6.3 Road Drainage Connectivity/Diversion

Road Drainage Connectivity Diversion Potential (calculates road segment drain points for GRAIP-Lite, WEPP and worst case drainage diversion scenarios).

Also see READI tool under "Road Surface Erosion"

Tool can utilize GPS road drain point data (as a point file).

Parameter Description: Tool

Data Type: Line (road layer)

(1) Layer name: roadGRAIP (for use with GRAIP-Lite)

(2) Layer Name: roadWEPP (for use with Road erosion WEPP analysis, WEPP:Roads)

(3) Layer Name: roadDrain (worst case, road drainage diversion)

Units: road length (m) and road gradient (L/L); distance of individual road segments to individual stream segments are included in NetMap

NetMap Module/Tool: Roads

Model Description:

Road layers that are imported into NetMap are comprised of line work with road segment lengths of typically multiple kilometers. To facilitate calculation of road drainage diversion potential and road surface erosion potential, the road segments are broken into smaller segments that reflect road drainage into streams and road drainage off of roads at other locations (e.g., road drain points). The road drainage tool is used to create three different road layers; one for use with the GRAIP-Lite tool (creating the ‘roadGRAIP’ road layer); one for use with the WEPP road surface erosion tool (creating the ‘roadWEPP’ road layer); and one to calculate road drainage diversion potential (creating the ‘roadDRAIN’ road layer).

When running the GRAIP-Lite and road WEPP erosion tools, the road drainage segment lengths that are calculated using the road drainage tool should reflect the actual road drainage conditions in the field. For example, an analyst may know that the average road drain spacing (culverts, rolling dips etc.) is 50 m. The road drainage tool is then adjusted (using various estimates of distance and change in elevation, see Figures 1 and 2, Table 1) to arrive at the estimated spacing value. In addition, a user may import their own GPS locations of road drainage structures (see below).

When estimating the worst case scenario of road drainage diversion potential, such as following wildfires or during storms, a user attempts to maximize road segment lengths (see Table 1).

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. NetMap’s road drainage tool allows users to custom set the drain spacing along road segments to use with road surface erosion predictions (using GRAIP-Lite or WEPP) and for estimating worse case scenarios when secondary drainage structures are non functioning because of large storms or following wildfires. This tool assumes that all road-stream crossings function, with drainage going from roads to streams at those locations. (1) A user selects the output layer desired (GRAIP, WEPP or road drainage diversion). (2) An elevation change is selected (the change in elevation along the road necessary to define road drain or pour points. (3) A road length distance is selected over which the elevation change (2) is calculated over. (4) A maximum road spacing can be specified. (5) A user can import their own GPS road drainage location data points (in the form of an Arc point shape file). (6) The average, minimum and maximum road segment lengths that are generated are displayed. (6) Attributes in the original road layer (such as road ID) can be carried over into the delineated segments. See Table 1 below for a sensitivity analysis of how change in elevation (2) and distance (3) influences the predicted road segment length.

Table 1. A sensitivity analysis that shows how changing elevation and distance will affect the average and maximum length of road segments for use in GRAIP, WEPP and drainage diversion potential. These values do not include user supplied GPS road drainage points.

Elevation Change (m)	Distance (m)	Average Segment (m)	Minimum Segment (m)	Maximum segment (m)
0.01	300	47	0.017	1276
0.1	300	60	0.017	1276
0.5	300	80	0.017	1727
1	300	95	0.003	1727
3	300	140	0.003	2366
5	300	165	0.003	3300

1	50	86	0.003	1727
1	100	94	0.003	1727
1	200	95	0.003	1727
1	300	95	0.003	1727
1	400	95	0.003	1727
1	500	95	0.003	1727
1	1000	95	0.003	1727

Figure 3. 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 4. 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 5. 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.

Made with help of Dr.Explain