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Road Drainage Diversion
Figure 1. NetMap’s road drainage tool divides long road segments into shorter ones that reflect topographic low and high points along the road that would create drain points and flow into stream channels. The change in elevation (between high and low points) and the distance over which that is calculated are used to determine the range and average road segment spacing (Table 1) and correspondingly the road segment gradient. Road segment length and gradient are required to estimate road surface erosion potential using GRAIP-Lite and WEPP roads. Note also that a user may import their own GPS locations of road drainage structures (as a point shape file). Road segments will be broken at those locations but also other breaks in the road network according to topography will still be included.
Figure 2. Example output from the road drainage tool showing road segment lengths (left) and corresponding road gradients (right). These two parameters are used to make road surface erosion predictions, in addition to other factors in the models.
Figure 3. In NetMap, road layers (lines) that may be kilometers long are broken at pixel cell boundaries (1). They are then re-aggregated into hydrologically discreet segments between topographic high and low (pour) points (2). These segments can be used to indicate the potential for road drainage diversion.
Figure 4. Predicted road hydrologic connectivity ranged between ten and 2500 meters (average 133 m) in the Clearwater Basin, Montana. This parameter could be viewed as an index of ‘road drainage diversion potential’ during large storms or following fires when secondary drainage structures may be compromised,. The drainage diversion index could be used to identify locations where field crews could check on drainage efficacy during or after storms or following fires.
