Agricultural PondPrivate Engineering Resources & Costs

Agricultural Water Resource Development Workshops

Presented by:Cavanaugh & Associates P.A.530 N. Trade Street, Suite 302

Winston-Salem, NC336.759.9001

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 = 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)

Q = CIA(Calculating peak flood discharge at a given point)

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

Commercial, C = 0.85

Wooded, Sparse Ground Litter, C = 0.20

Apartments, C = 0.60 Residences, 2 dwellings per acre, C = 0.35

Wooded, Deep Ground Litter, C = 0.10

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

Source: Dwane Jones/NCSU Co-op Extension

• 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?

Embankment Ponds

Source: Dwane Jones/NCSU Co-op Extension

• 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?

Embankment Earthen Dams• 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– 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 as-builts

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

– 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