HomeMy WebLinkAboutCT 88-03; Encinitas Creek; Master Drainage Plan; 1988-06-01.- 1~:
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Encinitas Creek Master Drainage Plan ‘, ‘? :I: :I:;~ .:: Volume “A” . :~
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HEX-2 HYDRAULIC ANALYSIS- ENCINITAS CREEX
PREVIOUS COUNTY STUDY
The previous County of San Diego HEC-2 computer analysis for Encinitas Creek was completed in 1981. The study was completed for several reaches of the creek and tributaries. The County study includes the reach west of El Camino Real which is of interest in this study.
The County HEC-2 study uses the design flows from the County of San Diego Hydrology Report for Encinitas Creek (1980).
A field topography check completed in June 1988 as part of this studv showed substantial variations in the Encinitas Creek geometry and invert elevation between the Countv HEC-2 studv and ', existins conditions. The invert of the creek has been filled , with a substantial amount of silt which causes concern about the loo-year water surface in the creek relative to the El Camino T,.. Real roadway. The loo-year water surface in the creek must also L'
be considered in the analysis of the capacity of the existing crossings under El Camino Real.
A revised HEC-2 analysis was completed for this study to evaluate the effects of the changes in the creek's geometry and invert on . the loo-year water surface elevation. The HEC-2 study reach is the 1.5-mile reach of Encinitas Creek in the City of Carlsbad which is on the west side of El Camino Real and flows northerly from Olivenhain Road to the Batiquitos Lagoon at La Costa Avenue.
DATA BASE
The input data for the revised HEC-2 analysis of the creek is based on current topography and developed land use design flows.
The cross section locations used in the HEC-2 analysis are the
same as in the San Dieqo County analvsis. The Manning 'InI' roughness values used for the creek are based upon the County analysis. The 'InI' values for the main channel of the creek have been increased slightly as requested by the City of Carlsbad. Additional cross sections were added between the County sections when additional detail was needed.
The starting water surface for the HEC-2 analysis downstream of La Costa Avenue in the Batiquitos Lagoon was derived from the U.S. Army Corps of Engineers report "Flood Plain Information San Marcos Creek" (USACOE, 1971). The "Batiquitos Lagoon Enhancement Project" (CH2M Hill, 1988) Draft Report does not address the loo-year water surface in the lagoon.
The Corps of Engineers report shows the loo-year water surface in the lagoon from the ocean upstream to the discharge point of San Marcos Creek. The water surface in Batiquitos Lagoon downstream of San Marcos Creek is the downstream control of Encinitas Creek at its discharge point into the lagoon.
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The cross sections used in the analysis are derived photogrammetrically from aerial photography dated May 3, 1988. The accuracy of the digitized cross sections exceeds FEMA standards. The cross section end point coordinates were digitized from the County Floodplain Maps for Encinitas Creek to accurately reproduce the section locations. The cross section of the bridge at La Costa Avenue was measured in the field.
The loo-year design flows were taken from the HEC-1 Model hydrologic analysis described in a separate report (Rick Engineering Company, 1988).
SUMMARY OF RESULTS- EXISTING CONDITIONS
The HEC-2 computer model output for Encinitas Creek for the loo-year storm under existing conditions is shown in Appendix 2 of Master Plan, Volume "B".
The analysis of the existing creek shows that the existing triple 8' X 12' R.C.B. crossing at La Costa Avenue is inadequate for the design flows given the existing headwater available on the La Costa Avenue and El Camino Real roadways near the culvert.
The HEC-2 analysis of the existing conditions also shows that the loo-year water surface in Encinitas Creek is below the El Camino .' Real roadway for the study reach except near the Olivenhain ,Road intersection. The backwater caused by the silt downstream of the El Camino Real culvert near Olivenhain Road results in the flooding of the roadway in this area. The capacity of the El Camino Real culvert also is impacted by the creek backwater.
The results of the HEC-2 hydraulic analysis of Encinitas Creek under existing conditions are summarized in the HEC-2 Comparison Table on page 11 of this report.
RECOMMENDATIONS
The following improvements are recommended to mitigate the existing inadequacies in Encinitas Creek from La Costa Avenue to Olivenhain Road:
1. Improve the existing triple 8' X 12' Reinforced Concrete Box (R.C.B.) under La Costa Avenue. We recommend adding an additional triple 8' X 12' R.C.B. and adding a berm along La Costa Avenue and El Camino Real to increase the available headwater at the culvert. The top of berm is proposed to elevation 20.2 (the top of the bridge above the culverts is at elevation 20.8) to provide 1.3 foot of freeboard to meet County of San Diego Criteria.
