Integrated Stormwater Facility Design to Address Hydromodification on a College Campus

Integrated Stormwater Facility Design to Address Hydromodification on a College Campus, Livermore, California

Lucas Paz1, William Beaman1, and Hans Kramer1

1Civil Engineering Group, LFR Inc. an Arcadis Company, 1900 Powell Street, Emeryville, CA 94608; PH (510) 652-4500; FAX (510) 652-4906; email: [email protected]

ABSTRACT LFR provided civil engineering services to the Chabot-Las Positas Community College District (“the District”) to support the proposed Las Positas College Facilities Master Plan (“the Master Plan”) development with respect to the design and integration of campus-wide low impact development (LID) stormwater management and drainage improvements. Activities included technical studies, the preparation of civil improvement plans, and technical services needed to support the detailed engineering design of the proposed watershed drainage improvements and associated tasks as required by local, state, and federal regulatory agencies (USACE, USFWS, CDFG, Alameda County, City of Livermore, etc.). The work included the development of conceptual design options for new stormwater management facilities required to satisfy Alameda County’s hydromodification and water quality treatment standards. The proposed stormwater facilities consisted of an integrated suite of measures including landscape based bioretention cells, vegetated swales, flow duration control basins, and subsurface storage elements. The Bay Area Hydrology Model (BAHM) was utilized to address hydromodification management goals and to determine sizing and design for each of the distributed, stormwater storage and treatment facilities. Following initial BAHM modeling, LFR developed a detailed Hydrology and Drainage Study (“the Study”) to support the conceptual design by characterizing existing hydrology and to model proposed conditions using the BAHM continuous simulation approach. Following issuance of the Study, LFR and the District met and reviewed the conceptual design. Consensus on revisions to the proposed conceptual design was reached and LFR prepared 75% civil design and grading plans for the proposed stormwater control facilities, including connections to the existing storm drain system, accommodations for design around existing utilities, re-runs of the modified BAHM model, and finalization of the Hydrology and Drainage Study. In parallel, in order to evaluate potential flooding impacts associated with the campus storm drain system and the new facilities, a StormCAD hydraulic model was developed and used to determine the extent to which the hydraulic grade line would be impacted. By modeling in this fashion, the effect of re-routing flows in the existing campus storm-drain system through the planned stormwater treatment and flow duration control

287Low Impact Development 2010: Redefining Water in the City © 2010 ASCE

facilities could be accounted for in the final facility design. Subsequently, LFR prepared final civil engineering design documents for the initial portions of the project slated for construction including the surrounding piping system and hydraulic structures. WATERSHED SETTING The 148-acre Campus is located within the greater Alameda Creek watershed, which extends from Altamont Pass and Livermore in the east, Mount Diablo and Dublin in the north, Mount Hamilton in the south, and Union City to the west where it discharges into San Francisco Bay. The Campus is located in the Collier Canyon Creek sub-basin of the 388-square-mile Livermore Drainage Unit just north of Interstate 580 near Springtown Boulevard (see Figure 1).

Figure 1. Vicinity Map

Historical land use practices in the project area have altered natural rainfall-runoff processes. Intensive and continued grazing on the hill slopes and valley floors of the tributary basins has led to the alteration and reduction in vegetative cover by converting native perennial grasslands to Mediterranean annual grasses. Grazing has also compacted and degraded the soil. The result of these changing physical conditions in combination with the effect of ongoing urbanization has increased peak runoff rates, destabilization of creek banks, and channel incision. To varying degrees, many of the tributary streams within the Livermore Basin are incised. Urbanized portions downstream of the Campus include flood control channels and piped storm drain systems to contain and direct stormwater runoff. Drainage conduits from the project area direct stormwater to Collier Creek and Arroyo Las Positas, which have been channelized in several reaches to accommodate flood flows. Where they pass through urbanized areas, the major stream channels are engineered as flood control facilities or are planned for flood control improvements in the near future. However, both Collier Creek and Arroyo Las Positas are earthen in the vicinity of the Campus (RJA 1998).


