Transcript Design of Maturation ponds, Aquaculture, and Wetlands for

Watershed Plan for Heredia, Costa Rica
Whitney Thomas, Matthew O’Malley, William Brown
Biological Systems Engineering, Virginia Tech
Introduction
•Problem Statement: The Public Utilities
Company of Heredia S.A. charges all customers a
green fee in order to pay land owners in the Heredia
watershed to keep their lands forested or to reforest
existing crop land. Currently any land owner that
wishes to take part in the program may enter into a
contract and all participants receive the same stipend.
The company would like to distribute funds based on
the amount of erosion and resulting total suspended
solids each land parcel contributes.
•Design Objective:
The goal of this study is
to identify the land areas that contribute the most to
erosion and total suspended solids in the Rio Tibas
and Rio Segundo Sub-Watersheds of the Heredia
Basin and determine the BMP options that will help
lower TSS to 10 mg/L at the watershed outlets.
Rio Segundo Watershed
Rio Tibas Watershed
GIS analysis
GIS analysis
The Rio Segundo Watershed is located in the western
region of the Heredia watershed. Currently, only
locations in the upper portion of the watershed have
been reforested as part of the Procuencas Project.
An analysis of the present situation within the Rio
Segundo watershed was assessed using GIS and the
USLE to determine whether reforestation from
previous years has reduced erosion and to assess
where critical areas of erosion are occurring.
The Rio Tibas Watershed is located in the Eastern
region of the Heredia watershed. Currently, the only
locations involved in the Procuencas Project are in
the upper mountainous portion of this watershed.
The areas that were previously reforested in this
upper region have shown to be efficient at reducing
erosion. An analysis of the present situation within
the Rio Tibas watershed was assessed using GIS
and the USLE.
The same mountain in Rio Segundo before (left)
and 5 years after (right) reforestation.
Heredia
•Costa Rica loses 860
million tons of top soil
annually.
• The Public Utilities
Company has reforested
1000 hectares of land
thus far and distributes
drinking water to all of
Heredia.
Intercropping
Design Results:
Design Results:
The highest areas of erosion were found to be
occurring in the middle regions of the watershed
closest to the streams. These regions contain
mostly permanent crop areas and urban areas with
slopes ranging from 0-15%. Assuming that all of
these crop areas implement vegetation filter strips
within 50 m of the stream and intercropping in the
permanent crop areas 50 m or more from the stream
the results indicate that the erosion will have been
reduced to low erosion levels. The only remaining
areas of high erosion are occurring in the Urban
areas.
The highest areas of erosion were found to be
occurring in urban areas near the watershed outlet.
The areas with medium potential erosion were found
throughout the middle and lower regions of the
watershed where majority of slopes are 0-15% and
the main land use is permanent crops. Here BMPs
such as vegetative filter strips are planned for the
high and medium erosive areas 50 m from the
stream and intercropping can be used 50 m and
further to reduce erosive potential.
Cost Analysis
Heredia Watershed
• Vegetative filter strips were chosen out of many
possible BMPs because they require little
maintenance and cost little to install.
• Filter strip length (parallel to flow) should always
equal the width of the contributing drainage
area.
• The limits of the lateral slope for embankment
are 2%-15%.
q = (0.00236/n)(Y1/5)(S1/2)*
Where:
q = discharge per foot of width of filter
strip (cfs/ft)
Y = allowable depth of flow (inches)
S = slope of filter strip (percent)
n = Manning’s “n” roughness coefficient
Costa Rica
•The population of Heredia
is about 125,000 people
in an area of 2657 square
kilometers.
Vegetative Filter Strip
WfMIN = Q/q*
Where:
WfMIN = minimum filter strip width
perpendicular to flow (feet)
Procuencas Project
The Procuencas Project is implemented by the
Public Utilities Company of Heredia, S.A. as an
incentive to promote better water quality throughout
the Heredia Watershed. Landowners are paid to
keep their land forested or to reforest barren land
or cropland. The goal of this program is to improve
water quality through flow regulation, water
filtration, erosion control and sedimentation, and
maintaining the hydrological functions provided by
forests.
Best Management
Practices
Currently all land owners are paid the same
amount regardless of how much erosion their
land produces. The funds to contribute to the
Procuencas Project have been divided so that
high erosive values were multiplied by 1.5,
medium by 0.75, and low by 0.25. These
multipliers were decided based on erosion
classification and percent of watershed.
Acknowledgements: Dr. Theo Dillaha - Biological Systems Engineering
Mr. Luis Gámez - Public Utilities Company of Heredia S.A., Costa Rica
Present Value (₡/ha/yr)
Suggested Value (₡/ha/yr)
Land Erosion
Percent of
Classification Reforestation Keep Forest Watersheds Reforestation Keep Forest
High
45,000
47,720
6.76
67,500
71,580
Medium
45,000
47,720
15.44
33,750
35,790
Low
45,000
47,720
77.79
11,250
11,930
Intercropping is the practice of cropping two
crops at the same time in the same place.
Some examples of intercropping in tropical
areas include; maize-bean, maize-pumpkin,
and sugarcane-bean. A case study from
southern China showed that intercropped
cassava in Arachis pentoi reduced soil loss 4fold. Intercropping has a similar effect to the
vegetation filter strip but it is placed within crop
fields.
Conclusions
• Forested areas located in the Upper reaches
of each watershed should continue to be
protected, as they have proven to be effective
at reducing erosion and soil loss
•The majority of the focus of the Procuencas
Program should be protecting new land in the
Rio Segundo Watershed because it has more
land with high erosion rankings.
•Implementation of the above BMP’s should
help to reduce soil loss. Vegetative Filter
Strips have found to be about 75% efficient at
reducing sediment and Intercropping has a
similar efficiency.
•Future field work should be done in Costa
Rica to establish the validity of the USLE
values used.