Project # 30
The site and associated drainage area is located in the eastern half of Section 13 of
Liberty Township (TI47N R45W) and western half of Section 18 of Garfield Township
(TI47N R44W) in Polk County, Minnesota. The area is a mix of wooded area, CRP, and
agricultural land with an associated drainage area (ADA) of 170 acres.
Extensive erosion has been occurring on the steeper slopes of the agricultural field
(currently in CRP) during the large rain events since 1997. This has led to gullies
forming on the steeper slopes and sedimentation on the flatter slopes and into the
receiving township ditch.
An ADA of 170 acres was determined using information supplied by the owner, FSA
slides, FSA photos, and USGS topo maps. Using NRC8's Hydrology Guide for
Minnesota, a peak flow for a lOyr,:,24hr storm event was estimated at 13.8 cubic feet per
second (cfs) with a runoff of 0.65 inches (runoff curve number = 64, 1.5 percent
watershed slope, and watershed length of3600 feet). The volume of runoff water from
the lOyr-24hr-storm was estimated at 9.2 acre-feet. A 25yr-24hr-storm peak flow was
estimated at 25.7 cfs with a runoff of 1.03 inches.
A grade stabilization structure in combination with a grassed waterway was designed
to reduce field erosion, reduce sedimentation in the field and the township ditch, and
improve water quality. Rainfall will be flood routed using the temporary upstream
storage area of 2.0 acre-feet created by the structure, the required storage for a 10 yr-24 hr storm
event for the design discharge rate.
The grade stabilization structure will use a 24-inch CMP that discharges at a design rate
ll-cfs. Based on an average discharge rate' of 4 cfs, a l0yr-24hr-storm event would drain
the storage area in approximately 6 hours. The designed structure has a ten-foot
top with 3H: 1 V slopes to be covered with suitable vegetation. A 20-foot wide emergency
spillway has been designed to direct flows greater than the design storm around the
structure and into the grassed waterway.
The grassed waterway is designed with an 8-foot bottom With 4H: IV sideslopes and a
minimum flow depth of 2 feet. The 8-foot bottom was selected to allow for use of
common excavating equipment (scrapers or dozers) to construct the channel. The 2-foot
depth has the capacity to handle a total flow of 82 cfs, large enough to handle a 100yr-
24hr-storm event (52-cfs). Based on NRCS's grassed waterway program, the design flow
of 11 cfs produces a channel velocity of 1.2 feet per second. Because this is below the
recommended minimum velocity of 1.5 feet per second, maintenance requirements have
been addressed in the Operation and Maintenance Plan.
In addition to the flow from the ADA, the Sand Hill Watershed District has installed a
24-inch CMP through the east-west township road on the north side of the section. It is
designed to allow overflow from the ditch on the north side of the township road
southward through the site. The CMP invert was installed approximately 3 feet below the
road surface (estimated flow of 11 cfs). It is assumed the flow through the designed
structure will handle the volume of water from the overflow CMP. For this design, it also
has been assumed that the total volume of runoff from a 10yr-24hr-storm event will pass
through the structure before an overflow through the township road will occur.