(A) The prevention of accelerated flooding and stream channel erosion in conjunction with urban development requires control of the discharge rate and volume of runoff prior to its release to off-site land. The purpose of this chapter is to: permit development without increasing the flooding of other lands, and
provide a basis for design of storm drainage systems on lands above or below undeveloped areas which will preserve the rights and options of both contributing and receiving property owners and assure the long-term adequacy of storm drainage systems.
(B) Accelerated channel erosion as well as increased flooding damage, could be controlled if the predevelopment hydrograph could be maintained. It is usually not practical, however, to retain the predevelopment rate of infiltration when a large part of the watershed is covered with impervious surfaces. The concept of reducing peak flows below those of predevelopment storms was developed as an alternative to maintaining the predevelopment hydrograph. The logic of this concept is to compensate for the increased erosiveness of:
(1) Increased volume of runoff due to loss infiltration;
(2) More frequent occurrences of the same flow. For example, a subdivision may have the peak flow of a predevelopment five-year storm occur once a year after development is complete; and
(3) Less sediment in the runoff water because the watershed is paved with nonerosive surfaces or good grass cover.
(C) The two-step standard requires that:
(1) The peak rates of runoff from an area after development shall be no greater than the peak rates of runoff from the same area before development for all 24-hour storms from one- to 100-year frequency. Designing for the one-, two-, five-, ten-, 25-, 50-, and 100-year storms is considered adequate to meet the requirement. Storm water requirements must be met at each point storm water is discharged from a development area;
(2) If the volume of runoff from an area after development will be greater than the volume of runoff from the same area before development, it shall be compensated by reducing the peak rate of runoff from the critical storm and all more frequent storms occurring on the development area to the peak rate of runoff from a one-year frequency, 24-hour storm occurring on the same area under predevelopment conditions. Storms of less frequent occurrence (longer return periods) than the critical storm up to the 100-year storm, shall have peak runoff rates no greater than the peak runoff rates from equivalent size storms under predevelopment conditions.
(D) The critical storm for a specific development area is determined as follows:
(1) Determine the total volume of runoff from a one-year frequency, 24-hour storm, occurring on the development area before and after development;
(2) From the volumes determined in division (D)(1) hereof, determine the percent of increase in volume of runoff due to development, and, using this percentage, select the 24-hour critical storm from this table:
If the Percentage of Increase in Volume Runoff Is: | The 24 Hour “Critical Storm” for Discharge Limitation Will Be: | |
Equal to or greater than | and less than | |
If the Percentage of Increase in Volume Runoff Is: | The 24 Hour “Critical Storm” for Discharge Limitation Will Be: | |
Equal to or greater than | and less than | |
0 | 10 | 1 year |
10 | 20 | 2 year |
20 | 50 | 5 year |
50 | 100 | 10 year |
100 | 250 | 25 year |
250 | 500 | 50 year |
500 | – | 100 year |
(E) The standard set forth in division (D) hereof basically requires peak flow to be reduced in proportion to increased runoff volume. If runoff volume is not increased, peak flows are only required to match predevelopment peak runoff rates for equivalent size storms for all storms from one-year to 100-year storms. This criterion is designed to require more control when the change in the runoff pattern caused by development is more drastic. The quantity and cost of preventative control features will need to be in proportion to the level of increase in runoff caused by the change in land use; a fair and equitable criterion.
(F) Concern is frequently expressed in regard to the extremes of the accelerated storm water control standard; the one- and the 100-year frequency storms. Justification for using the one-year storm is based on the erosivity of stream flow and on what is known as the “routing problem”. Small or frequent storms are very erosive because they occur so often. Damage doesn’t have time to heal before the next storm comes. Bankfull channel flow is essentially the most erodible state of stream flow on the channel itself. Any increase spills out over the floodplain with very little increase in the velocity of channel flow. Any flow less than bankfull generally has a corresponding decrease in velocity, and, therefore, causes less erosion damage to the channel. Where there is a significant increase in runoff volume, it is very important to have the runoff occur at something less than bankfull flow, especially in watersheds approaching complete development. Longer duration of bankfull flow than that which occurred under predevelopment conditions, can have disastrous effects on the stream channel and resulting sediment pollution. Since channels generally size themselves for about a one and one-half to a two-year frequency storm runoff event, the one-year storm was chosen as the control storm to prevent significant channel erosion.
(G) The “routing problem” is the problem of channel flow downstream from numerous smaller watersheds. As runoff from the smaller watersheds is altered, the effect on channel flow somewhere downstream, where all of the watersheds come together, can be favorable or adverse. Ideally, the entire large watershed should be hydraulically planned and the level of control of each sub-watershed should be based on the accumulative effects downstream. Since experience has proven the near impossibility, politically, of realizing that degree of organizational control, the one-year outlet rate is further justified in order to minimize adverse effects on the “routing problem” when compensation is necessary to control increased flow volumes.
(H) Many people suppose that runoff peaks and volumes do not increase from the large or infrequent storm because the ground is saturated by these storms and most of the rainfall will run off whether the land is used for forest land, cropland or urban purposes. Urbanization can, in fact, cause an increase in both peak and volume of runoff. Most of the increase may be in the early part of such a storm and perhaps the 25- or 50-year storm control may adequately address the situation. There are many variables which affect the storm water runoff pattern, including soil type, slope, kind of cover, percent of impervious cover, kind of disturbance and compaction. The 100-year storm needs to be examined in a development situation to determine its specific effect. Emergency flow areas and emergency outlets from structural work need to account for such storms if basements are not to be full of water and structures are not to fail. The additional control necessary for the 100-year frequency storm is not a highly significant factor in storm water management for accelerated runoff control.
(I) In addition to controlling channel erosion, this standard should make long range planning for storm water management easier. Storm sewers and stream channels can be designed in accordance with existing runoff conditions knowing that upstream runoff rates cannot be increased. This standard would also prevent an increase in flood levels and frequencies below future developments and, in many cases, reduce development costs because of reduced storm sewer size, particularly where multiple use detention facilities can be utilized in designated open space.
(J) A uniform procedure for determining runoff would be helpful in designing to meet this standard. The U.S. Soil Conservation Service procedures in Technical Release No. 55, 1986 edition, are recommended. These procedures consider watershed area, watershed shape, change in land use, soil type, hydrologic condition, impervious area, modification of hydraulic channels, increased intensity of rainfall near the middle of a storm, initial abstraction (surface storage, interception and infiltration prior to runoff), and decreased infiltration as a storm progresses.
(Ord. 1-91, passed 1-7-1991)