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Home My WebLink About; I-5 North Coast Floodplain; Agua Hedionda and Batiquitos Lagoon Hydraulic Study; 2008-02-01SAN DIEGUITO RIVER LOCATION HYDRAULIC STUDY 4.3 HEC-RAS INPUT The HEC-RAS River Analysis System v3.1.3 is a graphically based computer program which has been developed by the U.S. Army Corps of Engineers Hydrologic Engineering Center. Input required by the software included channel cross sectional geometry, channel roughness coefficients, starting water surface elevations, and a I OO-year storm event discharge. 4.3.1 Cross Sectional Geometry: The majority of the cross sections were placed perpendicular to the river flow path. Several cross sections have been skewed to allow channel reach lengths to accurately model the river. To better define the channel terrain, it was necessary for the cross sections in locations of bridges, depressions, islands or contractions/expansions to be more closely spaced. Ineffective flow areas were used at portions of the river which did not contribute significantly to the conveyance of the flood waters. The hydraulic model was based on the topography (including hydrographic elevations) obtained from Project Design Consultants, dated November 19, 2003 in the National Geodetic Veliical Datum of 1929 (NGVD 29). Along with this topography, the Grading Plan from the San Dieguito Wetland Project prepared by Southern California Edison Company was used for grading and excavation currently in progress (NGVD 29). Both topographic sheets were combined and scaled accordingly to create a complete topographic map of the floodplain. All results are provided in English units using NA VD 88. At the completion of the San Di eguito Restoration Project, this study may require revising if changes to the grading plan received, August 13, 2007, from Project Design Consultants occur during construction. 4.3.1.1 Bridges The existing [-5 Bridge is at cross section #7000. The Momentum Equation was used to calculate water surface elevations under the bridge. The primary concern of modeling the possible encroachment of the San Dieguito River from the [-5 Bridge replacement was the removal of the pier walls into the floodplain and construction of new concrete piers. The effective number obstructions will be halved from ten to five. How1ver, the diameter of the bridge supports will increase in diameter from N feet to 4 feet. ·The as-builts and the Advanced Planning Study used to model the [-5 Bridge are included in Appendix C. A clogging factor of +25% was added to the bridge piers to account for debris which _ could catch on the piers and provide additional blockage to the flow of the river. The proposed [-5 Bridge will be approximately 74 feet wider and approximately 40 feet longer than the existing [-5 Bridge. The existing channel (Figure 3) is proposed to be widened, from its existing location at the northern end of the [-5 Bridge, to the entire length of the widened bridge. The re-grading along with the reduction of bridge pier obstructions will reduce the constriction of the San Dieguito River flow beneath the [-5 Bridge crossing. ~DOKKEN ENGINEERING www.dokk •• t.,' .. ,rlnr·eoN 4-3 February 2008 COTTONWOOD CREEK CULVERT ANALYSIS SECTION 6 Cottonwood Creek Culvert Analysis 6-1 March 2009 COTTONWOOD CREEK CULVERT ANALYSIS SECTION 6: COTTONWOOD CREEK CULVERT ANALYSIS 6.0 PROJECT PURPOSE AND VICINITY MAP: Figure 1: Project Vicinity Map The Cottonwood Creek is located in the City of Encinitas, CA. The project site is located approximately 3,800 feet upstream of the creek mouth at the Pacific Ocean. (Thomas Guide page 1147: C-6 and C-7). This study was prepared for the California Department of Transportation (Caltrans) to determine the extents of any localized flooding from the 100-year storm event to the Interstate-5 (I-5). This study specifically addresses the following: 1) The existing extents of inundation upstream of the Cottonwood Creek/ I-5 crossing during the 100-year storm event. 