NORTHGATE CHANNEL REPORT final II 5 18 07
Transcript of NORTHGATE CHANNEL REPORT final II 5 18 07
NORTHGATE DIVERSION CHANNEL DRAINAGE STUDY
Presented to
City of El Paso
Prepared for
City Engineering Department
DORADO ENGINEERING, INC. in association with Professor H. S. Oey P.E., RPLS and
Strategies & Plus
MAY 2007 FINAL REPORT
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
2
TABLE OF CONTENTS
EXECUTIVE SUMMARY……………………………………………………………. 3
I. INTRODUCTION…….……………………………………………………. 5
Project Scope………….…………………………………………….. 6
II. RESEARCH SECTION…………………………………………………… 8 Existing Drainage Problems………………………………………. 8 Proposed Plans……………………………………………………… 9 Hydrology and Hydraulic Studies…………………………….….. 16 Hydraulic Designs…………………………………………………… 25
III. FIELD DOCUMENTATION……………………………………………… 35
Description………………………………..………………………….. 35 Physical Characteristics…………………………………………… 36 Site Investigation……………………………………………………. 37 Soils Report………………………………………………………….. 38
IV. CONSCLUSION AND RECOMMENDATIONS………………………. 39
V. CONSTRUCTION COST ESTIMATES………………………………... 41
REFERENCES..………………………………………………………..……………. 44
APPENDICES………………………………………………………………………… 45
Appendix A – Flood Hydrograph Package (HEC-1)
Appendix B – Plates (1 through 14) Appendix C – Soil Report
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
EXECUTIVE
SUMMARY
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
3
EXECUTIVE SUMMARY
In December of 2006, the City of El Paso retained the services of DORADO
ENGINEERING, INC. to develop a drainage study to resolve the flooding problems at
the Northgate Diversion Channel.
This document, Northgate Diversion Channel, examines and analyzes the drainage
problems in a specific region in Northeast El Paso. The study area is described as
bounded by the crest of the Franklin Mountains, Hondo Pass Drive, Galena Drive,
Moonlight Avenue and the Northgate Diversion Channel area. The Northgate Diversion
Channel is located adjacent to the intersection of Hondo Pass Avenue and Hondo Pass
Drive and is a dirt channel (arroyo) that suffered extensive soil erosion during the 2006
summer rains. The report investigates and assesses the most economical way to
reconstruct the existing channel and prepare the necessary engineering plans and
construction documents. Best management practices based on approved criteria is
delineated and structural and nonstructural improvements are recommended.
The report is comprised of the following major elements:
1. Executive Summary
2. Introduction and Project Scope
3. Hydrology and Hydraulic Studies
4. Field Documentation and Evaluation
5. Topographic Field Surveys
6. Conclusion and Recommendations
7. Construction Cost Estimates
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
4
8. Study Support Appendices
The study focuses on the ability to reasonably protect the study area from damage
which might accrue due to a storm of the order of the Criteria Storm adopted. Our study
indicates that all or the major portions of the proposed plan can be justified by a detailed
cost-benefit analysis. The estimated cost of works provided for in the report outlines
three alternate solutions which are; 1) retention dam, 2) concrete open channel, and 3)
closed conduit. Line item listings of the three cost estimates for the reconstruction of
the existing channel including a 10% contingency amount are depicted in Section V.
During the design phase the cost estimates may vary somewhat due to the inflationary
bid prices currently prevailing in the construction industry.
After a very thorough analysis and evaluation of the existing conditions at Northgate
Diversion Channel, it was concluded that using two – 66 inch reinforced concrete pipe
will be the best approach in conveying the 100 year criteria storm flow from the slopes
of the Franklin Mountains down to the existing Northgate Dam Impounding Area. The
cost of the recommended option is $736,060.
The recommendation is based on the following observations:
1. The most efficient and cleanest method of conveying storm runoff
2. The most approachable method of working on the steep slopes of existing arroyo
3. The least time to construct
4. The least work to maintain
5. The most environmental friendly since the storm pipes are covered in the arroyo
6. Elimination of existing arroyo
7. The most economical
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
DRAINAGE
STUDY
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
5
I. INTRODUCTION
The City of El Paso retained the services of Dorado Engineering, Inc. to perform the
necessary research in conjunction with the City Engineering Department, Street
Department, and the U. S. Corps of Engineers, to gather all available data needed to
design and reconstruct the existing channel in the Northeast Region of the City, also
know as the Northgate Diversion Channel.
