2016-09-02_HYDROLOGY - M1992069 - Laserfiche WebLink

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Mr. Roger Schmidt September 2, 2016 Page 2 the bank, thereby creating a rill (i.e., a small erosional channel) into which the flow began to focus. The concentrated flow in the rill caused headcutting which advanced the erosion of the channel in an upstream direction to the west. The eroding channel expanded downward and horizontally until the current ravine was formed. Water overflowing the bank of the Cache La Poudre River followed the newly formed ravine into the Al pit. During this dynamic erosion process, approximately 100 feet of the BIC pipeline, located just west of the pit,was breached by the velocity of the flow crossing the pipeline and eroding the west bank of the pit. • If the Al gravel pit was not present, the sheet flood flows from the river would have continued eastward as sheet flows and it would have been unlikely for the headcutting to occur over the F Street agricultural field and for the resulting breach of the pipeline to occur. Headcutting migrates in an upstream direction when the conditions downstream, including the velocity of the flowing water, cause erosion to occur and advance upstream. Based on a review of historic aerial imagery, no evidence of this type of headcutting occurring was observable prior to the presence of the pit. Other features in the field, such as prairie potholes, are observable in the historic aerial imagery before the Al Pit was constructed, though no headcutting was observable prior to the presence of the pit. In - other words, if the overland flow conditions had occurred without the breach down into the excavation of the Al Pit, some localized limited surficial erosion in the F Street agricultural field may have occurred, but without the headcut, creation of the ravine, associated mass wasting of soil material and breach of the pipeline that occurred in 2014. • Major physical features that likely altered the flow in the Cache La Poudre River and contributed to the amount of water flowing over the field during the flood event in June 2014 included the configuration of the Iverson/Martin Marietta gravel pit located to the west, between the Cache La Poudre River and the concrete bike path, and the Great Western Railroad bridge that crosses the Cache La Poudre River to the north. This letter report provides a summary of the analyses conducted as part of this evaluation. BACKGROUND INFORMATION This letter report is based on the following information and observations: • Review of documents provided by the City of Greeley. • Review of additional documents independently obtained by WWE. • Observations made during site visits conducted by WWE: o May 12, 2015 site visit by Jonathan Jones, P.E. and Ian Paton, P.E. Mr. Paton subsequently prepared this letter report with peer review from Mr. Jones.