PRENTISS CREEK (SUB-E) KENSINGTON PLACE CONCEPT PLAN VILLAGE OF DOWNERS GROVE, ILLINOIS AUGUST, 2011

Transcription

1 PRENTISS CREEK (SUB-E) KENSINGTON PLACE CONCEPT PLAN VILLAGE OF DOWNERS GROVE, ILLINOIS AUGUST, 2011 Submitted to: NATHANIEL HAWK, STAFF ENGINEER VILLAGE OF DOWNERS GROVE 5101 WALNUT AVENUE DOWNERS GROVE, IL Prepared By:

2 2 MAKING A DIFFERENCE FOR STREAMS, RIVERS & LAKES Stream Restoration Lake and Wetland Management Water Quality Control Stormwater Engineering August 29, 2011 Nathaniel Hawk Staff Engineer Village of Downers Grove 5101 Walnut Avenue Downers Grove, IL60515 Re: PRENTISS CREEK (SUB E) - KENSINGTON PLACE CONCEPT PLAN Village of Downers Grove Dear Mr. Hawk: Thank you for the opportunity to submit this Prentiss Creek (Sub E) Kensington Place Concept Plan to the Village of Downers Grove. The Prentiss Creek Project Site includes approximately 730 LF of stream channel and on-line impoundment area along Prentiss Creek. Three (3) Options have been provided that would enhance existing conditions. These Options range from 1) Dredging, 2) Shoreline Stabilization, and 3) Dam Removal with Stream and Wetland Restoration. These Options differ based on long-term cost effectiveness, whether or not they include an environmentally sound approach, and the degree to which water quality control, recreational opportunity, aesthetic value, wildlife benefits, and other objectives are provided. Based on all factors considered, we recommend either Option # 3a or # 3b Dam Removal and Stream Restoration, or a combination of options including Dam Removal and Stream Restoration (such as # 4a or Option # 4b) be provided. Please contact me at to answer any questions or to set up a meeting to discuss steps towards possible implementation. Best Regards, Ted Gray, PE, CFM, CPESC Eco-Hydrologist & Engineer

3 3 Background: The Village of Downers Grove (Village) is evaluating potential alternatives to provide long-term, cost-effective improvements for existing on-line impoundment facility located along Prentiss Creek in the Kensington Place Subdivision east of Woodward Avenue and south of Loomes Avenue (Project Site). Site impacts at the Project Site include sediment deposition, mud flats, low water levels and other undesirable conditions. The Village of Downers Grove is working with the homeowners associations at the Project Site, as well as adjacent properties, to review alternative improvement options and recommendations. Three Options were evaluated for the Project Site: Option # 1: Dredging and Sediment Removal, Option # 2: Bank Stabilization / Shoreline Enhancement, and Option # 3: Dam Removal with Stream and Wetland Restoration. This Narrative Report describes the Project Site, site impacts, causes and sources of impacts, a description of each option, as well as advantages, disadvantages, costs, and permit considerations for each option. Project Area Description: Prentiss Creek Watershed Prentiss Creek flows through Downers Grove, DuPage County, Illinois (Fig. 1). The stream flows from east to west as an intermittent channel. The entire length of the stream channel through Woodward Avenue is approximately 0.85 miles in length. The watershed area through the Project Site is approximately 3.9 square miles in area. Most of the watershed is developed. Land uses include residential, commercial, industrial, and open space. Prentiss Creek has been channelized upstream of the Project Site, at least since 1939 (see Fig. 2). Prior to agricultural development, the pre-settlement vegetation type was dominated by prairie. Downstream of the Project Site, Prentiss Creek flows through Woodridge and then through the Green Valley Forest Preserve. Prentiss Creek then discharges into the East Branch DuPage River.

