New Churn Creek Bottom Flood Risk Reduction Reconnaissance … · 2020. 2. 6. · conditions (corrected or current condition backwater model). The current effective FEMA FIS and FIRM

Churn Creek Bottom Flood Risk Reduction Reconnaissance Study

Shasta County

Prepared for:

Western Shasta Resource Conservation District 6270 Parallel Road

Anderson, CA 96007

Prepared by:

Pacific Hydrologic Incorporated 1062 Market Street

Redding, CA 96001

Norman S. Braithwaite

March 7, 2019

Churn Creek Bottom Flood Risk Reduction Reconnaissance Study Introduction: The reach of Churn Creek from the mouth to Churn Creek Road (between South Bonnyview Road and Rancho Road), locally known as Churn Creek Bottom, is subject to flooding on a relatively frequent basis. The Western Shasta Resource Conservation District (RCD) has assisted flood prone property owners to reduce flood risk by removing invasive vegetation in and adjacent to the Churn Creek channel thus increasing the flood carrying capacity of the channel. Although this project has reduced flood risk to some degree, the level of flood risk is still high and the RCD believes additional projects including further vegetation management and physical modifications may be of benefit in reducing flood risk to an acceptable level. Figure 1 identifies the study area on the current effective FEMA Flood Insurance Rate Map (FIRM). This study has been funded by the FEMA Cooperating Technical Partners (CTP) Program, contract EMF-2017-CA-00009. Above Churn Creek Bottom, Churn Creek drains a basin of 33.4-square miles ranging in elevation from approximately 460-feet at the upper end of Churn Creek Bottom to over 2000-feet on hills adjacent to Shasta Lake. Mean annual precipitation in the basin is reported to be 48-inches by the USGS. Land use in the basin consists mostly of urban development in the City of Redding and Shasta Lake City with some rural residential and undeveloped lands outside of the cities and on steep ground in the headwaters. City of Redding design standards have prevented development during the past 25-years from increasing the peak flow in Churn Creek, a standard more stringent than the “rule of reasonableness” establishing the standard of care for development in California. Vegetation in undeveloped areas consists primarily of chaparral. Many flood studies have been conducted within the Churn Creek basin in support of specific developments and for planning purposes. Two of these studies, the FEMA Flood Insurance Study (FIS) and the City of Redding City-Wide Master Storm Drain Study, address the entire basin and are of significance to the current study. Within Churn Creek Bottom, Churn Creek has a well defined meandering channel that is generally choked with non-native vegetation except where vegetation is being managed. A vegetation management program has been established for the Churn Creek channel from a point approximately ½-mile downstream of the middle Churn Creek Road crossing (near Green Acres Drive) to the north Churn Creek Road crossing (off South Bonnyview) after several flood events inundated farm land and structures in the vicinity of Green Acres Drive. Downstream of Meadowview Drive, the 100-year flood peak flow is generally contained within the active, vegetation choked channel. This reach of channel has incised since construction of Shasta Dam due to water surface elevations in the Sacramento River being maintained at artificially low elevations during times of high flow in Churn Creek. Knowledge of channel changes since the survey supporting the FEMA FIS including upstream propagation of incision and potential sedimentation (aggradation) in the upper reaches of Churn Creek within Churn Creek Bottom are of interest and have been addressed in this study. Since the ultimate goal of this project is to reduce flood risk and revise the published FEMA FIS and FIRM, this study has been prepared consistent with FEMA requirements for map revisions. The

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FEMA process generally consists of obtaining the original FEMA backwater model data set (current effective backwater model), rerunning the model data set in the current version of the original model program (duplicate effective backwater model) including recalibration to match the published FEMA 100-year flood (base flood) profile if necessary, and updating data to represent current conditions (corrected or current condition backwater model). The current effective FEMA FIS and FIRM were developed based upon a US Army Corps of Engineers’ HEC-RAS linear steady state backwater model. A two dimensional (2D) dynamic backwater model is required for this study. Therefore, as part of this study, a linear steady state duplicate effective backwater model was prepared before converting to a dynamic 2D model and rerunning to produce a dynamic 2D duplicate effective backwater model. Subsequent to preparation of the duplicate effective backwater models and prior to evaluation of alternatives, data in the dynamic 2D duplicate effective backwater model data was modified to represent the current condition. After completion of the current condition 2D backwater model, candidate flood risk reduction alternatives identified below were developed and in some cases evaluated using the dynamic 2D backwater model. These alternatives are described in greater detail later in this report.

