Hi Devansh, we must remember ...

Hi Devansh, we must remember that it is quite natural for rainfall to runoff and flow into the rivers and streams, and hence we must always ensure that at least an ecological minimum of flow reaches these water courses.  As you note roads do increase the % runoff vs infiltration, and having also modified the landscape runoff flow rates will be different to what would have happened under undisturbed conditions.  Because of this it is normally critical to design the drainage channels to reduce the energy of flow as much as possible to prevent erosion and to drop out larger silt particles.  Your question as to whether it is possible to filter the runoff which will generally be carrying a higher suspended solids load than under natural conditions is a great concept.  However you will realise than under any conditions above a light rainfall the flow rate coming off a reasonable stretch of road will be very high requiring a very large filter area.  Alternatives are to capture the runoff in a smaller dam an allowing this to pass through the filter at a constant lower rate - the dam will also allow settling out of the coarser particles.  The dam will need to be designed for flood conditions.  Alternative ideas also include putting solid flocculant blocks into the channels which dissolve under rainfall runoff conditions and will then allow some of the smaller suspended particles to coagulate, flocculate and settle out under low velocity conditions.  A third concept is to install constructed wetlands which the runoff is channeled through - these are natural systems which also act as sediment traps.