B. Summary of Key Design Criteria
In order for the function of new drain to be sustainable for the long-term, a stable outlet is required. The principal processes that could adversely affect its stability and function of the new drain include:
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Accelerated sedimentation within the drain
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Accelerated sedimentation at the outlet of the drain (at the Detroit River shoreline)
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Excessive accumulation of ice or floating debris (from the Detroit River) at the outlet of the drain.
The design of the drain outlet is critical to properly mitigating the above-noted phenomenon. The following sections of this report address the design criteria that have been adopted for this purpose.
Proposed Drain Profile
The method for establishing the vertical alignment of the new drain was set out in our Design Brief dated August 14, 2015. In summary, the vertical alignment was based largely on the requirement to provide proper outlet for the new sewers that will discharge to the new Broadway Drain. The required drain profile was established by fixing the drain elevation at its upstream limit (slightly lower than the lowest sewer invert) and extending the drain to the Detroit River at a practical minimum longitudinal slope of 0.1%, until it daylights above the existing nearshore Detroit River bottom. The proposed drain profile will allow sediment to move down the drain and outlet to the Detroit River.
Channel Outlet Protection
The basis of the original design of the drain outlet was summarized in the aforementioned Design Brief. It was noted therein that:
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The Detroit River shoreline at the outlet of the Broadway Drain is in a natural, unprotected state;
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A narrow, barrier beach consisting of sand and fine gravel existed across the mouth of the existing drain; and,
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The invert of the new drain will emerge into the Detroit River at an elevation below the nearshore river bottom.
In order to mitigate the potential for the new drain outlet to become fouled with river or littoral transport sediments, construction of two short training jetties (one on each side of the new drain) was originally proposed. Selection of an appropriate length for the jetties was based on an evaluation of the channel invert, nearshore bathymetry of the Detroit River, and the estimated width of the active littoral zone. No consideration was given during development of the original drain outlet design to the proposed infilling of the BBA shoreline.
Design Summary for Broadway Drain Outlet – January 12, 2016
Gordie Howe International Bridge