Conceptual Design Report for Cottonwood Slough Restoration Project

Transcription

1 Conceptual Design Report for Cottonwood Slough Restoration Project Prepared for: Skagit Conservation District Mt. Vernon, WA Prepared by: Northwest Hydraulic Consultants Seattle, WA January 11

2 Table of Contents Table of Contents... i List of Figures... ii List of Tables... ii 1 Introduction Goal and Objectives Previous Studies Methods Field Survey and Topographic Mapping Water Level Observations Hydraulic Modeling Site History Existing Conditions Hydrology and Hydraulics Flow Frequency Stage Discharge Sediment Alternatives Slough Channel & Outlet Options Backwater Channel Battelle Channel Minimum Channel Channel Comparison Inlet Options Sediment Management Location Flow Field Manipulation Maintenance Dredging Over excavation Inlet Type Preferred Alternative Channel Inlet Sediment and Hydraulic Performance of Preferred Alternative Hydraulic Performance Sediment Performance Summary Success in Meeting Project Objectives Risk and Uncertainty References... Cottonwood Slough Conceptual Design Report i

3 List of Figures Figure 1: Field Survey Points... 2 Figure 2: HEC RAS Typical Calibration Results at Slough Inlet... 3 Figure 3: Historic Map and Photographs... 4 Figure 4: Slough Photographs... 5 Figure 5: Site map... 6 Figure 6: Discharge Frequency Curve... 7 Figure 7: Stage Discharge Ratings at Cottonwood Slough... 8 Figure 8: Sediment Rating Curve at Mt. Vernon Gage... 8 Figure 9: Mean and Maximum Monthly Water and Sediment Flux at Mt. Vernon Gage Figure 10: Percent of Silts & Clays in Suspended Sediment... 9 Figure 11: Profiles of channel alternatives and low flow water surface profile Figure 12: Typical Cross Section Comparing Channel Alternatives Figure 13: Design Overview Figure 14: Inlet Design Figure 15: Cottonwood Slough Model Cross Sections Figure 16: Spring Low Flow Hydraulic Boundary Conditions Figure 17: Simulated Velocity, Flow and Depth in Proposed Channel Spring Low Flow (SLF) and 2 yr Flood Max... Figure 18: Assumed Sediment Fractions and Rating Curve Figure 19: Simulated Sediment Loading in Cottonwood Slough Figure : Initial and year channel thalweg elevations Figure 21: Change in Mean Effective Channel Invert Elevation over Time and Major Floods (Yang Equation) Figure 22: Flood Induced Channel Thalweg Variation (Yang Equation) List of Tables Table 1: HEC RAS Model Calibration Results March April Table 2: Mt. Vernon Gage Sediment Load WY in tons/yr... 8 Cottonwood Slough Conceptual Design Report ii

4 1 Introduction Cottonwood Slough is a relic channel of the Skagit River located at the junction of the North and South Forks of the Skagit River. Although the slough has filled with sediment over time, it has been identified in several previous studies as a good candidate for habitat restoration. The site is located in a tidally influenced freshwater zone, is owned entirely by WDFW, and the levee system is set back in this area. The purpose of this report is to evaluate a series of alternatives for restoration of the Slough and develop a conceptual design for a preferred alternative for the Skagit Conservation District (SCD). All elevations are reported in NAVD88 vertical datum. 2 Goal and Objectives The project goal and objectives are as follows: GOAL: Provide optimal side channel rearing habitat for juvenile chinook. Objectives that support this goal are as follows: Ecology Provide juvenile fish access March through June Provide water depths of 0.5 to 5.0 feet Provide water velocities of 0.5 to 1 feet per second Conserve existing wildlife habitat Retain diverse, mature forest and wetland conditions along the slough Engineering Keep deposition of river sediment into the slough to a manageable level Land Use Maintain (and ideally, improve) the existing local flood hazard management situation Provide continued public fishing access at the site Protect existing infrastructure, particularly the levees and gas pipeline 3 Previous Studies The Skagit Watershed Council contracted with Battelle to evaluate alternatives for Cottonwood Slough in 07. The Battelle report (Lee & Khangaonkar, 07) describes three alternatives that were evaluated and compared against baseline conditions for flows up to a year flood. The preferred alternative consisted of excavating the Slough with no further levee setbacks. This alternative is evaluated further in this report. Cottonwood Slough Conceptual Design Report 1

