Technical Report Draft

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1 Technical Report Draft of the Lower Stillaguamish River and Assessment of Selected Restoration and Flood Management Options Prepared for: Snohomish County Department of Public Works Surface Water Management Report Prepared by: R2 Resource Consultants, Inc. May 21, 2013

2 Technical Report Draft of the Lower Stillaguamish River and Assessment of Selected Restoration and Flood Management Options Prepared for: Snohomish County Department of Public Works Surface Water Management Report Prepared by: R2 Resource Consultants, Inc. Chiming Huang, Ph.D., P.E. Paul DeVries, Ph.D., P.E. May 21, 2013

3 CONTENTS 1. INTRODUCTION AND BACKGROUND DEVELOPMENT OF 2D MODEL AREA MODELED GEOMETRIC DATA PROCESSING...1 Defining General Floodplain Topography...2 Defining Finer Resolution Topography...2 Channel Bathymetry...3 Artificial Bathymetry...4 Floodgates...5 Drainage under the BNSF Railroad Grade MESH NETWORK DEVELOPMENT: EXISTING CONDITIONS ESTABLISHING BOUNDARY CONDITIONS FOR MODEL CALIBRATION TO EXISTING CONDITIONS LAND SURFACE CHARACTERIZATION D MODEL CHECKS AND REFINEMENTS...7 Accuracy of Mesh Geometry...7 Modified Boundary Conditions...9 Element Drain...9 Marsh Porosity...10 Time-Dependent Simulation MODEL CALIBRATION SIMULATION RESULTS FOR CALIBRATION (EXISTING) CONDITIONS SCENARIO SIMULATIONS SCENARIO 1: EXISTING CONDITIONS, WORST CASE TIDES FOR FLOODING SCENARIO 2: REMOVAL OF MATTERAND AND LEQUE ISLAND DIKES SCENARIO 3: REPLACEMENT OF A PORTION OF THE WTP LAGOON WITH A FLOODWAY...18 R2 Resource Consultants, Inc. iii May 2013

4 4.4 SCENARIO 4: WIDENING OF THE OLD STILLAGUAMISH RIVER CHANNEL MODELING USES AND LIMITATIONS REFERENCES FIGURES...23 FIGURES Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Simulation range of 2-D model domain. The upstream and downstream model extents were moved outwards to accommodate numerical inaccuracies that may occur near inflow and outflow boundaries, and limit them to outside the area of interest LiDAR data points on the floodplain before (top) and after (bottom) sampling. The railroad grade elevations were redefined subsequently Points on dikes surveyed by the County (red) and modeling domain (yellow) Breaklines, shown in light blue, derived from LiDAR and survey data. Top: Model domain. Bottom: Blow-up of red dashed circle area in top image, encompassing part of Old Stillaguamish River and Jorgenson Slough at Marine Drive, showing resolution used to define dike top widths in the model Coverage of surveyed channel bathymetry shown in light blue lines. The channels of Hatt Slough below Marine Drive and Old Stillaguamish River below Point A generally have good point coverage. A longitudinal profile survey was performed in the Old Stillaguamish River between Points A and B. The channel upstream of Point B has sparse crosssectional survey data Schematic of a cross-sectional profile derived for segments of the Old Stillaguamish River channel where only a longitudinal profile was available. The wetted channel was assumed to be trapezoidal, and the surveyed longitudinal channel elevation (red dot) was used to define the channel invert elevation. LiDAR data were used to derive landward elevations. Dike elevations were derived from breaklines R2 Resource Consultants, Inc. iv May 2013

5 Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Floodgate at the south-east corner of Stanwood water treatment plant. The gate is about 100ft long and consists of ten bays, each being 10 ft long and 5 ft deep. The photo was taken from the downstream side, and the upstream intake is approximately flush with the floodplain Jorgenson Slough at the BNSF railroad, looking toward the Lower Stillaguamish River floodplain south-west of the railroad. The opening is about 400ft wide Miller Creek at the railroad crossing, looking northwest of the trestle. The opening is about 200ft wide Miller Road railroad crossing, looking toward the floodplain south-west of the trestle. The channel stops a short distance (less than 100 ft) west of the trestle. The opening is about 45ft wide Unnamed slough between 48 th Street and Miller Road. There was no public access road to the railroad crossing, and the photo was taken from Norman Road Combined topography data for the model domain, composed of sampled LiDAR data, surveyed elevations, elevation breaklines, surveyed channel bathymetry, and artificial bathymetry in the vicinity of inflow and outflow boundaries Finite-element mesh network generated for the existing conditions model domain. The mesh has a total of 24,212 nodal points and 8,589 elements, including 2,070 triangular and 6,519 quadrilateral. Also depicted are the three inflow areas defined for the model Estimated inflow hydrographs from January 2009 flood event by nhc. The event lasted more than four days from 1/6/2009 to 1/10/2009. The peak flow is 80,666cfs combining all three hydrographs, including 58,411cfs, 910cfs, and 21,345cfs for Inflow 1, Inflow 2, and Inflow 3, respectively WXTide information page for the Crescent Harbor gage, Station ID (Top; N W) and Stanwood gage, Station ID TWC1131 (Bottom; N W)...38 Seattle tide gage datum, obtained from NOAA website p?stationid= R2 Resource Consultants, Inc. v May 2013

6 Figure 17. Figure 18. Figure 19. Tidal cycles at Crescent Harbor (top) and Skagit Bay (bottom) during the model calibration flood. Horizontal lines represent inverts at the mouth of each channel. The lowest sea dike elevation is about 7 ft higher than the high tides. The invert at the mouth of Hatt Slough is lower than the low tides. Datum = NAVD Specification of different types of land cover. There were 27 types set up in the model to account for the effect of different land covers on floodplain hydraulics Hydrographs and tides specified to define the inflow and outflow boundary conditions of the existing condition scenario. A 24-hour period was added prior to the beginning of the actual hydrograph starting from 1/6/2009 8pm or Hour 0. The peak flow depicted is 80,666 cfs from all three inflows combined Figure 20. Locations of high water marks (HWMs) surveyed after the January 2009 flood event. The numbers in light blue are HWM identifiers given by SWM, and the names in yellow are provided for reference in the text Figure 21. Figure 22. Figure 23. HWM elevations in light blue and model calibration errors in yellow for the January 2009 flood event. A calibration error is the difference between simulated WSE and HWM. A positive error means the simulated WSE is higher Lower Stillaguamish River flooding extent on January 9, 2009, a day after the flood peaked. Photo was provided by Mr. Max Albert of Stillaguamish River Flood Control District Modeled residual water depth on the floodplain at the start of the model calibration run (Hour 0). Channels are depicted in red Figure 24. Predicted water depths in the model calibration run at Hour 13 (1/7/2009 9am) when the total inflow is 48,600cfs, about midway on the ascending limb of the inflow hydrograph. Depths > 5 ft are colored in red simply to provide better color contrast over the shallower depth range. The black arrows depict flow directions across the railroad openings. Letters D, E, F, and G are used to indicate the locations of the unnamed crossing, Miller Road crossing, Miller Creek crossing, and Jorgenson Slough crossing, respectively...47 Figure 25. Predicted water depths in the model calibration run at Hour 40 (1/8/ pm) when the flood peaked at 80,666 cfs. Black arrows indicate flow directions at the four railroad openings D, E, F, and G, with velocities of approximately 2.5ft/s, 2.8ft/s, 3.5ft/s, and 5ft/s, respectively R2 Resource Consultants, Inc. vi May 2013

7 Figure 26. Predicted water surface elevations in the model calibration run at Hour 40 (1/8/ pm) when the inflow peaked at 80,666cfs. Leque Island and the farmland south of Hatt Slough were protected by dikes and not inundated during the January, 2009 storm. There were also three small areas on the floodplain corresponding to buildings obstructing the flood flow, including the Stanwood water treatment plant, the building near HWM (Shop), and the building near HWM (Lagoon) Figure 27. Predicted velocity vectors and water surface elevations in the model calibration run at Hour 40 (1/8/ pm) when inflow peaked at 80,666 cfs Figure 28. Predicted velocity contours in the model calibration run at Hour 40 (1/8/ pm) when the inflow peaked at 80,666 cfs Figure 29. Estimated tides at Stanwood in Port Susan Bay and Crescent Harbor in Skagit Bay around January 12, This was the highest set of tides found to occur during the Data were obtained from Figure 30. Hydrographs as boundary conditions for Scenario 1 through Scenario 4. Inflows at the upstream boundary are for the January 2009 flood event, and tides at the downstream boundaries are for the worst case January 1997 tide data. The time scale of the 1997 tides was shifted to approximately match the cycle of 2009 tides. As in the calibration run, a 24-hour period was added to the hydrograph to achieve realistic hydraulics on the floodplain when the actual hydrographs start at 1/6/2009 8pm Figure 31. Figure 32. Figure 33. Predicted WSE in the vicinity of Matterand property around the time of peak stage under Scenario 1. The effect of dikes holding back floodwater can be seen clearly Predicted velocities for Scenario 1 model run. Flow directions may reverse in the West Pass channel, flowing inland during high tide in Skagit Bay. The reverse flow acts to back up flood water and consequently reduce floodplain drainage Predicted velocity at peak inflow for Scenario 1 model run. There are four locations, as annotated, where the flow velocities are significantly greater than the rest of the floodplain. Note that the higher velocity west of sea dike is not real and is the result of the artificial ramp created to drain floodwater overtopping the sea dike R2 Resource Consultants, Inc. vii May 2013