Extend vegetation management downstream to the mouth of Churn Creek Develop off-channel detention at a location upstream of Churn Creek Bottom Construct a flood relief channel conveying Churn Creek flood flow to the Sacramento

River Construct a flood relief channel paralleling Churn Creek Construct a channel conveyance improvement (widen Churn Creek channel)

Other candidate alternatives including removal of a berm encroaching within the 100-year floodplain upstream of Knighton Road and construction of set back levees have not been evaluated in this analysis. Based on the results of the vegetation management alternative, no significant reduction in flood risk is believed to be associated with removal of the berm. The set back levee alternative was eliminated due to high costs associated with procurement of easements through private property and the fact that this alternative will increase flood risk downstream of the levees. Flood Hydrologic Analysis: Flooding in Churn Creek Bottom occurs as a result of large storm events, primarily cloudburst or nested cloudburst events now referred to as stationary convergence events, over the Churn Creek drainage basin. Developed in support of the City-Wide Master Storm Drain Study of 1993, the City of Redding maintains a rainfall-runoff model on the Corps of Engineers’ HEC-1 platform that is up to date and sufficient for evaluation of existing conditions in Churn Creek Bottom. A cursory review of flood hydrology in Churn Creek indicates that the FEMA FIS has relied upon the City of Redding rainfall-runoff model and that rainfall-runoff model results are consistent with the 100-year flood peak flow estimate by the USGS Streamstats web application (regional methodology) when considering the presence of urban development within the basin. All backwater model runs therefore relied on the FEMA published peak flows with the unsteady duplicate effective backwater models employing hypothetical partial flood hydrographs consisting of flow ramping up to the published FEMA 100-year flood flows over a period of 20-hours followed by constant flow at the published FEMA values over a period of 10-hours thus simulating steady state peak flow. Flood peak flows relied upon by FEMA for the Churn Creek FIS do not account for

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loss of flow from the channel due to overflow leaving the Churn Creek basin over Interstate 5 or for attenuation of peak flow by storage of flood water in the floodplain. The upstream boundary conditions for the current condition model runs consisted of 100-year flood hydrographs from the City of Redding rainfall-runoff model scaled to have the peak flows match the published FEMA 100-year flood flows. Loss of flood water from the basin and storage of flood water within the overbank floodplains are accounted for by the 2D backwater model. As such the peak flow in the Churn Creek channel is attenuated in the downstream direction by the 2D model. Simulated steady state and dynamic flood hydrographs for the upstream model boundary conditions are shown on Figures 2 and 3. The flood frequency relationship for Churn Creek at the upstream backwater model boundary (COR rainfall-runoff model “C1898” above Linden Drain) is identified in Figure 4. Duplicate Effective Backwater Models: Two duplicate effective backwater models were prepared for this study, one being a linear steady state backwater model (Duplicate Corrected) and the other being a linear dynamic backwater model (Duplicate Unsteady) in preparation of adding 2D domains representing shallow overbank flood flow. Unfortunately, simply re-running the current effective backwater model data set did not produce a flood profile meeting the FEMA standard for a duplicate effective backwater model (Duplicate Received). The reasons for substantial differences in 100-year flood profiles may be due to changes in HEC-RAS computational routines or more likely, the actual data set relied upon for preparation of the FIS was not submitted and saved by FEMA. Substantial differences in flood profiles were found at and upstream of bridges. Reasonable modifications of the Current Effective data set were therefore employed as necessary to produce the linear steady state Duplicate Corrected backwater model. Changes to the original data set included the following:

Meadowview Bridge – Convert computation method from pressure & weir to energy Change pier widths from 4.0-feet to 4.2-feet

Middle Bridge – Convert computation method from pressure & weir to energy X-sec 25968 – Define ineffective area in left overbank X-sec 25968 – Define ineffective area in left overbank X-sec 25984 – Revise encroachment limits

A comparison of the Current Effective, Duplicate Received, and Duplicate Corrected flood profiles is presented on Figures 5 and 6. In preparation of converting the model to employ overbank flow computations within two dimensional (2D) domains, a dynamic duplicate effective (Duplicate Unsteady) backwater model was prepared by employing the following steps:

Add horizontal coordinates for cross-section points to produce a georeferenced duplicate backwater model (Duplicate Georeferenced). Locations of georeferenced cross-sections are shown on Figure 7.

