1) The Ugum River watershed in Guam has experienced severe soil erosion, exceeding allowable rates and negatively impacting water quality and coral reefs. 2) A 5.5 acre section of the watershed with highly eroded slopes ranging from 30-60% was modeled using erosion prediction software and calculations. 3) Initial erosion rates were 2-3 times the allowable rate. A restoration plan was developed using slope regrading, terracing, mulching, and planting native ground covers to successfully reduce the erosion to within acceptable levels.

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Guam has been plagued for many years with soil erosion problems,
causing serious environmental problems, affecting navigation along the
rivers and the fresh water quality. The Ugum River watershed is a 4,672
acre watershed located in the southern region of Guam that is composed of
rolling hills and very steep slopes. The Guam Waterworks Authority is
allowed to pump 4 million gallons of water per day from the Ugum River
to their treatment facility; however, the operating cost has increased due to
high water turbidity. Water that is not treated in the facility makes its way
into the estuary and reef environment at the river outlet where a direct
decline in the coral reef health has been observed due to excessive
sediment loading upstream. Some secondary effects include decreased fish
populations and tourism. In this project, a 5.5 acre portion of the watershed
on a highly eroded portion of the Ugum River was isolated and analyzed
for current erosion rates using WEPP (Water Erosion Prediction Project)
Software with data collected from Web Soil Survey database and with
hand calculations. The project goal is to reduce rainfall erosion volumes to
significantly less than acceptable rates, which is 2 tons/acre/year for
Agfayan clay, by regrading the land and planting native ground covers.
Both of these work to reduce soil erodibility factor and the velocity of
runoff. Restoring the most damaged portions of the Ugum River should
reduce the total sediment loading and function to decrease stress on the
reef and water treatment plant.
Introduction
Materials and Methods
The average annual precipitation, runoff, and erosion are displayed in
Table 1; a snapshot from the model is shown as Figure 2. The Agfayan
clay soil type from the area of interest was not available in WEPP, so soil
composition parameters were specified. Hawaii was chosen for location
data because it has the closest latitude to Guam. Notably, the average
annual soil loss is 2 to 3 times the allowable quantity for Agfayan clay
soil.
Table 1: Comparison of Three Slope Profiles
Figure 2: WEEP Model for 60%-30% Mixed Slopes
Sediment loss during various storm events was also calculated by WEPP,
the data is displayed in Figure 2. The daily sediment loss is very high for
both 30% and 60% slopes, even for a 2 year return period storm. For a 50
year return period storm the daily sediment loss is a ridiculously high
average of 34 tons/acre/day. However, this data is even lower than
theoretically expected as the location information for Hawaii provides
lower runoff rainfall erosivity factor (R) and soil erodibility factor (K)
than Guam.
Figure 3: Daily Sediment Leaving for Three Different Slope Values
Results Conclusion
Initial erosion rates on the analyzed portion of the Ugum River were
magnitudes higher than the allowable annual and daily soil loss values.
Two restoration plans were derived to reduce these values to acceptable
levels. First, a new cover management plan was modeled which reduced
the maximum annual soil loss to 4 tons/acre/year. Since further reduction
was necessary, terraces were implemented resulting in a new maximum
annual soil loss of 1.95 tons, thus achieving the restoration goal.
Restoration should be completed to within a few feet of river level to
compensate for swelling during heavy storms.
For the revegetation aspect of the restoration, research was done on native
plants that would thrive in the riparian zone surrounding rivers and will
benefit the local ecosystem. Osmoxylon mariannense is a critically
endangered tree species in Guam and the Northern Mariana Islands.
Planting endangered species, especially along riversides will maximize
their capacity to propagate downstream. Coccinia grandis, the ivy gourd,
also known as baby watermelon is a vine species native to Guam that will
quickly provide ground cover and seasonal fruit for the fauna. Another
valuable groundcover would be a clubmoss, such as staghorn clubmoss
(Lycopodiella cernua), which primarily grows streamside. Additionally
Acrostichum aureum, the golden leather fern, is a large species of fern that
grows in mangrove swamps and other wet locations which will provide a
large vegetative surface area to impede direct rainfall impact.
This proposed restoration plan successfully remediated the excessive
erosion occurring along 700 feet of the Ugum river while also promoting
the overall health of the local ecosystem. Some assumptions had to be
made while modeling the environmental factors, but the results still
provide a reliable average representation of geologic and hydrologic
conditions. While this is only a small portion of the Ugum River, scaling
up this project would mitigate the elevated sediment and nutrient loading
that is damaging the estuary aquatic life and remove pressure from the
Guam Waterworks Authority treatment facility. Despite the fact that this is
a solution, the ever increasing amounts of severe weather events could
warrant more extreme remediation efforts.
References
1. Khosrowpanah, S. & Jocson, J. (2005) Environmental Assessment for
Non-Point Sources of Pollution for Ugum Watershed. University of
Guam, Water and Environmental Research Institute (WERI), Technical
Report 109.
2. Khosrowpanah, S. & Heitz, L. F. (2001) Rainfall Erosivity Erosivity
Factors (R-Factors) For Selected Islands in the Federated States of
Micronesia. University of Guam, Water and Environmental Research
Institute (WERI), Technical Report 92.
