Whoever violates or fails to comply with any of the provisions of this chapter shall be guilty of a misdemeanor of the fourth degree and shall be fined not more than two hundred fifty dollars ($250.00) or imprisoned not more than thirty days, or both. A separate offense shall be deemed committed each day during or on which a violation or noncompliance occurs or continues.

(Ord. 1978-8. Passed 1-19-78.)

CITY OF WESTLAKE

SMALL RETARDING BASIN

DESIGN PROCEDURE

MAXIMUM DISCHARGE RATE

On-site storm water storage (one of the options set forth in Ordinance 1978-8) is required of new developments within the City of Westlake to prevent the resultant increased storm run-off caused by the development from aggravating flooding conditions along the drainage streams in the watershed. For this reason, the duration of the design storm used to calculate allowable discharge rates from all on-site retarding basins is the concentration time of the main stream of the watershed on which the site to be developed is located. For convenience in calculations, the concentration times of all main streams in the City of Westlake is considered to be two hours. By ordinance, allowable discharge from developed sites or on-site retarding basins is limited to two-thirds of the run-off from the land in its present state of development. Hence, the maximum discharge rate for any on-site retarding basin is found by multiplying together: the factor two-thirds; the run-off coefficient for current land use; the tributary area in acres; and the rainfall intensity for a ten-year two-hour duration storm; i.e. Q10=2/3 x C X A X 1.02"/Hr. (Q10 in CFS). The allowable discharge rate of undeveloped land after development is therefore limited to 0.204 CFS per acre.

VOLUME OF STORAGE REQUIRED

In order to calculate the volume of storage required to achieve the allowable discharge rate, the total volume of run-off for ten-year storms of various durations has to be calculated to determine the maximum volume of storage the retarding basin must hold. For each storm duration, the total volume of run-off from this storm is calculated and from this volume is deducted the amount of water discharged from the retarding basin while the storm inflow into the retarding basin exceeds the maximum allowable discharge rate. The maximum remainder is the total volume of storage required for this particular retarding basin.

TEN-YEAR RAINFALL INTENSITY-DURATION TABLE

The following rainfall intensity-duration table is to be used for the calculation of ten-year inflow hydrographs to determine the maximum storage requirement in retarding basin design:

(A) Duration of Storm in Minutes | (B) Rainfall Intensity in Inches Per Hour |

10 | 4.95 |

11 | 4.78 |

12 | 4.63 |

13 | 4.48 |

14 | 4.34 |

15 | 4.21 |

16 | 4.09 |

17 | 3.98 |

18 | 3.87 |

19 | 3.76 |

20 | 3.67 |

21 | 3.57 |

22 | 3.49 |

23 | 3.40 |

24 | 3.32 |

25 | 3.25 |

26 | 3.17 |

27 | 3.10 |

28 | 3.04 |

29 | 2.97 |

30 | 2.91 |

35 | 2.64 |

40 | 2.41 |

45 | 2.22 |

50 | 2.06 |

55 | 1.92 |

60 | 1.80 |

70 | 1.60 |

80 | 1.43 |

90 | 1.30 |

100 | 1.19 |

110 | 1.10 |

120 | 1.02 |

RUN-OFF COEFFICIENTS

The following run-off coefficients and pervious area percentages are suggested for use in retarding basin design:

Pervious Area % | Impervious Area % | Run-off Coefficient “C” | |

Undeveloped land | 100 | 0 | 0.30 |

Rural Areas | 90 | 10 | 0.35 |

Residential Areas | 75 | 25 | 0.45 |

Apartment Areas | 25 | 75 | 0.75 |

Commercial Areas | 10 | 90 | 0.84 |

Industrial Areas | 25 | 75 | 0.75 |

DISCHARGE CONTROLS

There are two types of discharge controls for on-site retarding basins that have been considered for use in the City of Westlake. One discharge control type (off-stream retarding basins) permits all inflows less than the maximum discharge rate to pass downstream without storage. All inflows in excess of this maximum discharge rate are then diverted to storage in the retarding basin for release after the passage of the storm. Discharge under the other type of control (on-stream retarding basins) is dependent upon retarding basin storage depth, and discharge only reaches the maximum rate at the maximum retarding basin storage depth. This type of retarding basin therefore reaches its maximum storage depth when falling inflow rate drops down to equal maximum discharge rate.

The on-stream retarding basin-type of control is the one most generally used. The shape of the discharge rate curve for this type of control is generally an ogee curve, but for simplicity in analysis can be assumed to be a straight line from the origin of rainfall to its point of intersection with the inflow hydrograph at the maximum discharge rate.

