BIORETENTION HYDROLOGIC PERFORMANCE OF TEN FACILITIES LOCATED THROUGHOUT THE PUGET SOUND BASIN

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1 BIORETENTION HYDROLOGIC PERFORMANCE OF TEN FACILITIES LOCATED THROUGHOUT THE PUGET SOUND BASIN William J. Taylor, Taylor Aquatic Science Seattle, WA Doug Beyerlein PE, Clear Creek Solutions Bryan Berkompas, Aspect Consulting Jenny Saltonstall, Associated Earth Sciences Anne Cline, Raedeke Associates Christopher W. Wright, Raedeke Associates Funded by Washington State Stormwater Action Monitoring and Sponsored by the City of Bellingham INTRODUCTION Engineered bioretention facilities have been designed for stormwater quantity and quality control in Washington State under the Stormwater Management Manual for Western Washington (SMMWW) since The design of these facilities must use a continuous simulation hydrologic model to size the facility using input data on cell dimensions and overflow elevations, contributing area flows, soil media and subsurface infiltration rates (as opposed to rain gardens that can be designed using generalized criteria). Many hundreds of bioretention cells have been constructed since 2005, with very little detailed flow monitoring to assess actual performance of such designed facilities once built. It is essential that bioretention facilities meet hydrologic performance standards so that other benefits such as receiving water flow protection and water quality treatment can be realized. Accurate facility sizing is also important for to help ensure efficient use of limited space. For ten facilities within the Puget Sound basin, we present summary data collected from the wet season for rainfall, inflow, ponding, and outflow; soil media and subsurface core profiles; infiltration tests; shallow ground water fluctuation, and vegetation composition and cover. Figure 1 provides a map of the distribution of site locations around the Puget Sound basin. Comparing actual performance of bioretention facilities to their design expectations will support adjustments in the use of the hydrologic design model, and result in performance that better meets design expectations. This is also an important feedback project for Washington State s stormwater managers. PROJECT DESCRIPTION AND APPROACH The Bioretention Hydrologic Performance (BHP) Project, funded by the Washington State Department of Ecology and sponsored by the City of Bellingham, is measuring the actual hydrologic performance of constructed bioretention facilities throughout the Puget Sound basin. The measured performance of these facilities is then used to evaluate the consistency of operational conditions with the design model

2 Figure 1. Distribution of bioretention monitoring site locations.

3 conditions. This assessment of operational conditions compared to the design values evaluates the physical conditions (e.g. dimensions, overflow elevations, soil media composition, and underlying geologic unit) and dynamic parameters such as inflow rates and volumes, infiltration rates, and shallow ground water levels. The intended benefits of the project are to provide policy and implementation feedback across the Puget Sound Basin: 1. For design and construction guidance to jurisdictions reviewing site plans, and the engineering community to help ensure constructed conditions meet expectations, 2. To help ensure limited development space is efficiently and accurately used (not oversized or undersized), 3. To help ensure downstream beneficial improvements are achieved for instream flow and water quality protection. The project approach is to conduct in-field reviews of multiple candidate bioretention cells (over 70) identified by local jurisdictions and engineers for final selection to be monitored. A number of selection criteria were identified for qualifying the cells for monitoring, with the final distinguishing elements being: 1. The inflows and outflows could be distinctly identified for flow monitoring, 2. The cells were not lined (so infiltration into the native soil occurred), 3. The contributing areas were exclusively or largely impervious. Following in-field confirmation of site conditions, the ten selected sites physical conditions were measured, and they were equipped with instrumentation to record a variety of project hydrologic measurements. These included on-site rain gauges; inflow and outflow compound weirs and stage recorders to measure flow rates at 5-minute intervals; and shallow groundwater well points also with stage recorders. The sites were also sampled for bioretention soil profile composition grain size and organic content analysis; underlying geologic unit grain size; and sampled for plant community composition and cover. Controlled field infiltration testing of each cell was also conducted. Figure 2 provides a typical visual representation of a flow measuring weir used at the sites for inflow and outflow.

4 Figure 2. Weir flow monitoring installation with continuous stage recorder typical of flow monitoring in the ten project facilities. This location involved a custom curb cut inflow installation. INITIAL OBSERVATIONS Hydrologic Observations Continuous hydrologic data collected at the sites includes site-specific rainfall, inflow, outflow (if any), surface ponding, and shallow sub-surface groundwater levels. Inflow and outflow at each cell are measured using Thelmar Weirs. Seven of the sites receive inflow directly from storm drain pipes and were directly fitted with Thelmar Weirs. Three of the sites receive inflow from surface runoff via curb cuts and required custom modifications to route the inflow to the weirs (Figure 2). Data collection began in September 2016 and continues through June Outflow monitoring (where applicable) used the same weirs and pressure transducers but installed in the outlet pipe from the bioretention overflow structure. Ponding depth was measured using pressure transducers at key locations within the bioretention facility and measured both standing water within the cell as well as any overflow events where water levels exceeded the height of the overflow structure. Shallow groundwater was measured using pressure transducers in a monitoring well. Precipitation was measured using either a local municipal gauge or using a rain gauge installed locally at each site. All monitoring sites are maintained and downloaded twice a month. Each maintenance visit includes cleaning of all transducers and weirs (if needed), verifying that weirs and rain gauges are level and not clogged, and downloading all data from the transducers. Data QA/QC and analysis are underway. Example storm results from October 26, 2016 for three of the bioretention facilities highlight the range of conditions seen during this study. Figure 3 shows B145 (Bellevue, WA) received 1.68 inches of rainfall October 26-27, and roughly 10,600 cf of inflow with a peak discharge of almost 0.4 cfs. The