5 4 Methods 4.1 Field Survey and Topographic Mapping Cottonwood Slough was surveyed under the direction of the SCD. The survey included transects from the dike across the slough and additional survey in the WDFW parking lot and existing inlet area. The Skagit River mainstem and forks were surveyed by NHC using a survey grade depthsounder and GPS system (Figure 1). Base topography was obtained from the Skagit River System Cooperative 10 ft LiDAR elevation grid flown in 02. The various data sources were merged into a single composite topographic grid for use in modeling and design. 4.2 Water Level Observations SCD installed staff gages and stage recorders in the Skagit River at the inlet and outlet of the slough as well as a series of piezometers in the upper end of the slough channel itself. Manual observations were collected periodically from June 09 through April 10 and automated data in March and April Hydraulic Modeling NHC is under separate contract to update the Skagit River General Investigations Study HEC RAS model. This model was initially developed around 04 by the Corps of Engineers. For the Cottonwood Slough study, Figure 1: Field Survey Points the model from the Mt. Vernon gage downstream was extracted and a new branch was added to simulate the Slough itself separately from the river. New cross sections for the slough and river were cut from the topographic base map and added into the model. Calibration data was available from two stage recorders operated by SCD during March and April 10 at the inlet and outlet of Cottonwood Slough. Photographs taken at the WDFW parking lot during an approximate 2 yr flood on November 17, 09 (~73,000 cfs) were used to estimate water surface elevations as an approximate check on high flow calibration. Inflows for the calibration period were obtained from the USGS Skagit River near Mt. Vernon gage. Tidal boundary conditions were calculated by applying Skagit Bay correction factors to observed Elliot Bay tides. Channel roughness values were adjusted to match observed data; the calibrated Manning s n used was Calibration results are shown below in Table 1 and Figure 2. Table 1: HEC RAS Model Calibration Results March April 10 Site N Average Error (ft) Standard Dev of Error (ft) Inlet Outlet Legend NHC Bathymetry SCD Survey Cottonwood Slough Conceptual Design Report 2

6 Legend 14 Stage Obs Stage 13 Stage (ft) Mar10 Apr10 Time Figure 2: HEC RAS Typical Calibration Results at Slough Inlet A sediment transport analysis of the preferred alternative using the HEC RAS model was performed. Details of this effort are described in section 9.2. Cottonwood Slough Conceptual Design Report 3

7 5 Site History The 1866 General Land Office Map shows the Skagit River and North Fork channels forming a very sinuous S bend following the alignment of Cottonwood Slough, with the South Fork diverging at the midpoint. The 1889 USC&GS T Sheet shows a cutoff channel had formed, creating Cottonwood Island. The western channel (now Cottonwood Slough) was around times wider than the eastern cutoff channel. A 1911 map (not shown, see Figure 2 7 in Lee & Khangaonkar, 07) shows the same general form, but the channels bounding the island are approximately equal in width. By 1937 the mainstem channel had made the eastern cutoff its primary flow path. Cottonwood Slough was now mostly dry at low flows and has narrowed to around one quarter the width of the main channel. Cottonwood Slough continued to narrow as evidenced in the 1956 photograph, although it is clear it still conveyed significant flow. Landowner recollections of Cottonwood Slough prior to the mid 1960 s relate that it was about 40 feet wide with steep banks, was deep enough to swim in and had a constant flow. In the mid 1960 s the Department of Natural Resources built roads across both ends of the slough to allow sand and gravel extraction on Cottonwood Island. The culverts under the road were undersized and easily jammed, and the slough began to rapidly infill at this point, followed by vegetation encroachment. By the 1990 s no unvegetated channel is visible on orthophotos, a state which continues through the most recent photographs available. a b c.1956 d.05 Figure 3: Historic Map and Photographs Cottonwood Slough Conceptual Design Report 4

8 6 Existing Conditions Cottonwood Slough has continued to infill over time. Sedimentation rates have been greatest at the inlet and taper off downstream, resulting in a sediment wedge that prevents inflow to the Slough under normal flow conditions. The December 10 flood (90,300 cfs at the USGS Skagit River near Mt. Vernon gage) resulted in 4 6 inches of deposition in some areas near the inlet (personal communication, Tom Slocum). The upper end of the slough consists of fine sands and is open. Sediment size and slough elevations decrease downstream. The slough channel transitions from an open sand bed to being thickly covered in willows before perennial open water areas begin in the lower half (Figure 4). The lower half of the Slough is classified as a scrub shrub wetland (Pacific Ecological Consultants, 10). Bed material is comprised of riverine silts. Mature cottonwood and alder trees line the banks of the Slough. Bank slopes are mostly gentle with no clear topographic break at the top of bank. Upstream of the inlet there is a WDFW boat ramp and parking lot. Stakeholders have indicated the boat ramp is lightly used, but the parking lot and upstream levee are popular bank fishing locations. Running under the access road to the parking lot is a gas pipeline crossing of the Skagit River (Figure 5). A B C D Figure 4: Slough Photographs A) Looking upstream at inlet. B) Approx 1000 feet from inlet. C) Approx 1500 feet from inlet. D) Looking upstream near outlet. Cottonwood Slough Conceptual Design Report 5

9 Elevation High : 30 Note: Limits of elevation mapping at top of existing levees Low : ,000 2,000 3,000 Feet Skagit River Pipeline Crossing Zone Parking Lot Boat Ramp Cottonwood Slough Existing Slough Inlet Nor th Fork Skagit River South Fork Skagit River Figure 5: Site map Cottonwood Slough Conceptual Design Report 6

