MINNEHAHA GLEN FEASIBILITY STUDY

Transcription

1 THE KESTREL DESIGN GROUP, INC Prepared for: Prepared by: Minnehaha Creek Watershed District Minneapolis Park and Recreation Board The Kestrel Design Group, Inc. Wenck Associates

2 ISSUES AND STRATEGIES: Stream Channel: Stormwater Runoff: Erosion: Access: Recreation: Safety: 1) Channel Downcutting 2) Streambank Erosion/Collapse 3) Gabion Wall Collapse 4) Bridge Erosion 5) Urban Watershed 6) Stormsewer Pipe Outfalls 7) Gabion Basin Failure 8) Steep Slope Failure 9) Seeps 10) Mist Drip Line 11) Spiderweb of Trails 12) Obstructed Scenic Views 13) Handicap Accessibility 14) Water Access 15) Fishing 16) Biking 17) Picnicking 18) Facilities 19) Education 20) Crime 21) Vandalism Historic Preservation: 22) WPA Walls 23) Godfrey Mill Dam 24) Geologic Rock Outcroppings 25) Native American Presence Habitat: 26) Invasive Species 27) Loss of Habitat 28) Sensitive Habitat 29) Succession of Oak Forest 30) Aquatic Structure 31) Aquatic Spawning Beds

3 STREAM CHANNEL Channel Downcutting 1917 Ford Dam removes Mississippi River s sediment load Increased scour below the dam has lowered the Mississippi riverbed 5 to 8 feet Minnehaha Creek outlet perhaps lowered, causing downcutting in its channel Downcutting creates a steeper stream gradient with a faster current below the falls MINNEHAHA FALLS FORD DAM CREEK DOWNCUTTING RIVER SCOUR Boulder Cross Vanes Add boulder cross vanes to decrease gradient and slow current Catch sediment to raise creek bed and create riffles and pools Add rootwads and in-stream vegetation to enhance fish habitat STEEP GRADIENT = FAST CURRENT BOULDER CROSS VANES RIFFLE AND POOLS SEDIMENT CAPTURE

4 STREAM CHANNEL Streambank Erosion/Collapse Downcutting steepens the stream gradient, increasing its velocity and power to erode Fast current up against the shoreline undercuts the streambank Foot paths along streambank wear away vegetation that holds streambank in place Saturated streambanks are more likely to collapse, especially during freeze/thaw cycle Live-Staking/Rootwads/Rip Rap Add boulder cross vanes to slow current and keep it centered in the channel Define trails and keep them 25 from streambank edge while preserving views of creek Add live-stakes to revegetate shoreline and desaturate soils with evapotranspiration Add rootwads and in-stream vegetation to enhance fish habitat Add rip rap to hold shoreline in place

5 STREAM CHANNEL Gabion Wall Collapse Downcutting steepens the stream gradient, increasing its velocity and power to erode Fast current up against the shoreline undercuts the streambank Stone gabion retaining wall collapsed into the creek Vegetated Reinforced Soil System (VRSS) Add boulder cross vane to keep current in center of channel VRSS boulder base provides stability Geotextile fabric holds soil in place Living roots grow to anchor the entire assembly in place

6 STREAM CHANNEL Bridge Erosion Current is eroding outside bend of channel and undercutting bridge abutment THE KESTREL DESIGN GROUP, INC Boulder Cross Vanes Add boulder cross vanes to keep current in center of channel and under bridge

7 STORMWATER RUNOFF Urban Watershed Large, urban watershed with numerous impervious surfaces Impervious surfaces prevent infiltration and increase the volume of runoff Loss of wetlands to store, filter, and infiltrate stormwater runoff Volume of runoff in gorge contributes to flooding and erosion THE KESTREL DESIGN GROUP, INC Upland Rain Gardens Reduce runoff volume and peak rate while preserving flood storage capacity in watershed Disconnect impervious surfaces by daylighting select stormsewer pipes Treat and infiltrate stormwater runoff from two largest stormsewers in rain gardens Restore wetland habitats in upland areas to store and filter stormwater runoff Create animated shapes visible from above

