The White Clay Creek Flood Studies

Transcript

1 The White Clay Creek Flood Studies Authorized by the $17M flood mitigation ordinance (Ord. No ) Bread and Cheese Island study was completed in September 2005 at a cost of $85,100 River study was completed in March 2006 at a cost of $426,900

2 The White Clay Creek Flood Studies

3 The White Clay Creek Flood Studies General Approach Perform hydrologic analysis to estimate peak storm water flow amounts Perform hydraulic analysis to estimate stream, bridge, and culvert conveyance capability Develop a model that combines the hydrologic and hydraulic analyses and performs numerous calculations that estimate stream depth at each point along the reach of stream being studied Calibrate model by measuring high water marks in the field Run the model for both the existing and proposed conditions and assess the impact on critical areas

4 Bread and Cheese Island Study Purpose: To quantify expected flood levels To evaluate the potential for mitigating flooding at chronically flooding properties in the area (i.e., Shone Lumber and Metroform Medical Complex) Study a proposed flow bypass channel through Bread and Cheese Island Study a proposed excavation for storage volume along the upstream perimeter of Bread and Cheese Island

5 Bread and Cheese Island Study FIRST STATE BV OAK ST 5TH AV 6TH AV 7TH AV 8TH AV TELEGRAPH RD ASH ST 4TH AV CSX Railroad BALTIMORE & OHIO RAIL ROAD CSX Railroad 9TH AV 10TH AV Potential Flow Relief- Remove Old RT 7 Bridge 11TH AV Delaware Park 1 DELAWARE PARK BL 12TH AV Potential Flow Relief New Culverts at R.R. Bridge Potential Flow By-pass Stanton Christiana Road - Drainage Study/Improvements Amtrak RR Potential Flow By-pass OGLETOWN STANTON RD Amtrak Railroad RT 7 TWIN C LN RR STANTON CHRISTIANA RD Miles OLD CHURCHMANS RD

6 Shone Lumber / Metroform Center February 2007

7 Bread and Cheese Island Study Results: Bypass channel reduces the 100-year flood elevation at Shone Lumber by only 0.15 feet and at Metroform by only 0.13 feet (not worth the $1.15 million cost and environmental impact) The proposed floodplain excavation reduces the 100-year flood elevation by only 0.24 feet at Shone Lumber and only 0.28 feet at Metroform (not worth the $1.4 million cost and environmental impact)

8 Bread and Cheese Island Study Recommendations: Do not pursue these alternatives without other regional improvements Consider flood-proofing these properties (Shone Lumber has since been flood-proofed with FEMA and NCCD assistance)

9 Purpose: To develop concepts for reducing flood damage along an approximate 10-mile length of White Clay Creek between the Kirkwood Highway (Delaware Route 2) bridge and the confluence of White Clay Creek and Christina River

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12 Findings: The existing FEMA mapped 100-year floodplain boundary is outdated. Approximately 200 tax parcels are located in the FEMA- mapped 100-year floodplain (approximately 80 more are likely in the actual 100-year floodplain). Recent flood damage is primarily related to pre-regulation regulation construction in the floodplain and an increased frequency of large storm events. FEMA is currently updating the 100-year floodplain mapping for the White Clay, Red Clay, and Mill Creek watersheds. The White Clay and Red Clay Creek mapping is using the hydrology and hydraulic analysis completed in connection with the studies commissioned by Ordinance in January Revised FEMA maps will be available in 2012.

13 Findings cont d. d.: None of the approaches being considered will mitigate flooding and flood-related damages fully by themselves Combinations of these approaches will be required to maximize effectiveness Upstream flood storage is not considered a practical solution to the flooding issues in the study area due to: The storage volume that would be required The scarcity of undeveloped land in the New Castle County portion of the White Clay Creek watershed, and The designation of White Clay Creek as a Wild and Scenic River

14 Recommended Projects Location Flood Mitigation Recommendation Priority Preliminary Cost Estimate Red Mill Road Bridge Increase floodway flow area through 1 $127K - $150K sediment excavation Remove in-channel sediment deposits 3 $170K - $600K Provide additional flow relief through 6 $2.5M - $3M embankment Old Harmony Road Increase floodway flow area through 2 $510K - $535K Bridge sediment excavation Remove in-channel sediment deposits 3 $170K - $600K Bridge demolition and removal. Add 7 $400K - $850K walking paths. CSX Railroad Bridge Increase floodway flow area through 3 $170K - $193K sediment excavation Remove in-channel sediment deposits 3 $170K - $600K Provide additional flow relief through 4 $9M - $10M embankment AMTRAK Railroad Bridge Remove in-channel sediment deposits. 3 $600K Provide additional flow relief through 5 $9M - $10M embankment. Old Route 7 Bridge Bridge demolition and removal. 7 $400K - $850K Total $23M - $28M

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17 Recommendations cont d. d.: Geomorphic modification to the creek channel and floodplains to improve flood conveyance including removal of existing weirs (approx. $400 to $1,000 per foot of stream length) Continued public purchase of select flood-prone properties (approx. $10 million for flood-prone residential properties listed in Table 4 of the study) Public grants and loans for owner-directed flood-proofing of existing floodplain structures Installation of flood protection systems, such as levees and floodwalls (approx. $800 to $2,500 per foot of length of levee or wall) Changes to land use and storm water management within the watershed (thru the Department of Land Use s s and FEMA s s initiative to update the floodplain maps)