HYDROLOGIC, ENVIRONMENTAL AND REGULATORY COMPLIANCE ANALYSIS

 

APPLICATION FOR PRELIMINARY SUBDIVISION PLAT

 

SANCHEZ SUBDIVISION

 

SANTA FE, NEW MEXICO

 

 

 

 

Prepared for:

 

Southeast Neighborhood Association, Santa Fe, New Mexico

 

 

 

Prepared by:

 

Truchas Hydrologic Associates, Inc.

P.O. Box 541

Placitas, New Mexico   87043

 

 

September 27, 2005

 

 

 

 

 

 

 

 Preliminary Subdivision Plat Application

Sanchez Subdivision

Hydrologic, Environmental and Regulatory Compliance Analysis

 

 

In response to a request by the Southeast Neighborhood Association of Santa Fe, New Mexico, Truchas Hydrologic Associates, Inc. of Placitas, New Mexico has reviewed Preliminary Subdivision Plat plans and prepared the following analysis of hydrologic aspects of the proposed plans.  The Site of the proposed subdivision is located in Southeastern Santa Fe, east of the intersection of Old Santa Fe Trail and St. Michael's drive (Figure 1).   The proposed Sanchez Subdivision will encompass approximately 19 acres and will include 20 residential lots.  The Site is bounded on the southeast by Arroyo de los Chamisos, a major regional drainage which includes a designated regulatory floodplain and floodway as recognized by the National Flood Insurance Program.

 

Preliminary Subdivision Plat plans were reviewed with particular reference to guidelines and requirements provided in the Santa Fe Land Development Code (the Code), Stormwater Management Plan and various City Ordinances relating to terrain and storm water management, landscaping and site design.

 

A.  Stormwater Management and Flood Control

 

As required in Section 14-8.3 of the Code, the proposed development has presented a delineated flood hazard area (Figure A, Lot of Record).  According to Section 14-8.2, the plans include a Grading and Drainage Plan (Sheet C-2) and a Storm Water Pollution Prevention (SWPP) Plan (Sheets SW-1 and SW-2).  The Plans apply the TR-55 computer program to compute peak flood flows from drainage catchments both on and off-site.

 

Stormwater Management

 

Sheet C-2, Grading and Drainage Plan, presents the design criteria and implementation specifications for 4 on-site stormwater detention structures.  One pond, "Pond 1", collects stormwater runoff from Cristobal Lane public road and residential properties on the southeast side of the subdivision.  The remaining three ponds are bounded on the downstream end by structures designated as "temporary check dams," installed within the existing natural drainage that traverses the western part of the subdivision.

 

The "temporary check dams" should be re-designed as permanent structures for long-term function as storm water detention structures.  The dams should be constructed as gabions or provided with equivalent structural reinforcement.  Also, a regular maintenance plan should be put in place to ensure proper long-term function of these detention structures.

 

As a safeguard, additional drainage control structures should be put in place downstream of Pond 1.  In the event of a pond breach, flood waters from Pond 1 may pose a danger to residences located immediately downstream.  A rock-reinforced swale with berms should be constructed down slope of the pond to direct any overflow waters into the adjacent drainage, and the southern banks of the drainage should be built up and reinforced to protect any incursion of overflow waters into the properties located immediately downstream of Pond 1.

 

As with Ponds 2-4, a regular maintenance program should be implemented for Pond 1 to a)  keep the intake of the Arroyo Chamiso conduit clear, and b)  keep the ponds free of debris and silt, which may alter its holding capacity.

 

Flood Hazards

 

1.  Outdated Floodplain Map. 

 

Use of outdated floodplain designations may pose a significant risk to subdivision residents and downstream neighbors.  

 

Santa Fe Development Code Article 14-12 provides the following regulatory definition relevant to flood hazard management.

 

Floodway is defined as " That portion of the flood plain within the flood fringe which contains the higher velocity storm waters and is adjacent to the land areas that must be reserved in order to discharge the 100-year flood without cumulatively increasing the water surface elevation more than one foot in height."

 

With respect to federal flood insurance guidelines, the floodway is recognized as the primary floodplain, or the area marked as "Zone A3" on the Development Plan Survey.  This corresponds to the area so designated on the applicable National Flood Insurance Program's Flood Insurance Rate Map (FIRM), Panel No. 350070 0008 B, dated July 1980, included as Appendix A.

