Slide 7 of 43
Notes:
The grass channel consists of a broad, mildly sloped open channel designed to maintain a minimum residence time of about 10 minutes for the "water quality storm". Grass channels have traditionally been utilized only for stormwater conveyance purposes. In the past, designs ensured adequate capacity to carry a larger storm, usually the 10 year frequency storm and protection against erosion for smaller, more frequent storms, usually the 2 year event. Water quality treatment for the smaller, more frequent storms has only recently been a design consideration. The grass channel design is the only vegetated channel practice presented in this slide show which uses a flow rate as the principle design criteria variable. This is referred to as a rate based design.
To meet the water quality criteria, grass channels must have broader bottoms, lower slopes and denser vegetation than most drainage channels. Nominal pretreatment is created by placing checkdams across the channel below pipe inflows, and at various other points along the channel. The filter bed area in a grass channel is usually confined to the top inch of soil and thatch, since most runoff events will traverse the length of channel in about ten minutes.