FLOOD INSURANCE STUDY

Transcription

1 FLOOD INSURANCE STUDY DONA ANA COUNTY, NEW MEXICO AND INCORPORATED AREAS VOLUME 1 OF 2 COMMUNITY NAME COMMUNITY NUMBER ANTHONY, TOWN OF DONA ANA COUNTY (UNINCORPORATED AREAS) HATCH, VILLAGE OF LAS CRUCES, CITY OF MESILLA, TOWN OF SUNLAND PARK, CITY OF DONA ANA COUNTY REVISED Federal Emergency Management Agency Flood Insurance Study Number 35013CV001A

2 NOTICE TO FLOOD INSURANCE STUDY USERS Communities participating in the National Flood Insurance Program have established repositories of flood hazard data for floodplain management and flood insurance purposes. This Flood Insurance Study may not contain all data available within the repository. It is advisable to contact the community repository for any additional data. Part or all of this Flood Insurance Study may be revised and republished at any time. In addition, part of this Flood Insurance Study may be revised by the Letter of Map Revision process, which does not involve republication or redistribution of the Flood Insurance Study. It is, therefore, the responsibility of the user to consult with community officials and to check the community repository to obtain the most current Flood Insurance Study components. Initial Countywide FIS Effective Date: September 27, 1991 First Revised Countywide FIS Date: <To be determined> i

3 TABLE OF CONTENTS Table of Contents Volume 1 Page 1.0 INTRODUCTION Purpose of Study Authority and Acknowledgments Coordination AREA STUDIED Scope of Study Community Description Principal Flood Problems Flood Protection Measures ENGINEERING METHODS Hydrologic Analyses Detailed and Enhanced Approximate Type 1 Study Streams Redelineation MHIP Case 1 Study Streams Hydraulics Analyses Detailed and Enhanced Approximate Type 1 Study Streams Redelineation MHIP Case 1 Study Streams Vertical Datum Refinement FLOODPLAIN MANAGEMENT APPLICATIONS Floodplain Boundaries Floodways INSURANCE APPLICATION FLOOD INSURANCE RATE MAP OTHER STUDIES LOCATION OF DATA BIBLIOGRAPHY AND REFERENCES FIGURE Figure 1 Floodway Schematic ii

4 Table of Contents Volume 1 Continued TABLES Page Table 1 Scope of Study... 3 Table 2 LOMCs... 7 Table 3 Stream Name Changes... 7 Table 4 Summary of Discharges Table 5 Summary of Stillwater Elevations Table 6 Stream Hydrologic Methods Used Table 7 Stream Hydraulic Methodology Table 8 Summary of Roughness Coefficients Table 9 Summary of Roughness Coefficients for Fillmore Arroyo Table 10 Vertical Datum Conversion Table 11 Community Map History EXHIBITS Exhibit 1 - Flood Profiles South Fork (Tributary 1) Alameda Arroyo North Fork (Tributary 2) Alameda Arroyo North Fork (Tributary 3) Alameda Arroyo Baylor Canyon Arroyo Baylor Canyon Arroyo Tributary 1 Baylor Canyon Arroyo Tributary 2 Breedlove Arroyo Breedlove Arroyo - Divided Flow Corbett Arroyo Flow Path 1 (Sand Hill Arroyo) Flow Path 1 (Sand Hill Arroyo) North Diversion Flow Path 1 (Sand Hill Arroyo) Split Flow Flow Path 2 (Alameda Arroyo) Flow Path 3 (Alameda Main Arroyo) Flow Path 4 Flow Path 5 Flow Path 6 Panels 01P-03P Panels 04P-06P Panels 07P-08P Panels 09P-11P Panel 12P Panel 13P Panels 14P-16P Panel 17P Panels 18P-19P Panels 20P-30P Panels 31P-33P Panel 34P Panels 35P-40P Panels 41P-44P Panels 45P-47P Panel 48P Panels 49P-58P Table of Contents Volume 2 EXHIBITS Continued Flow Path 7 Flow Path 8 (North Fork Las Cruces Arroyo) Panels 59P-61P Panels 62P-65P iii

5 Table of Contents Volume 2 Continued EXHIBITS Continued Flow Path 9 (South Fork Las Cruces Arroyo) Flow Path 10 Flow Path 11 Flow Path 12 (Tellbrook Arroyo) Flow Path 13 Flow Path 15 North Branch of Gardner Arroyo South Branch of Gardner Arroyo Las Cruces Arroyo Lauson Arroyo Lauson Arroyo (Confluence) Lauson Arroyo Divided Flow 1 Lauson Arroyo Divided Flow 2 Lauson Arroyo Divided Flow 3 Lauson Arroyo (West Fork) Little Dam Arroyo North Fork Moreno Arroyo South Fork Moreno Arroyo South Fork Moreno Arroyo Split Flow at I-25 Stream Bilbo Stream 13 Stream 14 Stream 15 Stream 16 Stream 17 Stream 21 Stream 22 Stream 23 Stream 24 North Vado Arroyo South Vado Arroyo Panels 66P-69P Panels 70P-71P Panels 72P-73P Panels 74P-79P Panels 80P-81P Panels 82P-85P Panel 86P Panel 87P Panels 88P-89P Panels 90P-93P Panel 94P Panel 95P Panel 96P Panel 97P Panels 98P-100P Panels 101P-104P Panels 105P-111P Panels 112P-115P Panel 116P Panels 117P-119P Panels 120P-121P Panels 122P-123P Panels 124P-125P Panel 126P Panel 127P Panel 128P Panel 129P Panel 130P Panels 131P-133P Panel 134P Panel 135P Exhibit 2 - Flood Insurance Rate Maps iv