4 4 PROJECT SITE Fig. 1. Prentiss Creek sub-watershed boundary area and Project Site. The Project Site is located upstream of Woodward Avenue, south of 63 rd Street, and west of Prentiss Drive. The watershed area draining to the Project Site is approximately 3.9 square miles. Most of the watershed is urbanized. PROJECT SITE 1939 Fig. 2: 1939 aerial photograph of the Project site. East of Woodward Avenue, the stream had been channelized. West of Woodward Avenue, meanders occurred. Pre-settlement vegetation was prairie.

5 5 Project Site The proposed Project Site is located between Woodward Avenue to the west and downstream of Prentiss Drive. The riparian corridor at the Project Site is owned by the Village of Downers Grove and by the homeowners associations. The water surface area within the existing impoundment at the Project Site is approximately 1 acre in surface area. The length of stream channel within the impounded portion of the Project Site is approximately 730 linear feet (see also Concept Plans) Fig. 3: The 1974 aerial photograph depicts local development and the on-line impoundment. A small impounded area (depicted by arrow) also occurred upstream of the impoundment Fig. 4: By 2011, it is estimated that 3,000 cubic yards of silt became deposited in the impoundment. Existing water depths are typically 0.5 feet or less, due to the accumulation of silt. Algae blooms are visible (source: Google). By 2011, the small impounded area (see arrow) was also filled with silt.

6 6 Based on available aerial photographs, the impoundment was created between 1961 and Based on past topographic map information, water depths when the impoundment was constructed may have been 4 feet or deeper (Fig. 5). 4-FT+ Water Depths Fig. 5: Based on past topographic information, there are indications that water depths within the impoundment were 4 feet or deeper. By 2011 average water depths are reduced to 0.5 feet or lower (Photo 1). Since construction of the impoundment, large accumulations of silt have become deposited. Based on existing thickness of silt deposits, as well as past topographic maps, it is possible that approximately 3,000 cubic yards of silt have been deposited in the impounded area. Photo 1: Existing (2011) site conditions include silt deposits, aesthetic impacts, aquatic habitat impacts, and nuisance algae blooms. (Photo from Woodward Avenue, facing east.)

7 Resulting problems at the Project Site have included water quality impacts due to silt deposition, aesthetic impacts, degradation of aquatic habitat, nuisance algae blooms, odors due to extensive silt deposits, lack of dissolved oxygen, and other factors. 7 Photo 2: Silt deposits within the impoundment become exposed during water drawdown events. (Photo from east impoundment area, facing west Woodward Avenue in background.) Photo 3: The existing outlet structure at Woodward Avenue raises the water level to allow the impoundment to form. But this also reduces stream slope and acts to trap silt within the impounded area. These types of structures are also a barrier to fish migration from downstream areas. Causes of Problems: Causes of existing problems include the following. Streams convey both sediment and water as a natural process. However, on-line impoundments, such as Prentiss Creek at Kensington (Sub E), not only reduce the stream slope, but they also substantially reduce water velocity. As a result, sediment particles are no longer

8 conveyed, but they settle out of the water column and become deposited on the bottom of the impoundment. Due to an inability to generate scouring velocities, sediment deposits remain onsite. Typically heavier sediment particles (gravel and sand) become deposited at the upstream end of an impoundment. Lighter particles that remain suspended for a longer time period (silts and clays) become deposited farther downstream within the impoundment. These sediment deposition patterns have been observed at the Project Site. The stream channel upstream of the impoundment contains cobble and gravel materials on the stream channel bottom. As the impoundment is approached, smaller particles (sand) becomes predominant. Farther downstream within the impoundment, silt and clay deposition is common. The top of the sediment is covered by an organic matrix, due in part to decaying algae. According to available topographic data, the stream profile upstream of Woodward Avenue is approximately ft/ft. This represents a fall of approximately 0.38 ft for each 100 ft of longitudinal distance. In contrast, the slope within the impounded area is only approximately ft/ft. This is 400% reduction in slope compared with the free-flowing stream channel. Downstream of Woodward Avenue the stream slope is 800% higher (fall of 0.7 ft per 100 ft of longitudinal distance) compared with the impounded area. The low slope at the impoundment increases the efficiency with which the existing impoundment traps sediment transported from upstream areas. 8 Fig. 6: According to the DuPage County Soil Survey, the local soil type at the Project Site is Orthents, clayey, undulating. Clay soils are typically more resistant to erosion than other soil types, such as loam. However, even clay soils are subject to shoreline or streambank erosion. Sources of sediment flowing into the impoundment include the following. Severe streambank erosion was noted along the inlet channel that flows into the

