Layer: Overland Flow Path Spill Points from Roads 2021 (ID: 2)

Name: Overland Flow Path Spill Points from Roads 2021

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Description: <DIV STYLE="text-align:Left;"><DIV><DIV>RationaleA common problem with properties located immediately downstream of overland flow paths leaving the road is that the runoff is not well managed resulting in potential flood risk for the property. Typical situations are shown in the following Figures 3 to 5. The spill points provide a useful reference indicator when investigating flooding issues and can be used when assessing building consent applications to identify potential flood risks and the need to ensure suitable surface conveyance of stormwater through the affected properties.MethodRoad spill points are defined as the location where an overland flow path leaves the road parcel. Using Safe Software FME, a process was developed to automate the identification of road spill points by intersecting the OLFPs with LINZ Road Parcels. This results in a point dataset locating both road spill points and road entry points. Using the directionality inherent within the flow path spatial data, road entry points can be removed.By design, OLFP paths commonly follow road kerbs before entering the piped network or an open channel. Minor deviations from the road corridor are common due to limitations relating to the accuracy of the LiDAR data, and these should not be considered valid spill points. To account for these minor deviations, road parcels were first buffered by 1.5m. That is, to be considered a valid road spill point, a flow path must deviate from the road parcel by at least 1.5m. Road spill points are categorised by the flow path catchment size, which is indicative of the magnitude of flow. DisclaimerThe analysis provided here is a useful indication of road spill points, however there are number of limitations that should be understood. <UL STYLE="margin:0 0 0 0;padding:0 0 0 0;"><LI>Road spill points are located at points where OLFPs depart from flowing along or across a road parcel. Road parcels defined by LINZ may be misaligned or include additional roads (i.e. paper roads). Road spill points may also flag bridges and large culverts as road spill points. For these reasons, ground-truthing should be undertaken in areas of particular concern to inform decision making.</LI><LI>OLFPs and any related road spill points represent a synthetic flow path network and do not necessarily indicate where water will flow under specific storm events or represent specific flow rates. </LI><LI>Delineated flow paths and any related road spill points have one-dimensional geometry, i.e. they represent the centre-line of potential surface flow routes and do not provide information on flow width, depth or velocity. </LI><LI>OLFPs and any related road spill points are derived from a bare-ground DEM and therefore do not account for the effects of buildings or other built structures such as fences which may deflect surface flows. Ground-truthing of road spill points should be undertaken in areas of particular concern to inform decision-making. </LI><LI>Flow paths may follow the road parcel edge, resulting in multiple road spill points clustered within one area (see Figure 9). To prevent duplication of spill points, the most downstream spill point is provided for each reach segment. </LI><LI>Road spill points will also be identified where there are streams with bridges or culvert crossings conveying the overland flow, rather than the flow being directly on the road surface. These may be of interest for other reasons such as indicating potential fish passage issues where a culvert discharges to a stream.</LI></UL></DIV></DIV></DIV>

Service Item Id: 7a9fc3d738ea4b5a9611e1b7ca8170e8

Copyright Text: Morphum Environmental, July 2021

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OBJECTID ( type: esriFieldTypeOID, alias: OBJECTID )
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CATCHMENT_AREA ( type: esriFieldTypeDouble, alias: Estimated Contributing Catchment (m2) )
OLFP_ID ( type: esriFieldTypeString, alias: OLFP ID, length: 20 )

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