Stormwater Treatment in Cache Valley, Utah

Overview

Currently, Logan City has no stormwater treatment process in place. Therefore, NWEAU proposes to use the existing constructed wetland on the Northwest side of Logan as an evaporation basin with vegetation to treat stormwater. This evaporation basin will have the capacity to treat the stormwater from Logan, North Logan and Hyde Park (see Figure 2). The wetland is divided into three main sections (as shown in Figure 1.) The intake pond receives the stormwater via gravity flow. From the intake pond, the water is conveyed to the storage pond by the screw pumps in the intake pond. The water from the storage ponds is then gravity fed into the constructed wetland where the water will evaporate. The plants in the constructed wetland will also take up any excess pollutants and be harvested and recycled annually.

figure 1. constructructed wetland
figure 2. service area

Design

figure 4. graphical representation of the hydrological budget

Intake Pond Sizing. Sizing the intake pond was performed to ensure that sufficient capacity is provided within the pond so that during a maximum storm event the pond doesn’t overtop.The maximum storm event was a 48 hour 100 year design storm provided by JUB Engineers. The team performed a water balance of the incoming storm water with the water being pumped from the screw pumps up to the ponds. The spreadsheet was put through multiple iterations to optimize retention and minimal excavation of native material. The following pond outline was developed from the spreadsheet (Figure 5).
Hydrological Budget. Monthly evapotranspiration and runoff calculations were performed as a part of the hydrological budget as shown in Figure 4. These calculations are important to ensure the constructed wetlands ability to treat maximum storm events, prevent flooding, and minimize drought effects on plant life. To accomplish this, monthly water balances were performed by estimating runoff with a cumulative curve number that represented the Service Area (Figure 2). Evapotranspiration was estimated with various evaporation estimate methods and crop coefficients to estimate the transpiration.

Alternative Analysis

NWEAU conducted an alternative analysis using a decision matrix to determine the overall best solution. The decision matrix takes the alternatives (see Figure 3) and scores them in categories that are NWEAU and Logan City’s top priorities. The scoring system is on a sliding scale from 1-5 in each category for each alternative, with 1 being the most favorable and 5 being the least favorable.

figure 3. decision matrix

Using the decision matrix NWEAU found that the most ideal alternative is the “Evaporation ponds with vegetation”. NWEAU’s selected alternative will use the constructed wetland as an evaporation basin with vegetation and will not discharge any stormwater into Swift Slough.

Results

Sized Intake Pond.The existing pond overtopped at 23.1 hours into the storm. The spreadsheet solver found that the ideal pond size was an additional 166,000 square feet greater than the base pond‘s conditions. By continuing with the existing 8 ft depth, this results in 1,701,000 cubic feet of volume greater than the base pond's conditions. The outline of the upsized intake pond can be seen in Figure 5. The new intake pond will be excavated to 9.8 feet deep,have side slopes of 8.5:1, and will be graded at a 0.5% slope to the screw pumps in order to provide sufficient head for the pumps to operate. The cost upsizing the intake pond is estimated to be $1.27 million.
Hydrological Budget. The hydrological budget shows an increase in water storage during the winter months through May and June. During the month of July evaporation begins to exceed runoff and storage levels decrease. It is expected that the wetlands will begin to dry out in August and water storage will go to zero. November runoff will once again exceed evapotranspiration and each year will end with approximately 105 acre-ft of water storage. This excess water will be important to buffer against drought.

figure 5. outline of the ideal intake pond

Maintenance and Operation

Routine maintenance of the evaporation intake ponds, storage ponds, and constructed wetlands are essential to effectively treat stormwater and maintain a healthy pond ecosystem. The maintenance should include :

  • Annual Harvesting. Harvesting should occur when the evaporation basins are dried out after Spring runoff, which in a typical year is in mid August. The harvested vegetation should be tested for harmful levels of pollutant concentrations. If pollutant concentrations low enough, the vegetation can be sent to Logan City’s Green Waste Program to be composted. Harvesting should start in Cell 1 and end in Cell 9 (see Figure 6)
  • Embankment Maintenance. The vegetation along the embankments of the evaporation basins, storage pond, and intake ponds should be mowed once a month to avoid erosion along the embankments.
  • Control Structures. The gates, valves, and screw pumps should be inspected monthly for any damages.
  • Miscellaneous. Other routine maintenance should include water monitoring, constructed wetland property, and wildlife management.
figure 6. overhead view of the constructed wetlands