 

The use of a 1980 FIRM to establish the regulatory floodplain and floodway for the vicinity of the subdivision raises significant flood safety concerns for future residents of the proposed subdivision, and neighboring properties located immediately downstream of the subdivision. 

 

This existing Regulatory Floodplain map of the area ignores the effects of the last 25 years of changes in urban topography.  The increase in paved areas associated with upstream development in the watershed during this time frame will significantly alter watershed runoff estimates relative to levels computed in 1980.   As such, use of the 1980 FIRM may significantly underestimate the extent of the Arroyo Chamiso Floodplain in the vicinity of the Site.

 

Figure 2a is an aerial photograph of the Site vicinity taken in 1981 (EDAC, 2005).  Figure 2b is an aerial photograph of the Site vicinity taken in 2002 (EDAC, 2005).  Comparison of the two photographs illustrates significant changes in urban development during this period.

 

2.  Altered Stream Bed Topography

 

The Flood Insurance Study for Santa Fe (Federal Emergency Management Agency, Community 350070, Revised April 1993), computes water-surface elevations for floods with various recurrence intervals using standard Army Corps of Engineers hydraulic modeling software.  These elevations are then mapped onto the various FIRMs covering the City, such as Panel 8, included as Appendix A.  Analysis of this map in conjunction with the current site topography (Sheet C-1 and C-2) offers a means of comparing projected flood elevations with current land and Creek bed elevations.  Sheet C-1 indicates a current creek bed elevation for the Arroyo Chamiso of approximately 7080 feed above the National Geodetic Vertical Datum (NGVD) at the far southeastern corner of the property.  By locating this point on the FIRM using the map scale provided, the marked elevation of the base flood elevation in the FIRM at this location is 7076 feet.  In other words, the current measured elevation of the dry Creek bed is higher than the elevation of the 100-year flood as projected in 1980.  This analysis applies to the entire reach of the Arroyo Chamiso bordering the proposed subdivision.  As such, the flood stage elevation of the 100-year flood can be expected to exceed levels designated on the FIRM map, based on increased stream elevations alone.

 

These increased elevations of the Arroyo Chamiso bed relative to those marked on the FIRM indicates that significant aggradation, or accumulation of sediment, in the creek bed has occurred over the last 25 years.  Depending on the projected river stage associated with the 100-year flood, the projected flood elevation may increase by several feet, and the regulatory floodplain increase in area accordingly.

 

As such, the current regulatory floodplain map may significantly underestimate the extent of the actual floodplain based on current data.  Peak flood flows, floodways and the regulatory floodplain for Arroyo Chamiso should be re-computed on the basis of current topography prior to conducting any stormwater drainage and flood risk analysis of the proposed subdivision.

 

B.   Landscape and Site Design Standards

 

Sections 14-8.4(B)(a) and (b) prescribe that standards of this section apply to all applications for subdivision plat approval  and development plan approval.

 

1.  Section 14-8.4(E) details Water Harvesting and Irrigation Standards.  The section specifies that "Water conservation shall guide landscape and site planning, design, installation, and management. Landscape planning shall begin early in the development process in conjunction with the requirements of Terrain Management, §14-8.2. Landscape design shall apply the principles of xeriscape and achieve the highest industry standards for irrigation efficiency. Alternative sources of irrigation water shall be developed including harvested water from roof and site runoff. Graywater use is recommended where appropriate. Potable water shall be used as a back-up or temporary irrigation water source to the greatest extent possible. The purpose of these strategies is to develop drought tolerant landscapes, and to reduce the demand on the potable water system. " 

 

The subdivision plans should be revised to make provisions for water harvesting (including surface collection and in-ground storage), graywater management, on-site water features, and irrigation standards.  None of these items has been addressed in plans submitted to date.

 

2.  Section 14-8.4(F) details Plant Material Standards.  The section specifies "Plant material selection shall emphasize drought tolerant plant species and shall limit the use of high water use plant species. All required plant material shall be cold hardy to USDA Classification Zone 5 (-15° F) or colder. For appropriate plant material see the City of Santa Fe Plant List and consult local nurseries."

 

This section goes on to prescribe detailed plant material specifications, rules for protecting existing vegetation, soil management in conjunction with construction activities, and management of destroyed vegetation.  Only minimal references are provided regarding landscape and vegetation management, and the plans should be revised to include provisions for low water consuming vegetation.

 

 

 

 

 

 

 

 

 

 

 

 

APPENDIX A

 

FLOOD INSURANCE RATE MAP