When development started on the northwest side of Albuquerque, it was decided that the Calabacillas Arroyo would be maintained in a semi-natural state, rather than be concrete lined in the same manner as the arroyos on the east side of town.  Along the arroyo, erosion setbacks were established and the development occurred outside of these limits.  To keep the arroyo within the erosion setbacks, grade control structures and bank protection would have to be installed at various points along the arroyo.  The grade control structures control the depth of the arroyo while the bank protection controls the lateral movement of the arroyo.  The spacing of the grade control structures is determined by the equilibrium slope of the arroyo.  As water moves through the arroyo, sediment is both eroded and deposited.  For most of the arroyo, erosion exceeds deposition.  Once the deposition and erosion are equal, the equilibrium slope is achieved.  To date, over 20 grade control structures have been built and several miles of bank protection has been placed.

Currently under construction is Calabacillas Grade Control Structure 1a1.  This structure was identified in the recently completed Calabacillas Arroyo Facility Plan.  The purpose of the structure is to protect the outlet of Swinburne Dam, which carries Unser Blvd. on its top.  The project will also raise the south bank of the Calabacillas to Grade Control Structure 1a, just downstream, to prevent possible flanking of that structure in a high flow event.  The AMAFCA Project Schedule calls for the installation of a sediment retention structure in Swinburne Dam for compliance with our EPA permit.  As a result, the grade control structure must be made a little deeper to maintain the equilibrium slope.

Calabacillas Grade Control Structure 1a1 will have a pneumatically placed concrete face on the west side of the structure.  The bottom portion of the structure will be grouted rock and the sides will be a combination of grouted rock and placed rock for erosion protection.  In the middle of the structure, there will be a low flow bypass that will take the small flows through the structure.  The construction contract ends in September, but a majority of the construction should be completed in August.  The disturbed areas outside of the channel bottom will be seeded with a native vegetation mix to restore the site.