8 Figure 34. Figure 35. Figure 36. Figure 37. Figure 38. Figure 39. Figure 40. Figure 41. Figure 42. Figure 43. General predicted pattern of floodwater movement on the Matterand property at the time of peak stage under Scenario 1. Location A at the southeast corner has the lowest dike elevation, and is where overtopping flow is most prominent. After the Matterand floodplain fills, overflow into the South Pass channel is favored near the southwest corner near location B Section of dike at the southwest corner of the Matterand property that eroded during a flood and was subsequently rocked The parts of Matterand and Leque Island dikes removed as part of Scenario 2 are shown in yellow Predicted WSE at peak flow for Scenario 2 with the Leque Island and Matterand dikes removed. The arrows depict general floodwater movements across the floodplain. Dike removal is predicted to be associated with a lower water level in the Old Stillaguamish River, with attendant propagation of the lowering effect upstream Predicted velocity for Scenario 2 model run with both Matterand and Leque Island dikes removed. Arrows indicate flow directions, and the length of the arrow represents velocity magnitude Predicted velocity contour plot for the Scenario 2 model run at the time of peak stage, with both Matterand and Leque Island dikes removed Predicted changes in WSE from Scenario 1 at the time of peak stage associated with removal of the Matterand and Leque Island dikes under Scenario Predicted changes in velocity from Scenario 1 at the time of peak stage associated with removal of the Matterand and Leque Island dikes under Scenario Schematic of floodway modification to the Stanwood water treatment plant (WTP), suggested and designed by Mr. Max Albert of Stillaguamish Flood Control District. The southern half of the WTP lagoon is replaced by the floodway, which is designed to emulate the Irvine Slough prior to the lagoon was built and slough channel was narrowed and dammed. Elevation datum = NAVD Predicted WSE at peak flow for Scenario 3 model run with a floodway replacing the southern half of the WTP lagoon and with both Leque Island and Matterand dikes in place. The bolder arrow at the floodway indicates increased flood conveyance compared with Scenario R2 Resource Consultants, Inc. viii May 2013

9 Figure 44. Figure 45. Figure 46. Figure 47. Figure 48. Figure 49. Predicted velocity vectors and WSE contours for Scenario 3. The floodway helps drain the water east of the lagoon Predicted velocity contours for Scenario 3 at the time of peak stage. (not consistent, some locations use around and the others use at )...68 Predicted changes in WSE from Scenario 1 at the time of peak stage due to the addition of the proposed floodway under Scenario Predicted changes in velocity from Scenario 1 at the time of peak stage under the proposed floodway scenario Schematic of how channel widening was modeled for Scenario 4 in the upper section of the Old Stillaguamish River between Hatt Slough and Florence Road at 249 St NW. The widened channel has the same invert as the existing channel. The stream banks were not moved back to widen the channel to preserve existing dikes; only the tree lines on both sides were moved back. The channel was deepened only at its head, to remove materials that deposited at the junction with Hatt Slough Bathymetry changes implemented for Scenario 4 where channel widening occurred between Hatt Slough and Florence Road at 249 St NW. Channel deepening occurred only near the junction with Hatt Slough Figure 50. Predicted changes in peak WSE for Scenario 4 compared with Scenario 1. The maximum reduction in peak stage was less than 0.1 ft near the head of the Old Stilly Channel. The change was almost negligible (0.01 ft or less) on the floodplain Figure 51. Figure 52. Predicted changes in velocity at the time of peak flood stage associated with Scenario 4 compared with Scenario 1. The maximum change is about 0.4 ft/s near the head of the Old Stillaguamish River, indicated by the small area with red coloring Predicted temporal changes of the smallest stable grain size at various sites in the Old Stillaguamish River channel between the Seed Barn and the confluence with the West Pass and South Pass channels, using model output from Scenario 2. Sites are ordered in the legend from left ( upstream) to right (downstream) R2 Resource Consultants, Inc. ix May 2013

10 TABLES Table 1. Element Material Properties for Each Type of Land Use...8 Table 2. Element drain rate (cfs per square foot) of six flow sinks defined in the 2D model. The rates are designed to obtain a nearly dry condition in each region before flood water starts to spill over stream banks onto floodplain. The first 24 hours (Hour -24 to Hour 0) is the model warming up period. The actual flood hydrograph starts from Hour Table 3. Coordinates and surveyed elevations at the HWMs of the January 2009 flood event. Calibration errors are the differences between HWM elevations and simulated WSEs from the calibrated model. A positive error means simulated WSE is higher than HWM elevation Table 4. Predicted WSEs at HWM locations from simulation results of Scenario 1. The calibration run was based on lower tides from January 2009 and Scenario 1 on higher tides from January Table 5. Predicted WSEs at HWM locations for Scenario 2 and Scenario 1. A negative WSE difference indicate the WSE of Scenario 2 is lower. HWM locations around Norman Road are far away from where both dikes are removed and show no effect from dike removal R2 Resource Consultants, Inc. x May 2013

11 1. INTRODUCTION AND BACKGROUND Snohomish County Surface Water Management (County) contracted R2 to develop a 2- dimensional (2D) hydrodynamic model of the lower Stillaguamish River and floodplain, and to use the model to assess potential feasibility of conceptual measures to restore floodplain habitat for salmon and/or reduce flooding impacts on the Lower Stillaguamish River floodplain. The measures assessed included levee removal, channel excavation to increase conveyance, and floodway construction. The County desired to know potential flooding impacts associated with each measure. This technical report describes the development, calibration, and execution of the 2D model. 2. DEVELOPMENT OF 2D MODEL The model was developed using the SMS modeling system, which integrates RMA2, a two-dimensional depth-averaged finite-element model supported by the Army Corps of Engineers (USACE), with a user-friendly interface for data pre- and post-processing. 2.1 AREA MODELED The domain of the two-dimensional hydraulic model is depicted in Figure 1, which extends north to SR-532, north-east to the Pioneer Highway, east to about 1.3 miles downstream of the confluence of Cook Slough and Upper Stillaguamish River, south to the valley wall, west to the sea dike, and north-west to Leque Island. Floods enter from the east and drain into the bays on the west. During high flow events, low-lying areas along the SR-532 are dammed with sandbags (per correspondence with R. Aldrich, 12/5/2011) which keeps flood water from flowing north to the City of Stanwood. Both Pioneer Highway and the south valley wall are on high ground. There is a BNSF railroad grade running across the floodplain from east of the Stanwood Park & Ride (P&R) parking lot toward southeast-east of the project area. The railroad divides the floodplain into two parts. The floodplain on the northeast side of the railroad connects to the southwest side with four railroad openings within the project area, including Jorgenson Slough, Miller Creek, Miller Road, and an unnamed one between Miller Road and 48 th Ave NW. The railroad track remained dry during the 2009 flood except a portion near the P&R. 2.2 GEOMETRIC DATA PROCESSING The model topography was created using three types of data, including LiDAR, surveyed channel bathymetry, and surveyed levee/dike elevations. All data were provided by County R2 Resource Consultants, Inc. 1 May 2013

12 staff. LiDAR has a great data density but has less elevation accuracy while bathymetry and survey data have relatively lower data density but greater elevation accuracy. Due to the density and accuracy disparity, the three types of data were not directly combined in order to preserve the identity of each type. The bathymetry and ground survey data took precedence over the LiDAR data. Defining General Floodplain Topography The raw LiDAR points used had a spatial resolution of 6 ft, where the coverage for the entire project area corresponded to a file size of ~2 Gb. This size was too large for SMS and ArcInfo to operate practically. Consequently, LiDAR data were resampled to a coarser resolution of approximately one-tenth of the original density in each direction. After the sampling, the file size was reduced to a more manageable size for SMS to operate. An example is illustrated in Figure 2 near a segment of the railroad grade. The raw data were then reprocessed as needed to delineate distinct topographic features such as the railroad grade, roads, and dikes. Defining Finer Resolution Topography The resulting coarser resolution was considered adequate for modeling the floodplain, but insufficient to capture steep elevation changes in the vicinity of dikes, river banks, buildings (e.g., sewage treatment plant), and the railroad grade (which for simplicity are collectively called hydraulic structures in this memorandum). Locations with substantial slope breaks and steep elevation changes were modeled explicitly using finer scale topographic detail, and included the following hydraulic structures: Stanwood Wastewater Treatment Plant (WTP), part of left and right banks of Old Stillaguamish River channel, part of left and right banks of Hatt Slough channel, banks of West Pass and South Pass (Figure 1), part of Marine Drive, the BNSF railroad grade, and the Matterand Dike. Dike topography was modeled suing survey data collected by the County (Figure 3). For locations that that were not surveyed, and at locations where surveyed points were too far apart, LiDAR data were used together with surveyed data to derive elevation breaklines. Such locations included river banks, levees and the railroad grade. In each case, a denser line of points was defined at locations with substantial breaks in slope such that that mesh elements, when defined, did not cross over the line. The line effectively acted as a slope breakline when defining the mesh elements. The distance between two adjacent points on the delineated breakline was initially more than ~100 ft. A denser breakline was created by adding additional points linearly interpolated at ~ 3 ft intervals between two delineated points using a utility program written in Excel Macro. The interpolation resulted in hundreds to thousands of points on each delineated breakline. The interpolated points were then imported into SMS to define elevations for the finite-element mesh nodal points. R2 Resource Consultants, Inc. 2 May 2013

