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Reach No. 3 (Plate 7) The New Mercer Can~l carries the runoff from the upper portions of McClel- lands Basin to Mail Creek. OlIring periods of high rUllOff,thedepthofflowin the canal becomes greater than originally designed for. This results in the inundation of the ditch service road on the east and some associated erosion damage. At the confluence with Mail Creek, excessive discharge velocities could cause some additional erosion and/or deposition of silt. Finally, overtopping of the LarkboroughSubdi- vision detention pond along its east bank will also contribute to the flooding and erosion problem in this reach. ReaCh No.4 (Plates 8 and g) There is a substantial amount of overland flow through the streets and yards in Larkborough Subdivision during the 100-year storm. Although the flows are not above the first floor elevation for any of these units, there is the potential to have structure and content damage to seventeen (17) homes. Between Larkborough and Shields Street, the floodplain consumes a considerable amount of ground whiCh is presently in agricultural use. During floods here. there exists the potential for crop and/or flood damage. Reach No. 5 (Plate 9) The Shields Street crossing consists of one l5-inch corrugated metal pipe. As a result, Shields Street is overtopped during even minor storm events causing traffic interruption and some erosion problems. Upstream of Shields Street, con. siderableagriculturalgroundisconsumedbyfloodplain. Finally,duringflooding, the Pleasant Valley and Lake Canal would probably breach and thus require repairing and probable removal of some silt deposits. Reach No. 6 (Plates 10 through 13) The lower portions of the McClellands drainageway have extremely wide flood. plains as a result of backwater behind culverts at both Timberline Road and the Union Pacific Railroad. Both of these crossings would be overtopped by 1O.year stOrnlS or greater, and would be in danger of washing out. Because of the tall and steep em- bankment of the railroad, extensive damage to the railroad as well as adjacent pro. perty could result during a storm of 25-year frequency or greater. Upstream from Harmony Road, the McClellands drainageway h~s been improved as part of the residential development which has taken place. and all improvements are adequate for the lOO-year design flows. No flOOd hazards are apparent in either of these reaches. 1V.3 - , > - " ~ " ~ - . o - " . . < ~ :J o~, l-O:EL "'- .....,.. "" <D ..,. ..,. N ..,. 0 ,... "" '" 0 N <D "" "" "" '" ..... ..... 0 M M ." . >--"',,",... oo~ 00,... <D "" <D 00,.,,.,..,. 0,...,... M M,..,... N", '" "" '" "" , ""0 0 0,..,.. ~ ,... ~ N N N M ... ... ... ... ... ... 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