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HomeMy WebLinkAboutRFI16-0137 Ching-Stabel TM - Flood Plain StudyChing-Stabel Pond Hydraulic Impact Analysis 1 Technical Memorandum Ching-Stabel Pond Hydraulic Impact Analysis July 14, 2016 Prepared for: North Valley Engineering and Surveying Prepared by: Jacob Rowe, EIT Reviewed by: Chris Fritz, PE, CFM Purpose The purpose of this Technical Memorandum (TM) is to analyze the potential hydraulic impacts resulting from the construction of the Ching-Stabel Pond Project (Project). The project site is located in Butte County, California near the City of Chico, just south of Highway 32 and north of the Mud Creek and Sacramento River confluence (see Figure 1). The project features the construction of new raised berms and a pond within a FEMA designated Special Flood Hazard Area (SFHA) Zone A (see Figure 2). For the analysis, Peterson Brustad Inc. (PBI) conducted hydraulic modeling for the 100-year with- and without-project conditions using 1-dimensional and 2-dimensional hydraulic models developed by the California Department of Water Resources (DWR) Central Valley Floodplain Evaluation and Delineation (CVFED) program. Design plans for the project are included as Attachment 1. Horizontal and Vertical Datum The referenced horizontal projection for this analysis is the North American Datum 1983, Universal Transverse Mercator Coordinate System, Zone 10 North. The referenced vertical datum is the North American Vertical Datum of 1988 (NAVD88). All units are in survey feet. Existing Regulations The requirements of construction within a floodplain are set forth in Article IV Section 26 of Butte County Code. Code Section 26-23 (a) states that: The Department of Development Services and Department of Public Works shall review all development permit applications to determine that the proposed development does not adversely affect the carrying capacity of areas where base flood elevations have been determined but a floodway has not been designed. For purposes of this article, "adversely affects" means that the cumulative effect of the proposed development when combined with all other existing and anticipated development will increase the water surface Ching-Stabel Pond Hydraulic Impact Analysis 2 Technical Memorandum elevation on neighboring properties. The county will not allow development to increase the water surface elevation of the base flood more than one (1) foot at any point, nor will it allow any increase in the base flood elevation which adversely affects any neighboring property. In addition, Title 44 Code of Federal Regulations (CFR) states the following: For Zones A1-30 and AE, 44 CFR §60.3 (c)(10) states that when the Federal Insurance Administrator has provided a notice of final flood elevations for one or more special flood hazard areas on the community's FIRM and, if appropriate, has designated other special flood hazard areas without base flood elevations on the community's FIRM, but has not identified a regulatory floodway or coastal high hazard area, the community shall require until a regulatory floodway is designated, that no new construction, substantial improvements, or other development (including fill) shall be permitted within Zones A1-30 and AE on the community's FIRM, unless it is demonstrated that the cumulative effect of the proposed development, when combined with all other existing and anticipated development, will not increase the water surface elevation of the base flood more than one foot at any point within the community. For Unnumbered A-Zones, 44 CFR §60.3 (b)(10) states that when the Federal Insurance Administrator has designated areas of special flood hazards (A zones) by the publication of a community's FIRM, but has neither produced water surface elevation data nor identified a floodway or coastal high hazard area, the community shall obtain, review and reasonably utilize any base flood elevation and floodway data available from a Federal, State, or other source, including data developed pursuant to paragraph (b)(3), as criteria for requiring that new construction, substantial improvements, or other development in Zone A on the community's FIRM meet the standards. As noted previously, the project site is located within an unnumbered A-Zone with no identified regulatory floodway. Therefore, based upon the existing regulations as noted above, PBI has obtained, reviewed and reasonably utilized base flood elevation and floodway data available from Federal, State, or other sources to determine a) whether or not the project increases the base flood water surface elevation by more than one (1) foot at any