2006-06-12_REVISION - M1982015 (23)

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~ ~ MEMORANDUM TO: Kevin Jennings, Carroll & Lange, Inc. DATE: 21-March-2006 W~ ~ ~ ~ FROM: Brian Varrella, P.E., Public Works Department • SUBJECT: St. Vrain River Conditional Letter Of Map Revision (CLOMR) COLORADO Weld County Public Works Department has reviewed this CLOMR at the request of Carroll & Lange. Comments made during this intermediate review are not al]-inclusive, and must be addressed before the CLOMR will be approved by Weld County staff. Comments ^ A CLOMR for the St. Vrain River at St. Vrain Lakes PUD (PZ-1078) was sent to Public Works by Kevin Jennings at Carroll & Lange, Inc., on March 20, 2006 for an intermediate review by staff prior to the next submittal. ^ The following comments are not alt-inclusive. More comments may follow after the CLOMR has been re-submitted to Public Works. CLOMR Comments: 1. Reclaimed Mine Ponds (Gravel Pitsl: • a. It is recommended that a note be included in the report and in the model to discuss the n-value choice for the gravel pit reach. FEMA will be looking for some justification of n-values for these features. b. It appears there is a pond missing on the south overbank from Sections 56555, 55330, 53970, 53240, and 52370. The geometry of the cross section indicates deep overbank features, and the aerial photos at those sections suggest the presence of ponds. c. For the EXISTING LAKES reach: i. The previous submittal showed different proposed WSELs and operates independently of each other, and the release weir returning flow will operate independently of both of these ponds. All connections between ponds and to the river will likely have a short section of supercritical flow in large storm events. To simulate these behaviors in HEC-RAS, additional cross sections are required in the EXISTING LAKES reach of the model. ii. FEMA will require complete and accurate modeling of all proposed features constructed with fill in this reach, including islands, peninsulas, weirs between ponds, return weirs to the river, and others. This is critical for CLOMR approval and future LOMR submittals to FEMA. Please make sure all features have cross sections on both sides, and across the top (if necessary), with the exception of inline weirs (see the next comment). iii. The overtopping ground sections between the ponds at section 69640 and 68595 should be modeled as inline weirs to effectively model the rapidly-varied flow characteristics of these features. Please refer to Chapter 8 of the Hydraulic Reference Manual for specifics on modeling inline weirs. iv. Please be sure that Manning's-n values are appropriate for proposed erosion control features on the inline and lateral weirs, and that the geometries match those of the plan sheets submitted to Public Works. 2. Boundary Conditions: a. Each profile should have only one peak flow rate at the top of each reach for the entire study reach. The reviewed model had multiple flow changes at multiple locations, but since the model uses optimized • lateral weirs for flow budgeting, it is no longer necessary to have internal flow changes. _.-_-......... Page 1 of 3