6 1.0 INTRODUCTION FLOOD INSURANCE STUDY DONA ANA COUNTY, NEW MEXICO AND INCORPORATED AREAS 1.1 Purpose of Study This Flood Insurance Study (FIS) revises and supersedes the FIS reports and Flood Insurance Rate Maps (FIRMs) in the geographic area of Dona Ana County, New Mexico, including the Village of Hatch, City of Las Cruces, Town of Mesilla, City of Sunland Park, and unincorporated areas of Dona Ana County (hereinafter referred to collectively as Dona Ana County), and aids in the administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of This study has developed flood risk data for various areas of the community that will be used to establish actuarial flood insurance rates. This information will also be used by Dona Ana County to update existing floodplain regulations as part of the Regular Phase of the National Flood Insurance Program (NFIP), and by local and regional planners to further promote sound land use and floodplain development. Minimum floodplain management requirements for participation in the NFIP are set forth in the Code of Federal Regulations at 44 CFR In some states or communities, floodplain management criteria or regulations may exist that are more restrictive or comprehensive than the minimum Federal requirements. In such cases, the more restrictive criteria take precedence and the state (or other jurisdictional agency) will be able to explain them. 1.2 Authority and Acknowledgments The sources of authority for this Flood Insurance Study are the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of The hydro1ogic and hydraulic analyses for this study were performed by Mapping Alliance Partnership VI for the Federal Emergency Management Agency (FEMA) Region VI, under Task Order 11 and Task Order J028 of Contract No. EMT-2002-CO-0052, and by URS Corporation for the Dona Ana County Flood Commission under Contract No. DAC Mapping Alliance Partnership VI is a joint venture of URS Corporation, Greenhorne & O Mara, and Spectrum Mapping. These studies were completed in January An Enhanced Approximate Study Type II was conducted on the Rio Grande using FLO- 2D Flood Routing Model version , which was developed by Jim O Brien in October 2005, under contract by Tetra Tech, Inc. for the U.S. Army Corps of Engineers under contract DACW09-03-D Mapping Alliance Partnership VI was responsible for reviewing the model to ensure reasonableness and use the model to map the floodplains for the 0.1-percent-annual-chance flood. The detailed hydrologic and hydraulic analyses for Flow Path 1 (Sand Hill Arroyo) from Chandler Tank (Sand Hill Dam) to 1,300 feet upstream of State Highway 28 and for Flow Path 3 from Jornada Road South to 3.8 miles upstream of U.S. Government Dam was performed by Blackham, Roman, Cunaji and Associates, Inc., under contract EMT-2000-CA This study was completed in September The detailed hydrologic and hydraulic analyses for Flow Path 5 were performed by the City of Las Cruces under contract EMT-2000-CA This study was completed in August

7 The detailed hydrologic and hydraulic analyses for Flow Path 15 were performed by the City of Las Cruces under contract EMT-2000-CA This study was completed in October The detailed hydrologic and hydraulic analyses for Las Cruces Arroyo were performed by the City of Las Cruces under contract EMT-2000-CA This study was completed in September The detailed hydrologic and hydraulic analysis for the Lauson and Breedlove Flood Insurance Study was performed by Bohannan Huston, Inc. under contract EMT-2002-CA This study was completed in March The previous version of the FIS documents note that the hydrologic and hydraulic analyses for Flow Paths 1 through 13 and Flow Path 15 were prepared by the Albuquerque District of the U.S. Army Corps of Engineers (USACE) under Interagency Agreement No. EMW-E-0105, Project Order No. 1, Amendment Nos. 1 and 2. The hydrologic and hydraulic analyses for the remaining flooding sources in the unincorporated areas of Dona Ana County were prepared by Bohannon-Huston, Inc. for FEMA, under Contract No. EMT-87-C This work was completed in September In this revision to the Dona Ana County countywide FIS, hydrologic analyses for an unnamed pond in the City of Sunland Park were prepared by Molzen-Corbin & Associates and taken from the drainage study for the Anapra subdivision (Reference 1). The work was completed in March Base map information shown on this FIRM was derived from multiple sources. This information was compiled from the Dona Ana Flood Commission, 2004 and 2010; Bureau of Land Management, 2004; U.S. Geological Survey, 1989 and 2005; NGS, 2004; and U.S. Census Bureau, Additional information was compiled from the U.S. Department of Agriculture aerial photography, 2009 at a scale of 1:12,000. The projection used in the preparation of this FIRM was New Mexico State Plane, Central zone. The horizontal datum was NAD 83, GRS80 spheroid. Differences in datum, spheroid, projection or State Plane zones used in the production of FIRMs for adjacent jurisdictions may result in slight positional differences in map features across jurisdiction boundaries. These differences do not affect the accuracy of the FIRM. 1.3 Coordination The dates of previous Consultation Coordination Officer s (CCO) meetings held for Dona Ana County and the incorporated communities within its boundaries are tabulated below. Community Name Initial CCO Date Final CCO Date City of Las Cruces April 1976 February 16, 1983 Town of Mesilla * November 28, 1990 Village of Hatch * * City of Sunland Park * * Unincorporated Areas of Dona Ana October 22, 1986 November 28, 1990 County *Data not available or not applicable The initial CCO meetings were held with representatives from FEMA, the communities, and the study contractors to identify the streams to be studied by detailed methods. The final CCO meetings were held with representatives from FEMA, the communities, and the study contractors to review the results of the studies. For this revision, the final CCO meeting held on, and attended by representatives of 2

8 2.0 AREA STUDIED. All problems raised at that meeting have been addressed in this study. 2.1 Scope of Study This FIS covers the geographic area of Dona Ana County, New Mexico, including the incorporated communities listed in Section 1.1. The areas studied by detailed and approximate methods were selected with priority given to all known flood hazards and areas of projected development. Approximate analyses were used to study those areas having a low development potential or minimal flood hazards. The scope and methods of study were proposed to, and agreed upon, by FEMA and community officials. The scope and methods of study were proposed to and agreed upon by FEMA and Dona Ana County Flood Commission and the City of Las Cruces. The flooding sources studied by detailed and approximate methods along with the limits of study are shown in Table 1, "Scope of Study." Table 1 Scope of Study New Detailed Study Streams Stream Name Downstream Limit Upstream Limit Breedlove Arroyo Highway 460 Breedlove Dam Fillmore Arroyo Fillmore Dam 0.1 mile East of Remington Road Flow Path 1 (Sand Hill Arroyo) Flow Path 3 (Alameda Main Arroyo) 1,300 ft upstream of NM Highway 28 US Government Dam Immediately downstream from US miles upstream of US Government Dam Flow Path 5 Las Cruces Lateral Paxton Street Flow Path 12 (Tellbrook Arroyo) Flow Path 15 I-10 Southbound Frontage Road 1.4 miles East of Tellbrook Road Rim of Burn Lake Las Cruces Arroyo El Paseo Drive Walnut Street Lauson Arroyo West Fork Lauson Arroyo and Divided Flows Anthony Lateral 800 ft downstream from McClure Road 0.75 mi. upstream of Highway 460 Highway mi. upstream of I-10 Lauson Confluence Anthony Lateral Highway 460 3

