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1 <br /> <br /> in piping of finer material comprising a boundary into a <br /> more pervious zone. This process might consist of the <br />' following: <br />' a.) The head in a subsurface reservoir would increase <br />to a critical level. <br /> b.) Once that critical level is reached, piping would <br /> occur through the boundary creating a hole in the <br />' boundary and effectively draining the overlying <br /> reservoir. <br />t c.) As the pressures in the reservoir drop, the <br /> pressures in the pervious zones below the boundary <br /> would be expected to increase as they be;ir. to <br />' carry more water. <br />d.) Since a reservoir would likely consist of a syste-: <br />of interconnected pervious zones, drawdowr. of the <br />reserver would involve draining of these zones. <br />The result would be expected to include differing <br />' levels in vertically adjacent piezor..eters whict had <br />indicated the same levels prior to the drawdowr. <br />This is because the different zones would e:;tibit <br />differing drainage characteristics. <br />t e.) After the reservoir had sufficiently drained, <br />velocities and seepage forces would decrease, <br />' allowing for sediment deposition and p=__t:al <br />plugging of the hole in the boundary, estab'_ishing <br />conditions for another cycle <br />' The above conditions can perhaps partially account for <br />fluctuations in past measurements of the original cpen- <br />well piezometers. <br />' As car. be seen from the above discussion, because flew occurs <br />through confined zor:es and layers, it is impossible to create an <br />' accurate detailed model of pore pressure conditions. ??owever, tte <br />pore pressures can reasonably be estimated depending on whetter t::e <br />subsurface flow can be characterized as dominated by horizontal <br />' boundary conditions or by channel conditions. It should be noted <br />that the dominant characteristic varies significantly with both <br />elevation and horizontal location as well as over time. In zones <br />where a horizontal boundary appears to control subsurface flo~.~ <br />' characteristics, pore pressures can be reasonably depicted by a <br />phreatic surface with hydrostatically varying pressures. On t'-e <br />other hand, in zones where flow through channels is the domi^.a-`_ <br />' mode of groundwater flow, it is more reasonable to represent pare <br />pressures with a constant pressure value equal to the average cr <br />:::asimum values encountered in that zone. Modelling pore pressures <br />as described above includes a certain element of conservatis^ <br />' 13 <br />