Detention Pond Design: A Step-by-Step Guide for Land Development

When designing a subdivision or development site, managing stormwater isn’t just a regulatory requirement; it protects your layout, budget, and project success. This guide walks through a real-world example of how we design a detention pond using clear steps and plain language. We've included all the equations and explained each variable to make it accessible, even without an engineering background.

Site Summary:

• Size: 15 acres

• Project: Residential subdivision with 45 homes

• Slope: Gentle, flowing from northwest to southeast

• Soil: Moderate infiltration (Hydrologic Soil Group C)

• Drainage: All stormwater flows to one low point

• Goal: Ensure post-development runoff is no worse than before development.

Step 1: Estimate Pre-Development Runoff

We use the TR-55 method, developed by the USDA, to estimate how much stormwater runs off the land before it's developed. This will be our baseline.

Curve Number (CN)

We start with a value called the Curve Number (CN), which estimates how much water runs off versus how much soaks into the ground. A lower CN means more infiltration, while a higher CN means more runoff.

For open pasture on moderate soils (Hydrologic Soil Group C), the CN is 74.

CN range: 30 (more infiltration) to 98 (almost all runoff, like pavement).

We also consider the Hydrologic Soil Group (HSG), which reflects the soil’s ability to absorb water. Group A soils absorb the most water, while Group D soils absorb the least, leading to more runoff.

Group A: High infiltration, low runoff (e.g., sand, gravel).

Group B: Moderate infiltration and runoff (e.g., loam, sandy loam).

Group C: Low infiltration, higher runoff (e.g., clay loam).

Group D: Very low infiltration, high runoff (e.g., clay, high water table).

Potential Maximum Retention (S)

This tells us how much rainfall the land can hold before it starts producing runoff.

Formula: S = (1000 / CN) − 10

Variables:

S = Potential maximum retention, in inches.

CN = Curve Number, a dimensionless value.

Calculation: S = (1000 / 74) − 10 = 3.51 inches

This means the soil can hold about 3.51 inches of water before runoff begins.

Time of Concentration (Tc)

The Time of Concentration (Tc) is the time it takes for water to travel from the hydraulically most distant point in the watershed to the outlet. This is a crucial parameter in stormwater design because it dictates the timing of the peak runoff flow. The TR-55 method breaks the flow path into three distinct segments, each with its own calculation.

Sheet Flow: The initial stage of runoff, where water flows as a thin, uniform sheet. This typically occurs for a short distance, generally less than 300 feet. The TR-55 method uses the following formula to calculate the travel time for sheet flow:

Tt = (0.007(nL)^0.8) / (P2^0.5 × S_slope^0.4)

Variables:

Tt = travel time in hours.

n = Manning's roughness coefficient.

L = flow length in feet (max 300 ft).

P2 = 2-year, 24-hour rainfall depth in inches.

S_slope = land slope in ft/ft.

Shallow Concentrated Flow: Water begins to collect in small depressions or ruts, forming small channels. We calculate travel time using:

V = k × S_slope^0.5

Tt = L / V

Variables:

V = velocity in ft/s.

k = surface constant (unpaved = 16.1, paved = 20.3).

S_slope = land slope in ft/ft.

Channel Flow: Water flows in a well-defined channel. Velocity is calculated using Manning's equation:

V = (1.49 / n) × R^(2/3) × S_slope^(1/2)

Tt = L / V

Variables:

V = velocity in ft/s.

n = Manning's roughness coefficient.

R = hydraulic radius.

S_slope = channel slope in ft/ft.

To find the total Time of Concentration (Tc), you add the travel times from each of the three flow segments:

Tc = ∑Tt(sheet) + ∑Tt(shallow) + ∑Tt(channel)

For our 15-acre site, a detailed analysis using these methods yields a total Tc of 20 minutes.

Peak Flow Rate (Qpeak)

This tells us the maximum flow rate of water coming off the site during the heaviest part of a storm. We'll calculate this for a 10-year, 24-hour storm, which has a rainfall depth (P) of 4.0 inches in this region.