2) Changes in the inundation limits resulting from the 10+4 buffer alternative for the I-5 corridor. The Cottonwood Creek is not presently within a FEMA floodplain and has been highly channelized and undergrounded east of I-5 throughout the City of Encinitas. The proposed 10+4 buffer alternative for the I-5 corridor will have no effect on the inundation area caused from the existing backwater effects of the Cottonwood Creek Culvert flowing under the I-5. Project Site 6-2 March 2009 COTTONWOOD CREEK CULVERT ANALYSIS 6.1 DESCRIPTION OF WATERSHED: The Cottonwood Creek watershed is located within the Carlsbad Watershed, Hydraulic Unit 904.10-904.63 (Figure 2), and covers an area of 2,175 acres. The creek drains the western slopes of the ridge running parallel to and west of El Camino Real and the Encinitas Creek drainage. Cottonwood Creek discharges into the Pacific Ocean via a storm drain at Moonlight State Beach. The elevation within the watershed ranges from 400 feet to sea level. Figure 2: Regional Basin Area The Cottonwood Creek is a perennial stream fed by urban runoff, excess irrigation water from residential/commercial landscape, and agricultural uses, with some contribution from groundwater resurfacing into the creek. The dominant land use within the watershed is urban development. Downstream of the project site, the Cottonwood Creek flows through natural streams and underground culverts, discharging through a storm drain onto Moonlight State Beach and into the Pacific Ocean. The waters are not identified as an impaired water body on the California Section 303(d) Water Quality Limited Waters List. 6.2 CULVERT MODELING METHODOLOGY: The area of analysis ranges from approximately 200 feet upstream to 900 feet downstream of the I-5 crossing. In this region, the Cottonwood Creek would experience a peak flow of 1668.52 cfs during the 100-year storm event. The culvert crossing beneath I-5 changes shape twice before it outfalls into a natural channel section of the Cottonwood Creek (Figure 3). The cross culvert begins as a 10’ Pipe Arch Culvert (Figure 4) for 714 feet, then transitions to a 6’x 8’ Double Box Culvert (Figure 5) for 544 feet and finally transitions again to a 7’x 4’ Triple Box Culvert (Figure 6) for 116 feet before it ends downstream at a triple box headwall (Figure 7). A peak flow analysis determined the headwater elevation upstream of the cross culvert to be 91.8 feet. All elevations given in this report are based on the North American Vertical Datum of 1988 (NAVD 88). Project Site 6-3 March 2009 COTTONWOOD CREEK CULVERT ANALYSIS Figure 3: Cottonwood Creek Downstream of Culvert Figure 4: Upstream Headwall to Pipe Arch Culvert 6-4 March 2009 COTTONWOOD CREEK CULVERT ANALYSIS Figure 5: Double 6’x 8’ Box Culvert Figure 6: Triple 7’x 4’ Box Culvert 6-5 March 2009 COTTONWOOD CREEK CULVERT ANALYSIS Figure 7: Downstream Headwall of Triple Box Culvert 6.2.1 CulvertMaster: Bentley CulvertMaster v3.1 was used to analyze the culvert hydraulics and determine the headwater elevation upstream of the I-5. The culvert was analyzed as a 10’ Pipe Arch Culvert 1,374 feet long, because the system was found to be under inlet control (Figure 8) and the change in the culvert shape would have little effect on the computed headwater elevation. Under inlet control, the culvert barrel is capable of passing more storm water than the culvert inlet can accept. During the 100-year storm the headwater elevation upstream of the culvert inlet will