10 6.1 Hydrology and Hydraulics Flow Frequency A discharge frequency analysis was completed for mean daily flows at the USGS Mt. Vernon gage for the period Results for both the entire water year and March through June only were calculated. For the spring outmigration period, flows exceed 10,000 cfs 90% of the time and average about 17,000 cfs (Figure 6). Figure 6: Discharge Frequency Curve Stage-Discharge Water levels in the project area are influenced by both tides and freshwater inflow. Figure 7 shows simulated paired stage discharge data for water year 10 at both the inlet and outlet. At low flows, tidal influence causes over a 2 foot variation in stage for a given inflow. As flows increase, tidal influence is steadily diminished as shown by the decrease in scatter in the plot. Because water levels at the inlet and outlet rise and fall at similar rates, the overall water surface slope does not vary as much: the observed difference in water level between the inlet and outlet varied between 0.75 and 1. feet over a wide range of tides and flows from 7,000 cfs to 16,000 cfs. Cottonwood Slough Conceptual Design Report 7

11 Stage (ft NAVD888) Outlet Inlet Flow at Mt Vernon Gage (cfs) Figure 7: Stage Discharge Ratings at Cottonwood Slough Sediment The Skagit River below Mt. Vernon is a sand bed system. The USGS collected suspended sediment samples from at the Mt. Vernon gage, and restarted sampling in 06. A new sediment discharge curve based on USGS analysis (Curran, Grossman, & Mastin, 09) along with the older curve is shown in Figure 8. As a result of the recent sampling at higher flows, the estimates of sediment load are higher than previous. The 09 sediment rating curve was applied to mean daily flows at the gage for water years and the results shown in Table Sediment Discharge (tons/day) Y=2E 08X Sample Sample Collins Curve USGS Flow (cfs) Figure 8: Sediment Rating Curve at Mt. Vernon Gage Table 2: Mt. Vernon Gage Sediment Load WY in tons/yr Average Minimum (WY 01) Maximum (WY 1991) 2,430, ,000 6,880,000 Monthly results are shown in Figure 9. It can be seen that while average flows are higher during the spring snowmelt runoff, the maximum flow occurred in November. Because of the exponential nature of the sediment rating curve, the maximum sediment load for November (WY 1991) is almost ten times greater than the average November load, and even greater than the average annual load. Cottonwood Slough Conceptual Design Report 8

12 Mean bed sediment diameter is about 0.6 mm through the lower mainstem and the upper ends of both forks (Pentec Environmental, 02). Recent bedload sampling at the Mt. Vernon gage by the USGS (Curran, Grossman, & Mastin, 09) at 43,000 cfs showed a gradation dominated by medium and coarse sands at around 45% and 30% respectively of the total bedload. Two samples were collected at the upper end of Cottonwood Slough, near the inlet, by the SCD. The samples had mean diameters of 0.16 mm. The finer nature of these samples is consistent with the current inlet structure of the slough. With the sediment sill in place at the inlet, flows are drawn from the upper portion of the water column under flood conditions, where suspended sediment sizes are less. Suspended sediment with diameters less than 0.06 mm (silts and clays) that tend to remain in suspension within the river channel is defined as wash load. This fraction of the sediment load does not deposit within river channels. The remainder of the load (fine sands and larger) forms the material that makes up the river bed. Figure 10 shows the percent of wash load in suspended sediment samples on the Skagit River. All data are from the USGS. There is wide scatter in the data, especially at low flows. As flows increase, the range decreases to about % 60%. The single 78,000 cfs flood flow sample is similar to data from the Nooksack and Snohomish Rivers, which have about 55% and 65% percent wash load, respectively (Northwest Hydraulic Consultants, 04). Volume: Water (cfs days), Sediment (tons) Percent Suspended Sediment finer than 0.06 mm 5,000,000 4,000,000 3,000,000 2,000,000 1,000,000 Sediment is a concern for the sustainability of any side channel project such as this. While no specific design life is given as a project objective, a project that fails due to aggradation in a matter of years is clearly not desirable. The infilling and conversion of side channels to isolated wetlands and eventually floodplain is a normal river process that the historic record of Cottonwood Slough demonstrates, albeit accelerated by human activities. In natural systems, the loss of side habitat channel in one location can be replaced by the creation in another by channel migration and other processes. In a leveed river such as the lower Skagit, the opportunity for natural creation of side channels elsewhere is prevented, thus the importance of existing side channels functioning for the long term Flow (cfs) Max Sediment Max Flow Avg Flow Avg Sediment Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Figure 9: Mean and Maximum Monthly Water and Sediment Flux at Mt. Vernon Gage Mt. Vernon Gage Near Cottonwood Slough Inlet Figure 10: Percent of Silts & Clays in Suspended Sediment Cottonwood Slough Conceptual Design Report 9

