Figure 1 Cypress Street Study Area Location Map

July 20, 2016 TO: FROM: Jim Massarelli Director of Engineering Jeff Julkowski, PE Michael Burke, PE SUBJECT: Cypress Street Study Area Stormwater Analysis (CBBEL Project No. 16-0058) At the request of the Village of Arlington Heights (Village), Christopher B. Burke Engineering, Ltd. (CBBEL) has performed a detailed review and stormwater analysis of the flooding problem within the Cypress Street study area (Figure 1) in Arlington Heights. CBBEL has reviewed a previous study and corresponding modeling prepared by Brown and Caldwell (B&C) in July 2009. The B&C study report highlighted known flooding areas as well as storm sewer deficiencies revealed from their XPSWMM modeling analysis. The overall goal of their study was to develop an improvement alternative to provide 10-year flood protection for the entire Cypress Street study area. CBBEL has been directed to update and revise this analysis to provide 100-year flood protection in the known critical flooding areas. Figure 1 Cypress Street Study Area Location Map 1

Cypress Street Study Area Drainage An assessment of the existing storm sewer system and overland flow was performed using the Brown and Caldwell existing conditions XPSWMM model and Cook County 1-foot aerial topography. The sewer data in the Village s GIS database generally matched the XPSWMM model, although some areas had discrepancies with the sewer diameters. Subsequent to the initial modeling analysis, a utility survey was completed to verify the sizes and inverts from the B&C XPSWMM model. The surveying verified the sewer sizes and alignments, and showed the inverts were generally very close to those in the B&C model. One exception was at the point of connection from the Cypress Street basin to the Arlington Heights Road storm sewer, which is higher than shown in the B&C model. Stormwater drainage in the Cypress Street study area generally flows from west to east. The storm sewer system has three main trunk sewers draining towards Arlington Heights Road on White Oak St and Cypress St. A 24-inch trunk sewer on White Oak Street drains to a 36- inch storm sewer on Arlington Heights Road which continues north. There are two trunk sewers, a 36-inch and a 24-inch that flow east along Cypress Street, as shown on Figure 2. The main outfall for these two storm sewers is a 36-inch pipe that briefly heads north along Arlington Heights Road and continues east through a strip mall. One section of 36-inch pipe along this route is raised approximately 4.5 feet higher than the upstream and downstream pipes, likely to avoid utility conflicts. This raised pipe creates an inefficient hydraulic design and results in permanent storage of stormwater in the sewer system. To alleviate this problem the Village installed a 10-inch storm sewer down Cypress Street east of Arlington Heights road to allow the system to drain following a storm event. During any significant rainfall, high flows in the 36-inch Cypress Street storm sewer can access the 2.6 acre-foot Cypress Street detention basin at the northwest corner of Cypress Street and Arlington Heights Road through a diversion storm sewer. This basin drains to the 54-inch Arlington Heights Road storm sewer to the east. We understand that this basin has previously exceeded its capacity and overflows onto Cypress Street during large storm events. Other flood areas upstream in the study area are the result of inadequate conveyance and are discussed in the existing conditions modeling section. 2

Figure 2 Cypress Street Drainage Map (from Village GIS) Brown and Caldwell Study As part of the B&C study, an XPSWMM analysis of the existing and proposed conditions was performed. Upon reviewing the existing conditions XPSWMM model, CBBEL made the following observations: 91 subbasins with a total drainage area of approximately 200 acres. The 24-hour design storm was applied with an SCS Type II distribution No critical duration analysis was performed Existing depressional street and rear-yard storage was not explicitly modeled Only 1-foot of road flooding was allowed in selected areas An assessment of the B&C model found that it does not have the level of detail required to accurately assess the depth and limits of overland flooding. Flooding was simply defined as 3

