Case Study #1: Monitoring a dry detention pond with under-drains

C. F. Hussain, J. Brand, A. J. Erickson, J. S. Gulliver, and P. T. Weiss at the University of Minnesota.

Carver County dry detention pond is located along Highway 212 and lies one mile west of Cologne, Minnesota in the Carver Creek watershed. It drains a watershed that encompasses the corner of the Carver County’s new public works facility site, consisting of 45 acres with impervious area on the site of 10.2 acres. Future construction of County facilities may occur on the remainder of the site.

The dry detention pond is approximately 3 acres with a slope of 1% from inlet to outlet. It is designed to provide storage up to a 100 year – 24 hour event on the site. Stormwater runoff is directed through grass waterways to a small pretreatment pond (forebay) before it enters the pond. After entering the detention pond, the stormwater runoff infiltrates through the soil media. A series of rock-filled trenches holding perforated drain tile acts as an under-drain for the pond into which most of the stormwater runoff drains. Eight sets of 8-inch diameter, perforated polyethylene under-drain pipes (Y-shaped) are joined together with 8 inch by 8 inch by 4 inch polyethylene laterals oriented at 45 degrees. Every set of under-drain pipe consists of two arms, each 30 feet long with a diameter of 4 inches. A total of 140 feet of 8-inch diameter under-drain pipe and 480 feet of 4-inch diameter under-drain pipe were installed within the detention pond as shown in figure 1.

A cross section of the under-drain system is shown in figure 2. The under-drain pipe was surrounded by a mixture of soil and ASTM C33 fine sand, which was used as filter media for the Carver County pond. A filter fabric was used to wrap the soil-sand filter media and under-drain pipe. A layer of 6 inches of native soils (typically tighter clays for Carver County) was used to bury the filter fabric to avoid its exposure at the surface. The under-drains collect the infiltrated storm water and drain it into the outlet structure. The outlet structure of the Carver County dry detention pond is 5 feet in diameter and receives infiltrated runoff through an 8-inch under-drain pipe as shown in figure 3. This large outlet structure was provided so that rainfall in excess of the design storage volume could discharge downstream. An 18-inch (inner diameter) reinforced concrete pipe takes the runoff from the outlet structure and discharges it into the downstream watershed. Native plants were planted on the site, including the grass waterways (ditches) and areas around the parking lot.

Assessment Goals

The goals of this assessment were to (1) assess runoff volume reduction and (2) assess pollutant retention performance of total suspended solids, volatile suspended solids, total phosphorus, and particulate phosphorus. This pond was designed to drain within 48 hours after a runoff event by filtering the stormwater through sand trenches and into a perforated pipe collection system. An elevated outlet structure within the dry pond provided a secondary outlet, which prevented the ponds from flooding. In addition to filtration, a primary treatment process of dry detention ponds with under-drains is sedimentation, which occurs while the runoff is pooled in the pond.

Assessment Techniques

To meet the assessment goals, both inflow and outflow had to be measured and sampled at each pond. The pond was chosen for monitoring because it had one influent and one effluent location and limited overland inflow. Thus, only two flow measurement and sampling stations were needed.

A 6700 series portable ISCO water quality sampler, which contained a complete set of 24, 1-liter, wedge-shaped bottles, was installed at the inlet of Pond #3. The unit was programmed to collect flow-weighted samples and to record the depth, velocity, and discharge at 10-minute intervals. A tipping bucket ISCO rain gage was also installed near the inlet of pond to collect data on the total rainfall amount, antecedent dry days, and rainfall intensity for each storm event.

Initially, a 5-foot wide rectangular, sharp-crested weir was installed at the inlet of Pond #3, as shown in figure 4, with an ISCO 710 Ultrasonic Flow Module with a 6700 series sampler. The sensor on the 710 Ultrasonic Flow Module was installed over the water surface just upstream of the weir to measure depth behind the weir. The equipment continuously monitored and recorded the rainfall and water level at the inlet of Pond #3. It also estimated discharge based on the water level.

Results from preliminary monitoring showed that the rectangular weir did not provide accurate estimates of discharge at the relatively low discharge rates that were most common at the site. Therefore, the 5-foot wide rectangular weir was modified by cutting a 3-inch deep, 90 degree V-notch into the middle of the rectangular weir such that the result was a sharp-crested compound weir which could more accurately estimate low discharges.

At the outlet, another 6700 series portable ISCO sampler was programmed to take flow-weighted samples. Using a flexible circular spring ring, a 750 Area Velocity Flow Module was installed on the bottom of the outlet culvert. This type of module uses Doppler technology to measure average velocities at locations across the flow cross-section. A pressure transducer contained within the Module measured water depths and, based on conduit geometry, calculated flow areas. The total discharge was calculated by the ISCO sampler by summing the products of all recorded average velocities and their corresponding flow areas.

