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HomeMy WebLinkAboutRS_Flood_Risk_Analysis_R2B_SSDP_171128_v1 (2)ATTACHMENT 8 – FLOOD RISK ANALYSIS Introduction The following is a summary of the flood risk assessments that are being performed in association with the I-405, Renton to Bellevue Project (Stage 1) work. The project is being developed at a conceptual level in advance of final design and construction through design - build contracting. WSDOT has been developing conceptual designs in coordination with the City of Renton’s stormwater utility staff, with the intent to establish and verify performance criteria that will be met by the final design. The following is a summary of the work to date and is subject to change as the design and coordination continue. Hydraulic Model Input Data The channel geometry data for each stream were developed from topographic surveys performed by WSDOT surveyors from the Northwest Region in 2017.  Johns Creek survey data collected included existing culvert invert elevations, diameters, etc., as well as bridge dimensions for the Southport private access road over Johns Creek. No survey data was collected for the culverts under the Burlington Northern-Santa Fe railroad or Lake Washington Boulevard; rather, as-built data was used for the culvert input data.  May Creek survey data collected included existing culvert invert elevations, diameters, etc., as well as data from Lake Washington to approximately 200 feet east of I-405.  Unnamed tributary (UNT) 08.LW.0283 is a channel discharging to the storm drainage system under Seahawk’s Way and through the Virginia Mason Athletic Complex (VMAC) conveyance to Lake Washington. Sections of this UNT were developed based on three-dimensional terrain models that were generated from aerial survey conducted in 2016.  Unnamed tributaries 7.7A and 7.8 channels are located near the SE 76th Street. Model input for these smaller channels were developed based on three-dimensional terrain models that were generated from aerial survey conducted in 2016. As part of the input data for the model, a downstream known water surface elevation is used as the boundary condition. That downstream known water surface elevation is the Lake Washington water surface elevation. Lake Washington elevation ranges from 20 feet to 22 feet (Corps of Engineers Ship Canal Project Datum). The minimum elevation is maintained through the winter to provide storage space for high winter inflows, whereas the maximum elevation is maintained to meet increased summer water use, providing water necessary for fish passage, navigation, and salinity control. The I-405 project is using the North American Vertical Datum (NAVD) 1988 as the project control, so the Corps of Engineers datum must be converted to calculate the proper known water surface elevations. NAVD 88 datum is 3.25 feet above the Corps of Engineers datum, so 3.25 feet must be subtracted from the 20 foot and 22 foot elevations, resulting in elevations of 16.75 feet and 18.75 feet, respectively. Hydraulic Modeling The USACE HEC-RAS computer program, Version 5.0.3, is used to model each stream channel and the existing culverts and/or bridges under the roadways and/or railroad embankments from upstream of I-405 to the downstream location where the channels outlet to Lake Washington. The HEC-RAS hydraulic model for each stream is run using the peak flow values for the three storm events calculated for each sub -basin under existing and proposed conditions, in order to evaluate the proposed impacts. Each of these crossings has been designed to accommodate the I-405 Master Plan Project. Summary and Discussion of HEC-RAS Results For the flood risk water surface elevation calculations, the known water surface elevation boundary condition of 18.75 feet was used for Lake Washington. This assumes that the highest backwater effect will be realized in each stream channel. The following tables summarize the backwater analysis results based on the HEC-RAS modeling. Johns Creek The Stage 1 project proposes to separate WSDOT runoff from the existing drainage systems that convey runoff along Houser Way, North 8th Street, Garden Avenue, and under Park Drive to discharge into the upper reach of Johns Creek , where it parallels the south edge of Lake Washington Boulevard approximately 1,800 feet upstream from Lake Washington. WSDOT’s new separate discharge would relocate WSDOT runoff to a new discharge 800 feet downstream at River Station 990. Flows will decrease upstream of the new WSDOT discharge location, and flow will increase downstream as a result of the flow control exemption for new impervious development discharging to this lower part of the Johns Creek basin. The model evaluated the flood risks associated with this change. Summary Table River Station Existing Conditions (ft) Proposed Conditions (ft) Existing Versus Proposed Elevation Difference (ft) 148 20.03 20.08 0.05 405 22.19 22.25 0.06 570 23.63 23.70 0.07 702 25.05 25.21 0.16 759 25.15 25.30 0.15 832 25.36 25.53 0.17 908 25.37 25.54 0.17 989 25.41 25.63 0.22 1094 27.10 26.30 -0.80 River Station Existing Conditions (ft) Proposed Conditions (ft) Existing Versus Proposed Elevation Difference (ft) 1120 28.28 27.09 -1.19 1179 29.96 28.32 -1.64 Table 1. Johns Creek Water Surface Elevation Comparison – 100-year Discussion The maximum increase in water surface elevation in Johns Creek downstream of the proposed stormwater treatment outfall is 0.22 feet, or approximately 2.5 inches, with an average decrease of 1.21 feet upstream of the outfall. The overall impact to Johns Creek is negligible since these increases to the water surface are well within the existing Johns Creek channel. May Creek The Stage 1 project proposes to remove angular channel liner and existing bridge piers that influence May Creek flood flows at the I-405 crossing. May Creek is currently mapped as a FEMA floodplain at the crossing, so a conceptual channel design that involves minor regrading of the channel has been evaluated to confirm that the zero rise criteria can be met. Summary Table River Station Existing Conditions (ft) Proposed Conditions (ft) Existing Versus Proposed Elevation Difference (ft) 111 20.79 20.79 0.00 793 25.97 25.97 0.00 