2.3 OPTIMIZATION OF THE CONCEPT REPORT SCOPE …lapropo.org/sitefiles/westchester/WestFINALRepAUG08-2.pdf · Three potential trash and debris removal systems were evaluated, including

Concept Validation/Pre-Design Technical Memorandum Metcalf & Eddy | Geosyntec Consultants

Westchester Rainwater Improvement Project

westchester pd tm 20080812 final.doc 15 8/12/2008

2.3 OPTIMIZATION OF THE CONCEPT REPORT SCOPE ELEMENTS

PUMPING Although the gravity system presented in the Concept Report is technically feasible, the size and depth

of excavation necessary to install and operate such a system was deemed impractical, based on cost

and construction constraints. Various pumping configurations were evaluated during the Hydrologic

Analysis (See Section 3) to determine the optimal layout and pumping capacities with respect to

excavation, energy requirements, and process efficiency. Pumping capacities ranging from 20 to 255 cfs

were evaluated in combination with options to pump directly from the diversion structure or to use the

URST for flow rate equalization prior to the UIFs. The optimal configuration was determined to be the

installation of a pump after the URST to convey flows to the underground infiltration facility at a steady,

predictable rate. Optimal pumping capacity was determined based on the hydrologic analysis as

described in Section 3.

TRASH CAPTURE AND REMOVAL SYSTEM The Concept Report included provisions for a trash rack, hydrodynamic separator, and underground

settling basin. The selected alternative consolidates these functions to facilitate design, operations, and

maintenance and to reduce capital costs. Prior to entering the URST, the diverted stormwater flow is

pre-treated by passing through a trash and debris removal system designed to treat the 175 cfs design

flow while removing objects 5-mm in diameter and larger. Sediment and debris smaller than 5-mm in

diameter will settle out in the forebay of the URST. The removal of larger particles by the trash and

debris system installed up gradient will reduce the frequency and degree of URST maintenance.

Three potential trash and debris removal systems were evaluated, including a Gross Solids Removal

Device (GSRD) by Roscoe Moss, a Trash Net System by Fresh Creek Technologies, and a Continuous

Deflection Separation System (CDS) by Contech Stormwater Solutions. A GSRD is a perforated well

screen enclosed in a vault. Storm water enters one end of the screened pipe and exits radially through

louvered openings. The solids are retained in the pipe. The GSRD is built in sections, and each section

has a hinged access hatch for cleaning by vacuum hose. The trash net system relies on the natural

energy of the flow to drive the trash and debris into disposable mesh nets. Nets are lifted out and

replaced using a crane or boom truck. The CDS unit removes trash and debris as well as sediment, oil,

and grease by reliance on flow energy. Solids are captured in a sump at the bottom of the unit and are

removed by vacuum hose.

The design team evaluated the maintenance feasibility and frequency for each of the proposed trash

collection systems and discussed the options with the Bureau of Sanitation (maintenance operator).

Table 2-2 summarizes the advantages and disadvantages of each unit. The trash net system was

ultimately selected as the Preferred Alternative with respect to the site constraints, design flexibility,

and maintenance technique.




TABLE 2-2. TRASH AND DEBRIS REMOVAL ALTERNATIVES

Alternative Advantages Disadvantages

Gross Solids Removal Device

Large capacity per unit

Infrequent maintenance

No removable or replacement parts

Large footprint (5 ft of screen per 12 cfs), requires maintenance access along entire length of unit

Vacuum truck hose cannot reach required depth (>30’)

Confined space entry for maintenance

Trash Nets Limited confined space entry (if any) for maintenance

Disposable nets protect maintenance crew from direct contact with collected trash/debris

System design can be configured to better accommodate site

Low capitol cost

Smallest capacity per unit

Disposable nets must be replaced

Maintenance required at more frequent intervals than other systems

Additional units needed to provide equivalent capacity.