13 The top width of dikes in the project area is on the order of ~10 ft, which presented a special modeling issue. The RMA2 user s reference manual (King, 2011) recommends the length/width ratio of an element not exceed 10, which means the longer side of an element should be no more than 100 ft. An element this small may help reach a stable solution, but would also astronomically increase the total number of elements and nodal points of the mesh network and in turn result in multiple days of computer run time for each simulation. Such extended model run time is considered unreasonably long during the model development phase when numerous adjustments to both hydraulic parameters and distributions of elements are expected. To avoid this problem, the top widths of all such hydraulic structures were increased in the model topography to ~40 ft, which would allow the elements that represent the top of each hydraulic structure to be 400 ft on the longer side. With this element size, the computer time for each model run would be just slightly more than a day. To implement the top width, a second, parallel breakline was created 40 ft away from the delineated dike breakline, including along the banks of the Old Stillaguamish River, Hatt Slough, and the railroad grade (Figure 4). The artificial reduction in floodplain storage associated with this solution is small, and should not significantly affect decisions based on model predictions. Channel Bathymetry Figure 5 shows the extent of channel bathymetry surveyed by SWM in light blue. Hatt Slough below Marine Drive and downstream portions of the Old Stillaguamish River generally have good data coverage with adequate density. In the upper section of both streams, the coverage is not as complete, especially in the Old Stillaguamish River. For some portions, only longitudinal channel and/or cross-section survey profile were available. For channel segments in the Old Stillaguamish River channel with only a longitudinal profile survey, the cross-section profile was approximated by a trapezoidal geometry in the wetted portion, with the surveyed elevation representing the channel invert. The width of channel bottom was then estimated using aerial photos and unsampled LiDAR data. The LiDAR data was also used in determining the dry portion (e.g., sand bar) within the channel. A composite transect profile was completed by combining the trapezoidal assumption, channel invert, LiDAR data, and the surveyed bank elevations (Figure 6). A similar approach was used farther upstream where only periodic cross-sections were available. First the inverts of all cross-sections were connected using an arc extending upstream to the confluence with Hatt Slough. Elevations were linearly interpolated at points along the arc R2 Resource Consultants, Inc. 3 May 2013

14 spaced every 3 ft. This arc was then used to define the longitudinal thalweg, and the crosssection profile derived the same way as downstream. Artificial Bathymetry The model domain was created to be slightly larger than the project area of interest to ensure that velocity distributions and directions and depths near the project boundaries were hydraulically reasonable. Also, the boundary is required to be wet at all times during a simulation by RMA2. Accordingly, the upstream boundary geometry was extended upstream artificially with a low topography that gradually rises to meet the existing terrain at the actual project boundary. Such arrangement would ensure the extended boundary is wet during the entire period of flow hydrograph and allow inflows to have sufficient distance to adjust its direction distributions to reflect the local terrain when it reaches the project boundary. The downstream boundary required definition of artificial bathymetry on account of tidal cycling and overtopping of flood flow over sea dikes. During high tides, sea water inundates the river mouths and causes backwater in the channels. During low tides, the head of the tide may be a mile or more away from the sea dikes, separated from the river mouths by a mudflat. Outflows can change from subcritical flow in the channels to supercritical as floodwaters exit onto the mud flat, where channel width suddenly becomes unrestricted. The potential for flow regime change is critical to the 2-D model construction, because the super-critical flow condition would cause RMA2 solution to become unstable. To preclude this possibility, the channels were extended from the mouth to below the elevations of low tides by means of an artificial channel with low elevation bathymetry such that the extended mouth is always wet, including at low tide. Model predictions would not be realistic within this region, but errors are expected to be dampened out by channel backwater over a short distance within the modeling area of interest. It was not necessary to build an extensive artificial mesh network in the vicinity of a sea dike, however, which is high enough to keep tides out but is overtopped during rare-event floods when water plunges into the bay and goes through the critical flow condition. The net effect of this is that water on the floodplain in the vicinity of the dike is generally not influenced by backwater from the tides. Moreover, extending the mesh all the way to the head of low tide would increase the computer run time substantially without benefiting the solution in the key areas of interest, which are generally located away from the sea dike. To simulate plunging floodwater, the project boundary was only extended a short distance from the dike crest into the Port Susan bay by about 1600 ft to create a ramp for flood water to gradually enter the bay with a hydraulic condition just under critical flow near the sea dike. R2 Resource Consultants, Inc. 4 May 2013

15 Floodgates There is a floodgate at the south-east corner of Stanwood Water Treatment Plant (Figure 7). Measurements made on 12/21/2011 indicated the gate to be 100ft long with ten bays, each being 10ft long and 5 ft deep. The bottom elevation of the gate was estimated at ~8.2 ft, approximately the floodplain elevation just upstream of the gate intake. Drainage under the BNSF Railroad Grade There are four openings in the railroad grade prism within the project area, including (from north) Jorgenson Slough, Miller Creek, Miller Road (immediately south of Miller Creek), and an unnamed slough between Miller Road and 48 th Ave NW (Figures 8-11). The openings were measured at 400ft, 200ft, and 45ft for Jorgenson Slough, Miller Creek north, and Miller Creek south, respectively. The bottom elevation of each opening was simply estimated using railroad elevations from LiDAR and depth measurements obtained with a stadia rod. The unnamed slough opening had no public access road, so its geometry had to be estimated using LiDAR data for width and Miller Creek data for bottom elevations. 2.3 MESH NETWORK DEVELOPMENT: EXISTING CONDITIONS The project area is about 8.6 square miles, and is almost 10 square miles for the model domain that includes the artificial extension area. Due to the large area, mesh element sizes were carefully designed to reflect the effect of actual terrain on hydraulics with the least amount of elements. The elements are about 500ft long with almost 1:1 length to width ratio on the floodplain. Near hydraulic structures such as a dike or the railroad, the elements have a maximum length to width ratio of about 10, with the longer side oriented alongside the hydraulic structure. This arrangement takes advantage of the relatively mild hydraulic change in the longitudinal direction of the element. The mesh network was developed to capture the major topographic feature in the project area using the combined dataset described above (Figure 12). The merged data were triangulated and interpolated to develop mesh nodes. The resulting mesh consisted of many elements, either triangular with 3 sides and 6 nodes or quadrilateral with four sides and 8 nodes. All property variables such as velocity and depth were assumed to change quadratically along each side of the element. The mesh elements were developed to be larger and the distance between nodes longer at locations where flood hydraulic properties are not expected to change rapidly, such as on the floodplain. The elements were developed to be smaller and distance between adjacent nodes shorter where the terrain and/or hydraulics change rapidly in a short distance, such as in the vicinity of a dike and in channels. Figure 13 shows the finite-element mesh network for the whole model domain under existing conditions. The existing conditions mesh network is made up of 24,212 nodal points in 8,589 elements, including R2 Resource Consultants, Inc. 5 May 2013

16 2,070 triangular and 6,519 quadrilateral. A triangular element was mainly used as a transition element to connect two quadrilateral elements. 2.4 ESTABLISHING BOUNDARY CONDITIONS FOR MODEL CALIBRATION TO EXISTING CONDITIONS Boundary conditions are discharges and water surface elevations specified on the inflow and outflow portions of the model domain. The solution of a numerical simulation is the response of the model domain to the hydraulic conditions at the boundaries. For the current model, the flows enter from the east and drain into the bays on the west. No flows or water surface elevations are needed for the valley walls to the south and SR-532 to the north, where water flows along the boundary. The upstream boundary condition was set via inflow hydrographs of the January 2009 flood event estimated by nhc (Northwest hydraulic Consultants). Due to the complex hydraulic condition just upstream of the project area, nhc used three hydrographs to depict the inflow distributions (Figures 13, 14). Inflow 1 is the discharge entering Hatt Slough, Inflow 2 is the discharge entering the higher ground between Hatt Slough and the railroad, and Inflow 3 is the discharge entering the floodplain between railroad and the north valley wall. The combined peak flow of the three inflows was 80,666 cfs, including 58,411 cfs from Inflow 1, 910 cfs from Inflow 2, and 21,345 cfs from Inflow 3. The downstream boundary conditions were set as the water surface elevations (WSEs) of tides in Port Susan Bay and Skagit Bay. Tide data were obtained from WXTide 32, which used the same prediction algorithm used by NOAA s National Ocean Service ( The elevations predicted by the algorithm are relative to the MLLW (Mean Low-Low Water) of the reference station. The predicted elevations for the tide gage Station ID at Crescent Harbor, N. Whidbey Island ( N W) in the Skagit Bay were used for the tide elevations at the West Pass mouth. The predicted elevations for the tide gage Station ID TWC1131 at Stanwood, Stillaguamish River ( N W) were used for tide elevations at the South Pass mouth, Hatt Slough mouth, and the sea dikes. Figure 15 depicts the station information pages from WXTide that show the critical information about the gages. Elevations of both tide gages are referenced to the Seattle gage Station ID ( N, W). Figure 16 depicts the NOAA datum information sheet. The Seattle gage has an MLLW of 7.94 ft referenced to the local datum and the NAVD 88 datum is ft relative to the local datum. To convert to NAVD88 datum, 2.34 ft (10.28 ft-7.94 ft) was subtracted from MLLW-referenced elevations obtained from WXTide 32. R2 Resource Consultants, Inc. 6 May 2013

17 The model was calibrated to the tide levels that were measured during the period of the January 2009 flood event (Figure 17). It was assumed differences in tide elevation and timing were negligible between Crescent Harbor gage and the West Pass mouth, and between from Stanwood and the Hatt Slough mouth, South Pass mouth, or the toe of the sea dikes. Internal boundary conditions were not established in the model to account for floodplain drainage via tide gates and culverts on the floodplain after significant storm events. The drain rate and the capacity of the gates and culverts, if available, would be useful to improve the performance of the hydraulic model so that the 2-D model can be calibrated to the specific hydraulic conditions of the flood event. Efforts were made to try to obtain such information. We have contacted Tarang Khangaonkar at Battelle, Kat Morgan and Roger Fuller at TNC, Max Albert of Stillaguamish Flood Control District, and Timothy Walls of Snohomish County Public Works, and SWM staff, and scoured numerous publications. It appears, however, such information has not been documented. 2.5 LAND SURFACE CHARACTERIZATION In additional to hydraulic conditions on the model boundary, the water movement within the model domain is also affected by spatial variation in land cover on the floodplain. For example, wooded areas slow down flow velocity compared with grass or crop cover. Though much of the project area is alluvial floodplain used for agricultural purposes, the project area is divided into many regions by dikes, levees and railroad. To properly capture the effect of different land uses and surface characteristics on hydraulics in each region, a total of 27 types of land cover were defined to characterize the model domain, each with representative values of eddy viscosity, Manning s n roughness, and marsh porosity values (Figure 18; Table 1) D MODEL CHECKS AND REFINEMENTS Accuracy of Mesh Geometry The elevation contours of the model mesh element nodes were compared against the contours from the combined topographic data. The two data sets were found to have nearly identical elevation distributions. This similarity confirmed that the mesh preserved the characteristics of the original basin topography during triangulation and elevation interpolation process. R2 Resource Consultants, Inc. 7 May 2013

