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<br />0018i6 <br /> <br />2000-feet of the levee due to high flow velocities and steep <br />slopes required to fit the levee along the Riverwood Estates <br />subdivision. The Alternate 'B' alignment is shown on Sheet 1. <br />Figure 9 shows the typical tieback levee cross section. <br /> <br />This alternative is more costly than Alternate 'A', because of <br />the riprap requirements, but it removes Riverwood Estates from <br />the floodway. The improvements would restrict the usage of <br />existing platted lots which extend into the river. <br /> <br />L. Seepage Control <br /> <br />During the completion of this study, seepage control through <br />existing and upgraded levees has been a concern expressed by the <br />participating agencies. From what is currently known of the soil <br />conditions, all indications are that sandy soils exist throughout <br />the reach of the river and to an indeterminate depth. There is <br />presumed to be potential for seepage under the levee system for <br />all areas in the reach except those areas landside of the levee <br />that are above the elevation of the 100-year floodplain. <br /> <br />, <br />, <br />i <br /> <br />While it is generally felt that seepage under the levee system can <br />not be stopped entirely, measures may be needed to protect against <br />seepage where the flow of water through the soil could endanger <br />the structural integrity of the levee system and cause flooding of <br />properties landward of the levee. This protection could be <br />accomplished through the use of different control measures, <br />including: <br /> <br />I <br /> <br />o impervious cutoff trenches <br />o riverside impervious blankets <br />o landside seepage berms <br />o pervious levee toe trench <br />o pressure relief wells <br /> <br />l <br /> <br />In this study and cost comparison, seepage control has been <br />separated from the other types of improvements, as the extent and <br />type of seepage control improvements required is not known at this <br />time. It is recommended that the next phase of the project <br />include the necessary geotechnical investigations required to make <br />recommendations for specific seepage control measures required in <br />different locations along the project area. Design decisions <br />regarding seepage control measures should be coordinated with the <br />COE and FEMA so as to obtain concurrence on a proposed course of <br />action and the improvement design. <br /> <br />I' <br />~ <br />~ <br />r <br /> <br />For the purpose of including a cost estimate in this <br />reconnaissance study, seepage control was considered to be <br />required in all areas of the levee system, except those where the <br />levee is needed to provide for freeboard only. <br /> <br />-24- <br />