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— — Y3,y4 [X3;y3] [X4;y4]- - - - - - - - - - - - I I I I m I I v I I I Y2 - — — — — — — [► 2 Ot ; I ro I I y1- I I I I I I X-1-0 X2 X3 X4 Depth[feet] Figure 2-4: Schematic Lake Storage Capacity Graph Since xi, x2, x3 and y3 are known, only values for y, and y2 need to be approximated. The abbreviation `BS', meaning "Bottom Storage", refers to the storage volume below 5 feet, which is the normal water surface elevation. The abbreviation "TS", meaning "Top Storage", refers to the storage volume above 5 feet up to the top of the lake. Since the lakes' water surface will always be 5 feet or greater, the storage volume below 5 feet (the bottom storage) is not important for this analysis. It becomes important first when/if water is being pumped to decrease the lake water surface elevations below 5 feet. As the available data for Phase 2 lakes did not include the Bottom Storage (BS) volume, a value for y, was chosen.4 For simplicity the y, was chosen to be the same for all lakes and the value of 0.1 acres was chosen since the modelling software does not allow zero values. Equations 2.2-1a to 2.2-2b, derived from the calculation of the "area under the curve", were used to mathematically determinate values for y2. For more details on these calculations refer to Appendix B. TS = (x3 —x2) X Y2 + (Y3-Y2)X(X3-X2) Eq. 2.2-1 a Y2 - 2XTS+X2XY3-Y3XX3 Eq. 2.2-1 b (X3-X2) This mathematical model assumes that between these three coordinate points, the lake volume is increasing linearly. The additional fourth coordinate, [x4, y4], that can be seen in Figure 2-3, Figure 2-4 and Table 2-1 were added for modelling purposes. Since the purpose of the XPSWMM analysis is to determine if the lakes have sufficient stormwater storage capacity, the additional coordinate was included to capture the water surface elevation if any of the lakes should overtop, assuring no water was "removed" during the analysis due to an overtopping lake. This additional coordinate was placed two feet above the third coordinate ([x3, y3]) and has the same storage area as the third coordinate. ° For Phase 1 lakes, which are also a part of the Phase 2 models, the bottom storage is calculated based on available data which included bottom storage volume. See River Islands Phase 1 Stormwater Mitigation report dated April 2020 for more information on phase 1 lakes. River Islands Phase 2 Stormwater Mitigation Rev 2 10 1.� PACE