The monitoring systems at both the inlet and outlet of the pond were powered by heavy duty deep-cycle marine batteries and Global Tech (PRO 5W) solar powered battery chargers. Although ISCO 6700 samplers and 700 series modules are water-tight, corrosion resistant, and can be installed without additional protection, all the monitoring equipment was enclosed in lockable wooden environmental cabinets. A laptop PC with ISCO Flowlink 4 software was used to retrieve the data from the 6700 samplers.

An artificial head of water was created in the pipe by installing a 3-inch high plastic circular weir, as shown in figure 5, to ensure that the area velocity sensor used at the outlet had the required 2-inches or more of water depth needed to accurately measure the velocity profile. The area velocity sensor was located inside of the pipe, 6-inches upstream of the circular weir. Due to turbulence created by the weir, the velocity and resulting discharge reported by the area velocity probe were erroneous and could not be used to assess the pond. The depth measurement, however, reported by the probe was correct, and these values were used to calculate the head on the weir and the corresponding discharge.

Assessment Results

Data and samples were collected for twelve runoff events over two years. The results are presented in tables 1, 2, and 3. The data presented in tables 1 and 2 were used to estimate the performance of the pond for volume reduction and pollutant retention as listed in table 3. There was significant infiltration in the pond. Values ranged from 1/3 of the total influent volume at high discharges to greater than 2/3 of the total volume at lower discharges.

Overall, load-based efficiencies are assumed to be preferred for total load studies. Total load is determined by subtracting the sum of the outflow from the sum of the inflow and dividing by the sum of the inflow. These efficiencies for the twelve monitored storms were 88% for total suspended solids, 81% for volatile suspended solids, 58% for total phosphorus, and 52% for dissolved phosphorus. These load-based efficiencies incorporate infiltration as a treatment mechanism and are therefore less comparable between sites.

The average concentration-based retention efficiencies for the twelve storms at Carver County dry detention pond with under-drainage were 39% for total suspended solids, 32% for total volatile solids, 35% for particulate phosphorus, and 16% for total phosphorus. Retention efficiencies for dissolved phosphorus provided more variation and ranged between negative 18% to positive 60%, with an average retention efficiency of 3%. Dry detention ponds are focused on removing sediment and the associated pollutant concentration, such as particulate phosphorus. The primary retention mechanisms are not designed to retain dissolved phosphorus; thus, dissolved phosphorous retention is minimal.

Conclusions and Recommendations

Dry detention ponds have been widely used to temporarily store and treat stormwater runoff, but little is known about their effectiveness in terms of pollutant retention, particularly when they are equipped with under-drains. The Carver County dry detention pond with under-drains was selected and monitored from May 2004 to November 2004 and May 2005 to August 2005 to learn more about their performance. The performance of the pond in terms of pollutant retention efficiencies was estimated by comparing the influent and effluent pollutant concentrations. From the results obtained in this study, the following specific conclusions were reached:

  • The measured concentrations of most parameters in stormwater runoff that entered at the Carver County dry detention pond with under-drains were substantially lower than concentrations typically mentioned in other studies throughout the nation and influenced the pollutant retention efficiency of the pond. The lower values found at Carver County dry detention pond site are thought to be related to pre-treatment provided by the small pond near the inlet and also by the two grassy ditches/swales used to transport stormwater runoff to the detention pond site.
  • The use of a primary device for flow measurement is strongly recommended, especially in outlet under-drain pipes. These devices (V-notch, rectangular or circular weirs, and flumes) are easy to install and can be used to provide continuous flow hydrographs using measurements of water surface level. The study revealed that an AV sensor cannot measure any velocity unless there is at least 2.5 to 3 inches of water over it, which does not often occur in under-drain outlets.

This research study confirmed that dry detention ponds with under-drains are an effective option for water quality control. The Carver County pond provided moderate stormwater treatment and reduced the concentrations of total suspended solids, volatile suspended solids, particulate phosphorus, and total phosphorus, even with low influent concentrations.

Results from the Carver County dry detention pond with under-drains indicate that influent pollutant concentrations influenced the pollutant retention efficiencies. Higher total suspended and volatile solids influent concentrations for Storm Event 2 resulted in high total suspended and volatile solids retention. Similarly, dissolved phosphorus retention efficiencies were higher at high influent concentrations and lower at low influent concentrations. However, the trend between influent pollutant concentrations and retention efficiencies for all twelve monitored storms at Carver County pond was not consistent.

The filter under-drain system at the pond exhibited poor hydraulic performance and failed to keep the pond dry between the storm events. The runoff residence time in the pond for the twelve storm events monitored ranged from 2 days to 17 days, with an average of 5 days. The filter system requires continual maintenance to ensure that it is functioning properly. Field maintenance activities to maintain the hydraulic performance of the filter media may include replacement of filter media, filter backwashing, or scratching a few inches from the top of the filter media.

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