The City of El Paso requested that prior
to the preparation of design plans and
specifications of the reconstruction
project, A Preliminary Design Phase
Report be submitted for approval. The
report focuses on the ability to
reasonably protect the study area from
damage which might accrue based on the 100 year storm. This report will propose
three plans to alleviate and abate the storm water flood problem. In general it provides
for short period detention, diversion, conveyance, through channels and ultimate
delivery to dry lake sumps which will necessarily have to be excavated. The water will
be disposed of by a combination of evaporation and infiltration or seepage. This plan is
the most economical method to accomplish the required purpose of adequate protection
from a flood which might arise with the occurrence of a 100 year storm. Site
improvements are specified and related estimated construction costs are identified.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
6
PROJECT SCOPE
Northgate Diversion Channel Scope of Work Revised 12-4-06
The Northgate Diversion Channel is located adjacent to the intersection of Hondo Pass
Ave. and Hondo Pass Drive and is a dirt channel (arroyo) that suffered extensive soil
erosion during the 2006 summer rains.
The project scope, as delineated by the City of El Paso, consists of the following:
1. Perform the necessary research with the City Engineering Department, Street
Department and the U. S. Corps of Engineers, to gather all available data that
will assist in the design and reconstruction of existing channel.
2. Perform the necessary field work to gather the required information to locate
channel set grade, set channel cross sections and estimate cuts and fills.
3. Determine watershed area of channel and perform the hydraulic computations,
based on the 100 year storm, to determine expected flows through the channel.
4. Based on computed hydraulic computations design channel for size and grade to
safely carry the expected 100 year flows.
5. Based on gathered information, determine most economical way to construct
channel such as, concrete, gabions, rock rip rap, dirt or a combination.
6. Coordinate project with all effected City Departments, Utilities, U. S. Corps and
all other effected agencies.
7. Provide soil investigation.
8. Prepare the necessary engineering plans and construction documents to
advertise project.
a. Design drawings
b. Specifications
c. Cost estimates
9. Acquire required grading permit.
10. Provide water pollution control plan.
11. Reproduction
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
7
12. Assist owner in bidding process in determining the following:
a. Bid period and date
b. Respond to all questions from perspective bidding
c. Attend pre-bid conference
d. Prepare addenda
e. Evaluate bids
f. Provide recommendation on experience and acceptance of contractors
and sub contractors
g. Perform purchasing list
h. Issue Certificate of Substantial Completion
i. Provide As “Built” plans
j. Construction sequence plan
k. Construction schedule
13. Represent owner in all matters concerning the constructability of project such as:
a. Be in charge of construction management
b. Provide timely reports to owner
c. Respond to all matters concerning the project construction
d. Review and make recommendations to owners on
1. Submittals
2. Payment request
3. Change orders
e. Perform purchasing list
f. Issue Certificate of Substantial Completion
g. Provide As “Built” plans
h. Construction sequence plan
i. Construction schedule
Project Schedule *Preliminary Design Phase…..…………………………………………………45 Days
Final Design Phase…..………………………………………………………….15 Days * Provided that the City Engineering dept. has enough information on file, such as drawings and
topographic maps of the contributing water shed and existing drainage structures.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
8
II. RESEARCH SECTION
This study is made under the authority of a contract with the City of El Paso, Texas.
The City of El Paso of El Paso commissioned two previous drainage and flood control
studies for the Northeast Region of the city in 1960 and 1978. The purpose of the
previous studies was to develop a proposed master drainage plan and flood control
basin. Accordingly the city has taken some action in developing drainage and flood
control measures, however, drainage problems have continued.
EXISTING DRAINAGE PROBLEMS
Under existing conditions, flooding occurs at the intersection of
Hondo Pass Avenue and Hondo Pass Drive which are
adjacent to Mountain Hills Estates.
During the 2006 summer rains, the existing arroyo that carries
the flows from the Mountain Franklin slopes to the existing
Northgate Dam Impounding Area was over taxed causing the
rainfall water to go over the existing arroyo banks, resulting on
extensive erosion on the arroyo and the right-of-way of Hondo
Pass Drive. The arroyo overflows came very near to eroding
Hondo Pass Drive which eventually would have over spilled to
the existing residential subdivision. The existing condition of
the arroyo presents serious safety concerns to adjacent
properties if not corrected.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
9
DESCRIPTION OF PROPOSED PLANS
The plans herein being studied consist of detention dam, concrete open channel and a
closed concrete conduit. Each plan will be studied and together with the estimated cost
will be rated and recommendation made.