18 Table 1. Element Material Properties for Each Type of Land Use Isotropic Range of Manning s n Marsh Porosity Eddy Viscosity (lbsec/ft2) D1 Land Use/ Element Material Type min max (ft) D2 (ft) A1 A2 A3 1 Disable Element Type 2 Channel - Hatt (upper) Channel - Hatt (lower) Channel - Old Stilly (lower) Channel - Old Stilly (upper) Channel - Church Creek Banks - Hatt Banks - Old Stilly Banks - Church Banks Leque Island Dike - Coastal Water Treatment Plant Pond Floodplain (upper) Floodplain (lower) Floodplain (Leque Island) Forest Railroad Track Drainage - South High Land Drainage - Florence Island Drainage - Matterand Mud Flat Drainage - South of WTP Drainage North of Jorgenson Floodplain - West Pass Floodplain - North of Railroad Drainage South of Jorgenson Drainage - North of Railroad Note (1): See RMA2 reference manual for the definitions of A1, A2, A3 Note (2): Disabled elements are treated as a solid boundary. Note (3): D1 is the depth for no vegetation, D2 is the roughness by depth coefficient. See RMA2 user s manual for more details. R2 Resource Consultants, Inc. 8 May 2013

19 Modified Boundary Conditions When conducting simulations, it is easier for the model (i.e., RMA2) to find a solution with mild hydraulic conditions of low velocity and great flow depth. When depth is shallow and velocity fast, the model may not be able to find a solution without a proper initial solution, as in our simulation case. Accordingly, a special arrangement was made for the boundary conditions to help the model reach a convergent solution, in which the model domain was initially fully inundated with a great water depth at the beginning of the simulation, and the water level was dropped gradually over a 12 hour period at the downstream boundary to its actual level at the start of the actual flood hydrograph, with an intervening 12 hour period of steady flow (Figure 19). The constant hydraulic condition on the boundary during the second 12-hour period ensures the floodplain is dry and the effect of the artificial inundation during the initial 12-hour period is no longer significant when the actual simulation begins. Element Drain When the WSEs are drawn down from WSE 29.6ft at Hour -24 to the tide elevations at Hour -12 (Figure 19), the water levels on the floodplain are lowered to the elevations of stream banks and sea dikes. Between Hour -12 and Hour 0 when the WSEs on the outflow boundaries stay the same, the water levels in the channels continue to drop but the levels of the leftover water on the floodplain remain relatively unchanged. Culverts and pump stations were not included in the model due to lack of such information. To ensure the floodplain is adequately dry before floodwater starts to spill over stream banks, the element sink method available in RMA2 was used to remove leftover water off the floodplain, where a sink involves a number of mesh elements located at the lowest area of a region of interest. This method drains a pre-determined time-dependent rate of water from the elements designated for such draining. Using the water volume at the beginning of Hour -12 and the total area of the elements designated for the drainage/sink, the drain rates was estimated for those elements to achieve a nearly dry condition on the floodplain before the flood hydrograph arrived. A total of six sinks were accordingly deployed in six regions of floodplain that included the Matterand property, Florence Island, an area north of the railroad grade, the area south of the WTP, an area north of Jorgenson Slough, and an area South of Jorgenson Slough. The drain rates (cfs per square foot) for each of the 6 sinks are listed in Table 2. R2 Resource Consultants, Inc. 9 May 2013

20 Table 2. Element drain rate (cfs per square foot) of six flow sinks defined in the 2D model. The rates are designed to obtain a nearly dry condition in each region before flood water starts to spill over stream banks onto floodplain. The first 24 hours (Hour -24 to Hour 0) is the model warming up period. The actual flood hydrograph starts from Hour 0. Element Drain Rate (cfs per unit square foot) Hour Matterand Property Florence Island South of WTP North of Jorgenson South of Jorgenson North of Railroad Marsh Porosity As inflows ascend and descend and tides rise and ebb, predicted water levels within the model domain fluctuate in response to those changes. Some elements may become dry as local WSE falls below the ground elevation and re-wet as WSE increases. To take into account the dryingwetting process, RMA2 provides a method called Marsh Porosity. This method numerically maintains a thin layer of water on the ground at all time to avoid a completely dry surface, and thus would enhance the model stability. The thickness of the water is between 0.0 ft and 0.1 ft, with the actual depth dependent upon surrounding water surface elevation and ground elevation. The errors resulted from this thin layer of water should be acceptably insignificant. The parameters used in the Marsh Porosity method for each of the 27 materials are also summarized in Table 1. Time-Dependent Simulation The simulation was conducted with a dynamic, time-dependent process. The solution for the current time t is used as the initial condition for the next time step t+ t, with t being the time increment between two consecutive time steps. Multiple iterations are usually needed within a time step to reach a convergent solution that is then used as the initial condition for the next time step. Three time increments, 0.1 hour, 0.05 hour, and 0.02 hour, were tested to determine the adequacy of time increment and model stability. A comparison of the simulation results from the three time increments showed no significant differences resulting in predicted water depth or velocity at the time of peak stage. Therefore, the longest time step t = 0.1 hour was selected for all model simulations to reduce the required model run time (e.g., slightly more than a day on a duo core laptop computer with this time increment). R2 Resource Consultants, Inc. 10 May 2013

21 At the beginning of a simulation process (Model Hour -24), the initial condition consisted of zero velocities and depths that were the differences between assumed water surface elevation 29.6 ft and the local ground elevations. Each simulation was conducted for a total of 112 hours including the first 24 hours of artificial hydrograph to obtain a dry floodplain, followed by 88 hours (1/6/ :00 to 1/10/ :00) of flow and stage hydrographs. 3. MODEL CALIBRATION Model calibration is a process to ensure the model is capable of replicating the hydraulics of an existing flood event. This is a very important step prior to simulating scenarios because it is indicative of the accuracy of the hydraulic parameters and field data integrated in the model setup and the ability of the model to reproduce observed phenomena. The 2D model was calibrated to the January, 2009 flood event because this is a recent event with more post-flood survey data of high-water marks (HWMs) available than other recent floods within the project area. Calibration involved matching predicted HWMs with surveyed values and assessing model numerical stability. The calibration process generally required iterative adjustments of element properties such as Manning's n values, porosity, and eddy viscosity and changes of transitional element size to satisfy all hydraulic conditions imposed on the model boundary for the entire simulation period. In addition, re-orienting elements to the streamline directions for the transitional elements with refined element properties to reflect local hydraulics would often help achieve a stable and convergent solution. Figure 20 shows the locations of HWMs provided by the County and Table 3 summarizes their coordinates (NAD83/State Plane North) and elevations (NAVD88). Also listed in Table 3 are the simulated water surface elevations and calibration errors, the differences between the HWM elevations and the simulated WSE elevations. Figure 21 summarizes HWM elevations (in light blue) and calibration errors (in yellow). A positive difference means an over-prediction (i.e., simulated WSE is higher than HWM elevation), and a negative difference means an underprediction. Of the 12 locations, there are four calibration errors equaling less than 0.5 ft, three less than 1 ft, three less than 2 ft, and one more than 3 ft. The errors seem large at most of the locations. There are many factors that could contribute to the differences, but, in general, typical model errors are expected to be not too significant, say within half a foot or so. An error more than a foot suggests the adequacy of the data (boundary conditions, surveyed HWM elevations, basin topographic data, and etc.) used in comparison may be in question. R2 Resource Consultants, Inc. 11 May 2013

22 Table 3. Coordinates and surveyed elevations at the HWMs of the January 2009 flood event. Calibration errors are the differences between HWM elevations and simulated WSEs from the calibrated model. A positive error means simulated WSE is higher than HWM elevation. Easting Northing WSE (ft, NAVD88) Calibration Point (ft) (ft) HWM Calibrated Error (ft) Comment Norman Road (East) Norman Road (West) North Valley Wall Miller Road Park & Ride East Park & Ride Middle Park & Ride West Stilly Channel Matterand Stilly Channel Marine Drive Shop Lagoon Seed Barn The largest calibration error of 3.12 ft over-prediction occurs at HWM (Stilly - Matterand) located in the Old Stillaguamish River channel near the south-west corner of the WTP lagoon. Table 3 shows the surveyed elevation at this HWM is ft and Figure 19 shows the high tide in Skagit Bay is ft. Given the high flow event, it appears the difference of 0.36 ft (= ) between the two locations may be too small, suggesting error in one or both measurements. As potential corroboration, Figure 22 shows a photograph taken on January 9, 2009 and provided by Mr. Max Albert, showing the flood on the floodplain about a day after flow peaked at the time inflow had dropped to less than half of the peak. The right dike elevation near the HWM ( , Stilly-Matterand) is about 13 ft, and it appears the water level at HWM location is just above the local dike crest of 13 ft. The photo thus suggests the highest water level at this HWM location to be at least 13 ft, a value close to what the model has predicted. Part of the calculated error appears to lie in the field identification of the elevation of the high water mark. For example, the three surveyed HWMs at the Park & Ride parking lot are very close to each other, and are expected to have identical water surface elevations for the peak flow. However, Table 3 shows HWM (Park & Ride Middle) was 0.33 ft higher than HWM (Park & Ride East) with a distance of less than 200 ft between them. This apparent inconsistency raises the possibility of low survey accuracy at the HWMs. The HWMs listed in Table 3 were not surveyed at the time flow peaked, and thus there is some error in interpretation R2 Resource Consultants, Inc. 12 May 2013