8 STORMWATER RUNOFF Stormsewer Pipe Outfalls Stormsewer pipes convey large volumes of stormwater runoff into the gorge Outflow is eroding gorge bluffs, trails, and streambank and entering creek untreated Sediment and pollutants in runoff are degrading water quality Erosion removes vegetation creating disturbance and helping invasives species Two largest stormsewer pipes enter creek at Upper Glen and near creek mouth THE KESTREL DESIGN GROUP, INC Infiltration Cells Add energy dissipaters and live-staking at stormwater pipe outlets Create underground infiltration cells at two largest pipe outlets to treat and infiltrate runoff Create vegetated surface depressions for excess water as infiltrates St. Peter Sandstone Vegetation evapotranspirates runoff back into atmosphere Reduce runoff volume and peak rate while preserving flood storage capacity in watershed

9 STORMWATER RUNOFF Gabion Basin Failures Runoff volume from MN Veteran s Home is overwhelming gabions Erosion is undercutting gabions so no longer level and reducing storage capacity Erosion is degrading trail Little residence time in gabions and lack of fine material to filter stormwater runoff Gabions are aesthetically unappealing Infiltration Cells Dismantle gabions and bury rock material in underground infiltration cells Create vegetated surface depressions for excess water as infiltrates St. Peter Sandstone Vegetation evapotranspirates runoff back into atmosphere Look for ways to handle more of the MN Veteran s Home runoff up above on bluff Reduce runoff volume and peak rate while preserving flood storage capacity in watershed

10 EROSION Steep Slope Failures Stormwater runoff on steep slopes removes vegetation and contributes to slope failure Foot traffic on steep slopes removes vegetation and contributes to slope failure Crib walls are aesthetically unappealing Crib Wall Live-Staking Keep visitors on designated trails Use fiber rolls to break up energy of long steep slopes into shorter less steep grades Add live-stakes to strengthen existing crib walls and hide their presence Revegetate bare slopes with shade tolerant native species

11 EROSION Seeps Rare black ash seeps create muddy conditions along portions of the creek s west bank Mill pond sediments created an island that is now widening the channel Trails are too close to the streambank Existing boardwalk is not long enough and is falling apart Elevated Boardwalk 5 Wide helical screw boardwalk with wheel guard edges elevated less than 24 Wooden crib type elevated boardwalk with talus rock to guide visitor s path Remove island that is widening creek channel to create more room for boardwalk Add boulder cross vane/flow control device to restore historic Godfrey Mill Pond Extend boardwalk to full length of seep area and move back from streambank THE KESTREL DESIGN GROUP, INC. 2007

12 EROSION Mist Drip Line Drip line from mist of falls off the limestone ledge is saturating soils Saturated soils are contributing to slope failure especially during freeze/thaw cycle Vegetation is shallow-rooted turf grass and ash trees Crib Wall Live-Staking Most slope failures occur in top meter of soil Use fiber rolls to break up energy of long steep slopes into shorter less steep grades Add live-stakes to strengthen existing crib walls and hide their presence Revegetate bare slopes with deep-rooted native species

13 ACCESS Spiderweb of Trails Lack of trail definition encourages people to go off the trail Visitors create their own trails to rock outcroppings and the creek Creation of trails on steep slopes and streambanks cause erosion and slope failure Trail Definition/Controlled Access Keep traffic on designated trails, close and revegetate unauthorized trails Clearly define a system of designated trails with a compacted stone surface Clearly define a system of designated trails with a porous concrete surface Create limited number of controlled access points to rock outcroppings and the creek Provide visual cues to stay on trails with logs, benches, berms, bollards, stonework

14 ACCESS Obstructed Scenic Views Narrow, deep gorge with dense vegetation often blocks views of the creek and river Scenic Overlooks Create trail pullouts and scenic overlooks at appropriate locations Remove select trees to open views into the gorge

15 ACCESS Handicap Accessibility Numerous stairways and uneven trails limit handicap access into the gorge Wabun Access Clearly define a system of designated trails with a compacted stone surface Clearly define a system of designated trails with a porous concrete surface Uninterrupted handicap accessibilty through Upper Glen from Wabun