Formula for runoff depth:

Qdepth = (P − 0.2S)^2 / (P + 0.8S)

Variables:

Qdepth = Runoff depth, in inches.

P = Rainfall depth, in inches.

S = Potential maximum retention, in inches.

Calculation:

Qdepth = (4.0 − 0.2 × 3.51)^2 / (4.0 + 0.8 × 3.51) = 1.60 inches

Using the TR-55 method with the site’s Tc, CN, and runoff depth, the pre-development Qpeak is approximately 15 cfs.

Step 2: Post-Development Runoff

Now we estimate how much more water will run off after development (roads, roofs, etc.).

Weighted Curve Number (CNw)

We combine the Curve Numbers for the new surfaces—impervious areas and lawns—into a single average value.

Impervious (paved) area: 40% (CN=98)

Lawn (pervious): 60% (CN=84)

Formula:

CNw = (%Impervious × CNimpervious) + (%Lawn × CNlawn)

Variables:

CNw = Weighted Curve Number, a dimensionless value.

%Impervious = Fraction of the site with impervious surfaces (e.g., 0.40).

CNimpervious = Curve number for impervious surfaces (98).

%Lawn = Fraction of the site covered in lawn (e.g., 0.60).

CNlawn = Curve number for lawn (84).

Calculation:

CNw = (0.4 × 98) + (0.6 × 84) = 89.6

We round this to the nearest whole number, so our CNw is 90.

Potential Maximum Retention (S)

Using our new CN, we find the new retention value.

Calculation:

S = (1000 / 90) − 10 = 1.11 inches

The developed site can now only hold 1.11 inches of water before runoff begins.

Runoff Depth (Qdepth)

With less retention, more rain becomes runoff. We use the same 10-year, 24-hour storm (P = 4.0 inches).

Calculation:

Qdepth = (4.0 − 0.2 × 1.11)^2 / (4.0 + 0.8 × 1.11) = 2.92 inches

This shows a significant increase in runoff depth.

Post-Development Peak Flow Rate (Qpeak)

Using the TR-55 graphical method with the new CNw and runoff depth, the post-development Qpeak is approximately 45 cfs.

Step 3: Calculate the Required Detention Volume

Instead of simply subtracting the pre-development runoff volume from the post-development volume, we must determine the pond's required storage through a process called hydrograph routing. A hydrograph is a graph showing the flow rate (discharge) over time for a specific storm event.

The goal is to design an outlet structure that controls the flow of water out of the pond. We want this outflow to be no more than the pre-development peak flow. By restricting the outflow, the pond temporarily stores the excess runoff, and the volume of this stored water is the required detention volume.

The required detention volume is the area between the post-development inflow hydrograph and the controlled outflow hydrograph. This volume isn't a simple calculation; it's determined through iterative modeling using specialized computer software. The software simulates the storm event, routes the post-development runoff into the pond, and calculates how much water must be stored at any given time to keep the outflow below the pre-development peak flow rate. The maximum volume stored during this simulation is the required detention volume.

For our example, we are targeting an outflow rate of 15 cfs, which is the pre-development peak flow. By using a typical design storm hydrograph and modeling the outflow from our proposed orifice and weir, the necessary detention volume would be calculated by the software. For a 15-acre site with these specific parameters, a detailed hydrograph routing analysis might show a required detention volume of around 72,200 cubic feet, which is a value we can use for sizing the pond in Step 4.

Step 4: Size the Detention Pond

Once we've calculated the required detention volume (Vpond), the next step is to design a physical pond that can hold it. This process is iterative; you start with an initial design and then adjust the dimensions until the calculated volume meets or exceeds your target. Our goal is to design a pond that can hold at least 72,200 ft³.

Pond Geometry and Storage

Most detention ponds are designed with a trapezoidal shape, which allows the banks to be sloped for stability and safety. This slope is the reason the top of the pond is larger than the bottom. The relationship between the top and bottom dimensions is determined by the pond's depth and the chosen side slope ratio. For example, if a pond is 6 feet deep with a 3:1 side slope, the top dimensions would be 36 feet wider than the bottom dimensions (2×3 ft/ft×6 ft = 36 ft).