23 based on washlines and debris signs that can be affected by wave action and small scale hydraulics and local energy losses. The County is aware of the potential for inaccuracy of HWMs, and has launched a project to install flood stage recorders throughout the Stillaguamish River floodplain that can be used to further refine model calibration in the future. 3.1 SIMULATION RESULTS FOR CALIBRATION (EXISTING) CONDITIONS As in all scenarios, there was some residual water depth after the initial 24 hour artificial rampdown on the floodplain, prior to beginning the 2009 flood simulation (Figure 23). The water depth generally ranged between 0.1 ft and 2 ft with an average less than 0.3 ft. This residual depth should not be expected to have had a significant impact on the predicted floodplain hydraulics or timing of peak flow, given the scale of the modeling area. Figure 24 shows the flow depth at Hour 13 (1/7/2009 8am), about midway up the ascending limb on the inflow hydrograph. Water levels are predicted to continue to rise rapidly until Hour 20 (1/7/2009 4pm) and then slowly increase before reaching the peak stage around Hour 40 (1/8/ pm). Figure 25 shows the predicted flow depth at Hour 40 (1/8/ pm) around the time of peak stage. The arrow direction at opening D reverses from Hour 13 and water begins to enter from northeast of the railroad. Figure 26 shows the peak WSE. Leque Island and the farmland south of Hatt Slough were protected by levees and were not inundated during the entire period of the flood. The figure also shows there were three small areas staying dry during the flood event, including the building of Stanwood waste water treatment plant (WTP), the building near HWM (Shop), and the building near HWM (Lagoon). For those areas, there may be some small amount of water penetrating through the structures, but the top of the buildings were never submerged. That amount of penetrating water is insignificant and has negligible effect on flood level predictions in the vicinity of the buildings. Therefore, for practical purpose, these buildings were treated as disabled elements, equivalent to dry elements, at all times in the simulations. Figure 27 shows both predicted velocity vectors and WSE at around the time of peak stage (Hour 40, 1/8/ pm). The arrows indicate the local flow directions and their length represents predicted velocity magnitude. Figure 28 shows the velocity magnitude expressed as a contour plot, and clearly indicates locations where velocities are highest. 4. SCENARIO SIMULATIONS The calibrated model mesh was subsequently adapted to simulate hydraulics for four scenarios, including: R2 Resource Consultants, Inc. 13 May 2013

24 1. Replacing downstream WSE boundary conditions in Port Susan Bay and Skagit Bay with the January 1997 tides to be representative of a worst case flooding scenario for existing conditions as defined by the LiDAR and survey data; 2. Using 1997 tides and removal of both Matterand property and Leque Island dikes; 3. Using 1997 tides and modification of WTP lagoon to reflect a concept design proposed by Mr. Max Albert; and 4. Using 1997 tides and widening the upper section of the Old Stillaguamish River channel, especially the area at the Hatt Slough distributary, to increase flood conveyance. All four scenarios involved the same boundary conditions created to simulate worst case hydraulic conditions contributing to peak flooding impacts. The condition combined the January 2009 flood event with the highest recorded tide in the Port Susan and Skagit bays since To do so, 32 years of high/low tide data between Year 1970 and Year 2011 were downloaded from It was determined January 12, 1997 had the highest tides in Port Susan and Skagit bays. Figure 29 shows the tidal time series from January 10, 1997 through January 13, 1997 at these two stations. Combination of this extreme tide with the high flood event occurring in January 2009 would be expected to result in flooding on the floodplain that is worse than actually happened in The higher tides in Skagit Bay and Port Susan Bay would cause more significant backwater in the West Pass and South Pass channels, respectively, and would reduce drainage capacity of the channel and result in higher flooding level on the floodplain. This may also cause salt water to flow into the Old Stillaguamish River channel.. Maintaining the same boundary conditions in all scenarios permitted making relative comparisons of impacts of different actions where the differences in predicted stage and velocity distributions reflected the influence of each action relative to a common baseline. The worst case existing conditions scenario was used to represent the baseline against which habitat restoration and flood impact reduction measures could be compared. Comparisons were made chiefly in terms of conditions at peak stage, when flood impacts are generally greatest. It is possible to review model output at different stages of a flood and evaluate other attributes such as timing and duration of inundation at specific locations, depending on the question being asked. However, because of the time to set up and run a 2D hydrodynamic model of this scale and resolution and to post-process the model results, as well as the large volume of information that can be generated, we note the importance of formulating model questions as carefully, simply, and specifically as possible. R2 Resource Consultants, Inc. 14 May 2013

25 4.1 SCENARIO 1: EXISTING CONDITIONS, WORST CASE TIDES FOR FLOODING This model run differs from the calibration run in that it has extreme high tides from January 1997 as the downstream WSE hydrographs for boundary condition (Figure 30). The time scale of the 1997 tide event was shifted to approximately fit the 2009 event tidal cycle (cf. Figure 19). As before, the initial 24-hour period involves an artificial hydrograph prior to the model run. There are no other differences in modeling setup between Scenario 1 run and calibration run. Scenario 1 is designed to simulate the possible worst case flooding scenario on the floodplain resulting from the highest tides under the storm event of January, The predicted water surface elevations at surveyed HWM locations are summarized in Table 4 for both Scenario 1 and calibration model runs. The primary difference in predicted WSE is a 0.12 ft increase at the HWM (Stilly-Matterand) location near the south-west corner of Stanwood WTP lagoon. Away from the Old Stillaguamish River channel and farther upstream, the differences are negligible. This is consistent with the observation of tidal influence zone in the Old Stillaguamish River, where higher tides result in greater backwater effect. Since the effect moderates in the upstream direction, the water surface elevation increases due to the higher tides at the mouths diminish up the valley. Table 4. Predicted WSEs at HWM locations from simulation results of Scenario 1. The calibration run was based on lower tides from January 2009 and Scenario 1 on higher tides from January WSE (ft, NAVD88) Point Easting (ft) Northing (ft) Calibration Run (2009 Tides) Scenario 1 (with 1997 tides) WSE (ft) Diff. Description Norman Road (East) Norman Road (West) North Valley Wall Miller Road Park & Ride East Park & Ride Middle Park & Ride West Stilly Channel Matterand Stilly Channel Marine Drive Shop Lagoon Seed Barn R2 Resource Consultants, Inc. 15 May 2013

26 The model results indicate the following key observations: The dike on the east side of the WTP lagoon is a barrier holding back water and slowing down the drainage at the Park & Ride parking lot location at the north-east corner of the modeling area (Figure 31). In addition, the sharp color contrast across the Old Stillaguamish River south of Jorgenson Slough indicates floodwater is held back by the dike on the right side (i.e., east side) of the Old Stillaguamish River. Flow directions in the West Pass may reverse during high tides in the Skagit Bay (Figure 32). The reverse flow could back up the water in the Old Stillaguamish River and slow down the drainage on the floodplain. Figure 33 indicates locations in the Old Stillaguamish River where velocities are greatest and thus would be greatest concern for possible channel erosion. Figure 34 illustrates the hydraulics on the Matterand property at peak flow. Point A, located at the south-east corner, is where the elevation of the Matterand Dike is lowest above the junction of the West Pass and South Pass channels. As the floodwater rises, this lowest point is where floodwaters preferentially top the dike. After the Matterand floodplain fills, water begins to spill over into the South Pass channel, with more overflow occurring at the southwest corner denoted as point B in the figure. This route is associated with the shortest path with greatest elevation difference for floodwater drainage, and is thus is the location on the Matterand property that is at highest risk of avulsing. The stage difference across the western dike near point B can be as much as ~7 ft depending on the tide. The energy released by the plunging water may erode the dike footing and was likely a contributing factor to a dike toe failure that occurred at that location that was subsequently fortified with rock (Figure 35). 4.2 SCENARIO 2: REMOVAL OF MATTERAND AND LEQUE ISLAND DIKES This scenario involved complete removal of the Matterand and Leque Island dikes (Figure 36), representing the most extensive restoration work that might be implemented in the foreseeable future. The boundary conditions were similar to Scenario 1. To simulate removal of the dikes, the elevations of points representing the two dikes were lowered to the levels of the adjacent floodplains. The predicted water surface elevations at surveyed HWM locations for this scenario are summarized in Table 5 along with the results of Scenario 1. The results indicate a significant change in flood stage on the Matterand property and immediate surroundings as the result of dike removal. The drop in WSE at the HWM (Matterand-Stilly) location is almost 2ft. Away from where dikes are removed, the influence of the dike removal, as represented by predicted reduction in peak stage, diminishes in the upstream direction. The HWM location with R2 Resource Consultants, Inc. 16 May 2013

27 the second greatest WSE drop is at the Shop (HWM ) location (about 900ft upstream from the extent of dike removal), and is associated with a predicted 0.17ft decrease. Predicted reductions in peak stage were minor in the vicinity of the Park & Ride lot, on the order of 0.05ft. On Florence Island (floodplain between Hatt Slough and Old Stillaguamish River), the modeling predicted an average drop of about 0.1ft. Farther upstream to the Norman Road near the Hatt Slough, the predicted changes are negligible. Note that Leque Island is dry under existing conditions; hence, no results are presented for there. Table 5. Predicted WSEs at HWM locations for Scenario 2 and Scenario 1. A negative WSE difference indicate the WSE of Scenario 2 is lower. HWM locations around Norman Road are far away from where both dikes are removed and show no effect from dike removal. WSE (ft, NAVD88) Point Easting (ft) Northing (ft) Scenario 1 (with Dike) Scenario 2 (Dike removed) WSE (ft) Difference Description Norman Road (East) Norman Road (West) North Valley Wall Miller Road Park & Ride East Park & Ride Middle Park & Ride West Stilly Channel-Matterand Stilly Channel-Marine Drive Shop Lagoon Seed Barn The model results indicate the following key observations: The dikes on the east side of the WTP lagoon and along the right bank of the Old Stillaguamish River channel south of Jorgenson Slough continued to hold back floodwater, similar to Scenario 1 (Figure 37). Also, the water level in the Old Stillaguamish River is reduced and the zone in which the channel and floodplain WSEs at the peak are equivalent moves upstream compared to Scenario 1 (cf. Figure 31). Velocities at the south end of Matterand dike become higher in the vicinity of the southeast corner of the property compared with Scenario 1 (Figures 38, 39). Consequently, bank protection may be needed at this high velocity zone to reduce the potential risk of erosion or avulsion. R2 Resource Consultants, Inc. 17 May 2013

