Page -1-

Modeling Underground Stormwater Storage

by Peter Smart, CEO, HydroCAD Software Solutions

BackgroundDetention ponds are a frequently used tool for managing stormwater runoff. By detaining runoffin a storage area, peak flows can be reduced in order to approximate the pre-developmentcharacteristics of a site. However, traditional above-grade ponds require valuable open space,and may create safety hazards or health risks on some sites. To maximize land utilization,designers are increasingly turning to underground storage alternatives that can be placed underparking lots, playing fields, or other open spaces. On appropriate sites, underground storage alsoprovides a cost-effective means of restoring the infiltration capability that is usually reduced bydevelopment activities. In addition to providing groundwater recharge, this helps to offset theincrease in runoff volume that accompanies most site development, and enables the design ofstormwater management systems that more closely approximate the pre-development sitebehavior, in terms of peak flow and volume.

Box-of-RocksOne of the simplest means of providing underground storage is an excavation filled with cleanstone, sometimes referred to as a “box of rocks”. The storage is provided by the void spacebetween the stone, which can be as much as 40% of the overall volume. Typically, the stone iswrapped in filter fabric to prevent the migration of soil into the stone voids, or even a waterproofmembrane for sites where infiltration is undesirable. The introduction of water into the stone istypically accomplished with an embedded drywell or perforated pipe. In most cases, the systemalso includes outlet provisions to handle overflow conditions or achieve a desired discharge intoa storm sewer or other design point.

While a box-of-rocks can be a viable solution on certain sites, the high cost of stone and openspace often calls for a more sophisticated storage solution.

Page -2-

Underground Tanks and Rectangular ChambersThe most space-efficient means ofunderground storage is often a rectangulartank or chamber. These products range fromsmall injection-molded “cells”, to largeconcrete tanks or vaults, as shown inFigure 1. The chambers are typicallyinstalled in a rectangular grid, using whatevernumber of rows and chambers are required toachieve the required storage. Since there islittle or no gap between these chambers,storage efficiency can be as high as 95%,depending on the thickness of the chamber material. For open-sided chambers, the system istypically wrapped in filter fabric to prevent soil infiltration, while other chambers systems areself-sealing and do not require a fabric barrier.

Embedded Chambers and PipeThe third category of underground storageinvolves a combination of stone andembedded chambers. By using perforatedchambers, water is also stored in the voidsof the surrounding stone, producing anoverall storage efficiency of 60-74%. Thedesigner has many options for embeddedstorage, ranging from perforated pipe toarched stormwater chambers. (See Figures2 and 3.)

For situations requiring a sealed storage solution (i.e. detention-only without infiltration), theentire chamber-and-stone system can be wrapped in a waterproof membrane. An alternative is touse a closed-chamber solution, such as non-perforated pipe, with a typical space efficiency of35-55%, depending on the type and size of pipe.

Figure 1 - Concrete Stormwater ChambersPhoto courtesy of Northeast Concrete Products Association

Figure 2 - Perforated CMP storage with porous backfillPhoto courtesy of Contech Construction Products

Page -3-

Modeling underground storageRegardless of how the storage volume is created, most underground detention or retentionsystems are modeled like a traditional above-grade pond. This is based on the assumption thatwater can flow freely within the storage system, creating a level pool that can be modeled withconventional pond routing procedures, such as the Storage-Indication method.

For tanks or rectangular chambers, the storage calculation is based on the overall dimensions,with possible adjustments for reinforcing webs or other intrusions into the storage space. For thebasic box-of-rocks, the storage at any depth is calculated by multiplying the volume of the boxby the stone voids. In both cases, the result is a linear stage-storage curve, which can be readilycalculated and entered into any pond routing model.

However, the situation becomes considerably more complex when using a curved storagechamber, or a chamber-plus-stone storage combination. These situations require a detailedanalysis of the chamber volume at any given depth, plus the inclusion of stone storage at eachwater surface elevation.

Figure 3 - Typical Plastic Chamber InstallationPhoto courtesy of Cultec, Inc.

Page -4-

The result of these calculations is astage-storage curve, as shown in Figure4. Note the linear storage relationshipover the first ½ foot, provided by theuniform stone bed on which thechambers are placed. Above the stonebed there is a sudden increase instorage-per-foot caused by the openchamber, gradually returning to a linearrelationship for the stone cover overthe top of the chamber.

Stage-storage calculationsStage-storage calculations are often performed with a spreadsheet provided by the chambermaker. After specifying the basic layout parameters, a typical spreadsheet will provide a stage-storage table that can be transferred to the modeling software. A better approach is to use amodeling program with native support for chamber calculations. This avoids the need to use adifferent spreadsheet for each storage product, and eliminates the time-consuming and error-prone process of transferring spreadsheet data into the modeling program. Native chambersupport also allows immediate changes to the layout and design parameters, without having tomanually update the stage-storage data or re-run a separate “chamber tool”.

Other software featuresFinally, look for a program that provides automated layout of the drainage field, including theability to automatically apply recommended chamber spacing, bedding, and cover. Make surethe software includes a large selection of chambers, including a full range of pipe storageoptions. The ability to provide cost estimates based on the cost of chambers, stone, andexcavation is another time-saving feature worth looking for.

Peter Smart is the founder and CEO of HydroCAD Software Solutions,developer of the HydroCAD Stormwater Modeling Systemwww.hydrocad.net

Figure 4 - Stage-Storage for Plastic Chambers & Stone