Several of my papers mention road cross drainage frequency, and techniques, general guides, etc. that I learned as a hydrologist for the US Forest Service. Some of the early studies were at Coweeta Hydrologic Station in North Carolina, evaluating broad based dips. Broad based dips are needed when logging or large trucks need to negotiate dips. I saw many instances of problems when I worked in Oregon and mentioned them in my paper dealing with flood risk in forest management. I did notice that some of the old mining roads seemed to function fairly good in the long term. These roads had what I call rolling dips, which rolled with the terrain, with areas with sloping at the dips to promote frequent drainage. The frequency needed may depend somewhat on high rainfall intensity, erodibility of road materials, road gradient, and perhaps stability and cover factors of materials. But if the inslope road gradient has dropped between 6-10 feet in elevation or maybe averaging 8 feet, I would install a cross drain as a dip or small culvert. It is good to avoid installing cross drains in areas with deep fills, and use energy dissipation if using culvert on deep fills to avoid the shot gun culvert appearance from erosion or failure. Most US state forestry or highway have best management practices published that may be helpful. One must remember that the road is capturing water both from its surface, but also from the drainage area above 5e road, so one must also ensure placement of culvert or other drainage associated with the perennial, intermittent and ephemeral channels crossed. Many of my road surveys for adequate culvert or drainage frequency have been walking along the road with what we called a string chain, which helps to measure distance. Sometimes we drove and used mileage changes of 0.1 mile (estimating to nearest 0.05 miles) and various trail bike or ATV to speed surveys. I would mark or flag areas using desired frequency, looking for suitable areas where water could be diverted without causing problems. One must learn to recognize sections of road insloping, road outsloping, areas with confined ditches. When culverts are used, sizing criteria when crossing channels or streams consider their drainage area, and sometimes bankfull indicators may be helpful. As with any new or recent road, drive or walk to survey function and assess damage during or following storms, especially bankfull or large storms. Extreme events, though infrequent, may be 10 times the bankfull flow (which occurs about every year). You might find some useful wording or guidance in the Soil and Water Conservation Guides in the Road section Which is included in my researchgate.

I failed to mention bridges, fords and other types of crossings, that might be needed when crossing larger streams, rivers, floodplains, etc. Watershed sizing is typically used associated with the area being roaded. Licensed engineer and possibly hydrologist may be required to adequately assess the situation, apply any available info, site indicators, flood marks, local resident observations, etc.

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Firstly you should know the reason of flood on roads with information of catchment / contributing area. Then topographic survey /toposheet is needed to setup drainage point. Peak discharge can be calculated, based on runoff coefficient, rainfall intensity and catchment area by rational method or any other method. Based on topographic survey (or with the help of toposheets) drainage point can be fixed and based on peak discharge /runoff proper section can be selected on roads for safe disposal of flood.

You could also calculate a flow direction/accumulation based on a DEM and then extract drainage network. That info can be crossed examined with road data to see the most likely flooding points (water always drains through lowest point). The catch is to do a good preparation of your DEM so it is hydrologically coherent and, ideally, takes into account culverts on territory.

Try to use the Road Surface itself to play the role of a Drain by introducing a slightly parabolic shape and then direct the flow to local drains crossing the road.

There are several ways to identify drainage points along roads:

1. Visual inspection: One of the simplest methods is to visually inspect the road for any visible drainage points such as gutters, catch basins, or storm drains.

2. Maps and blueprints: Consult maps and blueprints of the area to identify the location of drainage points.

3. Local authorities: Contact local authorities such as the city or county public works department or the department of transportation to obtain information about drainage points.

4. Field surveys: Conduct field surveys to identify drainage points by walking along the road and looking for any signs of water flow or drainage structures.

5. Aerial imagery: Use aerial imagery such as satellite images or drone footage to identify drainage points along roads.

6. Water flow monitoring: Install sensors or conduct water flow monitoring to identify areas where water accumulates or flows during rain events, which can indicate the location of drainage points.