Query
The project team has considered the Rain Water Harvesting and Conservation Manual-Govt. of India for the rainwater harvesting calculations. However some differences have been observed in the calculation methodologies according to the RWHC Manual and the GRIHA manual. These differences are as follows:
1.The run-off coefficients provided in RWHC Manual (Table No.2) differ from the ones provided by the GRIHA Manual, especially in reference to that of hard paved surfaces (GRIHA Manual : 0.95 / RWHC Manual : 0.7-0.95).
2. A coefficient for spillage and evaporation is considered as per the RWHC Manual (2.7.1 (ii) on Page 13). However, the GRIHA Manual does not address this factor.
3. The peak rainfall has been calculated with reference to the IMD Data (Peak month rainfall / number of rainy days in the same month) and found to be 22.5 mm. According to the calculations as per the RWHC Manual, the volume of the rainwater recharge pit required is 28 KL whereas according to the calculations as per the GRIHA Manual the volume required is 42 KL.
It is also to be noted that the Haryana Pollution Control Board suggests one recharge pit per acre of total plot area whereas the plot area for the project is 0.52 acre. Therefore, the project currently complies with all local bye laws.
Please find attached all relevant references. Clarification is requested on whether the project may move forward with pre certification based on the calculations according to the Rain Water Harvesting and Conservation Manual.
Response
Note that RWHC is a guideline developed to understand rain water harvesting in detail. This guideline was developed in 2000. Project can use the process defined in the manual, however, data proposed should be updated from the relevant resources:
1. In RWHC manual (guidelines) a range of run off co-efficient have been provided 0.7 - 0.95 as the coefficient varies with different types of hard paving. Depending on the finish and base of the paving the run off may vary. To design rain water system to handle peak run off 0.95 has been used for all hard paving in GRIHA.
2. The manual clearly states that the coefficient can be considered, however, if the project wishes to consider this in the calculation, relevant data from a peer reviewed journal will be required. Please note that evaporation losses will be minimal during rains, and the calculation is being done to calculate run off generated during rainy season. Hence, the same was not considered in the run off calculation formula currently used by GRIHA.
3. As stated here the project wishes to consider peak rainfall. The same can be obtained directly from IMD rather than calculating the average daily/ average hourly value.