North Branch of Cypress Creek Ecological Restoration: A Comprehensive Approach to Stream Restoration Rocky Powell Clear Creeks Consulting, LLC Keith D. Tate, P.E. BayLand Consultants & Designers, Inc. Mid-Atlantic Stream Restoration Conference October 31, 2013 Consultants & Designers, Inc. Integrating Engineering and Environment 1321 Mercedes Drive, Suite A Phone: (410) 694-9401 Hanover, Maryland 21076 Fax: (410) 694-9405
Project Location
Project Location - Severna Park, MD Anne Arundel County 9 Acre Restoration Site
Primary Design Constraints Highly impervious watershed with altered hydrology. Limited stormwater management. Highly efficient conveyance system stormwater pipes and concrete channels.
Watershed - 40% Impervious 475 Acre Watershed
Primary Design Constraints A series of on-line ponds were constructed in the 1930s. 1960 s urbanization increased sediment loadings to the creek causing sedimentation of the ponds. The resulting down-valley floodplain elevations varied significantly from one impoundment area to the next. Pond embankments breached in the1970s stream downcut through the pond deposits. By the 1990s, the creek was deeply incised, with active head-cuts and severe bank erosion.
Ponds 1943
Pond sedimentation, drastic floodplain elevation changes, dams breach, channel incision, bank erosion and filling of tidal creek
Headwater Wetland Design Approach Reduce peak discharge runoff rate for the 1- and 2-year storm flows. Provide water quality management for the first flush of pollutants. Three design alternatives were evaluated. The selected alternative 2.1 acre-feet stormwater wetland.
Headwater Wetland Peak Discharges for Design Storms 1-Year 2-Year 10-Year 100-Year Inflow (cfs) 93.8 142.1 334.1 588.4 Outflow (cfs) 57.7 107.8 322.2 586.9 Attenuation 38% 24% 4% 0.2%
Upper Reach Step-Pool Channel Design Approach Channel design in this area had to accommodate the narrow valley and steep slope. Accomplished by constructing the upper reach as a B2 channel with a series of log/boulder step pools. Construction of this reach required that the floodplain along the reconstructed channel be raised as well.
Upper Reach E3 Channel Design Approach Floodplain widens and valley gradient flattens downstream of the step pool reach. A short transition E3 channel was constructed. This was accomplished by raising the existing streambed and stabilizing the stream banks.
Anastomosed Reach Design Approach Area of old impoundment. Remnants of old dam in place. Reconstructed as anastomosed or DA channel - multiple channels and stable islands. Primary channel in anastomosed system existing channel. Secondary channels excavated along existing forested floodplain. Trees preserved on islands. Streambed along primary channel raised to reconnect the floodplain. Stream banks stabilized.
Anastomosed Reach Design Approach Minimal sediment supply (Headwater Wetland). Maximum stream power and shear stress consistent with reference reach and research literature. Low gradient Wide floodplain
Floodplain Wetlands Design Approach General The middle reaches along the mainstem were excavated/filled to create floodplain wetlands. Multiple floodplain wetland impoundments provided a method for transitioning from one old floodplain level to the next. Shallow Wetland and Open Water Impoundment Areas Wetland impoundments maximize water quality treatment of storm flows. Depth = 0.5-1.0 foot in shallow wetland areas Depth = 2.0-3.5 feet in open water areas Sized to maximize water quality volume 2.43 acre-feet
Floodplain Wetlands Design Approach Wetland Embankments Broad landform constructed to impound water in the wetlands. Top elevation 1.0 foot above upstream permanent pool. Embankment width equal to width of existing floodplain to minimize flow velocity and shear stress. Boulder Cascades Water conveyed from one wetland area to the next via boulder armored single thread channel constructed through wetland embankment. Boulder cascade sized to convey 1-year storm all other flows spreading across the entire width of the embankment/floodplain.
Floodplain Wetlands Design Approach Islands and Peninsulas Fine grading established islands, peninsulas, and microtopography. Produced a variety of hydrologic regimes within the wetland areas. Top elevations set at 0.5 to 1.0 foot above permanent pool.
Tidal Channel Reach Design Approach Last floodplain wetland embankment raised crest of outlet several feet above tidal area. Existing alignment maintained. Streambed raised by installing constructed riffles. Provided a gradual transition from crest of embankment to tidal area. Reconnected the tidal channel to its floodplain. Provides fish passage from tidal area into floodplain wetland impoundments.
Tributaries and Stormwater Design Approach Drainages Addressed storm flow from outfalls along perimeter of project and along tributaries. Provided water quality treatment and conveyed in a non-erosive manner. Utilized Regenerative Step-pool design.
Environmental Benefits Restored 2,750 LF of mainstem and 880 LF of tributaries Water Quality Volume 5.6 acre-feet 60 acres of Equivalent Impervious Treatment Reduction in sediment and nutrient loadings Created/Restored 7.4 acres of wetlands with a net gain of 3.3 acres including: In-line headwater wetland Forested wetlands Emergent wetlands Tidal wetlands Seepage wetlands Creation of 4.5 acre riparian forest In-stream and riparian habitat enhanced/diversified
Construction Cost $1.7 million