2002 Annual Report
Research
Evaluation and Design of Blind Inlets
Bruce Wilson, Associate Professor
Gary Sands, Assistant Professor
Bradley Hansen, Assistant Scientist
Kristina Oveson, Graduate Student
Udai Singh, Research Associate
Funding Source
Minnesota Pollution Control Agency; Center for Agricultural Impacts on
Water Quality; Minnesota Department of Agriculture
Objective
The overall goal of this research is to develop design criteria for blind
inlets. The specific objectives are:
- Evaluate design criteria for rock inlets.
- Evaluate effectiveness of high-density drainage tube installations
as blind inlets.
Project Description
Surface inlets are installed in fields where the low permeability of
the soil limits the removal rate of ponded surface water. These inlets
allow the water to be removed rapidly, but the outflow bypasses the natural
filtering processes of the soil. An alternative approach is to replace
the surface tile inlets with blind inlets, that is, replace the soil with
a coarse media of high permeability. Farmers frequently prefer these inlets
because they are easier to use with their tillage equipment. In addition,
they have the potential to remove sediment and other pollutants. Unfortunately,
blind inlets often become ineffective with time as the deposited sediment
plugs the filter. Design procedures are needed to account for the time-varying
permeability. Research is also needed to assess the effectiveness of blind
inlets in removing potential contaminants.
The framework for the proposed approach is based on representing the
processes by dimensionless variables. This approach is widely used in
engineering design. For example, the friction loss in pipe flow is computed
by using a dimensionless friction factor obtained from the Moody diagram
for a dimensionless Reynolds number (hydraulically smooth flow) or for
a dimensionless roughness height (hydraulically rough flow). The effectiveness
of grass filters to remove sediment is also well represented by dimensionless
variables. Dimensionless representation allows relatively inexpensive
laboratory and/or prototype studies to be used to determine field response.
Results
A unique site at the Southern Research and Outreach Center at Waseca,
Minnesota, is being used to gather field data. The site was subdivided
into four adjacent fields of approximately 3/4 acre each. Berms were created
to define clearly the watershed boundaries. To represent field depressions,
rectangular basins were constructed at the field outlets. Four surface
drains were installed consisting of (1) flush drain pipe, (2) a rock inlet
filled with coarse gravel, (3) a rock inlet filled with fine gravel or
pea rock, and (4) a rock inlet filled with a 50/50 mixture of the coarse
and fine gravel. Unique data collection methods have been developed and
are being used at the site. This includes a water-quality sampling algorithm
based on a calculated curve number, which fluctuates not only with soil
moisture conditions but also with crop cover. Cumulative runoff volume
was used to ensure that samples are gathered for large events. The design
of sampling cores to determine sedimentation depth within rock inlets
was an important component of the study.
All of the rock cores plugged because of storms in 2001. Two of the rock inlets have been replaced with alternative-sized rocks. The third rock inlet has been replaced with high-density drainage tube installation to serve as a blind inlet. Very few runoff events were observed in 2002. Data collection is continuing at the site.