What are Drainage Plans & why are they Required?

Introduction

When it rains, water falls on the roof and flows into the downspouts and gutters. Then it’s finally directed to the ground. Where this water flows from here will depend on the drainage plan design. So, a drainage plan refers to the proposed technique of controlling rainwater on your lot. This helps to prevent the runoff from flowing into the adjoining property or flooding your yard. 

Besides, most counties in California require drainage plans when you’re undertaking a new construction project. But, why is this so important? Well, we’ll tell you all about it in this article! First, here is what you need to know about drainage plans. 

What are Drainage Plans? 

As we’ve already mentioned, a drainage plan is the proposed approach for containing and controlling surface runoff in your yard. In California, drainage plans are required to control the increased amount of runoff from rainwater. This increase in surface runoff usually occurs in the construction of a new structure on that property. 

Creating a drainage plan involves the implementation of a land grading program. Grading your land area properly will ensure that rainwater flows away from every structure on your property efficiently. After establishing the drainage plan, excavators and other machinery will be used to grand the land according to the specifications. 

A proper drainage system will prevent water from damaging structures in your lot. Moreover, it protects the environment by preventing soil erosion which can be caused by poor drainage.

Why are Drainage Plans Required?  

Almost every new structure increases the amount of water running off your property after heavy rainfall. Besides, activities like land grading eliminate depressions and natural pockets in the soil. The role of these pockets is to hold water until the soil absorbs it. Therefore, eliminating them will cause the water to runoff, instead of being absorbed into the soil. 

Similarly, such activities remove trees, bushes, and other vegetation. As a result, the amount of water that’s absorbed by roots is reduced. In addition, the construction of a house, driveway, pathways, and other impervious surfaces, prevent the underlying soil from absorbing surface water. The same case applies to a grass lawn since it is less absorbent than bare land. Not to forget other activities like driving trucks over the ground compacts the surface. 

All these factors affect the native soil’s ability to absorb water from rainfall. As a result, the excess water will ‘pool’ or ‘stand’ on the surface, if the land area doesn’t have sufficient drainage systems. Worst of all, the pooling can spill over into the adjoining property or the crawl space. 

On the bright side, a drainage plan will demonstrate how this issue has been addressed. Besides, this type of plan is required by the building department before a construction permit is issued. This applies when you’re planning to enlarge an existing structure or build a new structure. 

The Standard Method Drainage Plan 

Most homeowners use downspouts to direct the stormwater runoff from the roof into dry wells. To use this method, you have to submit a Plot Plan, which should include a pre-approved drainage plan. 

At the same time, this method can only be used if you meet the following criteria; 

• The property must have a site that’s dedicated to a dry well. However, the site shouldn’t conflict with any public wells, structure foundation, or septic drain field
• The soil on your land area must be adequately permeable. If your soil allows the use of a pressurized or conventional sewage system, it has adequate permeability. 
• The remaining part of your lot should be mainly covered by vegetation or landscaping. In other words, it should not be mainly covered with relatively impervious surfaces like driveways, concrete, or asphalt. Most counties may require submission of a drainage evaluation if such impervious surfaces take up more than 10% of the lot size. 

These guidelines help to simplify the evaluation of drainage plans. If your site meets the guidelines of this method, make sure the site plan clearly shows how the surface runoff flows. That will be enough to act as a drainage plan. In that case, you don’t need further drainage plans or evaluations, so your permit will not be delayed.

What if my development doesn’t meet the above guidelines? 

If you don’t prefer using dry wells or if your soil isn’t sufficiently permeable, there are alternative drainage plans you can use. For instance, you can use infiltration trenches retention berms. Alternatively, you can direct the stormwater runoff into underground storage tanks or detention ponds. 

However, to use an alternative method, the proposed development has to be assessed for its impact on stormwater runoff. Also, the Building Division must inspect the drainage system design before certifying the building for occupancy. More notably, a licensed expert in civil engineering has to provide a certification letter indicating that the site topography and soil permeability will not increase the stormwater runoff. 

Drainage Plan Requirements 

In general, drainage plan requirements can be grouped into 3 categories, depending on your situation. These categories include; 

• Non-residential development, land divisions, or Multi-family residential developments. When planning to undertake any of these developments, a drainage plan created by an experienced & licensed civil engineer is required. 
• Residential development in an area governed by a pre-approved drainage plan. In this case, you’ve to conform to the pre-approved drainage plan when building in such areas. But if you intend to deviate from the plan, the proposed drainage plan must be created by an experienced and licensed civil engineer. 
• Single-family residential development. This applies to areas that aren’t governed by pre-approved drainage plans. If you’re building a house in such an area, you can simply use the standard drainage method, which involves installing the appropriate dry well. 

Note, the Road Department must approve all drainage plans before a construction permit is issued. 

Final Word 

Having a Drainage Plan is very essential when it comes to controlling and containing stormwater runoff in your property. However, the plan must be comprehensive and include all the relevant details for it to be effective. If implemented properly, it will play an important role in your site pollution prevention and environmental management plan.