BOARD01971

• Board Meeting Date: 3/22/2005
• Description: WSP Section and CF Section - Feasibility Study Status Report - La Plata Water Conservancy District - Long Hollow and Red Mesa Ward Reservoirs
• Board Meetings - Doc Type: Memo

TABLE OF CONTENTS

Page

1.0 EXECUTIVE SUMMARy....,..........,.........,........,.,..................................,,.......................1

2.0 PROJECT DESCRIPTION AND PURPOSE ................................................................6
2.1 Project Location and Description .........................................................................................6
2.2 Project Purpose .... .... ." ................... ..... ...... ................ ............ ... ............................... ............ 7
2.3 Study Objective... ................... ...... ......... ......... .................................. ....... ................... ..........8

3.0 DESIGN DA T A......................................................................................,...........,..........10
3.1 Topography and Feasibility Level Study Drawings ............................................................10
3.2 Reservoir Inflow........... ......... .... ............................. ....................... ... ....... ............... .... .... ....11
3.2.1 Water Rights ......................................................................................................11
3.2.2 Surface Water ....................................................................................................11
3.2.3 Groundwater Recharge........ ................ ............... ............. .............. ....................13
3.3 Dam Hazard and Size Classification .................................................................................14
3.4 Hydrology.................... ............ ............................................................... .................... ........ 15
3.4.1 Background ........................................................................................................15
3.4.2 Probable Maximum Precipitation ...................................... .................... ...........15
3.4.3 Unit Hydrographs ...............................................................................................16
3.4.4 HEC-1 Rainfall Runoff Simulations ....................................................................18
3.5 Outlet Operating Criteria.......................... ...................... ........ ................... ................. ........20
3.6 Engineering Geology.......... ............. ........ ........ ..... .......................... ...... ............... ............. .20
3.7 Geotechnical Investigation . .................. .................. .... ........................................... ............. 21

4.0 INSTITUTIONAL ASPECTS................................,................,.............................,......... 22
4.1 National Environment Protection Act.................................................................................22
4.2 Threatened and Endangered Species...............................................................................22
4.2.1 Federally Listed Species Considered and Evaluated.........................................22
4.2.2 Sensitive Species.................. ......... ........................ ...... ..................................... .24
4.2.3 San Juan River Basin Recovery Implementation Plan.......................................25
4.2.4 Summary of Conclusions of Threatened and Endangered Species Evaluation 25

4.3 Wetlands................ ... .... ........ ........ ........... ........ ...... ..... .............. ......... ... ............... ....... ...... .26
4.3.1 Direct Impacts .......... ............................................................. .............................26
4.3.2 Preliminary Wetlands Mitigation Plan.................................................................27

4.3.3 Permitting....... ............................ ..' ...................................... ............. ................. .27
4.4 Cultural Resources.................. ...... ......................................................... .................... ...... .28
4.5 Local Consultations......................... ........................ ........ ......... ............................ ......... ....29
4.6 Indian Trust Assets........................... ............ .............................. .......................................30
4.7 Findings of Regulatory and Institutional Investigations............................................ ..........31

5.0 DAM AND APPURTENANT STRUCTURE ALTERNATIVES....................................32

5.1 Reservoir............................. ................... ............................................................. ..............32
5.2 Spillway................. ...... ............... ............................ ........... ................................................. 32
5.2.1 Spillway Type .....................................................................................................33
5.2.2 Hydraulic Control Sections .................... ........ ...... .......................................... ..... 35
5.2.3 Construction Materials.. .......... ...... .............................................. ............... ........36
5.2.4 Discharge Efficiency.................................................................. ............. ..........36

5.2.5 Spillway Flow. .......... ......... .... ............... .......................... ....... ...................... ....... .37
5.3 Embankment..... ........ ................ ....................................... ....... ....... ......... .......................... 37
5.3.1 Topsoil Stripping, Stockpiling, and Placing ........................................................40
5.3.2 Placing the Embankment............................. ............. ...... ................................ ...40
5.3.3 Internal Drainage................ ....... .......... ............... ................................................40