16 RECREATION Water Access Narrow channel, steep banks, and fast current makes the creek inaccessible in most places Waders have eroded the streambank at popular access point in the Upper Glen Streambank erosion adds sediment into the creek Waders like to pile up stones that divert the flow which then widens the channel Upper Glen Wading Area Add hardscape edge to wading area to prevent futher erosion and widening of channel Include steps or multiple levels to accomodate fluctuating water levels Add cross vane/flow control structure to maintain water level and reduce current Maintain access to sun and add seating with logs, stones, benches Keep sediment from entering the creek

17 RECREATION Fishing Shallow water and fast current combine for little habitat in creek below falls Fisherman create their own trails to the creek which increases erosion Erosion adds sediment into the creek and removes native vegetation THE KESTREL DESIGN GROUP, INC Fishing Stations Add boulder cross vanes to create riffles and pools Create fishing stations at pools with defined access from designated trails Keep sediment from entering the creek THE KESTREL DESIGN GROUP, INC. 2007

18 RECREATION Biking Bicyclists create trails in the woods, increasing erosion and removing native vetgetation Bike Trail Clearly define a system of designated trails with a compacted stone surface Clearly define a system of designated trails with a porous concrete surface Provide visual cues to stay on trails with logs, benches, berms, bollards, stonework Close and revegetate unauthorized trails

19 RECREATION Picnicking Narrow, shady vegetated gorge leaves little room for picnicking The sunny and open Upper Glen is a popular gathering spot for picnicking The restroom building has been closed after being vandalized Upper Glen Picnic Area Add a picnic shelter, picnic tables and grills Repair and reopen the restroom building Maintain access to sun in Upper Glen THE KESTREL DESIGN GROUP, INC. 2007

20 RECREATION Facilities Park infrastructure in the gorge is in disrepair after use and abuse Multiple aesthetic styles of park architecture in gorge Improvements Repair historic WPA stonework in gorge Add new stonework council rings and sitting stones that are more durable Maintain prairie school aesthetic style of architecture in gorge

21 RECREATION Education Minnehaha Falls is rich in historic, ecologic, hydrologic, natural and cultural resources Vandalism and theft have destroyed many of the original brass interpretive plaques Interpretive Signs and Public Art Add interpretive signs to explain the rich history and unique resources present in gorge Add public art to interpret human experience Minimize the number of signs to prevent sign pollution in the park

22 SAFETY Vandalism Dark, secluded conditions with dense vegetation limit sightlines Graffiti is present in the gorge Appearance of lack of upkeep encourages more graffiti and vandalism Presence of graffiti encourages more crime THE KESTREL DESIGN GROUP, INC Graffiti Removal Add lighting to deter vandalism and crime Remove graffiti promptly Maintain appearance of upkeep in gorge Infrastructure in open, highly visible locations whenever possible THE KESTREL DESIGN GROUP, INC. 2007

23 SAFETY Crime Dark, secluded conditions with dense vegetation limit sightlines in gorge Groups of unsupervised youth (drinking, smoking, and fighting) are present in the gorge Lighting in the gorge is limited to a few street lights in the Upper Glen THE KESTREL DESIGN GROUP, INC Improve Lighting Maintain trails on both sides of creek so always two routes for escape in emergency Add lighting along designated trails Extend access from Wabun through Upper Glen for public safety vehicle access Uninterrupted emergency and public safety vehicle access in Upper Glen from Wabun

24 HISTORIC PRESERVATION WPA Walls Increased volume of runoff in creek can t expand channel horizontally Channel downcutting has undermined and collapsed some of the historic WPA walls Debris from failed slope is diverting current from center of channel into WPA walls THE KESTREL DESIGN GROUP, INC Boulder Cross Vanes Work with USACOE to develop support and reinforcement of WPA walls Work with USACOE to restore collapsed WPA wall sections Add boulder cross vanes to keep current in center of channel Remove talus debris and prevent future slope failures from slumping into creek channel Add large limestone slabs as cross vane to create plunge pool at Falls THE KESTREL DESIGN GROUP, INC. 2007

25 HISTORIC PRESERVATION Godfrey Mill Dam Channel downcutting is undermining the historic Godfrey Mill Dam Mill pond sediments created an island that is now widening the channel Channel widening erosion of streambank is undercutting Godfrey Mill Dam Boulder Cross Vanes Remove island which divides creek and widens channel Add boulder cross vane to restore mill pond and keep current in center of channel Uncover portions of the mill dam to reveal history THE KESTREL DESIGN GROUP, INC. 2007