the hydraulic grade line (HGL) exceeding the rim elevation. This method is acceptable assuming the end goal is to provide a 10-year level of protection provided within the storm sewers for the entire study area. The next section describes how CBBEL updated the overland flow network in the B&C to more accurately describe the existing conditions flood problem. In total, the B&C study proposed four alternatives in their report, ranging from an $111,000 project to lower a section of 36-inch storm sewer, to a $11.5 million project to overhaul much of the existing storm sewer system within the study area, expand the detention basin at the northwest corner of Cypress Street and Arlington Heights Road, and construct a new lift station. CBBEL has enhanced the existing conditions model and made revisions to assess the extent of flooding for the 100-year storm event. CBBEL Existing Conditions Analysis This section describes CBBEL s existing conditions analysis of the 100-year storm event, and the changes made to the B&C model to facilitate our analysis. CBBEL reviewed the subbasin delineations shown in Figure A-1 of the B&C report and compared it to the Cook County 1- foot aerial topography. In general, the subbasin delineations appeared to be accurate, but a more detailed review is recommended if this project moves forward to a design phase. The hydrology in the B&C analysis used SCS methodology with calculated Curve Numbers (CN) and Time of Concentration (Tc) unique to each subbasin. These hydrologic parameters were reasonable based on a review of the land use and topography. CBBEL did not make any changes to the subbasin areas and hydrologic parameters developed in the B&C study. As mentioned previously, the sewer sizes and inverts in the CBBEL analysis are based upon the B&C model. A CBBEL utility survey verified the sewer sizes and alignments shown in the B&C model. Sewer inverts were also verified, and most were shown to be very close to the elevations in the B&C model. The Arlington Heights Road storm sewer was found by the CBBEL survey to be approximately 0.6 higher than the B&C model showed, and the pond outfall was found to be 1.5 higher. It also showed minor changes to the storm sewer routing in the area of the Cypress Street basin outfall to the Arlington Heights Road storm sewer. Based on the survey, it appears the 54 storm sewer on Arlington Heights Road turns west toward the basin to accept the outfall sewer, and a 36 storm line from the south appears either blind connects to the 54 or has a buried manhole. The 36 line shown in the Village s GIS was not located in any of the surveyed manholes on Arlington Heights Road. Figure 3 shows a revision to the Village s GIS data based on the CBBEL survey. The CBBEL XPSWMM model was based upon Figure 3: 4

Figure 3 Updated Cypress Street Drainage Map (from Field Survey and Village GIS) The B&C study used an SCS Type II rainfall distribution with a 24-hour storm duration in its XP-SWMM modeling. The SCS Type II distribution is commonly used throughout the country, however it can be very conservative in that it applies the majority of the 24-hour rainfall depth within a one hour time period. In the Chicago region, a more commonly accepted methodology uses the Huff rainfall distributions, which are a set of four distributions developed specifically for northeastern Illinois that change with different storm durations. A critical duration analysis should then be performed to determine the storm duration that produces the highest flood elevations and overland flow rates throughout the study area. The critical duration for the Cypress Street Study Area was determined to be the 2-hour storm. In general, water surface elevations (WSEL) throughout the Cypress St study area were lowered for a given storm frequency when using the Huff distribution compared with the SCS Type II distribution. In our experience, using the Type II distribution is not unreasonable but is less accurate than the Huff distributions and may result in oversizing of project elements. We have based this updated study and its recommendations on the modeling completed with the Huff distributions. 5

Depressional flood storage was calculated and inputted into the XPSWMM model at three locations: Vail Avenue, the rear yards between Vail Avenue and Dunton Avenue, and on Dunton Avenue near Dunton School. Overland flow routes along streets were also added in areas where sewer surcharge occurred, but no overflow was previously defined. These changes to the overland flow network were required to accurately assess the depth and extent of flooding for the 100-year storm event as seen in Exhibit 1. Based on discussions with Village Staff, critical flood areas were identified and then verified by the existing conditions XPSWMM model. Table 1 describes these flood areas and the associated 100-year flooding. Flood Area Table 1 Existing Conditions Summary Table Low Elevation at ROW Boundary Modeled 100-Year Flood Elevation Flood Depth at ROW Boundary Highland Ave North of White Oak 703.3 704.2 0.9 Vail Ave Depression 701.8 703.1 1.3 White Oak St and Vail Ave 703.0 703.1 0.1 Dunton Ave Depression 701.0 701.8 0.8 White Oak St and Dunton Ave 702.5 703.0 0.5 White Oak St and Evergreen Ave 702.5 702.9 0.4 Cypress St South of Basin 698.5 699.7 1.2 As seen in the table above, the low elevation at the right of way (ROW) line was provided and is based on a best estimate using Cook County 1-foot aerial topography. This elevation serves as a target elevation when developing the proposed 100-year improvement concept plan. The general objective for stormwater design within the Village is to provide a 10-year level of service in the storm sewers for conveyance, and a 100-year level of service with the combination of storm sewers and overland flow paths. The next section outlines an improvement concept to achieve the Village s overall goal of a 100-year level of service for the Cypress Street Study Area. CBBEL Proposed Conditions Analysis CBBEL developed one improvement concept plan which incorporates certain aspects of the alternatives proposed in the B&C study to prevent flooding for the 100-year storm event. The goal of the improvement concept is to provide 100-year flood reduction by storing all excess stormwater within Village ROW boundaries. The improvement concept which is shown in Exhibit 2 includes the following flood reduction measures: Expanded Cypress Street detention basin from 2.6 acre-feet to 30.2 acre-feet. Lower 24-inch outlet pipe from detention basin 1.5 feet to maximize basin volume while maintaining a gravity outfall. 6

