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Page 2 of 3 2. For the Prospect Ditch, please explain the relationships of the model elements. In particular, it is unclear how subwatersheds were delineated. Some are very large (subwatershed L), and some are only on one side of the channel (L, M, K). Also, why does subwatershed L all drain to Structure 939? Some of it should possibly drain Structure #30. Colowvo 's Response: When setting up a SEDCAD model, the first demarcation is usually the drainage boundary. Next, if a particular watershed has areas of constant runoff characteristics (eg. a single reclamation status condition and therefore a single CN), then that will likely become a candidate for a sub - watershed. It may be one of several sub - watersheds tributary to a SEDCAD "structure ", or it may be the only sub - watershed tributary to that structure. The reason for this is that the collection point (the SEDCAD "structure') is selected as the point at which flow can arrive subject to the restriction that the flow path must generally cross the contours at a perpendicular angle. In fact, that flow path is used by SEDCAD to determine the time of concentration for that sub - watershed. In the case of Subwatershed L, it is entirely reclaimed, all at 3+ years (CN 74). Crossing the contours at right angles takes the flow down to Structure 39 as overland flow, almost flowing parallel to Reach 2 ofprospectDitch, but several hundred feet to the north. Watersheds M and K are limited to the other side of the channel, but not because they are different CN's; they are the same CN— 74 for 3+ years of vegetation growth as the land on Subwatershed L on the north side of Prospect Ditch. However, looking at the contours it is clear that Sub - watershed K drains directly into Pond PDl (perpendicular lines to the contours), while Sub - watershed M must drop into the channel (Str. # 30) somewhere around Station S, again constrained by the requirement to cross contours at a right angle as close as practicable. It is recognized there is some judgment involved in building the model. But in this case it would be unlikely than much of Subwatershed's runoff would report to Str. 30 ( channel from Pond PDI to Pond PD2) because it would have to flow parallel to the contours to get th the channel, which violates the physics). In fact, some small portion of the runoff form Sub - watershed L probably does find its way into Reach 2, but the model is constrained by the physics, which are controlled by the contours. Current Division comments: Some of the sub - watershed areas are odd shapes and do not, in fact, follow the restriction that flow paths must cross the contours at a perpendicular angle. For example, for Sub- watershed L the eastern portion of this area appears to not drain to Structure 39, unless there is a contour ditch that causes flows to deviate from the basic flow pattern. Another example is sub - watershed K; the eastern portion of this area does not appear to drain to Pond PD 1, per the topography. Please indicate if contour ditches are the reason for these odd shapes or if there is another explanation. 3. For the North Tributary of East Pit Ditch, please explain why drainage areas are modeled as "structures." Colowvo 's Response: On complex drainages or drainages that vary in size over several modeled time frames, we have elected to show the actual SEDCAD structures on the watershed maps to make it easier to follow the model. The sub - watersheds themselves are not actually modeled as "structures ". Rather, the collection points (eg., nodes, channels, channel junctions, stock ponds, etc) are modeled as "structures ". In some cases, a