1. What is Detention Storage?

Detention storage refers to the volume of a stormwater facility (for example, a detention basin or tank) designed to temporarily hold stormwater runoff and release it at a controlled rate. This practice helps reduce peak flow entering downstream systems and mitigates flooding or erosion risks.

2. Key Concepts for Calculating Detention Volume

• Inflow Hydrograph: A time-series (flow vs. time) of runoff entering the detention facility.
• Outflow Hydrograph: A time-series of runoff leaving the facility, governed by the outlet structure (for example, an orifice or a weir).
• Peak Flow Mitigation: The outflow is typically limited to meet certain regulatory or design criteria, such as matching pre-development peak flows.
• Design Storm: A standard storm event (for example, a 10-year, 24-hour storm) used for sizing the basin. The basin is sized so that the outflow doesn’t exceed the maximum allowable rate during this design storm.

The detention volume is essentially the largest difference between the cumulative inflow volume and the outflow volume at any point in time. Engineers usually use hydrologic software or manual methods (like the Modified Rational Method) to calculate this.

3. Overall Procedure

3.1 Hydrologic Modeling to Generate Inflow

1. Select the Design Storm: Determine the frequency and duration (for example, a 2-year, 10-year, or 100-year storm event).
2. Compute Runoff:
   • Curve Number (SCS/NRCS Method): Uses land use, soil group, and antecedent moisture conditions to compute a runoff hydrograph.
   • Rational Method: A simpler formula Q = C × i × A, sometimes extended into the “modified rational method” to create a runoff hydrograph.

3.2 Determining the Outlet Structure and Outflow

Engineers propose outlet structures (orifices, weirs, or multi-stage outlets) to limit the outflow to a target rate. The correct sizing of these outlets is crucial to control the release of stored water.

3.3 Iterative Design to Size the Detention

1. Trial Basin: Assume an initial volume and outlet configuration.
2. Route the Inflow: Calculate how water surface elevation changes over time, which gives the outflow hydrograph.
3. Check Peak Release Rates: Ensure they stay within required limits.
4. Check Water Surface Elevation: Confirm that the basin’s freeboard (the vertical distance from peak water surface to basin crest) meets regulations.
5. Iterate: Adjust the basin or outlet until all conditions are satisfied.

4. Core Methods in More Detail

4.1 Storage-Indication (Level Pool) Method

The storage-indication method (level-pool routing) is based on the continuity equation:

I – O = dS / dt

• I = inflow rate
• O = outflow rate
• S = stored volume
• t = time

By evaluating this equation in small time steps, you can determine the storage volume and outflow at each step. The maximum storage found during the simulation represents the detention storage volume required.

4.2 Modified Rational Method

For smaller projects, some jurisdictions allow a simplified “modified rational method.” Here, the Rational Method provides peak inflow, and a simplified hydrograph shape (often triangular) is used to estimate total runoff volume.

1. Compute the peak flow (Q = C × i × A).
2. Shape a hypothetical inflow hydrograph around that peak flow.
3. Set a target outflow rate (for example, equal to pre-development peak flow).
4. Calculate the inflow volume and outflow volume. The maximum difference is the required detention volume.

5. Detailed Example (Conceptual)

Assume a 10-acre site, a 10-year 24-hour storm, and an inflow hydrograph with a peak of 15 cfs at the storm’s midpoint. Suppose local regulations require an outflow of no more than 5 cfs.

1. Create an initial design for the basin and size the outlet to pass 5 cfs at a certain water level.
2. Route the inflow: for each time step, use (Inflow – Outflow) × time step = Change in Storage to find how storage volume changes.
3. Identify the maximum stored volume during the entire storm. That value is your detention storage requirement.
4. If it doesn’t meet the outflow or freeboard criteria, adjust the design and repeat.

6. Additional Considerations

• Multiple Storm Events: Many areas require checking several storm return periods (2-year, 10-year, 100-year).
• Freeboard: Most basins need a certain distance between peak water level and the top of the berm (embankment).
• Emergency Spillway: Large basins often include spillways for events larger than the design storm.
• Storm Duration and Distribution: Different rainfall distributions (for example, SCS Type I or II in the U.S.) can affect how quickly runoff arrives.
• Downstream Constraints: Local regulations often set maximum release rates or volumes based on pre-development conditions.

7. Summary of Key Steps

1. Identify design storms and any local requirements for peak flow or discharge rates.
2. Select a hydrologic method (for example, SCS Curve Number or the Rational Method) to generate the inflow hydrograph.
3. Propose or size the outlet structure (orifice, weir) to limit outflow.
4. Perform hydrograph routing (for example, storage-indication) to find the water surface elevation in the basin over time.
5. Calculate the maximum storage volume. That value is the detention storage requirement.
6. Iterate until you meet peak flow constraints and freeboard requirements.

8. Conclusion

Calculating detention storage volume combines hydrologic estimates of inflow with hydraulic design for outflow control. By using time-stepped routing to compute inflow minus outflow, engineers determine how much volume must be provided to prevent flooding and protect downstream systems.