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I I I I I I I I I I I I I I I I I I I Table 1 also shows those discharges calculated by Wilbur Engineering for mapping of the flood hazard boundaries. As previously stated, the Dolores River peak discharge was estimated using USGS 17B procedures and adjusted based on historical data, while Silver Creek peak discharge was based on area/discharge relationships for similar basins in the area. The Dolores River peak discharge agrees well with the peak discharge found in this study. However, the Silver Creek discharge is half of that found in this study. The Silver Creek basin area is nearly three times smaller than the smallest basin in the area/discharge relationship. In general, peak discharges increase as basin size decreases. Therefore, the area/discharge relationship used in the Wilbur Engineering study is likely underestimating peak discharge. Therefore, it is recommended that the discharge calculated in this study of 700 cfs be utilized for planning purposes. Because Silver Creek is only shown as an approximate floodplain on the flood hazard map, no changes to the map are required at this time. However, it should be recognized that utilization of the higher discharge will increase the floodplain width from lower discharges, and development near the floodplain may require additional analysis to determine more accurate floodplain boundaries. Peak flow records are available at the Dolores River Below Rico gage (USGS Gage No. 09165000) from 1952 through 1996. Using the gage records, USGS Bulletin 17B procedures were used to calculate discharges for specific return intervals at the gage. These discharges were then used to estimate peak flows for the Dolores River at the Incorporation Limits using CWCB Technical Memorandum No. 1 procedures. The estimated loo-year discharge using these procedures is 2,200 cfs. This is 20 percent less than the estimate of 2,800 cfs using the regression equations. Due to the relatively short period-of- record, it is recommended that 2,800 cfs be used for planning purposes. Table 2 presents the recommended planning level discharges for the Dolores River and Silver Creek in the Town of Rico. Rico should regulate the approximate floodplains on a case-by-case basis. FEMA requires that a detailed floodplain analysis must be performed by a professional engineer and submitted by the developer or development proponent for development projects involving more 5 acres or more than 50 units (whichever is more restrictive). T bl 2 R a e . ecommen annIDl! ISC arl!es or Location 100-year Peak Discharl1:e (cfs) Dolores River 2,800 Silver Creek 700 ded PI D' h 1" Rico Hvdraulic Analvsis Hydraulic methods used for the approximate floodplain delineations shown on the Rico ARM panels are not available or have not been documented by FEMA. The Wilbur Engineering study has performed hydraulic calculations using the Corps of Engineers' HEC-2 computer program. For future studies where the floodplains may be affected, the HEC-2 computer model should be updated with the new information. Channel and floodplain cross-sections may be obtained from detailed topographic mapping or from field surveys. Cross-sections obtained from USGS quadrangle maps are not advised. 5