2016-09-02_HYDROLOGY - M1992069

• Permit No: M1992069
• IBM Index Class Name: Hydrology
• Doc Date: 9/2/2016
• Doc Name: Correspondence
• From: City of Greeley
• To: DRMS
• Email Name: PSH
• Media Type: D
• Archive: No

Mr. Roger Schmidt
September 2, 2016
Page 8
overtop the concrete bike path and flood the F Street agricultural property. Again, the
2015 peak flow rate was slightly greater than the 3,400 cfs 10-year flow rate published in
the 2016 FIS (Figure 5). (Note: As discussed in point 1 above, in 2010 when a flow rate
of approximately 3,500 cfs caused damage to the Boyd-Freeman ditch, it occurred in a
location where the ditch turned from east to north and was formerly open channel. The
damage from that event was the result of the high flow in the channel, not from overland
flow across the field, based on the recollection of City staff. After that event, the south to
north portion of the ditch was modified from open channel to buried pipe).
The Al Pit also receives inflow from the North Eagleview drainage channel, which has an outfall
on the south side of the eastern portion of the Al Pit(Figure 1). The location of the outfall into
the pit is armored with an engineered concrete cellular mattress to provide scour protection
where the discharge flows down the slope of the pit. The North Eagleview drainage channel
conveys urban runoff from the area south of the Al Pit into the eastern portion of the Al Pit. The
j outlet of the Al Pit consists of an armored overflow channel on the north side in the eastern
portion of the pit(see Figure 1) and three culverts (approx. 42-inch diameter) underneath the
railroad. The North Eagleview drainage channel and the pit outlet are not significantly related to
the breach of the Boyd-Freeman Ditch or the headcutting that occurred across the F Street
agricultural field.
TOPOGRAPHIC MAPPING
Topographic data and mapping for the area of interest were provided by the City for 1995 (Figure
6), 2005 (Figure 7), 2013 (Figure 8) and 2014 (Figures 9.0 and 9.1). The data and mapping were
reviewed and the following general findings were made:
• The topography in the area of the Iverson/Martin Marietta Pit has changed substantially
from 2005 to 2013.
o Prior to the Iverson/Martin Marietta Pit being excavated, the 2005 topography
shows a relatively level ground surface elevation of approximately 4688 feet in
the area separating the bends of the oxbow in the Cache La Poudre River west of
the F Street agricultural field. At the narrowest point between the bends, they are
spaced approximately 750 feet apart from west to east(Figure 7).
o After the Iverson/Martin Marietta Pit was excavated (see 2013 topography shown
on Figure 8), a relatively direct flow path was created from the pit to the Boyd-
Freeman Ditch wasteway for any flow from the Cache La Poudre River entering
the pit along its perimeter (see Figure 1). Figure B.22 (Appendix B), shows an
aerial image taken on June 19, 2014 of this condition, where a breach in the
embankment between the river and the northwest corner of the Iverson Pit/Martin
Marietta pit created a hydraulic connection that routed water from the river
through the pit toward the Boyd-Freeman Ditch wasteway. Overflow from the
ditch is captured by the wasteway and routed to the Cache La Poudre River during