CDS Unit Large capacity

Limited confined space entry for maintenance

Infrequent maintenance

No removable or replacement parts required

Large footprint

Vacuum truck hose cannot reach required depth (>30 feet)

High capital cost (per unit)




3 HYDROLOGIC ANALYSIS

3.1 INTRODUCTION AND PURPOSE

A hydrologic analysis focusing on the wet weather Project scope optimization was completed to support

pre-design of the proposed facility by simulating long-term performance of multiple facility

configurations over a representative period of historical conditions. These simulations are intended to

support pre-design efforts by predicting likely facility performance against TMDL standards and identify

optimized solutions through an iterative process.

The overall method described herein represents an iterative approach to facility pre-design, whereby

initial assumptions are made and then adjusted based on observed performance in the model results.

Thus, the preliminary phase of this effort involved the analysis of several alternative configurations to

develop a matrix of preliminary results. Evaluation of these preliminary results allowed for

development of conceptual relationships between design parameters (i.e. diversion rate, storage

volume, drawdown, etc.) and system performance and was used to develop the Preferred Alternative

design.

METHODOLOGY OVERVIEW The type of facilities proposed and the nature of the TMDL requirements present a number of

challenges in design and analysis. Conventional design storm sizing methods, which are commonly used

in storm water facility sizing, are not appropriate for this Project for the reasons listed below.

Simulation of bacteria concentrations in storm water is a difficult task to undertake with

confidence. Measured bacteria concentrations in stormwater are highly variable both

spatially and temporally and limited monitoring data exist upon which to develop

predictions. Additionally, the types of models required to dynamically predict bacteria

concentrations are generally complex and not well-suited to design applications.

The compliance criteria stipulated by the Santa Monica Bay Beaches Bacteria TMDL are

based on exceedance-days. While these criteria are conducive to monitoring for compliance,

there are no clearly established methods from which design criteria can be based.

Because the compliance criteria are based on a 90th percentile TMDL compliance year and a

reference watershed approach, the approach for quantifying performance and compliance

should incorporate hydrologic variabilities so that appropriate management decisions can be

made. Characteristics such as antecedent (inter-event) watershed conditions become

significantly more critical.

Because there are a limited number of Best Management Practices (BMPs) that can meet

water quality standards for bacteria, from a hydrologic and hydraulic perspective, the types

of facilities proposed will potentially exhibit both volume-limited and flow-limited bypass




events2 and will exhibit transient storage effects (i.e. filling of storage during storm events

and draining after storm events).

For simulation of water quality concentrations, a conservative assumption was made. It was

assumed that whenever an untreated bypass occurred, the concentration of bacteria in the

bypass flow was higher than the TMDL limits. This assumption was made with the intent of

simplifying the analysis without over-predicting system performance.

To address many of the challenges discussed above, a continuous simulation approach was used.

Continuous simulation hydrologic and hydraulic modeling has emerged over the last 30 plus years as a

robust alternative and complement to event-based simulation. The basic hydrologic principles that

govern continuous simulation models are no different than event-based models: a rainfall hyetograph is

converted to a runoff hydrograph employing time variant mathematical relationships intended to mimic

hydrologic processes (Adams and Papa, 2000). This hydrograph is then routed through the stormwater

conveyance system and through stormwater detention and/or treatment facilities. The main differences

between continuous and event-based models lie in the hydrologic input and the time scale considered.

Where event-based models consider only one event and must assume antecedent conditions,

continuous simulation models utilize observed historic meteorological conditions and account for

antecedent watershed and storage conditions from one storm to the next.

The primary advantage of continuous simulation models are that 1) they can provide predictive results

where there are complex design constraints requiring an iterative process, 2) they are based on

observed long-term hydrologic patterns representative of the region, and 3) the output from

continuous simulation models provides a statistical description of hydrologic response and system

performance. These factors make continuous simulation models a more appropriate tool, and a more

robust approach for predicting future system performance and variability.

