6 - Drainage Improvements
No Build Option
Recommendations are based on multiple options set forth for the roadway section. They address cross drainage culverts under US 60 and areas where water overtopping and ponding occurs.
The no build option although being the least expensive route does not address any pending drainage issues throughout the project. This will cause further sedimentation of existing structures and increase water overtopping the roadway.
Cross Drainage Culvert Structures
Recommendations for culverts are shown in the table below. Four of the existing culverts are inadequate to handle the flows. Recommendations have been made for additions and/or replacements at locations where existing culverts were not capable of handling peak flows. The inlets and outlets of many of the existing crossing culverts have experienced significant silt and debris accumulation and it is recommended that they be dug out and cleaned. In order to accommodate the standard drainage structure protection systems required, each structure will need to be extended farther out than they are currently while still being within the right of way. If this is not possible culvert ends within the clear zone will require concrete slope blankets with safety grates to minimize the hazard to errant vehicles.
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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.
Roadside Ditches
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.
It is recommended for the rural section to build a v-shaped ditch with 3:1 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.
Storm Drain
The addition of curb and gutter to the west of the urban section has been proposed in order to counteract several issues including drainage. The Urban Typical section shows the addition of curb and gutter to channel water into drop inlets. Installation of storm drain system under the roadway to designated areas would eliminate water overtopping the road. The storm drain would channel the flow into existing outlets to reduce the flows over the road.
This would require the construction of a minimum 36” trunk line starting from about station 662+00.00 all the way to the outlet of DS-06 at about station 684+00.00. This would handle all of the flow from Basin C2.
Retention Area
One site has been selected as possible locations to retain flows. This would allow a reduction in peak volume and velocity running down adjacent streets and allow for a more controlled release. The site identified currently acts as a pond where the flows from DS-04 and DS-05 converge. The figure below shows the location and size.