9 Table 1 Scope of Study (Continued) Stream Reaches Studied by Enhanced Approximate Methods Stream Name Downstream Limit Upstream Limit Baylor Canyon Arroyo Upstream face of US mile upstream of confluence with Baylor Canyon Arroyo Tributary 1 Baylor Canyon Arroyo Tributary 1 Baylor Canyon Arroyo Tributary 2 Confluence with Baylor Canyon Arroyo Confluence with Baylor Canyon Arroyo 0.7 mile upstream of confluence with Baylor Canyon Arroyo 1.3 miles upstream of confluence with Baylor Canyon Arroyo Rio Grande American Dam Caballo Dam North Branch of Gardner Arroyo South Branch of Gardner Arroyo Stream Bilbo North Vado Arroyo South Vado Arroyo Intersection of McNutt Road and Alvarez Road Intersection of McNutt Road and Alvarez Road Bilbo Pond Spillway at Mesilla Hills Road Confluence with Unnamed Drainage Channel Confluence with Unnamed Drainage Channel West of Valley Vista Road West of Valley Vista Road Lake Maureen Spillway Immediately west of I-10 Immediately west of I-10 Redelineation Detailed Study Streams Stream Name Downstream Limit Upstream Limit South Fork (Tributary 1) Alameda Arroyo Confluence with Alameda Arroyo 200 feet upstream of City of Las Cruces and Dona Ana County boundary North Fork (Tributary 2) Alameda Arroyo North Fork (Tributary 3) Alameda Arroyo Corbett Arroyo Fillmore Arroyo Tributary 1 Fillmore Arroyo Tributary 3 Confluence with Alameda Arroyo Confluence with North Fork (Tributary 2) Alameda Arroyo Espina Street 0.3 mile West of Alamo Mine Trail 0.4 mile West of Alamo Mine Trail 2,450 feet upstream of confluence with North Fork (Tributary 3) Alameda Arroyo 320 feet upstream of City of Las Cruces and Dona Ana County boundary 1,600 feet upstream of Telshor Boulevard 0.6 mile East of Lost Padre Mine Road 0.1 mile East of Soledad Canyon Road 4

10 Table 1 Scope of Study (Continued) Redelineation Detailed Study Streams (continued) Stream Name Downstream Limit Upstream Limit Fillmore Arroyo Tributary 4 Flow Path 2 (Alameda Arroyo) Confluence with Fillmore Arroyo State Route 28 (Dona Ana Road) 1 miles Northeast of Talavera Avenue 1,680 feet upstream of I-25 Flow Path 4 Flow Path 6 2,200 feet downstream from State Route 28 (Alameda Boulevard) 34 feet downstream from Linden Avenue Flow Path 7 Farney Drive Main Street Flow Path 8 (North Fork Las Cruces Arroyo) Flow Path 9 (South Fork Las Cruces Arroyo) US Government Dam US Government Dam 1,070 feet upstream of US Routes 70 and 82 (Main Street) 0.45 mile upstream of Palmer Road 840 feet upstream of City of Las Cruces and Dona Ana County boundary 260 feet upstream of Unnamed Road Flow Path 10 I-25 Culvert Outlet 560 feet upstream of Las Alturas Flow Path 11 Flow Path 13 Little Dam Arroyo North Fork Moreno Arroyo South Fork Moreno Arroyo South Fork Moreno Arroyo Split Flow At I-25 Stream 1 Culvert under Las Cruces Lateral Las Cruces Lateral Spillway of US Government Dam El Camino Real El Camino Real Confluence with South Fork Moreno Arroyo at I-25 Quirke Lake Stream 2 Playa 3 Stream 3 Confluence with Stream feet upstream of I-10 westbound frontage road 0.48 mile upstream of University Avenue 200 feet upstream of City of Las Cruces and Dona Ana County boundary 2.03 miles upstream of I-25 North Bound 1.13 miles upstream of confluence with South Fork Moreno Arroyo-Split Flow 110 feet upstream of Del Rey Boulevard Approximately 1,400 feet upstream of State Route 213 Approximately 0.8 mi. upstream of Playa 3 Approximately 1.1 miles upstream of Playa 3 5

11 Table 1 Scope of Study (Continued) Redelineation Detailed Study Streams (Continued) Stream Name Downstream Limit Upstream Limit Stream 4 Confluence with Stream 3 Stream 5 Playa 4 Stream 6 Playa 4 Stream 7 Quirke Lake Stream 8 Confluence with Stream 7 Stream 9 Stream 10 Quirke Lake Quirke Lake Stream 11 Playa 5 Stream 13 Stream 14 Stream 15 Stream 16 Stream 17 Picacho Drain Picacho Drain Picacho Drain Picacho Drain Picacho Drain Stream 20 Playa 2 Stream 21 Stream 22 Stream 23 Stream 24 Picacho Drain Picacho Drain Picacho Drain Picacho Drain Approximately 0.44 mile feet upstream of Stream 3 Approximately 0.8 mi. upstream of Playa 4 Approximately 0.8 mi. upstream of Playa 4 Approximately 1,700 feet upstream of State Route 213 Approximately 1,400 feet upstream of State Route 213 Approximately 1,800 feet upstream of State Route 213 Approximately 2,400 feet upstream of State Route 213 Approximately 1,000 feet north of Mesilla Drive 0.66 mile upstream of Picacho Drain 0.52 mile upstream of Picacho Drain 0.78 mile upstream of Picacho Drain 0.46 mile upstream of Picacho Drain 0.49 mile upstream of Picacho Drain Approximately 1,700 feet upstream of State Route 213 1,950 feet upstream of Picacho Drain 1,000 feet upstream of Picacho Drain 700 feet upstream of Picacho Drain 1.06 miles upstream of Picacho Drain 6

12 All stream reaches previously studied by detailed study methods were redelineated for this countywide update. All Zone A Special Flood Hazard Areas that were not subsequently studied by either detailed or enhanced approximate methods were refined. Table 2, "LOMCs" Letter of Map Changes lists those revisions that have been incorporated into the countywide update for Dona Ana County. Table 2 LOMCs Community Case Number New Panel Number City of Las Cruces P 35013C1092G, 35013C1111G City of Las Cruces, and Dona Ana County City of Las Cruces, and Dona Ana County P P 35013C0183G, 35013C1084G, 35013C1091G, 35013C1092G, 35013C1094G 35013C0183G, 35013C1084G, 35013C1091G, 35013C1092G, 35013C1094G City of Las Cruces P 35013C1111G City of Las Cruces P 35013C1091G City of Las Cruces P 35013C1103G City of Las Cruces P 35013C1084G City of Las Cruces P 35013C1081G, 35013C1082G City of Las Cruces P 35013C1101G City of Las Cruces P 35013C1101G Table 3, "Stream Name Changes," lists those streams whose names have changed or that differ from those published in the previous FIS for Dona Ana County or any of the communities within. Table 3 Stream Name Changes Community Old Name New Name City of Las Cruces Flow Path 1 Flow Path 1 (Sand Hill Arroyo) City of Las Cruces Flow Path 12 Flow Path 12 (Tellbrook Arroyo) 2.2 Community Description Dona Ana County is located in the south-central portion of New Mexico. Comprised of 3,804 square miles, the county is bordered by the unincorporated areas of Luna County to the west, the unincorporated areas of Sierra County to the north, the unincorporated areas of Otero County to the east, the unincorporated areas of El Paso County, Texas to the 7