26 HISTORIC PRESERVATION Geologic Rock Outcroppings Visitors wear trails to exposed limestone outcroppings, talus piles, and soft sandstone Controlled Access Define trails and give visitors controlled opportunities to approach rock outcroppings

27 HISTORIC PRESERVATION Native American Presence Historic Native American presence is not visible in the landscape Interpretive Signs Create interpretive and artistic opportunities to reveal Native American experience THE KESTREL DESIGN GROUP, INC. 2007

28 HABITAT Invasive Species Invasive plants and diseases are present in the park Invasive plants rapidly crowd out valuable native plants Disturbed edge conditions along forest patches favor invasive plants Lack of disturbance regime allows invasive plants to become established Disturbance Regime Remove favorable conditions for invasive plants and diseases Limit fragmentation of intact forest patches Mimic disturbance regime in park landscape to favor native species Remove invasive plants with mowing, and trimming Consider prescribed burns as a way to return fire to the landscape THE KESTREL DESIGN GROUP, INC. 2007

29 HABITAT Loss of Habitat Minnehaha Gorge is a key conservation area in the highly urbanized Twin Cities region Loss of wetlands and their rich habitat has been extensive throughout the region Remnant patches of mature oak and cottonwood forest survive in the gorge Restoration Restore wetlands to reduce runoff volume and peak rate, preserve flood storage capacity Preserve and enhance mature forest ecosystems Minimize the removal of vegetation

30 HABITAT Sensitive Habitat The Mississippi River corridor is a major migration route for birds The Twin Cities are a heavily urbanized obstacle with little habitat for migrating birds Presence of rare black ash seep habitat in the gorge Protect Core Areas, Nesting Sites, Rare Seeps Protect rare black ash seep habitat Preserve core forest patches and nesting sites in migration corridor Restore wetland and forest habitats

31 HABITAT Succession of Oak Forest Lack of fire in disturbance regime favors maple basswood succession over oak THE KESTREL DESIGN GROUP, INC Selective Plantings Plant and protect oak seedlings to promote future generations of oak forest in park Consider prescribed burns as a way to return fire to the landscape THE KESTREL DESIGN GROUP, INC. 2007

32 HABITAT Aquatic Structure Little vegetation or structure exists in creek to provide cover and habitat Lunker Boxes, Rootwads, Bank Cuts Add lunker boxes, root wads and bank cuts to improve fish holding areas

33 HABITAT Aquatic Spawning Beds Fast current in creek limits spawning habitat Boulder Cross Vanes, Riffles and Pools Add root wads and bank cuts to improve fish holding areas Add boulder cross vanes to decrease gradient, slow current and create riffles and pools

34 SUMMARY ISSUES AND STRATEGIES: Stream Channel Stabilize channel Reduce downcutting and erosion Improve structure and habitat value Stormwater Runoff Volume Reduce runoff volume and peak rate entering stormsewer pipes Preserve flood storage capacity in watershed by disconnecting stormsewer pipes Redesign gabion basin treatments at end of stormsewer pipes Erosion Keep people on trails Keep water off steep slopes Use native plants to hold slopes in place Access Designated network of low impact trails Engage environmental elements Recreation Increase opportunities Minimize impact Safety Reduce sense of isolation Historic Preservation Preserve and promote historic elements Habitat Restore habitat in Minnehaha Gorge as a key conservation area Encourage conditions which favor mature oak forest habitat

35 NEXT STEPS PROJECT SCHEDULE: July Collect Public Input Incorporate Public Input into Management Plans August Draft Feasibility Study Reviewed by MCWD and MPRB Staff Draft Feasibility Study Presented to MCWD and MPRB Boards September Final Feasibility Study Reviewed by MCWD and MPRB Staff Final Feasibility Study Presented to MCWD and MPRB Board Approval and Acceptance of Final Feasibility Report by MCWD Board? October Final Plans? November Bids? January, 2008 Funding? Construction of In-Stream Improvements during Winter Season?