MODEL SELECTION The EPA Stormwater Management Model (SWMM) Version 5.0 was used for continuous simulation

analysis of the various facility configurations. SWMM is a dynamic rainfall-runoff simulation model used

for single event or continuous simulation of runoff from primarily urban areas. The model accounts for

various hydrologic processes that combine to produce stormwater runoff from urban areas. The model

also contains a flexible set of hydraulic modeling capabilities used to route runoff and external inflows

through a drainage system network of pipes, channels, storage/treatment units and diversion structures

(USEPA, 2008). SWMM was selected because of its proven capabilities in simulation of urban hydrology

and hydraulics, and its flexibility in representing the proposed systems.

2 Volume-limited bypass events occur when facility is full and cannot accept any more volume, causing

stormwater to bypass the facility untreated. Flow-limited bypass events occur when stormwater flow

rates exceed the diversion capacity of the facility, causing stormwater to bypass the facility untreated.




MODEL DEVELOPMENT

The baseline model for the Project was developed based on information obtained from the Los Angeles

International Airport Drainage Master Plan (LAWA, 2005). Representations of proposed facilities were

developed from process flow diagrams developed by the project design team based on knowledge of

site opportunities and constraints. Meteorological inputs (i.e. precipitation and evapotranspiration (ET))

were developed from local gages to be representative of historic weather patterns in the region. Note

that due to spatial variability of regional precipitation patterns, these inputs represent approximations

of actual rainfall at the project sites. Assumed model input parameters are provided in Table 3-1.

Assumptions were intended to maintain a reasonable degree of conservatism in the evaluation of

facility performance. As presented in the Westchester-LAX Stormwater Best Management Practices

Project Concept Report (2007), the North Westchester Drainage Area and Project location are

illustrated in Figure 3-1.

Imperviousness of the watershed was based on values provided in the LAX Master Plan which are

consistent with land use-based imperviousness assumptions provided in the Los Angeles County

Hydrology Manual. These values are believed to be appropriate for estimating land use-based

imperviousness depending on residential density. For the project watershed, these values were

believed to be fairly accurate. All imperviousness was simulated as “directly connected” because

estimates of “disconnection” could not be made based on available information about the watershed.

Disconnection of imperviousness is believed to be important for facility performance as it may have

significant effects in small storm events. Future efforts may evaluate the impact of disconnection of

imperviousness on facility design requirements.

FROM WESTCHESTER‐LAX STORMWATER BEST

MANAGEMENT PRACTICES PROJECT CONCEPT REPORT MARCH, 2007,

North Westchester Drainage Area

Project Location

NORTHWESTCHESTER DRAINAGE AREA

Pre‐Design Technical MemorandumN

DATE AUGUST 2008

PROJECT NO. LA0175

NORTH WESTCHESTER DRAINAGE AREA

FIGURE3‐1Image: Google Earth Pro, 2008




TABLE 3-1. BASELINE SWMM INPUTS - HYDROLOGY

Parameter Value Units Source/Rationale

Rainfall LAX Gauge in/hr Representative of rainfall pattern at project location; long period of record; good resolution; minimal missing data

ET CIMIS Zone 4, 60% ETo

in/mo CIMIS ET map; 60% ETo is typical of urban development

Imperviousness 69% composite; varies by subarea

% Based on LAX Drainage Master Plan

Slope Generally 0.02 in airport and transportation land uses; 0.04 in residential areas

ft/ft From a digital elevation model (National Elevation Dataset) and knowledge of land use characteristics. Intended to represent overland flow slope, not drainage network slope. (minor sensitivity to analysis)

Impervious Roughness

0.01 - Literature1 (not sensitive to analysis)

Pervious Roughness

0.1 - Literature1 (not sensitive to analysis)

Impervious Depression Storage

0.02 inches Literature1 (sensitive to analysis, selected conservatively)

Pervious Depression Storage

0.06 inches Literature1 (sensitive to analysis, selected conservatively)

