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2.4 Hydrographs and Rainfall Data The input data for XPSWMM that represents the rainfall and drainage from surrounding watershed into each lake is a hydrograph. The hydrograph shows the amount of water going into the lake over time (the rain event). A technical report, River Islands Stage 1, 2A and 2B Full Buildout Internal Hydrology Analysis dated October 23 2019, contained the analysis of all Phase 1 lakes (lakes 1-13) for the 48-hour storm event. The 48-hour storm event data (hydrographs) for lakes 10-13 were directly used as input in the XPSWMM 48-hour storm event hydraulic models. The hydrographs for Phase 1 lakes are also described in detail in River Islands Phase 1 Stormwater Mitigation report dated April 2020. Since an HEC-HMS analysis was not available for the Phase 2 watershed, the below methods was used to retrieve hydrographs for the lakes in Phase 2. The analysis was done for the hydrographs given by both methods below to allow a comparison. More exact hydrographs would be obtained by a detailed HEC-HMS analysis of the Phase 2 development, which is assumed to be completed at a later date. See Appendix D for detailed calculations from the below described scaling methods. 2.4.1 Scaling based on approximated drawn contributory area The hydrograph for each lake and design storm was scaled from the hydrograph for lakes 1-2-3 based on their approximated contributory area, thus assuming the ratio of developed/undeveloped area as well as runoff coefficients etc. to be the same in Phase 2 as in Phase 1 Stage 1. See Figure 2-7 for a graphic representation of the contributory areas for each lake. Contributory Area for Lake X _ Hydrograph Value Lake X Eq. 2.4-1 Contributory Area for Lake 1-2-3 Hydrograph Value Lake 1-2-3 ➢ Solve for Hydrograph Value Lake X e I 4 a =: G J5 .kms 2 _I 015 / N � c w 7 _ fM1 � rC •,i P4ggDI5F CVT _ - 7-7 Figure 2-7: Approximate contributory drainage area for phase 2 lakes River Islands Phase 2 Stormwater Mitigation Rev 2 14 PACE