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Replace steady state flow data with flow hydrographs and run the model in dynamic mode (Duplicate Unsteady)

A comparison of the Current Effective and Duplicate Unsteady flood profiles is presented in Figure 8. Some further adjustment of the duplicate unsteady model will be required prior to any FEMA map revision due to differences in water surface elevations exceeding 0.5-feet at one location. Channel Morphology: As a result of controlled flows in the Sacramento River during times of local flooding, the Churn Creek channel has incised from the mouth to near Meadowview Drive. According to residents in the vicinity of Green Acres Drive, sediment accumulation was observed in the channel prior to implementation of the vegetation management program. Given these recognized and potentially continuing processes, a backwater model run was prepared to identify the significance of continuing geomorphic processes. This model run was developed by replacing channel data in a copy of the Duplicate Georeferenced model employing the FEMA data set (2004±) with new field surveyed channel data collected at the FEMA cross-section locations. No other data from the Duplicate Georeferenced model was modified so as not to contaminate the ability of the Morphology Check model results. Both model runs were linear steady state runs. Data and results of the Morphology Check model were compared to the duplicate effective model run to determine the magnitude and significance of changes to the channel since the FEMA survey. A comparison of flood profiles from the Morphology Check and Duplicate Georeferenced backwater models is presented in Figure 9. Comparisons of channel geometry at cross-sections selected to have a spacing of approximately 5000-feet are shown on Figures 10 through 15. If the geometry of Churn Creek has experienced substantial changes since the FEMA survey, the trend would be reflected by an increase or decrease in the 100-year flood profile over a number of cross-sections. Both the flood profiles and the selected cross-sections tend to support an argument that the channel has not experienced significant geomorphic changes since the FEMA cross-section survey. Any sediment which may have accumulated in the reach of Churn Creek near Green Acres Drive prior to implementing the vegetation management program has since been conveyed downstream. Current Condition Backwater Model: After completion of the duplicate effective models and morphology check, two current condition backwater models were prepared by adding 2D domains east and west of the Churn Creek channel, revising top of bank stations, revising channel roughness coefficients, adding the Knighton Road bridge (not in FEMA current effective model), and adding internal and external boundary conditions to the 2D domains (Current 2D). Top of bank stations were moved to reflect the active portion of the Churn Creek channel rather than the flood channel. Adjacent to the 2D flow domains, overbank areas were defined as ineffective flow areas on the linear domain cross-sections to prevent the backwater model from double accounting for flow in the overbank areas. Channel roughness coefficients employed in the current condition backwater model were based on field observations of factors contributing to overall channel roughness and ranged from 0.035 to 0.050. Roughness coefficients within the 2D domains were based on 2011 National Land Cover Database (2011 NCLD) using roughness coefficients suggested in the HEC-RAS v5 2D Modeling User’s Manual. External boundary conditions were established between Interstate 5 (I-5) and the Sacramento River

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at two locations where overflow in the west 2D domain overflow during the most probable 100-year flood will to overtop I-5. The normal depth method with a hydraulic slope of 0.005 was used for all external boundary conditions. Internal boundary conditions were defined for the crowns of Smith Road, Knighton Road, and Churn Creek Road north and south of Green Acres Road. The internal boundary conditions were included to prevent the backwater model from indicating flow past these road prisms unless the water surface elevation exceeds the crown elevation. Flow over the road prisms was computed using normal 2D computations rather than weir equations. The Knighton Road bridge was modeled by adding a copy of Cross-section 15593 to the model at a location 400-feet downstream of Cross-section 15593 and adding the bridge geometry data from as-built drawings between the two cross-sections. Unsteady flow contraction and expansion coefficients were set to 0.1 and 0.3 respectively except in the vicinity of bridges where the unsteady flow roughness coefficients of 0.3 and 0.5 respectively were employed. The first current condition backwater model was run using the simulated steady state hydrograph (Current Condition) and the second was run using full flood hydrographs from the City of Redding rainfall-runoff model1 (COR 100-year). A comparison of the current condition backwater model profiles to the current effective profile is shown on Figure 16. Differences between the current condition flood profile and the current effective flood profile represent differences in data between the two models including roughness coefficients reflecting channel vegetation, the Knighton Road bridge, and most significantly ground data representing the overbank floodplains. Figure 17 identifies a significant floodplain elevation difference between the FEMA current effective model and the LiDAR terrain data used in the 2D models at cross-section 28903. Similar differences exist at other cross-sections. The COR 100-year profile is below the current condition flood profile due to the limited volume of water represented in the COR flood peak as opposed to an effectively unlimited volume of water available in the simulated steady state flood hydrograph. Peak flow for both dynamic 2D model runs is attenuated by overflow leaving the Churn Creek basin over I-5 and by storage of flood water in the floodplain. A comparison of peak flows along Churn Creek for the current effective, current condition, and COR 100-year model runs is presented in Figure 18. Dips in the peak flow followed by increases in a downstream direction represent flow leaving the channel as overflow then re-entering the channel. The overall decrease in peak flow of the dynamic models represents flow lost from the Churn Creek basin and flood water stored (delayed) in the floodplains. The maximum extents of inundation for the two current condition runs are presented in Figures 19 and 20. The current condition backwater models indicate incipient overflow in the vicinity of Green Acres Drive at a flow of approximately 7000-cfs, a flood estimated to have a statistical recurrence of approximately 10-years.

