$解：如圖，過點?A?作?AF⊥BC?于點?F，?過點?D?作?DG⊥BC?于點?G?$
$?(1)?∵?AF⊥BC ?$
$∴?∠AFB=90°?$
$∵?∠B=60° ?$
$∴?AF=AB×sin B=5\sqrt 3m?$
$∴?S_{△DCE}=\frac 12×CE×AF=\frac 12×4×5\sqrt 3=10\sqrt 3(\mathrm {m^2})?$
$∴?10\sqrt 3×120=1200\sqrt 3(\mathrm {m^3})?$
$∴需要填土?1200\sqrt 3\ \mathrm {m^3}?$
$?(2)?四邊形?ABCD?是梯形\ \ \ \ $
$∴?AD//BC?$
$∵?AF⊥BC，??DG⊥BC?$
$∴?∠AFG=∠DGF=∠FAD=90°?$
$∴四邊形?ADGF?是矩形$
$∴?DG=AF=5\sqrt 3m?$
$在?Rt△CDG?中，?CG=\sqrt {CD^2-DG^2}=15m?$
$∴?EG=CG+CE=19m?$
$在?Rt△EDG?中，?tan E=\frac {DG}{EG}=\frac {5\sqrt 3}{19}?$
$∴新大壩背水面?DE?的坡度為?5\sqrt 3∶19?$