28 The only location where peak stage is predicted to increase compared with Scenario 1 is in the South Pass channel, where the channel receives much greater flow than West Pass due to lower tides in Port Susan Bay (Figure 40). Also the figure shows the channel on the east side of Matterand property has a significant WSE drop of approximate 2.5 ft, and the drop on the Matterand property is at least 3 ft with the greatest drop of more than 4 feet occurring at the south-west corner. The peak stage in the Old Stillaguamish River immediately upstream of the dike removal area drops more than 2 ft. The predicted drop gradually diminishes in the upstream direction. Away from the Matterand property, the floodplain has a relatively small drop in WSE of about 0.1 ft, as indicated in Table 5. The lower WSE in the channel results in higher velocities at the peak stage (Figure 41). The velocity next to the dike at the south end of the Matterand property increases by about 3 ft/s, suggesting engineered bank protection may be needed at this general location. The results indicate a strong avulsion potential exists over the south end of the Matterand property. Also, the flow velocity in the Old Stillaguamish River channel at the north end of the Matterand property is reduced by 2-3 ft/s. This is because floodwater takes a shorter path to move out of the system by flowing over Matterand property to enter South Pass channel, as indicated by the velocity arrows in Figure 38. Since the majority of floodwater is no longer using the Old Stillaguamish channel, flow velocity there is reduced (Figure 39), and it is expected that aggradation could occur in this section of the channel where the channel bed may rise over the long term. Over the long term, the model results suggest that the increased erosion potential may lead to the South Pass channel becoming a primary channel, and the increased deposition leading to the West Pass becoming a secondary channel. Other than in the vicinity of Matterand property, predicted velocity changes are minimal to negligible for the rest of the modeling area. 4.3 SCENARIO 3: REPLACEMENT OF A PORTION OF THE WTP LAGOON WITH A FLOODWAY Mr. M. Albert of the Stillaguamish Flood Control District proposed a modification to the lagoon of the Stanwood WTP by replacing the southern half of the lagoon with a floodway to allow floodwater east of the WTP to drain faster during high flow events (Figure 42). Mr. Albert argues the floodwater drained more quickly before the WTP construction and Irvine Slough was narrowed and gated. Mr. Albert s idea is to use the floodway to replace the lost flow conveyance of the Irvine Slough. According to Mr. Albert, the proposed floodway has a channel width similar to that of the original Irvine Slough. As part of this concept, the southern half of the lagoon is occupied by the floodway. The upstream floodway entrance is a sill with an elevation of 10ft and a width of 250ft. The width remains unchanged for most of the floodway length, and then is reduced to 120ft at the confluence with the Old Stillaguamish River. The floodway R2 Resource Consultants, Inc. 18 May 2013

29 bottom elevation is about 7ft NAVD88, roughly the elevation of the surrounding floodplain. The bank elevation of the proposed floodway is 15 ft NAVD88, roughly the same elevation as banks of the present WTP lagoon. For this scenario, it is assumed both the Leque Island and Matterand dikes remain in place. The model results indicate the following key observations: The floodway is predicted to convey an increased amount of flood flow as proposed. Otherwise, the general spatial distribution of peak stage is similar to that predicted for Scenario 1, including the location where the channel and floodplain peak stage are equivalent (Figure 43), suggesting the floodway may not have a spatially extensive effect on overall floodplain inundation level. Velocities through the floodway at the time of peak stage are predicted to be greater than the surrounding floodplain, and increase by about 3 ft/s compared with the lagoon under current conditions, suggesting the floodway would help mitigate the flood level on the east side of WTP lagoon (Figures 44, 45). The peak inundation level on the east side of the WTP lagoon is reduced by about 0.15 ft from Scenario 1 (Figure 46). The floodway is predicted to have a minor to negligible effect on peak flood stage on the floodplain to the south toward Hatt Slough. Peak stage is predicted to increase by about 0.7 ft downstream of the floodway due to the added flow. The peak stage in the Old Stillaguamish River upstream of the floodway exit is predicted to be ~0.2 ft higher than under Scenario 1 and velocity is predicted to be lower as the result of backwater and higher WSE at the junction (Figures 46, 47). The peak flood stage on the Matterand property is consequently also increased by about 0.2 ft. This appears to be due to (i) added water in the channel downstream of the floodway exit, and (ii) a sharp angle of almost 90 degrees as the floodway water drains into the Old Stillaguamish River channel. Because of the large angle, the floodway and river do not combine flows efficiently as configured. As a result, the floodway flow does not have momentum in the direction of the mainstem flow, causing the velocity to slow down and WSE be elevated upstream of the junction. 4.4 SCENARIO 4: WIDENING OF THE OLD STILLAGUAMISH RIVER CHANNEL The upper section of the Old Stillaguamish River channel between Hatt Slough and Florence Road at 249 St NW is narrower than downstream. There is also effectively a natural levee deposit at the entrance to the channel where it splits with Hatt Slough, which is indicative of a lower energy environment flowing into the Old Stillaguamish River where sediments tend to settle out. The average low-flow channel width within the constricted portion is about 46 ft as measured on an orthophotograph. The channel is wider downstream, thus it has been R2 Resource Consultants, Inc. 19 May 2013

30 hypothesized that the narrower width may not provide adequate conveyance during floods with consequent increased flooding over the adjacent floodplain. This scenario was conceived to evaluate whether channel widening in this upper section would help reduce floodwater levels. In the scenario, the channel was widened approximately 14 ft from 46 ft to 60 ft, as constrained by existing levees which were not set back (Figure 48). The channel was not deepened over most its length, however. The only substantive deepening occurred at the confluence with Hatt Slough, where the bed elevation was lowered by up to 5 ft (Figure 49). The model results indicate the following key observations: Effects of channel widening on peak flood stage are localized to the vicinity of the confluence with Hatt Slough (Figure 50). The maximum change in peak flow is predicted to be no more than 0.1ft near the head of the Old Stillaguamish River. The results suggest that increasing channel cross-section area without setting back levees and roads would have limited benefit to flood damage reduction. It was not determined at this time if the lack of effect is due to insufficient widening or is an artifact of backwater from downstream. It is suspected, but not confirmed at this time, that the increased conveyance may have a more meaningful effect on peak stage during smaller, more frequent flood events. The maximum change in velocity is a ~0.4 ft/s increase occurring near the head of the Old Stillaguamish River (Figure 51), where the most extensive removal of channel deposits was required (Figure 49). In the rest of the reach where widening occurred, velocities were predicted to be no more than 0.1 ft/s higher compared with Scenario 1. Changes in floodplain velocities were predicted to be negligible. It is likely based on existing morphology that the entrance would simply refill again through deposition as has occurred previously. The following attributes of the model are noted: 5. MODELING USES AND LIMITATIONS The 2D model potentially can be used to evaluate any restoration or flood reduction scenario that involves earthwork, and can evaluate water levels and velocities at any point and time of interest during a flood. Running a scenario and post-processing results for a particular time of interest takes time and effort, with a model run potentially costing between $5,000-$10,000 to set up the model geometry, check the results and refine the model as needed, and post-process the results. Thus, we recommend that each scenario for modeling be defined carefully and strategically, and agreed upon by all stakeholders. R2 Resource Consultants, Inc. 20 May 2013

31 The accuracy of the model appears to be on the order of +/- 1.0 ft with respect to predicting peak flood stage. Part of this range appears to include errors in field survey measurements. A network of flood stage recorders would provide the best data for future recalibration (if deemed necessary depending on modeling objectives). The scale and resolution of the model is too large to be able to simulate effectively the results of modifications involving small scale hydraulic structures such as floodgates and localized stormflow inputs. For example, modifications to improve drainage from Irvine Slough are probably modeled more effectively and efficiently using a method such as HEC-RAS. The 2D model can be used to establish the appropriate boundary conditions for such an analysis. The model can be used to simulate the effects of projected changes in sedimentation patterns. However, the model does not directly simulate sedimentation processes, and there is a high degree of uncertainty in simulating these processes using any modeling method. Future runs evaluating sedimentation should be based on projected changes in bed elevation using other methods; logical reasoning is one acceptable approach defining likely upper bounds to sedimentation (e.g., worst case scenario). The model output can be used to evaluate various criteria such as incipient motion and transport rate over a range of flows. For example, Figure 52 depicts estimates of the largest size particle transportable at various locations and at different times during the modeled flood hydrograph under Scenario 2; the implications of the results are depicted in Figure 39, where locations of expected deposition are indicated. R2 Resource Consultants, Inc. 21 May 2013

32 6. REFERENCES Collins, V., and Turner, T Draft hydraulic modeling report Dwayne Lane Docket Amendment, Assessment of Potential Flood Impacts to the Stillaguamish River Valley. Surface Water Management Division, Snohomish County Department of Public Works, Snohomish County, Washington. FEMA (Federal Emergency Management Agency) Water on the Wrong Side of the Levee? Best Practices. FEMA Region X Mitigation Division, Disaster Mitigation Working in Washington. March HEC HEC-RAS River Analysis System, Hydraulic Reference Manual, v4.1, U.S. Army Corps of Engineers, Hydraulic Engineering Center, Davis, CA. King, I User s Guide to RMA2, v4.5. U.S. Army, Engineering and Development Center, Waterways Experiment Station, Coastal and Hydraulic Laboratory. Snohomish County Surface Water Management (SWM) Draft Stillaguamish Hydrology and Hydraulic Report for the Section 205 Flood Damage Reduction Study, Snohomish County, Washington, Submitted under Authority of Section 205 of the 1948 Flood Control Act, April Yang, Z., Sobocinski, K., Border, A., Khangaonkar, T., and Thom, R Hydrodynamic and Ecological Assessment for Port Susan Bay Restoration Project, Battelle. Yang, Z., Sobocinski, K., Heatwole, D., Khangaonkar, T., Thom, R., and Fuller, R Hydrodynamic and ecological assessment of nearshore restoration: A modeling study. Ecological Modelling, 221, R2 Resource Consultants, Inc. 22 May 2013

