5700 23RD DR W 2016-01-01 MF Import

_. ._ _ _ __.__ . __ 1 ( 7•0 "RO ION A .ARnS �' In order to evalusu poxntial erosion nnpacts (and subsequendy provide midgadon) as a result of I the proposcd development, it is itnpottant to uiderstand erosion and scdime�transport processea. The seciiment begins motion by a pracess cailed gross erosion (Table 1), which can be bro,ken + down into shat erosion and channel (gully) erosion. I Sheet erosion is a rtlativel slow rocess and is caused Y p by shallow "shcets" of water flowiag over ( the land surface and transpoting soil particles that have ban detached by iain drops. The shsllow surface flow rarely moves as a uniform s6at for more than a few fat before concentrating ia surface irregularities and resulting in rill erosion. Additional sediment is thus picked up and 1 transported. This erosion process is continuous over several storm or normal rainfali events. If the rills become mon than a few in;-hes deep, then the erosive regime changes to a much more raoid gully erosion regime where concentrated water flow can uanspoa large quancities of ; sedime�during a single storm e��ent. This usually occ�us on slopes greater than 20 percent a� � for slope lengths greater than 20 feet. � The surficial soils and parent maurial at the site have different susceptibilides toward shat a� gully erosion regimes. Advance outwash or equivalent gta�rular soils are less susceptible to shat � erosion than lodgemem till or silt because they have smaller percentages of€mes (parflcies smaller , than 0.02 millimeurs) and therefore surface water tends to infiltrate. U�er concentrated flow i regimes (gully erosion), the erosional hazards in gramAar soils are much gresur when slopes � j excad 10 percent. Areas underlain by lodgement till are considered susceptible to sheet erosion / whea slope gradieats ,.zcad 20 percent due to a high percentage of fines and low infiltration \. . capacity. However, the cohesive nature of the parent material attcmiates erosion potential under � �oncena�aua flow. 7.1 ShePt Erosion jlnpa c ( The various degrees of shcet erosion potentiai and sedime�hansport of surficial soils encoumered at the site can be calculated fmm the Uaiversal Soil Loss Equstion(U.S.L.E.). However, the soil Ierodibility can vary significandy between natural co�itions, when the soil is pmtecttd by a vegetadve cover, and consuvcuon condidoas when the vegetaaon has ban removed. IOne compo�nt of the equadon, lmown as the soil erodibiliry factor "K', is nlated to the type of material subjectcd to erosion. The K factor recomme�ed for use in the U.S.L.E. for these soil � rypes under nanual and developed conditions is presen[ed below. � I 1 8 I