2. (Alternative A) Improve the existing Encinitas Creek channel immediately downstream of the El Camino Real culvert near Olivenhain Road. The proposed improvements consist of an IlOO-foot long trapezoidal channel which is 100 feet wide on the
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main part of the creek and 50 feet wide on the creek downstream of the culvert. The trapezoidal channel proposed to vary from approximately 3 feet to 5 feet.
directly depth is
3. (Alternative B) If Alternative A -(abovek-_-is not ~feasible because environmental clearance can not be obtained, then we .F recommend the El Camino Real roadway be raised above the loo-year water surface. The roadway should be raised to the elevation ~4
.-- 84.0 and a quadruple 4' x 12' R.C.B. culverts be constructed under the road to match the existinq creek invert. $ P
The HEC-2 computer model output for the loo-year storm with the 3 proposed improvements in place is shown in Appendix 3 of Master b Plan Volume B. The results of the HEC-2 analysis considering the ' recommended improvements are shown in the HEC-2 Comparison Table below. The results of the recommended improvements analysis for Alternative A are also compared to the edge of pavement elevation on the El Camino Real roadway and the available freeboard height is shown.
HEC-2 COMPARISON TABLE
Comparison of loo-year Water Surface in Encinitas Creek between San Diego County (1981) and Rick Engineering Company (1988) HEC-2 Runs (values in feet)
Section loo-yr. loo-year loo-year Approx. Freeboard
NO. WSEL WSEL (REC) WSEL (REC) Rd. edge To Road- County Existing Proposed Elev. Prop. run (Alt-B) (Alt-A)
0.098 la.2fj/."'.~~~1;*a la.9 20.1 1.2 0.129 la.0 19.9 19.6 25.3 5.7 0.202 27.2 28.9 29.0 42.4 13.4 0.310 36.5 38.9 38.9 50.1 11.2 0.451 44.7 45.7 45.7 58.3 12.6 0.522 46.4 48.5 48.5 56.3 7.8 0.615 50.5 52.1 52.1 69.1 17.0 0.678 55.5 55.5 55.5 78.6 23.1 0.780 59.8 60.3 60.3 80.0 19.7 . 0.834 62.0 62.6 62.6 74.4 11.8 0.918 63.4 65.6 65.6 66.6 1.0 0.998 64.9 .66.8 66.8 69.6 2.8 1.096 67.5 67.9 67.9 79.2 11.3 1.164 69.3 69.9 69.9 85.7 15.8 3.236 72.0 76.9 72.8 89.6 16.8 1.313 74.0 78.8 75.2 83.6 8.4 1.365 -- 81.4 76.2 80.2 4.0
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=Y EXISTING DRAINAGE CONDITIONS
ADEQUACY OF EXISTING STORM DRAIN FACILITIES
The existing storm drain and culvert facilities were evaluated for their adequacy for the loo-year design flows. The capacity of the facilities was based upon approximate methods and engineering judgment. The existing culvert capacities were based upon inlet control unless specific downstream control was knotin. The only instance where downstream control is known to 'be significant is west of El Camino Real in Encinitas Creek.
The existing storm drain facilities within the watershed study area are, in general, adequate to convey the loo-year design storm. Most of the development in the basin has occurred recently and the drainage facilities associated with the developments were adequately designed.
Some of the existing road culverts constructed with the major roadways were found to be inadequate for the loo-year design flows. Some of these culverts may have been sized for historic flows rather than developed flows.-
The following existinqCinadequate for the loo-year desion flows:
Bi 1. The bridae crossins of Encinitas Creek at El Camino Real is silted and will not pass the design flow without over-topping the
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roadway.
2. The existing 60" C.M.P. under El Camino Real north of Levante
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Street is ,inadequate for the loo-year design flow.
3. The existing 24" R.C.P. under El Camino Real north of
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Olivenhain is inadequate for future developed flows.
4. The existing dual 24" C.M.P under Olivenhain Road is inadequate for the loo-year design flow.
oa(l 5. The existing dual 4' X 10' Concrete Box Culvert crossina of Encinitas Creek under Ranch0 Santa Fe Road-is inadequate for the loo-year design flow.
w1 6. The existing triple 8' X 12' R.C.B. culvert under La Costa
I\venue is inadequate for the design flow due to the limited available headwater on the roadways.
Several of, the existing road culverts have collected silt which
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reduces their capacity. The existing silt volume in the majority of the culverts will be flushed out during any major storm. As the remainder of the watershed is developed, smaller volumes of
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silt will be delivered to the culverts and the future problems should be minimised.
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Two of the existing major crossings which are presently silted will require special attention during final design to minimize the effects of the existinq silt on the facility capacity. The crossings affected are:
1. The Encinitas Creek crossinq of El Camino Real south of Olivenhain Road which is almost entirely filled with silt. The downstream creek bed has also collected silt.
2. The dual 5'~ X 6' Concrete Box Culvert under El Camino Real near the proposed Calle Barcelona intersection (Facility Number 11) is silted about 2'. Downstream silt conditions in Encinitas Creek cause the culvert to collect silt.
The preliminary design for these crossings are discussed in the "Recommended Drainage Infrastructure Improvements" section of this report.
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RECOMMENDED DRAINAGE INFRASTRUCTURE
INTRODUCTION
The recommended facility improvements for
IMPROVEMENTS
the Encinitas Creek watershed area in the City of Carlsbad is intended to provide a guideline for design of the storm drainage infrastructure for the area. The analysis addresses storm drains greater than or. equal to thirty (30") inch diameter pipe size. Storm drain collection systems including inlets, and storm drains less than thirty (30") inches in size will be provided as part of the individual developments and projects within the watershed.