33 7. FIGURES R2 Resource Consultants, Inc. 23 May 2013

34 To Skagit Bay Leque Island West Pass South Pass Sea Dike SR-532 Old Stillaguamish River Jorgenson Slough Stanwood P&R Pioneer Highway Miller Creek Inflow 3 Mud Flat BNSF Railroad Inflow 2 Outflows Hatt Slough Stillaguamish River Inflow 1 Port Susan Bay Figure 1. Simulation range of 2-D model domain. The upstream and downstream model extents were moved outwards to accommodate numerical inaccuracies that may occur near inflow and outflow boundaries, and limit them to outside the area of interest. R2 Resource Consultants, Inc. 24 May 2013

35 Figure 2. LiDAR data points on the floodplain before (top) and after (bottom) sampling. The railroad grade elevations were redefined subsequently. R2 Resource Consultants, Inc. 25 May 2013

36 Figure 3. Points on dikes surveyed by the County (red) and modeling domain (yellow). R2 Resource Consultants, Inc. 26 May 2013

37 WTP Marine Drive Figure 4. Breaklines, shown in light blue, derived from LiDAR and survey data. Top: Model domain. Bottom: Blow-up of red dashed circle area in top image, encompassing part of Old Stillaguamish River and Jorgenson Slough at Marine Drive, showing resolution used to define dike top widths in the model. R2 Resource Consultants, Inc. 27 May 2013

38 Old Stillaguamish River A Marine Drive B Miler Creek confluence Hatt Slough Figure 5. Coverage of surveyed channel bathymetry shown in light blue lines. The channels of Hatt Slough below Marine Drive and Old Stillaguamish River below Point A generally have good point coverage. A longitudinal profile survey was performed in the Old Stillaguamish River between Points A and B. The channel upstream of Point B has sparse cross-sectional survey data. R2 Resource Consultants, Inc. 28 May 2013

39 Dike (breakline) Dike (breakline) Floodplain dry land Floodplain LiDAR data used for dry land (e.g., sand bar) Wetted Channel Wetted channel estimated from aerial photos Figure 6. Schematic of a cross-sectional profile derived for segments of the Old Stillaguamish River channel where only a longitudinal profile was available. The wetted channel was assumed to be trapezoidal, and the surveyed longitudinal channel elevation (red dot) was used to define the channel invert elevation. LiDAR data were used to derive landward elevations. Dike elevations were derived from breaklines. R2 Resource Consultants, Inc. 29 May 2013

40 Figure 7. Floodgate at the south-east corner of Stanwood water treatment plant. The gate is about 100ft long and consists of ten bays, each being 10 ft long and 5 ft deep. The photo was taken from the downstream side, and the upstream intake is approximately flush with the floodplain. R2 Resource Consultants, Inc. 30 May 2013

41 Figure 8. Jorgenson Slough at the BNSF railroad, looking toward the Lower Stillaguamish River floodplain south-west of the railroad. The opening is about 400ft wide. R2 Resource Consultants, Inc. 31 May 2013

42 Figure 9. Miller Creek at the railroad crossing, looking northwest of the trestle. The opening is about 200ft wide. R2 Resource Consultants, Inc. 32 May 2013

43 Figure 10. Miller Road railroad crossing, looking toward the floodplain south-west of the trestle. The channel stops a short distance (less than 100 ft) west of the trestle. The opening is about 45ft wide. R2 Resource Consultants, Inc. 33 May 2013

44 Figure 11. Unnamed slough between 48 th Street and Miller Road. There was no public access road to the railroad crossing, and the photo was taken from Norman Road. R2 Resource Consultants, Inc. 34 May 2013

45 Figure 12. Combined topography data for the model domain, composed of sampled LiDAR data, surveyed elevations, elevation breaklines, surveyed channel bathymetry, and artificial bathymetry in the vicinity of inflow and outflow boundaries. R2 Resource Consultants, Inc. 35 May 2013

46 Inflow 3 Inflow 2 Inflow 1 Figure 13. Finite-element mesh network generated for the existing conditions model domain. The mesh has a total of 24,212 nodal points and 8,589 elements, including 2,070 triangular and 6,519 quadrilateral. Also depicted are the three inflow areas defined for the model. R2 Resource Consultants, Inc. 36 May 2013

47 Hydrograph of January 2009 Flood Event Inflow 1 inflow hydrograph (cfs) Inflow Inflow 2 0 1/6/09 12:00 1/7/09 0:00 1/7/09 12:00 1/8/09 0:00 1/8/09 12:00 1/9/09 0:00 1/9/09 12:00 1/10/09 0:00 1/10/09 12:00 Figure 14. Estimated inflow hydrographs from January 2009 flood event by nhc. The event lasted more than four days from 1/6/2009 to 1/10/2009. The peak flow is 80,666cfs combining all three hydrographs, including 58,411cfs, 910cfs, and 21,345cfs for Inflow 1, Inflow 2, and Inflow 3, respectively. R2 Resource Consultants, Inc. 37 May 2013

48 Figure 15. WXTide information page for the Crescent Harbor gage, Station ID (Top; N W) and Stanwood gage, Station ID TWC1131 (Bottom; N W) R2 Resource Consultants, Inc. 38 May 2013

49 Figure 16. Seattle tide gage datum, obtained from NOAA website R2 Resource Consultants, Inc. 39 May 2013

50 Tide Elevations at Crescent Harbor During the Jan 2009 Flood Event 12 Tide Elevation (ft, NAVD88) Crescent Harbor Tide Invert of West Pass 6 1/6/09 12:00 1/7/09 0:00 1/7/09 12:00 1/8/09 0:00 1/8/09 12:00 1/9/09 0:00 1/9/09 12:00 1/10/09 0:00 1/10/09 12:00 Tide Elevations at Stanwood Station During the Jan 2009 Flood Event Lowest Sea Dike Elevation Tide Elevation (ft, NAVD88) Invert of South Pass Stanwood Tide 3 Invert of Hatt Slough 6 1/6/09 12:00 1/6/09 18:00 1/7/09 0:00 1/7/09 6:00 1/7/09 12:00 1/7/09 18:00 1/8/09 0:00 1/8/09 6:00 1/8/09 12:00 1/8/09 18:00 1/9/09 0:00 1/9/09 6:00 1/9/09 12:00 1/9/09 18:00 1/10/09 0:00 1/10/09 6:00 1/10/09 12:00 Figure 17. Tidal cycles at Crescent Harbor (top) and Skagit Bay (bottom) during the model calibration flood. Horizontal lines represent inverts at the mouth of each channel. The lowest sea dike elevation is about 7 ft higher than the high tides. The invert at the mouth of Hatt Slough is lower than the low tides. Datum = NAVD88. R2 Resource Consultants, Inc. 40 May 2013

51 Figure 18. Specification of different types of land cover. There were 27 types set up in the model to account for the effect of different land covers on floodplain hydraulics. R2 Resource Consultants, Inc. 41 May 2013

52 90000 Hydrograph of Jan 2009 Flood Event (hours) inflow hydrograph (cfs) /5/09 8:00 PM 1/6/09 4:00 AM 1/6/09 12:00 PM 1/6/09 8:00 PM 1/7/09 4:00 AM 1/7/09 12:00 PM 1/7/09 8:00 PM 1/8/09 4:00 AM 1/8/09 12:00 PM 1/8/09 8:00 PM 1/9/09 4:00 AM 1/9/09 12:00 PM 1/9/09 8:00 PM 1/10/09 4:00 AM 1/10/09 12:00 PM Tide elevation (ft, NAVD88) 2 6 Inflow 1 Inflow 2 Inflow 3 Total Inflows Stanwood Tide Crescent Harbor Tide Figure 19. Hydrographs and tides specified to define the inflow and outflow boundary conditions of the existing condition scenario. A 24-hour period was added prior to the beginning of the actual hydrograph starting from 1/6/2009 8pm or Hour 0. The peak flow depicted is 80,666 cfs from all three inflows combined. R2 Resource Consultants, Inc. 42 May 2013

53 Figure 20. Locations of high water marks (HWMs) surveyed after the January 2009 flood event. The numbers in light blue are HWM identifiers given by SWM, and the names in yellow are provided for reference in the text. R2 Resource Consultants, Inc. 43 May 2013

54 Figure 21. HWM elevations in light blue and model calibration errors in yellow for the January 2009 flood event. A calibration error is the difference between simulated WSE and HWM. A positive error means the simulated WSE is higher. R2 Resource Consultants, Inc. 44 May 2013

55 HWM Figure 22. Lower Stillaguamish River flooding extent on January 9, 2009, a day after the flood peaked. Photo was provided by Mr. Max Albert of Stillaguamish River Flood Control District. R2 Resource Consultants, Inc. 45 May 2013

56 Figure 23. Modeled residual water depth on the floodplain at the start of the model calibration run (Hour 0). Channels are depicted in red. R2 Resource Consultants, Inc. 46 May 2013

57 G F E D Figure 24. Predicted water depths in the model calibration run at Hour 13 (1/7/2009 9am) when the total inflow is 48,600cfs, about midway on the ascending limb of the inflow hydrograph. Depths > 5 ft are colored in red simply to provide better color contrast over the shallower depth range. The black arrows depict flow directions across the railroad openings. Letters D, E, F, and G are used to indicate the locations of the unnamed crossing, Miller Road crossing, Miller Creek crossing, and Jorgenson Slough crossing, respectively R2 Resource Consultants, Inc. 47 May 2013