13 southeast, and the State of Chihuahua, Mexico, to the south. The City of Las Cruces is located in the heart of Dona Ana County. Dona Ana County has a population of 189,444 (Reference 2), and is one of the fastest growing areas of New Mexico. The county contains four incorporated communities: the City of Las Cruces (population 76,990); the City of Sunland Park (population 13,309); the Village of Hatch (population 1,673); and the Town of Mesilla (population 2,180) (Reference 3). Presently, the main economic sources in the county are: agricultural products, such as cotton, chili, pecans, and alfalfa; aerospace research and manufacturing; tourism; and New Mexico State University in Las Cruces. The Rio Grande, which is regulated by Elephant Butte Reservoir, flows through the central portion of Dona Ana County in a north-south direction. The Rio Grande, along with an extensive system of irrigation canals, provides the water for the county s agricultural industry. The remaining arroyos in Dona Ana County are ephemeral and have runoff as a result of short, intense summer thunderstorms. The mountain ranges within Dona Ana County are typically aligned in a north-south direction, and include the San Andres, San Augustin, Organ, and Franklin Mountains. The climate of the county can be described as arid, except for semiarid areas at higher elevations where precipitation is greater and temperatures are cooler. The average annual precipitation in Las Cruces is approximately 8 to 9 inches per year. The mean annual temperature in Las Cruces is 60.5ºF, with monthly temperatures ranging from 41.9ºF in January to 79.8ºF in July (Reference 4). Due to excessive land use activities such as agriculture, grazing, and development, the native vegetation in many portions of the county has been greatly depleted. Much of the acreage that was once desert grassland is now dominated by shrubs and annual forbs (Reference 5). 2.3 Principal Flood Problems Most of the storms that produce large amounts of runoff occur in the months of June through September. Fall, winter, and spring are the dry seasons because much of the moisture in eastward circulation from the Pacific Ocean is removed as the air passes over the mountains west of New Mexico. In the summer, moisture-laden air from the Gulf of New Mexico enters southern New Mexico. Strong surface heating and upslope flow of the air cause brief, and often heavy, showers. In the unincorporated community of Chaparral, the flooding is generally very shallow due to the lack of well-defined floodpaths. Flooding on streams on the West Mesa (west of Las Cruces) is characterized by high-velocity flows heavily charged with sediment and debris, which generally results from brief, intense thunderstorms. Flood hazards are further aggravated as these high-velocity sediment flows enter the Rio Grande valley. Upon reaching the escarpment, these flows spread out to form alluvial fans, which are cones of sediment at the base of the escarpment deposited by floods. The areas with alluvial fans are characterized by high-velocity flows, large amounts of sediment deposition, and unpredictable flow paths that may change during the same flood event. During the week of August 3 10, 1984, Dona Ana County experienced heavy rainfall that caused flooding in many locations. In the Chaparral area, homes were surrounded by water and dirt streets turned into lakes that were impassible due to siltation and flooding. Pumping was required to alleviate the problems. The county was also hit by heavy rain on August 23 and 24, 1987, when over three inches of rainfall were recorded. Floodwater swamped homes, businesses, schools, and hundreds of acres of farmland, and overtaxed irrigation canals. The county reported damages of approximately $667,000. The arroyos west of the Rio Grande in Las Cruces were among the areas affected by the floodwaters. 8

14 In July 1994, heavy rains produced flooding in several Las Cruces businesses, apartments, and a daycare center. Damages were estimated at $2 million. Las Cruces was hit by flash floods in June 1999, when three to four inches of rain fell in the East Mesa area. Water flowing off the Organ Mountains caused considerable damage to mobile homes, houses, and washed out roads. Property damage was estimated to be approximately $300 thousand. In June 2004, thunderstorms repeatedly moved over southern and central Dona Ana County, dropping more than 3 inches of rain between Las Cruces and El Paso. The unincorporated community of Vado, which lies in a somewhat sunken basin, was hit by flash flooding, and most of the town was under 2 to 3 feet of water, with a few areas under 6 feet of water. The governor eventually declared Vado a disaster area. The county reported damages of approximately $500 thousand. In August 2006, Dona Ana County was declared a disaster. Hardest hit was the Village of Hatch which experienced heavy rains beginning in July 2006 and continuing through August. Residents applied for approximately $1.4 million in federal disaster assistance (Reference 6). 2.4 Flood Protection Measures U.S. Government Dam, constructed by the USACE in the City of Las Cruces, extends across Alameda Arroyo to Las Cruces Arroyo. It is located just east of Interstate Route 25 and is 15,250 feet long and 69 feet high. The reservoir has a capacity of 12,500 acre-feet at the maximum water surface elevation of 4,139.8 feet. The outlet controls release of a maximum of 275 cubic feet per second (cfs), with the reservoir at spillway crest, elevation 4,130 feet. Outflows are conveyed westward for a distance of 17,200 feet to the Rio Grande in an unlined channel. The Rio Grande Levee System was constructed by the International Boundary and Water Commission (IBWC) between 1937 and 1938, and consists of an East and West Levee. The levee is primarily used to contain and control discharges from the Caballo and Elephant Butte Reservoirs. In 1958, the City of Las Cruces constructed a detention dam on one of the small arroyos that drains a portion of the area between Alameda and Las Cruces Arroyos. It has been designated as the Country Club Detention Dam. The capacity of the reservoir is 225 acre-feet at maximum water surface elevation and the maximum capacity of the outlet is 54 cfs. The IBWC constructed a slack water channelization improvement from Caballo Dam to El Paso, consisting of a channel, bank protection, control works, and levees that provide conveyance ranging from 12,000 to 17,000 cfs through the Mesilla Valley. New Mexico State University constructed an earthen dike upstream from the university to divert Campus Arroyo flows into a small, excavated channel through the southern portion of the campus. The system of ditches (drains and laterals) in Las Cruces is principally for the conveyance of irrigation water and drainage of subsurface water, but also conveys some flood flows. The Natural Resources Conservation Service (NRCS) has constructed two floodwaterretarding structures on Tortugas Arroyo having an aggregate capacity of 1,326 acre-feet for floodwater detention, and 2,929 feet of channel with appurtenant structures for conveyance of outlet discharges from the retardation structures and the runoff from the uncontrolled area below the structure to an existing drain. 9

