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METHOD TO DETERMINE LATERAL EFFECT OF A DRAINAGE DITCH ON ADJACENT WETLAND HYDROLOGY

R.W.Skaggs, G.M Chescheir, and B.D. Phillips

A method was developed
to estimate the lateral effect of a single drainage ditch on wetland
hydrology. The method can be used to calculate the distance of
influence of a single ditch constructed through a wetland, where the
distance of influence is defined as the width of a strip adjacent to
the ditch that is drained such that it will no longer satisfy the
wetland hydrologic criterion. Simulation analyses were conducted with
DRAINMOD to define the minimum, or threshold, drainage intensity that
would result in failure of a site to satisfy the wetland hydrologic
criterion. Analyses were conducted for five hydric soils spanning a
wide range of profile hydraulic transmissivities. DRAINMOD was used to
predict water table fluctuations between parallel ditches for a
50-year period of climatological record. For each soil, simulations
were conducted for a range of ditch spacings and depths to determine
the combinations that would result in the land midway between the
ditches just barely satisfying the wetland hydrologic criterion.
Analyses were conducted for climatological conditions for three
locations in eastern North Carolina. Results for Wilmington, North
Carolina, showed that the threshold drainage intensities would result
in water table drawdown from an initially ponded surface to a depth of
25 cm in approximately 6 days. That is, ditch depths and spacings
sufficient to lower the water table from the surface to a depth of 25
cm in a threshold time of about 6 days would result in hydrologic
conditions that would just barely satisfy the wetland hydrologic
criterion for that location. The threshold time is denoted T_{25} and is used as a surrogate for quantifying the water table drawdown
rate of sites that barely satisfy the wetland hydrologic criterion. T_{25} was found to depend somewhat on drain depth, but it was essentially
constant for all five of the soils examined. Similar results were
obtained for the other two locations, but because of differences in
weather and in the growing season, the threshold time (T_{25})
was found to be dependent on location. The T_{25} value is
also dependent on surface depressional storage, decreasing with
increasing storage. The discovery that water table conditions barely
satisfying the wetland hydrologic criterion are well correlated to the
time required for water table drawdown of 25 cm (T_{25} values) makes it possible to predict the effects of subsurface drains
on wetland hydrology. The lateral effect of a single ditch on wetland
hydrology can be computed by using T_{25} values in solutions
to the Boussinesq equation for water table drawdown due to drainage to
a single drain. While the method was developed for drainage ditches,
it may also be used for subsurface drains.

Project Objective

Quantify the lateral effect (distance of hydrologic influence) of a single ditch using proper drainage theory, climatic variables, and soil properties.

Related Publications

Phillips, B.D., R.W. Skaggs, G.M. Chescheir. 2007. Determination of Lateral Effects of Borrow Pits on Hydrology of Adjacent Wetlands. Written for presentation at the 2007 ASABE International Meeting in Minneapolis, Minnesota.

Phillips, B.D. 2006. Methods to Determine Lateral Effect of Drainage Ditch on Wetland Hydrology. Masters Thesis. Deptartment of Biological and Agricultural Engineering, N.C. State University, Raleigh, N.C.

Phillips, B.D., R.W. Skaggs, and G.M. Chescheir. 2005. Determination of Lateral Effect of a drainage ditch on Wetland Hydrology for two field sites. Written for presentation at the 2006 ASABE International Conference on Hydrology and Management of Forested Wetlands in New Bern, North Carolina.

Skaggs, R.W., G.M. Chescheir, and B.D. Phillips. 2005. Methods to determine lateral effects of a drainage ditch on wetland hydrology. Transactions of the ASAE. Vol. 48(2).

Skaggs, R.W., and G.M. Chescheir. 2001. Methods to Determine Lateral Effects of a Drainage Ditch on Wetland Hydrology . Written for presentation at the 2002 ASAE Annual International Meeting / CIGR XVth World Congress.

Tables (PDF)

T25 values for all North Carolina counties (Based on 5% of Growing Season)- Updated July 1, 2006. Revisions in RED

Figures

Plot for determining Boussinesq Parameter