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.
14
Embed
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
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
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
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.
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