Floodplains Are for Floods – did Grandma know?
5.14.Watershed.Photo.Mayfair 1933 aerial (showing wet areas)
Above is an aerial photo (Denver GIS) from about 80 years ago, showing what part of the Montclair Creek watershed looked like in Grandma and Grandpa’s day. The photo extends from Leyden to Ivy streets, and from 13th Avenue to Colfax Avenue.
Areas that convey or hold water frequently (creeks, swales, ponds, wetlands) show up on aerial photos. I’ve outlined what I see, to make it easier to see the pattern. A dark swath, ranging in width from ¼ of east-west block (¼ of 330’) to more than a block (330’+), extends southeast to northwest from 13th and Leyden to 14th and Kearney, and on to Colfax and Ivy.
Between 13th and 14th avenues, two things caught my eye:
• An agricultural or light industrial facility south of 14th between Leyden and Krameria;
• One or more buildings south of 14th on the east side of Kearney.
Between 14th and Colfax avenues, three things:
• Between Leyden and Krameria, wetlands extend pretty far north of 14th Avenue;
• The plant nursery City Floral has been around for quite some time along Kearney;
• A lot of regrading activity has occurred south of Colfax, probably including a good deal of filling;
More significant than those “details” were two overarching reactions:
1. The proportion of land in those 10 city blocks that was “wetlands” or “bottomlands” was quite sizeable, despite all of the obvious “islands” and “peninsulas” of fill.
2. If a time machine could transport them back to 1933, scientists, landscape architects, and engineers of today could readily identify a logical open space corridor, including some candidate sites for 1933 riparian restoration.
Denver’s Storm Drainage Master Plan calls for a drainage system that can accommodate a 5-year flow (a flow one would expect statistically to be equaled or exceeded at least once in any 5-year period). When a 100-year flow occurs, the pipes for Montclair Creek would carry the 5-year flow, and the remainder would flow overland. The table below shows that between 13th Avenue and Colfax Avenue open channel capacity to convey flows between 340 cfs and 1360 cfs would be needed.
Simplistically assuming a flow velocity of 3 or 4 feet/second (about 2 – 2.6 mph), we need a channel cross-section of about 100 sq. ft. at Niagara and Severn and a cross-section of 400 sq. ft. to convey that floodwater at Dahlia Street and 16th Avenue. At Niagara and Severn, a rectangular channel three feet deep by 35 feet wide would do that job. At Dahlia and 16th, a channel five feet deep by 80 feet wide would be needed. From 13th to Colfax we could assume 200 – 300 square feet, or four feet deep by 50-75 feet wide.
Picture greenways you already know in Denver. There is a small low-flow channel with trails and open space forming the rest of the flood channel/floodplain. You can start seeing what Grandma and Grandpa could have done in 1933. If only we could have talked to them about “watershed love” for Montclair Creek.
Brian Hyde is an expert in floodplain management and stream restoration. He wants your feedback at westerly_connect_brian@comcast.net or 720-939-6039.