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I <br />where, <br />Q =flow rate <br />A =cross-sectional area of the drain or spoil <br />n =spoil or drain porosity, and <br />Vv = velocity calculated with Eq. 4.7 using the 50 percent <br />particle diameter. <br />4.2.2 DESIGN OF UNDERDRAINS <br />The regulatory requirements for underdrains are that: drains be located <br />in all natural or man-made water courses, lateral drains be provided to <br />areas of springs or wet-weather seeps, drains be sized to transport peak <br />flows from all water and protected with a filter System to prevent piping <br />and clogging of the drain, and underdrains must be constructed of durable, <br />non-degradable, non-acid or toxic-forming rock. <br />The sizing of an underdrain depends upon the quantity of water (Q), the <br />cross-sectional area of the drain or pipe hydraulic radius, if used, (A) <br />and the average velocity of the flow within the drain (V) from Eq. 4.7. <br />These parameters are related by the equation <br />Q = A V = KiA (Eq. 4.9) <br />Thus, with an estimate of the quantity of seepage and the velocity, the <br />minimum size of the drain may be calculated. <br />The estimate of flow velocity is dependent upon whether flow is laminar or <br />turbulent. Turbulent flow should generally be anticipated in underdrains <br />constructed of large rock. Cedergren (1981) reports success with designs <br />for underdrains under both turbulent and laminar conditions using Darcy's <br />equations and provides examples. leps (1913) indicates that flow through <br />rockfill may be classified using a Reynold's number <br />Re = Vm (Eq. 4.10) <br />v <br />-139- <br />