Article III — SUBDIVISION AND IMPROVEMENT REGULATIONS

Accompanying the preliminary plans of each proposed subdivision, there shall be furnished the following information and data:

A.

Contour Map. A topographic map with appropriate contour intervals of the land to be subdivided and such adjoining land whose topography may affect the layout on drainage of the subdivision. On such a map, there shall be shown the following:

The location of streams and other floodwater runoff channels, their normal channels, and the extent of the floodplains at the established high water elevations and the limits of the floodway (if available), all properly identified;

Storm drains and sewers;

Septic tank systems and outlets, if any;

Wells and springs.

B.

Preliminary Drainage Plan. A preliminary drainage plan is intended to describe the present conditions and the general method and features proposed for handling storm-water runoffs after development. The plan will be accompanied by preliminary maps or other descriptive materials prepared by and bearing the seal of a registered civil engineer showing the following:

Drainage subareas;

General alignment of storm sewers and other drains;

Areas where special provisions may be necessary to reduce the impacts of high stormwater flows;

Existing streams and floodplains and any pro-posed realignments or modifications;

Locations of existing and proposed culverts, detention ponds and basins, and other features which now affect, or will affect, stormwater runoffs and areas inundated by high stream flows.

C.

Site Plan. A plan showing the dimensions of the site with existing and currently proposed structures prop- erly located, together with elevations and/or contours of the terrain before and after the proposed grading and fill-ing, if any, has been completed.

(Ord. 508 §3.01, 1991).

Accompanying the final plans of each proposed subdivision, there shall be furnished the following information and data prepared by and bearing the seal of a registered civil engineer:

A.

Contour Map. As described in Section15.28.120(A).

B.

Comprehensive Drainage Plan. A comprehensive drainage plan is a plan designed to handle safely the stormwater runoff following the rainstorms which exceed the predevelopment capacity of storm sewer

systems by detention of the increased stormwater runoff. The plan shall provide or be accompanied by maps or other descriptive material showing the following:

The extent and area of each watershed tributary to the drainage channels in the subdivision;

The storm drains to be built, the basis of their design, the outfall and outlet locations and elevations, receiving stream or channel and its high water elevation, and the functioning of the drains during high water conditions;

The parts of the proposed street system where pavements are planned to be depressed sufficiently to con- vey or temporarily store overflow from storm drains and runoff over the curb resulting from the heavier rainstorms and the outlets for such overflow;

Existing streams and floodplains to be main-tamed, enlarged, altered, and eliminated, and new channels to be constructed, their locations, cross-sections and pro-files;

Existing culverts and bridges, drainage areas, elevation and adequacy of waterway openings, and new culverts and bridges to be built, their materials, elev-ations, waterway openings and the basis of their design;

Existing detention ponds and basins to be main-tamed, enlarged and altered and new ponds on basins to be built with dams, if any;

The estimated location and extent of impervious surfaces existing and expected to be constructed when the subdivision is completely developed;

The slope, type and size of all storm drains, culverts and other waterways;

For all detention basins, a plot on tabulation of storage volumes with corresponding water surface elevations and of the basin outflow rates for those water sun-face elevations;

For all detention basins, design hydrographs of inflow and outflow for the twenty-five-year design run-off events for the site under developed conditions and the calculated twenty-five-year peak flows from the site under natural and developed conditions, unless the hydrograph requirement is waived by the city engineer.

C.

Site Plan. As described in Section15.28.120(C).

(Ord. 508 §3.02, 1991).

The following rules shall govern the design of improvements with respect to controlling the runoff of stormwaters:

A.

Design. Streets, lots, parks and other public grounds shall be located and laid out in such a manner as to reduce the velocity of overland flow and allow the maximum opportunity for infiltration of stormwaters into the ground, and to preserve and utilize natural streams, channels and detention basins.

B.

Channel Straightening. Meandering streams on channels may be partly straightened and minor changes made in other channels where it can be demonstrated that down-stream flooding will not be increased in frequency on depth.

C.

Streets Designed for Detention. Cross-sections of streets selected by the developer for the detention of stormwater runoffs shall be constructed above flood elevations and shall be designed to temporarily store and convey flows in excess of storm drain capacities, together with over-the-curb runoff. Those streets also shall be provided with adequate outlets for the safe disposal of flows. Where grading of lots is necessary to avoid damage to adjoining buildings, the lot grade adjacent to each building should be at least two feet higher than the crown of the pavement, and the minimum sill elevation of openings in the outer walls of such habitable buildings shall be at least two and one-half feet above the crown elevation.

D.

Manholes. All sanitary sewer and storm drain manholes constructed in a floodplain, in a street designed for detention, or in an area designed for the storage or passage of flood or stormwaters, shall be provided with either a watertight bolted manhole cover, or be constructed with a rim elevation of a minimum of one foot above the high water elevation of the design flood or the high water elevation of the design storm, whichever is applicable to the specific area.

(Ord. 508 §3.03, 1991).

Basins may be construct-ed to temporarily detain the stormwater runoff which exceeds the maximum peak flow rate authorized by this chapter.

A.

Storage Volumes. The volume of storage provided in these basins, together with storage as may be authorized in other on-site facilities, will be sufficient to control the runoff from the twenty-five-year storm of four-hour duration.

B.

Maximum Depth. The maximum planned depth of stormwaters stoned shall not normally exceed six feet.

C.

Side Slopes. The side slopes of the basin will conform as closely as possible to regraded or natural land contours and preferably shall be under ten percent. If the side slopes exceed twenty percent, erosion control and safety measures shall be provided.

D.

Outlet Control Structures. Outlet control structures shall be designed to operate simply and automatically. They will limit discharges into existing or planned downstream channels or conduits so as not to exceed predetermined maximum authorized peak flow rates.

E.

Emergency Spillway. Emergency overflow facilities must be provided in all instances so that stoned waters will not exceed the safe capacity of the basin.

(Ord. 508 §3.04, 1991).

For basins designed with permanent pools:

A.

Wet bottom basins shall not be permitted unless specifically approved by the city council.

(Ord. 508 §3.05, 1991).

For detention basins designed to be completely dewatered:

A.

Interior Drainage. Provisions must be incorporated to facilitate complete interior drainage of dry bottom basins, to include the provision of natural grades to out-let structures, longitudinal and transverse grades to per-imeter drainage facilities, pumps or the installation of subsurface drains.

B.

Multipurpose Features. These shall be designed to serve secondary purposes for recreation, open space or other types of use which will not be adversely affected by occasional or intermittent flooding.

C.

Aesthetics. Designs should result in aesthetical-ly pleasing configurations which will enhance public acceptability.

(Ord. 508 §3.06, 1991).