5 ponding station near the inlet shows that during more intense periods of rain there are brief periods of standing water near the inlet while the ponding station near the outlet indicates that the ponding never extends across the entire facility. Shallow groundwater also responds to the rainfall and during the most intense rain period it is high enough to generate outflow. Note that there was not ponding near the outflow indicating that this outflow event was driven by groundwater leaking into the outlet structure. Figure 3. Bioretention hydrologic response at station B145. Figures 4 and 5 show a pair of sites located in Mill Creek, WA. MCCA1 and MCCA 2 both collect runoff from the same building and were constructed in a similar manner but their performances differ. The October 26, 2016 storm generated 1.16 inches of precipitation and generated 107 cf of runoff into MCCA1 and 345 cf of runoff into MCCA2. Figure 4 shows that MCCA1 infiltrated all of the inflow with no ponding or outflow and the shallow groundwater level responding in line with storm intensity. The inflow into MCCA2 generates sustained ponding and for a brief period the ponding is deep enough to generate overflow into the outlet pipe (Figure 5). Standing water is maintained within MCCA2 beyond the end of the storm event and shallow groundwater levels remain elevated throughout.

6 Figure 4. Bioretention hydrologic response at MCCA 1 Figure 5. Bioretention hydrologic response at MCCA 2

7 Hydrogeologic and Geotechnical Observations Hydrogeologic parameters for this study were evaluated at each site by reviewing the regional geologic and hydrogeologic setting, design documents, including grading plans where available, and then completing site specific exploration and testing including: multiple shallow hand borings, installation of shallow ground water observation well points, geotechnical soil T probe gridding, grain size distribution and organic matter testing, and controlled field infiltration testing per the Ecology 2014 guidelines for pilot infiltration testing. Figure 6 provides an example view of the excavation process for soil profile sampling and installation of the well points within the bioretention facilities. Of the 10 bioretention cells, only one is underdrained to discharge to surface water, while the other nine must reliably infiltrate through the subsurface geologic unit to properly function. From initial hydrogeologic/ geotechnical characterization, 6 of 10 the cells are high-performing, situated in glacial outwash, and will likely meet or exceed design targets by infiltrating all of the water directed into the cells. For the remaining 4 cells, the cell with an underdrain likely does not provide the intended flow control due to short-cutting of flow to the underdrain (pending review of monitoring data). The short-cutting of flow is a function of the inlet location being adjacent to the outlet, with very little retention in bioretention soil and quick access to the underdrain, combined with a very low permeability geologic unit. It remains to be determined whether the two cells situated in weathered till will meet design targets if so, meeting the design performance may be due to lateral flow which can connect to an adjacent permeable pavement reservoir. The cells are situated over shallow perching layers (glacial till) where prior to cell construction, only a thin intermittent water-bearing zone was likely present (low transmissivity). Also remaining to be determined is whether one cell situated in recent alluvium with very shallow ground water will meet design targets, and the degree of infiltration rate reduction due to ground water mounding beneath one of the high-performing outwash cells. For all 10 cells, the imported bioretention soil is very well draining, and had a higher infiltration rate than the design infiltration rate in all cells, so would not be considered a controlling factor in hydrologic performance relative to the design. The high infiltration rate of the bioretention soil was not surprising, since the infiltration rate of the bioretention soil is directly proportional to the amount of finer-grained particles and compaction, and most of the soil did not contain the required amount of finer-grained particles. Two cells are likely oversized by a factor of 10 or more due to required restrictions on allowable bioretention soil infiltration rate at the time of design (1.5 inch/hour) for sizing, as required at the time by local guidance. Finally, 2 of the 10 cells had field infiltration tests similar to the updated Ecology Pilot Infiltration Test conducted either for design or during construction, allowing for optimized bioretention sizing (reduced footprint for one, and higher flow control for the other).

