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Rocky Branch Watershed Urban StudyFinal Report SummaryJune 6, 2012
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Ground Rules and Agenda
This is about AMEC’s technical studiesNot policies or politics or economics or metaphysics
I will walk through the presentationThere will be a free form Q&AWait to be recognizedOne at a timeWe will try to take all similar questions in one groupWe will write them down If we cannot answer we will get back to you
First a Quick Recap from Last Night
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Reference Points
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234
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Questions we were asked to answer
1. Comparison with past studies?2. Would proposed site fill cause a rise?3. What does improving downstream crossings do?4. Can water quality be addressed?5. What impact does Congaree River have on Rocky Branch?6. Can the watershed be restored by site improvements?
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1. Comparison with past studies?
Were the PACE and PB studies reasonable for their purposes?
PACE used effective FEMA model with lower flows (1970’s)
PB developed new model with higher flows (2006)
Cross-sections needed to be extended laterally by AMEC
AMEC HEC-RAS model floodplain limits similar to the effective FIRM
The PB Study modeled the Bluff Rd. RR crossing as a dam/lake
Calculated lower flows (in HEC-HMS) to downstream areas
But, did not include lower flows in HEC-RAS…
…downstream baseline stages too high
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2. Would proposed site fill cause a rise?
Would Site Fill Cause A Rise Due To Lost Conveyance?
NO - site lies in the backwater area from abandoned Bluff Rd. RR culvert.
Would Site Fill Reduce Floodplain Storage Volume & Cause Impacts?
NO – not significant enough to make an impact–Includes volume QC estimates using new topography.
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Hydraulic Impact
Station 1 Station 2 Station 3 Station 4 Station 5 Station 6Scenario above
Assembly St.at site above Bluff
Rd.below RR above Olympia
Ave.below Olympia
Ave.
1 Existing 175.7 175.3 175.3 152.1 149.4 147.9
2 With Fill 175.7 175.3 175.3 152.1 149.4 147.9
3 Existing w/o RR
4 Existing w/o RR or Olympia
5 Fill w/o RR or Olympia
6 Existing w/o u/s constrictions
7 Fill w/o RR only
What about hydrology not hydraulics?
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Main Stem
Site+
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• 3 cfs increase to main stem• 2.7 acre foot increase in R.O. volume• But this is less than 0.3% of the storage
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Key Questions Answered…
Can Improving RR/Bluff Rd.Crossings Reduce Assembly St. Area Flooding?
YES – significantly for Assembly St. area, as identified by PACE u/s benefits increase (stage drop), d/s impacts increase (rise) u/s benefits (stage drop) diminish with each flood event:
–100-yr: stops 600 ft u/s of Assembly St.–50-yr: stops at Assembly St.–2- to 10-yr: stops 500 ft d/s of Assembly St.
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With Fill and Without RR/Bluff
Station 1 Station 2 Station 3 Station 4 Station 5 Station 6Scenario above
Assembly St.at site above Bluff
Rd.below RR above Olympia
Ave.below Olympia
Ave.
1 Existing 175.7 175.3 175.3 152.1 149.4 147.9
2 With Fill 175.7 175.3 175.3 152.1 149.4 147.9
7 Fill w/o RR only 172.9 162.6 161.9 151.9 149.4 147.9
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Flooded Acres at Assembly
90 acres24 acres
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Key Questions Answered…
Would Improving RR/Bluff Rd. Crossing Increase Flooding Downstream?
YES slightly –Bluff Rd. RR crossing serves as a dam and detention pond.
–Increased opening increases flows & stages to Olympia Ave:–100-yr: ~1.5 ft– 50-yr: ~1 ft–10- to 25-yr: <0.5 ft–1- to 5-yr: no rise
*BUT…Impacts could be mitigated
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Key Questions Answered…
Would Improving RR/Bluff Rd. Crossing Increase Flooding Downstream? (continued)
BUT…
Improving Olympia Ave. fixes rises between Olympia & Bluff Rd.Modifying additional crossings d/s of Olympia Ave. could mitigate
those rises:–100-yr floodplain ~50-60 ft. wider than currently predicted–Further d/s, stages contained within channel
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Hydraulic Impact
Station 1 Station 2 Station 3 Station 4 Station 5 Station 6Scenario above
Assembly St.at site above Bluff
Rd.below RR above Olympia
Ave.below Olympia
Ave.