15 Fillmore Dam and Sand Hill Dam are ungated, dry dams located on Fillmore Arroyo and Flow Path 1 (Sand Hill Arroyo), respectively. Fillmore Dam is maintained regularly by the Dona Ana County Flood Commission. No historical records regarding the construction or design of the dam are available. Historical accounts from the Dona Ana County Flood Commission indicate that the Fillmore Dam has never overtopped. Nonstructural measures of flood protection are also being used to aid in the prevention of future flood damage. These are in the form of land-use regulations, adopted from the Code of Federal Regulations, that control building within areas that have a high risk of flooding. All levees shown on the previous FIRMs are shown on the current DFIRMs. Levee certification was investigated as part of this restudy. The levees are located alongside the Rio Grande at certain locations. Within Dona Ana County, the Rio Grande is designated as Zone A. These levees fall under the jurisdiction of the IBWC. The IBWC was unable to provide certification against the 1-percent annual-chance flood for the Rio Grande Levee. As a result, the 1-percent annual-chance floodplain boundaries will widen approximately 5 miles along the Rio Grande. 3.0 ENGINEERING METHODS For the flooding sources studied by detailed and enhanced approximate Type 1 methods in the community, standard hydrologic and hydraulic study methods were used to determine the floodhazard data required for this study. Flood events of a magnitude that are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having special significance for floodplain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and 0.2-percent chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the long-term, average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood that equals or exceeds the 1-percent-annual-chance in any 50-year period is approximately 40 percent (4 in 10); for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish peak discharge-frequency relationships for each flooding source studied by detailed methods affecting the community. Peak discharge-drainage area relationships for Dona Ana County are shown in Table 4, "Summary of Discharges." Flooding Source and Location Table 4 Summary of Discharges DRAINAGE AREA (in mi 2 ) 10% - Annual- Chance PEAK DISCHARGES (cfs) 2% - 1% - Annual- Annual- Chance Chance 0.2% - Annual - Chance SOUTH FORK (TRIBUTARY 1) ALAMEDA ARROYO Upstream of confluence with Alameda Main Arroyo ,104 10

16 Flooding Source and Location Table 4 Summary of Discharges (Continued) DRAINAGE AREA (in mi 2 ) 10% - Annual- Chance PEAK DISCHARGES (cfs) 2% - 1% - Annual- Annual- Chance Chance 0.2% - Annual - Chance At a point approximately 0.72 mile upstream of Alameda Main Arroyo ,036 NORTH FORK (TRIBUTARY 2) ALAMEDA ARROYO Upstream of confluence with Alameda Main Arroyo , , ,324 4 Downstream of confluence with North Fork (Tributary 3) Alameda Arroyo , ,780 4 NORTH FORK (TRIBUTARY 3) ALAMEDA ARROYO Upstream of confluence with North Fork Alameda Arroyo (Tributary 2) BAYLOR CANYON ARROYO Main Channel At US * * 2,030 * Upstream of Confluence with Tributary * * 1,270 * Upstream of Confluence with Tributary * * 686 * Tributary * * 1,040 * Tributary * * 1,270 * CORBETT ARROYO Downstream of I * * * 500 At Sharon Circle 1.12 * * * 930 FILLMORE ARROYO Main Arroyo At Fillmore Dam * * 3,510 5,720 At Achenbach Canyon Rd * * 1,960 3,100 At Confluence with Tributary * * 1,380 2,130 Tributary mile downstream from Lost Padre Mine Trail 1.89 * * 1,050 1,590 Tributary mile downstream from Alamo Mine Trail 0.87 * * 699 1,040 Tributary 4 At Confluence with Fillmore Arroyo 3.05 * * 1,340 1,040 FLOW PATH 1 (SAND HILL ARROYO) Upstream of I-25 * 910 4,400 6,100 7,600 At Sand Hill Arroyo Dam 2.96 * * 1,533 * At N Jornada Road 1.65 * * 829 * At Mesa Grande Road 1.54 * * 791 * FLOW PATH 1 (SAND HILL ARROYO) SPLIT FLOW At diversion location * * * 411 * 11

17 Table 4 Summary of Discharges (Continued) DRAINAGE AREA (in mi 2 ) 10% - Annual- Chance PEAK DISCHARGES (cfs) 2% - 1% - Annual- Annual- Chance Chance 0.2% - Annual - Chance Flooding Source and Location FLOW PATH 1 (SAND HILL ARROYO) NORTH DIVERSION At Oro Vista Ponds 9.26 * * 3,590 * Downstream US * * 920 * Upstream US * * 4070 * FLOW PATH 2 At Dona Ana Road FLOW PATH 3 (ALAMEDA MAIN ARROYO) At US Government Dam , , , ,689 4 Downstream from confluence with South Fork (Tributary 1) , , , ,792 4 At confluence with North Fork Alameda Arroyo (Tributary 2) ,739* 4,551* 6, ,6884 Upstream of confluence with North Fork Alameda Arroyo (Tributary 2) ,327 3,483 4,707 9,365 FLOW PATH 4 At Alameda Boulevard ,016 1,253 1,965 At a point approximately 730 feet downstream from US Routes 70/82 * ,160 At a point approximately 400 feet downstream from US Routes 70/82 * FLOW PATH 5 At confluence with Las Cruces Lateral ,060 1,260 1,840 Solano Drive * FLOW PATH 6 At a point approximately 0.7 mile downstream from Union Avenue * * * At a point approximately 160 feet downstream from Valley Drive * * * 472 1,002 At Avenida de Mesilla * * At a point approximately 200 feet downstream from Willoughby Avenue * * At a point approximately 30 feet downstream from Madrid Road * * FLOW PATH 7 At cross-section B ,465 1,900 3,160 FLOW PATH 8 At US Government Dam ,050 2,760 3,730 7,400 12

