▁m ▁( two ▁feet ) ▁vert ically ▁and ▁ 1 . 5 ▁m ▁( five ▁feet ) ▁hor izont ally ▁on ▁level ▁ground . ▁ ▁If ▁the ▁soil ▁is ▁s atur ated , ▁or ▁if ▁the ▁ra inf all ▁rate ▁is ▁greater ▁than ▁the ▁rate ▁at ▁which ▁water ▁can ▁in fil tr ate ▁into ▁the ▁soil , ▁surface ▁run off ▁occurs . ▁If ▁the ▁run off ▁has ▁sufficient ▁flow ▁energy , ▁it ▁will ▁transport ▁lo os ened ▁soil ▁particles ▁( sed iment ) ▁down ▁the ▁slope . ▁She et ▁er os ion ▁is ▁the ▁transport ▁of ▁lo os ened ▁soil ▁particles ▁by ▁over land ▁flow . ▁ ▁R ill ▁er os ion ▁refers ▁to ▁the ▁development ▁of ▁small , ▁e ph em eral ▁concentr ated ▁flow ▁paths ▁which ▁function ▁as ▁both ▁sed iment ▁source ▁and ▁sed iment ▁delivery ▁systems ▁for ▁er os ion ▁on ▁hills lop es . ▁Gener ally , ▁where ▁water ▁er os ion ▁rates ▁on ▁dist urbed ▁u pl and ▁areas ▁are ▁greatest , ▁r ills ▁are ▁active . ▁Flow ▁depth s ▁in ▁r ills ▁are ▁typically ▁of ▁the ▁order ▁of ▁a ▁few ▁cent im eters ▁( about ▁an ▁in ch ) ▁or ▁less ▁and ▁along - channel ▁s lop es ▁may ▁be ▁quite ▁ste ep . ▁This ▁means ▁that ▁r ills ▁exhib it ▁hyd ra ul ic ▁physics ▁very ▁different ▁from ▁water ▁flow ing ▁through ▁the ▁deeper , ▁wider ▁channels ▁of ▁streams ▁and ▁rivers . ▁ ▁Gul ly ▁er os ion ▁occurs ▁when ▁run off ▁water ▁accum ul ates