1 The lateral inflow hydrograph was scaled such that the magnitude of lateral inflow at the time of peak flow in Churn Creek matched the FEMA steady state inflow.

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Vegetation Management: Management of vegetation is a commonly considered first tier approach for reducing flood risk. A vegetation management program has been established for the Churn Creek channel from a location approximately ½-mile downstream of the middle Churn Creek Road crossing of Churn Creek (near Green Acres Drive) to the upper Churn Creek Road crossing of Churn Creek (near South Bonnyview Road). This vegetation management program has reduced flood risk in the vicinity of Green Acres Drive but the area remains subject to inundation on a relatively frequent basis. From downstream of the reach for which vegetation is being managed to the mouth of Churn Creek, however, the channel is presently choked with vegetation. A backwater model run representing extension of the vegetation management program to the mouth of Churn Creek was prepared (CC2D – Vegetation) by reducing Manning’s roughness coefficients in the channel to 0.033 from a range of 0.045 to 0.060 in order to determine the potential flood risk benefit of extending the program to the mouth of Churn Creek. A comparison of the 100-year flood profile for the extended vegetation management program to the current condition 100-year flood profile is shown on Figure 21. The comparison indicates a substantial potential flood risk benefit downstream of the current vegetation management reach but no significant potential flood risk benefit along the current vegetation management reach. Upstream Off-channel Regional Detention: Off-channel regional detention consists of a detention basin separated from the Churn Creek channel by a side channel weir and having a small downstream outlet. The weir elevation is set such that flood water only enters the detention basin when flow in the channel exceeds a design threshold. The small downstream outlet drains keeps the detention basin from ponding water prior to the flood (local runoff) and to drain the detention basin after the flood. This type of detention basin will truncate the flood peaks exceeding the design threshold and is therefore much more efficient than conventional on channel detention facilities. The size of the detention basin is determined from the available hydraulic head between the upstream design threshold water surface elevation and the downstream low flow (1-year flood ±) water surface elevation and from the volume of flood water in excess of the design threshold during the flood peak. The length of the side channel weir is determined by the peak flow to be diverted and the difference in water surface elevation between the threshold flood profile and the maximum water surface profile. Off-channel detention facilities can be designed as multiple use facilities hence they can accommodate parks, community gardens, environmental mitigations, and other uses that are not sensitive to inundation. This study took the approach of determining the volume of detention available in the area of low intensity land use between the Churn Creek channel and Kids Kingdom Park then estimating how much this volume may attenuate the Churn Creek flood peak. The available hydraulic head was determined to be approximately 11 feet between cross-sections 39790 and 38790 near the up and downstream ends of the candidate detention basin area. Without substantially encroaching in the area required by Churn Creek to convey flood flows, the surface area available for the detention basin was estimated to be approximately 15 acres. The useable volume of an off-channel detention facility at this location is therefore approximately 190 acre feet. This volume is sufficient to truncate the 100-year flood hydrograph upstream of Churn Creek Bottom from

