6 - Drainage Improvements

Outlet protection will be designed where outlet channels have steep slopes or where velocities are greater than recommended. Outlet protection will primarily consist of wire-enclosed riprap and will extended depending on site conditions. The supporting calculations for the cross-drainage culverts can be found in Appendix D.

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Roadside Ditches (Retained)

Channelization of flows along the roadway can be achieved by providing roadside ditches graded towards the structures.  Roadside ditches have also been silted over to the point they are do not handle the water flow collecting in them.

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It is recommended for the rural section to build a V-shaped ditch with 4:1 slopes up to the edge of the Right-of-Way. Roadside ditches will be designed to meet the NMDOT Drainage Design Criteria. Ditch lining will be installed when the velocities are greater than the allowable for the native vegetation and/or if soils are erodible. The minimum ditch grade will be 0.5%. Drainage from the road will follow the same patterns as they currently exist, with runoff sheet flowing off of the pavement onto the adjacent land on the south part of the road. 

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Storm Drain (Selected Alternative)

The proposed addition of curb and gutter to urban segments of the roadway would be best paired with underground storm drain systems and surface inlets to safely collect and convey runoff to suitable outfall locations. The storm drain systems could be sized to collect runoff generated in the US 60 right of way, as well as offsite areas within the Town of Mountainair and reduce the occurrence of overtopping. The storm drain system would also reduce flooding in developed areas south and east of US 60.

 

US 60 SOUTH SD

Preliminary assessments for storm drain networks were completed assuming offsite runoff from the 100-year event will be collected by storm drain systems in US 60. Analysis indicates that two storm drain systems will be necessary – one storm drain system to convey runoff north and one to convey runoff south, from the high point in the roadway profile, just north of Main Street. Trunkline diameters for the US 60 South Storm Drain will range between 24-inch and 48-inch, while the entire US 60 North Storm Drain will require a minimum 42-inch diameter trunkline along the entire system. The South Storm Drain is intended to collect runoff from Basins A1, B1, and B2, while the North Storm Drain is intended to collect runoff from Basins B3, B4, C2 and C1 (See Figure IV-6). It should be noted that runoff from a portion of Basin C1 could be collected and conveyed under US 60 via the existing DS-7, which would reduce the capacity needed in the North Storm Drain System. Preliminary storm drain trunkline analyses are provided in Appendix D.

Further evaluation of the storm drain system, including collection systems (ie. laterals and inlets) in connecting side streets and along US 60 will be necessary during preliminary and final design to ensure spread and depth criteria is met. Design of outfalls for both storm drains will also be necessary to ensure that outfall locations are not adversely impacted and that erosion control measures are designed to manage erosion.