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ADVANCE SURVEYING & ENGINEERING CO. 5811 Cedar Lake Road, Minneapolis, MN 55416 December 31, 1987 Mr. Michael P. Gaffron Assistant Planning & Zoning Administrator City of Orono Post Office Box 66 Crystal Bay, Minnesota 55323 Dea Mike: Phone 541 0500 We received your letter dated December 23, 1987 today. We would like to clear up some misunderstandings that the letter seems to indicate. We enumerate our responses to correspond to the numbering in your letter: 1. We are aware that the longitudinal axis of a typical mound system should be aligned with the contour lines on site and that is our normal practice. The underlying reason for this provision of the code is that this practice gi:-s a maximum amount of ground surface under the mound which is in the path of downward percolating effluent (lets call that surface the "wetable interface"). The effluent from the rock bed percolates straight down from the rock bed until it rraches the sand topsoil interface and then if the underlying soil is less permeable than the sand, the effluent will have to spread out to some extent to be absorbed. Each drop of effluent which cznnot be absorbed directly under the rock beds will proceed in a downhill direction (at right angles to the underlying contour lines) until the area wetted is sufficient to absorb all the effluent. One goal of a mound design is to provide the maximum "wetable interface" since if the wetted arc« --equired to absorb the effluent exceeds the "wetable interface" provided then hydraulic failure will occur at the toe of the mound. In the rather unusual case of this double moun:i, the goal of providing the maximum "wetable interface" best met by aligning the diagonal rather than ,e longitudinal axis of the mound with the contour linc.6. We enclose a diagram illustratir-4 this relationship. This w - our motivation for the proposed alignm, and the fact that this better fits the boundary lir: with this alignment is a fortunate coincidence.