9 impoundment from Loomes Avenue. According to the U.S. Geological Survey, up to 50% of the inflowing silt can originate from streambank erosion. In addition to this source of silt, washoff of solids can occur from sewered roads, parking lots, rooftops, commercial/industrial areas, and other urban areas of the watershed. In addition, onsite algae blooms contribute to decaying organic matter that contributes to filling of the impoundment. According to methods by Illinois EPA, it is possible that the 3.9 square mile watershed may contribute up to 750 cubic yards (wet volume) of inflowing sediment into the impoundment, each year. If this entire volume were deposited, this represents enough silt to fill up the available storage capacity of the impoundment within just a 4-year time period. When an impoundment is excavated, the efficiency for trapping sediment only increases. Therefore, even if the impoundment were re-excavated, it is likely the dredged volume would re-fill with silt perhaps within just several years. 9 Photo 3: Existing severe streambank erosion observed along the inlet channel that flows from near Loomes Avenue into the impoundment. In some watersheds, up to 50% of the inflowing sediment can be attributed to streambank erosion. Watershed development such as the installation of impervious surfaces as well as storm sewer systems has substantially contributed to existing degraded conditions at the Project Site. For instance, urbanization of the watershed reduces water levels during dry-season conditions. Drawdown of water levels contributes to exposing large mud flat deposits during low-water periods. This can occur because urbanization reduces the amount of rainfall that can infiltrate into the ground, thereby reducing replenishment of subsurface groundwater supplies. Without sufficient groundwater, stream flow and water depths can become so low that they threaten aquatic life present in the stream. The impounded conditions also worsen

10 water quality by raising water temperatures, reducing dissolved oxygen, and reducing desirable aquatic habitat for stream-dwelling species. 10 Proposed Alternative Solutions: Three alternative solutions were evaluated for Prentiss Creek at Kensington Place (Sub E). These alternatives included: 1) Dredging for Sediment Removal, 2) Bank Stabilization / Shoreline Enhancement, and 3) Dam Removal with Stream and Wetland Restoration. A description of each proposed alternative is as follows. Option 1: Dredging for Sediment Removal Option 1 Dredging for Sediment Removal was evaluated. It was assumed that enough sediment removal would be provided to maintain an average water depth of 2 feet in the approximately 1-acre impounded area. It is estimated that at least 3,000 cubic yards of sediment would need to removed in order to accomplish this objective. As a second option, over 5,500 cubic yards of sediment was evaluated for removal to provide an average 3.5 foot depth in the impoundment. In either case, it is likely that a safety shelf would need to be installed around the perimeter of the pond. Advantages: - Returns water depths to past conditions - Large area of open water for those wanting the historic condition - If permit agencies consider the site to be a detention facility, then dredging may be considered a maintenance project which can reduce permitting requirements. - Construction of the Sediment Forebay upstream of the impoundment will assist with trapping sediment from upstream areas before it enters the impoundment.

