From Dr. Moreno, as posted on Nextdoor
A little background information: the neighborhood known as Pueblo Alto was built ca. 1950. It has an oral history over the years of very-occasional flooding that has affected a small number of properties. The flooding is the result of undersized storm-drainage pipes: most of the neighborhood rainfall is collected from the streets into a single 24-inch pipe that exits west and passes under Twin Parks to reach a 48-inch pipe under Morningside Drive. If the flow of water reaching the exit pipe exceeds the pipe’s flow capacity, the excess accumulates on the streets and may get deep enough to flood adjacent low-lying properties.
This happened most recently in 2015, affecting one property on Jefferson Street. Ironically that was just after the completion of a city effort to address the flooding problem by adding and relocating some of the street-grating entry points into the (still) 24-inch pipes. After the 2015 flood, two speed-bump-like flow diverters were installed across Adams and Jefferson Streets at Mountain Road, to reduce the flow of water reaching the exit pipe, by forcing more water to exit Pueblo Alto via Mountain Road. In this time frame, City Counselor Dianne Gibson became involved, and a hydrological study was piggybacked onto a much-wider-area study commissioned by the Albuquerque Metropolitan Arroyo Flood Control Authority (AMAFCA).This study was carried out by the Smith Engineering Company, and was released in November 2017. A major point that this study made was that over the wide area it considered, the storm-water drainage system is generally undersized, and would be prohibitively expensive to up-size. Secondly, the study recommended instead the construction of detention ponds, to detain excess water that would otherwise flood properties, with subsequent gradual drainage into the existing pipes over a few days at most.
For the Pueblo Alto drainage, the Smith Engineering study considered two scenarios: a detention pond at the lower end of Pueblo Alto, or a detention pond at Twin Parks. From the City’s perspective, the advantage of the Pueblo Alto site might be that the expensive up-sizing of the exit pipe would not be necessary, since any storm-water flow exceeding the exit-pipe capacity would be detained in the neighborhood. The disadvantage of the Pueblo Alto site might be the immediate unavailability of a site for the pond, and the cost of acquiring same. Similarly, the advantage of the Twin Parks site for the city is almost certainly the existing open space, while the disadvantage would be the up-sizing of the drainage pipe from Pueblo Alto to the park. What the respective residents might think is another matter.
The Smith Engineering study proposed the following detention-pond plan for the Twin Parks lower field: a maximum design storage volume of 5.8 acre feet of water, with a maximum depth of 11 feet and a maximum surface area of 34,597 square feet (0.794 acres). Various design constraints result in this less than 100% use of the field area. In addition, the sides must be sloped for safety, and the bottom must be sloped for drainage to the exit point, reducing the maximum possible volume. Since the pond bottom is well below the downstream pipes, it also seems that pumping would be required to drain the pond. Using the upper field as well as the lower field would introduce additional complications.
For the record, I did a hydrological analysis of Pueblo Alto’s 2015 storm-flood that raised some questions in my mind about the Smith Engineering study (I am a PhD mechanical engineer). I forwarded my results and questions to AMAFCA and the City two years ago. The response was total silence. For whatever reason, the project has been quiet (though not idle) until recently. Perhaps it was waiting on the recent bond election. It isn’t clear yet how much of the Smith Engineering work is going into the Bohannnan-Huston proposal. We only know that the city has mentioned a 10 acre-feet storage goal rather than the figures given above.
Also from Dr. Moreno – here is a correction to a statement (on Nextdoor) that the home on Jefferson flooded after every rain. The flooding was bad on one occasion and worrying on others, but actual flooding was rare.