Ksat 0.15

in/hr Literature1 (representative of B/C soils) (moderately sensitive to analysis

IMD 0.20 in/in Literature1 (representative of B/C soils) (moderately sensitive to analysis, not highly variable)

Suction Head 8 inches Literature1 (representative of B/C soils) (not sensitive to analysis)

% of Imp area w/o DS

25% (residential) 10% (transportation/ airport)

- Residential is SWMM default; transportation is based on knowledge of land use characteristics (moderately sensitive to analysis)

Path Length Varies by catchment (approx. 500 – 2000)

ft Path length measured from hydrology maps; for aggregated watersheds it includes overland path lengths plus portion of conveyance length (SWMM guidance1) (moderately sensitive to analysis)

Routing Imp and Perv routed directly to outlet

- Conservative representation; in reality some imperviousness will be routed over pervious area, resulting in diminished volumes for small storm events

Dry Weather Flow 0 cfs DWF not observed in Argo Ditch

1 – Based on James and James, 2000.




Soils assumptions based on Natural Resource Conservation Service (NRCS) definition of B and C soil

horizons were considered appropriate for the project drainage area. Per hydrology map number 1-H1.7

(Venice) from the Los Angeles County Hydrology Manual, the predominant soil type in the drainage area

is 010 (Oakley Fine Sand) with minor but significant incidence of 014 (Ramona Sandy Loam). Inspection

of the Hydrology Manual runoff coefficient charts for each soil indicates that both soil types have less

potential to generate runoff than many other soil types within Los Angeles County. Ramona Sandy

Loam is included in NRCS Soil Survey 675 and is reported to be a B soil with infiltration rates ranging

from approximately 0.6 in/hr to 2.0 in/hr. Comparison of the Hydrology Manual runoff coefficient

charts shows that Oakley Fine Sand is likely more infiltrative than Ramona Sandy Loam, hence the

assignment of B/C soil properties for both soils is thought to be appropriate while accounting for

potential reductions in natural infiltration rates as a result of soil disturbance.

Conveyance features were simplified to represent system performance without incorporating detailed

feature attributes (except near the diversion structure). Conveyance losses and in-channel attenuation

effects, which may be significant in Argo Ditch, were not accounted for in the model. This simplification

tends to increase peak flows and decrease volumes, which would provide conservative estimates of

required diversion flow rates. The facility representation considered hydraulic effects relevant to the

purpose of the analysis including diversion rates, diversion shut-down due to limited storage, varying

storage capacity, pump controls, and drawdown rates.

Infiltration rates under the proposed facility were not simulated explicitly, but it was assumed that a

steady discharge rate would be provided to draw down the storage over a set period of time. While

infiltration rates of the underlying soil were still under investigation at the time of this analysis, this

uncertainty was not included in the analysis. Modifying the area of the infiltration gallery and/or

spacing of dry wells would allow the design to compensate for uncertainties in infiltration rate without

nullifying model results. Likewise, other components of the facility design could be adjusted to

compensate for uncertainties in infiltration rate.

MODEL CALIBRATION Calibration of the models was not possible due to an absence of flow data from which to base

calibration and verification efforts. Storm runoff data were not available for the modeled watershed.

Because of the project timeframe, acquisition of runoff data to support model calibration efforts was

not considered feasible. As such, validation of model assumptions relied on literature review of

appropriate inputs, comparison of results to locally-derived empirical relationships (i.e. runoff

coefficient formulas), and limited sensitivity testing of selected model parameters.




The following rationale supports the ability of uncalibrated models to produce meaningful results for

this effort:

First, uncalibrated models are generally more reliable in watersheds with higher

imperviousness because simulation of impervious area relies on fewer parameters and these

parameters are less variable than pervious area parameters. This supports the use of

uncalibrated models for the project watersheds (69% impervious).