58 G F E D Figure 25. Predicted water depths in the model calibration run at Hour 40 (1/8/ pm) when the flood peaked at 80,666 cfs. Black arrows indicate flow directions at the four railroad openings D, E, F, and G, with velocities of approximately 2.5ft/s, 2.8ft/s, 3.5ft/s, and 5ft/s, respectively. R2 Resource Consultants, Inc. 48 May 2013

59 Leque Island WTP Shop Lagoon south high land Figure 26. Predicted water surface elevations in the model calibration run at Hour 40 (1/8/ pm) when the inflow peaked at 80,666cfs. Leque Island and the farmland south of Hatt Slough were protected by dikes and not inundated during the January, 2009 storm. There were also three small areas on the floodplain corresponding to buildings obstructing the flood flow, including the Stanwood water treatment plant, the building near HWM (Shop), and the building near HWM (Lagoon). R2 Resource Consultants, Inc. 49 May 2013

60 Figure 27. Predicted velocity vectors and water surface elevations in the model calibration run at Hour 40 (1/8/ pm) when inflow peaked at 80,666 cfs. R2 Resource Consultants, Inc. 50 May 2013

61 Figure 28. Predicted velocity contours in the model calibration run at Hour 40 (1/8/ pm) when the inflow peaked at 80,666 cfs. R2 Resource Consultants, Inc. 51 May 2013

62 Figure 29. Estimated tides at Stanwood in Port Susan Bay and Crescent Harbor in Skagit Bay around January 12, This was the highest set of tides found to occur during the Data were obtained from R2 Resource Consultants, Inc. 52 May 2013

63 Figure 30. Hydrographs as boundary conditions for Scenario 1 through Scenario 4. Inflows at the upstream boundary are for the January 2009 flood event, and tides at the downstream boundaries are for the worst case January 1997 tide data. The time scale of the 1997 tides was shifted to approximately match the cycle of 2009 tides. As in the calibration run, a 24- hour period was added to the hydrograph to achieve realistic hydraulics on the floodplain when the actual hydrographs start at 1/6/2009 8pm. R2 Resource Consultants, Inc. 53 May 2013

64 Figure 31. Predicted WSE in the vicinity of Matterand property around the time of peak stage under Scenario 1. The effect of dikes holding back floodwater can be seen clearly. R2 Resource Consultants, Inc. 54 May 2013

65 Flow Direction Can Reverse Here Depending On Tide Height And Flow Rate Quiet Water Area Figure 32. Predicted velocities for Scenario 1 model run. Flow directions may reverse in the West Pass channel, flowing inland during high tide in Skagit Bay. The reverse flow acts to back up flood water and consequently reduce floodplain drainage. R2 Resource Consultants, Inc. 55 May 2013

66 Bend Scour Flow Accelerates Prior To Channel Expansion Flood Gate Converging Flow Model Boundary Condition Artifact Sea Dike Figure 33. Predicted velocity at peak inflow for Scenario 1 model run. There are four locations, as annotated, where the flow velocities are significantly greater than the rest of the floodplain. Note that the higher velocity west of sea dike is not real and is the result of the artificial ramp created to drain floodwater overtopping the sea dike. R2 Resource Consultants, Inc. 56 May 2013

67 Figure 34. General predicted pattern of floodwater movement on the Matterand property at the time of peak stage under Scenario 1. Location A at the southeast corner has the lowest dike elevation, and is where overtopping flow is most prominent. After the Matterand floodplain fills, overflow into the South Pass channel is favored near the southwest corner near location B. R2 Resource Consultants, Inc. 57 May 2013

68 Figure 35. Section of dike at the southwest corner of the Matterand property that eroded during a flood and was subsequently rocked. R2 Resource Consultants, Inc. 58 May 2013

69 Figure 36. The parts of Matterand and Leque Island dikes removed as part of Scenario 2 are shown in yellow. R2 Resource Consultants, Inc. 59 May 2013

70 Flood Water Dike Backs Up Flood Flood Water Water Overflow Flood Water Overflow Dike Backs Up Flood Water Dikes Back Up Flood Water Channel WSE Floodplain WSE Figure 37. Predicted WSE at peak flow for Scenario 2 with the Leque Island and Matterand dikes removed. The arrows depict general floodwater movements across the floodplain. Dike removal is predicted to be associated with a lower water level in the Old Stillaguamish River, with attendant propagation of the lowering effect upstream. R2 Resource Consultants, Inc. 60 May 2013

71 Periodic Flow Reversal As Well, But Becomes Less Concentrated in South Pass Channel Flow Expansion Area Quiet Water Area Figure 38. Predicted velocity for Scenario 2 model run with both Matterand and Leque Island dikes removed. Arrows indicate flow directions, and the length of the arrow represents velocity magnitude. R2 Resource Consultants, Inc. 61 May 2013

72 Reduced Channel Erosion Expect Sediment Deposition Based on Permissible Velocity Criteria Flood Gate Former Dike Breach/Scour Hole Location (See Next Slide) Bank Toe Protection Required? Expect Channel Avulsion Flow Accelerates Due To Channel Expansion Converging Flow Figure 39. Predicted velocity contour plot for the Scenario 2 model run at the time of peak stage, with both Matterand and Leque Island dikes removed. R2 Resource Consultants, Inc. 62 May 2013

73 Only Location Where Peak Flood Level is Predicted to Increase Figure 40. Predicted changes in WSE from Scenario 1 at the time of peak stage associated with removal of the Matterand and Leque Island dikes under Scenario 2. R2 Resource Consultants, Inc. 63 May 2013

74 Lower Velocity Due To Less Flow And Lower Energy Slope Higher Velocity And Flow Due To Greater Energy Slope And Shorter Flow Path Flow Accelerates Due To Downstream Channel Expansion Resulting From Removal Of Matterand Dike Figure 41. Predicted changes in velocity from Scenario 1 at the time of peak stage associated with removal of the Matterand and Leque Island dikes under Scenario 2. R2 Resource Consultants, Inc. 64 May 2013

75 Bank Elevation=15ft Weir Sill=10ft Figure 42. Schematic of floodway modification to the Stanwood water treatment plant (WTP), suggested and designed by Mr. Max Albert of Stillaguamish Flood Control District. The southern half of the WTP lagoon is replaced by the floodway, which is designed to emulate the Irvine Slough prior to the lagoon was built and slough channel was narrowed and dammed. Elevation datum = NAVD88. R2 Resource Consultants, Inc. 65 May 2013

76 Flood Water Artificial Bathymetry Used To Model Downstream Boundary Conditions Dike Flood Water Overflow Floodway Flood Water Overflow Floodgate Dike Backs Up Flood Water Dikes Back Up Flood Water Channel WSE Floodplain WSE (no significant change in location from existing condition) Figure 43. Predicted WSE at peak flow for Scenario 3 model run with a floodway replacing the southern half of the WTP lagoon and with both Leque Island and Matterand dikes in place. The bolder arrow at the floodway indicates increased flood conveyance compared with Scenario 1. R2 Resource Consultants, Inc. 66 May 2013

77 Flow Direction Can Reverse Here Depending On Tide Height And Flow Rate Quiet Water Area Quiet Water Area Figure 44. Predicted velocity vectors and WSE contours for Scenario 3. The floodway helps drain the water east of the lagoon. R2 Resource Consultants, Inc. 67 May 2013

78 Figure 45. Predicted velocity contours for Scenario 3 at the time of peak stage. (not consistent, some locations use around and the others use at ) R2 Resource Consultants, Inc. 68 May 2013

79 Numerical Boundary Errors Higher WSE due to added flow from floodway Floodway Accelerating flow with lower WSE P&R WSE=0.15ft lower Floodway sill Dike Channel WSE backed up by floodway flow Upstream end of backwater Floodplain WSE=0.07ft lower Figure 46. Predicted changes in WSE from Scenario 1 at the time of peak stage due to the addition of the proposed floodway under Scenario 3. R2 Resource Consultants, Inc. 69 May 2013

80 Slightly higher velocity due to added flow from floodway Floodway Accelerating flow at floodway exit P&R Vel=0.15 ft/s higher Accelerating flow at floodway entrance Dike Velocity slightly slowed down by floodway flow Floodplain velocity=0.05ft/s slower Figure 47. Predicted changes in velocity from Scenario 1 at the time of peak stage under the proposed floodway scenario. R2 Resource Consultants, Inc. 70 May 2013

81 25 20 Bank Bank Elevation (ft) Widened Bathymetry Existing Bathymetry Station (ft) Figure 48. Schematic of how channel widening was modeled for Scenario 4 in the upper section of the Old Stillaguamish River between Hatt Slough and Florence Road at 249 St NW. The widened channel has the same invert as the existing channel. The stream banks were not moved back to widen the channel to preserve existing dikes; only the tree lines on both sides were moved back. The channel was deepened only at its head, to remove materials that deposited at the junction with Hatt Slough. R2 Resource Consultants, Inc. 71 May 2013

82 Florence Road 249 St NW Hatt Slough Figure 49. Bathymetry changes implemented for Scenario 4 where channel widening occurred between Hatt Slough and Florence Road at 249 St NW. Channel deepening occurred only near the junction with Hatt Slough. R2 Resource Consultants, Inc. 72 May 2013

83 Figure 50. Predicted changes in peak WSE for Scenario 4 compared with Scenario 1. The maximum reduction in peak stage was less than 0.1 ft near the head of the Old Stilly Channel. The change was almost negligible (0.01 ft or less) on the floodplain. R2 Resource Consultants, Inc. 73 May 2013

84 Figure 51. Predicted changes in velocity at the time of peak flood stage associated with Scenario 4 compared with Scenario 1. The maximum change is about 0.4 ft/s near the head of the Old Stillaguamish River, indicated by the small area with red coloring. R2 Resource Consultants, Inc. 74 May 2013