SECTION 5.0 DETAILED ANALYSIS OF LITTLE MINNESOTA RIVER FLOODWAY AND TOELLE COULEE FLOOD MITIGATION ALTERNATIVES 5.1 LITTLE MINNESOTA AND TOELLE COULEE ALTERNATIVES Based upon the information presented within Section 4.0, Range of Flood Mitigation Alternatives Considered, discussions with the BVFMTF, and consultation with the UMRWD, the range of Little Minnesota River flood mitigation alternatives became reduced to: 1) a Little Minnesota River floodway; and, 2) storage, a floodway, a levee or some combination thereof for Toelle Coulee. This section provides descriptions, detailed information on the engineering design, and an assessment of hydraulic performance for each of the flood mitigation alternatives selected for evaluation. Section 7 provides the rationale for selection of the preferred floodway alternatives for the Little Minnesota River and the design for Toelle Coulee. 5.1.1 Little Minnesota River Floodway Alternatives Six different alignments and two inlet types (i.e., gate-controlled or “active” and fixed crest or “passive”) for a total of twelve different floodway alternatives were considered for the Little Minnesota River to provide flood protection for the City of Browns Valley. Floodway Option 3 was added by Houston Engineering, Inc. subsequent to the November 1, 2007 BVFMTF meeting. Option 3 is intended to better address potential concerns relative to maintaining the historic proportion of flow to Lake Traverse and concerns about modifying low flows through the City of Browns Valley. All floodway designs focused on containing the design discharge (plus 1-foot of freeboard) within the floodway channel, rather than using levees to provide additional capacity. Material from floodway construction will be placed in spoil banks adjacent to the floodway and used to reconstruct Roberts CR 24. The spoil banks will function to contain flow for floods exceeding the freeboard capacity and some portions of the spoilbanks will serve as levees where the 100-year water surface profile is higher than the ground elevation (near Traverse CSAH 4 for example). The alignments primarily differ relative to the location of the floodway inlet from and outlet to the Little Minnesota River (and therefore slopes and dimensions also differ). Floodway Project No. 5304-002 January 2008 Browns Valley Flood Mitigation Page 5 - 1 designs were completed using the steady-state HEC-RAS model as described within Section 3.0. Performance was then further evaluated by simulating several historic flood events, also described within Section 3.0. Each alignment has advantages and disadvantages. 5.1.1.1 General Description of the Alternatives Each alternative requires the construction of a weir inlet at the entrance to the floodway. The dimensions of the weirs differ, depending upon whether water from the Little Minnesota River is allowed to passively flow over the weir into the floodway (i.e., a passive inlet), or a dam and gated structure is used to restrict the Little Minnesota River, increasing head and the amount of water entering the floodway (i.e., an active inlet). Each alignment includes plans for relocating Roberts CR 24 along the south side of the floodway and the construction of one new bridge on Traverse CSAH 4/Bigstone CSAH 31 across the floodway near the boundary of Big Stone and Traverse Counties. Each alignment uses the existing bridge south of Browns Valley on Traverse CSAH 4. The spoil placed along the northerly portion of the floodway is expected to be used as an access trail. Each option also requires the construction of an outlet to create a transition from the floodway into the Little Minnesota River, south of Browns Valley. The transition is needed for hydraulic and erosion protection purposes. The advantages and disadvantages of the various floodway alignments and inlet controls are shown in Table 5-1. Four categories were used to compare the alternative floodway alignments and alternatives: hydrologic effects, engineering/cost, operation/maintenance and environmental issues. Alternative CS4, Floodway Option 1 Two alignments share a similar location for the floodway inlet located within Section 31, Becker Township 125N, R 49W (Roberts County, South Dakota). The floodway alignment is generally oriented in a south-easterly or easterly direction through Section 5, T 124N, R 49W (Roberts County, South Dakota). The terminus of the floodway joining the Little Minnesota River is located within either Section 20, T 125N, R 49W (Traverse County, Minnesota) or Project No. 5304-002 January 2008 Browns Valley Flood Mitigation Page 5 - 2 Table 5-1 Little Minnesota River Floodway Alternatives Analysis Matrix de of City (i.e., environmental flows) environmental (i.e., eaching North Si Fishermen) Relative Cost Relative Relative