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Home My WebLink AboutA28 February 27, 2015 City of Ames Attn: Charlie Kuester 515 Clark Avenue Ames, Iowa 50010 Dear Charlie, Task Order No. 1 of the Master Services agreement between the City of Ames and HDR Engineering, Inc. (HDR) (executed February 6, 2015) included evaluating the hydraulic effects the proposed Riverside Manor development project on water surfaces near the Oak-Riverside neighborhood (upstream from the proposed project)for the effective Federal Emergency Management Agency(FEMA) 0.2 percent annual probability flood event(500-year). Information from the effective FEMA 1 percent annual probability flood event(100-year) evaluation, developed as part of the City's Grand Avenue extension project(HR Green 2014), is also presented. A key assumption in this evaluation is that the analysis will be used as a decision support tool to evaluate potential hydraulic impacts only and will not be used for permitting or as part of a greater flood risk evaluation. Following the 2010 flooding event in Ames,the Iowa Department of Transportation (Iowa DOT) staff developed and calibrated a series of two-dimensional (2D) hydraulic models to investigate the Interstate 35 (1-35) South Skunk River crossing, and evaluated flood risk mitigation alternatives (Iowa DOT 2013). This series of models included a detailed model of Squaw Creek and the Skunk River as they are conveyed through Ames. The model was developed using TUFLOW (BMT-WBM 2014), a proprietary software package, and multiple light detection and ranging (LiDAR) (Iowa Department of Natural Resources and Iowa DOT) data sets as well as survey data. The intent of this evaluation was to execute the 2D hydraulic model for the 0.2 percent annual probability flood event, to modify the 2D hydraulic model with proposed grading from Riverside Manor, to execute the modified 2D hydraulic model for the 0.2 percent annual probability flood event, to develop water surface information near the Oak Riverside neighborhood for both simulations, and to compare the differences. Elevations from the effective FEMA 0.1 percent annual probability flood event(100-year) evaluation that were performed as part of the Grand Avenue Extension study(HR Green 2014)are also presented. The model was executed for the effective 0.2 percent annual chance flood event(500-year). The 0.2 percent annual chance flood discharge is 26,300 cubic feet per second (cfs) on Squaw Creek (FEMA 2008). Flood extent and velocity contours are shown in Figure 1. Comparison locations are depicted in Figure 1. Modeled water surface elevations were compared to the effective 0.2 and 1 percent flood profiles (see Table 1)just upstream from South 4`h Street. The modeled water surface elevations compare within plus or minus 0.5 foot to the FEMA model. While not expected to compare exactly, since the information was developed using different methods for different hdrinc.com 300 E Locust Street,Suite 210,Des Moines,IA 50309-1823 (515)280-4940 purposes, the water surface elevations should show reasonable agreement, or warrant a discussion if they do not compare well. The model is considered appropriate for the purposes of comparing water surfaces from an existing condition to a proposed condition for this task order. The proposed model included only updating the proposed Riverside Manor development(fill to be 3 feet above the base flood elevation). The proposed grading contours for Riverside Manor are shown in Figure 2 and were obtained from the developer. The proposed model was executed for the effective 0.2 percent annual chance flood event(500- year)with the flood extent and velocity contours shown in Figure 3. The differences between water surface elevations in the two conditions are shown in Figure 4. Table 2 shows the differences in water surface elevation between the existing condition and proposed condition for the 0.2 and 1 percent annual chance flood. The difference plot(see Figure 4)combined with photographs from the site (see Figures 5, 6, and 7) provide the following evaluation of model results at the 0.2 percent annual chance event. Inspecting Figure 4,the model results show that due to changes in grading at Riverside Manor, water surface elevations upstream during the 0.2 percent annual chance event will range from 0.6 foot higher(right upstream from Riverside Manor, in the 4th Street right-of-way),to less than 0.3 foot higher in the park to the north of Riverside Manor, and less than 0.1 foot higher at Lincoln Way. The site inspection shows that as of February 20,2015, some proposed grading has been performed (final grading can be confirmed by the City of Ames after As-Built submittal) and results (as expected) in ground elevations that are higher or nearly equal to the South 4`h Street embankment(see the area circled in red in Figure 6). Before this area was filled, floodplain flows overtopped South 4th and continued across the developed area back to the Squaw Creek. This area serves as a new obstruction to floodplain flows during the 0.2 percent annual chance flood event. The model shows that with this fill, more water continues along South 41h Street than in the existing condition. Figure 7 confirms that this is a viable flow path. The fill also leads to slightly higher water surface elevations upstream from the development for the 0.2 percent annual chance flood. References BMT-WBM. 2014.TUFLOW[computer software] Build 2013-12-AC.www.tuflow.com. Federal Emergency Management Agency(FEMA). 