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12,700-cfs to 11,000-cfs. The length of the side channel weir is estimated to be 1750-feet based on a need to divert 1700-cfs and an elevation difference of 0.45-feet between the maximum water surface profile and the 11000-cfs water surface profile. The low intensity land use area on the opposite side of the Churn Creek channel downstream of the site adjacent to Kids Kingdom Park has a similar potential for attenuation of the flood peak. If developing both sites as off-channel regional detention, the combined volume available will be approximately 350 acre feet sufficient to attenuate the 100-year flood hydrograph upstream of Churn Creek Bottom from 12,600-cfs to 10,300-cfs. Figures 22 through 24 identify the areas available for potential off-channel regional detention, the available hydraulic head, and the City of Redding Churn Creek 100-year flood hydrograph with potential attenuation identified. The estimated potential benefit of the off-channel regional detention options at cross-section 32960 located downstream of the upper Churn Creek Road crossing of Churn Creek (near South Bonnyview) is identified on Figure 25 (includes local lateral inflow). Flood Bypass to Sacramento River: At their closest point upstream of Green Acres Drive, the Churn Creek channel is within 1600-feet of the Sacramento River channel. Diversion of flow from Churn Creek in excess of the incipient overflow near Green Acres Drive (approximately 7000-cfs) to the Sacramento River can be an effective approach to reduce flood risk in Churn Creek. The added flow to the Sacramento River will not increase flood risk along the Sacramento River because flow in the Sacramento River is reduced at times of high flow in the tributaries entering the river below Shasta Dam. Diversion of the Churn Creek flood peak to the Sacramento River will require construction of a side channel weir entrance structure, 2100-feet of channel or culvert including crossings at the ACID canal and I-5, and some type of outlet structure or erosion protection. The entrance structure will require approximately 250-feet of side channel weir with a top elevation approximately 3.5-feet below the 100-year flood profile and taper to the channel or culvert entrance width. The alignment considered for this analysis is identified on Figure 26. If an open channel is to be considered for the bypass, the channel will consist of two segments, one of approximately 700-feet from the Churn Creek channel through I-5 with a slope of 0.013 and the other of 1400-feet from the west side of I-5 to the Sacramento River with a slope of 0.0017. The upper segment will consist of a non-prismatic section having a 250-foot wide by 3.5-foot deep side channel weir entrance transitioning to a 70-foot wide by 5-foot deep downstream section. The narrow end of the upper segment will be under Interstate-5. The lower segment will consist of an 80-foot wide by 5-foot deep prismatic channel. Flow in the open channel will be supercritical for the entire length with velocities reaching 18-feet per second in the narrow portion of the upper segment and averaging 13-feet per second in the lower segment. Freeboard has not been included in the channel depths reported above. If a culvert structure is to be considered for the bypass, an entrance structure consisting of a side channel weir having a top elevation approximately 3.5-feet below the 100-year flood profile and a length of approximately 250-feet will be required. The entrance structure will taper to the width and depth of the culvert entrance at a distance approximately 100-feet downstream of the side channel weir. The culverts should have a hydraulically efficient entrance, be placed under the ACID canal (or pipe), and have a uniform slope to the Sacramento River. The slope of

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culverts having this configuration will be approximately 0.002 and velocities will range from 6- to 10-feet per second. The average hydraulic slope will be approximately 0.005. Three 6’ x 20’ barrels or fourteen 6’ diameter pipes will be required to convey the 5000-cfs flood peak between Churn Creek and the Sacramento River. Flow in the box culverts will be supercritical. Flood Relief Channel Parallel to Churn Creek: Unlike a flood relief channel from Churn Creek to the Sacramento River, a flood relief channel parallel to Churn Creek must be designed prismatic with a slope equal to the slope of the floodplain. The alignment of the candidate flood relief channel considered for this analysis is shown on Figure 27. Based on Manning’s Equation, the width of the upper segment (solid line) of channel sufficient to convey the 5000-cfs difference between the 100-year peak flow in the Churn Creek channel and the incipient overtopping flow are identified for a variety of depths and boundary materials in Table 4.

Table 4: Churn Creek to Churn Creek Flood Relief Channel Geometry, Prismatic Channel

Segment Slope Manning’s n Depth1 (feet) Width (feet) Segment 1, concrete, 4' deep 0.0019 0.011 4 89.1Segment 1, concrete, 5' deep 0.0019 0.011 5 63.9Segment 1, concrete, 6' deep 0.0019 0.011 6 49.4Segment 1, earth, clean, 4' 0.0019 0.028 4 219.3Segment 1, earth, clean, 5' 0.0019 0.028 5 153.9Segment 1, earth, clean, 6' 0.0019 0.028 6 116.2Segment 1, earth, reeds, 4' 0.0019 0.04 4 311.1Segment 1, earth, reeds, 5' 0.0019 0.04 5 217.2Segment 1, earth, reeds, 6' 0.0019 0.04 6 163.0