The detention dam will function by temporarily impounding the 100 year criteria storm
run-off in a reservoir and releasing them at maximum non-damaging rates of flow. Thus
reduce the extremely large and extensively damaging peak flows or discharge rate.
The concrete open channel and close conduits will function as control conveyances to
move the expected peak flows from the mountain alluvial washes to the existing
Northgate Dam impounding area. All flows will be contained within a proposed concrete
channel or closed concrete conduits to eliminate all erosion on existing soils. The
proposed channel and closed conduits are designed to convey the 100 year criteria
storm.
In determining the rate of run-off, several methods could be considered. Estimating the
time of concentration and use of the rational formula was considered, but was not used,
since the center of the mass of the storm, time wise, is in the late portion of the storm.
Instead through the use of HEC-1 computer program we were able to develop a
hygrograph of the actual storm on the area using the 100 year criteria storm. The storm
that was used in this study was the City of El Paso 100 year storm for contributing
watershed areas larger than 200 acres as shown on the City of El Paso Subdivision
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
10
Improvements Design Standards Plate 2-9 “Rainfall Curves”. Based on Plate 2-9 the
100 year criteria storm for watershed area of 805 acres the intensity is 3.80 inches in
three hours. This information was inputted into the HEC-1 program that uses the United
States Soil Conservation Surfaces (U.S.S.C.S.) unit hydrograph to compute the
hydrographs used for this design. This is the storm that was used in computing the
hydrographs shown on pages 24 and 26.
Rainfall Criteria Used
The “criteria storm” of a return frequency of 100 years that was be used in the design of
the detention reservoir (dam) open channel and closed conduits. The storm duration is
three hours with a total rainfall of 3.80 inches in the eye of the storm, for the watershed
area of 805 acres. The rainfall sequence is as follows;
1st hour …………………….. 0.50 inch
2nd hour…………………….. 0.78 inch
3rd hour…………………….. 2.55 inch
The rainfall intensity in inches for the criteria storm versus the area in acres is shown on
the subdivision design standards plate 2-9 and was used in the design.
Detention Reservoir Capacity (Dam)
It is recommended and used as criteria that the storage capacity of the proposed dam
be designed to contain one hundred percent (100 %) of the runoff for the criteria storm
as calculated plus a 10 year silt volume.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
11
Spillway Capacity
It is recommended and used as criteria that the spillway be designed for the passage of
the maximum rate of flow for the 100 year criteria storm, assuming the reservoir full and
the outlet flow closed. A minimum of five feet of free board is used.
Outflow Pipe
The outflow pipe will be designed to empty the detention reservoir (dam) within 72 hours
maximum. The outflow pipe will also be designed to minimize erosion at the lower
reaches of the detention reservoir (dam).
Runoff Factor
Mountainous watersheds 85%
As shown on plate 2-10 of the subdivision standards and was used in the design.
Silt and Debris
Mountainous watersheds 0.003 ac. ft. / ac
As shown on the plate 2-5 of the subdivision standards and was used in the design.