11 11 Disadvantages: - High long-term costs, since sediment deposition will continue to occur, necessitating probable future re-dredging. - Dredging is a temporary, costly solution. For instance, according to methods by Illinois EPA, it is possible that the 3.9 square mile watershed may contribute up to 750 cubic yards (wet volume) of inflowing sediment into the impoundment, each year. If this entire volume were deposited, this represents enough silt to fill up the available storage capacity of the impoundment within just a 4-year time period. When an impoundment is excavated, the efficiency for trapping sediment only increases. Therefore, even if the impoundment were re-excavated, it is likely the dredged volume would re-fill with silt perhaps within just several years. - Does not address impacts to water quality or impacts to aquatic life caused by on-line impoundments - If sediment toxicity is present, it may escalate disposal costs, permitting requirements, and/or permit-related construction costs beyond the existing cost projections. Permit Considerations: - See Disadvantages above. - Agencies likely to require permit review for Option # 1: U.S. Army Corps of Engineers plus joint review agencies (IDNR, FWS, IEPA, SHPO) Illinois EPA - Materials Data Analysis will be required. If sediment toxicity is present, it may escalate disposal costs, permitting requirements, and/or permit-related construction costs beyond the existing cost projections. Kane/DuPage Soil and Water Conservation District Illinois Department of Natural Resources - Office of Water Resources Village of Downers Grove - No hydraulic modeling should be necessary, since the existing outlet structure would not be modified. Option 1 simply proposes to change the section below the hydraulic control at Woodward and will have little impact with the possible exception of lowering the hydraulic profile a small amount. From a permitting perspective, this potential lowering

12 should not be modeled nor should it be counted on since the dredging activities would fall under a maintenance activity and future sedimentation would then impact the hydraulic profile negatively. - It remains unclear whether the Project Site was designed as a stormwater management facility. If so, then dredging would likely be considered a maintenance activity provided that a) sediment is controlled, b) downstream impacts are controlled, and c) shoreline naturalization is a component of excavation, and d) no long-term impacts would occur. Furthermore, wetlands formed within stormwater management facilities may not be subject to wetland regulations. - Any work within Prentiss Creek would at a minimum, require authorization from the U.S. Army Corps of Engineers. If a Sediment Forebay is constructed, then the Corps requests that only silt be removed, that as much existing wetland as practicable be maintained, and that the adjacent shoreline be restored to native plantings. Activities that involve filling or discharging or dredging within the waterway are subject to regulation under the federal Clean Water Act. The area of regulation typically includes the watercourse up to the Ordinary High Water Mark (OHWM), including any natural wetland areas extending beyond the OHWM. - A Wetland and Riparian submittal (Tab 4 and 5, respectively) would also need approval from the Village. 12 Other Factors: - Option 1 Dredging is considered an unsustainable, short-term option. It is relatively expensive, and it will require additional expenditures in the near future, potentially within several years, in order to sustain the constructed conditions. Therefore, long-term costs in order to sustain Option 1 Dredging are considered high. - Certain elements of Option # 1, possibly including construction of a Sediment Forebay upstream of the impoundment to trap sediment from upstream areas, may be desirable regardless of which Option is selected. Costs Two cost options were evaluated based on the amount of sediment removed:

13 - Cost Option # 1a = $354,298 to remove 3,000 cubic yards of silt and provide a temporary average water depth of 2 feet. - Cost Option # 1b = $502,345 to remove 5,500 cubic yards of silt to provide a temporary average water depth of 3.5 feet 13

14 Option # 2: Bank Stabilization / Shoreline Enhancement Option # 2 Bank Stabilization / Shoreline Enhancement includes Bioengineering stabilization for existing eroding shoreline areas as well as replacement of invasive shrubs and trees with deep-rooted native species to enhance shoreline stabilization. However, Option # 2 does not address the exposed mud flats and sediment deposition within the impoundment. 14 BEFORE AFTER Photos 4 through 5: Example conversion of eroding shoreline with invasive shrubs (left) into a stabilized shoreline (right) with native plantings and shoreline wetland vegetation. Advantages: - Shoreline stabilization and enhancement - Reduction of sediment deposition within the impoundment from shoreline erosion Disadvantages: - No removal of sediment deposits - No stabilization of sediment deposits - No resolution for odors, aesthetic impacts within the impoundment, water quality impairment, or other factors.