Notes: 1) Freeboard not included. The length of the upper segment of channel is approximately 5800-feet. The lower segments, if needed, will require similar dimensions as the upper segment. The lengths of lower segments are approximately 1250-feet for the middle segment and 1400-feet for the lower segment (dashed lines). The side channel weirs at the upstream end of each segment will have the same dimensional requirements as for the diversion from Churn Creek to the Sacramento River. There may also be requirements at the downstream ends of the flood relief channels to prevent stranding of fish as flood water subsides. Channel Conveyance Improvement: Channel conveyance improvements consist of widening the channel to accommodate greater flow thus reducing the frequency and magnitude of overflow. Geomorphic considerations dictate that the conveyance improvement not deepen the channel and environmental considerations dictate that the widening be above the water surface elevation of some threshold flow. The edge of perennial vegetation should be considered a lower limit of the elevation of a conveyance improvement. Vegetation in any reach of channel having a channel conveyance improvement will need to be managed in order for the conveyance improvement to remain

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efficient. This study has evaluated the potential requirements and flood risk benefit of a channel conveyance improvement by adjusting the channel width in a copy of the Current Condition CC2D COR 100-year hydrograph model data set. The width of the channel was increased by various factors between top of bank stations using the adjust stations option in the HEC-RAS cross-section editor. A factor of 1.5 or 50% increase in the width of the Churn Creek channel was found to substantially reduce overflow in the vicinity of Green Acres Drive. Channel widths were adjusted from just downstream of Knighton Road (Cross-section 15273) to a point downstream of the upper Churn Creek Road crossing of Churn Creek (Cross-section 32154). Plots of changes in the geometry of selected cross-sections are presented in Figures 28 and 29. Table 5 identifies the changes in channel area associated with the increased channel width. An actual channel conveyance improvement project (constructed widening as opposed to an assumption of a wider channel) will require similar increases in area in order to have similar benefits. A comparison of the 100-year flood profile with the conveyance improvement compared to the Current Condition CC2D COR hydrograph 100-year flood profile is presented on Figure 30. The maximum extent of inundation for the channel conveyance improvement run is presented in Figure 30.

Table 5: Conveyance Improvement, Increases in Channel Areas

Cross-section Area (sq ft) Cross-section Area (sq ft)

32154 327 25894 568 31138 572 25563 490 30189 359 24129 318 28903 369 22273 468 28345 383 19481 533 26626 490 18480 451 26004 378 18250 481 25968 359 15593 851 25924 536 15273 845

Summary of Results: Changes to the Churn Creek channel since publication of the current effective FEMA FIS and FIRM have not been great enough to affect a significant change to flood risk in Churn Creek Bottom. Evaluation of flood risk for current conditions considering dynamic flow and loss of flood water from the basin using a 2D modeling platform indicates flood risk in the lower reaches of Churn Creek within Churn Creek Bottom to be as represented by the FEMA FIS and FIRM but indicates flood risk in the vicinity of Green Acres Drive to be greater than represented by the FEMA FIS and FIRM. The higher flood risk for current conditions is due to mischaracterization of floodplain ground elevations as being lower than actual in the backwater model relied upon by FEMA for the current effective FIS and FIRM. Of the flood risk reduction alternatives investigated, only diversion of peak flows in Churn Creek directly to the Sacramento River is likely to be capable of addressing flood risk concerns in the vicinity of Green Acres Drive on its own. Other alternatives may be used in combinations