18 Table 4 Summary of Discharges (Continued) Flooding Source and Location DRAINAGE AREA (in mi 2 ) 10% - Annual- Chance PEAK DISCHARGES (cfs) 2% - 1% - Annual- Annual- Chance Chance 0.2% - Annual - Chance FLOW PATH 9 At US Government Dam ,460 3,820 5,100 9,906 FLOW PATH 10 At a point approximately 380 feet downstream from Interstate Route 25 * 135 2,880 6,025 10,560 At Las Alturas , , ,460 3 FLOW PATH 11 At confluence with Las Cruces Lateral ,924 5,200 8,900 FLOW PATH 12 (TELLBROOK ARROYO) At Las Alturas Drive ,000 1,230 1,890 Downstream from Interstate Route 10 * 480 1,250 1,715 1,800 At Stern Drive * 480 1,250 1,715 2,325 FLOW PATH 13 At confluence with Las Cruces Lateral FLOW PATH 15 At confluence with Burn Lake ,535 2,050 3,700 At US Routes 70/80 * ,750 River Station/Flood Frequency Munson Pond * 635 1,118 1,516 2,295 Espina Street * ,316 1,988 Solano Drive * ,116 1,690 Walnut Street * ,000 1,514 GARDNER ARROYO North Branch At intersection of McNutt and Alvarez Roads South Branch At intersection of McNutt and Alvarez Roads 3.51 * * 1,440 * 3.50 * * 1,440 * LAS CRUCES ARROYO River Station/Flood Frequency Walnut Street * ,000 1,514 Solano Drive * ,116 1,690 Espina Street * ,316 1,988 Munson Pond * 635 1,118 1,516 2,295 13

19 Table 4 Summary of Discharges (Continued) Flooding Source and Location DRAINAGE AREA (in mi 2 ) 10% - Annual- Chance PEAK DISCHARGES (cfs) 2% - 1% - Annual- Annual- Chance Chance 0.2% - Annual - Chance LAUSON AND BREEDLOVE ARROYOS WITH BERM Lauson Arroyo At confluence with Breedlove Arroyo At I At Dam Outlet At Dam Inlet ,452 1,850 2,948 Breedlove Arroyo At confluence with Lauson Arroyo At I At Dam Outlet At Dam Inlet ,351 Breedlove/Lauson At Anthony Lateral ,147 At confluence ,136 LAUSON AND BREEDLOVE ARROYOS - WITHOUT BERM Lauson Arroyo At confluence with Breedlove Arroyo * At split (North Branch) * At I At Dam Outlet At Dam Inlet ,452 1,850 2,948 Breedlove/Lauson At Anthony Lateral * At confluence * Lauson Arroyo At Highway 404 (West) * At Highway 460 (West) * At split (West Branch) * LITTLE DAM ARROYO Downstream from Foothills Road NORTH FORK MORENO ARROYO At El Camino Real 0.90 * * 1,135 1,441 At Elks Road 0.50 * * Downstream from I-25 SOUTH FORK MORENO ARROYO At confluence of North and South Forks of Moreno Arroyo 1.26 * * 851 1,109 At Kennedy Road 0.80 * * At 1,100 feet upstream of Arabela Drive 0.40 * * At I * *

20 Table 4 Summary of Discharges (Continued) Flooding Source and Location DRAINAGE AREA (in mi 2 ) 10% - Annual- Chance PEAK DISCHARGES (cfs) 2% - 1% - Annual- Annual- Chance Chance 0.2% - Annual - Chance STREAM BILBO At Bilbo Pond 0.45 * * 496 * STREAM 1 At Quirke Lake 8.08 * * 2,103 * At Playa * * 3,463 * At confluence of Stream * * 877 * STREAM 2 At confluence with Stream * * 417 * STREAM 3 At confluence with Stream * * 1,482 * At Playa * * 919 * STREAM 4 At confluence with Stream * * 377 * STREAM 5 At Playa * * 524 * STREAM 7 At Quirke Lake 7.84 * * 3,964 * At confluence of Stream * * 3,183 * STREAM 8 At confluence with Stream * * 932 * STREAM 9 At Quirke Lake 2.40 * * 1,650 * STREAM 10 At Quirke Lake 6.18 * * 4,215 * STREAM 11 At Playa * * 973 * STREAM 13 At a point approximately 0.6 mile above Picacho Drain STREAM 14 At a point approximately 2,400 feet above Picacho Drain STREAM 15 At a point approximately 1,100 feet above Picacho Drain 1.87 * * 407 * 0.49 * * 249 * 1.08 * * 305 * 15

21 Table 4 Summary of Discharges (Continued) Flooding Source and Location DRAINAGE AREA (in mi 2 ) 10% - Annual- Chance PEAK DISCHARGES (cfs) 2% - 1% - Annual- Annual- Chance Chance 0.2% - Annual - Chance STREAM 16 At a point approximately 0.5 mile above Picacho Drain STREAM 17 At a point approximately 0.7 mile above Picacho Drain 0.27 * * 375 * 0.45 * * 574 * STREAM 20 At Playa 2 * * * 450 * STREAM 21 At a point approximately 0.5 mile above Picacho Drain STREAM 22 At a point approximately 2,400 feet above Picacho Drain STREAM 23 At a point approximately 0.5 mile above Picacho Drain STREAM 24 At a point approximately 0.9 mile above Picacho Drain 0.20 * * 412 * 0.11 * * 186 * 0.15 * * 226 * 5.66 * * 3,031 * NORTH VADO ARROYO Downstream from tributary at confluence with Unnamed Drainage Channel 1.13 * * 800 * Upstream of tributary confluence 0.46 * * 502 * SOUTH VADO ARROYO At confluence with Unnamed Drainage Channel * * 2,600 * NOTES: *Data not computed 1 Includes drainage area above Las Cruces Dam. 2 Local drainage areas only. 3 Discharges decrease because of flood control structures. 4 lncludes flows diverted from Posse Creek (10-percent-annual-chance, 262 cfs; 2-percent-annual-chance, 684 cfs; 1-percent-annual-chance, 940 cfs; and 0.2-percent-annual-chance, 1,289 cfs) 5 Conveyance capacity of Box Culverts under US-70 at Sand Hill Arroyo. 6 Including Ponding Areas 16