point, and b) whether or not the project has potential to increase the flood hazard on other properties. FIGURE1Ching-Stabel Pond Project Location1180 Iron Point Rd., Suite 260Folsom, CA 95630 Phone: (916) 608-2212Fax: (916) 608-2232 CHING-STABEL POND Service Layer Credits: Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, SACRAMENTO RIVER MUD CREEK A A X X X X SHA D E D X SHA D E D X SH A D E D X X FIGURE2Ching-Stabel Pond Existing FEMA Flood Zones 1180 Iron Point Rd., Suite 260Folsom, CA 95630 Phone: (916) 608-2212Fax: (916) 608-2232 Ching-Stabel Pond Hydraulic Impact Analysis 4 Technical Memorandum Hydraulic Analyses The project site is potentially impacted by flooding from two different flooding sources: Mud Creek and the Sacramento River. Individual analyses were thus conducted to determine the potential hydraulic impacts of the project on both of these potential sources. Individual analyses were necessary due to differing flood conditions resulting from the two different flooding sources. The residual flooding for Mud Creek (assuming a ‘without levee’ condition) consists of shallow overbank flooding, which has the potential to result in effective flow at the project location. However, when considering 100-year residual flooding from Sacramento River, the project site is located within an ineffective flow area. The following sections document the individual hydraulic analyses that were conducted for analyzing the potential impacts considering each of these two potential flooding sources. Mud Creek Analysis To analyze the potential impacts for Mud Creek, PBI conducted a hydraulic analysis assuming that the right (northern) bank of Mud Creek is completely removed, which is consistent with FEMA’s ‘without levee’ requirements for mapping flood risk behind non-accredited levees. However, it is important to note that this levee system is currently shown as being provisionally accredited on the existing FEMA FIRM panel. Therefore, this analysis represents a conservative approach for determining the hydraulic impacts of the project since, if the levee owner submits the required data and documentation necessary to maintain accreditation of the right bank levee, it will effectively eliminate the Mud Creek potential flooding source for base (100-year) flood conditions. For the analysis, the CVFED Mud Creek/Big Chico HEC-RAS model1 was truncated to only include the section of Mud Creek south of Nord Highway bridge. The upstream boundary condition was then coded in to match the current 100-year flow at the bridge. The 100-year flow hydrograph was developed by scaling the 1997 flood hydrograph by 0.8 so that the peak flow aligned with the 100-year peak flow. The 100-year peak flow was identified as 10,700 cfs2. The right bank lateral structures within the HEC-RAS model were lowered to the surrounding ground elevation to simulate where bank overtopping would occur if the levee was not there. All of the other HEC-RAS model conditions were maintained. 1 Task Order 32 CVFED Development of SB 1278/AB 1965 ULOP Information Maps for the Chico Urban Area: 200-year SPFC Levee Breach Composite Floodplain. Prepared by CH2M Hill for the State of California, Department of Water Resources. July 2013. 2 Flood Insurance Study, Butte County, California and Incorporated Areas. FEMA. January 2011. Ching-Stabel Pond Hydraulic Impact Analysis 5 Technical Memorandum Overtopping flows were then coded into the CVFED FLO-2D model3. Two FLO-2D simulations were run; one run for with-project conditions, and one run without. The with-project simulation captured the influence of the pond by coding the perimeter pond embankment elevations into the model (Figure 3). The FLO-2D results were then imported into ArcGIS to map and compare the with- and without-project floodplains. Figure 3: With-Project Analysis FLO-2D Screenshot Sacramento River Analysis The influence of Ching-Stabel Pond on the Sacramento River floodplain was determined by modeling the rise in water surface elevation (WSE) as a result of the storage volume lost. HEC- RAS simulations were developed for both the with- and without-project scenarios to determine the effect of the project on the 100-year Sacramento River floodplain. This method is appropriate 3 Task Order 32 CVFED Development of SB 1278/AB 1965 ULOP Information Maps for the Chico Urban Area: 200-year SPFC Levee Breach Composite Floodplain. Prepared by CH2M Hill for the State of California, Department of Water Resources. July 2013. Ching-Stabel Pond Ching-Stabel Pond Hydraulic Impact Analysis 6 Technical Memorandum because area in the vicinity of Ching-Stabel Pond is identified as an ineffective