2008. Flood Insurance Study. Story County, Iowa and Incorporated Areas. Federal Emergency Management Agency. 19169CV000A. HR Green (2014). Grand Avenue Extension. Progress Meeting Minutes dated November 25, 2014 Iowa Department of Transportation (Iowa DOT)2013. 1-35 Over South Skunk River, Hydraulic Analysis-General.Technical Memorandum. May 14, 2013. hdrinc.com 300 E Locust Street,Suite 210,Des Moines,IA 50309-1823 (515)280-4940 2 Velocity Vector t° Existing Conditions Effective 500-yr Flood Inundation Extent 5,00 ftts 0,00 ftts � � � yak X 2D Model Boundary i� h t 4 s = Effective 100-yr Floodplain :_ i ' r / Flobdway ��Tx ' � - _ $ i � � t "a w XV Id �� :iff �, A•'":•� � � Yqi"'.ft '-S �.v a ,Rrl/�, ..,, _, _,. _d�„ ...—° .•� as"r�J,`s Figure 1: Modeled existing conditions 500-year inundation extent and comparison points Table 1:Water surface elevations during 0.2 (500-year)and 1 percent(100-year)annual chance events compared to effective FIS water surface elevations (NAVD 1988). Point Effective 500-year Modeled Existing Effective 100-year Modeled Existing WSEL' Condition 500-year WSEL' Condition 100-year WSEL' WSEL' 1 897.8 897.69 896.1 896.04 2 897.8 897.38 896.1 895.67 3 898.9 898.63 897.0 897.01 4 1 898.9 1 898.68 1 897.0 1 897.04 1. Water Surface Elevation(WSEL)measured in feet. hdrinc.com 300 E Locust Street,Suite 210,Des Moines,IA 50309-1823 (515)280-4940 3 l fSBae FIQGd Regutarir Ftoodptalro E30tin�iary� r st ��Etftietiva i�loedway / ""4° � � a � ? 1 !• Figure 2: Riverside Manor Grading plan compared to effective floodway and base flood floodplain boundary hdrinc.com 300 E Locust Street,Suite 210,Des Moines,IA 50309-1823 (515)280-4940 4 v�i��aYve��o 'Proposed Grading Conditions Effective 500-yr Flood Inundation ExtentIJW ,' Lio, 0fLs ) �� z ^ � Te z l f� kMI „ Ef#ective 100 r Flood lain A 14 � y �: ^a y Figure 3: Riverside Manor grading-post grading 500-year inundation extent and comparison points hdrinc.com 300 E Locust Street,Suite 210,Des Moines, IA 50309-1823 (515)280-4940 5 Impact(ft) .,. Impact to Efteetrve 500 yr Fload WSEL a 0.4k, f 0,3 0.2 0.1 0.0 k , t a / I y ,t ?wv 9 / r e> 771-1 y M Figure 4: Riverside Manor grading -impact on 500-year water surface elevation Table 2: Existing condition water surface elevations during 0.2 (500-year)and 1 percent(100- year) annual chance event compared to proposed conditions(NAVD 1988). Point Modeled Proposed Impact on Modeled Proposed Condition Impact on 100- Existing Condition(With 500-year Existing (With Riverside Manor year WSEL' Condition Riverside WSEL' Condition 100- Grading)100-year 500-year Manor Grading) year WSEL' WSEL' WSEL' 500- ear WSEL' 1 897.69 897.91 0.22 896.04 896.11 0.07 2 897.38 897.50 0.12 895.67 895.72 0.05 3 898'63 898.71 0.08 897.01 897.03 0.02 4 898.68 898.76 0.08 897.04 897.07 0.03 5 897.26 897.87 0.61 896.00 896.10 0.10 1. Water Surface Elevation(WSEL)measured in feet. hdrinc.com 300 E Locust Street,Suite 210,Des Moines,IA 50309-1823 (515)280-4940 6 I p�, \ i $y Jlk _ . Figure 5: Riverside Manor looking north to south across South 4`h Street F z , try��„� ,� ��',q.• m ,,,.. .., Figure 6: Riverside Manor looking north to south(the fill is higher than South 4th pavement) hdrinc.com 300 E Locust Street,Suite 210,Des Moines, IA 50309-1823 (515)280-4940 7 FH Figure 7: Riverside Manor—South 4`"to the east hdrinc.com 300 E Locust Street,Suite 210,Des Moines,IA 50309-1823 (515)280-4940 8 I hereby certify that this engineering document was prepared �.�+�+.�.•+ , by me or under my direct per sonol supervision and that lam xt� SStC3 0M' <wQ� ,..•« »•ti �1` a duly licensed Professional Engineer under the lows of the o s� State of low v ANDREW W. (t VMcCOY z ,# to 1 B963 Mrn (/( 's r i toWA ' , Signoture Oate ANDREW W. Mc Y Printed or Typed Name My license renewal dote is December 31, 2015 Pages or sheets covered by this seal. 41 If you have any questions, do not hesitate to contact me at 515.280.4950 or andrew.mccoy@hdrinc.com. Sincerely, HDR Engineering, Inc. Andrew McCoy, PhD, PE Senior Water Resources Engineer hdrinc.com 300 E Locust Street,Suite 210,Des Moines,IA 50309-1823 (515)280-4940 9