15 15 Permit Considerations: - No hydraulic modeling may be necessary, since the existing outlet structure would not be modified. Option 2 proposes to restore the corridor through stabilization and restoration of the banks and vegetation along the shoreline of the facility. Such modification will not have an appreciable impact on the Manning's 'n', or resistance to flow, for this reach. Also, the sections associated with the facility are significantly wider than the rest of the stream, which will result in low velocities through this reach. This means any proposed changes to the vegetation will have little, if any, hydraulic impact and should not require modeling. - Agencies likely to require permit review for Option # 1: U.S. Army Corps of Engineers notification (permit submittal likely not necessary if no fill or discharge occurs in waters of the U.S.) Kane/DuPage Soil and Water Conservation District Village of Downers Grove Wetland and Riparian submittal (Tab 4 and 5, respectively) would also need approval from the Village. Other Factors: - Due to the Disadvantages described above, Option # 2 Shoreline Stabilization is not considered to be a comprehensive solution for existing problems. However, it does provide a desirable shoreline enhancement that would also reduce sediment deposition within the impoundment. Therefore, at least portions of Option # 2 may be desirable to implement to supplement one of the other selection Options. Costs - Cost Option # 2 = $254,421 for Bank Stabilization / Shoreline Enhancement. - This includes conversion of the shoreline from invasive species into native species, as well as over 556 linear feet of biotechnical bank stabilization, such as the eroding Loomes Avenue inlet area.

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17 Option 3: Dam Removal with Stream and Wetland Restoration Option 3 Dam Removal with Stream and Wetland Restoration is considered an environmentally sound solution for existing problems. Option # 3 provides cost effective long-term elimination of exposed sediment and mud deposits within the impoundment. With Option # 3, in place of mud flats and nuisance algae blooms, a restored wetland would be constructed. Sediment deposits would be stabilized. A meandering stream channel would flow through the open space area. Native plant seed and plant plugs would be installed throughout the basin area (see Photos 6 through 10 below). 17 BEFORE AFTER BEFORE AFTER Photos 6 through 10 above: Example dam removal and stream restoration project with conversion of mud flats into meandering stream channel with native wetland and prairie.

18 Methods for dam removal could include the following. Dam removal could be accomplished by a phased removal of the center of the existing concrete outlet structure. In order to achieve a desirable stream gradient, comparable to upstream and downstream areas, the concrete invert of the central box culvert (of three existing box culverts) below Woodward Avenue would also be phased. The culvert invert would be sawcut and the stream invert elevation lowered to provide an acceptable stream gradient that will enable a balance between water and sediment transport. This will also protect downstream and upstream areas from disturbance. The channel bottom below Woodward Avenue would be armored at the new, lower elevation, using large rock materials. The outlet area would also be stabilized. 18 Advantages: - Environmentally sound elimination of mud flats and sediment deposits - High long-term cost effectiveness for sediment control - Stream restoration - Wetland restoration - Water quality improvement - Reduced nuisance algae blooms - Reduced odors - Sediment stabilization during the dam removal process using Option # 3a. - Fish passage would be restored through the Project Site - Increased diversity of birds, stream fishes, and other wildlife - Permit agencies may not require sediment testing since the sediment would be stabilized in-place with native wetland and prairie vegetation. - As a possibility, if the project creates new wetland, wetland buffer, or riparian habitat, these may be used to off-set either adverse impacts or mitigation responsibilities for this and/or other projects in the watershed. Enhancement of existing wetlands, buffers, and riparian habitat may also be used for mitigation purposes. - No removal of sediment necessary