22 The stillwater elevations for the 1-percent and 0.2-percent-annual-chance floods were determined for Burn Lake, Quirke Lake, and unnamed pond in the City of Sunland Park, Playas 1 through 6, and all are summarized in Table 5, "Summary of Stillwater Elevations." Flooding Source and Location Table 5 Summary of Stillwater Elevations ELEVATION (feet) 1%-Annual- Chance 0.2%- Annual- Chance BREEDLOVE DAM Entire shoreline within County 3, ,939.2 BURN LAKE Entire shoreline within County 3, ,888.0 FILLMORE DAM Entire area within County 4, ,062.4 LAUSON DAM Entire shoreline within County 3, ,965.8 QUIRKE LAKE Entire shoreline within County 4,034.3 * UNNAMED POND 3,730.1 * PLAYA 1 Entire area within County 4,063.2 * PLAYA 2 Entire area within County 4,060.9 * PLAYA 3 Entire area within County 4,056.7 * PLAYA 4 Entire area within County 4,066.1 * PLAYA 5 Entire area within County 4,036.5 * PLAYA 6 Entire area within County 4,032.4 * TWO UNNAMED PONDING AREAS Area bounded by Interstate 10 to the southwest, Valley Drive to the northeast, Barker Road to the southeast, and Avenida de Mesilla to the northwest 3,885 3,887 NOTE: *Data not computed. 17

23 3.1.1 Detailed and Enhanced Approximate Type 1 Study Streams FEMA Indefinite Delivery, Indefinite Quantity (IDIQ) Contract and Cooperating Technical Partner (CTP) Mapping Activity Statement (MAS) Fillmore and Tellbrook Arroyos The discharges for the new detailed and enhanced approximate Type 1 study streams, with the exception of Sand Hill Arroyo, were calculated using the U.S. Geological Survey (USGS) Regression Equations. New Mexico is divided into eight hydrologic regions. Dona Ana County falls within the Southwest Desert Region. The Southwest Desert Regression Equations are valid for drainage areas that range from 0.20 to 2,830 square miles. Drainage area is the only input variable required for these equations. The National Flood Frequency (NFF) Program, Version 3.2, a FEMA-approved software program, was used for computation of the 1-percent-annual-chance storm event discharge values (Reference 7). Sand Hill Arroyo hydrology was analyzed using the Rainfall-Runoff Model, which was developed using Hydrologic Engineering Center - Hydrologic Modeling System (HEC-HMS) (Reference 8). The model used the NRCS curve number for the loss rate, the NRCS method for the runoff transform, and the NRCS unit hydrograph for the precipitation. The Fillmore Arroyo hydrology was initially analyzed using the rainfall-runoff methodology. A hydrograph was derived from the regressional equations and compared to the HEC-HMS results. Ultimately, the regression equation results were used since the HEC-HMS results overpredicted the discharges based on historic data. Stream gage data were not used for hydrologic analysis on any of the streams studied in Dona Ana County as stream gage data were available for three locations, none of which were within a close proximity of the study reaches. Due to the lack of comparable data, the models were not calibrated based on the stream gage data. In order for the Rainfall Runoff Model to predict peak discharges, the model requires drainage area, channel slope, and mean annual precipitation as the input variables. Topographic data obtained from Dona Ana County Flood Commission were used in conjunction with USGS Digital Elevation Model (DEM) data to create a terrain model of Dona Ana County. The watersheds were delineated using field reconnaissance, available topographic data and data obtained from the Watershed Concepts, Inc. WISE software, a FEMA-approved software program (Reference 9). The WISE Hydrology module delineated basins using a streamline file and a hydro-correct terrain model. The hydro-correct terrain model is a DEM with a grid size of 50 feet by 50 feet. The WISE delineation was evaluated and revised as necessary to reflect available contour data, USGS Quadrangle maps, new development, and/or topographic changes documented as part of the field reconnaissance. The centroid of each watershed for the study reaches was approximated based on the watershed delineation. The coordinates of the centroid were input into the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 website (Reference 10). The NOAA Atlas website uses the coordinates of a fixed location 18

24 to interpolate the average precipitation values for that location. Updated Precipitation Frequency Data for the state of New Mexico were released by NOAA in August These data supersede NOAA Atlas 2 data. Watershed characteristics, such as sub-basin area and cumulative basin area, were obtained through field investigation, available topographic data, and available orthophotogrammetric data. Table 6, "Stream Hydrologic Methods Used," lists the study streams and shows the type of study that was performed for each stream and the methodology selected. Table 6 Stream Hydrologic Methods Used Study Stream Name Study Type Hydrologic Method Used Baylor Canyon Arroyo Enhanced Approximate Study Type I USGS Regression Equations Fillmore Arroyo Detailed Study Without Floodway USGS Regression Equations Flow Path 1 (Sand Hill Arroyo) Enhanced Approximate Rainfall Runoff Model Study Type I NRCS Method Flow Path 12 (Tellbrook Arroyo) Detailed Study Without Floodway USGS Regression Equations Gardner Arroyo Enhanced Approximate Study Type I USGS Regression Equations Stream Bilbo Enhanced Approximate Study Type I USGS Regression Equations Vado Arroyo Enhanced Approximate Study Type I USGS Regression Equations CTP MAS Flow Path 1 (Sand Hill Arroyo), Flow Path 3 (Alameda Main Arroyo), Flow Path 5, Flow Path 15, Las Cruces Arroyo, and Breedlove and Lauson Arroyos The hydrologic analyses of Sand Hill Arroyo to determine the 10-, 2-, 1- and 0.2- percent-annual-chance flood event were prepared using HEC-1 (Version 4, 1990) (Reference 11) hydrologic model. The Sand Hill Arroyo watershed was divided into five subbasins: Sand Hill Arroyo at I-25 (0.39 mi 2 ), Sand Hill Arroyo between I-25 and US Highway 70 (5.00 mi 2 ), Sand Hill Arroyo above US Highway 70 (1.10 sq. mi.), Posse Creek above US Highway 70 (1.90 mi 2 ), and Mine House Arroyo above US Highway 70 (6.60 mi 2 ). These subbasins were delineated on two USGS Quadrangle maps: Las Cruces, New Mexico, dated 1978, at a scale of 1 inch = 2,000 feet, with a contour interval of 10 feet; and Tortugas Mountain, New Mexico, dated 1955, at a scale of 1 inch = 2,000 feet, with a contour interval of 20 feet. Based on a review of these work maps, the physical input parameters (drainage areas, lengths, and slopes) were entered into the HEC-1 model. The 10-, 2-, 1- and 0.2-percent-annual-chance, 6-hour rainfall depths (inches) were obtained from the NOAA's Precipitation Frequency Atlas of the United States, Vol. 4. The 0.2-percent-annual-chance rainfall was obtained through extrapolation. The initial and constant infiltration losses were repeated from the previously effective FIS, dated September 6, USACE developed synthetic unit hydrographs for each sub-basin using Snyder's method, as outlined in the USACE Engineering Manual EM , titled 19