REGULATORY SETTING The Clean Water Act National Pollutant Discharge Elimination System (NPDES) Phase II stormwater program expands on the Phase I program by requiring additional operators of Small Non-traditional MS4s to implement programs and practices to control polluted stormwater runoff. Stormwater discharges throughout California are regulated through NPDES permits issued by the State Water Resources Control Board (SWRCB) administered locally by one of the nine respective RWQCBs. Municipal permit coverage is required for operators of Non-traditional Small MS4s, including universities and colleges. The municipal permit requires development and implementation of a Stormwater Management Plan (SWMP). The SWMP identifies pollutant sources potentially affecting the quality and quantity of stormwater discharges, defines Best Management Practices (BMPs) to be implemented by the college, and provides measurable goals for implementation of the SWMP to reduce discharge of identified pollutants. The goal of the SWMP is to reduce the discharge of pollutants to the Maximum Extent Practicable (MEP), and to identify activities or structural improvements that control the quantity and improve the quality of stormwater runoff. BMPs are developed for Minimum Control Measures for the SWMP to reduce the discharge of pollutants to the storm drain system to the MEP. The San Francisco RWQCB incorporated updated requirements into Alameda County’s (“the County’s”) NPDES Permit in February 2003 (NPDES PERMIT NO. CAS0029831, Provisions C.3.c, C.3.f, and C.3.i, ORDER R2-2003-0021, issued February 19, 2003; “the Permit”). These updated stormwater control requirements are predominantly in the category of new development discharge controls. The permit requires that permanent, post-construction stormwater quality control measures be implemented as part of development projects to address potential hydrograph modification and stormwater quality impacts. The Permit requires the County to implement a Hydromodification Management Plan (HMP) for new and redevelopment projects that add or replace 1 acre or more of impervious surface area (“Group 1 Projects”). The Permit allows certain projects to be exempt from HMP requirements, including projects that discharge to channels that are hardened to the Bay, projects located in areas that are influenced by tidal activity, and infill projects located in highly developed watersheds. Currently all areas in eastern Alameda County automatically require default application of the HMP provisions (ACPWA 2005). INTRODUCTION AND BACKGROUND The Campus Master Plan development required site grading and creation of additional impervious surfaces in the three primary drainage areas (Area 1, Area 2, and Area 3). Estimated increases in impervious surface were calculated comparing the current and proposed level of development. Impervious surface under existing (pre-development) conditions and proposed (post-development) conditions was estimated based on gross square footage of future buildings, proposed land-use


elements (i.e., athletic facilities), and an accounting of the current level of site development. It is estimated that the proposed Master Plan will increase total impervious surface by approximately 26 acres. Without design, construction, and operation of on-site LID stormwater treatment and storage facilities, impacts on the downstream Collier Creek and Arroyo Las Positas corridors were considered potentially significant. LFR worked closely with the District to develop an overall workable LID approach to managing on-site stormwater runoff by designing multiple distributed facilities throughout the Campus within the constraints of an already built environment. Design development for the distributed LID facilities was conducted to address new development requirements as stipulated by the San Francisco RWQCB, the Alameda County Clean Water Program (ACCWP), the City of Livermore, and Zone 7 of the Alameda County Flood Control and Water Conservation District (“Zone 7”). Based on the results of the Study, a series of on-site stormwater treatment and storage facilities (i.e., bioretention areas, flow duration control basins, and storage vaults) were designed using the BAHM to assure that stormwater runoff leaving the Campus will not unacceptably increase due to the proposed new development. MIMICKING PRE-DEVELOPMENT SITE HYDROLOGY The primary goal of the proposed LID methods is to mimic the pre-development site hydrology by using site design techniques and structural control measures that store, infiltrate, evaporate, and detain runoff. All aspects of site development affect the hydrologic response of the site and use of these techniques helps to reduce off-site runoff and increase groundwater recharge. Previous traditional approaches to urban stormwater hydrology have focused on conveying runoff off site and controlling peak flow (and flooding) and velocity. Today’s focus on stormwater quality and protecting natural channels from erosion has led to recognition that effective LID approaches require an acknowledgement of potential changes to total volume and timing/distribution of runoff as opposed to simply evaluating peak discharge rates. HMPs are now being developed to determine and control geomorphologically significant (critical) flows to avoid increases in channel erosion. Hydromodification Management (HM) or Flow Duration Control (FDC) differs from traditional “design storm” approaches to design of detention facilities for flood control or water quality treatment. Instead of specifying static holding times for one or a few discrete events, the flow duration standard manages runoff discharge over the full range of runoff flow levels predicted through continuous simulation modeling, based on a long-term precipitation record. In the past, stormwater management facilities were typically designed to maintain peak flow rates at or near their pre-development levels to prevent increases in the frequency of flooding due to new development. However, facilities that control peak flow rates allow the cumulative duration of elevated flows to increase over time, which can still cause increased erosion of the downstream system (Beyerlein 2005). Protecting stream systems from hydromodification requires HM measures that control the duration of a