Cost Factor Relative Degree of Maintenance Degree Bridge Span (and Cost) Span (and Bridge Transportation Features Transportation Operational Requirements Operational (i.e., overtopping Hwy. 28) overtopping (i.e., Extent of Debris and Ice Issues Ice and of Debris Extent Relative Environmental Impacts Environmental Relative Control Flood Flow to Diversion Length of Diversion Channel (ft) Channel of Diversion Length Non-Flood Flow Through Diversion Percieved Ease of Obtaining Permits Obtaining of Ease Percieved Diversion Channel Excavation (cu.yd.) Excavation Channel Diversion Replicate Historic Flows to Lake Traverse Traverse Flows Historic to Lake Replicate Likelihood of FloodingLikelihood from South of City Ease of Road Realignment / Traffic Patterns Sediment Issues, Low Flows, Issues, Sediment Geomorphology Ability to address flooding from Lake Traverse Lake from flooding to address Ability Use of Land by Landower (e.g. bisecting parcels) bisecting (e.g. by Landower of Land Use Morphology to Channel Related Issues Perceived Perceived Magnitude of Aquatic Resource Impacts Resource of Aquatic Magnitude Perceived Safety Issues (to Swimmers, Boaters, Snowmobilers, Snowmobilers, Boaters, Swimmers, (to Issues Safety Control Flood Flows 500 cfs) Through City (to about Non-Flood Flows Through City Ability to Prevent Flows from R Potential for Adverse Downstream Impacts (Big Stone Lake) Impacts Downstream for Adverse Potential HYDROLOGY ENGINEERING, DESIGN & CAPITAL COST OPERATION & MAINTENANCE ENVIRONMENTAL Passive Floodway Inlet (Inoperable Weir) 1 BRIDGE WITH HIGHER Alternative CS4, ROAD LESSER LEAST FEWER FAIR GOOD REDUCED THAN NO GOOD LEAST POOR 4,500 400,000 OK POOR 1 HIGHEST NO OPERATION HIGH LOW SAME GREATEST GREATEST GREATEST GREATEST REALIGNMENTS & SPAN LIKELY ISSUES DESIRED Option 1A CLOSURE 1 BRIDGE WITH HIGHER Alternative CS4, ROAD NOT AS GREATEST MOST FEWER FAIR GOOD REDUCED THAN MAYBE MAYBE MODERATE GOOD 6,500 630,000 POOR 2 HIGHEST NO OPERATION HIGH MODERATE SAME GREATEST GREATEST GREATEST MODERATE REALIGNMENTS & GOOD SPAN LIKELY ISSUES DESIRED Option 2B CLOSURE 1 BRIDGE WITH HIGHER Alternative CS4, ROAD NOT AS SMALLEST LEAST FEWER FAIR GOOD REDUCED THAN YES GOOD GREATEST GOOD 7,000 575,000 POOR 2 HIGHEST NO OPERATION HIGH MODERATE SAME GREATEST GREATEST GREATEST MODERATE REALIGNMENTS & GOOD SPAN LIKELY ISSUES DESIRED Option 3 CLOSURE Passive Floodway Inlet (Bladder Weir or Gate on Floodway) 1 BRIDGE WITH LOW TO ACHIEVE REDUCED SAME AS GATED Alternative CS4, ROAD LESSER SIMPLE LOW TO LEAST FEWER GOOD EXCELLENT MODERATE DESIRED NO GOOD LEAST POOR 4,500 400,000 OK POOR 2 LOWEST LOW SAME MODERATE COMPARED TO STRUCTURE IN GREATEST REALIGNMENTS & SPAN OPERATION MODERATE LIKELY ISSUES SAME RANGE INOPERABLE WEIR RIVER Option 1A CLOSURE 1 BRIDGE WITH LOW TO ACHIEVE REDUCED SAME AS GATED Alternative CS4, ROAD NOT AS GREATEST SIMPLE LOW TO MOST FEWER GOOD EXCELLENT MODERATE DESIRED MAYBE MAYBE MODERATE GOOD 6,500 630,000 POOR 3 LOWEST MODERATE SAME MODERATE COMPARED TO STRUCTURE IN MODERATE REALIGNMENTS & GOOD SPAN OPERATION MODERATE LIKELY ISSUES SAME RANGE INOPERABLE WEIR RIVER Option 2B CLOSURE 1 BRIDGE WITH LOW TO ACHIEVE REDUCED SAME AS GATED Alternative CS4, ROAD NOT AS SMALLEST SIMPLE LOW TO LEAST FEWER GOOD EXCELLENT MODERATE DESIRED YES GOOD GREATEST GOOD 7,000 575,000 POOR 3 LOWEST MODERATE SAME MODERATE COMPARED TO STRUCTURE IN MODERATE REALIGNMENTS & GOOD SPAN OPERATION MODERATE LIKELY ISSUES SAME RANGE INOPERABLE WEIR RIVER Option 3 CLOSURE Active Floodway Inlet (Gated Structure in River) 1 BRIDGE WITH ACHIEVE COMPLEX IN SAME AS GATED Alternative CS4, FLOW RANGE ROAD LESSER LEAST DAM IN RIVER MORE EXCELLENT EXCELLENT DESIRED NO GOOD LEAST POOR 4,500 400,000 OK POOR 3 LOWER ADVANCE OF LOW LOW SAME LEAST STRUCTURE IN GREATEST SAME REALIGNMENTS & SPAN LIKELY MANAGEABLE ISSUES RANGE FLOOD FLOODWAY Option 1A CLOSURE 1 BRIDGE WITH ACHIEVE COMPLEX IN SAME AS GATED Alternative CS4, FLOW RANGE ROAD NOT AS GREATEST MOST DAM IN RIVER MORE EXCELLENT EXCELLENT DESIRED MAYBE MAYBE MODERATE GOOD 6,500 630,000 POOR 4 LOWER ADVANCE OF LOW MODERATE SAME LEAST STRUCTURE IN MODERATE SAME REALIGNMENTS & GOOD SPAN LIKELY MANAGEABLE ISSUES RANGE FLOOD FLOODWAY Option 2B CLOSURE 1 BRIDGE WITH ACHIEVE COMPLEX IN SAME AS GATED Alternative CS4, FLOW RANGE ROAD NOT AS SMALLEST LEAST DAM IN RIVER MORE EXCELLENT EXCELLENT DESIRED YES GOOD GREATEST GOOD 7,000 575,000 POOR 4 LOWER ADVANCE OF LOW MODERATE SAME LEAST STRUCTURE IN MODERATE SAME REALIGNMENTS & GOOD SPAN LIKELY MANAGEABLE ISSUES RANGE FLOOD FLOODWAY Option 3 CLOSURE Ability of Option 2 ability to replicate historic flows depends upon where ice and debris jams form relative to the floodway inlet. Option 3 requires the addition of stop log bays in culverts through Th 28 to