The effect of these two types of control systems is illustrated by the hydrographs on the next page.

HYDROGRAPH

ON-STREAM RETARDING BASIN

LEGEND

1. Beginning of rainfall (Inflow = zero)

2. Concentration time of tributary watershed

(Inflow = rational equation with rainfall intensity at end of storm)

3. End of rainfall or duration of storm

(Inflow - rational equation with rainfall intensity at end of storm)

4. Calculated time of end of run-off (Inflow = zero)

5. Inflow hydrograph

6. Volume of storage required

7. Discharge curve

8. Maximum allowable discharge

Note: The trapezoidal shape of the above hydrographs will become a triangle if the time of concentration of the tributary watershed equals the duration of rainfall.

DESIGN EXAMPLE

DATA

WATERSHED AREA = 36.4 ACRES

CONCENTRATION TIME = 15 MIN.

EXISTING LAND USE = UNDEVELOPED

ALLOWABLE DISCHARGE = 0.204 CFS/AC.

FUTURE LAND USE = RESIDENTIAL

I10 @ 15 MIN. = 4.21 "/HR.

C = 0.45

INFILTRATION RATE = 0.5 "/HR.

CALCULATION OF MAXIMUM ALLOWABLE DISCHARGE RATE

Q10 = 0.204 A

Q10 = 0.204 x 36.4

Q10 = 7.43 CFS

CALCULATION OF REQUIRED STORAGE VOLUME

(1) RETARDING BASIN TYPE = ON-STREAM

DURATION OF STORM = 15 MIN.

PEAK INFLOW RATE

Q10 = CIA

Q10 = 0.45x4.21x36.4

Q10 = 69 CFS @ 15 MIN.

RUN-OFF RATE

I10 = (0.25x4.21)+(0.75x(4.21-0.5))

I10 = 1.05+2.78

I10 = 3.83 "/HR.

TOTAL INFLOW VOLUME

VT = 3.83 x 15 x 36.4 x 43,560

12 60

VT = 126,520 C.F.

END OF RUN-OFF TIME

TE = 126520x2

69x60

TE = 61.1 MIN.

VOLUME OF STORAGE REQUIRED

TOTAL INFLOW = 126,520 C.F.

LESS DISCHARGE = 7.43 x 61.1x60 = 13,620 C.F.

2

STORAGE VOLUME = 112,900 C.F.

OR 2.59 AC-FT

RETARDING BASIN TYPE = ON-STREAM

(2) DURATION OF STORM = 30 MIN.

I10 @ 30 MIN. = 2.91 "/HR.

PEAK INFLOW RATE

Q10 = CIA

Q10 = 0.45x2.91x36.4

Q10 = 47.7 CFS

RUN-OFF RATE

I10 = (0.25x2.91)+(0.74(2.91-0.5))

I10 = 0.73+1.81

I10 = 2.54 "/HR.

TOTAL INFLOW VOLUME

VT = 2.54 x 30 x 36.4 x 43,560

12 60

VT = 167,810 C.F.

END OF RUN-OFF TIME

TE = (167.810x2) - 15

( 47.7x60 )

TE = 117.3-15

TE = 102.3 MIN.

VOLUME OF STORAGE REQUIRED

TOTAL INFLOW = 167,810

LESS DISCHARGE = 7.43 x 102.3x60 = 22,800

2

STORAGE VOLUME = 145,010 C.F.

OR 3.33 AC-FT

RESULTS OF SIMILAR CALCULATIONS MADE

(3) FOR STORMS OF INCREASING DURATIONS

ARE TABULATED BELOW:

Storm Duration (Min.) | Peak Inflow Rate (cfs) | Volume of Inflow (c.f.) | Volume of Discharge (c.f.) | Storage Required (c.f.) |

15 | 69.0 | 126,520 | 13,620 | 112,900 |

30 | 47.7 | 167,810 | 22,800 | 145,010 |

40 | 39.5 | 179,320 | 28,180 | 151,140 |

50* | 33.7 | 186,040 | 33,170 | 152,870* |

60 | 29.5 | 188,380 | 37,450 | 150,930 |

70 | 26.2 | 188,710 | 41,240 | 147,470 |

* MAXIMUM STORAGE REQUIRED

THE MAXIMUM STORAGE REQUIRED FOR THIS ON-STREAM RETARDING BASIN WITH A MAXIMUM ALLOWABLE DISCHARGE OF 7.43 CFS IS THEREFORE A VOLUME OF 152,870 C.F. OR 3.51 AC.-FT.

(Ord. 1978-47. Passed 5-18-78.)