Sizing of the recommended facilities are based upon approximate methods and engineering judgment. Final design of any storm drain or open channel should be performed by a qualified engineer.
The existing and recommended drainage facilities are tabulated in the Drainage Facility Tables beqinning on page 18 of this report. The location of the-drainage facilities referenced are'shown on Plates B and C following the tables.
DESIGN CRITERIA
The loo-year frequency storm was used as the basis the recommended improvements in the watershed.
in the tables
for design of Hydrologic methodology is discussed in the l'Hydrologic Methodology and
Description" section of this report.
Closed conduits are recommended in most cases for systems forty-eight (48") inches or less. Closed conduits larger than 4a-inch are used if a channel is impractical in the reach. Storm drains are designed to connect into existing systems found adequate for the ultimate capacity as defined by this study.
Reinforced concrete pipe (R.C.P.) is assumed for closed conduit design. For road crossings, either R.C.P. or reinforced concrete box culverts are used. R.C.P. sizes from 30-inch to 78-inch diameter in increments of 6-inches are used in the study. A Manning's roughness coefficient (n value) of 0.012 is used for R.c.P. storm drain design.
Pipe slopes are based one the existing ground slope where applicable with a minimum gradient of 0.5 percent.
Box culvert sizing is based on a minimum 3 foot height for ease of maintenance. Height sizing is based' on one foot vertical increments and width on two foot horizontal increments. A Manning's roughness coefficient of 0.014 is used for design.
Where existing pipes were found to be inadequate, they were assumed removed if metal and removed or paralleled depending on the situation for concrete pipes. ., b 14
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P Floodplain management including natural channels and vegetation lined graded channels is used for open channel reaches of the drainage system in lieu of lined channels. Natural channels which are narrowed to accommodate development and graded channels will be designed to meet allowable velocity criteria. The finished floor of structures shall be built a minimum of one foot above the loo-year water surface in the channel. All channels shall be in compliance with current flood plain criteria.
EL CAMINO REAL CULVERTS
Desiltinq basins are recommended at the upstream side of the two culverts under El Camino Real draininq towards Encinitas Creek These culverts have collected a significant amount of silt. We also recommend that the culverts be cleaned out and the creek downstream of the culverts be dredged to provide positive outflow for the culverts under all flow conditions.
We propose to clean out the dual 5' X 6' R.C.B. under El Camino Real near the proposed Calle Barcelona Intersection to its invert. It is also recommended that a 15-foot wide trapezoidal channel be constructed from the culvert invert draininq to the, creek flowline to provide an outfall. The depth of excavation needed to implement this recommended solution is approximately 2 feet to 3 feet.
In Alternative A, we propose to lower the invert of the existing bridge under El Camino Real south of Olivenhain Road to provide adequate capacity and positive drainage. The invert is proposed to be lowered to elevation 72 at the downstream side of the bridge from the existing invert elevation of approximately 75. The proposed channel improvement downstream of the bridge is discussed in the "HEC-2 Hydraulic Analysis - Encinitas Creek" section of this report.
In Alternative B, we propose to raise the El Camino Real roadway to elevation 84.0 and build a new quadruple 4' X 12' R.C.B. culvert under the road to match the existing creek invert.
The land for the desilting basin at the dual 5' X 6' R.C.B. is a part of the development upstream and is zoned as open space. The land upstream of the bridge crossing south of Olivenhain Road is within the Citv of Encinitas and will need to be coordinated with Encinitas. The land acquisition cost is not included in the - preliminary cost estimates.
The proposed desilting basins and channel dredging should solve the problem since the upstream drainage areas are being developed and silt volumes delivered to the culverts will be reduced in the future under developed conditions. The proposed solutions to the silt problem in the culverts will, however, need to monitored
periodically after completion to insure that the design is maintained.
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PRIORITY OF IMPROVEMENTS
The recommended improvements as outlined in this Plan have been given a priority rating in the Drainage Facilities Tables. The ratinqs vary from 1 as the hiqhest priority to 3 as the lowest priority. Criteria for determining the ratings are:
1. Improvement is needed due to an endangerment to life or public health and safety.
2. Improvement is needed to mitigate potential damage to existing property or structures.
3. Improvements will be needed to protect future development.
A summary of the total cost of improvements broken out by priority rating is shown on page 17. Refer to the cost estimate portion of this study for total cost of improvements in each study sub-basin.
DRAINAGE FACILITY TABLE DESCRIPTION
The existing and recommended drainage facilities in the Encinitas Creek watershed study area are shown on the Drainage Facility . Tables. These tables list preliminary facility location, tributary drainage area, size, length, capacity, loo-year design flow and recommended improvements (where applicable).
The location of the drainage facilities are shown on Plates B and C included with the tables. Confluence points of storm drains shown on the plates are approximate. Location and size of inlets and lateral storm drains smaller than 30 inch diameter pipe size will be designed and provided as part of the individual developments within the watershed.
conveyance of flow in streets was neglected for preliminary design of storm drains. In final design, street conveyance capacity should be considered in the analysis.
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