Design Criteria Used
1. Drainage Area ………………………………………… 805 acres 2. Design Storm (100 Yr) ……………………………….. 3.80 inches in 3 hours
3. Runoff Factor.………………………………………….. 85 %
4. Total Calculated Rainfall Volume…………..………… 127 Ac. - Ft.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
12
5. Peak Calculated Runoff ………………………….…… 1400 c. f. s.
6. Required Silt Storage …………………………………. 3.0 Ac. - Ft.
7. Total Required Reservoir Capacity …………………. 130 Ac. - Ft.
8. Spillway Capacity ……………………………………… 1400 c. f. s.
9. Width of Spillway ……………………………………… 80.0 Feet 10. Depth of Flow at Spillway …………………………… 3.0 Feet 11. Spillway Elevation……………………………………. 4365 12. Empty Reservoir Time ………………………………. 72 Hours 13. Outflow Pipe………………………………………….. 24 Inch RCP 14. Top of Dam Elevation ……………………………….. 4370 15. Maximum Highwater Elevation at Dam ..…………. 4365
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
16
Hydrologic Study Studies of the 100-year floods (according to the city of El Paso this corresponds to a
storm intensity of 3.80 inches over a period of 3 hours) for Northgate Drainage Area,
and the watersheds feeding them, are conducted using the U.S. Corps of Engineers
software HEC1 (Visual Hec) and available reports on the records. The study area is
located in North East El Paso. The subsoil in this study area is characterized by steep
soils that are shallow over the caliche, or are deep and gravelly and also belongs to the
Harkey-Glendale Association which is characterized by deep, nearly level loamy soils,
very fine sand to silty, clay loam. The presence of fissures and Karst formations are
also observed in the area. Watershed boundaries are identified from U.S. Geological
Survey (Topographic) Triangles. Characteristics of the dam and the watersheds in this
study are summarized in the following table.
The total calculated storage capacity of the dam without an active outflow pipe is 208
acre-feet. When a twenty four (24) inch outflow pipe is introduced into the hydrographs
and the inflow and outflow are simultaneously calculated the required capacity of the
dam is 126.70 acre feet as shown, on Table 22-1, page 22 and on the Outflow/Storage
Curves on Page 23.
Calculated storm runoff values in Karst formations are always higher than actual values
observed in the field. This is due to the cracks and fissures in the surface that enables
water to flow into the ground. Photographs taken of the terrain show the presence of
Karst formations and land fissures (openings) that are not taken into account when
hydrology and hydraulic calculation are performed. It is a known fact in this area that
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
17
Karst and Fissures do exist in the Franklin Mountains and they account for some of the
storm runoff never reaching the lower portion of the Franklins because some of the
storm runoff flows disappears into underground cavitations (Karst and Fissures). These
underground flows that occurred in the Franklin Mountains served as an additional
factor of safety into the design of storm carrying structures. Silt produced by storm
runoffs in mountainous terrain is estimated at 0.003 ac-ft per acre of watershed area.
Table 17-1 on Page 17 shows the watershed area, maximum runoff volume, maximum
outflow volume, maximum storage capacity, volume of silt, peak stage, and time of peak
after the start of rain.
TABLE 17-1 Runoff simulation results are shown in Appendix A (from HEC-1 output).
The input data that was used to calculate the hydrograph shown on Page 24, which was
used in the design of the dam, and the hydrograph shown on Page 26, which was used
in the design of the concrete channel and concrete pipes, was seven (7) inches of
rainfall in twenty four (24) hours, this storm is equivalent to the City of El Paso 100 year
storm of 3.80 inches in three (3) hours. A six minute input interval was used to do the
storm simulation using the U.S.S.C.S unit hydrograph and curve number 65 in the HEC-
1 program number.
Based on the results of the simulated storm hydrograph the dam was designed to have
Dam Watershed Area, Acres
Concrete. Pipe dia. in inches
Flow capacity cfs
Max Out-Flow, cfs
Storm Runoff Storage, Ac-ft
Calculated10-year Volume of Silt, Ac-ft
Peak Stage, ft
Peak Time from start, hrs.
Dam 1 805 24 328 96 208 3.0 30 17
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
18
a maximum storage capacity of approximately 208 ac-ft with no outflow and storing 100
percent of the storm. The peak ponding volume at the highest stage is 127 ac-ft with a
simultaneous maximum outflow of 96 cfs. The dam is designed to detain the runoff
water temporarily to reduce the downstream outflow through the community below. The
required ponding capacity for the dam is calculated according to the runoff volume that
has to be detained for this purpose. Using the topographic map provided to us the
reservoir capacities and rating curves are calculated and tabulated.
The following Table shows what the probabilities of a certain year storm occurrences
are.
The probability that an event of given recurrence interval will be equaled or exceeded
during periods of various lengths.
In the absence of dams and/or culverts, the maximum stage is not calculated but it is
estimated not to be significantly high. The max storm flow is calculated to be 1400 cfs
as shown on the hydrograph on Page 26. This is the max uninterrupted storm runoff
flow through the existing arroyos and berms.
Hydraulic Study
Storm
Recurrence -
Period Years
5 10 25 50 100
5 0.67 0.89 0.996 1.00 1.00
10 0.41 0.65 0.093 0.995 1.00
50 0.10 0.18 0.40 0.64 0.87
100 0.05 0.10 0.22 0.40 0.63
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
19
Two methods that were used for conveying runoff flows through the open channel and
pipe or rectangular cross section closed conduits. The flow capacity of the conduits are
computed using Manning’s formula with roughness coefficient n = 0.013 as
recommended by the Texas Department of Transportation.