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to achieve the same goal. Descriptions of the potential flood risk benefit of each alternative along with additional considerations for each alternative are described below. Vegetation Management: Extension of the vegetation management program for the Churn Creek channel down to the mouth of Churn Creek was found to provide no direct flood risk benefit in the vicinity of Green Acres Drive. However, implementation of this alternative may be necessary to prevent any increase in flood risk from near Knighton Road to the mouth of Churn Creek associated with implementation of other alternatives including the Churn Creek to Churn Creek flood relief channel and the channel conveyance improvement. Both of these alternatives will increase the peak flow in Churn Creek at their downstream end by eliminating flood water from leaving the basin over I-5 and by reducing storage of flood water on the floodplain. If a vegetation management program is to be relied upon for reduction in flood risk recognized by FEMA, the program must be administered or overseen by a public agency and the land subject to management must be in a vegetation management easement. Upstream Off-Channel Detention: Although off-channel regional detention is efficient, the area necessary for a detention basin volume sufficient to address the flood risk concerns in the vicinity of Green Acres Drive is not available. A reasonable assumption of the maximum area available to implement an off-channel regional detention basin is only sufficient to reduce peak flows in Churn Creek to approximately 10,000-cfs. Incipient overtopping in the vicinity of Green Acres Drive occurs when flow in the Churn Creek channel exceeds approximately 7000-cfs. The reduction in Churn Creek peak flow associated with implementing off-channel detention results in a reduction in flood water leaving the basin over I-5 and a reduction in east overbank flow circumventing the Green Acres Drive area rather than reducing peak flows near Green Acres Drive. Off-channel detention may be used to reduce the size of flood relief channels or channel conveyance improvements. If off-channel detention is to be considered, the facility must be under the ownership and jurisdiction of a local agency. Design and knowledge of the actual flood risk benefit of off-channel detention facilities will require preparation of a backwater model with extra attention to detail in the vicinity of the proposed facility. The potential benefits of off-channel detention reported here are only rough estimates. Flood Relief Channel to Sacramento River: By design, a flood relief channel conveying Churn Creek flows in excess of the incipient overtopping flow of approximately 7000-cfs near Green Acres Drive will be capable of addressing the flood risk concerns in the vicinity of Green Acres Drive without requiring other candidate alternatives. The reduction in flood risk will extend downstream to the mouth of Churn Creek. In addition to the physical requirements of a diversion channel, challenges with this alternative include environmental concerns along both channels, crossings of the ACID canal and I-5, and property ownership. A reduced capacity flood relief channel may be used in conjunction with other alternatives to address the flood risk concerns in the vicinity of Green Acres Drive. Flood Relief Channel Parallel to Churn Creek: Also by design, a flood relief channel conveying Churn Creek flows in excess of the incipient overtopping flow around the vicinity of Green Acres Drive can be capable of addressing the flood risk concerns in the vicinity of Green Acres Drive. In addition to eliminating overflow in the vicinity of Green Acres Drive, the water surface elevations upstream in Churn Creek will be reduced enough to prevent flood flow from

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leaving the basin over I-5 and to prevent overflow in the east floodplain. Consequently, the incidental flood risk benefit associated with loss of flow from the Churn Creek basin over I-5 and storage of flood water in the floodplain under current conditions will be eliminated resulting in higher 100-year peak flows in the Churn Creek channel downstream of the parallel flood relief channel project. Residential structures near Meadowview Drive that are in the 100-year floodplain and already subject to relatively frequent flooding will be at higher risk of flood damage if implementing the parallel flood relief channel project without mitigating the increased downstream flood risk. The vegetation management alternative might be sufficient to mitigate increased downstream flood risk associated with implementing the parallel flood relief alternative. Channel Conveyance Improvement: Construction of a channel conveyance improvement consisting of widening the channel above an environmental threshold elevation can provide a significant reduction of overflow in the vicinity of Green Acres Drive but may not entirely eliminate overflow due to environmental and practical considerations. The potential flood risk benefit of a channel conveyance improvement was estimated by increasing the existing channel width and hence channel area by 50%. Considering the environmental requirement of staying above a threshold elevation such as may be defined by the edge of perennial vegetation, the actual width of a conveyance improvement representing a 50% increase in total channel area will be approximately equal to the existing channel width (100% increase in channel width). At this increase in width, it may be prudent to construct the conveyance improvement on both sides of the active channel. The existing middle Churn Creek Road bridge over Churn Creek (near Green Acres Drive) will have to be replaced with an appropriately longer bridge. Combined with other alternatives the conveyance improvement alternative may be sufficient to address the flood risk concerns near Green Acres Drive. Like the parallel flood relief channel alternative, the channel conveyance improvement alternative will prevent flood water from leaving the basin over I-5 and will prevent overflow in the east floodplain thus increasing peak flow in the Churn Creek channel downstream of the project. The increased flood risk associated with the increased peak flow downstream might be mitigated by implementing the vegetation management alternative. Conclusions: A combination of flood risk reduction alternatives may be the best approach to address the flood risk concerns in the vicinity of Green Acres Drive. Physical details identified by this analysis for the alternatives may be used for preliminary cost estimates necessary to further refine definition of candidate projects to meet the project objectives. Recommendations: Results of this analysis may be used to develop preliminary cost estimates of alternatives and combinations of alternatives however confident knowledge of the potential flood risk benefits and design details of the alternatives and combinations of alternatives will require additional backwater model runs representing the specific projects.