rise above the top of the culvert. The storm water passing into the culvert will occur as orifice flow, once the inlet is submerged. The flow will pass through the critical depth near the culvert entrance and transition to supercritical throughout the culvert. The existing cross culvert has an average slope of approximately 1.3% and one 45 degree bend between the transition from the double box culvert to the triple box culvert. The invert elevations were taken from metric digital topography (0.5 meter contours, NAVD 88), provided by Caltrans. Figure 8: Inlet Control Flow Condition 6-6 March 2009 COTTONWOOD CREEK CULVERT ANALYSIS 6.2.2 StormCAD: Bentley StormCAD v5.6 was used to analyze the culvert system crossing beneath I-5. The three sections of the culvert system were modeled to study the transitions between and the characteristics of each section as they affect each other. See Appendix B for the culvert system Summary Table. Because the system is under inlet control and the downstream outlet is anticipated to be inundated, the system will experience pressure flow during the peak 100-year storm. The culvert system will experience pressure flow in two locations during the peak 100- year storm. 1) The transition between the 10’Pipe Arch and Double 6’x 8’ Box Culverts. 2) The transition between the Double 6’x 8’ Box and Triple 7’x 4’ Box Culverts and throughout the length of the Triple 7’x 4’Box Culvert. The flow regime within all three culvert sections is supercritical with velocities ranging from approximately 20 to 29 ft/s. The greatest velocities would occur towards the middle of each culvert section. 6.2.3 Tailwater Elevation: Trials for varying tailwater elevations were performed, ranging from a free outfall to a tailwater elevation of 65 feet. The large head on the system will overcome any backwater effects from the tailwater elevation. The tailwater elevation used in the analysis was based on the Triple 7’x 4’ Box Culvert section flowing at capacity with a normal depth of 4 feet. The tailwater elevation was input as 56.5 feet. 6.2.4 Discharge: Discharge information was obtained from the Cottonwood Creek / Moonlight Beach: Final Hydrology Report, by Nolte Associates, Inc.for the City of Encinitas, dated July 2003. For the purposes of the culvert analysis, 1,668.52 cfs was identified as the peak 100-year storm discharge at the upstream headwall to the 10’ Pipe Arch Culvert. This location relates to node number 808 of the Hydrology Map (see Appendix C). For the StormCAD analysis, two additional discharges were obtained from the hydrology report. Node number 812 relates to the transition from the section of 10’ Pipe Arch Culvert to the section of Double 6’x 8’ Box Culvert, with a 100-year storm discharge of 1,979.59 cfs. Node number 907 relates to the transition point from the section of Double 6’x 8’ Box Culvert to the section of Triple 7’x 4’ Box Culvert, with a 100-year storm discharge of 2,133.45 cfs. 6-7 March 2009 COTTONWOOD CREEK CULVERT ANALYSIS 6.3 SUMMARY AND CONCLUSION: The anticipated flow in the cross culvert is received from urban runoff from the City of Encinitas and released into the Cottonwood Creek which flows directly to the Pacific Ocean. Since the cross culvert operates under inlet control the flow will backup at the upstream end of the culvert to a maximum elevation of 91.8 feet, during the peak of the 100-year storm event. This elevation will be contained within the natural depression located in the southeast quadrant of the Encinitas Blvd./ I-5 Interchange and no damage to adjacent properties is expected. December 2007 COTTONWOOD CREEK CULVERT ANALYSIS APPENDIX ‘A’ Inundation Exhibit December 2007 COTTONWOOD CREEK CULVERT ANALYSIS APPENDIX ‘B’ CulvertMaster Report StormCAD Report Culvert Designer/Analyzer Report Cottonwood Creek Arch Culvert Title: I-5 Floodplain Studies p:\...