Analyzing Flood Risk from the I&M Canal s Non-Levee Embankments A Modified Application of FEMA s Natural Valley Procedure

Transcription

1 Analyzing Flood Risk from the I&M Canal s Non-Levee Embankments A Modified Application of FEMA s Natural Valley Procedure Tiffany Coleman, PE Senior Project Engineer Stantec/STARR September 10, 2015

2 How do we analyze risk to a community from a non-levee embankment upstream?.

3 Location Joliet, Will County, Illinois Chicago

4 Background Updated Countywide Study in Progress Contracted FEMA to review an area called out by USACE as weak link in flood protection system There are non-levee embankments along the Des Plaines River and I&M Canal Asked to identify risk to Joliet due to potential for embankment failure and define the regulatory floodplain

5 Agenda 1 Overview of Embankments 2 Natural Valley Procedure Joliet, Will County, Illinois Chicago 3 Hydraulic Structures/Rating Curves 4 Results 5 Next Steps

6 I&M Canal/ Des Plaines R. Embankments Historic Canal Built in 1848 Concrete Lock Wall along Des Plaines River ends at Ruby Street There are non-levee embankments along the Des Plaines River and I&M Canal

7 What is a Non-Levee Embankment? Levee A manmade structure, usually an earthen embankment, designed and constructed in accordance with sound engineering practice to contain, control, or divert the flow of water so as to reduce risk from temporary flooding (Source PM 43, FEMA) Non-Levee Embankment Typically highways or railroads built on fill in low lying areas that impose lateral constraints on flood flows. (Source Floodplain Mapping of Non-Levee & Non-Dam Embankments, ASPFM)

8 Are the I&M Canal Embankments Levees? A structure is a levee and subject to FEMA s LAMP procedure if it meets the following conditions: It was designed as a levee An owner has been identified for it It is operated and maintained as a levee It is hydraulically significant

9 Are the I&M Canal Embankments Levees? A structure is a levee and subject to FEMA s LAMP procedure if it meets the following conditions: It was designed as a levee - NO An owner has been identified for it It is operated and maintained as a levee It is hydraulically significant

10 Are the I&M Canal Embankments Levees? A structure is a levee and subject to FEMA s LAMP procedure if it meets the following conditions: It was designed as a levee - NO An owner has been identified for it - NO It is operated and maintained as a levee It is hydraulically significant

11 Are the I&M Canal Embankments Levees? A structure is a levee and subject to FEMA s LAMP procedure if it meets the following conditions: It was designed as a levee - NO An owner has been identified for it - NO It is operated and maintained as a levee - NO It is hydraulically significant

12 Are the I&M Canal Embankments Levees? A structure is a levee and subject to FEMA s LAMP procedure if it meets the following conditions: It was designed as a levee - NO An owner has been identified for it - NO It is operated and maintained as a levee - NO It is hydraulically significant - YES LAMP does not apply. ->Follow Guidance for Non-Levee Embankments

13 How do we analyze risk to a community from a non-levee embankment upstream?.

14 Natural Valley Natural Valley Criteria: Levee Doesn't Meet and Doesn t Impact the Flood Elevation Mapping Approach: Natural Valley Floodplain Analysis Only to Map Special Flood Hazard Area

15 Natural Valley Only provides flood elevations adjacent to the embankment Does not compute discharge into downstream areas

16 Modified -Natural Valley Modified Natural Valley Procedure Based on steady-state effective hydraulic study Disregard hydraulic impact of non-levee embankment segment Provide an inflow to route downstream through the City Account for flow constriction at culverts and flow into City sewer system

17 Approach Disregard hydraulic impacts of embankment Assume landward side of embankments acts as a bathtub Compute WSEL and flows at each outlet (neglect small conduits) Account for constrictions that would reduce flow Q Storm Sewer Q Overland

18 Hydraulics Features/ Flow Paths LEGEND Normal Flow Direction Potential Flow Reversal Hydraulic Feature Embankment Crest

19 Modified -Natural Valley Procedure 1. Identify hydraulic structures 2. Compute rating curves for a range of tailwater elevations 3. Iterative process Compute Q at each structure based on tailwater from previous iteration

20 Modified -Natural Valley Hydraulic Features Feature Number Description 1 Power Plant Culvert 2 Railroad Culvert 3 Lateral Box Culvert 4 Main Culvert (conveys flow from ditch to storm sewer) 5 Storm Sewer Inlet 6 Overland Flow into City at Columbia Street

21 Modified -Natural Valley I&M Canal Outlet to Des Plaines River The I&M Canal Channel Is Not a Major Constriction (neglected)

22 Modified -Natural Valley Hydraulic Features 1 The Power Plant Culvert is a major constriction (Flow from upstream embankments were neglected)

23 Modified -Natural Valley Hydraulic Features Lateral Box Culvert 6 Span, 7 Rise 2 Railroad Double Box Culvert (Rating Curves were developed to analyze flow through this culvert)

24 Modified -Natural Valley Hydraulic Features 3 Lateral Box Culvert (6 X7 ) (Rating Curves were Developed to analyze flow through this culvert)

25 Modified -Natural Valley Hydraulic Features 4 Main Culvert (Rating Curves were Developed to analyze flow through this culvert)

26 Modified -Natural Valley Hydraulic Features 5 Storm Sewer Inlet (Neglected due to small opening)

27 Modified -Natural Valley Main Culvert Plan/Profile 5 4 Plan Profile

28 Modified -Natural Valley Storm Sewer in Concrete Lock Wall

29 Step 1 Compute River Elevation in HEC-RAS Step 2 Compute Flow into Each Outlet based on tailwater Step 3 Recompute River Flow

30 Modified -Natural Valley ITERATIONS Iteration River (NAVD 88) Main Culvert TW/ Flow Lateral Culvert TW/ Flow Railroad Culvert TW/ Flow Sum Outflow /287 0/326 0/773 1, / / /735 1, / / /737 1,328

31 Modified -Natural Valley RESULTS Feature Number Flow 1 Power Plant Culvert (NEGLECTED) 2 Railroad Culvert (740 cfs) 3 Lateral Box Culvert (310 cfs) 4 Main Culvert (280 cfs) 5 Storm Sewer Inlet (NEGLECTED) 6 Overland Flow into City at Columbia Street (1,050 cfs) Flow Into City

32 Next Steps Illinois State Water Survey is continuing study. Develop an unsteady hydrograph using a gage at Ruby Street Route flow downstream using 2D hydraulic modeling Develop flood mapping with a combination of AH and AO Zones

33 How do we analyze risk to a community from a non-levee embankment upstream?.

34 Analyzing Risk to DS Community Review Site Specific Information Discuss Risk and Study Approach with Community Compute Flow and Route Downstream

35 References Floodplain Mapping of Non-Levee & Non-Dam Embankments, ASFPM, 2011 Procedure Memorandum No. 51, FEMA, 2009

36 Questions?