Watershed Achievements Report

Statewide Watershed Achievements Report 2015
Side Inlets to Improve Water Quality
The primary focus of this study is the effectiveness of
alternative side inlet designs in reducing the sediment
contributions of croplands and decreasing downstream
peak flow rates.
The key objectives were to: 1) conduct an inventory of
side inlet locations in several selected watersheds to
help develop siting method(s), 2) perform hydrologic
modeling on large and small scales to evaluate the
potential for side inlets and water quality benefits,
3) conduct side inlet control experiments at U of M,
Southwest Research and Outreach Center (SWROC) to
evaluate several alternative side inlet designs, 4) develop
design guidance, and 5) to demonstrate the use of
alternative side inlets in field conditions, with associated
outreach. The types of side inlets considered in this study
were the existing straight sloped pipe and alternative
designs consisting of a drop inlet with flush pipe,
Hickenbottom perforated riser on a drop inlet, rock
inlet, perforated tile coil inlet, and rock weir.
Three GIS-based methods were developed and evaluated
for identifying the location of side inlets. Overall, the best
method predicted locations of side inlets, or potential
need for a side inlet, with a fair level of accuracy.
Field experiments were conducted at the U of M SWROC
in Lamberton, Minnesota to experimentally assess
five alternative side inlets to obtain hydraulic and
sedimentation parameters that are needed to model
them and to define field scale pros and cons of these
inlets. The data indicated that side inlets with a longer
detention time have greater sediment removal and that
the perforated tile coil did not function well.
Modeling results support the use of alternative types
of side inlets along with potential earth reshaping to
establish short-term detention. When performance, cost,
maintenance, life expectancy and modeling results for
individual sites were all considered, the Hickenbottom
riser (or similar product) on a drop inlet was indicated as
the best alternative side inlet choice. A drop inlet type
of side inlet has a number of advantages over designs
like straight sloping pipes, rock weirs and rock trenches,
including:
a. Typically can be constructed closer to a berm along
the top of the ditch or streambank compared to
a straight sloping pipe, reducing the distance the
side inlet extends into the field and improving the
potential for location of the inlet in a perennial
buffer.
b. Typically reduces the flow velocity at the outlet of
the structure, reducing associated outlet erosion
potential, by dissipating more energy in the drop
pipe and reducing the slope of the horizontal pipe.
c. Provides for optional types of intakes on top of
the drop pipe that can be tailored to the residue
accumulation conditions at the intake, including
easy replacement, to ensure adequate flow capacity,
as well as adequate detention time for sediment
trapping. A wide variety of intake styles and
capacities for drop inlets are available from various
manufacturers.
Watershed level modeling indicated trapping efficiencies,
reduction efficiencies, watershed reduction efficiencies
and net reduction efficiencies were generally dependent
on the magnitude of the runoff event and the influent
size distribution, as well as on-field detention time.
Four local partners demonstrated alternative side inlets.
The events led by local, state agency and university
partners engendered positive responses to alternative
side inlet designs, as well as some producer investigation
of the practice for their property.
Goals
• Develop a GIS methodology for identifying side inlet
from Light Detection and Ranging (LiDAR) data.
• Develop design guidance material based on results
of experimental work at SWROC and modeling done
using Basin Analysis of Sediment-laden Inflow
(BASIN).
• Demonstrate side inlets for water quality in four
partner local governmental unit (LGU)s.
Results that count
• GIS methodology was found that has a fair degree
of accuracy in locating and prioritizing sites for side
inlet control structures. However, more research
and development will be required to make the
methodology available to and usable by LGU staff.
• Supplemental design guidance was developed
clearly defining the potential of side inlets to increase
Minnesota Pollution Control Agency www.pca.state.mn.us 36