13 7 Alternatives The alternatives discussion is divided into two parts: options for the slough channel itself and, of greater complexity, options for the inlet. 7.1 Slough Channel & Outlet Options Backwater Channel It was suggested at the stakeholders meeting that a backwater channel connected only at the lower end be considered. There are several advantages to this alternative. Since most of the sedimentation has occurred in the upper quarter of the slough, construction costs to excavate a backwater channel below the sediment wedge would be substantially lower, and there would be no inlet construction costs, which are also substantial. Leaving the sediment wedge in place also reduces much of the risk and uncertainty of rapid sediment infilling associated with a flow through channel. There are also concerns with this option. First, it fails to meet of one of the primary project objectives of providing flow and velocity in the channel. Some in and outflow would occur with tidal fluctuations, but as the range is only around one foot, the velocities would be very low. Second, the water quality of such an alternative is uncertain. Inspection of various aerial photographs showed numerous backwater channels on the Skagit River floodplain with the characteristic orange brown color indicative of high iron content, including Cottonwood Slough itself and Britt Slough across the river. In the North Meander restoration project on the Stillaguamish River, a restored side channel was initially connected only at the lower end. Groundwater inflow to the channel contained high iron concentrations and resulted in dissolved oxygen levels dropping to lethal levels for fish. Due to these concerns, this alternative was not developed further Battelle Channel The preferred alternative (Alternative 3) in the Battelle report (Lee & Khangaonkar, 07) consisted of a 50 foot bottom width channel with 3:1 side slopes. Invert elevations were set to 0.8 ft at the inlet and 0.5 ft at the outlet. This results in a channel that would be a minimum of 9 feet deep during the spring outmigrant period, and have an approximate wetted top width of feet. Based on a reported maximum velocity of 0.5 ft/s, flows would be around 340 cfs. The bankfull wetted top width would exceed 0 feet Minimum Channel NHC developed a minimum channel configuration using the current hydrologic and hydraulic analyses. This configuration is designed to meet all habitat objectives while minimizing excavation quantities and hence costs. Channel thalweg elevations of 8 feet at the inlet and 7 feet at the outlet will provide year round function of the channel and a minimum of 2 3 feet of depth during the targeted spring outmigration period. A minimum bottom width of 30 feet was selected; widths less than this are more prone to blockage due to tree fall, flood debris, or beaver activity. A 30 foot bottom width with 3:1 side slopes also generally fits into the existing slough channel well, maximizing width without requiring significant excavation into the banks and loss of mature forest canopy. This results in a wetted top width of 42 feet under minimum spring flow conditions and around 100 feet in bankfull conditions. This channel configuration was modeled with a variety of inlet options (discussed in more detail in the following sections) in order to confirm its ability to meet habitat objectives. Cottonwood Slough Conceptual Design Report 10

14 7.1.4 Channel Comparison Figure 11 and Figure 12 below show the Battelle and NHC minimum channel geometries in profile and for a typical cross section. The much greater size of the Battelle channel can be clearly seen. The river is approximately 700 feet wide at the inlet location. Figure 11 shows how the Battelle channel would need to be dredged more than halfway across the mainstem channel in order to remove bar deposits to the design elevation. Estimated excavation quantities for the Battelle channel are 3,000 cubic yards and for the minimum channel 80,000 cubic yards. Cottonwood Slough Profile 15 Mainstem Skagit River North Fork Skagit River Elevation (ft NAVD88) NHC Minimum Channel Battelle/LBS Alt Station (ft) Figure 11: Profiles of channel alternatives and low flow water surface profile Cottonwood Slough Conceptual Design Report 11

15 30 Elevation (ft NAVD88) Existing Ground Battelle Channel Minimum Channel Station (ft) Figure 12: Typical Cross Section Comparing Channel Alternatives 7.2 Inlet Options Sediment Management Design of the inlet is driven primarily by sediment issues. The most likely mode of sediment impact is the reestablishment of a sediment wedge at the inlet. Over time it will grow and extend downstream, steadily reducing the frequency of flows that can overtop it and flow into the slough. No alternative modeled has velocities that approach those of the main channel, so any sand size sediments ingested are likely to be deposited within the channel. The Battelle modeling consistently showed sediment deposition of up to 6 feet in a single yr flood in the vicinity of the Slough entrance. While the magnitude of the deposition may be overstated due to inaccurate model bathymetry, the overall pattern is consistent with field observations. These include the presence of sand spoils at the boat ramp from repeated WDFW maintenance; the existence of the sediment wedge in the upper Slough channel; and the natural levees visible in Figure 13 on both banks of the main channel, formed by the deposition of suspended sediment borne overbank during flood flows. A series of inlet options are available that alone or in combination will reduce the risk of unwanted sedimentation. In particular, a review of design recommendations for irrigation intakes was undertaken. Here, the basic project goal of diverting water while minimizing sediment ingestion is the same Location Point bars are not preferred locations for inlets as there is increased risk of sedimentation. The current Slough inlet is located on the large right bank point bar attached to Cottonwood Island (Figure 5). The farther upstream the inlet is moved, the lower the risk of bar growth impacting it. While an ideal location would be at the upper end of the parking lot, a gas pipeline crossing prevents this. Locations Cottonwood Slough Conceptual Design Report 12