range of geomorphically significant flows, those capable of moving sediment and eroding stream bank material. These processes are most affected by small, frequent storms whose runoff is elevated a greater amount of the time after development occurs in the watershed. Throughout North America, observations that peak flow controls alone are not adequate for erosion control have initiated revisions to stormwater requirements to incorporate FDC standards, where pre- and post-development flow duration curves must be matched according to locally specified criteria. FDC requires that increases in surface runoff created by the installation of impervious surfaces be retained on site with gradual discharge either to groundwater through infiltration, reduction by evapotranspiration, and/or controlled releases to downstream receiving waters in order to minimize accelerated bed and bank erosion. Qc is defined as the stream flow threshold that produces the critical shear stress initiating channel bed or bank movement/erosion; the duration of channel flows below Qc may be increased indefinitely without significant contribution to hydromodification impacts. The size of a FDC facility is primarily dictated by the un-mitigated increase in runoff volume between pre- and post-development conditions. Flow duration curve matching is also accomplished through optimization of specialized outlet flow control structures. As mandated by the San Francisco RWQCB, final implementation of the Master Plan will require an innovative, integrated approach to minimize permanent post-construction impacts associated with increased stormwater flows, sedimentation, and potentially contaminated runoff. Due to the proposed land use changes and the existing inadequacies and susceptibility associated with downstream creek channels, stormwater management measures were developed to address Campus drainage elements, sediment management, stormwater quality treatment, and management of the duration and volume of stormwater discharges generated from the Campus. These measures will allow pre-development site hydrology to be maintained despite the increase in impervious surface area. RESEARCH STAGE OF THE PROJECT LFR collected and reviewed background information and met with the District’s civil engineer and other District staff and consultants to discuss the design and construction of drainage elements and other pertinent information regarding the history of the Campus and details associated with the Master Plan. LFR researched and procured the necessary background files including boundary and topographic mapping, building plans, soil studies, grading and drainage plans, and utility plans. This large collection of documents had to be organized and then synthesized into an existing base map representing not only surface features like buildings and other hardscape but also the underground utility system. Once the horizontal locations of these utilities were defined, vertical positions were determined by reconciling information from the various plans for a multitude of prior and ongoing Campus projects.


An initial site visit was conducted to directly observe field conditions and to better understand site constraints associated with the existing built environment (i.e., topography, facilities, and existing utilities). Site inspections, review of aerial photography, review of existing plans, and review of associated documentation allowed for accurate characterization of existing conditions. Detailed site topography base mapping and drainage area land cover distribution estimates were developed. An assessment of existing utility infrastructure design and as-built conditions and facility/building layout was also conducted. Information pertaining to the following elements was collected:

• existing storm drain system (piping and structures) as-built data and follow-up to address data gaps

• as-built drawings for Collier Creek culvert, outfall to Collier Creek, and campus drainage facilities