flow area according to CVFED 14601 Upper Sacramento HEC-RAS model4 (Figure 4). Figure 4: The CVFED 14601 Upper Sacramento HEC-RAS model identifies the left over bank flow as ineffective flow in the vicinity of Ching-Stabel Pond. The storage volume removed from the floodplain was determined by calculating the volume of the Pond within the area of the new embankments. The volume was conservatively estimated by roughly outlining the Pond and assuming a uniform landside toe elevation of 138 ft (the minimum landside toe elevation). An adjusted stage volume relationship was developed for the storage area surrounding Ching-Stabel Pond to account for the volume lost as result of the project. The adjusted storage area was then coded into the HEC-RAS model to develop the with-project 100-year flood scenario for the Sacramento River. The 100-year hydrograph for the Sacramento River was coded into the model at the upstream boundary (Station 199). The 100-year hydrograph was selected to be the 1997x1.10 flow pattern 4 CVFED Upper Sacramento River System HEC-RAS Model Documentation. Prepared for State of California, Department of Water Resources, by CH2MHILL. January 2014 Ching-Stabel Ching-Stabel Pond Hydraulic Impact Analysis 7 Technical Memorandum per current CVHS5 data and guidance. The 100-year flow at station 199 on the Sacramento River is 172,500 cfs (Figure 5). Figure 5: The CVHS regulated rain flood peak flow frequency curve at Sacramento River station 199. Results The results of the hydraulic analysis conclude that the construction of Ching-Stable Pond has an insignificant effect on the overall floodplain hydraulics for the base (100-year) flood event. Figure 6 and Figure 7 present the flood depth with- and without-project scenarios for the Mud Creek analysis. Results show that construction of the pond increases the immediate surrounding water surface elevations no greater than 0.2 ft. for 100-year event (Figure 8). When considering the conservative assumptions utilized for the hydraulic analysis (i.e. the ‘without levee’ conditions and the provisionally accredited levee status) in combination with the relative accuracy of the hydraulic models, in can be concluded that the project does not measurably affect upstream or downstream flood hazards as they relate to Mud Creek. In addition, the minor increases are consistent with the criteria and within the limits as specified in 44 CFR §60.3. 5 Central Valley Hydrology Study. Department of Water Resources. November 2015. Ching-Stabel Pond Hydraulic Impact Analysis 8 Technical Memorandum The results of the Sacramento River analysis show that the development of Ching-Stable Pond has no effect on the 100-year river channel and floodplain hydraulics. The simulated 100-year peak stages both within the Sacramento River and within the modeled storage areas showed no measureable increase for both the with- and without-project conditions (Figure 9 and Figure 10). While some floodplain storage volume is lost due to the project, the change is shown to be insignificant when compared to the relative size of the Sacramento River floodplain at this location. FIGURE6Ching-Stabel Pond With-Project Flood Depth1180 Iron Point Rd., Suite 260Folsom, CA 95630 Phone: (916) 608-2212Fax: (916) 608-2232 Service Layer Credits: Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS,AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User CommunityEsri, HERE, DeLorme, MapmyIndia, © OpenStreetMap contributors Legend Study AreaFlood Depth (ft) 0 - 1 1 - 2 2 - 3 3 - 5 5 - 10 FIGURE7Ching-Stabel Pond Without-Project Flood Depth 1180 Iron Point Rd., Suite 260Folsom, CA 95630 Phone: (916) 608-2212Fax: (916) 608-2232 Service Layer Credits: Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS,AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User CommunityEsri, HERE, DeLorme, MapmyIndia, © OpenStreetMap contributors Legend Study AreaFlood Depth (ft) 0 - 1 1 - 2 2 - 3 3 - 5 5 - 10 FIGUREChing-Stabel Pond Change in Flood Depth1180 Iron Point Rd., Suite 260Folsom, CA 95630 Phone: (916) 608-2212Fax: (916) 608-2232 Service Layer Credits: Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, Legend Study AreaChange in Depth (ft) -0.21 - -0.10 -0.09 - -0.06 -0.05 - 0.05 0.05 - 0.10 0.10 - 0.20 8 Figure 9: No changes were shown within the storage area. Ching-Stabel Pond Hydraulic Impact Analysis 12 Technical Memorandum Figure 10: No changes were shown in the Sacramento River water surface elevation. Ching-Stabel Pond Hydraulic Impact Analysis 13 Technical Memorandum Ching-Stabel Pond Hydraulic Impact Analysis Technical Memorandum ATTACHMENT 1 CHING-STABEL POND DESIGN PLANS