31-25 DRAINAGE; MINIMUM REQUIREMENTS.

All portions of the subdivision shall be protected from flood hazard, inundation, sheet overflow and ponding of storm waters, springs and all other surface waters. All finished floors shall be above the water surface of a one hundred (100) year frequency storm runoff.

31-25.2 Adequate Storm Drainage Facilities.

The design of all improvements within the subdivision shall be such that all surface waters occurring within the subdivision, as well as all surface waters flowing onto and/or through the subdivision, shall be conveyed through the subdivision without damage to any improvement, building site, or dwelling which may be constructed within the subdivision. Storm drainage facilities within the subdivision shall be designed to adequately convey the storm water runoff from the ultimate development of the drainage basin or watershed.

a. Surface waters flowing from the subdivision in any form or manner shall be conveyed without damage to any improvement, building, or dwelling to a natural watercourse having a definable bed and banks, or to an existing adequate storm drainage facility. Storm drainage facilities to be constructed outside of the subdivision shall be designed to adequately convey the storm water runoff from the ultimate development of the drainage basin or watershed lying within and above the subdivision.

b. Whenever surface waters must be conveyed beyond the boundaries of the subdivision in order to discharge into a natural watercourse or to an existing adequate storm drainage facility, the subdivider shall comply with either paragraphs c., d. or e. of this subsection, prior to filing of the final map or parcel map.

c. The subdivider shall deposit with the Engineering Department a copy of a duly recorded drainage release from the adjacent property owners in a form and content acceptable to the Town Counsel, accepting the flow of surface waters from the subdivision onto and over that property, or the detention and pending of surface waters upon that property, without liability by the Town for damages occurring therefrom. Such releases shall be obtained from all property owners between the boundaries of the subdivision and the point at which the surface waters will enter a natural watercourse having definable bed and banks or an existing adequate storm drainage facility.

d. The subdivider shall deposit with the Engineering Department a copy of a duly recorded grant of a drainage easement to the Town from the property owner(s). Such a document shall be obtained from all property owners between the boundaries of the subdivision and the point at which the surface waters will be discharged into a natural watercourse or an existing adequate storm drainage facility, or the area upon which surface waters will be detained and ponded.

e. The subdivider shall present written evidence which proves to the satisfaction of the Engineering Department that it is not feasible to obtain by negotiation from the property owner(s) either a drainage release or a grant of drainage easement. The Engineering Department may then authorize the subdivider to institute condemnation proceedings to obtain the easement.

f. Whenever it shall appear to the subdivider that those heretofore mentioned matters relating to the acquisition of drainage releases and easements, and/or the institution of condemnation proceedings, may be more expeditiously administered by the Town, the subdivider may request that the Town Council direct the Engineering Department to assume the responsibility for proceeding with these matters. The Council, upon such terms and conditions as they specify, may authorize the Department to act on behalf of the subdivider in complying with paragraph b. of this subsection.

Whenever surface waters flowing to or onto the subdivision in any form or manner will be detained or ponded on adjacent property(s) as the result of improvements constructed by the subdivider, the subdivider shall comply with subsection 31-25.3 prior to the filing of the final map or parcel map.

Storm waters flowing from the subdivision in any form or manner shall not be permitted to flow into any water conveyance facility of the Contra Costa Canal, nor into any other water conveyance or impounded facility for domestic water consumption.

Runoff quantities shall be determined by methods consistent with current engineering practices using basic data supplied by the Engineering Department for the frequency of the average recurrence interval stipulated in subsection 31-25.7.

Drainage facilities directly affecting the subdivision shall have the following minimum capacities:

a. Major drainage channels and conduits shall have sufficient capacity to contain a fifty (50) year frequency of average recurrence interval runoff;

b. Secondary drainage channels and conduits shall have sufficient capacity to contain a twenty-five (25) year frequency of average recurrence interval runoff;

c. Minor drainage facilities, channels and conduits shall have sufficient capacity to contain a ten (10) year frequency of average recurrence interval runoff.

Waters within street areas shall be placed in closed conduits when the maximum depth of computed flow exceeds the capacity of the gutter or creates a traffic hazard or endangers property. Storm water in natural or artificial drainage channels shall be placed in closed conduits or concrete lined channels where the quantity does not exceed eighty (80) cubic feet per second except that the advisory agency may recommend that an existing natural watercourse endowed with significant natural beauty in the form of trees, shrubs, or scenic attraction may be utilized for an open drainage facility with such drainage improvements as may be designated by the Engineering Department. Such areas shall either be dedicated as easements or designated on the final map or parcel map as drainage reserves upon and within which no improvements or structures of any nature shall be constructed without the written consent of the Engineering Department.

Design of drainage channels, conduits, and appurtenances shall conform with design standards of the Engineering Department.