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Home My WebLink About3338; Agua Hedionda & Calavera Creek Dredging; Agua Hedionda Creek Hydraulic Study; 1996-11-10Howard 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 November 10, 1996 Mr. Lee Anderson Ms. SueLoftin Mr. Steve Weed Rancho Carlsbad Country Club Estate 5200 El Camino Real Carlsbad, California 92008 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. / i In the review, I have found some basic mistakes in the study. Such mistakes resulted in significantly higher 100-yr flood elevations in Agua Hedionda Creek near El Camino Real. The major mistakes include incorrect roughness coefficients, incorrect definitions of the mam 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 example, 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: NC0.04 0.05 0.08 0.3 0.5 This data record defines the n value of 0.04 for the left overbank, 0.05 for the right bank, and 0.08 for the main channel. The XI 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 limit 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 FEMA. Please free to call me if you have any questions regarding this matter. Howard H. Chang Ph.D., P.E. it I! Hi lii Fig. 1. View of main channel for Agua Hedionda Creek just downstream of El Camino Real JZ> * 3. 5- — / - V, 75" , - 7, 75- / 7, -9 5- / ^,3o 3i.\ o 5 ' "7, 3 - 3 ^ , -7 V - J ^ . a BEST ORIGINAL ^ed to develop and validate the cunes of figure 14 >)stcnutic regional pattern. Duration turned out to be .tor. rhe>curvcs shown are based on data for the 2-yr I. Within the accuracy of the data available, it could Jt neither magnitude nor return period was a signifi- ce of Snow in Estimating y Values itrjbutii ii i-i" Miow .:in*'Ui;ts U> the ;>re»ipitalion-:'re- •> for Jaratioiis of 14 hours or 'ess ha.s been invcsii- t of the western United States, hi many parts of this ularly .it higher elevations, snow accounts for over M) e normal annual precipitation. Thus, the importance ) short-duration (6- to 2-4-hr) precipitation-frequency interest for a more complete understanding of the frequency regime. inual .precipitation containing a ..high percentage of nces does not necessarily mean that• sipovrcontributed > the annual series of maximum 6- or 24-hr procipita- [ his problem was investigated by'tubulating two sets ill stations where snowfall observations were made j first set of data contained the greatest 24-hr (and at recording-gage stations) precipitation 'amount for :ardless of typo of precipitation (water equivalent for unts). The'second series was restricted solely to rain- some eases, the second series contained amounts as fth highest for a particular year. Results of these are reported in the section for each state. Reliability of Results The term "reliabilit;. " is used here as an indication of the degree of confidence that t-m be placed in tbe accuracy of the results obtained from the maps. The reliability at these results is influenced by the sampling errors in time and space, and by the manner in which the maps were constructed. Sampling errors in time and space result !r, .:"iom.i!oiis storm '.s!r. h h . Ixs for one >t;:;ic<:i. . !U r. !) !he chance '...;.x' piecipitation e' v.1 'in over .1 limited event (especially <>f itj*. II -'lld i .1 -:.:.. iii In '.he .;:•....! .! ::.:. ^). - . ill e ^o.;-iJer.d !• gr..j:fve .:.a. Chiii. i . cetivc n-iLire) at a sr easijy have occurred at a neighboring station or between Results from a generalized anal-, sis based on spaccsivenjgHJg tech- niques are considered more nearly correctthan rojits determined from an analysis of only individual stalioA diy. (it the'more nwrtniaiiious regions, orography has grejlfet"cWttrototf the 'oration and magnitude of the largest storms and Dimple *fUv$ <^-r;~i:1g between neighhoiing •.i.itj.-ns is in:ippropri;He: Cfniskler.ition nn.it be given to effects of the slopes of surrounding terrain, station elevations, the intervening barrier between- station location and moisture source, etc. The locations of the stations used in the analyses aic shovvn in figures 3 and 4. This geographic networlf of stations Joes u u reveal with complete accuracy the very detailed structure of the isopluviul patterns in the mountainous regions of the West. The multiple regression equations discussed earlier were used to help in interpolation between values computed for these stations. The standard error of estimate for these relations should be consiiieied when using the precipitation-frequency values shown on the maps In general, the accuracy of the estimates obtained from the maps of this Atlas varies from a inmtmtfp.^^^^^|^|!$|'cj||t>iolMhii shorter return periods in relatively nonof^|f>iphic ,regjoos to 20 percent lor the longer return periods jfe ib?4R9E* rugfi«d orp- graphic regions. \ The values shown on these maps are in general :;grceni.'!it with those of Weather Bureau Technical Paper No. 40 (U.S. Weather Bureau I9M). Differences are found because f the gteuter attention paid to physiographic features in the present study Even though the precipitation-frequency maps presented are prepared considering physiographic factors, only those of a major scale could be considered. There are some basins, therefore, that are more sheltered or exposed than a generalized topographic map would indicate. The rriup values may not be representative of the precipitation regimes in »t^ch basins.