• original Collier Creek culvert design • site soils data • proposed parking area layout and drainage design • supporting calculations on existing storm system design and proposed campus

improvement projects • proposed distribution/area of artificial turf vs. natural turf • specific locations designated for stormwater infiltration or detention • preliminary design for sizing and layout of stormwater treatment and storage

facilities • stormwater treatment and storage facility design options for below or above

ground • drainage routing and off-site discharge • existing topographic mapping, design drawings, and existing hydrology and

hydraulics reports (from the District, Zone 7 of the Alameda County Flood Control and Water Conservation District, and the City of Livermore) defining the current Collier Creek system in the immediate vicinity of the Las Positas College

CONCEPTUAL DESIGN LFR worked closely with the District from the start to the develop preliminary conceptual design options to improve or replace an existing 108” road culvert where the main college entrance road crosses Collier Creek and to develop potential design options for the replacement and/or retrofit of an adjacent “bubble up” outfall structure located on the bank of Collier Creek. Retrofit and/or replacement of these existing drainage structures was initially recommended to accommodate future build-out associated with the Master Plan. The overall scope of work expanded to the preparation of a detailed Hydrology and Drainage Study and engineering design for a series of on-site stormwater treatment and storage systems associated with the full build-out of the Master Plan. The Study was conducted in order to meet the


requirements of Alameda County Hydrograph Modification Management Plan using the BAHM. Following initial research, LFR developed a conceptual design for new HM facilities to support the Master Plan. This was accomplished using the BAHM software, newly adopted by Alameda County and still in beta-test mode at the time (ACCWP 2006). The BAHM software allows for stormwater facility sizing to meet the County’s HMP criteria by controlling post-development flows. The BAHM iteratively develops a solution to establish flow duration curves between 10% of the 2-year flow (Qc) and the 10-year flow. Application of the critical flow criterion, Qc, was key in minimizing stormwater impacts associated with the Master Plan build-out. New FDC facilities had to be sized and sited to work within the existing Campus, approximately two thirds of which was already developed. The constraints within which the new facilities had to be designed can be summarized as follows:

• Maintain runoff at/below pre-development rates (between Qc & 10-year flow) • Site facilities within existing topography • Maintain pre-project drainage areas • Utilize existing utility infrastructure • Locate facilities in the midst of the existing building layout

These constraints represented opportunities to integrate LID objectives into the project. Facilities were each designed to meet these design constraints, including bioretention facilities allowing for infiltration into site soils to provide additional treatment and flow attenuation. Application of the BAHM required that the Campus be divided up into three hydrologic drainage areas. Each drainage area was further subdivided to detail aerial estimates for land cover classifications required for modeling including impervious features such as roads, parking, driveways, roof areas, sidewalks, and impervious athletic surfaces, and pervious features such as open grass areas (hydrologic soil group C/D), urban landscaping, and pervious athletic surfaces. Slope designations from 0% to greater than 20% were applied to the acreages and input into the model. These conditions were simulated for both pre- and post-project scenarios. In addition, off-site areas within the watershed contributing stormwater runoff to the Campus system were included in the model. The results of the BAHM, an example of which is shown in Figure 2, below, allow direct comparison of a given scenario’s runoff to pre-project conditions over the full continuous simulation period (20+ years of precipitation data). Local, site-specific precipitation, soils, and land cover databases were used by the model to calculate runoff. Flow duration curves are generated for both the pre- and mitigated post-development conditions at a downstream Point of Compliance (POC) and facility sizes are iteratively adjusted until post-development flows are equal to or less than the pre-development flows above the Qc. Flow duration curves associate the range of flows with the percent time exceeded, throughout the period of record. The BAHM


also allows extensive control over the facility outlet structures; riser height, riser diameter, weir notch sizes, and low-flow orifice diameter and height are all features that may be adjusted to fine tune the flow attenuation results. If post-development flows are not adequately controlled, facility sizing and outlet structure dimensions may be adjusted and the model can be re-run.