Proposed new and replacement storm sewers to increase conveyance to the expanded detention basin. 4 high capacity inlet locations at critical locations to intercept overland flow. A summary of the resulting benefits from the proposed improvement concept plan is provided in Exhibit 3. In general, the overall concept of relief sewers and expanded detention storage are similar to the B&C concept. The CBBEL plan provides 100-year protection at a lower cost than B&C estimated for 10-year protection. Some of the differences between the two plans are described below. One difference is the design criteria and how it was applied. As mentioned previously, the B&C study was concerned only about the 10-year event and sized their storm sewers to eliminate any surface surcharge. They did not evaluate the 100-year event. Our plan does not eliminate 100-year flooding, instead street ponding is allowed within the right of way to minimize the required pipe sizes. Therefore, the difference between 10-year and 100-year design criteria is not as significant as it may initially appear. The B&C 10-year design may have kept 100-year ponding within the right of way, however it was never evaluated. One of the major difference between B&C alternatives and the CBBEL improvement concept is the route location of the proposed storm sewers. Alternative 4 from the B&C study includes a new 60-inch storm sewer down White Oak Street and continuing down Dunton Avenue. To utilize this route for a 100-year design, this proposed storm sewer would have to be larger than a 60-inch, and may cause constructability issues as well as being a cost-prohibitive design. To minimize the sections of large diameter proposed pipe (>60-inch), CBBEL opted to propose relief sewers along known flooding streets draining north to a main trunk sewer on Cypress Street which drains directly to the expanded detention basin. The high capacity inlets proposed along these relief sewers intercept overland flow at critical flooding locations to ensure stormwater can effectively be displaced to the larger detention basin. Other differences between the CBBEL and B&C recommendations concern the proposed detention basin. It may be expected that since the CBBEL design is for 100-year protection compared to the 10-year in the B&C report, that the required volume would increase significantly. However, the Type II and Huff rainfall distributions discussed previously can generate runoff rates that are very different, but runoff volumes that are similar. Therefore, changing from the Type II to Huff distribution does not significantly change the required storage volume for the Cypress Street basin. In addition, the B&C design allowed approximately 2 of freeboard between the calculated high water elevation and Cypress Street; we have chosen to eliminate any freeboard and allow minor street ponding in the 100- year event. The maximum calculated 100-year depth on Cypress is approximately 0.8 feet from a 12-hour storm event. This change allows the required 100-year storage volume to be provided with a gravity outfall and without the use of a lift station. 7

CBBEL also analyzed the effect of lowering the section of raised 36-inch storm sewer just east of Arlington Heights Road. This improvement has minimal benefits for the 100-year storm with respect to lowering water WSELs (< 0.1 feet) in critical flood areas. Therefore, we have not included it as part of the overall 100-year improvements. However, lowering the 36- inch storm sewer does provide benefits to areas along Cypress Street and Dunton Avenue for storm events less than the 10-year if a lower standard of protection is ultimately pursued. The estimated cost of this proposed improvement plan is $6.2 million. A detailed cost estimate of the improvement plan is provided in Appendix 1. Conclusion CBBEL reviewed the study report prepared by B&C and enhanced their XPSWMM modeling to define the existing 100-year flooding problem in the Cypress Street study area. The B&C model lacked a sufficient overland flow network to fully understand the scope of flooding in the Cypress Street study area. CBBEL added overland flow paths and depressional storage areas where required to develop a comprehensive existing conditions XPSWMM model. CBBEL also updated the rainfall distribution from an SCS Type II distribution to Huff distribution, and performed a critical duration analysis to determine which storm event produced the greatest flooding. Overall, simulated flood elevations decreased throughout the Cypress Street study when using the more commonly utilized Huff rainfall distribution compared with the SCS Type II distribution. The existing conditions model was then used as the base to develop an improvement concept plan to alleviate flooding for the 100-year storm. Proposed storm sewers will intercept potential floodwater and convey it to the expanded Cypress Street detention basin. The proposed basin would be gravity drained and expanded from 2.6 to 30.2 ac-ft. This improvement concept intends to lower WSELs along flood areas to an acceptable level by confining excess stormwater within the right of way limits for the 100-year event. The estimated cost of the proposed improvement plan is $6.2 million. List of Enclosures Exhibit 1 Existing Conditions 100-Year Inundation Map Exhibit 2 Proposed Improvement Plan Exhibit 3 Proposed Conditions 100-Year Inundation Map Appendix 1 100-Year Improvement Plan Cost Estimate MJB/JJJ N:\ARLINGTONHEIGHTS\160058\Water\Docs\M.Cypress Study Area_rev070616.docx 8