Q = 1.49 A R2/3 S1/2 / n
where n = 0.013 and S = 0.0025
A = cross-sectional area
P = wetted perimeter
R = hydraulic radius
For the submerged openings, the calculations are based on the following formula,
Q = C A (2gh)0.5
C is given the value 0.70
g = 32.2 ft/sec2
The outflow volumes of an 18 inch and 24 inch concrete pipes were calculated for
different stage at different storage volumes at Table 20-1. The relationship between
highwater elevation, volume of water stored and volume on outlet pipe is shown on
Table 22-1, page 23, on stage Outflow/Storage Curves.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
20
TABLE 20-1
In the hydrology simulations of the arroyos the stage elevation is taken as 0.00 (zero) at
the culvert invert, which is assumed 4 ft above the low point of the ponding area before
silting occurred. The highest water stage is approximately 30 ft above the culvert invert
elevation. The berm should allow at least a 5 ft freeboard.
Outflow capacity of an 18" & 24" concrete pipe opening, cfs
Depth Head,H
(ft) h=H-1.5/2 C A(18")
Q=CA (2gh)^0.5 A(24")
h=H-2/2
Q=CA (2gh)^0.5
3 2.25 0.7 1.766 14.88 3.14 2 24.95 4 3.25 0.7 1.766 17.88 3.14 3 30.55 5 4.25 0.7 1.766 20.45 3.14 4 35.28 6 5.25 0.7 1.766 22.73 3.14 5 39.44 7 6.25 0.7 1.766 24.80 3.14 6 43.21 8 7.25 0.7 1.766 26.71 3.14 7 46.67 9 8.25 0.7 1.766 28.49 3.14 8 49.89 10 9.25 0.7 1.766 30.17 3.14 9 52.92 11 10.25 0.7 1.766 31.76 3.14 10 55.78 12 11.25 0.7 1.766 33.27 3.14 11 58.50 13 12.25 0.7 1.766 34.72 3.14 12 61.10 14 13.25 0.7 1.766 36.11 3.14 13 63.60 15 14.25 0.7 1.766 37.45 3.14 14 66.00 16 15.25 0.7 1.766 38.74 3.14 15 68.32 17 16.25 0.7 1.766 39.99 3.14 16 70.56 18 17.25 0.7 1.766 41.20 3.14 17 72.73 19 18.25 0.7 1.766 42.38 3.14 18 74.84 20 19.25 0.7 1.766 43.53 3.14 19 76.89 21 20.25 0.7 1.766 44.64 3.14 20 78.88 22 21.25 0.7 1.766 45.73 3.14 21 80.83 23 22.25 0.7 1.766 46.79 3.14 22 82.73 24 23.25 0.7 1.766 47.83 3.14 23 84.59 25 24.25 0.7 1.766 48.85 3.14 24 86.41
30 29.25 0.7 1.766 53.65 3.14 29 94.99 35 34.25 0.7 1.766 58.06 3.14 34 102.85
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
21
Volume of Silt Accumulation
Ten year silt volume is calculated in accordance with the City of El Paso ordinance, Title
19, “Subdivision Improvement Standards”. The area is mountainous, and the silt
volume is 0.003 ac-ft per acre of watershed area.
Recommendation
Results of the Hydrologic and Hydraulic studies lead us to the following
recommendations:
• A 35 ft dam should be built at the indicated location.
• An outlet tower should be provided at the toe of the upstream side of the dam
with a 24” concrete pipe thru the dam at an invert elevation of at least 2 ft above
the bottom of the dam.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
22
3 1 2
NORTHGATE DIVERSION DAM
Time
Elevation
Stage, ft
Storage, ac-ft
24” Pipe Outflow, cfs
0:00 4335 0 0 0
2 4336 11.6 12.1 58
4 4365 29.1 117.4 94
6 4366 30.2 126.7 96
8 4366 30.2 126.6 96
11 4365 29.4 119.9 94
13 4353 18.7 42.2 75
18 4340 5.3 1.6 37 20 4335 0.1 0 1
RE = Stage = Water Depth at Dam
TABLE 22-1
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
23
3
2
1
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
25
HYDRAULIC DESIGNS
1. Detention/Retention Basin (35 Ft High Dam) Plates 1 and 2
The proposed detention basin will be designed to impound the 100 year criteria storm
for an 805 acre watershed. As shown on the Northgate Diversion Table 22-1, Page 22
on the Outflow/Storage Curves on Page 23, calculations using the U.S.S.C.S.
