2006-06-12_REVISION - M1982015 (23)

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Weld County Public Works Attn: David Bauer, PE • January 30, 2006 J N: 3075 Page 2 Colorado, published in April 1981, contains a table which was also referenced for "Year 2000 Urbanization at Longmont and Lyons." This table lists 100-year peak flows for the St. Vrain River at Idaho Creek (located just upstream of I-25) of 17,070 cfs. On page 38 of this report, there is discussion of the results under section C. Comparison of the Peak Flows for Different Development Conditions. Within this discussion, WRC states, "...[VVje can see that the percentage increase in peak flows resulting from the year 2000 development conditions is minimal. Such increases ranging from none to 1.55% will have a minimal impact on the water surface elevations resulting from the 100-year frequency flood." They continue on to say, "In some cases for the 50-year and 500-year floods, because of flood peak desynchronization, the year 2000 urbanized discharges are slightly lower than those for the existing conditions." The referenced pages of this report are attached. Due to this `minimal difference,' the Army Corp of Engineers Study was used in the CLOMR as this study contained a level of detail not found in any other study including adjusted flow rates at all stations along the St. • Vrain River accounting for the routing effect of a flood in a river, as well as the contributions from smaller tributary streams along the reach of the river. However, Carroll & Lange, Inc. performed an exercise as a check to the sensitivity to the peak discharge on the water surface elevation. In order to obtain an increase of one foot to the 100-year water surface elevation an increase in flow of approximately 6,000 cfs would be needed. This is an increase of approximately 38% to the total peak flow. 4. The applicant should verify and demonstrate in the 12/12/05 CLOMR report that the estimated peak flows passing by the proposed development property accurately reflect current conditions. Response: See discussion above. 5. 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 conitnue 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. Response: Concrete debris was placed along the river embankment during the previous mining operation. However, we are currently working with our • geotechnical consultant regarding the stability of the existing river embankment during flood events. Stability recommendations, if any, will