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With regard to any future FEMA map revision, the linear backwater model runs prepared for this study are substantially sufficient to meet the FEMA requirements for a map revision. Prior to relying on the dynamic backwater model runs, the following will need to be addressed:

1. FEMA should be consulted regarding changing the modeling platform and flood hydrologic analysis.

2. Lateral inflow hydrographs need to be revised. The backwater model relied upon for the current effective FEMA FIS and FIRM included only one lateral inflow representing all contributing flow downstream of the upstream boundary condition. This was preserved for all linear model runs used in this study and was replaced with only two lateral inflow hydrographs for the dynamic (City of Redding hydrographs) model runs.

3. FEMA should be consulted regarding redefinition of the designated floodway. At present, although theoretically possible, computation of a designated floodway cannot always be accomplished using a 2D backwater model.

The concept of a bypass conveying flood flow in Churn Creek to the Sacramento River has been considered for many years. More recently, the possibility of commercial development of the parcel located between I-5 and the Sacramento River has been discussed. Development of the parcel without regard to flood risk concerns along Churn Creek will likely eliminate any opportunity to develop what may be the most cost effective approach to reducing flood risk along Churn Creek to an acceptable level. Negotiation with potential developers of the property may be prudent to preserve opportunity to reduce flood losses or possibly to develop the Sacramento River bypass option.

Figure 1: FEMA FIS

Figure 2: Churn Creek Flood Hydrographs at Backwater Model Boundary upstream of Churn Creek Bottom

Figure 3: Lateral Inflow Hydrographs at Linden Drain upstream of Churn Creek Bottom

Figure 4: Churn Creek Flood Frequency Relationship at Backwater Model Boundary upstream of Churn Creek Bottom

Figure 5: Comparison of Duplicate Received and Duplicate Corrected 100-year Flood Profiles to Current Effective (Rev Encr) Flood Profile

Figure 6: Detail of 100-year Flood Profiles identified in Figure 5

Figure 7: Locations of Georeferenced Cross-sections

Figure 8: Comparison of Duplicate Unsteady 100-year Flood Profile with Current Effective (Rev Encr) Flood Profile

Figure 9: Comparison of Morphology Check 100-year Flood Profile with Duplicate Georeferenced 100-year Flood Profile

Figure 10: Cross-section 4294 Morphology Check

Figure 11: Cross-section 10605 Morphology Check

Figure 12: Cross-section 14582 Morphology Check

Figure 13: Cross-section 19481 Morphology Check

Figure 14: Cross-section 24129 Morphology Check

Figure 15: Cross-section 30189 Morphology Check

Figure 16: Comparison of Current Condition 2D Simulated Steady State and CC2D COR Hydrograph Backwater Model 100-year Flood Profiles to Current Effective (Rev Encr) Flood Profile

Figure 17: Comparison of FEMA Cross-section 28903 to LiDAR Terrain

Figure 18: Comparison of Current 2D Simulated Steady State and CC2D COR Hydrograph 100-year Flood Peak Flows in Churn Creek Channel to FEMA Current Effective 100-year Flood Peak

Figure 19: Area of Inundation Estimated for Current Condition Simulated Steady State 100-year Flood

Figure 20: Area of Inundation Estimated for CC2D COR Hydrograph 100-year Flood

Figure 21: Comparison of Vegetation Management (CC2D – Veg) 100-year Flood Profile to Current Condition Simulated Steady State Flood Profile (Both profiles computed using simulated steady state hydrograph)

Figure 22: Candidate Detention Basin Locations

Figure 23: Approximate Hydraulic Head Available for Upper Candidate Detention Basin Site

Figure 24: Approximate Potential Flood Peak Attenuation at Candidate Detention Basin Sites

Figure 25: Estimated Benefit of Off Channel Regional Detention Options at Cross-section 32960

Figure 26: Alignment Considered for Churn Creek to Sacramento River Flood Relief Channel

Figure 27: Alignment Considered for Flood Relief Channel Parallel to Churn Creek

Figure 28: Simulated Channel Conveyance Improvement, Cross-section 18250

Figure 29: Simulated Channel Conveyance Improvement, Cross-section 26626

Figure 30: Comparison of Channel Conveyance Improvement 100-year flood profile with Current Condition CC2D COR 100-year flood profile

Figure 31: Area of Inundation Estimated for Channel Conveyance Improvement