Second, to address the frequency-based nature of the TMDL, the proposed facilities are

intended to capture and treat runoff from small storms, while permitting portions of

moderate to large storms to bypass untreated. In small events, the contribution of pervious

area is expected to be fairly minimal for a broad range of soil conditions. Small events

represent the critical operational range of the facilities; therefore the sensitivity of the

analysis to pervious runoff parameters is expected to be significantly reduced in comparison

to other types of analyses in which pervious runoff may be more important.

MODEL ASSUMPTIONS AND PARAMETERS Preliminary analyses of alternative configurations were based on Storm Years3 1990 through 1999 as

defined by the TMDL (also referred to as compliance years herein). While a period of record of more

than 50 years is available from the local precipitation gage, a subset was used to reduce model runtimes

thereby allowing for a greater number of simulations. The 1990s were selected because they are

approximately representative of long-term average conditions, include both El Niño and La Niña years,

and include the 90th percentile storm year (1993) by number of storms per year. A 55 year period of

record was used for extended simulation of the preferred configuration, discussed below.

Initial configurations simulated for each project were based on Project Concept Reports produced by

the City for each project being conducted by the City to meet the TMDL requirement, in advance of this

effort. These reports specified a diversion flowrate and off-line storage volume for each project which

were developed from watershed characteristics. In addition to these values, baseline rates of facility

drawdown and specifics of facility operation were provided by the project design team. Subsequent

analyses investigated various combinations of diversion rate, storage volume, and drawdown time.

Additional model parameters were added as necessary to account for modifications to facility concepts

(e.g. supplemental in-channel storage, etc). These are provided in Table 3-2.

3 A storm year is defined as November 1 through October 31.




TABLE 3-2. MODEL ASSUMPTIONS

Watershed Characteristics

Drainage Area, ac 2375

Composite Imperviousness 69%

Approx Runoff Coefficient in Small Events 0.55

Modeled Configuration

Option 4A-3 Configuration

Diversion Rate from Channel, cfs 175

Approximate Equivalent Rainfall Intensity, in/hr 0.13

Off-Channel Equalization/Storage Volume, MG 5

Approximate Equivalent Storm Depth, in 0.14

Pumped Diversion Rate from Equalization, cfs 20

Approximate Equivalent Rainfall Intensity, in/hr 0.02

Offline Detention/Infiltration Volume, MG 4

Approximate Equivalent Storm Depth, in 0.11

System Drawdown Time, days 6

Assumed Dry Weather Flow, cfs 0

Required Infiltrative Loss Rate, cfs 2.32

The following parameters were considered during design optimization:

Diversion flow rates (75, 100, 150, 250, and 350 cfs)

In-line, pre-infiltration storage volumes (0, 3, 4, and 5 MG)

Pump diversion rates (0, 20, 30, 40, 50 cfs)

Off-line storage volumes (3, 4, 5, 6, 7, 10, 20, 40 MG)

Dewatering/equalizing time frames (6, 12 hours)

Total number of design permutations >30

PERFORMANCE CRITERIA For each modeled configuration, results were queried from model output to provide a performance

summary. The wet-weather TMDL regulation language and technical documentation does not support

an absolute interpretation of the requirements that could be directly applied to evaluation of

performance the proposed alternatives in meeting the TMDL requirements. For the purposes of this

project, criteria were developed that defines the term ‘exceedance days’ in a way that can be easily

quantified and directly related to BMP performance and is based on the following assumptions:

Exceedance days were counted as flow events with 24-hr inter-event time;




It was assumed that any untreated flow event would cause an exceedance of bacteria

standards; and

Compliance with the exceedance frequency standards was determined by “Storm Year” or

“Compliance Year”, defined in the TMDL as November 1 through October 31. By comparison,

a standard water year is defined as October 1 through September 30.

Using these criteria, untreated flow events separated by an inter-event time of at least 24 hours were

extracted from the continuous flow records at the project outfalls and tabulated by storm year.

