Agricultural Pond
Private Engineering
Resources & Costs
Agricultural Water Resource
Development Workshops
Presented by:
Cavanaugh & Associates P.A.
530 N. Trade Street, Suite 302
Winston-Salem, NC
336.759.9001
Why Hire An Engineer?
• Is the pond in the best location?
• How much water do I need?
• How much water will it yield?
• Will it hold water?
• Which will work - Excavated or
embankment? Does it matter?
• Are there wetlands?
• What about all these permits?
This looks like a good spot!
How much water will it yield?
Inflow Hydrograph for Small
Watersheds
• Need:
– Peak Discharge, Qp
– Time to Peak Discharge, Tp
– Shape of the Hydrograph
The Rational Method
Q = CIA
(Calculating peak flood discharge at a given point)
• Q = estimated design discharge (cfs)
• C = Composite Runoff Coefficient for
the drainage area
• I = Rainfall intensity for the design
storm (inches/hour)
• A = Drainage Area (acres)
Delineating Drainage Area, A
• The first step in determining if your pond
is in the “right spot”
• Orange Pond A = 25 acres
• Blue Pond A = 125 acres
• Purple Pond A = 350 acres
Determine Time of Concentration, tc
• Depends on
– Hydraulic Length of the drainage area
– Height of the most remote point on the watershed
above the outlet point
• Orange Pond tc = 5 min
• Blue Pond tc = 7 min
• Purple Pond tc = 8.3 min
Determine Rainfall Intensity, I
• Depends on
– Statistical
rainfall for the
design location
– Designated
return period
for the design
Determine
Composite Runoff Coefficient, C
• Depends on land use
in the drainage area
– Building types
– Ground coverings
– Soil types
– Building densities
Wooded, Sparse
Ground Litter, C = 0.20
Wooded, Deep Ground
Litter, C = 0.10
Commercial,
C = 0.85
Apartments, C = 0.60
Residences, 2 dwellings
per acre, C = 0.35
Time to Peak, Tp
• Based on Qp & Volume of Water Under
the Hydrograph
– Qp is estimated by Rational Method
– Volume is estimated based on
•duration of the design storm
•Soil types
•Cover conditions
Shape of the Hydrograph
• Accepted practice is to assume a step
function unit hydrograph to estimate the
shape
Use the Inflow Hydrograph to
determine if the pond is
sufficient for your needs.
Excavation Ponds
• Is there adequate drainage to fill?
• What is the level of the water table?
• What is the impact of the water table?
• Will the soil types sustain water
storage?
Source: Dwane Jones/NCSU Co-op Extension
Embankment Ponds
• Is there adequate
drainage to fill?
• Will the soil types sustain
water storage?
• Is an earthen dam
adequate?
• What type of outlets are
needed?
• What is the downstream
impact?
Source: Dwane Jones/NCSU Co-op Extension
Embankment Earthen Dams
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•
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Built in natural topographic draw
Low flow outlet device
Riser/Barrel Outlet
Emergency spillway
Outlet size calculated using Stage-Storage Function to control
pond level and downstream needs
• Dam Construction must meet Dam Safety Requirements
General Steps in Pond Design
• Base Topography
– Existing Conditions
– Height of Dam Required
– Earthwork Required
• Geotechnical Investigation
– Dam Core Design
– Seepage Control
– Compaction & Materials Testing
General Steps in Pond Design
• Hydrologic Design
– Rainfall Event Investigation
– Peak Discharge
– Watershed Yield/Water Supply
• Hydraulic Design
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Primary Spillway Design
Riser/Barrel Design
Emergency Spillway Design
Overtopping Control
Construction Considerations
• Preparation of Construction Documents
– Plans & specifications to be sealed by
Professional Engineer
– Permits required from NC Division
of Land Quality/Dam Safety
• Bid Acquisition
– Qualifying construction contractors
– Hold bid opening
– Ensuring contractor has proper bonds
• Construction Administration
– On-site observation of construction to protect
investment and safety
– Engineer certification of construction and asbuilts
What’s the Engineering Cost?
• Site dependant
• Two phases:
– Preliminary Site Evaluation
– Design: Engineering Design, Permitting & Construction
Document Preparation
• Rough estimate of design cost is 15-20% of
construction cost
• Preliminary engineering evaluation
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Preliminary hydraulic & hydrologic calculations
Cost estimate of geotechnical & wetland investigations
Preliminary design concept
Preliminary cost estimate of engineering costs, permitting
fees and construction costs
Why Hire An Engineer?
• Professional recommendation on
construction alternatives
• Familiar with Dam Safety Rules and other
permitting needs
• Protect your interest during construction
• Reduce your liability and risk
• In long run will save money and time