2005-11-02_REVISION - M1977348 - Laserfiche WebLink

Home Browse Search

~~ Applegate Group, ~~~. Consultants for Land, Minerals, and Water Memorandum Date: October 13, 2005 File No. 02-199 To: Bill SchenderleiD. P.E. From: Jason Ullmann, P.E. Subject: Headcut Erosion Protection Design This memo s~~*r+*n~*+zes the calculations used to desigo erosion protection for two headcuts on the Holcim Boettcher Mine property. Standazd engineering practices for design of erosion protection were used to determine the peak flows at the headcut location, size of the necessary riprap apron, maximum slope for placement of riprap and the size of riprap particles necessary to protect the slope. HYDROLOGY In order to design the erosion protection, a design flow rate needed to be established. The basin draining to the downstream (South) headcut azea was delineated using the site survey in Autocad. The drainage basin azea was determined to be approximately 53-acres. Because the azea is less than 160-acres, the Rational Method was used to calculate peak flows at the headcut location. According to the Larimer County Soil Survey, most of the basin (93%) is underlain by Kim series soils in NRCS hydrologic soil group B while the other 7% is underlain by Laporte series soils in hydrologic soil group C. A weighted average of runoff coefficient values from Table RO-5 of the Urban Drainage and Flood Control District (UDFCD) Criteria manual was used to determine the basin runoff coefficient for the 10-yeaz and 100-yeaz recurrence interval storms (0.18 & 0.37 respectively). Time of concenttafion was computed using methods outlined on page RO-8 of the UDFCD Criteria Manual. Rainfall intensities for the 10-yeaz and 100-year storm (2.3 in/lu & 4.7 in/hr respectively) were obtained from the Larimer County 5tormwater Design Standazds, which for the Holcim azea aze sourced from the City of Fort Collins 1999 rainfall criteria revisions. This produced the following peak flows: • 10-yeaz recurrence interval peak flow, Qt° = 22 cfs • 100-yeaz recurrence interval peak flow, Q~~ = 92 cfs EROSION PROTECTION SIZING The peak flows stated above were used to size the riprap based on Abt's Equation for sizing angulaz riprap on embankments. Erosion protection for both areas was designed using the peak flow from the downstream azea because the drainage azea and accompanying peak flow is higher and thus yields a more conservative design. Using Abt's equation a Dso riprap size (average riprap particle size) for the 10- and 100-yeaz events was calculated to be 8-inches and 16-inches respectively. Neither of these corresponds to a standazd UDFCD riprap size and using riprap with a 18-inch average size appeazs to be overly conservafive for this application. Therefore, it was determined that using UDFCD Type M riprap, which has a DS° of 12-inches would be more appropriate. Abt's equation is only valid for slopes at a 30-degree angle (3.3:1 slope) or less, consequently, the apron design includes laying the slope of the headcut on all sides back to 4:1. The attached exhibit shows the proposed grading at each headcut locafion, a typical cross secfion of the riprap protected azeas, and pertinent constmction notes for the contractor. Riprap sizing was performed with a design riprap layer thickness of twice the DS°, or 2-feet thick. In order to protect the downstream toe of the riprap, the apron design includes extending the riprap 15-feet downstream from the toe. Riprap placed downstream of the toe should also be 3-feet thick to provide added protection against undercutting. Bedding for the riprap will conform to UDFCD standazds and will include a 4-inch layer of UDFCD Type I riprap (CDOT 703.01) followed by a 4-inch layer of UDFCD Type II (CDOT 703.09 Class A) bedding. This will assist in preventing removal of the fine material from underneath the apron during storm events. 1499 West 120th Avenue, Suite 200 • Denver, Colorado 80234-2728 • (303) 452-661 I • Fax: (303) 452-2759