8 Figure 6. Example excavated soil profile material (on plastic sheeting) and screened 6-foot long well point in preparation for installation in remaining hole. Vegetation Community Observations Bioretention facility plant composition and density was measured for selected monitoring sites in one of three possible approaches depending on site conditions. Only the bottom (area subject to inundation) of the bioretention cell was sampled for vegetation. Woody plants and stems were counted, and herbaceous vegetation was measured using a quadrat along a transect. At a minimum, 25% of the cell was sampled for herbaceous vegetation. Figure 7 provides a view of an example transect and quadrat sampling process. Overall, shrubs perform well in bioretention cells, and have high survival rates. Many common shrubs observed in the bioretentions cells are red-osier dogwood (native and non-native varieties), native ninebarks, Douglas spireas, black twinberry, and the non-native dwarf arctic blue willow. The red-osier dogwoods provide the most stem density within the cells, since they tend to branch low. Most of the shrubs observed tend to have a wetland indicator status of facultative or wetter, but native shrubs tend to be very versatile and are able to tolerate the wet winter and withstand drought conditions in the summer. Even in two unirrigated sites, the native shrubs were performing well. The herbaceous vegetation appears to be less adaptable than the shrubs. Much of the herbaceous vegetation specified on the planting plans has a wetland indicator status of obligate or facultative wetland. In general, the herbaceous vegetation that requires prolonged inundation has not survived or

9 Figure 7. Example transect and visual sampling characterization of herbaceous vegetation species and density with a bioretention cell. thrived within the bioretention cells. Interestingly, Carex obnupta, slough sedge, an obligate wetland species, appears to be the primary obligate species that survives and even thrives within the facilities observed. The only cells where slough sedge was on the planting plan but not observed or was sparse was in the unirrigated cells. However, the unirrigated cells are maintained by volunteers, so it is possible the volunteers accidently weeded the slough sedge out of the cells. We documented two cells that were planted with a variety of native herbaceous plants and a variety of landscape plants, such as golden sedge and corkscrew rush. Our initial impression is that the landscape plants appear to have greater survival rates than the native plants, and might be more adaptable to the wet/ dry conditions. Recommended guidance for bioretention planting plans emphasize planting drought tolerant species, many of the sites original planting plans included a variety of less drought resistant species resulting in apparent low survival rates of these plants. A narrower range of recommended herbaceous plant species based on observed survival may be beneficial for increased survival rates in future projects and possibly reduced costs of plant material. Plants play varying roles in a bioretention cell. Stem density is thought to be important, because the movement of stems allows for water movement through a siltier medium. We noted that the dwarf arctic blue willow is an extremely dense shrub which shades out all the vegetation underneath it. Some of the other shrubs, such as dogwood, are more sparse, allowing for herbaceous vegetation to grow

10 around and through the stems, increasing the amount of stems in contact with the soil. In addition, the greater quantity of plants in a cell implies a greater overall root mass in the soils, and research is indicating that roots play an important role in the long-term tilth of the soil, thus maintaining infiltration capacity. Modeling Observations The ten bioretention facilities monitored as part of the Bioretention Hydrologic Performance Study have been reviewed in terms of the computer models used to design and size these facilities. Most of these facilities were designed prior to the availability of stormwater software that accurately modeled the movement of the water through the bioretention s amended soil layers. As a result, they used a variety of techniques, software, and modeling assumptions to design and size their facilities. The various approaches can be divided into three major groups: 1. Western Washington Hydrology Model, Version 3 (WWHM3) using either the gravel trench element or the trapezoidal pond element to represent the bioretention facility. At the time the gravel trench element was the recommended solution; the trapezoidal pond element had to be adjusted to accurately represent the reduced storage volume available due to the presence of the amended soil volume in the pond. 2. WWHM4 (and WWHM3 Pro) using the state-of-the-science soil-water movement algorithms that are now available in WWHM2012. Even with the use of these more advantaged software programs the designers did not always set up the models correctly (although in the end the facilities still appear to perform as planned). 3. Various other stormwater software programs including King County Runoff Time Series (KCRTS) and Santa Barbara Unit Hydrograph (SBUH) using creative interpretation of modeling options that were never designed for modeling (or sizing) bioretention facilities. These bioretention facilities, designed and constructed with really no stormwater standard as guidance, are most susceptible to failure. In summary, the fact that all ten bioretention facilities were designed to provide full (100%) infiltration on the site may compensate for the errors, incorrect assumptions, and inappropriate software used in sizing them. That said, there is less room for error in the design and sizing of bioretention facilities that do not rely on infiltration to the native soils and instead produce a downstream discharge through the inclusion of underdrains. Additional study of such facilities may provide more clarity on this situation. Figure 8 provides a schematic of the modeling process using WWHM 2012 for the bioretention facilities.

11 Figure 8. Bioretention facility modeling process in WWHM SUMMARY OF INITIAL OBSERVATIONS Intensive monitoring of the hydrologic performance of ten bioretention cells throughout the Puget Sound basin has been very successful during the wet season. Initial indications reveal the influence of the highly variable Puget Sound subsurface geologic conditions on the performance of the facilities. Preliminary data indicate that two facilities may be significantly over-sized, and at least one facility will not meet flow control goals. Imported soil media of the selected sites are generally very well-drained, and coarser-grained than the recommended soil gradation; however, including more fine-grained soil per recommendations in the current SWMMWW may result in slower infiltration (but increased contact time) in future projects. Vegetation survival success of shrub species is good, while herbaceous species survival can likely be improved by selecting from a more narrow range of plants for species tolerant of highly drained drought conditions. Review of facility design modeling approaches revealed creative approaches to representing the sites. The accuracy of these approaches may be masked by the facilities being oversized to some degree. Final analysis of the complete data sets is expected to provide more detailed findings and include recommendation for future bioretention design, plan review, and construction.