19 19 Disadvantages: - Reduced area of open water (although this open water is being converted at present into exposed mud flats). - Permit requirements would require hydraulic modeling to be performed for permit approval. - At least some downstream discharge of sediment would occur. However, streams naturally convey both water and sediment. Permit Considerations: - See Hydraulic Modeling Feasibility Analysis below Option # 3 would require hydraulic modeling to be performed for permit approval, since the outlet structure would be modified. Option 3 will have a hydraulic impact on the flood profile for this reach due to the proposal to lower the thalweg of the existing box culvert, which conveys flow under Woodward Avenue. The lowering of the thalweg through the center box culvert by approximately one foot with no other modifications to the other box culverts would result in a lowered hydraulic profile for the 100-year event by almost 0.2 feet at the confluence just upstream of the on-line facility. Modification of the outer two box culverts under Woodward Avenue through construction of a concrete slab over the inverts would mitigate for the lowering of the hydraulic profile by preserving the limited open area of the existing box culverts. - The Village of Downers Grove is seeking to preserve existing stormwater storage capacity (volume above the existing normal water level control elevation) in a proposed condition. - Agencies likely to require permit review for Option # 1: DuPage County (floodway modifications review) Illinois Department of Natural Resources - Office of Water Resources U.S. Army Corps of Engineers plus joint review agencies (IDNR, FWS, IEPA, SHPO) Illinois EPA U.S. Fish & Wildlife Service Kane/DuPage Soil and Water Conservation District Village of Downers Grove

20 - Permit approvals for the proposed work will require a permit from DuPage County since the construction activities will occur within the Floodway of Prentiss Creek. The permit submittal will likely require two separate hydraulic models, one series of models using the regulatory information (1978-era HEC-2 converted to HEC-RAS and then modified to reflect the existing condition) and one series of models using the DuPage County FEQ information with modifications to reflect the existing and proposed conditions, which would then be compared to the original FEQ model. Further, it is likely that a separate Floodway model for at least the HEC- RAS regulatory information will need to be created, if not for both the HEC-RAS and FEQ models. - It remains unclear whether the Project Site was designed as a stormwater management facility. If so, then wetlands formed within stormwater management facilities are generally not subject to wetland regulations. - Regarding possible excavation of a Sediment Forebay alone, a preliminary discussion with the U.S. Army Corps of Engineers (ACOE) suggest that minor excavation of accumulated silt (Sediment Forebay) would be permitted, without mitigation, if water quality and wetland habitat is not significantly impacted. - Any work within Prentiss Creek would at a minimum, require authorization from the U.S. Army Corps of Engineers. Activities that involve filling or discharging or dredging within the waterway are subject to regulation under the federal Clean Water Act. The area of regulation typically includes the watercourse up to the Ordinary High Water Mark (OHWM), including any natural wetland areas extending beyond the OHWM. - A Wetland and Riparian submittal (Tab 4 and 5, respectively) would also need approval from the Village. 20 Other Factors: - Option # 3 Dam Removal with Stream and Wetland Restoration is considered the most environmentally sound, long-term, cost-effective solution available for this Project Site. - Desirable components of other Options discussed, such as a) Option # 1 Sediment Forebay installed upstream of the impoundment to trap sediment from upstream areas, and/or b) Shoreline Stabilization per Option # 2 may be desirable regardless of which Option is selected

21 21 Costs Two cost options were considered for Dam Removal with Stream Restoration - Cost Option # 3a = $468,726 includes not only dam removal, stream restoration, installation of Fiber Roll to retain some sediment, and installation of plantings, but also installation of structures such as Rock Vane Grade Control or Rock Points to provide sediment stabilization during the dam removal process (sediment destabilization could otherwise be more substantial during dam removal phase). Moreover, these same structures will facilitate long-term stream stabilization at the Project Site. - Cost Option # 3b = $326,814 will accomplish similar objectives as Option # 3a (dam removal, stream restoration, installation of Fiber Roll to retain some sediment, and installation of plantings). However,

22 Option # 3b does not include structures such as Rock Vane Grade Control or Rock Points to provide short-term and long-term stabilization of sediment deposits. It is more likely that increased discharge of sediment deposits from the impounded area will be discharged downstream under Option # 3b. 22 Other Options & Considerations In addition to the three basic Options discussed above, other combinations of options could also occur. For instance, a combination of Option # 3 Dam Removal with Stream and Wetland Restoration, as well as installation of a Sediment Forebay and stabilization of the Loomes Avenue inlet channel (severe erosion) could occur. This Option # 4a would have a probable cost of $531,973 if grade control were included with stream stabilization. If grade control were not included, then Option # 4b would have a probable cost of $391,786.