Tabulations of ‘exceedance days’ by storm year were evaluated against the allowable exceedance day

standards set by the wet weather TMDL. Dockweiler Beach, the project receiving water, is allowed 17

exceedance days per year4. Model results were tabulated by ‘years in violation’ (>17 exceedance days

per year), and ‘years close to violation’ (16 or 17 exceedance days per year) for each modeled

configuration. The results showed a range of ‘years in violation’ and ‘years close to violation’ depending

on respective facility design assumptions (i.e. diversion rate, storage volume, drawdown time, etc.).

RESULTS OF PRELIMINARY ANALYSIS OF ALTERNATIVE CONFIGURATIONS The results of the hydrologic analysis of the various alternative configurations modeled support the

following observations:

The results show that storage volumes and diversion rates are both important in facility

design and cannot be viewed separately. As discussed previously, bypass events may occur

as a result of volume-limited conditions or peak-limited conditions.

Model results show sensitivity to drawdown rate which is a function of infiltration rate of

underlying soils. The sensitivity of underlying infiltration rate is mitigated substantially by

the flexibility in facility design which could provide more or less infiltrative area to maintain

specified drawdown times. Results also show that other design parameters may be adjusted

to compensate for changes in drawdown rate.

In general, significantly larger and more expensive facilities would be required to meet TMDL

standards in the worst case year (in this case, compliance year 1998), compared to facilities

that could meet TMDL standards in every other compliance year.

Providing in-channel or off-channel (connected by high-capacity line) flow equalization

storage results in lower total storage volumes and lower pumping rates while achieving

comparable performance.

4 It is recognized that for regulatory compliance purposes, the number of exceedances has been adjusted

to account for monitoring frequency at each ocean outfall. This analysis is intended to be valid regardless

of monitoring frequency, and as such the 17-day criteria was adopted.




SELECTION OF PREFERRED ALTERNATIVE CONFIGURATION Based on preliminary hydrologic model results and discussion of policy considerations, a Preferred

Alternative configuration was selected (see Section 4 for description). A tradeoff point between

anticipated facility performance and facility cost was agreed-upon by the City. The design criteria

considered potential extreme El Niño years (e.g., 1998), which would have likely resulted in significant

exceedances at the reference watershed location, to be outliers. Full compliance was required for the

90th percentile year, which was the basis for TMDL requirements.

EXTENDED SIMULATION OF PREFERRED CONFIGURATION The Preferred Alternative configuration was simulated using a 55-year period of record that included

compliance year 1952 through 2006. The results of these analyses are summarized in Table 3.3. It was

estimated that 13 of 55 analyzed years were moderate to severe El Nino years (76% were non-El Nino

years) (http://ggweather.com/enso/oni.htm). The analysis predicted that during all non-El Nino years,

and during the majority of historical El Nino years, the Project as conceived would have been in full

TMDL compliance.

TABLE 3-3. SUMMARY OF RESULTS

Summary of Analysis Results (SY 1952 - 2006)

Total Exceedance Days (Project) 654

Total Allowable Exceedance Days (17/yr) 935

% of Years in Compliance 95%

% of Non-El Nino Years in Full Compliance 100%

DISCUSSION SUMMARY

As described earlier in the methodology overview, the analysis performed for the proposed project

made use of continuous simulation concepts as a key part of the pre-design process. This approach is

considered the most robust method of measuring anticipated performance of the proposed facility in

meeting the TMDL requirements, as event-based models cannot capture the interrelated impacts of

multiple variables. The analysis was completed with the intent of accurately representing exceedance

days while maintaining flexibility and balancing underlying uncertainties caused by lack of data for

calibration and verification.

Accuracy and flexibility were maintained in the models by continually testing sensitivity in modeling

assumptions and working closely with the design team to understand the design constraints and

probable detailed design features. Model representations did not represent specific detailed design

features but relied on general simplifications to allow flexibility in subsequent detailed design efforts.

