{"id":2849,"date":"2021-05-05T16:02:43","date_gmt":"2021-05-05T16:02:43","guid":{"rendered":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/?post_type=ht_kb&p=2849"},"modified":"2026-04-09T16:58:33","modified_gmt":"2026-04-09T16:58:33","slug":"connecting-a-pond-to-a-storm-sewer-system","status":"publish","type":"ht_kb","link":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/knowledge-base\/connecting-a-pond-to-a-storm-sewer-system\/","title":{"rendered":"Connecting a Pond to a Storm Sewer System"},"content":{"rendered":"\n

You may encounter site development drainage plans that include both storm sewer networks and detention ponds. One such scenario is shown below where a storm sewer system outfalls into a pond.<\/p>\n\n\n\n

\"\"
Site plan with storm sewer system that outfalls into a detention pond<\/figcaption><\/figure>\n\n\n\n
\"\"
Standard Report in Stormwater Studio<\/figcaption><\/figure>\n\n\n\n

Although Hydrology Studio lacks storm sewer system design capabilities, its counterpart, Stormwater Studio, offers this feature. If you have Stormwater Studio installed on your desktop, you will need to utilize it for designing your storm sewer system.<\/p>\n\n\n\n

\n

The challenge becomes, “How do I create an inflow hydrograph from this storm sewer for use in the detention pond?”<\/p>\n<\/blockquote>\n\n\n\n

It’s standard practice to use the Rational method for storm sewer design, but not for detention pond design. Most drainage authorities require the use of the Unit Hydrograph method, namely the NRCS method, for this type of analysis which takes into account volume and<\/em> peak Qs. Regardless of which method you use, Rational or NRCS, here’s how to connect your storm sewer to a detention pond.<\/p>\n\n\n\n

    \n
  1. Design your storm sewer system using the Rational method in Stormwater Studio.<\/li>\n\n\n\n
  2. From the Reports tab in Stormwater Studio, note the Total Drainage Area and System Tc in the Standard Report at the most downstream Line (Outfall). In the case above the total area is 5.7 acres and a Tc at the Outfall of 16.30 minutes.<\/li>\n\n\n\n
  3. Launch Hydrology Studio and add a runoff hydrograph of your choice, Rational or NRCS methods.<\/li>\n\n\n\n
  4. Rational Method:<\/em> Use the Total (C x A) output at the Outfall to get the composite C value. C = (C x A)\/A. In this case C = 3.80\/5.7 = 0.67. Now you can use these inputs, (C, Area and Tc) in Hydrology Studio to create an equivalent inflow hydrograph.<\/li>\n\n\n\n
  5. NRCS Method:<\/em>\u00a0Use the Total (C x A) output at the Outfall to get the composite C value. C = (C x A)\/A. In this case C = 3.80\/5.7 = 0.67. Convert the C to an equivalent CN value. We’ll choose 80 based on soil type, etc. (See Calculating Equivalent CN from C below)

    Now you can use these inputs, (CN, Area and Tc) in Hydrology Studio to create an NRCS inflow hydrograph.<\/li>\n\n\n\n
  6. Create and model your detention pond in Hydrology Studio.<\/li>\n<\/ol>\n\n\n\n

    Here are the two inflow hydrographs created in Hydrology Studio. Interestingly the Q peaks, in both inflow hydrographs as well as Stormwater Studio are very similar.<\/p>\n\n\n\n

    \"\"
    Red is the Rational, blue is NRCS 6-hour<\/figcaption><\/figure>\n\n\n\n

    Below is the simple Basin Model for this scenario using the NRCS method for the Inflow hydrograph. Simply use the inputs generated in Step 4 or 5 to create the inflow hydrograph for the detention pond.<\/p>\n\n\n\n

    \"\"<\/figure>\n\n\n\n

    Calculating Equivalent CN from C<\/h2>\n\n\n\n

    Below is a straightforward approach to derive an equivalent CN from a Rational Method C. It is logical to conclude that the Rational Method C indicates a percentage of imperviousness; for instance, a C value of 0.60 signifies 60% impervious surface. An equivalent CN can be calculated as:<\/p>\n\n\n\n

    \"\"<\/figure>\n\n\n\n

    Where:<\/p>\n\n\n\n

    IMP = % impervious
    X is taken from the table below and is based on the Soil Type.<\/p>\n\n\n\n\n\n\n\t\n\n\t
    Soil Type<\/th>A<\/th>B<\/th>C<\/th>D<\/th>\n<\/tr>\n<\/thead>\n
    X =<\/td>39<\/td>61<\/td>74<\/td>80<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n\n\n

    For a C = 0.60 and a Soil Type C, an equivalent CN is 98(.60) + 74(1-0.60) = 88.40<\/p>\n\n\n\n

    <\/p>\n","protected":false},"excerpt":{"rendered":"

    Shows how to connect your storm sewer to a detention pond<\/p>\n","protected":false},"author":1,"comment_status":"closed","ping_status":"closed","template":"","format":"standard","meta":{"footnotes":""},"ht-kb-category":[24],"ht-kb-tag":[],"class_list":["post-2849","ht_kb","type-ht_kb","status-publish","format-standard","hentry","ht_kb_category-building-your-basin-model"],"_links":{"self":[{"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb\/2849","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb"}],"about":[{"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/types\/ht_kb"}],"author":[{"embeddable":true,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/comments?post=2849"}],"version-history":[{"count":9,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb\/2849\/revisions"}],"predecessor-version":[{"id":4887,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb\/2849\/revisions\/4887"}],"wp:attachment":[{"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/media?parent=2849"}],"wp:term":[{"taxonomy":"ht_kb_category","embeddable":true,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb-category?post=2849"},{"taxonomy":"ht_kb_tag","embeddable":true,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb-tag?post=2849"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}