hydrographs the maximum highwater mark (stage) will be 30.20 feet (4366 elevation)
from invert of dam with a maximum impounding (storage) of 126.70 acre feet and
maximum outflow of 96 cubic feet per second (cfs). The emergency spillway will be
designed to pass the 100 year criteria storm of 1400 cfs. As calculated by the
hydrograph shown on Page 21. The dam is an earthen dam with side slopes of one
vertical over three horizontal. A 20 feet wide road on top is provided for maintenance.
The intake for out-flow is placed 5 ft above the low point to allow the pond to store some
water before releasing it. The opening/outflow conduit is designed to allow the pond to
store water over a one week period and reduce the downstream flow. A spillway is
constructed for emergency flow if the water elevation in the pond rises above a level
providing freeboard to the dam. The outlet through the dam is a 24” concrete pipe
releasing water downstream. The road will be reconstructed to form an inverted
triangular cross-section capable of conveying the runoff overflow of the spillway.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
27
2. Rectangular Concrete Open Channel Plates 4 to 8 The second option to consider is a concrete open channel from the top of existing berm
to the bottom North of Hondo Pass. As shown on the hydrograph on Page 26, the
expected storm runoff is 1400 cfs as per the 100 year criteria storm and from the 802
acre watershed flows through the existing berms and into the existing Northgate Dam.
As shown on pages 28 and 29 the proposed rectangular channel will be Ten (10) feet
wide by five (5) feet high and will flow at approximately 2.50 feet in depth at the super
critical flow with a velocity of sixty (60) feet per second. In order to minimize the velocity
force and to slow down the flows three (3) energy decipitators were introduced. The
channel will be approximately 550 feet long. In order to minimize rocks and boulders
flowing down the channel and scouring and possibly chipping the concrete a desilting
basin will be constructed on the intake to catch all the boulders before entering the
channel.
A. Watershed Area……………………………………….. 805 AC
B. Runoff Coefficient……………………………………… 0.85
C. Peak Calculated Runoff (100yr)……………………… 1400 cfs
D. Bottom Width…………………………………………… 10 ft
E. Average Slope………………………………………….. 15%
F. Normal Depth…………………………………………… 2.30 ft
G. Velocity………………………………………………….. 60.23 ft/sec
H. Manning’s “n”…………………………………………… 0.013
Three energy dissipitators are proposed to alleviate the high velocity and to minimize
erosion. Manning’s formula was used to compute all flowing results.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
30
3. Closed Conduit – 2-66 Inch Reinforced Concrete Pipes Plates 9 thru 13
The third option that was considered is the placement of two (2) sixty six (66) inch
reinforced concrete pipes from the top of the existing berms to the bottom of Hondo
Pass Drive. The two pipes were be sized using the hydrograph on Page 26 that was
developed through the use of the United States Soil Conservation Surfaces
(U.S.S.C.S.) unit hydrograph, with an input of 3.80 inches in three (3) hours, 100 year
criteria storm and 802 acres of watershed area. As shown on Page 23 hydrograph, the
total calculated peak runoff is 1400 cfs and the two (2) sixty six (66) inch reinforced
concrete pipes were sized to handle the calculated peak runoff. The hydraulic
calculations for the two (2) sixty six (66) inch pipes are shown on pages 32, 33 and 34.
As shown on calculations sheet the expected velocity on the pipes carrying 1400 cfs at
a normal depth of 3.90 feet is thirty nine (39) feet per second which might be considered
slouring velocity. According to the pipe manufacture (Rinker Materials) the thirty nine
(39) feet per second velocity affects the durability of the pipe, what the effect the pipe is
the amount and size of mountain alluvian that may be carrying on the pipe. The alluvian
gravel may be scoured inside of the pipes. To minimize the amount and size of alluvian
gravel that may be flowing in the pipe, a desilting basin is recommended at the inlet
structure of the pipes. This will minimize scouring of the pipes. Also, in order to avoid
an erosion problem the outlet structure should be designed as an energy dissipitator.