1432 31.75 31.75 0.00 2232 38.09 38.09 0.00 2301 38.74 38.53 -0.21 2445 41.67 40.12 -1.55 2508 41.90 40.41 -1.49 2619 42.49 42.06 -0.43 2743 43.17 42.96 -0.21 2832 44.17 44.17 0.00 Table 2. May Creek Water Surface Elevation Comparison – 100-year Discussion The Stage 1 project spans the May Creek channel (River Station 2301) and is providing May Creek with a floodplain under the new spanning structure for I-405. As a result, water surface elevations are reduced at the bridge (-0.21 feet at River Station 2301), and for a distance upstream of 400 feet. Water surface elevations remain the same downstream of the I-405 bridge. Unnamed Tributary (UNT) 08.LW.0283 The Stage 1 project proposes to remove the Washington Department of Fish and Wildlife (WDFW) documented fish barriers at the I-405 crossing. The existing culverts are severely undersized with a history of flooding upstream of I-405. The conveyance systems downstream of the I-405 crossings were designed considering full development in the basin as well as the future I-405 widening. Hydraulic models were developed to meet fish crossing guidelines for I-405 and the NE 44th Street culvert replacements. The sizing of removes the last remaining component that is causing flooding in the channel, so these improvements assure that flood risks are addressed with the replacement of the existing channel between NE 44th Street and Seahawks Way. Summary Table River Station Existing Conditions (ft) Proposed Conditions (ft) Existing Versus Proposed Elevation Difference (ft) 37 18.78 18.78 0.00 923 (907) 20.80 21.28 0.48 1750 (1780) 30.68 30.29 -0.39 2096 (2069) 48.10 34.42 -13.68 2260 (2231) 48.10 36.85 -11.25 Table 3. UNT 08.LW.0283 Water Surface Elevation Comparison – 100-year Discussion The assessment of increases in water surface elevation downstream of the I -405 crossing were difficult, as the location and configuration of the stream is altered from its existing alignment. Comparing the water surface elevations downstream I -405 shows that there is an increase of 0.48 feet from the existing condition. In the existing condition, the flow exits the I-405 culvert and enters a poorly defined channel where storm event flows act more like sheet flow than concentrated flows within a defined channel. The proposed stream realignment creates a well-defined stream channel with floodplains that concentrate flows into the channel, which in turn elevates the water surface. Existing conditions upstream of the NE 44th Street crossing show that storm event flows are backwatered upstream of the crossing, whereas in the proposed condition, that backwater effect is eliminated. Unnamed Tributary (UNT) 7.7A The Stage 1 project proposes to remove a WDFW documented fish barrier at this I-405 crossing. The existing culvert is undersized based on a conceptual level assessment of hydrology from the contributing area upstream of I-405. The new fish crossing will have a much greater capacity to pass runoff through the crossing. Since this capacity increase raises concerns about impacting downstream channels and culverts, the conceptual design proposes to include a temporary flow constriction to protect the downstream properties. The performance requirement for the flow restriction will be to maintain existing conveyance under I-405 without increasing flood risk to the upstream properties. Summary Table River Station Existing Conditions (ft) Proposed Conditions (ft) Existing Versus Proposed Elevation Difference (ft) 22 18.78 18.78 0.00 112 20.67 20.69 0.02 197 22.42 22.42 0.00 363 25.35 25.35 0.00 690 38.93 38.93 0.00 757 39.08 39.08 0.00 790 39.33 39.33 0.00 1052 (1047) 48.18 46.71 -1.47 1110 48.28 48.28 0.00 Table 4. UNT 7.7A Water Surface Elevation Comparison – 100-year Discussion The water surface elevations downstream of I-405 are similar in the existing and proposed conditions. The existing I-405 culverts act as flow regulators because they are slightly undersized relative to the predicted runoff from the upstream basin . Installing fish passable culverts under I-405 will result in increased the capacity under I-405 and has the potential to increase downstream flows for larger storm events. Since the increased flows is expected to exceed the capacity of the downstream private conveyance facilities, additional measures to restrict flows are planned. The restrictor will mimic the existing pipe diameter to replicate the existing culvert, so flood risk will not change as a result of the Stage 1 changes. The restrictor will be designed to be removed from the fish passage culvert when downstream capacity constraints (flood risk) have been addressed. Unnamed Tributary (UNT) 7.8 The Stage 1 project proposes to remove a WDFW documented fish barrier at this I-405 crossing. The existing culvert is undersized based on a conceptual level assessment of hydrology from the contributing area upstream of I-405. The new fish crossing will have a much greater capacity to pass runoff through the crossing. Since this capacity increase raises concerns about impacting downstream channels and culverts, the conceptual design proposes to include a temporary flow constriction to protect the downstream properties. The performance requirement for the flow restriction will be to maintain existing conveyance under I-405 without increasing flood risk to the upstream properties. Summary Table River Station Existing Conditions (ft) Proposed Conditions (ft) Existing Versus Proposed Elevation Difference (ft) 22 18.78 18.78 0.00 130 21.00 20.81 -0.19 197 22.42 22.42 0.00 297 23.89 23.89 0.00 338 24.17 24.21 0.04 584 41.22 30.87 -10.35 606 41.22 31.25 -9.97 669 41.22 34.53 -6.69 787 41.22 39.17 -2.05 Table 5. UNT 7.8 Water Surface Elevation Comparison – 100-year Discussion Like UNT 7.7A, the existing I-405 culverts act as flow regulators, and the Stage 1 new fish crossing structure would potentially increase downstream flows for larger storm events. A temporary restriction is planned to mimic the existing pipe diameter and avoid increasing downstream inundation. Flood risk will not change as a result of the Stage 1 changes. The restrictor will be designed to be removed from the fish passage culvert when downstream capacity constraints (flood risk) have been addressed .