2006-06-12_REVISION - M1982015 (23)

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~~~,~~~ MEMORANDUM O Ci COLORADO TO: Kim Ogle, Planner FROM: David Bauer, P.E., Public Works Department SUBJECT: CLOMR for PZ-1078 St. Vrain Lakes PUD DATE: 12-January-2006 Weld County Public Works Department has reviewed the CLOMR report and application for the St. Vrain Lakes - Carma project. Comments ^ St. Vrain Lakes PUD is planned for up to 5,13] single family residential and commercial building sites. The south edge of the proposed project borders the 100-year floodplain of the St. Vrain River. ^ Carma / St. Vrain Lakes PUD (applicant) has submitted, to Weld County and FEMA, a 12/12/05 CLOMR report requesting a revision of the 100-year floodplain map. The report relies on an un-published 1977 U.S. Army Corps of Engineers hydrology report for determination of the peak flow in the St. Vrain River. The peak flow in the St. Vrain River is influenced by seasonal snowmelt in addition to contributions from rainstorm runoff. ^ A substantial amount of change to the St. Vrain River drainage basin up-gradient of the Carma/St. Vrain Lakes site • due to new development has occurred since 1977. Those structural changes likely contributes a significant amount of runoff to the river. On-going sand and gravel mining up-gradient of and adjacent to the Carma/St. Vrain Lakes site also likely influence the flood response of the river. The modeling approach presented in the CLOMR report is an acceptable approach to evaluation the 100-year floodplain, however the hydrologic assumptions need verification. ^ The applicant should verify and demonstrate in the ] 2/12/05 CLOMR report that the estimated peak flows passing by the proposed development property accurately reflect current conditions. ^ The report assumes that the existing berms, that were left by prior gravel and sand mining along the north side of the St. Vrain River channel, will continue to exist in their present configuration during the 100-year flood. This assumption may not be justified for un-armored, geotechnically unstable sand and gravel deposits. Additional supporting information is needed to justify this assumption. ^ The report does not address the length of time that the peak flow occurs. The 12/12/05 CLOMR report assumption, that the ponds located at the St. Vrain Lakes site have capacity to receive the proposed overflows, needs to be evaluated. It may be that the ponds' storage capacity is soon exhausted and that the remaining peak flows pass downstream unattenuated in the main river channel causing higher water levels than projected. ^ The CLOMR report cannot be accepted until it has been stamped, signed and dated by a professional engineer licensed in the State of Colorado Recommendation ^ The Public Works Department does not recommend approval of this CLOMR application. Public Works is amenable • to meet with the applicant to discuss the above noted concerns. Page I of 1