Figure 2. BAHM Flow Duration Curve Matching for Area 3

Stormwater treatment and FDC facilities were iteratively sized using the BAHM for each drainage area. Figure 3, below, shows the three drainage areas and the eleven proposed facilities. Area 1 included the following facilities: FDC Basin A, Bioretention Basin B, Storage Vault C, and Storage Vault D. Area 2 included: Bioretention Basin 1a, Bioretention Basin 1b, Bioretention Basin 2, Bioretention Basin 3, Bioretention Basin 4, and Storage Vault 5. Area 3 included FDC Basin X. The facilities were positioned in a distributed manner throughout the Campus so as to minimize changes to existing hydrology and drainage area boundaries. Conceptual drawings showing the location and preliminary dimensions of each facility were then produced for discussion with District staff and other stakeholders.


Figure 3. Campus Drainage Areas and Proposed Facilities

ENGINEERING DESIGN DOCUMENTS

Once the backbone of the proposed system was laid out, LFR prepared overall system hydraulic profiles from upstream to downstream to check the design feasibility. This allowed for a better understanding of the overall system, supporting modifications of physical relationships between elements in the system. The BAHM was re-run using new information regarding existing utility constraints gleaned from construction documents and detailed hydraulic profiles. Facility sizing and outlet structure design was refined to meet FDC and water quality treatment criteria. A StormCAD model was also created using information from the hydraulic profiles to ensure that a 10-year design storm event could be conveyed/metered through the system while still meeting the HMP FDC objectives. StormCAD results were used to support final design sizing (i.e. pipe slopes/diameters) and invert elevations for conveyance and storage facilities. In addition, the hydraulic grade lines (HGLs) were compared to the rim elevations for the facilities to evaluate the potential for overflow. Engineering plans and specifications were developed following the hydraulic profile and StormCAD analyses and after reaching consensus with the District and other stakeholders on the conceptual layout and design. Proposed stormwater features included both conveyance facilities such as pipes and culverts as well as storage features (bioretention facilities, vaults, and FDC basins). Bioretention and FDC basins served the dual purpose of both flow attenuation and water quality treatment. Table 1, below, conveys the type of information available as output from the BAHM and also indicates the required storage volume provided by each facility.


Table 1. Flow Duration Control Proposed Facility Sizing Results

Description

Facility Volume Facility Dimensions Outlet Structure Dimensions

Soil Zone Ponding Zone

Total Storage

Volume*(af)

Total Treatment Volume*

(af)

Area* (sf)

Depth (ft) Riser Notch

Orifice Diameter

(in) Empty Volume

(af)

Pore Space

Volume (af)

p=0.40

Empty Volume*

(af) Soil Ponding Diameter

(in) Height

(ft) Height

(ft) Width

(ft)

AREA 1 Flow Duration Control

Basin A - - 0.45 0.45 0.45 5,092 - 5 18 5 2.69 1.13 8.6

Bioretention Basin B1 0.68 0.27 0.22 0.49 0.49 10,568 4 1 36 5 1.00 1.00 3.0

Bioretention Basin B2 0.36 0.15 0.12 0.27 0.27 5,886 4 1 15 5 1.00 1.00 3.0

Storage Vault C - - 0.73 0.73 - 6,749 - 4.7 24 4.7 2.00 1.00 3.0

Storage Vault D - - 0.64 0.64 - 8,488 - 3.3 15 3.3 2.00 0.50 3.0

AREA 2

Bioretention Basin 1a 0.82 0.33 0.55 0.88 0.88 13,158 4 2 36 6 2.00 1.50 3.0

Bioretention Basin 1b 0.06 0.03 0.03 0.06 0.06 2,082 4 1 18 5 1.00 0.50 3.0

Bioretention Basin 2 0.05 0.02 0.03 0.05 0.05 1,704 4 1 18 5 1.00 0.50 3.0



Storage Vault 5 - - 1.91 1.91 - 17,691 - 4.7 24 4.7 2.00 1.00 3.0

AREA 3 Flow Duration Control

Basin X - - 2.81 2.81 2.81 36,400 - 4 18 4 1.80 0.90 1.7

Notes: * = with water surface at riser rim, ft = feet, in = inches, af = acre-feet, p = porosity, sf = square feet, vert = vertical

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Facilities were located with respect to existing watershed boundaries and other physical constraints such as existing retaining walls. Augmented detail in base mapping prevented utility conflicts during the design stage. Both 75% and 100% construction design documents were prepared to support detailed grading and associated improvements for each of the eleven proposed on-site stormwater facilities. Figure 4 shows a typical outlet riser structure, sized using the BAHM software and detailed by LFR in the civil design plans.