Formula:

V = ((Atop + Abottom) / 2) × D

Variables:

V = Estimated pond volume, in cubic feet (ft³).

Atop = Surface area of the pond at the top of the water level, in square feet (ft²).

Abottom = Surface area of the pond at the bottom, in square feet (ft²).

D = Water depth, in feet.

Initial Calculation

Let's try an initial design with a 3:1 side slope and a 6-foot depth. We'll start with a bottom size of 85 ft by 85 ft.

Bottom size: 85 ft × 85 ft = 7,225 ft²

Top size:

Top Length = 85 ft + (2 × 3 ft/ft × 6 ft) = 121 ft

Top Area = 121 ft × 121 ft = 14,641 ft²

Depth: 6 ft

Using the formula, the estimated volume for this design is:

V = ((14,641 + 7,225) / 2) × 6 = 65,598 ft³

Analysis and Adjustment

Our initial design has an estimated volume of 65,598 ft³, which is less than the 72,200 ft³ required. We need to increase the pond's storage capacity. Let's try adjusting the depth to 7 feet while keeping the same bottom size.

Revised Calculation

Bottom size: 85 ft × 85 ft = 7,225 ft²

Top size:

Top Length = 85 ft + (2 × 3 ft/ft × 7 ft) = 127 ft

Top Area = 127 ft × 127 ft = 16,129 ft²

Depth: 7 ft

The revised volume calculation is:

V = ((16,129 + 7,225) / 2) × 7 = 81,720 ft³

This revised volume of 81,720 ft³ is greater than our required volume of 72,200 ft³, so this design is acceptable. This iterative process of calculating and adjusting ensures the pond can effectively manage the stormwater runoff from the developed site.

Step 5: Design the Outlet Structure

The outlet structure is the control center of the detention pond. Its job is to precisely regulate the flow of water out of the pond, making sure the total outflow never exceeds the pre-development peak flow rate. To do this, it typically uses two key components: a low-flow orifice and a high-flow weir.

Orifice (Low Flow Release)

The orifice is a small opening, often a pipe, located at the pond's lowest point. It handles the initial stages of a storm and is designed to release water at a controlled rate for small to medium storm events. The flow through the orifice is directly related to the water depth, or head, above it. As the pond fills, the flow through the orifice increases. The formula for this flow is:

Q = Cd × A × √(2gh)

Variables:

Q = Flow rate (in cfs)

Cd = Discharge coefficient (typically 0.6)

A = Area of orifice opening (in ft²)

g = Acceleration due to gravity (32.2 ft/s²)

h = Head, or water depth above the orifice center (in feet)

Weir (Overflow Release)

The weir is an overflow structure, essentially a notch or wall that activates when the water level rises above its crest. It's designed to manage the high-volume flows from larger, less-frequent storms. The weir works in conjunction with the orifice. The key design challenge is ensuring that the combined flow from both the orifice and the weir at the pond's maximum water depth does not exceed our target outflow of 15 cfs.

The flow over a weir is calculated using the following formula:

Q = Cw × L × H^(3/2)

Variables:

Q = Flow rate (in cfs)

Cw = Weir coefficient (typically 3.33)

L = Weir length (in feet)

H = Head over the weir (in feet)

In a real-world scenario, you don't calculate these values in isolation. The orifice and weir are modeled together using hydrograph routing software. The software simulates the storm event and the resulting pond inflow, then calculates the combined outflow at every time step. The dimensions of the weir (its length and crest elevation) are adjusted in the software until the total peak outflow hydrograph is successfully controlled at or below the pre-development peak flow rate. The final design is a successful combination of these two components that achieves your stormwater management goal.

Final Thought

When done right, stormwater design protects both the land and your investment. By understanding these key calculations and methods, you can design a detention basin that not only meets compliance requirements but also helps the overall project succeed.