A. Watershed Area……………………………………….. 805 AC
B. Runoff Coefficient……………………………………… 0.85
C. Peak Calculated Runoff (100yr)………………… 1400 cfs
D. Pipe Diameter………………………………………….. 5.50 FT
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
31
E. Number of Pipes Proposed…..……………………….. 2.0
F. Design Slope……………………………………………. 6.0%
G. Velocity………………………………………………….. 38.87 FT/SEC
H. Manning’s “n”…………………………………………… 0.013
Manning’s formula was used to compute all flowing results.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
35
III. FIELD DOCUMENTATION
DESCRIPTION The study area is about 805 acres. The
entire area is rectangular in shape. The
western boundary is the crest of the
Franklin Mountains. The northern
boundary follows down alluvial slopes of
the Franklin Mountains and connects to
Hondo Pass Drive. The eastern boundary
is Galena Drive. The southerly boundary
is essentially Moonlight Avenue as it
conceptually extends to the crest of the
Franklin Mountains.
The topography ranges from the extremely steep
slopes of the Franklin Mountains through the
relatively moderate slopes of the alluvial terraces
and fans to the almost flat floor of the desert.
Vegetation is sparse desert type consisting of yucca,
grease wood, mesquite and desert weeds in the
lower and flat portions.
A large mass of stones and rocks has
accumulated at the eastern edge of the
channel (dirt arroyo).
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
36
PHYSICAL CHARACTERISTICS Geologically the area is composed
of the Eastern slope of the Franklin
Mountains which is the escarpment
of a typical block fault. The
escarpment rises as much as 3000
feet above the lower portion of the
desert floor. The mountains
traversed by numerous cross faults,
along which some of the major
canyons have developed. The rocks
are principally sediments ranging from pre-Cambrian to Cretaceous, although there are
some igneous rocks ranging in age from pre-Cambrian to Tertiary.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
37
SITE INVESTIGATION Site visits to the Northgate Diversion
Channel were conducted in December
2006. The objectives were to
investigate and verify the contributing
drainage areas, assess obstructions
within the waterways, document
structural distress to any structures and
determine the need for improvements
and construction.
Photo 2 – Erosion Photo 1 – Berm
Photo 3 – Erosion Photo 4 – Canyon Wall
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
38
SOILS REPORT Pending (Appendix C)
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
RECOMMEN
DATIONS
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
39
IV. CONCLUSION AND RECOMMENDATIONS After a very thorough analysis and evaluation of the existing conditions at Northgate
Diversion Channel, it was concluded that:
A. Using two – 66 inch reinforced concrete pipe will be the best approach in
conveying the 100 year criteria storm flow from the slopes of the Franklin
Mountains down to the existing Northgate Dam Impounding Area.
Our recommendation is based on the following observations:
8. The most efficient and cleanest method of conveying storm runoff
9. The most approachable method of working on the steep slopes of existing arroyo
10. The least time to construct
11. The least work to maintain
12. The most environmental friendly since the storm pipes are cover in the arroyo
13. Elimination of existing arroyo
14. The most economical
B. Northgate Diversion Dam is our second recommendation. Our recommendation
is based on the following:
1. Will be able to detain all calculated maximum peak runoff.
2. Will have an uncontrolled outflow. Pipe capable of evacuating dam reservoir in
seventy two (72) hours.
3. Will require extensive excavation to create storage reservior and actual dam.
4. Difficult terrain to excavate- rock – mountain alluvian.
5. Diffucult site to approach by consruction equipment and contruction material.
6. More expensive to construct.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
40
C. Concrete open channel is our third and last recommendation. Our
recommendation is based on the following:
1. High degree of diffuclty to construct form concrete (cast in place).
2. Extremely diffucult terrain to manuver with construction equipment.
3. The concrete open channek steepness may create a high safety risk due to its
high velocity.
4. Not environment friendly to have an exposed concrete open channel on the
slopes of the mountain.
5. More expensive to construct and maintain than closed conduits.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
COST
ESTIMATES
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
41
V. CONSTRUCTION COST ESTIMATES Northgate Diversion Dam
March 21, 2007
COST ESTIMATE No.