25 Flood Runoff Analysis, dated October 11, The unit hydrograph parameters were independently verified. The peak discharge results of the hydrologic analyses are shown in Table 4. Flows for the 10-, 2-, 1-, and 0.2-percent-annual-chance events are given at each concentration point. The hydrology for Flow Path 5 was developed using the 1992 Feasibility Study Hydrology and the hydrology for the 2000 Conditional Letter of Map Revision by the USACE Albuquerque District. The hydrology for the 2000 Conditional Letter of Map Revision was based on the discharge-frequency relationships in the study area. HEC-1 computer program was used to obtain the peak discharges for the 10-, 2-, 1-, and 0.2-percent-annual-chance events are listed in Table 4. The hydrology for Las Cruces Arroyo was developed using the 1992 Feasibility Study Hydrology and the hydrology for the 2000 Conditional Letter of Map Revision by the USACE - Albuquerque District. The hydrology for the 2000 Conditional Letter of Map Revision was based on the discharge-frequency relationships in the study area. HEC-1 computer program was used to obtain the peak discharges for the 10-, 2-, 1-, and 0.2-percent-annualchance events. Since the flow path starts below the top of the basin and extends to the bottom of the basin, the peak flow rates changed at several locations. All the water from the basin entered Munsin Pond at several locations. Water also enters the flow path at several locations above these locations. Table 4, "Summary of Discharges," shows the locations where the flows are reduced. The September 6, 1995 FIS hydrology was used for Flow Path 15. The peak discharges for the 10-, 2-, 1-, and 0.2-percent-annual-chance events are provided in Table 4. Hydrologic analyses were carried out to establish the peak discharge-frequency relationships for the Lauson and Breedlove Arroyos following the construction of flood control structures in the arroyos. The hydrologic analyses for the Lauson/Breedlove watershed were performed using USACE HEC-HMS Version The watersheds for the Lauson and Breedlove Arroyos originate in the Franklin Mountains. Rainfall runoff flows in a southwesterly direction toward the Town of Anthony, NM. The flood control structures were constructed upstream of I-10. Downstream from the flood control structures, the Lauson and Breedlove Arroyos and their tributary basins cross 1-10 via several existing culverts. From I-10, the Breedlove Arroyo then flows to the southwest toward Highway 460. Runoff from Basins 8 and 9 contribute to the Breedlove Arroyo at Highway 460. Downstream of I-10, the Lauson Arroyo is diverted by an existing berm and diverted to the northwest where it, too, merges with the Breedlove Arroyo at Highway 460. The combined discharge then crosses over Highway 460 at a low point and sheet flows across relatively level, partially developed land toward the Anthony Lateral and Highway 478. Anthony Lateral forms the downstream end of the study area for this project. Because the existing berm, which diverts flows from the Lauson Arroyo does not meet FEMA standards for a levee, a model was developed without the berm in place. Without the berm, the Lauson Arroyo splits. The north branch converges with the Breedlove Arroyo at Highway 460 and continues toward Highway

26 The west branch continues toward the 18 culvert crossing in Highway 460 and reaches the Anthony Lateral just south of Highway 404. The NRCS Curve Number Loss Method was used to calculate the rainfall loss for each basin. This method requires the input of area, curve number, initial loss, and percent impervious for each basin. Upstream of I-10, data used to determine the basin areas were obtained from USGS Quad Maps. Downstream of I-10, 2-foot contour mapping was used. The Soil Survey of Dona Ana County Area New Mexico was used to determine the hydrologic group (A, B, or C) of the soil in each basin. The following NRCS Curve Numbers were then applied to the hydrologic groups: 63 for A, 77 for B, and 85 for C. Initial losses were calculated using an empirical relationship from the HEC-1 User's Manual. The equation computes initial loss based on curve number. The percent of impervious area for each basin was calculated based on values from the New Mexico State Highway and Transportation Department Drainage Manual and City of Albuquerque Development Process Manual for various land uses. The NRCS Dimensionless Unit Hydrograph was used to calculate the hydrograph for each basin. Data from the 2-foot contour mapping were used to determine length, slope, and channel shape for each routing reach. A Manning's "n" value of was used for each channel to describe an earth-lined channel. The precipitation model requires the input of a storm distribution and a point precipitation value. The NRCS Type II-60 pattern was used to distribute the storm over a 24-hour time period. The point precipitation information for the 10-, 2-, and 1-percent-annual-chance storms was obtained from the NOAA Atlas 2. The 0.2-percent-annual-chance storm data were extrapolated based on storm data from the NOAA Atlas. Flows were routed through the dams based on stage-storage-discharge curves. Stage-storage data were calculated using the 2-foot contour mapping. The effect of sediment transport on the elevation-storage data were taken into account. The sediment volumes transported from the basins upstream of the dams for the 10-, 2-, and 1-percent-annual-chance storms were estimated in the Pre-Design Phase Report for the Lauson-Breedlove Arroyo Flood Control Project. The sediment volumes for the 0.2-percent-annual-chance storm were extrapolated based on the given data. The sediment volumes were subtracted from the existing storage volumes in the reservoirs to determine the net volumes of available storage. Elevation outflow data were calculated using information on the outlets and the emergency spillways from the Supplemental Design Report for the Lauson- Breedlove Arroyo Flood Control Project. Diversion from the system occurs at two culverts along Highway 460. At each culvert, incoming flows are divided between the roadside ditch and the culvert. At Culvert 1, the flow capacity of the roadside ditch was calculated based on the ditch geometry, slope, and Manning's n. InRoads Storm & Sanitary Drainage Structure Analyzer, which utilizes inlet and outlet control equations to determine the capacity of a given culvert based on the headwater depth, was used to calculate the flow capacity of the culvert at varying depths. For each depth, the capacity of the roadside ditch and the capacity of the culvert were added to determine the incoming flow at that depth. This relationship of incoming flow to diverted flow was then input into HEC-HMS. At Culvert 2, two feet of water will pond before spilling over the berm into the roadside ditch. InRoads Storm & Sanitary Drainage Structure Analyzer was used to calculate the flow capacity of the culvert at a constant depth of two feet. In HEC-HMS, Culvert 2 receives all flows up to the 21