1 Existing 175.7 175.3 175.3 152.1 149.4 147.9
2 With Fill 175.7 175.3 175.3 152.1 149.4 147.9
7 Fill w/o RR only 172.9 162.6 161.9 151.9 149.4 147.9
5 Fill w/o RR or Olympia 172.9 162.6 161.9 151.7 147.7 147.9
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Key Questions Answered…
Can Water Quality of Rocky Branch Be Addressed?
Yes and No…NO – not by preserving floodplain storage u/s of Bluff Rd. RR:
–2-yr (or less) storage volumes too small for in-stream treatment YES – by reducing Bluff Rd RR culvert exit velocities:
–Reduced streambed scouring and streambank destruction–Reduced sedimentation (Rocky Branch & Congaree River) –Supports stream restoration efforts.
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Existing Downstream Exit Velocities (fps)
Storm Event 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr
Bluff Rd. RR 11 14 17 21 24 27
Olympia Ave. 14 12 11 10 9 8
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Key Questions Answered…
Can Water Quality of Rocky Branch Be Addressed?
Yes and No…NO – water backed up behind RR/Bluff provides NO water quality
benefit YES – by reducing Bluff Rd RR culvert exit velocities:
–Reduced streambed scouring and streambank destruction–Reduced sedimentation (Rocky Branch & Congaree River)
YES – new site may employ Water Quality BMPs
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There may be BMP Requirements or Opportunities
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Rain Garden or…
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Key Questions Answered…
What Impact Does Congaree River Flooding Have On The Area? 100-yr: extends up to Olympia Ave. 50-yr: extends to quarry access road <25-yr: below stages in Rocky Branch
Can These Improvements “Fix” the Watershed? Partially…nothing u/s of the Assembly St. area d/s can be improved with improved culverts (stage & velocity)
Bigger Picture Observations
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Urban Streams are Economic and Aesthetic Drivers
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Recommendations
Improve Downstream Crossings…responsibly Promote Green Infrastructure (GI) Techniques Consider Integrated Stream Restoration & Greenway Features Assess Potential Upper Watershed Improvements Collaborate With Local Watershed Associations
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Recommendations
Improve Downstream Crossings: advantages, impacts & remedies: Significant reduction in u/s flood stages up to Assembly St. Increased LOS for Bluff Rd. & Olympia Ave.Reduced velocities from Bluff Rd. RR culvert Increased flow rates from Bluff Rd. RR culvert:
–Olympia Ave. culvert improvement can prevent impacts–The d/s flood stage increases have moderate impacts:
– In many locations, flows are within the streambanks–Potential rises mitigated by improving additional downstream
crossings
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Recommendations
Integrate Stream Restoration and Greenway Features Bluff Road/RR & Olympia Avenue improvements support watershed
restoration:–Pedestrian walkway/greenway connects USC housing to site–Support for greenways &restoring the watershed –Support in-stream restoration
Consider Alternative Upper Watershed ImprovementsCulvert inlet modificationsUnderground storage for “peak shaving”Use a dynamic model (SWMM) previously developedUse available USGS gage data to calibrate the model
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Recommendations
Adopt Green Infrastructure (GI) Techniques “Green Roofs”; pervious pavement; and bioretention featuresCisterns and “Blue Roofs” that use water for other purposesNon-concentrated flow through downspout disconnection and open
swales
Collaborate with Local Watershed Associations: RBWA Technical Committee, Congaree Riverkeeper Integrate public outreach for support of watershed improvementsOpportunities for stream restoration/flow reduction
–Five Points & USC Identify and apply new resources:
–Cooperating with USGS and NOAA (rainfall & stream stage/flow info)
–Finding potential locations for diversion/capture & GI features
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Questions?
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Hydraulic Impact
Station 1 Station 2 Station 3 Station 4 Station 5 Station 6Scenario above
Assembly St.at site above Bluff
Rd.below RR above Olympia
Ave.below Olympia
Ave.
1 Existing 175.7 175.3 175.3 152.1 149.4 147.9
2 With Fill 175.7 175.3 175.3 152.1 149.4 147.9
3 Existing w/o RR 172.9 162.4 161.9 151.9 149.4 147.9
4 Existing w/o RR or Olympia 172.9 162.4 161.9 151.7 147.7 147.9
5 Fill w/o RR or Olympia 172.9 162.6 161.9 151.7 147.7 147.9
6 Existing w/o u/s constrictions 177.4 177.4 177.2 151.3 149.1 146.9
7 Fill w/o RR only 172.9 162.6 161.9 151.9 149.4 147.9
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