16 between the existing boat ramp and middle of the parking lot would provide better conditions for the inlet Flow Field Manipulation By constructing various features in the inlet vicinity, river flows can be manipulated to provide better inlet conditions. Two options have been identified Hard Points/Log Jams Extending the inlet out into the channel a short distance by using a rock deflector or log jam will induce a scour hole at the inlet head. By locally deepening the main channel while keeping the inlet elevation at a higher elevation, less bed load and suspended sediment (which increases in concentration with depth) will be ingested. This also provides habitat value within the main channel and may serve as an attractant for fish to utilize the slough Floodplain Grading The inlet of the Slough is located at a sudden expansion in available flow area: The width between levees upstream is around 1,100 feet which expands to over 5,000 feet across Cottonwood Island. The rightward bend in the mainstem coupled with this flow expansion is responsible for the large point bar on Cottonwood Island. Minor local reductions to the available expansion area would be expected to erode and shift the head of the bar downstream somewhat, creating more favorable inlet conditions. This can be accomplished by elevating the existing parking lot several feet. In doing so, some of the flows that now are directed out of the main channel through the parking lot would be kept in the channel, increasing local velocities on the head of the bar. The parking lot elevation would be limited such that in large floods it would still be inundated and no adverse impacts on flood levels would occur. This would also reduce sedimentation in the parking lot, reducing WDFW s maintenance costs Maintenance Dredging The project can be designed to allow excavation of deposited sediments within the slough. This would be accomplished by designing a section of channel very near the inlet to induce deposition. Permanent access roads would allow removal of sediments using standard construction equipment. This option requires an entity to assume maintenance responsibility and the associated costs. Volumes removed and costs would vary, possibly significantly, depending on floods that occurred in a given year Over-excavation The slough channel can have greater volume excavated than required to meet habitat objectives; the excess volume would serve as a sediment deposition area while allowing continued function of the project as a flow through side channel. The over excavated area would need to be located in the upper end of the channel where sediment deposition will be greatest. Increasing channel width rather than depth will lessen construction costs and increase usable wetted habitat area, at the cost of losing additional mature riparian forest Inlet Type Using a culvert rather than natural inlet allows a variety of strategies to manage sediment. Culverts can be sized such that they provide nearly the same inflow during normal flow conditions, but substantially less so during flood conditions, thereby reducing sediment ingestion. Culverts also provide a structure to attach various gates to, giving further options for inflow regulation. A self regulating tidegate (SRT) is one such example. A system similar to that installed on Fisher Slough would allow full connection during normal flows, with the gates closing as river flows and stages rose to Cottonwood Slough Conceptual Design Report 13

17 flood levels. Such a system has mechanical complexity and potentially larger maintenance costs, but allows unattended flow regulation. Seasonal gate operation is another option culverts allow. Under this scenario, a responsible entity would open and close gates on an agreed to schedule to regulate the amount of flow into the Slough. This would most likely only need to occur 2 3 times a year. Gates could either be permanently mounted side hinge gates or a simpler timber stop log system. The reasoning behind such a system is shown in Figure 9. It can be seen that while mean sediment loads in the Skagit River show a relatively balanced distribution with approximately equal fall and spring peaks, maximum loads are strongly skewed towards the fall. This is reflective of the extremely large sediment loads that occur during large floods which are most common in November to December. Thus a system where the gates are closed during the fall and winter months greatly reduces the risk of a large flood borne sediment pulse. The gates can then be opened to allow full access during the spring outmigrant window. 8 Preferred Alternative The recommended alternative has been selected based on evaluation of the options discussed above, and discussions with landowners, WDFW and SCD. Key elements are shown in Figure 13, Figure 14 and described below. 8.1 Channel The recommended channel configuration is based on the minimum channel alternative evaluated: a 30 foot bottom width with 3:1 side slopes and invert elevations of 7 and 8 feet at the outlet and inlet, respectively. Maximum invert elevations may be increased up to one foot; this will not substantially affect channel hydraulics during the spring outmigrant period. This alternative fits into the existing channel well and requires only one third of the excavation quantity the Battelle alternative requires. It should be noted that the dimensions given are the minimum: there are numerous locations, especially in the lower end, where there will be opportunity to expand the channel width without a significant cost penalty. Immediately downstream of the inlet structure, an expanded channel is proposed to function as a sediment trap. This allows the option of active sediment removal as a management strategy for the restored channel. Channel layout should be completed in the field by the project designers to maximize habitat area, channel form variability, and utilization of existing features such as ponds and shade. Trees and shrubs that must be removed for construction should be kept on site and placed in and around the channel to increase habitat. 8.2 Inlet The proposed inlet is located immediately downstream of the existing parking lot. This location is well upstream of the existing inlet, reducing point bar impacts, but avoids loss of significant parking lot area which would need to be mitigated. A regulated inlet is recommended. Modeling indicates a bank of three 10x10 ft box culverts provides sufficient flow to meet habitat objectives. The culverts are set in an abutment with mechanically stabilized earth (MSE) walls. This allows vegetated near vertical abutment walls that permit culvert lengths to be minimized. The culvert inverts should be set at an elevation of 8 9 feet. A 9 foot invert provides two feet of depth at a 90% exceedance spring low flow and free surface flow up to nearly 40,000 cfs, or about a 4% exceedance flow. An 8 ft invert would allow year round flow in most years. Removable gates should be attached to the culvert to allow seasonal operation. The gates can be simple wood or metal structures and need not be watertight. A slot or other opening should be left in one gate to allow some flow into the slough at all times. It is recommended the gates be attached from October through the end of January to minimize sediment ingestion risk during flood season. Cottonwood Slough Conceptual Design Report 14