DESCRIPTION QTY UNIT UNIT PRICE TOTAL
1. Clearing & Grading 15 A.C. 3,500.00 52,500.00
2. Grading temporary access roads 2300 F.T. 12.00 27,600.00
3. Excavation 90,680 L.Y. 9.00 816,120.00
4. Compacted dam embankment 58,807 C.Y. 10.50 617,474.00
5. Dam embankment loose rock 6450 TON 50.00 322,500.00
6. Dam embankment imperious soil material 17,559 C.Y. 30.00 526,770.00
7. Dam embankment 6 inch bedding material 1752 S.Y. 15.00 26,280.00
8. 24 inch reinforced concrete pipe – concrete encased 280 L.F. 300.00 84,000.00
9. Emergency concrete spillway 1100 C.Y. 450.00 495,000.00
10. Concrete outlet tower 1 E.A. 30,000 30,000.00
11. 46 inch pressured concrete manhole 1 E.A. 7,500.00 7,500.00
12. Gauge pole 1 E.A. 2,000.00 2,000.00
SUB TOTAL 3,007,744.00
Contingency (5%) 150,387.00
GRAND TOTAL 3,158,131.00
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
42
Northgate Diversion Channel March 21, 2007
COST ESTIMATE No.
DESCRIPTION QTY UNIT UNIT PRICE TOTAL
1. Clearing & Grading 590 L.F. 25.00 14,750.00
2. Fill 1459 C.Y. 30.00 43,770.00
3. Cut 460 C.Y. 30.00 13,800.00
4. Concrete Channel 965 C.Y. 600.00 579,000.00
5. 2 – Headwalls 20 C.Y. 500.00 10,000.00
6. Safety Gate 1 E.A. 25,000.00 25,000.00
7. Temporary road grading 850 L.F. 25.00 21,250.00
8. Loose rock dissipitator 500 TON 50.00 25,000.00
SUB TOTAL 732,570.00
Contingency (10%) 73,257.00
GRAND TOTAL 805,827.00
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
43
Northgate Diversion Pipe March 21, 2007
COST ESTIMATE No.
DESCRIPTION QTY UNIT UNIT PRICE TOTAL
1. Clearing & Grading 590 L.F. 25.00 14,750.00
2. Fill 7498 C.Y. 30.00 224,940.00
3. Cut 2909 C.Y. 30.00 87,270.00
4. Bedding 805 C.Y. 15.00 12,075.00
5. Pipe – 66” 1086 L.F. 260.00 282,360.00
6. 2 – Headwalls 20 C.Y. 500.00 10,000.00
7. Safety Gate 1 E.A. 25,000.00 25,000.00
8. Temporary road grading 850 L.F. 15.00 12,750.00
SUB TOTAL 669,145.00
Contingency (10%) 66,915.00
GRAND TOTAL 736,060.00
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
REFERENCES
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
44
V. REFERENCES
1. “Drainage & Flood Control Studies, Northeast Area of El Paso”, Lance
Engineers, Inc., June 1960, - for the City of El Paso, Texas.
2. “Preliminary Engineering Study for Northeast Drainage Improvements”, Parkhill,
Smith and Cooper, Inc., March 1978, - for the City of El Paso, Texas
3. Texas Water Code – Subsection 26.027, Texas Natural Resource Conservation
Commission.
4. Title 19 – Subdivision Ordinance: Subdivision Improvement Design Standards.
Retention Basin Design.
5. Storm Water Management Ordinance: Ordinance No. 013477, Revision 1-27-98.
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
HEC-1
PRINTOUTS
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
45
APPENDICES
Appendix A – Computed Hydrograph HEC-1
Appendix B – Plates
1. Dam (Plates 1,2,3) 2. Closed Conduit (Plates 4,5,6) 3. Open Channel (Plates 7,8,9) 4. Channel Cross Section (Plates 10,11) 5. Closed Conduit Cross Section (Plates 12,13) 6. Open Channel Dissipator (Plate 14)
Appendix C – Soil Report
CITY OF EL PASO-NORTHGATE DIVERSION CHANNEL
DORADO ENGINEERING, INC.
PLATES
E A
R T
H E
N B
E R
M
HONDO PASS
RA
NC
ICH
GA
LEN
A
BE
LK
MA
GN
ETI
C D
R.
SANDS
VIR
GO
SWANSON
GABEL
CRAIGO
MOONLIGHT
MAR
BLE
GA
LEN
A
PROP. CHANNEL
PR
OP
. DA
M
PIPE
OUTLET PIPE
WA
TER
SH
ED
805
AC
RE
S
1 4
0 0
C F
S(E
XIST
ING
)
TYPICAL CHANNEL PLAN