Slide 32 of 36
Notes:
The design process is for some, including this author, the most rewarding part of the process. Here, the concept is converted from a dream to a construction drawing. Design of retrofit projects incorporate the same elements as any other BMP project including: adequate hydrologic and hydraulic modeling, detailed topographic mapping, property line establishment, site grading, structural design, geotechnical investigations, erosion and sediment control design, construction phasing and staging to name a few. But there is one very big difference. Normal BMP design usually follows a prescribed design criteria (e.g., control of the 2 year storm or sizing for a specified water quality volume), retrofit designers must work backwards from a set of existing site constraints to arrive at an acceptable stormwater control obtainable.
Sometimes this process yields facilities that are too small or ineffective, and therefore not practical for further consideration. One such project in Gaithersburg, Maryland was recently proposed as a major stormwater wetland (upstream from an existing road culvert) to control a 1000 acre watershed. The only problem was that only one-twentieth of an inch of total storage (.05") was obtainable. Clearly this facility would have been a maintenance nightmare and likely would have done little to remove pollutants or control downstream channel erosion. The City of Gaithersburg correctly decided not to pursue the project even though they had already retained a consultant and spent significant time and money on preliminary design.
The key to successful retrofit design is the ability to balance the desire to maximize pollutant removal and channel erosion protection while limiting the impacts to adjacent infrastructure, residents or other properties. Designers must consider issues like avoiding relocations of existing utilities, minimizing existing wetland and forest impacts, maintaining existing floodplain elevations, complying with dam safety and dam hazard classification criteria, avoiding maintenance nuisance situations, and providing adequate construction and maintenance access to the site.