18 The culvert and abutment are set to face upstream and extend into the channel through the use of a deflector attached to the downstream abutment. The structure should induce scour while minimizing the risk of collecting floating wood and building a blocking jam in the inlet. Induced scour cannot be allowed to undermine the culvert and abutment. For this reason, a composite wood/rock based structure is recommended. Large woody debris can be anchored to the bank and deflector at lower elevations to provide habitat during normal flow periods. The higher elevations of the abutment should be armored as necessary and then transition to vegetated slopes, but no anchored wood is recommended in order to help shed any floating LWD that may strike it. The boat ramp is relocated upstream to the parking lot. The proposed inlet may induce some local aggradation downstream that could further exacerbate the current boat ramp maintenance issues. Relocating the boat ramp will reduce maintenance costs as it can be designed to be less prone to sedimentation. It also will let WDFW prevent uncontrolled vehicle access across the inlet to Cottonwood Island, helping to conserve the area. The ramp is built out from the existing bank in order to reduce sedimentation on it. The parking lot should be elevated several feet to divert more flow into the main channel past the inlet opening. This will help to keep the inlet clear of sediment and also reduce maintenance costs for the parking lot. The elevated parking lot, boat ramp, and inlet structure/deflector are designed to function together to locally reverse the current rightward flow curvature at the inlet that is a function of the overall river meander. The design elements will induce a sweeping flow away from the bank, resulting in bend scour at the inlet location. The deflector will induce further local abutment scour at the inlet. The intent is to scour local bed elevations to several feet below the culvert invert at the inlet mouth and induce transverse bottom currents to transport sediment away from the inlet. Cottonwood Slough Conceptual Design Report 15

19 Skagit Conservation District Cottonwood Slough Conceptual Design - Overview Scale - 1:6,000 Ü Feet Scale - 1:6,000 Northwest Hydraulic Consultants project no Nov 10 See Inlet Detail Sheet Legend Channel Toe Limits of Channel Excavation \ \ \ \ Pipeline Crossing Zone 5 Ft Contours Elevation High : 30 Low : -5 DRAFT DRAFT Figure 13: Design Overview Cottonwood Slough Conceptual Design Report 16

20 30 Legend Proposed Project 0 LWD Toe of Channel # Proposed Contours (5 Ft) Abutment Boat Ramp Culvert Deflector LimitsChanExcWithSedBasin Existing Features Edge Road/Parking Lot \ \ \ \ Pipeline Crossing Zone 5 Ft Contours 1 Ft Contours Elevation High : ### # ## # # # # ####### Low : -5 Skagit Conservation District Cottonwood Slough Conceptual Design - Inlet Area DRAFT Scale - 1:1,0 Ü Feet Scale - 1:1,0 Northwest Hydraulic Consultants project no Nov 10 DRAFT 35 Section A A 30 Elevated Parking Lot Existing Ground Culverts 5 Raise Parking Lot (3) 10x10 Box Culverts w/ gates Abutment Station (ft) Sediment Basin A A Elevation (ft) Figure 14: Inlet Design Cottonwood Slough Conceptual Design Report 17

21 9 Sediment and Hydraulic Performance of Preferred Alternative The preferred alternative was evaluated for meeting hydraulic design objectives and long term function considering likely sediment ingestion in the slough. Figures in the following sections are based on a slough profile and cross sections shown below. Stream profile stationing increases in the upstream direction. Figure 15: Cottonwood Slough Model Cross Sections 9.1 Hydraulic Performance Hydraulic performance of the preferred alternative was evaluated using the HEC RAS model for two scenarios: Spring Low Flow The calibration flow period of March 28 to April 12, 10 was simulated. River flow and tidal conditions during this period are shown in Figure 16. The average flow is 12,100 cfs for this period, compared with average spring flows of 17,000 cfs. 2 yr Flood The flood event of November 17, 09 was simulated. This flood had a peak of 73,0 cfs at the Mt. Vernon gage, which is about a 2 yr flood event. Tidal influence is negligible at this flow. Channel velocity, flow, and depth for the Spring Low Flow condition and the 2 yr flood are shown in Figure 17. This figure shows the variation in hydraulic conditions within the channel as a result of the interaction of varying flows and tidal influence. During Spring Low Flow conditions, the range in flow, depth and to a greater extent, velocity, increases at the downstream end of the channel; this is due to Cottonwood Slough Conceptual Design Report 18

