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HomeMy WebLinkAboutMINUTES - 06201989 - 2.5 r TO: BOARD OF SUPERVISORS, AS THE BOARD OF SUPERVISORS OF CONTRA COSTA- COUNTY AND AS THE GOVERNING BODY OF THE CONTRA COSTA COUNTY FLOOD CONTROL AND WATER CONSERVATION DISTRICT FROM: J. MICHAEL WALFORD, PUBLIC WORKS DIRECTOR DATE: JUNE 20, 1989 SUBJECT: MAY 23 , 1989 REFERRAL - REPORT ON ALHAMBRA VALLEY FLOOD WATER DETENTION BASINS. SPECIFIC REQUEST(S) OR RECOMMENDATIONS) & BACKGROUND AND JUSTIFICATION I. Recommended Action: 1. ACCEPT .report from the Public Works Director, and 2 . INDICATE the Board's intent to require mitigation of the accumulative storm water runoff impact from new development, and 3 . INDICATE the Board's intent to not accept financial responsibility for maintenance of detention facilities in the Alhambra Valley unless said facilities are of a regional nature having sufficient capacity to mitigate many developments at one time, and Cont'd on Attachment: x yes Signature: _ Recommendation of County Administrator _ Recommendation of Board Committee Approve Other Signature(s) Action of Board on June 20, 1989 Approved as recommended x and Other x APPROVED Recomendations 1 through 4, and APPROVED Recomendation 5 as amended to direct Flood Control District staff to discontinue investigation of regional detention basins in the Alhambra Valley area and to work with the developer and the community to address alternatives in resolving flooding problems. Vote of Supervisors x Unanimous (Absent — ) I HEREBY CERTIFY THAT THIS Ayes: Noes: IS A TRUE AND CORRECT COPY Absent: Abstain: OF AN ACTION TAKEN AND ENTERED ON THE MINUTES OF THE BOARD OF SUPERVISORS ON DATE SHOWN. ATTESTED �x� Q'0, 1989 PHA BATCHELOR, Clerk of the Board of Supervisors and County Administrator MFK:rs BO:201.t6 BY D UTY Orig. Div: Public Works (Flood Control) cc: County Administrator's Office County Counsel County Auditor-Controller Community Development Director Public Works Director Flood Control Engineering Services Alhambra Specific Plan Committee Mr. Jack Garner, City Manager, City of Martinez Board of Supervisors June 20, 1989 Page Two 4. CONFIRM Public Works Policy that mitigation detention basins required by development conditions of approval are beyond the scope of normal County general drainage maintenance and such facilities shall have perpetual maintenance assured through an established government entity and service charge, and 5. DIRECT the Flood Control District staff to discontinue their investigation of regional detention basins in the Alhambra Valley area. II. Financial Impact: Detention basin maintenance is expensive. Maintaining numerous small detention basins is significantly more expensive than maintaining several large (regional) detention basis. In 1984 when the Deadhorse and Stonehurst develop- ments were approved we agreed to recommend to the Board that a Flood Control Zone be formed to maintain regional basins in the Alhambra Valley in order to not worsen the flooding in downtown Martinez. County and District policy is now to require all regional mitigation basins to be maintained by the new developments through drainage benefit assessment areas. However, we recommend that the District honor the 1984 offer if a regional detention basin is constructed. III. Reasons for Recommendations and Background: The Deadhorse (17 lots) and Stonehurst (47 lots) developments (Subdivision Nos. 6443 and 4687 respectively) were approved in 1984 after considerable public input. One of the issues at the time was the mitigation of the accumulative hydro- logic impact from these and other developments. The final conditions of approval required the Deadhorse development to participate or contribute 25 cents per square foot of impervious surface ($45,400) toward the construction of a detention basin on the Stonehurst property. The Stonehurst development was conditioned to construct a detention basin and associated control structures subject to the approval of the Public Works Department. Since then a new tentative map has been filed on this property (Subdivision No. 7091) with the same condition. Subsequent to the approval of these developments, the Flood Control District staff prepared the attached concept paper for reducing the peak discharge quantities in Alhambra Creek. The report suggests four possible detention basin sites and indicates a preliminary hydrologic evaluation of the effec- tiveness of the detention basins. This paper was presented to the City of Martinez several years ago. During the past year, staff has further developed the concept for Site D. the East Bay Regional Park District's "pear orchard property" . The proposed basin at this site is designed to work in conjunction with the Stonehurst basin utilizing a common control structure and emergency spillway. The combined facility is capable of mitigating all development in the Alhambra Valley area while providing some limited reduction in the peak flows in Martinez. On April 24, 1989, staff presented the Site D Basin layout to the Alhambra Valley Specific Plan Committee. The Committee Chairman's letter of May 17, 1989, to the Board states that the Committee recommends that the Site D Basin not be pursued further. On May 24, 1989, Mr. Jack Garner, City Manager for Martinez, sent a letter to the Board requesting that the concept of requiring the Stonehurst and Deadhorse develop- ments to mitigate their storm water runoff be retained and the Flood Control District be directed to find a replacement basin site if the Site D location is not useable. Board of Supervisors June 20, 1989 Page Three Site D is the best detention site of the four sites ident- ified in the report. The other sites do not perform as well from a hydraulic-hydrology standpoint and two of the other sites have significant social and economic drawbacks. If Site D is eliminated, we recommend that the District's planning for basins in the valley be discontinued. This action would require each development to install its own mitigation facility and to form or join a government entity having responsibility for future maintenance of such . facilities. All future maintenance costs would be borne by the new lots through some form of annual service charge. IV. Consequences of Negative Action: Board action is needed to set policy for staff.