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24 24 Hydraulic Modeling Feasibility Analysis Dam Removal: The proposal to modify the on-line facility east of Woodward Avenue, described under Options for Dam Removal and Stream Restoration, necessitated the preparation of hydraulic models to understand the potential hydraulic impacts of the solutions proposed for this section of Prentiss Creek. The hydraulic models were created in HEC-RAS using the existing hydraulic data provided in the original DuPage County FEQ model for Prentiss Creek. Four (4) models were created to better understand the hydraulic situation at the on-line facility and the potential permitting considerations for the project options discussed in this report. The four models prepared are listed as follows: 1. The original model was created as a "direct" conversion from the FEQ model data supplied by the Village to provide baseline conditions for comparison. 2. The second model proposed to lower the invert of the central box culvert at Woodward Avenue with no other improvements to the upstream reach. 3. The third model proposed to lower the invert of the central box culvert at Woodward Avenue with reduction of capacity in the other two culverts through a layer of concrete on the bottom and no other improvements to the upstream reach. 4. The fourth model proposed to lower the invert of the central box culvert at Woodward Avenue with reduction of capacity in the other two culverts through a layer of concrete on the bottom and also improvements in the form of rock vanes and weirs to provide for the establishment of a defined stream channel in the upstream reach. The results from the hydraulic models indicate that the proposed improvements will not significantly impact the hydraulic character of the stream corridor and that any of the proposed options can be permitted. It should be noted that, in the case of Option # 3, care will need to be exercised in order to preserve the hydraulic character of the stream and maintain the stream storage presently being utilized by the on-line facility. We note that the original FEQ model is unclear with regard to the hydraulic character of the Woodward Avenue box culvert and weir complex. The elevations in the FEQUTL input file seem to describe the upstream end of the box culverts as having the same elevation as the upstream weir. Field observation indicates that this is not the case. It is likely that future permitting, for this project or others on this corridor, will require clarification of this hydraulic condition, through

25 additional survey and updated FEQ modeling. This concept study did not require revisions to establish a more exact hydraulic understanding, since the concepts are comparative in nature and seek only an understanding of potential permitting considerations. Hydraulic Modeling Results/Conclusions The proposed restoration and stabilization work on the Prentiss Creek facility east of Woodward Avenue should not affect the hydraulic characteristics of Prentiss Creek, with the exception of Option Option 1 simply proposes to change the section below the hydraulic control at Woodward and will have little impact with the possible exception of lowering the hydraulic profile a small amount. From a permitting perspective, this potential lowering should not be modeled nor should it be counted on since the dredging activities would fall under a maintenance activity and future sedimentation would then impact the hydraulic profile negatively. Option 2 proposes to restore the corridor through stabilization and restoration of the banks and vegetation along the shoreline of the facility. Such modification will not have an appreciable impact on the Manning's 'n', or resistance to flow, for this reach. Also, the sections associated with the facility are significantly wider than the rest of the stream, which will result in low velocities through this reach. This means any proposed changes to the vegetation will have little, if any, hydraulic impact and should not require modeling. Option 3 will have a hydraulic impact on the flood profile for this reach due to the proposal to lower the thalweg of the existing box culvert, which conveys flow under Woodward Avenue. The lowering of the thalweg through the center box culvert by approximately one foot with no other modifications to the other box culverts would result in a lowered hydraulic profile for the 100-year event by almost 0.2 feet at the confluence just upstream of the on-line facility. Modification of the outer two box culverts under Woodward Avenue through construction of a concrete slab over the inverts would mitigate for the lowering of the hydraulic profile by preserving the limited open area of the existing box culverts. Hydraulic Modeling Permit Considerations Permit approvals for the proposed work will require a permit from DuPage County since the construction activities will occur within the Floodway of Prentiss Creek. The permit submittal will likely require two separate hydraulic models, one series