22 the increasing effect of tidal variation closer to the outlet. Flows average around 190 cfs, depths are typically between 3 4 feet, and velocities between 1.3 and 1.5 ft/s. While the depths and flows during the 2 yr flood are substantially larger, velocity maximums only increase to 2.4 ft/s. The decrease in velocity in the downstream direction is due to increasing backwater effects near the outlet. Based on the model results, the preferred alternative will meet project hydraulic performance objectives. 16,000 15,000 USGS AT MT VERNON PROVISIONAL DATA FLOW 14,000 Flow (cfs) 13,000 12,000 11,000 10,000 9,000 8, TIDES NAVD88 STAGE Slough Outlet Stage Slough Inlet Stage 10 Stage (ft) Mar10 Apr10 Figure 16: Spring Low Flow Hydraulic Boundary Conditions Cottonwood Slough Conceptual Design Report 19

23 Velocity (ft/s) SLF Max SLF 75% SLF Mean SLF % SLF Min 2 yr Flood Flow (cfs) Depth (ft) Station (ft) Station (ft) Station (ft) SLF Max SLF 75% SLF Mean SLF % SLF Min 2 yr Flood SLF Max SLF 75% SLF Mean SLF % SLF Min 2 yr Flood Figure 17: Simulated Velocity, Flow and Depth in Proposed Channel Spring Low Flow (SLF) and 2 yr Flood Max 9.2 Sediment Performance The greatest uncertainty with the proposed project is sediment. The project has been designed to minimize sediment ingestion into the Slough but substantial quantities will nevertheless enter every Cottonwood Slough Conceptual Design Report

24 year. A sediment transport analysis was conducted to evaluate the ability of the project to provide desired habitat conditions over the long term. The intent is to ensure there are no indications that the project will undergo rapid sedimentation after construction. It must be emphasized that any sediment transport analysis will contain large uncertainties. The amount of sediment that may be ingested into the Slough was estimated by first developing a rating curve to relate the percent of Skagit River flow the proposed alternative diverts under various conditions. This curve was developed from modeling up to a 2 yr event. The curve was extrapolated to enable use under the full range of historic flows observed, but there is greater uncertainty at floods larger than a 2 year event, as the model was not set up to accurately simulate conditions where overbank flow is significant. Sediment loading into the slough was then calculated by multiplying the percent flow diverted by the Skagit River sediment load. This assumes that sediment will be ingested in the same proportion as flow. The paired slough flow sediment data was used to develop a total load sediment rating curve (Figure 18). Sediment size fractions were roughly apportioned based on bed material, bedload sampling and percent fines data discussed in section 6.2 (Figure 18). A fixed gradation was used for modeling although it is likely that gradations would be finer under lower flows. Estimates of sediment loading (Figure 19) show similar patterns to the mainstem Skagit (Figure 9). The average annual load (including wash load) is 56,000 tons. Average monthly loads exceed 9,000 tons for November and June. The maximum November monthly load is over 10 times the monthly average and exceeds the average annual load by a factor of two. Sediment Fraction % of Total Load Silts/Clays 50 Very Fine Sand 5 Fine Sand 5 Medium Sand Coarse Sand Sediment Load (tons/day) Flow (cfs) Figure 18: Assumed Sediment Fractions and Rating Curve Only the Cottonwood Slough portion of the HEC RAS model was used for the sediment transport analysis. The upstream boundary condition used flow based on the Mainstem Slough diversion rating curve, and the downstream boundary was a tidally averaged stage flow rating curve extracted from the main model outputs. An averaged downstream boundary was required as the sediment transport module of HEC RAS only runs in quasi steady state mode. As tidal influence is limited at the site during low flows and becomes negligible under higher flow and sediment loads this should have only minor influence on the results. A minimum baseflow of 15 cfs was enforced as the model will not run with zero flow. Sediment loading inputs were developed as described above and input at the upper end of the Slough along with flow. Cottonwood Slough Conceptual Design Report 21

25 A year period from water year was simulated. This period was chosen because it contains water year 1991, which has the largest annual, monthly and single flood sediment load within the available data record. In addition to the November 1990 floods, large floods occurred again in 1995, 03, and 06. Two transport equations and two loading scenarios were used for a total of four simulations. One scenario input the entire sediment load including silts/clays, and allowed overbank deposition. The other scenario input only the sand fraction of the load, excluding wash load, and did not allow overbank deposition. The Engelund Hansen and Yang transport equations were used on both scenarios. In all runs no seasonal gate closures were simulated, that is, it was assumed the inlet was fully open yearround. At the end of the year simulation period, the Yang simulations showed up to two feet of aggradation in the upper end and less farther downstream in the channel. Engelund Hansen based simulations showed no change to one half foot of degradation (Figure ). However, interannual variation, especially in the upper portion of the channel, is significantly greater than the end condition might imply. Figure 21 shows changes in the mean effective invert elevation for three cross sections located in the upper, middle, and lower channel. This parameter represents overall channel change, not just thalweg adjustment. It shows around half of the total aggradation in the Yang simulations occurred in the first year, which may be related to simulation starting conditions, and then during the November 1990 floods. Rates after 1990 range from 0.5 to 1 foot over years. Figure 21 also demonstrates the variation in response to sediment inputs along the channel. At the uppermost cross section, bed changes have the largest magnitude and response to sediment and flow inputs. In the middle cross section, bed changes are dampened and smoothed out, although individual floods still cause aggradation of over one half foot. The lowermost cross section shows a much attenuated response, with flood borne sediment pulses taking several months to be transported to the lower end. Sediment Load (tons) Average Load Max Month Minimum Load Average Annual Load Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Figure 19: Simulated Sediment Loading in Cottonwood Slough Cottonwood Slough Conceptual Design Report 22

