Within this location, there was also limited right of way, undulating and outcropping bedrock, and high stream velocities due to storm-water runoff from the surrounding developed areas. Erosion protection is paramount when designing flood protection projects in any environment, but it’s even more critical in an urban setting, due to limited space and the proximity of structures.
In addition to these challenges, the design of the Molly Ann’s Brook project varied from a trapezoidal riprap section to a rectangular section formed by precast concrete walls. Conventional solutions such as riprap, wire mesh gabions, cabled concrete mats, and more than 3,000 linear feet of precast concrete retaining walls were used to protect the banks of the brook and contain floods on adjacent areas.
Given the savings and ease of construction of a geocell system, the Corps of Engineers employed this innovative approach at two difficult locations. The geocellular approach in these two channel sections saved the Philadelphia District $395,000 overall. In addition, the District estimates that it designed and constructed the geocell wall for less than half the cost of an equal length of reinforced concrete retaining wall. The channel lining was completed at about 25% of the cost of a 54-in. riprap section of equal length.