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HomeMy WebLinkAbout3338; Agua Hedionda & Calavera Creek Dredging; Calavera Creek Channel Protection-Hydraulic Analyses; 2005-08-11HYDRAULIC ANALYSES FOR CALAVERA CREEK CHANNEL PROTECTION August 30,2005 ehanm@@msmD~~? Clvil Engineering Hydrology - Hydraulics Sedimentation P.O. Box 9496 Rancho Santa Fe, CA 92067 (858) 692-0760 -TABLE OF CONTENTS . Introduction ................................................................................................................................ 1 Channel Protection ........................................................................................................................ 2 ................................................................................................................................... Conclusion 4 APPENDIX A . Riprap Force Calculations and vmax3 Calculations B . 100-Year HEC-RAS Analysis INTRODUCTION A portion of Calavera Creek is located along the northerly boundary of the Rancho Carlsbad Mobile Home Park (RCMHP) in the city of Carlsbad (see Vicinity Map). The creek flow passes over a recently constructed reinforced concrete weir wall near the northerly end of RCMHP prior to entering the mobile home park. Within RCMHP, the creek primarily consists of a vegetated trapezoidal channel. Figure 1. Vicinity Map During a recent site visit, riprap was observed in Calavera Creek near the weir wall outlet. The riprap was present on the banks immediately below the weir wall (see Figure 2) and also on the Calavera Creek channel bank opposite the weir wall. Riprap has been placed on the opposite bank because the creek flow outletting the weir wall is directed towards the bank at an angle of approximately 45 degrees. The riprap was installed as part of the weir wall construction. Additional channel protection is now proposed below the weir wall to provide a greater factor- of-safety against erosion resulting fiom high velocity flow exiting the weir wall. Resource Agency permits are required for construction of the additional protection. The San Diego Regional Water Quality Control Board and US Army Corps of Engineers have indicated a preference for protection measures that support vegetative growth rather than more traditional hard armoring such as riprap. This report presents research and hydraulic analyses used to recommend erosion control measures that address the resource agency's criteria. Figure 2. Looking Upstream in Calavera Creek towards Weir Wall CHANNEL PROTECTION During a site meeting, Mr. Chris Means fiom the San Diego Regional Water Quality Control Board, stated that the Board's reference is for erosion protection measures such as vmax3 fiom P North American Green. Vmax is a turf reinforcement mat consisting of composite layers (matrix material between UV-stabilized polypropylene high strength, permanent nets) that prevent erosion when a channel is in an initial unvegetated, partially vegetated, and filly vegetated state. In a channel's initial unvegetated state, vmax3 contains a matrix material (straw, coconut, or polypropylene) that completely covers bare soil to prevent erosion. As vegetation establishes and matures, the matrix material continues to provide structural support for the developing plants. Ultimately, the vegetation and permanent netting provides the required erosion protection. Discussions with North American Green revealed that the vmax3 has been tested and installed in many channel applications. However, the technical staff indicated that the material has not been tested nor certified in situations similar to that at the weir wall where flow fi-om two creeks confluence at a sharp angle. As a result, vmax3 was not recommended for protection of the channel bank opposite the weir wall due to the lateral force impact. Furthermore, vmax3 is not recommended immediately below the weir wall, where the shear force from the high velocity outlet flow is large. Based on these considerations, Figure 4 (included after the report text) illustrates areas near the weir wall where riprap and vmax3 should be installed to provide erosion protection from the flow outletting the weir wall. Riprap is proposed immediately downstream of the weir wall and along the opposite channel bank. The riprap on the opposite bank extends laterally beyond the effective flow area from the weir wall. vmax3 is proposed in the remaining channel areas below the weir wall outlet. Note that vmax3 is also proposed on the Little Encinas Creek bed and banks immediately upstream of the weir wall. Figure 3 shows that the creek is currently concrete-lined in this area. The concrete is cracking and has failed in one area. Mr. Means stated that the Board prefers that the concrete be replaced with vmax3. Figure 3. Looking Upstream in Little Encinas Creek Above Weir Wall Previous force calculations indicated that the riprap on the southerly Calavera Creek bank should be 1-ton rock class (see Appendix A). A 100-year HEC-RAS analysis was performed using updated (May 12, 2005) topographic mapping at a 2-foot contour interval to confirm the riprap sizing. There are several hture upstream improvements that will impact the ultimate 100-year flow rate. In order to account for the potential range of conditions in the watershed, HEC-RAS analyses were performed based on the conditions that would generate the least (84" RCP installed, Lake Calavera improvements, Basin BJB modifications, and Basin BJ construction) and greatest 100-year flow rates (existing conditions and 84" RCP installed). The analyses indicate that the greatest outflow velocity (10 feet per second) fiom the weir wall will occur with lower 100-year flows. This analysis is included in Appendix B and the HEC-RAS cross-sections are shown on Figure 4. While it may be contrary to intuition that a lower flow will generate a higher velocity, this occurs because the backwater below the weir wall is lower with the lower flows. The 1-ton riprap can withstand this 100-year flow velocity and is, therefore, appropriate below the weir wall. Analyses were also performed to verifL the stability of vmax3 in this application. North American Green provides "Erosion Control Materials Design Software" for the analysis of vmax3. The input parameters include the flow rate, channel dimensions, channel slope, and vegetation development phase. Based on the topographic mapping, the channel was assumed to have a 2 percent longitudinal slope, 8-foot bottom width, and 2:l (horizonta1:vertical) side slopes. The anal ses for a vegetated and unvegetated channel are included in Appendix A and Y show that Vmax P550 will be stable. CONCLUSION Hydraulic analyses have been performed to outline criteria for erosion protection of Calavera Creek from 100-year flow outletting the weir wall. These analyses are not intended to address the potential for erosion in other areas of Calavera Creek with the exception of the concrete-lined area immediately upstream of the outlet. The analyses show that a combination of 1-ton class riprap and vmax3 P550 can be used to provide protection. Riprap is proposed in areas subject the greatest forces. Long-term channel protection with vmax3 is dependent on vegetative growth within the vmax3 lining. Therefore, vegetation (turf or other ground cover) that can be sustained within the site environment must be selected. The channel design must be in accordance with the channel dimensions and slope modeled in the design software, as indicated above. Any deviations in the channel design must be verified. North American Green provides detailed installation guidelines and requirements that must be followed by the contractor. Finally, it is recommended that the vmax3 be keyed at least 5-feet below the upstream channel bed and banks to prevent undermining.