\culvertmaster\cottonwood_arch.cvm 03/09/09 12:00:03 PM DE San Diego c Bentley Systems, Inc. Haestad Methods Solution Center Watertown, CT 06795 USA +1-203-755-1666 Project Engineer: Glen Parker CulvertMaster v3.1 [03.01.010.00] Page1of 2 Analysis Component Storm Event Design Discharge 1,668.52 cfs Peak Discharge Method: User-Specified Design Discharge 1,668.52 cfs Check Discharge 2,133.45 cfs Tailwater Conditions: Constant Tailwater Tailwater Elevation 56.50 ft Name Description Discharge HW Elev. Velocity Culvert-1 1-120 x 114 inch Arch 1,668.52 cfs 91.84 ft 27.40 ft/s Weir Not Considered N/A N/A N/A Culvert Designer/Analyzer Report Cottonwood Creek Arch Culvert Title: I-5 Floodplain Studies p:\...\culvertmaster\cottonwood_arch.cvm 03/09/09 12:00:03 PM DE San Diego c Bentley Systems, Inc. Haestad Methods Solution Center Watertown, CT 06795 USA +1-203-755-1666 Project Engineer: Glen Parker CulvertMaster v3.1 [03.01.010.00] Page2of 2 Component:Culvert-1 Culvert Summary Computed Headwater Eleva 91.84 ft Discharge 1,668.52 cfs Inlet Control HW Elev.91.84 ft Tailwater Elevation 56.50 ft Outlet Control HW Elev. 87.33 ft Control Type Inlet Control Headwater Depth/Height 2.30 Grades Upstream Invert 70.00 ft Downstream Invert 52.50 ft Length 1,374.00 ft Constructed Slope 0.012737 ft/ft Hydraulic Profile Profile S2 Depth, Downstream 6.52 ft Slope Type Steep Normal Depth 6.48 ft Flow Regime Supercritical Critical Depth 8.71 ft Velocity Downstream 27.40 ft/s Critical Slope 0.007931 ft/ft Section Section Shape Arch Mannings Coefficient 0.013 Section Material Concrete Span 10.00 ft Section Size 120 x 114 inch Rise 9.50 ft Number Sections 1 Outlet Control Properties Outlet Control HW Elev. 87.33 ft Upstream Velocity Head 7.18 ft Ke 0.20 Entrance Loss 1.44 ft Inlet Control Properties Inlet Control HW Elev.91.84 ft Flow Control Submerged Inlet Type Cottonwood Culvert Area Full 73.6 ft2 K 0.00180 HDS 5 Chart 1 M 2.50000 HDS 5 Scale 1 C 0.02894 Equation Form 1 Y 0.74000 Performance Curves Report Cottonwood Creek Arch Culvert Title: I-5 Floodplain Studies p:\...\culvertmaster\cottonwood_arch.cvm 03/09/09 12:00:58 PM DE San Diego c Bentley Systems, Inc. Haestad Methods Solution Center Watertown, CT 06795 USA +1-203-755-1666 Project Engineer: Glen Parker CulvertMaster v3.1 [03.01.010.00] Page1of 1 Range Data: Minimum Maximum Increment Discharge 0.00 1,668.52 300.00 cfs Performance Curves Discharge (cfs)(ft)Headwater Elevation 70.0 75.0 80.0 85.0 90.0 95.0 0.0 200.0 400.0 600.0 800.0 1000.0 1200.0 1400.0 1600.0 1800.0 HW Elev. SectionDescriptionUpstreamNodeDownstream NodeLength(ft)NumberofSectionsSectionSizeTotal Flow(cfs)FullCapacity(cfs)AverageVelocity(ft/s)Velocity In(ft/s)Velocity Out(ft/s)Upstream InvertElevation(ft)DownstreamInvert Elevation(ft)10' Pipe Arch Node #808 Node #812 714 1 10 x 9.5 ft 1,668.52 2,061.49 28.66 21.39 14.21 70 60.3Double 6'x8' Box Node #812 Node #907 544 2 6 x 8 ft 1,979.59 1,711.43 20.62 20.62 20.62 60.3 53.85Triple 7'x4' Box Node #907 Cottonwood Creek 116 3 7 x 4 ft 2,133.45 1,220.94 25.4 25.4 25.4 53.85 52.49SectionDescriptionUpstreamNodeDownstream NodeConstructedSlope(ft/ft)Depth In(ft)Depth Out(ft)Avg EndDepth / Rise(d/D) (%)HydraulicGrade LineIn (ft)HydraulicGrade LineOut (ft)HydraulicSlope(ft/ft)Energy GradeLine In(ft)Energy GradeLine Out(ft)Energy Slope(ft/ft)10' Pipe Arch Node #808 Node #812 0.014 8.68 11.64 106.9 78.68 71.94 0.009 85.79 75.08 0.015Double 6'x8' Box Node #812 Node #907 0.012 11.64 9.46 131.8 71.94 63.31 0.016 78.54 69.92 0.016Triple 7'x4' Box Node #907 Cottonwood Creek 0.012 6.8 4.01 135.2 60.65 56.5 0.036 70.68 66.52 0.036StormCAD Summary Table