Technical Memorand u m to d et er m i ne the se d imen t load ing ant ic ipa t ed along t hese f low paths and to determi n e the n e ed fo r sed i ment basi ns al ong F l o w Paths 38A and 38B. 3.0 RECOMMENDATIONS The f ollowi ng ar e r eco m me n dations f or potential m o difi ca tions t o the Land Study, l i st e d in no parti c ula r order : • Update the hydrolog i c modeling using the la tes t v er sion ofHEC - HMS to utilize t he 1DO- yea r , 24-hour rainfall depths in the DDM, the rainfall d i s ribution provided in the DDM, and the SCS Curve Nu m ber met hod. • Upda t e t h e hy d r a ulic m od el i n g u sing the latest version ofHEC-RAS to re f lec curr e nt topographic in formation when the Land Stud y are a pr ogresses in to the de ve lopment phase. •• E nsur e th at th e co n cret e lin e d c hann e l s eg me nts pro p o s d for Flo w P a ths 38 A a n d 38B a l ig n p r op e rly wi th the e xi s t ing rundo w ns int o Loma s d el Sol Pond # 3. •• Consider revising the loca t ion of the p a rk/pond adjacent to Flow Path 38B and the Land Study b oun d ar y to the a r e a be twe en Flo w Paths 38A and 38B and me r g i ng the proposed p a rk / pond wi th the exi stin g Lomas del Sol Pond #3. • Re e va l uat e pl ac ement of p a rkJponds located on Flow Path s 39 and 4 0 . • M a i n tain Flow P at h s 38A, 3 8B, a n d 4 0 as na t ural a rro y os t hro u gh t he L and St udy are a t o con f orm with the Gr ee n I n frastructur e Plan. o E va l u at e p la c em en t o f s e di m e nt b asi n s ne a r th e up stream li m i ts o f t h e f l o w p a ths wi t h i n th e L and Study a rea. • Incorporate th e open space land dedication to the F r a nklin Mo u ntai n State Park. C C : J a mes Sh el t o n- EP W U Rick French - URS F IN A L _ Te c hnic a l M e m o _ 20 0 9AprIO . d oc 50f5 U RS P r o je ct N o . 41 0 08 8 1 6 Centr al Fi l e Le e . 06 . 0 3
and 38B where they leave the Land Study area. Additionally, due to it's proximity to the Land Study area,
there may be future potential to expand the limits of this pond into the Land Study area, and thus increase
its capacity, and have it serve duel purposes: to reduce runoff downstream and to detain the future
increased runoff from the Land Study developed area.
Hunt Communities and Plexxar envision the need for a pond on both Flow Paths 39 and 40 to allow for
development downstream along these reaches. There are no immediate plans for development or
construction of these ponds. However, both parties believe that a shared ponding facility or a series of
ponding facilities with the Land Study area would be preferred. Currently, the Land Study calls for a
park/pond to be located adjacent to flow paths and to Hunt property. Both parties noted that the proposed
location of the Land Study parkJponds fall on a 'ridge'.
2.1.2 Stormwater Conveyance Network
Due to the location of the Lomas del Sol Pond #3, the channel discharge locations for Flow Paths 38A
and 38B will need to align with the existing rundowns. During the design of the channels, the rundowns
should be evaluated to determine if improvements to the rundowns are needed as a result of the Flow
Paths being altered from their natural arroyo state.
It is currently anticipated that Flow Path 39 will be channelized downstream of the Land Study to allow
for development of the area. There are not immediate plans for improvements to Flow Path 40
downstream of the Land Study.
2.2 OPPORTUNITIES FOR OPEN SPACE
One of the key objectives by the local governing agencies within the City of E I Paso is to preserve and
maintain open space areas. The value of maintaining and preserving the natural drainage paths is
documented in the Green Infrastructure Plan for EI Paso as well as the SMP.
2.2.1 Natural Arroyos
Flow Paths 38A, 38B, and 40 are identified on a figure as 'Lands to Preserve' within the Open Space
Master Plan Recommendations section, page 5-8, of the Green Infrastructure Plan. However, these flow
paths are not specifically identified within any of the recommended open space preservation tables found
within the chapter. Based on discussions with the City of E I Paso Parks & Recreation Department, the
preference is to maintain natural arroyos upstream of the PNG pipeline with a particular emphasis on
maintaining Flow Paths 41 and 42 in their natural state.
2.2.2 Open Space
Figure 3- 1 of the Land Study indicates a large swath of open space proposed along the eastern boundaryof the study lands, adjacent to the Franklin Mountain State Park. This swath comprises a majority of the
recommended open space within the Land Study. This band is not currently required or prioritized within
any of the documents listed above and reviewed for this memorandum. However, recently a large portion
ofthis land has been dedicated to the Franklin Mountain State Park. The boundary of this dedication is
The Parks and Recreation department provided URS with a Mountain to River Trail alignment that they
would like to see incorporated into the plan. The alignment currently goes east up Flow Path 42 from theRiver to the crest of the Franklin Mountains, where it follows the ridge south, crosses Trans-Mountain
Road, descends west down Flow Path 39 to where the flow path meander comes within close proximity to
Trans-Mountain Road, and then follows the Trans-Mountain Road Right-of-Way west.
The Trail Amenities Plan within the Land Study shows an undeveloped hike/bike trail following a similar
alignment. However, near the upper portion of Flow Path 39, the proposed trail heads south, opposite the
direction of the proposed Mountain to River Trail alignment. The Land Study should be updated to
incorporate the Mountain to River Trail alignment within the planning document.
2.3 COMPLIANCE WITH THE DRAINAGE DESIGN MANUAL
The City ofEI Paso issued the DDM as an official document authorized under the City ofEI PasoSubdivision Ordinance Chapter 19.19 Storm Water Management Requirements in June, 2008.
As noted in the Land Study, the flow paths traverse alluvial deposits with poorly defined channels. As
such, changes in flow path alignment and size are anticipated to occur over time. It is anticipated that the
flow paths could have been substantially altered as a result of the 2006 storm events that caused
significant flooding to west EI Paso. These drainage corridors through the Land Study area should be
reevaluated when the Land Study area progresses into the development phase.
2.3.1 Hydrology
The Land Study utilized the 1~O-year storm frequency for the hydrologic criteria. It is not clearly
identified within the Land Study what storm duration was used. Discussion with MCi noted that the
6-hour duration was used for the Land Study. The current criterion within the DDM is to use thel Otl-year, 24-hour storm for design of channels and ponding facilities.
The rainfall depth source utilized within the Land Study was US Department of Commerce Technical
Paper No. 40 (TP40) dated May 1961. However, as part of the DDM, new rainfall data has been
developed for the El Paso area. The following table shows a comparison of the rainfall values from the
two source s.
TP40 2.35 . ·· ···2.95 3.90.95 2.50 3.45
4 .18 .D M 2 . 3 1 2.96 3.12 3.47 3.83
Due to the difference in storm durations and rainfall depths, it is anticipated that this will result in
substantially higher runoff values than identified within the Land Study.
The DDM recommends the use of the SCS Curve Number Method to account for runoff losses, The Land
Study utilized the Initial and Constant Rate Loss Method. Guidance for the HEC-I-nvIS software indicates
that the Initial and Constant Rate Loss Method may be "difficult to apply to ungaged areas due to lack of
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