To account for uncertainties, the analysis was based on reasonably conservative assumptions in both

hydrologic and hydraulic model representation consistent with the requirements of the preliminary

design phase of the projects. The overall impact of these assumptions is that infrastructure

requirements developed from this analysis may be somewhat conservative. Because of the

conservative assumptions built into the model, it is anticipated that refinements to the hydrologic and




hydraulic parameters, as a result of additional site-specific data, would likely reduce facility sizing

requirements.

A key hydrologic uncertainty lies in representation of soils. Soil properties in the tributary watershed

were estimated based on typical urban conditions and may be somewhat conservative based on

information from the Los Angeles County Hydrology Manual as discussed above. However, the

Hydrology Manual contains only coarse soil delineations and may not account for compaction of soil in

typical urban settings. To ensure that the model representation does not under-predict runoff from the

watershed, the sensitivity of infiltration rate on model results was explored by reducing the infiltration

rate by 50 percent. This analysis showed that the reduction in infiltration rate caused no perceptible

change in exceedance day results. Infiltration rate is the most sensitive of soil parameters. A full

hydrologic sensitivity analysis is beyond the scope of this effort, but knowledge of parameter

importance and typical ranges allowed the design team to balance parameter uncertainty with

reasonably conservative estimates where appropriate.

As a check on the water balance in the watershed, SWMM results were compared to the runoff

coefficient method described in the Hydrology Manual. The Hydrology Manual specifies the following

equation for computation of runoff coefficient:

CD = (0.9 Imperviousness) + (1.0 – Imperviousness) CU

Where: CD = Developed Runoff Coefficient Imperviousness = Proportion Impervious (0 to 1) CU = Undeveloped Runoff Coefficient The undeveloped runoff coefficient (CU) in this equation is a function of soil type and rainfall intensity

which may be obtained from the runoff coefficient charts in the Hydrology Manual. For the soils found

in the project watershed (010 & 014), the range of rainfall intensities associated the vast majority of the

cumulative rainfall volume (0.1 to 1.0 in/hr) result in a CU of 0.1. Substituting this value into the

equation above yields:

Runoff Coefficient = 0.008 % Impervious + 0.1

Substituting the watershed imperviousness of 0.69 into the equation above yields a runoff coefficient of

0.65. By comparison, the period of record runoff coefficient for SWMM is approximately 0.66. While

this comparison is not sufficient to fully validate the SWMM model, it provides support for the overall

balance between runoff and losses predicted by the model and the comparability of the model to

regionally accepted methods.

Attenuation effects in the conveyance network and the Argo Ditch were likely underestimated in the

model representation. The resulting effect is that peak events may be higher in magnitude and shorter

in duration than would actually be observed in the channel. No attempt has been made to quantify the

potential impact of this effect on facility performance, but it may be addressed qualitatively. The most

important facility design parameters influenced by shorter, higher peaks would be diversion rate and




equalization storage volume. It is anticipated that refinements to the routing assumptions would be

more likely to reduce these requirements than to increase them.

All discussions of exceedance days assume that any stormwater discharge that is not treated will

constitute a violation of receiving water standards. While this is an appropriate assumption for planning

and design, it is recognized that should a “first flush” effect be observed it is possible that flows at the

tail end of runoff events may not contribute to exceedances of receiving water standards.

Overall, the modeling approach and selection of parameters were intended to be appropriate with

tendency toward conservatism in certain parameters. Underlying uncertainties such as climatic

variation, concentration of bacteria in stormwater, and bacterial re-growth and die-off processes which

may be very difficult to quantify or predict could potentially result in facility performance that deviates

significantly from predicted values.

Documents

2.3 OPTIMIZATION OF THE CONCEPT REPORT SCOPE …lapropo.org/sitefiles/westchester/WestFINALRepAUG08-2.pdf · Three potential trash and debris removal systems were evaluated, including