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HomeMy WebLinkAbout3338; Agua Hedionda & Calavera Creek Dredging; Flood Control; 1996-11-104. Howard H. Chang Consultants Hydraulic, Hydrologic and Sedimentation Engineering P.O. Box 9492 Rancho Santa Fe, CA 92067 (619)756-9050,594-6380. FAX: (619)756-9460 *) i I t x. November 10, 1996 Mr. Lee Anderson Ms. Sue Loftin Mr. Steve Weed Rancho Carlsbad Country Club Estate 5200 El Camino Real Carlsbad, California 92008 4,. Dear Friends: Subject: Technical Review for the Hydraulic Study of Agua Hedionda Creek by Ensign & Buckley in 1992 Pursuant to your request, I have reviewed the hydraulic study for Agua Hedionda Creek in the City of Carlsbad made by Ensign & Buckley in 1992. The hydraulic study was for the purpose of determining the 100-yr flood level and floodplain and floodway boundaries. My review was to assess the accuracy of the study results for Agua Hedionda Creek at Rancho Carlsbad Country Club. In the review, I have found some basic mistakes in the study. Such mistakes resulted in significantly higher 1Wyr flood elevations in Agua Hedionda Creek near El Camino Real. The major mistakes include incorrect roughness coefficients, incorrect definitions of the main channel and overbank areas, and improper split flow analysis to account for weir flows over the roadway of El Camino Real as described below. Incorrect roughness coefficients (or n value) were used for certain cross sections. For exampie, the n value of 0.08 was used for the cross section just downstream of El Camino Real. The NC record for this section is as follows: NC 0.04 0.05 0.08 0.3 0.5 1 This data record defms the n value of 0.04 for the left overbank, 0.05 for the right bank, and 0.08 for the main channel. The X1 record defines the main channel to be from station 25 feet to 170 feet for a total width of 145 feet. In fact, the main channel at this location has a bed width of about 100 feet. The channel bed is basically free of vegetation but trees do grow near the channel bank. Figure 1 shows physical conditions of the channel as viewed from the El Camino Real Bridge. This picture was taken in the summer of 1996. Since the main channel is clean, the proper n value should be about 0.03 but not the value of 0.08 used in the Ensign & Buckley study. The incorrect definitions of the main channel and roughness coefficient contributed to a significantly higher water-surface elevations near El Camino Real Bridge. Because of the higher water-surface elevations, there would also be significant flood discharge overtopping the roadway of El Camino Real. Most of the overtopping discharge would occur on the west side of the channel where the roadway elevations are lower. If correct n values had been used in Ensign & Buckley study, computed water-surface elevations would have been lower and the overtopping discharge would have been much less and most likely insignificant. At this time, the Cannon Road project is moving forward. After the completion of Cannon Road, the overtopping flow to the west will be blocked by the higher road embankment. As a conservative measure, one should disregard the overtopping flow in the hydraulic computation. Without split flow due to overtopping, the starting water-surface elevation at the downstream lit of study would also be lower according to the Ensign & Buckley study. In conclusion, I have found some basic mistakes in the Ensign & Buckley study. Such mistakes contributed to significantly higher 100-yr flood elevations near El Camino Real. In view of the mistakes, it is necessary to revise the study before any official floodplain mapping is adopted by the city of Carlsbad and FEh4.4. Please free to call me if you have any questions regarding this matter. I W.D., P.E. 2 Fig. 1. View of main channel for Agua Hedionda Creek just downstream of El Camino Real 3 0 .I -. . .. stud?. Even chough the precipitation-frequency maps pre:sen;d arc prCphred considering physiographic facton. only those of a m+x scale could be considerd. There are some basins. thercfore. that are more sheltcrcd or uxpod than a generalized topographic map would indicatc. The dup valucs may not be represellalive of thc precipiration regimes in rgL'h basins. - -- - 0- .. .c .-.