16.76.010 Drainage.

A. Drainage Criteria. The Pueblo County Drainage Criteria Manual will use a storm duration of six hours for hydrologic computations. A return frequency of five years will be used for determining runoff for minor collection systems (drainage areas less than four hundred (400) acres and peak flows less than five hundred (500) cfs). A return frequency of one hundred (100) years will be used for determining runoff for major collection systems (drainage areas four hundred (400) acres and larger and for all peak flows equal to or exceeding five hundred (500) cfs).

B. Drainage Control. Each development shall provide for the on-site or off-site detention of excess stormwater runoff from that development and ensure that:

1. All stormwater storage facilities shall be designed with sufficient capacity to accommodate all runoff caused by the development in excess of the runoff which would have resulted from the site if left in its natural, existing or undeveloped condition;

2. No development shall cause downstream property owners, water courses, channels or conduits to receive stormwater runoff from proposed developments at a higher peak flow rate than would have resulted from the same storm event occurring over the site of proposed development with the land in its existing, natural or undeveloped condition;

5. The drainage system shall be designed to consider the drainage basin as a whole and shall accommodate not only runoff from the proposed development area, but also, where applicable, the system shall be designed to accommodate the runoff from those areas adjacent to and upstream from the subdivision itself, as well as its effects on lands downstream;

6. In areas in which calculations have been developed, by the U.S. Geological Survey, Soil Conservation Service, or County studies or reports, those figures shall be used for purposes of calculation. All proposed surface-drainage structures shall be indicated.

All appropriate designs, details, and dimensions necessary to clearly explain proposed construction materials and elevations shall be included in the drainage plans.

C. Detention Storage. All development must restore runoff characteristics to at least natural conditions. The following formula is an example to calculate the volume of detention required:

V = the change in volume (in cubic feet per second) from the site. This is the base volume of excess stormwater flows that would result from the development.

Rd = the volume of stormwater runoff (in cubic feet per second) flowing from the site after its development. This determination shall include runoff from pervious and impervious surfaces, changes in areas of forest, changes in soils due to compaction, and changes in the time of concentration, for a one hundred (100) year storm of twenty-four (24) hour duration.

Rn = the volume of stormwater runoff (in cubic feet per second) flowing from the site in its natural state. This determination shall include runoff from the site with its natural cover, grassland, or woodland for a one hundred (100) year storm of twenty-four (24) hour duration. Farm fields shall be calculated as grassland.

Ros = the volume of stormwater runoff (in cubic feet per second) flowing onto the site from upstream properties in their present state of development for a one hundred (100) year storm of twenty-four (24) hour duration.

S = the change in capacity (in cubic feet per second) of the natural on-site detention areas of the site. This indicates any drainageways, small depressional areas, or other areas which would naturally retain water during a one hundred (100) year storm of twenty-four (24) hour duration.

GW = the change in subsurface flows due to dewatering techniques which would add to the total surface water runoff during a one hundred (100) year storm of twenty-four (24) hour duration. Included here are dewatering devices such as drain tiles, curtain drains, or sump pumps. (This calculation may carry either a plus or minus value.)