Figure 4. Typical Outlet Riser Detail

CUSTOMIZED DESIGN FEATURES AND FINAL DETAILS During the design process several notable challenges required the following customized solutions. Inlet Risers were necessary to allow flow from upstream pipes to release into bioretention basins at a specific elevation to avoid surcharge and allow water to release without the need for excessive hydraulic head on the upstream end to drive the flow. The inlet risers also allow for flow to enter the basins in a more passive condition to prevent disturbing the three-layer engineered fill matrix. High Flow Bypasses were introduced to allow low flows to enter proposed facilities unimpeded and high flows at or above the 10-year flow-rate to bypass proposed facilities, protecting side slopes and the soil matrix by not allowing for more erosive conditions. Each bypass feature was customized according to site-specific constraints and had to be sized and located so that new piping could be set to connect the high flow bypass structure to the existing storm drainage system. Equalizer Culverts were needed to allow flow to pass from one storage feature to an immediately adjacent storage feature. These are instances where the Campus either had a pre-existing landscape feature or the landscape feature was already part of existing final improvement plans for a portion of the site. Bioretention Basins were designed to retain specified storage volumes per the BAHM. This was partly accomplished by using an engineered matrix consisting of 18” of sandy loam over 18” of sandy gravel over 18” of gravel resting on


uncompacted subgrade. In some narrow instances, proposed basin subgrade slopes could be designed at 1:1 or steeper due to the fact that they would be backfilled immediately with the engineered matrix. Storage volume within the voids of the soil matrix had to match the volume requirements for the particular facility. Stepped basins were used in narrow landscape areas that had longitudinal slopes greater than 5% to prevent higher velocity flows from eroding the upper sandy loam soil layer. Bioretention basins also required emergency overflow elements to facilitate/control overflow during extreme conditions or due to plugging, minimizing potential undirected surface flooding near doorways or other areas on the Campus. Outlet Risers were sized/dimensioned in accordance with BAHM results. Design head on the riser, riser barrel diameter, weir notch height and width, and low flow orifice diameter were used to control the maximum allowable outflow. Stainless steel screens were used to cover the low flow orifice and hold back the surrounding ¾” clean drain rock that was mounded around the base of the riser and above the orifice. A Vertical Headwall was required along a flow duration control basin due to space constraints between an important existing roadway and Collier Creek. By using a vertical headwall element storage volume objectives were met and the basin was appropriately sized within a relatively narrow 25-foot-wide work area. CONCLUSION LFR worked closely with the District to develop a workable LID approach to managing on-site stormwater runoff by designing multiple distributed FDC facilities throughout the Campus within constraints of the existing built environment. Alameda County HMP provisions stipulated that a continuous simulation modeling approach be used to size/design proposed FDC facilities. The BAHM was successfully utilized to evaluate pre- and post-development conditions, developing a solution to establish flow duration curves between 10% of the 2-year flow and the 10-year flow. Specialized outlet risers were designed to control water surface elevation accommodating both FDC and flood control objectives. After refinement of the BAHM and StormCAD models, adjusted results were used to support final design. REFERENCES Alameda County Clean Water Program (ACCWP). (2006). Bay Area Hydrology

Model (BAHM), Alameda County Beta Version Release Notes. May 19. Alameda County Public Works Agency (ACPWA). (2005). Hydrograph Modification

Management Plan. May 15. Beyerlein, Douglas. (2005). “Flow Duration-Based Stormwater Mitigation

Modeling.” Stormwater - The Journal for Surface Water Quality Professionals. May/June.

Ruggeri, Jensen, Azar & Associates (RJA). (1998). “Hydraulic Study of Collier Canyon Creek (Line M) from I-580 North to the Crossing of the Proposed Collier Canyon Road, City of Livermore, Alameda County.” August 12.


Integrated Stormwater Facility Design to Address Hydromodification on a College Campus

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