3. Khosrowpanah, S. (2018) Soil erosion Monitoring program. University
of Guam, Water and Environmental Research Institute (WERI).
4. Lucas, J. & Gawel, A. M. (2018) Plants of Guam and CNMI.
iNaturalist Network. http://www.inaturalist.org/guides/4082
5. . Haddock, R. (2016) Urolithiasis, Urinary Cancer, and Home Drinking
Water Source in the United States Territory of Guam, 2006–2010. Int J
Environ Res Public Health.
6. Ward, P. & Hoffard S. (1965) Ugum River in The Hydrology of Guam
(pp. H7-H8). Washington D.C. U.S. Government Printing Office.
7. Ridgell, C. (2016) Santa Rita Residents Face Erosion of Property From
River. Pacific News Center. https://pacificnewscenter.com/santa-rita-
residents-face-erosion-of-property-from-river/.
Acknowledgements
Dr. Tom Owino
Materials:
• USDA Web Soil Survey
• Water Erosion Prediction Project (WEPP)
Methods
1. Perform preliminary analysis of Ugum Watershed using Google Earth.
2. Locate heavily eroded portion of the Ugum River using USDA Web
Soil Survey.
3. Determine soil type, composition, depth,
4. Determine rainfall erosivity, watershed traverse lengths, slope angles,
and profile width.
5. Average traverse lengths
6. Input data into WEPP to calculate daily sediment leaving and volume
runoff for each segment of the survey area.
7. Check values by calculating erosion and volume runoff using
Equations 1 and 2.
8. Model remediation options in WEPP and using hand calculations
including grading of terraces, lowering the cover management factor
with ground covers.
9. Consider supplemental ecosystem restoration like planting endangered
native plant species.
10. Determine optimal restoration plan and compare new erosion and
runoff rates to original profile.
Equation 1
Q=
(𝑃−0.2𝑆)2
(𝑃+0.8𝑆)
Equation 2
T=RKLSCP
Where T= total soil loss
R= Runoff Erosivity
K= Erodibility
LS= Length/Slope Factor Figure 1: Area of Interest Soil Types
C= Cover Management
P= Preservation Practices
BE 3220, EEES, Clemson University, Clemson, SC, 29634
Devon Beesley, Pa-Sweet Betancourt, Jacob Simmons
Ugum River Restoration Project
Average Annual
Precipitation (mm)
Average Annual Runoff
(mm)
Average Annual Soil Loss
(tons/acre)
30% Slopes 501.91 152.9 4.135
60% Slopes 501.91 153.09 6.098
50% Split 501.91 153.07 4.926
Percent Slope
Rate of Erosion
(tons/acre-year)
Average Runoff (in)
Average Precipitation
(in)
30% 56.43 82.59 85
60% 107.34 82.59 85
Mix 81.89 82.59 85
Erosion
Calculations
R K LS C P T
30% 600 0.24 4.9 0.003 1 2.12
60% 600 0.24 9.32 0.003 1 4.03
30%-60%
mix
600 0.24 7.11 0.003 1 3.07
Percent Slope
Soil Loss with 1
Terrace (tons/acre-
year)
Soil Loss with 2 Terraces
(tons/acre-year)
30% 1.58 1.24
60% 2.88 1.95
30%-60% mix 2.23 1.73
Table 2, below, shows the hand calculated Erosion and Runoff values.
Runoff erosivity (R), erodibility (K), and annual precipitation values were
found in reviewed literature (Khosrowpanah et al, 2001, 2005, 2018). Hand
calculated values are much larger and more realistic than the WEPP values.
The erosion tolerance of the soil was 2 tons/acre-year, therefore both
models call for significant remediation efforts to be made
Table 2: Hand Calculated Values
The first method for remediating this area was to mulch the current
vegetation and plant new endangered species of plants and plants that
would succeed in this environment. Since there will be a vegetative canopy,
the modified universal soil loss equation could not be used. Instead a cover
management factor of 0.003 was used assuming a 1.5 foot canopy with
100% ground cover, which is simulated by the thick mulch layer. The
calculations for this approach are shown in Table 3. This implementation
reduced total annual soil loss to a maximum of 4 tons per acre. This is 26
times less erosion than before restorative efforts, but is still more than the
allowable 2 tons/acre/year for the Agfayan Clay soil type. Terraces are the
next factor that can be implemented to reduce erosion. Data could not be
found for terrace sediment delivery ratios on slopes greater than 25% so the
highest ratio was assumed to apply to 30% and 60% slopes. The soil loss
with one or two terraces is shown in Table 4 below.
Table 3: Soil Erosion Value with Mulching
Table 4: Soil Erosion with Mulching and Terracing
Two terraces combined with mulching and replanting of native plants
effectively reduced the erosion to acceptable levels. A third terrace would
further reduce on site erosion but may not be possible considering the
average traverse length is 88 ft. Despite the fact that this is a solution, the
ever increasing amounts of severe weather events could warrant more
extreme remediation efforts.