essentially any space where water can flow unimpeded when the drainage systems are overwhelmed.

If you have landed on this page, it is likely that a Local Council authority or building certifier has identified a flood overlay, which is typically found on a Planning Scheme Policy Flood Overlay Map, or you have sighted this on interactive mapping/flood report. Our processes to identify an OFP, typically includes topographic analysis, hydrologic modelling and stormwater infrastructure data. Items that are critical to consider when assessing for an OFP include:

  • Review Local Council Maps: As demonstrated on our website, most local council’s will have an interactive mapping planning system that will identify any suspected overland flow paths. For example, Brisbane City Council has both a Planning Scheme interactive map, and also a Flood Awareness Map.
  • Study of Topographic Maps: Topographic maps (ie. Contours) provide detailed information about the physical features of the land, including elevation and slope, which can help identify potential OFPs.
  • Hydrologic Modelling: Sophisticated software is often used to simulate different design rainfall scenarios (provided by the BOM) and observe where water accumulates and flows, with some guidance from national and local standards. This is what we do here at Stormflood. We will identify the flood regime of an OFP, including characteristics such as water level, depth, velocity and hazard indicators of the Overland Flow Path to guide how your development will need to address the flooding issue.
  • Field Verification: While maps and models are useful (noting that Aerial survey data is quite practical to use), there’s no substitute for observing the land directly, particularly during or immediately following a rainfall event.
  • Review of Historical Flood Data: Previous flood events can give insight into potential OFPs, especially in urban areas where man-made structures may influence water flow.

There are many reasons why you should consider engaging a specialist flood engineer when you suspect a property to be affected by an Overland flow path, we’ve included a few below:

  • Flood Risk Assessment: Understanding overland flow paths can help identify areas at higher risk of flooding. This knowledge allows for the design and implementation of appropriate flood mitigation strategies or flood resilience design when planning a new building or extension.
  • Infrastructure Design: The layout and design of a building or extension can significantly influence how water flows around the property. Knowing and understanding the overland flow paths can guide the placement of structures around infrastructure to minimise flood impacts, and thus reducing potential flood risks and damage.
  • Landscape Design: Awareness of overland flow paths can inform the design of landscaping and other outdoor features. For example, gardens and driveways can be designed to help channel water away from buildings and into appropriate drainage areas. On the other hand if the OFP is considered high velocity, the assessment can help avoid building within these high scour areas, to avoid constant maintenance and nuisance to landscaped areas.
  • Compliance with Regulations: In most local government areas, consideration of overland flow paths is integral in the planning and construction process. Failure to do so can result in increased financial costs, planning and construction delays, and even penalties if nuisance is created external to your site.
  • Insurance Purposes: In many cases, insurance companies require an understanding of flood risks, including overland flow paths, before providing coverage. Additionally, awareness and proactive demonstration of mitigation of these risks can potentially lower insurance premiums.
  • Long-term Property Value: Buildings or extensions that take overland flow paths into consideration can maintain or increase their property value over time, by proactive approaches to reduce risk. Properties with a lower risk of flooding, or a demonstration via flood documentation of flood risks, are generally more desirable and can command higher prices in the real estate market. It should be noted that flood overlay maps are easily locatable, and attempting to ignore these when transacting property can be seen as deceitful as a seller, therefore it is recommended you speak to us to determine how impactful the OFP is.

Understanding and identifying Overland Flow Paths is a critical aspect of flood engineering. It helps in effective flood risk management by predicting areas of potential flooding and informing the design of infrastructure to handle excess water. By studying the topography, using hydrological models, and examining historical flood data, one can map out these paths and make strategic decisions that could help build flood resilience into your property, reduce flood insurance risks/premiums and prepare against the devastating effects of floods.

Want Stormflood to look at a property for you? Get in touch with us today.

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