Four Seasons

Four Seasons

Gambrills, Maryland

BioHabitats Collaboration Project

This design/construction management project, completed in 2012, included a series of cobble weirs and a network of sand berms in order to retain water to simulate the geology and hydrology found in pristine coastal plains stream valley flood plains.  The design dissipates storm water energies upon exit from the twin 36” pipes as follows.  The first flush of water spreads along the perimeter of the project site through a moat or seepage pool thereby retaining water at the highest possible elevation within the landscape of the project site and re-establishing a wetted perimeter to more than an acre of floodplain forest and reducing peak flows. The water surface in the moat rises commensurately with the water surface elevation over the first grade control structure or weir situated about 25’ downstream of the pipes discharging the problematic storm water.

Water stored in the moats during storm events releases back into the stream as peak storm flows diminish, thereby increasing the duration of stream flow significantly. The moat is contained by sand berms on the channel ward side. The berms act as a level spreader when water depths over the first weir exceed a foot with larger storm events. The result is a non-erosive sheet flow and inundation of the flood plain. The water remaining in the moats slowly filters through the sand berms creating sandy seepage slopes replicating the geology and hydrology found on the highest quality wetlands and saturating the flood plain over an extended period of time.  The water infiltrates then enter the stream as cool clean ground water. Additionally the stream spills water onto the floodplain on either side of the channel at several points along the restored reach as the grade controls or weirs are positioned within 1 foot of the adjacent floodplain. The site originally disconnected from the floodplain and occupied by the non-native and invasive Wisteria chinensis. This design not only removes the non-native invasives but it re-establishes a number of components of a vital, self-sustaining coastal plain stream valley floodplain.