26 of models using the regulatory information (1978-era HEC-2 converted to HEC- RAS and then modified to reflect the existing condition) and one series of models using the DuPage County FEQ information with modifications to reflect the existing and proposed conditions, which would then be compared to the original FEQ model. Further, it is likely that a separate Floodway model for at least the HEC-RAS regulatory information will need to be created, if not for both the HEC- RAS and FEQ models. 26 Hydraulic Modeling Results and Comparisons: Model Description 100-year 10-year Approx. 2-year Discharge Elev. Discharge Elev. Discharge Elev. Location: Downstream of Woodward Ave Culvert 1 - Correctedcs Base Conversion Corrected- Opt3O Drop Center Culvert Corrected- Opt3 Drop Center Culvert, Fill Inverts Corrected- Opt3A Drop Center Culvert, Fill Inverts, Rock Vanes Location: Upstream of Woodward Ave Culvert 1 - Correctedcs Base Conversion Corrected- Opt3O Drop Center Culvert Corrected- Opt3 Drop Center Culvert, Fill Inverts Corrected- Opt3A Drop Center Culvert, Fill Inverts, Rock Vanes Location: Confluence 1 - Correctedcs Base Conversion Corrected- Opt3O Drop Center Culvert Corrected- Opt3 Drop Center Culvert, Fill Inverts Corrected- Opt3A Drop Center Culvert, Fill Inverts, Rock Vanes Notes: 1. The 10 and 100-year information was based in flows and starting WSEL taken from the FIS.

27 2. The approximate 2-year information was based in flows from the FEQ and an estimated starting WSEL. 27 Summary Recommendation: Based on all information as provided above, we recommend that either Option # 3a, # 3b, # 4a, or # 4b be considered for implementation at the Project Site. This is because these options provide a long-term cost effective approach, environmentally sound techniques, and because multiple objectives and benefits will be provided to the community.

28 PRENTISS CREEK (SUB E) - KENSINGTON PLACE CONCEPT PLAN VILLAGE OF DOWNERS GROVE, ILLINOIS INDEX TO SHEETS SHEET 1 - COVER SHEET SHEET 2 - EXISTING CONDIITONS SHEET 3 - OPTION # 1 DREDGING SHEET 4 - OPTION # 2 SHORELINE STABILIZATION SHEET 5 - OPTION # 3 DAM REMOVAL & STREAM RESTORATION SHEET 6 - DETAILS OWNER VILLAGE OF DOWNERS GROVE 5101 WALNUT AVENUE DOWNERS GROVE, IL CONTACT: NATHANIEL HAWK ENGINEER LIVING WATERS CONSULTANTS, INC. 1 S. 132 SUMMIT AVE, SUITE 304 OAKBROOK TERRACE, IL OFFICE FAX CONTACT: TED GRAY, PE, CFM, CPESC PROJECT LOCATION PRENTISS CREEK (SUB E) - KENSINGTON PLACE. SOUTH OF 63RD STREET, EAST OF WOODWARD AVENUE, NORTH OF PRENTISS DRIVE, VILLAGE OF DOWNERS GROVE, DUPAGE COUNTY, ILLINOIS. WETLAND CONSULTANT JAMES J. BENES & ASSOCIATES 950 WARRENVILLE ROAD SUITE 101 LISLE, IL OFFICE FAX CONTACT: DAVID KOLDOFF Call LIVING WATERS CONSULTANTS, INC. 1 S. 132 SUMMIT AVE, SUITE 304 OAKBROOK TERRACE, IL TEL: FAX: SHEET 1: COVER SHEET PRENTISS CREEK - KENSINGTON PLACE VILLAGE OF DOWNERS GROVE SHEET 1 OF 6

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