(A) If during construction the plans require a spring to be crossed or encroached upon, a stormwater conveyance system may be installed; however, the natural course of the spring must not be redirected or altered.
(B) Post-development peak runoff rates for 24-hour storm events must not exceed the following peak runoff rates for the same event (exclusion: Sites where one-year post-development peak discharge is less than 2.0 cfs):
(1) Overbank flood protection (Qp). Post-development peak runoff rates for the two-, ten-, and 25-year, 24-hour storm event must not exceed the pre-settlement peak runoff rate for the same event. Post development peak rates may be further restricted by available capacity of downstream drainage systems.
(2) Extreme flood protection (Qf). Post-development peak runoff rates for the 100-year, 24-hour storm event must not exceed the pre-development peak runoff rate for the same event. Post-development peak rates may be further restricted by available capacity of downstream drainage systems.
(3) Unified stormwater sizing criteria. Summary of the city unified stormwater sizing criteria for management of stormwater quality and quantity is as follows:
Table 1 | |
Sizing Criteria | Recommended Method |
Extreme Flood Protection (Qf) | Evaluate the effects of the 100-year storm on the stormwater management system, adjacent property, and downstream facilities and property. Manage the impacts of the extreme storm event through detention controls and/or floodplain management. |
Water Quality Volume (WQv) | The runoff resulting from a rainfall depth of 1.25 inches or less. Equal to Rev. Goal is to reduce average annual post-development total suspended solids loadings by 80%. Goal is met if Rev is completely infiltrated. WQv = (Rv)(A)(P)/12; Rv = site runoff volume coefficient, A = site drainage area (acres), P = design rainfall depth = 1.25 inches. |
(4) Intakes. Storm sewer intake points (inlet grates, endwalls, and the like) should have a minimum capacity to convey the post-development condition ten-year peak flow rate. Intakes shall be designed in such manner that the flooded street width shall not exceed the following:
Table 2: Allowable Pavement Encroachment and Depth of Flow | |
Street Classification | Maximum Encroachment |
Collector/Minor Arterial | No curb overtopping. Flow spread must not encroach to within eight feet of the centerline of a two-lane street. The flow spread for more than two-lane streets must leave the equivalent of two 12-foot driving lanes clear of water; one lane in each direction. For one- way streets, a single 12-foot lane is allowed. |
Local | No curb overtopping. Flow may spread to crown of street. |
Major Arterials (four, or more, lanes) | No curb overtopping. Flow spread must not exceed ten feet from the face of the curb of the outside lane. The flow spread for streets with more than two lanes must leave the equivalent of two 12-foot driving lanes clear of water; one lane in each direction. For one-way streets, two 12-foot lanes are required. For special conditions, when an intake is necessary in a raised median, the flow spread should not exceed four feet from the face of the median curb for an inside lane. |
(5) Storm sewers. Storm sewers should have a minimum capacity to convey the post-development condition ten-year peak flow rate. Provisions should be made for the 25- and 100-year peak flow rate when overland flow is not allowed or available. A minimum cleaning velocity of three fps should be used for the design storm. Storm sewer and surface water conveyance easements should be dedicated to the public. For those storm sewers that will handle footing drains, the following additional discharge (Q) values should be used.
(a) For less than 50 houses, Q = five gpm (0.011 cfs) per house.
(b) For greater than 50 houses, Q = 250 gpm (0.556 cfs), plus 2.5 gpm (0.0056 cfs) per house for each additional house over 50.
(6) Culverts. Culverts should have capacity to convey the following:
(a) Post-development condition 25-year peak flow rate without the headwater depth exceeding the diameter of the culvert.
(b) Post-development condition 50-year peak flow rate without the headwater depth exceeding one foot over the top of the culvert.
(c) Post-development condition 100-year peak flow rate without the headwater depth exceeding one foot below the low point of the roadway/embankment, unless there are other, more restrictive elevations.
(d) Private culverts should be designed with an overflow so that no damage is caused upstream of the proposed culvert.
(7) Ditches. Ditches should have capacity to convey the post-development condition 50-year peak flow rate within the ditch banks. Provisions should be made for the post-development condition 100-year peak flow rate to flow overland. Surface water flowage easements should be provided to the general public for all designed drainage ways and overland flow paths.
(8) Outlet stabilization. Stable outlets must have the capacity to handle the designed outflow from the pond outlet or pipe conveyance structures they serve. If the outlet is to be vegetated, it should be constructed and established before installation of other stormwater or erosion control structures. Outlet stabilization shall be designed based on the expected outlet discharge from the ten-year, 24-hour storm event.
(9) Open channel stabilization. To prevent channels from eroding, the channel lining must be adequate to carry the design velocity and volume. Where velocities are higher than five feet per section or where the channel must carry prolonged flow, the channel should be lined with riprap or other armoring material. Channel linings shall be designed based on the expected channel velocity from the ten-year, 24-hour storm event.
(10) Low impact tools, BMPs. In addition to the previously described stormwater detention and conveyance requirements previously described, the stormwater management systems designed as part of this chapter shall be developed using one or more of the low impact development tools or other best management practices identified in the Iowa Stormwater Manual, as approved by the city, or their designated representative.
(Prior Code, § 6-21-8)