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I I I I I I I I I I I I I I I I I II ENGINEERING METHODS Hvdrolol!ic Analvsis The Rico FIRM presents approximate floodplain delineations. Hydrologic information used for the FEMA approximate floodplain delineations are not available or were not documented by FEMA. The Wilbur Engineering report contains a detailed floodplain for the Dolores River and an approximate floodplain for Silver Creek. The 100-year discharge for the Dolores River is based on a gage analysis while the 100-year discharge for Silver Creek is based on a regional comparison of other gages. In order to verify discharges in the Wilbur Engineering report, the discharges were compared to those calculated using CWCB methods described below. The CWCB has developed acceptable hydrologic information for undeveloped basins using regional regression methodologies for computing 100-year flood-flow values. The CWCB methodology is fully described in a publication by the CWCB entitled "Guidelines for Determining 100-year Flood Flows for Approximate floodplains in Colorado". The Dolores River Subregion (DLR-l) is used for streams in the Dolores River basin. The CWCB equation is: Equation 1 Regional Regression Equation for Dolores River Basin QIOO = 213.8(A)o.601 (Only for drainage areas between 2 and 1080 square miles) For Equation 1, the QIOO is the 100-year peak flow in cubic feet per second, and A is the upstream (contributing) drainage area in square miles. The regression equation equations are only valid for the range of drainage areas as shown next to each equation. The l00-year flow for Rico within the Town Limits were estimated by utilizing the regional regression equations shown in Equation 1. Table 1 presents lOO-year flow estimate for these locations. As shown, three of the watersheds are too small to utilized the CWCB equations. For these watersheds, other methods should be used in development of flood discharges. sIma e ISC arl!es or ICO Drainage tOO-year Discharee (cCs) Drainage Area Description Area (mi) CWCR Method Wilbur Report Dolores River - Upstream of Silver Creek 72.43 2,800 Dolores River - At Rico Downstream 82.75 3,000 2,800 Incorporation Limits Silver Creek - Upstream of Mouth 7.13 700 350 Iron Draw - Upstream of Mouth 0.84 Outside Bounds (1) Spear Creek - Upstream of Mouth 0.30 Outside Bounds (1) Sulfur Creek - Upstream of Mouth 0.83 Outside Bounds (1) Table 1 Eft d D' h ~ R' Notes: (I) Streams are shown on the FHBMs. However, because the watersheds are less than I square mile, these streams are not included in the CWCB floodplain designation. II 4