26 Elevation (feet) E H E H No Overbank Yang No Overbank Yang Initial Profile Channel Station (ft) Figure : Initial and year channel thalweg elevations Elevation (ft) XS 404 XS 27 XS / / / /04 10/09 Year Figure 21: Change in Mean Effective Channel Invert Elevation over Time and Major Floods (Yang Equation) Simulated changes in bed profile over one flood event (November 1995) are shown in Figure 22. The bed aggrades 2.5 feet at the upper end of the channel at its maximum on December 3 compared to conditions before the flood and develops a sediment wedge that extends well down the channel. Six days later the sediment wave is beginning to be moved down the channel, and three weeks after the flood peak, the new sediment is fairly uniformly distributed over the upper two thirds of the channel. Cottonwood Slough Conceptual Design Report 23

27 Channel Invert Elevation (Feet) Nov 95 3 Dec 95 9 Dec 95 1 Jan Channel Station (Feet) Figure 22: Flood Induced Channel Thalweg Variation (Yang Equation) In general, the channel showed acceptable long term average rates of sedimentation that will ensure long term function of the project. Large floods input a sediment pulse that would deposit in the upper end, and then be transported downstream over time under lower flows until the bed profile was similar to pre flood conditions. The scenarios that allowed overbank deposition and included wash load showed significant overbank deposition in the simulation period. 10 Summary 10.1 Success in Meeting Project Objectives The preferred alternative meets the project design objectives. Ecological objectives are all directly met. Note that mean channel velocities are slightly higher than the objective range, but the constructed channel will have much greater variation in width and other dimensions than the simple trapezoidal channel modeled, which consequently will provide extensive areas with velocities in the desired ft/s range. The sediment transport analysis and prior work in the Battelle report indicate the project should provide long term function as designed, although this is the least certain part of the analysis (see the next section). The preferred alternatives maintains existing public access and also provides a lower maintenance boat ramp for WDFW, while allowing better control of vehicular access onto the wildlife conservation area of Cottonwood Island proper. The project has been designed to protect the existing infrastructure including the adjacent dike and pipeline upstream Risk and Uncertainty The largest risks and uncertainties with the project revolve around long term sedimentation. The sediment transport modeling indicates the channel will have a long functional life, but there is large uncertainty with the modeling results. A particular concern shown in both the Battelle modeling and that presented here is the deposition of a large sediment wedge at the inlet during a large flood. The simulations do not account for the decrease in flow that will result from a sediment wedge being downstream of the inlet, nor is the height of the wedge certain. One risk is that the sediment wedge is of enough height to block inflow to the channel once the river recedes. The modeling shows it is the Cottonwood Slough Conceptual Design Report 24

28 lower post flood flows that redistribute and eventually transport the flood deposited sediment out of the channel. If post flood flows are blocked, the sediment wedge may stay in place and continue to build during each flood, quickly transforming the channel into a backwater system and allowing silts to begin depositing in the channel. All pieces of the inlet design have been developed in order to minimize sediment ingestion into the Slough. Seasonal gate operations are one of the key pieces that will reduce the uncertainty, as it is the larger floods that occur in the November February timeframe that create the largest deposition. Due to the uncertainties regarding sediment, project monitoring and adaptive management plans should include sediment monitoring as an important component. 11 References Curran, C., Grossman, E., & Mastin, M. (09). Measurements of suspended sediment and flow distribution with implications for habitat restoration in the Skagit River Delta, Washington. 09 Puget Sound Georgia Basin Ecosystem Conference. Seattle, WA. Lee, C., & Khangaonkar, T. (07). Cottonwood Island Restoration Feasibility Study Hydrodynamic and Sediment Transport Analysis. Richland, WA: Battelle Pacific Northwest Division. Northwest Hydraulic Consultants. (04). Stillaguamish River North Meander Reconnection Alternatives Analysis Report. Seattle, WA. Pacific Ecological Consultants. (10). Cottonwood Island Slough Wetland & Habitat Conservation Area Reconnaissance Report. Pentec Environmental. (02). Geomorphology and Sediment Transport Study of Skagit River Flood Hazard Mitigation Project Phase 1 Interim Report. Cottonwood Slough Conceptual Design Report